Archive for vertical flux – Page 2

WBC Series: Fine-scale biophysical controls on nutrient supply, phytoplankton community structure, and carbon export in western boundary current regions

Posted by mmaheigan 
· Friday, November 10th, 2017 

Sophie Clayton1, Peter Gaube1, Takeyoshi Nagai2, Melissa M. Omand3, Makio Honda4

1. University of Washington
2. Tokyo University of Marine Science and Technology, Japan
3. University of Rhode Island
4. Japan Agency for Marine-Earth Science and Technology, Japan

Western boundary current (WBC) regions are largely thought to be hotspots of productivity, biodiversity, and carbon export. The distinct biogeographical characteristics of the biomes bordering WBC fronts change abruptly from stable, subtropical waters to highly seasonal subpolar gyres. The large-scale convergence of these distinct water masses brings different ecosystems into close proximity allowing for cross-frontal exchange. Although the strong horizontal density gradient maintains environmental gradients, instabilities lead to the formation of meanders, filaments, and rings that mediate the exchange of physical, chemical, and ecological properties across the front. WBC systems also act as large-scale conduits, transporting tracers over thousands of kilometers. The combination of these local perturbations and the short advective timescale for water parcels passing through the system is likely the driver of the enhanced local productivity, biodiversity, and carbon export observed in these regions. Our understanding of biophysical interactions in the WBCs, however, is limited by the paucity of in situ observations, which concurrently resolve chemical, biological, and physical properties at fine spatial and temporal scales (1-10 km, days). Here, we review the current state of knowledge of fine-scale biophysical interactions in WBC systems, focusing on their impacts on nutrient supply, phytoplankton community structure, and carbon export. We identify knowledge gaps and discuss how advances in observational platforms, sensors, and models will help to improve our understanding of physical-biological-ecological interactions across scales in WBCs.

Mechanisms of nutrient supply

Nutrient supply to the euphotic zone occurs over a range of scales in WBC systems. The Gulf Stream and the Kuroshio have been shown to act as large-scale subsurface nutrient streams, supporting large lateral transports of nutrients within the upper thermocline (Pelegrí and Csanady 1991; Pelegrí et al. 1996; Guo et al. 2012; Guo et al. 2013). The WBCs are effective in transporting nutrients in part because of their strong volume transports, but also because they support anomalously high subsurface nutrient concentrations compared to adjacent waters along the same isopycnals (Pelegrí and Csanady 1991; Nagai and Clayton 2017; Komatsu and Hiroe pers. comm.). It is likely that the Gulf Stream and Kuroshio nutrient streams originate near the southern boundary of the subtropical gyres (Nagai et al. 2015a). Recent studies have suggested that nutrients in the Gulf Stream originate even farther south in the Southern Ocean (Williams et al. 2006; Sarmiento et al. 2004). These subsurface nutrients can then be supplied to the surface through a range of vertical supply mechanisms, fueling productivity in the WBC regions.

We currently lack a mechanistic understanding of how elevated nutrient levels in these “nutrient streams” are maintained, since mesoscale stirring should act to homogenize them. While it is well understood that the deepening of the mixed layer toward subpolar regions (along nutrient stream pathways) can drive a large-scale induction of nutrients to the surface layer (Williams et al., 2006), the detailed mechanisms driving the vertical supply of these nutrients to the surface layer at synoptic time and space scales remain unclear. Recent studies focusing on the oceanic (sub)mesoscale (spatial scales of 1-100 km) are starting to reveal mechanisms driving intermittent vertical exchange of nutrients and organisms in and out of the euphotic zone.

Recent surveys that resolved micro-scale mixing processes in the Kuroshio Extension and the Gulf Stream have reported elevated turbulence in the thermocline, likely a result of near-inertial internal waves (Nagai et al. 2009, 2012, 2015b; Kaneko et al. 2012, Inoue et al. 2010). In the Tokara Strait, upstream of the Kuroshio Extension, where the geostrophic flow passes shallow topography, pronounced turbulent mixing oriented along coherent banded layers below the thermocline was observed and linked to high-vertical wavenumber near-inertial internal waves (Nagai et al. 2017; Tsutsumi et al. 2017). Within the Kuroshio Extension, measurements made by autonomous microstructure floats have revealed vigorous microscale temperature dissipation within and below the Kuroshio thermocline over at least 300 km following the main stream, which was attributed to active double-diffusive convection (Nagai et al. 2015c). Within the surface mixed layer, recent studies have shown that downfront winds over the Kuroshio Extension generate strong turbulent mixing (D’Asaro et al. 2011; Nagai et al. 2012). The influence of fine-scale vertical mixing on nutrient supply was observed during a high-spatial resolution biogeochemical survey across the Kuroshio Extension front, revealing fine-scale “tongues” of elevated nitrate arranged along isopycnals (Figure 1, Clayton et al. 2014). Subsequent modeling work has shown that these nutrient tongues are ubiquitous features along the southern flank of the Kuroshio Extension front, formed by submesoscale surface mixed layer fronts (Nagai and Clayton 2017).

Microscale turbulence, double-diffusive convection, and submesoscale stirring are all processes associated with meso- and submesoscale fronts. The results from the studies mentioned above support the hypothesis that WBCs are an efficient conduit for transporting nutrients, not only over large scales but also more locally on fine scales, as isopycnal transporters, lateral stirrers, and diapycnal suppliers. It is the sum of these transport processes that ultimately fuels the elevated primary production observed in these regions.

Figure 1. Vertical sections of nitrate (μM) observed across the Kuroshio Extension in October 2009. The panels are organized such that they line up with respect to the density structure of the Kuroshio Extension Front. Cyan contour lines show the mixed layer depth (taken from Nagai and Clayton 2017).

Phytoplankton biomass, community structure, and dynamics

WBCs separate regions with markedly different biogeochemical and ecological characteristics. Subpolar gyres are productive, highly seasonal, tend to support ecosystems with higher phytoplankton biomass, and can be dominated by large phytoplankton and zooplankton taxa. Conversely, subtropical gyres are mostly oligotrophic, support lower photoautotrophic biomass, and are not characterized by a strong seasonal cycle. In turn, these subtropical regions tend to support ecosystems that comprise smaller cells and a tightly coupled microbial loop. As boundaries to these diverse regions, WBCs are the main conduit linking the equatorial and polar oceans and their resident plankton communities. Within the frontal zones, mesoscale dynamics act to stir water masses together and can transport ecosystems across the WBC into regions of markedly different physical and biological characteristics. Furthermore, mesoscale eddies can modulate vertical fluxes via the displacement of ispycnals during eddy intensification or eddy-induced Ekman pumping, or generating submesoscale patches of vertical exchange. At these smaller scales, vigorous vertical circulations ¾ with magnitudes reaching 100 m/day ¾ can fertilize the euphotic zone or transport phytoplankton out of the surface layer.

Numerous studies have hypothesized that the combination of large-scale transport, mesoscale stirring and transport, and submesoscale nutrient input leads to both high biodiversity and high population densities. Using remote sensing data, D’Ovidio et al. (2010) showed that mesoscale stirring in the Brazil-Malvinas Confluence Zone brings together communities from very different source regions, driving locally enhanced biodiversity. In a numerical model, in which physical and biological processes can be explicitly separated and quantified, Clayton et al. (2013) showed that high modeled biodiversity in the WBCs was due to a combination of transport and local nutrient enhancements. And finally, in situ taxonomic surveys crossing the Brazil-Malvinas Confluence (Cermeno et al. 2008) and the Kuroshio Extension (Honjo and Okada 1974; Clayton et al, 2017) showed both enhanced biomass and biodiversity associated with the WBC fronts. Beyond these local enhancements, WBCs might play a larger role in setting regional biogeography. Sugie and Suzuki (2017) found a mixture of temperate and subpolar diatom species in the Kuroshio Extension, suggesting that the boundary current might play a key role in setting downstream diatom diversity.

However tantalizing these results are, they remain relatively inconclusive, in part because of their relatively small temporal and spatial scales. Extending existing approaches for assessing phytoplankton community structure, leveraging emerging ‘omics and continuous sampling techniques, larger regions might be surveyed at high taxonomic and spatial resolution. Combining genomic and transcriptomic observations would provide measures of both organism abundance and activity (Hunt et al. 2013), as well as the potential to better define the relative roles of growth and loss processes. With genetically resolved data and appropriate survey strategies, it will be possible to conclusively determine the presence of these biodiversity hotspots. A better characterization and deeper understanding of these regions will provide insight into the long-term and large-scale biodiversity, stability, and function of the global planktonic ecosystem.

Organic carbon export via physical and biological processes

Export, the removal of fixed carbon from the surface ocean, is driven by gravitational particle sinking, active transport, and (sub)mesoscale processes such as eddy-driven subduction. While evidence suggests that WBCs are likely hot spots of biological (Siegel et al. 2014; Honda et al. 2017a) and physical (Omand et al. 2015) export fluxes out of the euphotic zone, only a small handful of studies have explored this. Recent results from sediment trap studies at the Kuroshio Extension Observatory (KEO) mooring, located just south of the Kuroshio Extension, suggest that there is a link between the passage of mesoscale eddies and carbon export (Honda et al. 2017b). They observed that high export events at 5000 m lagged behind the passage of negative (cyclonic) sea surface height anomalies (SSHA) at the mooring by one to two months (Figure 2). In other regions, underway measurements (Stanley et al. 2010) and optical sensors on autonomous platforms (Briggs et al. 2011; Estapa et al. 2013; Estapa et al. 2015; Bishop et al. 2016) have revealed large episodicity in export proxies over timescales of hours to days and spatial scales of 1-10 km.

Figure 2. Time series of ocean temperature in the upper ~550 m (less than 550 dbar) at station KEO between July 2014 and June 2016. The daily data shown in the figure are available on the KEO database. White contour lines show the temporal variability in the daily satellite-based sea surface height anomaly (SSHA). White open bars show the total mass flux (TMF) observed by the time series sediment trap at 5000 m (based on a figure in Honda et al. 2017b).

Another avenue of carbon export from the surface ocean results from grazing and vertical migration. Vertically migrating zooplankton feed near the surface in the dark and evade predation at depth during the day. Fronts generated by WBCs produce gradients in zooplankton communities, both in terms of grazer biomass and species compositions (e.g., Wiebe and Flierl, 1983), and influence the extent and magnitude of diel vertical migrations. Submesoscale variability in zooplankton abundance can be observed readily in echograms collected by active acoustic sensors, but submesoscale variability in zooplankton community structure and dynamics remains difficult to measure. Thus, the nature of this variability remains largely unknown.

Future research directions

Building a better understanding of how physical and biogeochemical dynamics in WBC regions interact relies on observing these systems at the appropriate scales. This is particularly challenging because of the range of scales at play in these systems and the limitation of existing in situ and remote observing platforms and techniques. As has been outlined above, the ecological and biogeochemical environment of WBCs is the result of long range transport from the flanking subtropical and subpolar gyres, as well as local modification by meso- and submesocale physical dynamics in these frontal systems.

Another challenge in disentangling the relationships between physical and biogeochemical processes in WBCs is the difficulty in measuring rates rather than standing stocks. In such dynamic systems, lags in biological responses mean that the changes in standing stocks may not be collocated with the physical process forcing them. Small-scale lateral stirring spatially and temporally decouples net community production and export while secondary circulations contribute to vertical transport. As much as possible, future process studies should include approaches that can explicitly quantify biological rates and physical transport pathways. New platforms are beginning to fill these observational gaps: BGC-Argo floats, autonomous platforms (e.g., Saildrone), high-frequency underway measurements, and continuous cytometers (including imaging cytometers) are all capable of generating high-spatial resolution datasets of biological and chemical properties over large regions. Gliders and profiling platforms (e.g., WireWalker) are making it possible to measure vertical profiles of biogeochemical properties at high frequency. Operating within a Lagrangian framework, while resolving lateral gradients of physical and biogeochemical tracers with ships or autonomous vehicles, may someday allow us to quantitatively partition the observed small-scale variability in biogeochemical tracers between that attributable to biological or physical processes.

 

 

 

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The Ross Sea deep microbial community’s role in sequestering CO2

Posted by mmaheigan 
· Thursday, November 9th, 2017 

Antarctic shelf systems generate the densest waters in the world. These shelf waters are the building blocks of Antarctic Bottom Water, the ocean’s abyssal water mass. These bottom waters have the potential to sequester carbon out of the atmosphere for millennia. One such form of marine carbon is dissolved organic carbon (DOC). DOC is produced in the surface ocean via primary production and is the global ocean’s largest standing stock of reduced carbon.

In a recent study, Bercovici et al (2017) used hydrographic and biogeochemical measurements to assess the mechanism that brings DOC into the shelf waters of the Ross Sea, the shelf system in the Pacific sector of Antarctica. These mechanisms include sinking particles, brine rejection caused by katabatic winds in the Terra Nova Bay polynya, and vertical mixing. This study revealed that DOC is primarily introduced into the deeper shelf waters via convective overturning and deep vertical mixing upon the onset of austral winter. Substantial DOC enrichment of shelf waters suggests that this carbon is exported off the shelf into Antarctic Bottom Water. However, this study finds much of the excess Ross Sea shelf DOC is actually consumed and remineralized to CO2 by deep microbial communities at the slope of the Ross Sea shelf, ultimately sequestering this carbon into the ocean’s interior.

Physical and biological processes have the potential to introduce carbon into the dense shelf waters (blue) in the Ross Sea. Incoming waters (yellow) are modified from the Southern Ocean’s circumpolar waters. At the onset of winter, cooler temperatures and katabatic winds cause brine rejection. The rejection of brine, sinking particles and vertical mixing are all potential mechanisms for bringing DOC to the dense shelf waters. At the shelf slope, outflowing shelf waters ultimately contribute to Antarctic Bottom Water formation. This research furthers our understanding of global carbon cycling through demonstrating that Antarctic shelf systems have the potential to sequester organic carbon into the abyssal ocean.

Authors:
Sarah K. Bercovici (Rosenstiel School of Marine and Atmospheric Science, University of Miami)
Bruce A. Huber (Lamont Doherty Earth Observatory, Columbia University)
Hans B. Dejong (Stanford University)
Robert B. Dunbar (Stanford University)
Dennis A. Hansell (Rosenstiel School of Marine and Atmospheric Science, University of Miami)

Tiny marine animals strongly influence the carbon cycle

Posted by mmaheigan 
· Thursday, August 31st, 2017 

What controls the amount of organic carbon entering the deep ocean? In the sunlit layer of the ocean, phytoplankton transform inorganic carbon to organic carbon via a process called photosynthesis. As these particulate forms of organic carbon stick together, they become dense enough to sink out of the sunlit layer, transferring large quantities of organic carbon to the deep ocean and out of contact with the atmosphere.

However, all is not still in the dark ocean. Microbial organisms such as bacteria, and zooplankton consume the sinking, carbon-rich particles and convert the organic carbon back to its original inorganic form. Depending on how deep this occurs, the carbon can be physically mixed back up into the surface layers for exchange with the atmosphere or repeat consumption by phytoplankton. In a recent study published in Biogeosciences, researchers used field data and an ecosystem model in three very different oceanic regions to show that zooplankton are extremely important in determining how much carbon reaches the deep ocean.

Figure 1. Particle export and transfer efficiency to the deep ocean in the Southern Ocean (SO, blue circles), North Atlantic Porcupine Abyssal Plain site (PAP, red squares) and the Equatorial Tropical North Pacific (ETNP, orange triangles) oxygen minimum zone. a) particle export efficiency of fast sinking particles (Fast PEeff) against primary production on a Log10 scale. b) transfer efficiency of particles to the deep ocean expressed as Martin’s b (high b = low efficiency). Error bars in b) are standard error of the mean for observed particles, error too small in model to be seen on this plot.

In the Southern Ocean (SO), zooplankton graze on phytoplankton and produce rapidly sinking fecal pellets, resulting in an inverse relationship between particle export and primary production (Fig. 1a). In the North Atlantic (NA), the efficiency with which particles are transferred to the deep ocean is comparable to that of the Southern Ocean, suggesting similar processes apply; but in both regions, there is a large discrepancy between the field data and the ecosystem model (Fig. 1b), which poorly represents particle processing by zooplankton. Conversely, much better data-model matches are observed in the equatorial Pacific, where lower oxygen concentrations mean fewer zooplankton; this reduces the potential for zooplankton-particle interactions that reduce particle size and density, resulting in a lower transfer efficiency.

This result suggests that mismatches between the data and model in the SO and NA may be due to the lack of zooplankton-particle parameterizations in the model, highlighting the potential importance of zooplankton in regulating carbon export and storage in the deep ocean. Zooplankton parameterizations in ecosystem models must be enhanced by including zooplankton fragmentation of particles as well as consumption. Large field programs such as EXPORTS could help constrain these parameterisation by collecting data on zooplankton-particle interaction rates. This will improve our model estimates of carbon export and our ability to predict future changes in the biological carbon pump. This is especially important in the face of climate-driven changes in zooplankton populations (e.g. oxygen minimum zone (OMZ) expansion) and associated implications for ocean carbon storage and atmospheric carbon dioxide levels.

 

Authors:
Emma L. Cavan (University of Tasmania)
Stephanie A. Henson (National Oceanography Centre, Southampton)
Anna Belcher (University of Southampton)
Richard Sanders (National Oceanography Centre, Southampton)

Biophysical drivers of vigorous carbon cycling near the Kuroshio Extension

Posted by mmaheigan 
· Thursday, April 27th, 2017 

The Kuroshio Current and its Extension jet in the western North Pacific Ocean form a dynamic western boundary current (WBC) region characterized by large air-sea exchanges of heat and carbon dioxide gas (CO2). The jet is known to oscillate between stable and meandering states on multi-year timescales that alter the eddy field and depth of winter mixing in the southern recirculation gyre. These dynamic state changes have been shown to imprint biogeochemical signatures onto regional mode waters that can be distributed widely throughout the North Pacific and remain out of contact with the atmosphere for decades.

Figure. ~7 years of (a) AVISO daily sea surface height (SSH) anomalies and (b) upper-ocean temperature from the NOAA Kuroshio Extension Observatory (KEO) surface mooring. Black and gray lines in b show the mixed layer depth (MLD) and 17C contour, respectively. Spring bloom periods are indicated in blue in a. The semi-regular upwelling of cold water and corresponding depression of SSH is caused by cold-core eddies that pass the KEO mooring. Winter ventilation depths increase by ~100 m after 2010 when the extension jet entered a stable phase.

To better characterize carbon cycling in this region, ~7 years of daily-averaged autonomous CO2 observations from NOAA’s Kuroshio Extension Observatory (KEO) surface mooring were used to close the mixed layer carbon budget. High rates of net community production (NCP; >100 mmol C m-2 d-1) were observed during the spring bloom period, and a mean annual NCP of 7±3 mol C m-2 yr-1 was determined. Biological processes near KEO largely balance the input of carbon that occurs annually through winter mixing; however, physical processes that deviate from climatology were not resolved in this study. Therefore, it remains unclear how transient features such as eddies influence biological carbon production and export through altered nutrient supply and active vertical transport of organic material. Further work is required to determine how biophysical interactions during mesoscale and submesoscale disturbances contribute to local carbon cycle processes and variability in regional mode water carbon inventories.

Ocean Carbon Hot Spots, an upcoming workshop focused on understanding biophysical drivers of carbon uptake in WBC regions, will be held September 25-26, 2017 at the Monterey Bay Aquarium Research Institute (MBARI) in Moss Landing, California. The primary objective of the workshop is to develop a community of observationalists and modelers working on the topic, and to identify critical observational needs that would improve model parameterizations. Ocean Carbon Hot Spots will be co-sponsored by US CLIVAR, US OCB, MBARI, and OMIX.

Written by Andrea J. Fassbender, Monterey Bay Aquarium Research Institute

 

Mixed-layer carbon cycling at the Kuroshio Extension Observatory (Global Biogeochemical Cycles) 

Authors:
Andrea J. Fassbender (Monterey Bay Aquarium Research Institute)
Christopher L. Sabine (NOAA Pacific Marine Environmental Laboratory)
Meghan F. Cronin (NOAA Pacific Marine Environmental Laboratory)
Adrienne J. Sutton (Joint Institute for the Study of the Atmosphere and Ocean, University of Washington)

Oceanic fronts enhance carbon transport to the ocean’s interior through both subduction and amplified sinking

Posted by mmaheigan 
· Wednesday, March 1st, 2017 

Mesoscale fronts are regions with potentially enhanced nutrient fluxes, phytoplankton production and biomass, and aggregation of mesozooplankton and higher trophic levels. However, the role of these features in transporting organic carbon to depth and hence sequestering CO2 from the atmosphere has not previously been determined. Working with the California Current Ecosystem Long Term Ecological Research (CCE LTER) program, we determined that the flux of sinking particles at a stable front off the coast of California was twice as high as similar fluxes on either side of the front, or in typical non-frontal waters of the CCE in a recent study by Stukel et al. (2017) published in Proceedings of the National Academy of Sciences.

This increased export flux was tied to enhanced silica-ballasting by Fe-stressed diatoms and to an abundance of mesozooplankton grazers. Furthermore, downward transport of particulate organic carbon by subduction at the front led to additional carbon export that was similar in magnitude to sinking flux, suggesting that these fronts (which are a common feature in productive eastern boundary upwelling systems) are an important conduit for carbon sequestration. These enhanced carbon export mechanisms at episodic and mesoscale features need to be included in future biogeochemical forecast models to understand how a changing climate will affect marine CO2 uptake.

Authors

Michael R. Stukel (Florida State University)
Lihini I. Aluwihare, Katherine A. Barbeau, Ralf Goericke, Arthur J. Miller, Mark D. Ohman, Angel Ruacho, Brandon M. Stephens, Michael R. Landry (University of California, San Diego)
Hajoon Song (Massachusetts Institute of Technology)
Alexander M. Chekalyuk (Lamont-Doherty Earth Observatory)

ENSO impacts on ecosystem indicators in the California Current System

Posted by mmaheigan 
· Thursday, February 16th, 2017 

El Niño-Southern Oscillation (ENSO) events activate long-distance teleconnections through the atmosphere and ocean that can dramatically impact marine ecosystems along the West Coast of North America, affecting diverse organisms ranging from plankton to exploitable and protected species. Such ENSO-related changes to marine ecosystems can ultimately affect humans in many ways, including via depressed plankton and fish production, dramatic range shifts for many protected and exploited species, inaccessibility of traditionally fished resources, more prevalent harmful algal blooms, altered oxygen and pH of waters used in mariculture, and proliferation of pathogens. The principal objective of the Forecasting ENSO Impacts on Marine Ecosystems of the US West Coast workshop was to develop a scientific framework for building an ENSO-related forecast system of ecosystem indicators along the West Coast of North America, including major biological and biogeochemical responses. Attendees realized that a quantitative, biologically-focused forecast system is a much more challenging objective than forecasting the physical system alone; it requires an understanding of the ocean-atmospheric physical system and of diverse organism-level, population-level, and geochemical responses that, in aggregate, lead to altered ecosystem states.

In the tropical ocean, important advances have been made in developing both intensive observational infrastructure (Global Tropical Moored Buoy Array) and diverse dynamical and statistical models that utilize these data in ENSO forecasting. These forecasts are made widely available (e.g., NOAA’s Climate Prediction Center). The most sophisticated ENSO-forecasting efforts use global, coupled ocean-atmosphere climate models that extend ENSO-forecasting skill into seasonal climate forecasting skill for other regions, including the California Current System (CCS). However, both these measurement systems and forecast models are restricted to the physical dynamics of ENSO, rather than biotic and biogeochemical consequences.

Primary modes of influence of El Niño on marine organisms

In this brief discussion, we focus primarily on the warm (El Niño) phases of ENSO, which can have large and generally negative ecosystem consequences, although changes accompanying the cold phases (La Niña) can also be significant. We primarily address pelagic ocean processes, which merely reflect the expertise of the participants at the workshop. Physical mechanisms by which ENSO impacts the U.S. West Coast are more completely explained in Jacox et al. (this issue).

El Niño affects organisms and biogeochemistry via both local and advective processes (Figure 1). ENSO-related changes in the tropics can affect the CCS through an atmospheric teleconnection (Alexander et al. 2002) to alter local winds and surface heat fluxes, and through upper ocean processes (thermocline and sea level displacements and geostrophic currents) forced remotely by poleward propagating coastally trapped waves (CTWs) of tropical origin (Enfield and Allen 1980; Frischkencht et al. 2015; Figure 1). It is important to recognize that ecosystem effects will occur through three primary mechanisms: (1) via the direct action of altered properties like temperature, dissolved O2, and pH on the physiology and growth of marine organisms; (2) through food web effects as changes in successive trophic levels affect their predators (bottom up) or prey (top down); and (3) through changes in advection related to the combination of locally forced Ekman transport and remotely forced geostrophic currents, typically involving poleward and/or onshore transport of organisms. Advective effects can be pronounced, transporting exotic organisms into new regions and altering the food web if these imported species have significant impacts as predators, prey, competitors, parasites, or pathogens.

Figure 1. Schematic illustration of dominant mechanisms through which ENSO impacts biological and biogeochemical processes in the California Current System. Processes include both local effects (e.g., heat budget, winds) and advective effects. Such processes can influence organisms via: (1) (yellow arrow) direct physiological responses to changes in temperature, O2, pH, etc.; (2) (orange arrows) effects that propagate through the food web, as successive trophic levels affect their predators (bottom up, upward-facing orange arrows) or prey (top down, downward-facing orange arrows); (3) (blue arrows) direct transport effects of advection. Top predators are not included here. CTW indicates coastally trapped waves.

 

I. Poleward and onshore transport

Active, mobile marine fishes, seabirds, reptiles, and mammals may move into new (or away from old) habitats in the CCS as ENSO-related changes occur in the water column and render the physical-chemical characteristics and prey fields more (or less) suitable for them. Planktonic organisms are often critical prey and are, by definition, subject to geographic displacements as a consequence of altered ocean circulation that accompanies El Niño events. Most commonly, lower latitude organisms are transported poleward to higher latitudes in either surface flows or in an intensified California Undercurrent (Lynn and Bograd 2002). However, some El Niño events are accompanied by onshore flows (Simpson 1984), potentially displacing offshore organisms toward shore (Keister et al. 2005).

Two of the most celebrated examples of poleward transport come from distributions of pelagic red crabs (Pleuroncodes planipes) and the subtropical euphausiid (or krill, Nyctiphanes simplex), both of which have their primary breeding populations in waters off Baja California, Mexico (Boyd 1967; Brinton et al. 1999). Pelagic red crabs were displaced approximately 10° of latitude, from near Bahia Magdalena, Baja California, northward to Monterey, California (Glynn 1961; Longhurst 1967) during the El Niño of 1958-1959. This early event was particularly well documented because of the broad latitudinal coverage of the California Cooperative Oceanic Fisheries Investigations (CalCOFI) cruises at the time. Such El Niño-related northward displacements have been documented repeatedly over the past six decades (McClatchie et al. 2016), partly because the red crabs often strand in large windrows on beaches and are conspicuous to the general public. The normal range of the euphausiid Nyctiphanes simplex is centered at 25-30°N (Brinton et al. 1999). N. simplex has been repeatedly detected far to the north of this range during El Niño, extending at least to Cape Mendocino (40.4°N) in 1958 (Brinton 1960), to northern Oregon (46.0°N) in 1983 (Brodeur 1986), and to Newport, Oregon (44.6°N; Keister et al. 2005) and northwest Vancouver Island (50.7°N; Mackas and Galbraith 2002) in 1998. In spring of 2016, N. simplex were extremely abundant in the southern California region (M. Ohman and L. Sala, personal communication) and detected as far north as Trinidad Head (41.0°N) but not in Newport, Oregon (W. Peterson, personal communication). Sometimes such El Niño-related occurrences of subtropical species are accompanied by declines in more boreal species (e.g., Mackas and Galbraith 2002; Peterson et al. 2002), although this is not always the case.

Among the organisms displaced during El Niños, the consequences of transport of predators are poorly understood but likely significant in altering the food web.  Subtropical fishes can be anomalously abundant in higher latitudes during El Niño (Hubbs 1948; Lluch-Belda et al. 2005; Pearcy and Schoener 1987; Pearcy 2002; Brodeur et al. 2006), with significant consequences for the resident food web via selective predation on prey populations.

II. Habitat compression

Many species are confined to a specific habitat that may compress during El Niño. This phenomenon has been observed repeatedly for species and processes related to coastal upwelling in the CCS. During major El Niño events, as the offshore extent of upwelled waters is reduced and becomes confined close to the coast, the zone of elevated phytoplankton (observed as Chl-a) compresses markedly to a narrow zone along the coastal boundary (e.g., Kahru and Mitchell 2000; Chavez et al. 2002). For example, during the strong El Niño spring of 1983, the temperate euphausiid Euphausia pacifica was present in low densities throughout Central and Southern California waters, but 99% of the biomass was unusually concentrated at a single location (station 80.51) very close to Point Conception, where upwelling was still pronounced (E. Brinton, personal communication). The spawning habitat of the Pacific sardine (Sardinops sagax) was narrowly restricted to the coastal boundary during El Niño 1998, but one year later during La Niña 1999, the spawning habitat extended a few hundred kilometers farther offshore (Lo et al. 2005). Market squid, Doryteuthis opalescens, show dramatically lower catches during El Niño years (Reiss et al. 2004), but in 1998, most of the catch was confined to a small region in Central California (Reiss et al. 2004). During the El Niño in spring 2016, vertical particle fluxes measured by sediment traps were reduced far offshore but remained elevated in the narrow zone of coastal upwelling very close to Point Conception (M. Stukel, personal communication).

III. Altered winds and coastal upwelling

Upwelling-favorable winds along the US West Coast may decline during El Niño conditions (Hayward 2000, but see Chavez et al. 2002) and vertical transports can be reduced (Jacox et al. 2015), mainly during the winter and early spring (Black et al. 2011). Independent of any changes in density stratification (considered below), these decreased vertical velocities can lead to diminished nutrient fluxes, reduced rates of primary production, and a shift in the size composition of the plankton community to smaller phytoplankton and zooplankton (Rykaczewski and Checkley 2008). Such changes at the base of the food web can have major consequences for a sequence of consumers at higher trophic levels, as both the concentration and suitability of prey decline.

However, there are potential compensatory effects of reduced rates of upwelling. Diminished upwelling also means less introduction of CO2-rich, low-oxygen waters to coastal areas (Feely et al. 2008; Bednaršek et al. 2014), with potential benefits to organisms that are sensitive to calcium carbonate saturation state or hypoxic conditions. Furthermore, reduced upwelling implies lower Ekman transport and potentially reduced cross-shore fluxes far offshore within coastal jets and filaments (cf., Keister al. 2009).

IV. Increased stratification and deepening of nutricline

El Niño-related warming of surface waters and increased density stratification can result from advection of warmer waters and/or altered local heating. Evidence suggests that the pycnocline (Jacox et al. 2015) and nitracline (Chavez et al. 2002) deepen during stronger El Niños. This effect, independent of variations in wind stress, also leads to diminished vertical fluxes of nitrate and other limiting nutrients and suppressed rates of primary production. Decreased nitrate fluxes appear to explain elevated 15N in California Current zooplankton (Ohman et al. 2012) and decreased krill abundance (Lavaniegos and Ohman 2007; Garcia-Reyes et al. 2014) during El Niño years. For example, the 2015-16 El Niño resulted in a pronounced warming of surface waters and depressed Chl-a concentrations across a broad region of the CCS (McClatchie et al. 2016).

V. Direct physiological responses to altered temperature, dissolved O2, pH

Most organisms in the ocean—apart from some marine vertebrates—are ectothermic, meaning they have no capability to regulate their internal body temperature. Heating or cooling of the ocean therefore directly influences their rates of metabolism, growth, and mortality. Most organisms show not only high sensitivity to temperature variations but nonlinear responses. A typical temperature response curve or “thermal reaction norm” (e.g., of growth rate) is initially steeply positive with increasing temperature, followed by a narrow plateau, then abruptly declines with further increases in temperature (e.g., Eppley 1972). Different species often show different thermal reaction norms. Hence, El Niño-related temperature changes may not only alter the growth rates and abundances of organisms, but also shift the species composition of the community due to differential temperature sensitivities.

Similarly, El Niño-induced variations in dissolved oxygen concentration and pH can have marked consequences for physiological responses of planktonic and sessile benthic organisms and, for active organisms, potentially lead to migrations into or out of a suitable habitat. Interactions between variables (Boyd et al. 2010) will also lead to both winners and losers in response to major ENSO-related perturbations.

Altered parasite, predator populations, and harmful algal blooms

ENSO-related changes can favor the in situ proliferation or introduction of predators, parasites, pathogens, and harmful algal blooms. Such outbreaks can have major consequences for marine ecosystems, although some are relatively poorly studied. For example, a recent outbreak of sea star wasting disease thought to be caused by a densovirus adversely affected sea star populations at numerous locations along the West Coast (Hewson et al. 2014). While not specifically linked to El Niño, this outbreak was likely tied to warmer water temperatures. Because some sea stars are keystone predators capable of dramatically restructuring benthic communities (Paine 1966), such pathogen outbreaks are of considerable concern well beyond the sea stars themselves.

Domoic acid outbreaks, produced by some species of the diatom genus Pseudo-nitzschia, can result in closures of fisheries for razor clams, Dungeness crab, rock crab, mussels, and lobsters, resulting in significant economic losses. While the causal mechanisms leading to domoic outbreaks are under discussion (e.g., Sun et al. 2011; McCabe et al. 2016), warmer-than-normal ocean conditions in northern regions of the CCS have been linked to domoic acid accumulation in razor clams, especially when El Niño conditions coincide with the warm phase of the Pacific Decadal Oscillation (McKibben et al. 2017).

ENSO diversity, non-stationarity, and consequences of secular changes

There is considerable interest in understanding the underlying dynamical drivers that lead to different El Niño events (Singh et al. 2011; Capotondi et al. 2015). Although there appears to be a continuum of El Niño expression along the equatorial Pacific, some simplify this continuum to a dichotomy between Eastern Pacific (EP) and Central Pacific (CP) events (Capotondi et al 2015). Whether EP and CP El Niños have different consequences for mid-latitude ecosystems like the California Current Ecosystem is an area of open research, but some evidence suggests that differences in timing and intensity of biological effects may exist (cf. Fisher et al. 2015). While some studies (e.g., Lee and McPhaden 2010) suggest that the frequency of CP El Niños is increasing, the evidence is not definitive (Newman et al. 2011). In addition to questions about the ecosystem consequences of El Niño diversity, there are unknowns regarding interactions between El Niño, decadal-scale variability (Chavez et al. 2002), and secular changes in climate (Figure 2, Ohman, unpubl.), which suggest a non-stationary relationship between California Current zooplankton and El Niño. An index of the dominance of warm water krill from CalCOFI sampling in Southern California shows that for the first 50 years there was a predictable positive relationship between these warm water krill and El Niño. This relationship held during both EP and CP El Niño events from 1950-2000. However, the relationship appeared to weaken after 2000. The warm water krill index was negatively correlated with the moderate El Niño of 2009-10. While the krill index again responded to the major El Niño of 2015-16 and the preceding year of warm anomalies (Bond et al. 2015; Zaba and Rudnick 2016), the magnitude of the response was not comparable to what had been seen in earlier decades. It is unclear whether such results are merely the consequence of interannual variability in the mode of El Niño propagation (Todd et al. 2011) or a change in the relationship between El Niño forcing and ecosystem responses.

 

Figure 2. Covariability of California Current euphausiids (krill, blue lines) with an index of ENSO off California (de-trended sea level anomaly [DTSLA] at San Diego, green lines). Note the markedly different relationship between euphausiids and DTSLA after 2000. Sustained excursions of DTSLA exceeding one standard deviation (i.e., above upper dotted red line) are expressions of El Niño (or of the warm anomaly of 2014-2015). Red arrows indicate specific events categorized as either eastern Pacific (EP) or central Pacific (CP) El Niño events (Yu et al. 2012), apart from 2015-2016 which could be either CP or EP. The Warm-Cool euphausiid index is based on the difference in average log carbon biomass anomaly of the four dominant warm water euphausiids in the CCS minus the average anomaly of the four dominant cool water euphausiids (species affinities from Brinton and Townsend 2003). Euphausiid carbon biomass from springtime CalCOFI cruises off Southern California, lines 77-93, nighttime samples only. Dotted blue lines indicate years of no samples (Ohman, personal communication).

Conclusions

While the potential modes of El Niño influence on biological and biogeochemical processes in the CCS are numerous, not all processes are of first order consequence to all organisms. Forecasting ENSO effects on a given target species will likely focus on a limited number of governing processes. Table 1 illustrates some of the specific types of organisms susceptible to El Niño perturbations and the suspected dominant mechanism. We look forward to developing a framework for forecasting such responses in a quantitative manner.

Ecosystem indicator Region and season Change during El Niño Time scale of response Regional ocean processes
Primary production Entire CCS

winter, spring, summer

 Declines Variable lag;

Instantaneous or time-lagged

 Reduced upwelling, nutrient fluxes; Deeper nutricline and weaker winds
Pseudo-nitzschia diatoms; Domoic Acid Entire CCS

spring-summer

 Blooms  

1-3 month lag

 Elevated temperature; Altered nutrient stoichiometry
Copepod assemblage NCCS

spring-summer

 Warm water species appear Nearly instantaneous Poleward advection; Reduced upwelling, warmer temperature
 

Subtropical euphausiids

  

SCCS

spring-summer

  

Increase

 Nearly instantaneous; persists beyond Niño event Poleward advection
Cool water euphausiids Entire CCS

spring-summer

 Decrease Time-lagged Reduced upwelling; Anomalous advection
Pelagic red crabs SCCS & CCCS

winter, spring, summer

 Increase Nearly instantaneous Poleward advection
Market squid CCCS & SCCS

winter & spring

 Collapse Instantaneous for distribution; time-lagged for recruitment Warmer temperature/deeper thermocline; Reduces spawning habitat
Pacific sardine Entire CCS

winter-spring

 Changes in distribution;

Compression of spawning habitat

 Instantaneous for spawning and distribution, recruitment time-lagged, biomass is time-integrated Wind stress, cross-shore transport

 
Northern anchovy CCCS & SCCS

winter-spring

 Changes in distribution;

Compression of spawning habitat

 Instantaneous for spawning and distribution, recruitment time-lagged, biomass is time-integrated Reduced upwelling; Anomalous advection

 
Juvenile salmon survival NCCS

spring-summer

 Decrease in Pacific NW Time-integrated Reduce river flow, decreased food supply in ocean
Adult sockeye salmon

(Fraser River)

 NCCS

summer

 Return path deflected northward to Canadian waters Time-integrated Ocean temperature, including Ekman controls
Warm assemblage of mesopelagic fish SCCS

spring (?)

 Increase Lagged 0-3 months Poleward and onshore advection
Common murre

(reproductive success)

 CCCS

winter-spring

 Decrease Time-Lagged, time-integrated Prey (fish) availability; Thermocline depth; Decreased upwelling?
Top predator reproduction and abundance Entire CCS Species-dependent Time-integrated Advection of prey, altered temperature, upwelling, mesoscale structure
Top predator distribution Entire CCS Altered geographic distributions Instantaneous or time-lagged Advection of prey, altered temperature, upwelling, mesoscale structure
Table 1.   Examples of water column biological processes and organisms known to be affected by El Niño in the California Current System. Columns indicate the type of organism; approximate geographic region and season of the effect; direction of change in response to El Niño; temporal pattern of response (immediate, time-lagged, time-integrated); and the hypothesized oceanographic processes driving the organism response. CCS = California Current System; NCCS, CCCS, and SCCS denote northern, central, and southern sectors of the CCS.

 

Authors

Mark D. Ohman (Scripps Institution of Oceanography)
Nate Mantua (NOAA Southwest Fisheries Science Center)
Julie Keister (University of Washington)
Marisol Garcia-Reyes (Farallon Institute)
Sam McClatchie (NOAA Southwest Fisheries Science Center)

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Singh, A., T. Delcroix, and S. Cravatte, 2011: Contrasting the flavors of El Niño-Southern Oscillation using sea surface salinity observations. J. Geophy. Res., 116, doi:10.1029/2010JC006862.

Sun, J., D. A. Hutchins, Y. Y. Feng, E. L. Seubert, D. A. Caron, and F. X. Fu, 2011: Effects of changing pCO2 and phosphate availability on domoic acid production and physiology of the marine harmful bloom diatom Pseudo-nitzschia multiseries. Limnol. Oceanogr., 56, 829-840, doi: 10.4319/lo.2011.56.3.0829.

Todd, R. E., D. L. Rudnick, R. E. Davis, and M. D. Ohman, 2011: Underwater gliders reveal rapid arrival of El Nino effects off California’s coast. Geophy. Res. Lett., 38, doi: 10.1029/2010gl046376.

Yu, J. Y., Y. H. Zou, S. T. Kim, and T. Lee, 2012: The changing impact of El Nino on US winter temperatures. Geophy. Res. Lett., 39, doi: 10.1029/2012gl052483.

Zaba, K. D., and D. L. Rudnick, 2016: The 2014–2015 warming anomaly in the Southern California Current System observed by underwater gliders. Geophy. Res. Lett., 43, 1241-1248, doi: 10.1002/2015GL067550.

Subtropical gyre productivity sustained by lateral nutrient transport

Posted by Katherine Joyce 
· Saturday, November 5th, 2016 

Vertical processes are thought to dominate nutrient resupply across the ocean, however estimated vertical fluxes are insufficient to sustain observed net productivity in the thermally stratified subtropical gyres. A recent study by Letscher et al. (2016) published in Nature Geoscience used a global biogeochemical ocean model to quantify the importance of lateral transport and biological uptake of inorganic and organic forms of nitrogen and phosphorus to the euphotic zone over the low-latitude ocean. Lateral nutrient transport is a major contributor to subtropical nutrient budgets, supplying a third of the nitrogen and up to two-thirds of the phosphorus needed to sustain gyre productivity. Half of the annual lateral nutrient flux occurs during the stratified summer and fall months, helping to explain seasonal patterns of net community production at the time-series sites near Bermuda and Hawaii. Figure from Letscher et al. (2016).

Trace metal uptake and remineralization and their impact on upper ocean stoichiometry

Posted by mmaheigan 
· Wednesday, July 6th, 2016 

1. Stoichiometry of metals in the ocean

The close relationship between the stoichiometry of nutrients dissolved in the upper ocean and the planktonic organisms that grow in these waters has long been recognized (1, 2). The stoichiometry of 106 C:16 N:1 P first summarized by Redfield has become a fundamental concept of marine biogeochemistry, with numerous studies using the ratio as a benchmark to assess ecosystem function. Decades after the work of Redfield, with the implementation of trace metal-clean techniques, oceanographers produced the first meaningful measurements of dissolved trace metals in the open ocean (3-5), and they found that many of the bioactive metals such as Fe, Zn, Ni, Cu and Cd are also depleted in surface waters and enriched at depth, similar to the macronutrients. Such nutrient-like behavior supported not only a growing understanding of the physiological roles that these metals play in phytoplankton physiology (6), but it also indicated that biological uptake and sub-surface remineralization were important processes for controlling the distributions of these bioactive metals in the ocean. Thus, the biogeochemical behavior of the micronutrient metals is in many ways analogous to that of the macronutrients N, P and Si.

In the open ocean far from coastal and shelf influences, dissolved concentrations of bioactive metals increase with depth at relatively consistent ratios to macronutrients (5), and these metal:nutrient remineralization ratios have been used to approximate the composition of sinking biogenic material and euphotic zone phytoplankton (7, 8). These ‘extended Redfield ratios’ have been compared to average compositions of marine phytoplankton species grown in culture (9-12),  and the general agreement between these approaches further supports the importance of biological uptake and subsequent remineralization of trace metals in the upper ocean as key processes impacting trace metal geochemistry. Average metal:nutrient stoichiometries for phytoplankton have also been compared to dissolved stoichiometries in the ambient water, and relationships between these fractions have been used to estimate nutrient limitation and deficiency in the ocean (13). Thus, there is significant interest in controls on upper ocean metal stoichiometries, as well as the relationships between cellular/biological, particulate and dissolved fractions.

Analogous to macronutrients, there are also relationships between metal stoichiometries in phytoplankton and those in deeper waters of the ocean. Departures from these relationships are likely to provide insights into the internal biogeochemical cycling of metals in the ocean. Morel and Hudson (7) noted differences in the extended stoichiometries of plankton and the water column and concluded that they must reflect the relative efficiency of remineralization of the elements, as well as the propensity of elements to be scavenged onto sinking particles in the sub-surface ocean. Similarly, the rapid remineralization of trace metals from sinking plankton was addressed in seminal work by Collier and Edmond (14). Using carefully collected data on surface plankton material, and with more computational rigor than (7), they compared surface particle stoichiometries to deep water dissolved stoichiometries and calculated the relative remineralization of plankton-associated elements in sinking biogenic material. They noted significant differences among the behaviors of biogenic metals such as Cd, Ni and Fe due to their scavenging and remineralization behaviors. More recently, Morel (15) mused about these processes and their relationships to cellular biochemistry and evolution of phytoplankton physiology and ocean biogeochemistry.

Through the GEOTRACES program, the data to test and extend these early, relatively simple box models and stoichiometric comparisons are now available. Metal concentrations and stoichiometries for phytoplankton, bulk and size-fractionated particulate material, and co-located dissolved species have been measured in the North Atlantic and South Pacific Oceans thus far. Combined with data for non-bioactive metals such as Ti and Th, these data also provide the opportunity to discern the behavior and contributions of lithogenic vs. biogenic matter, as well as the processes of remineralization and scavenging.

2. Processes affecting dissolved and particulate stoichiometries of trace metals

Vertical profiles of dissolved macronutrients show characteristic depletion at the surface and enrichment at depth due to remineralization, and dissolved micronutrients often show the same behavior. However, the internal cycling of metals in the ocean is expected to differ from that of macronutrients for a few salient reasons. Some metals such as Fe are significantly less soluble than macronutrients and are prone to abiotic adsorption onto particulate surfaces (16). This process is driven by thermodynamics, and the accompanying process of desorption also occurs; the net observed process is typically called ‘scavenging’ (Fig. 1). Scavenging in the deep ocean causes concentrations of less soluble metals such as Fe and Al to decrease along the path of thermohaline circulation, in contrast to macronutrients and more soluble metals that may mimic macronutrient behavior such as Cd and Zn (17). In the absence of significant lateral nutrient inputs, the balance of scavenging and remineralization will influence the resulting vertical profiles of dissolved elements (18).

Another key difference between macronutrients and metals is the importance of abiotic particulate fractions such as lithogenic (e.g., aeolian dust and sediment) and authigenic (e.g., Fe- and Mn-oxyhydroxide) phases. While biogenic phases are almost universally produced at the surface and remineralized with depth, abiotic phases can exhibit very different and dynamic internal cycles (19). Dust events, lateral transport and poorly constrained scavenging processes can both deliver and remove specific metals alongside biological processes. Lithogenic phases are generally denser and more refractory than biogenic particles and detritus and are thought to sink more rapidly and remineralize more slowly and at greater depth (Fig. 1; 20). Lithogenic particles may also (re)scavenge metals differently than biogenic material. Efforts to examine these processes in sinking material have been extremely limited to date, with only a few studies examining metals in trace metal-clean sediment traps (21, 22). However, recently published datasets from the GEOTRACES program are shedding new light on the multiple facets of metal partitioning and how they affect subsurface remineralization and scavenging.

A comparison of metal:phosphorus ratios in the upper ocean illuminates some of these processes. Figure 2 displays Cd:P, Fe:P, Co:P and Ni:P ratios in particles in the upper 100m, 100-300m, and 300-1,000m of the water column in the middle of the North Atlantic basin. Particulate material is sub-divided into ratios for phytoplankton cells and non-lithogenic particles (corrected for lithogenic minerals using Ti; 19). Also plotted are dissolved remineralization ratios (that is, the slope of a linear regression between the dissolved metal and phosphate) for these upper ocean depth ranges. The close coupling of Cd and P biogeochemistry has long been recognized (4), and indeed we observe very close agreement (within a factor of about 2) between dissolved Cd:P remineralization and Cd:P in surface ocean particles, as well subsurface particles. Clearly these elements are remineralizing from sinking particles at similar rates. Such comparisons of particulate and dissolved constituents need to carefully consider the different residence times of these fractions and the likelihood for lateral inputs. Here, we have chosen to focus on stations from the mid-North Atlantic gyre, where the upper 700m of the water column consists primarily of a single water mass (23).

In contrast, the remineralization of Fe and P are quickly decoupled in the water column (Fig. 2). Between 100 and 300m, typically the depth of most rapid regeneration of sinking organic material, labile particulate Fe:P has more than doubled from that in surface waters, and the Fe:P ratio of remineralized dissolved elements (0.98 mmol/mol) is more than 10-fold below that of the labile material that is sinking into these waters. Looking deeper into the water column, Fe and P continue to decouple in labile (i.e., non-lithogenic) particulates, with Fe:P of 300-1,000m particles increasing 10-fold and the dissolved remineralization ratio being nearly 1,000-fold lower (0.35 mmol/mol). Additionally, organic ligands play an important role in stabilizing dissolved Fe (24), so dissolved Fe and P ratios may be further decoupled by biological processes impacting the production and fate of these ligands (20).

A strength of GEOTRACES datasets is their wide coverage of the periodic table, and additional insights can be gained from looking at the behaviors of other bioactive trace metals that are also incorporated into sinking biogenic material. Co:P ratios in particles and remineralized dissolved fractions in the water column follow the same trend as Fe, but the decoupling of Co and P is much more subtle than with Fe, presumably due to differences in ligand coordination and Co co-oxidation with Mn (25, 26). Dissolved Co:P remineralization ratios at 100-300m generally match those found in phytoplankton and drop only 3-fold below 300m. Similarly, labile particulate Co:P ratios don’t change between 0-100m and 100-300m, also indicating that Co and P remineralize in tandem in the upper 300m. Below 300m, labile particulate Co:P increases approximately 3-fold (in contrast with Fe:P, which increases 12-fold), and this depth effect matches the effect in dissolved remineralization ratios. Thus, even though Fe and Co are considered hybrid metals that display both biological uptake and scavenging, there are clear differences in the behaviors of these metals. Nickel provides yet another perspective on the coupling of metals and P. Dissolved remineralization ratios in both subsurface depth ranges closely resemble surface ocean labile particles, supporting the biological coupling of Ni and P (5). However, residual labile particulate Ni:P increases 2- to 4-fold in successive depth ranges, indicating that remineralization is rather decoupled. Given that Ni seems to be associated with both organic material and opal frustules in diatoms (27), it may be that Ni and P are remineralized from particulate organic matter in tandem, but some Ni remains associated with sinking biogenic silica in the ocean.

3. Additional tools to explore and differentiate remineralization processes

The GEOTRACES program has welcomed the application of new analytical approaches that further enable us to study the cycling of metals in the ocean. Spectroscopy and quantitative imaging methods using synchrotron radiation have become more common in the past decade (28), and these allow us to analytically distinguish the behaviors of different fractions of particle assemblages. During the FeCycle II project, a GEOTRACES process study, the fate of Fe was tracked during a spring diatom bloom (29). Diatom cells from the dominant bloom species (Asterionellopsis glacialis) were collected in surface waters and from trace-metal clean sediment traps at 100m and 200m in the 48h following the decline of the bloom. Synchrotron x-ray fluorescence (SXRF) analyses of individual cells showed that constituent elements were lost from sinking cells at notably different rates (Fig. 3). Phosphorus was rapidly released from sinking cells, with mean P quotas decreasing 55% and 73% from surface values by 100m and 200m, respectively (30). However, only 25% of cellular Fe was lost from cells sinking through the upper 200m, while 61% of cellular Ni was remineralized. This supports the story told by the bulk biogeochemical data from the North Atlantic: Ni is remineralized largely to a similar degree as P, while Fe is lost more slowly from sinking biogenic material.

Application of microanalytical techniques such as SXRF can be combined with bulk approaches to further advance understanding of subsurface metal remineralization and cycling. In FeCycle II, Fe:P of sinking A. glacialis cells increased, on average, only 2.3-fold in the upper 200m, while Fe:P in bulk particulate matter increased more than 13-fold (30). This indicates that the behavior of sinking cells was not representative of the full particle assemblage. Iron and P were likely more completely decoupled in sinking fecal pellets and detrital material (which appears to have contributed significantly to the particulate Fe pool during FeCycle II; 31) than in intact sinking cells. Further application of this approach will allow us to not only distinguish between the behavior of biogenic and lithogenic fractions (Fig. 1), but potentially also between detrital particles. By considering metals such as Mn that are prone to oxidation and scavenging in the subsurface ocean (32, 33), it may also be possible to separate abiotic scavenging from net biological remineralization (Fig. 1). Additionally, 2D (and potentially 3D) mapping of elements within cells and particles also provides information about the spatial and potentially chemical associations of elements with particles (30, 34).

The GEOTRACES program is generating unprecedented data, both in terms of quality and quantity, regarding the cycling of bioactive trace metals in the ocean. Syntheses of these data, and integration of insights from novel microanalytical tools, as well as transcriptomic and proteomic approaches, are resulting in substantial advances in our understanding of metal biogeochemistry. No longer are we limited to a few painstakingly collected dissolved metal profiles. There is now painstakingly collected full-depth coverage of most ocean basins, including in many cases dissolved and particulate fractions of nearly all biogenic elements, enabling testing of early hypotheses about trace metal cycling and parameterization of these processes into next-generation ocean biogeochemical models.

Authors

Benjamin S. Twining, Daniel C. Ohnemus, Renee L. Torrie (Bigelow Laboratory for Ocean Sciences)

Acknowledgments

This work was funded by NSF grant OCE-1232814 to BST. RLT was funded by NSF REU grant 1460861 to Bigelow Laboratory for Ocean Sciences.

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