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Industrial era climate forcing drives multi-century decline in North Atlantic productivity

Posted by mmaheigan

· Wednesday, October 2nd, 2019

Phytoplankton respond directly to climate forcing, and due to their central role in global oxygen production and atmospheric carbon sequestration, they are critical components of the Earth’s climate system. There are however few observations detailing past variability in marine primary productivity, particularly over multi-decadal to centennial timescales. This limits our understanding of the long-term impact of climatic forcing on both past and future marine productivity.

Multi-century decline of subarctic Atlantic productivity. From top: standardized (z-score units relative to ad 1958-2016) indices of Continuous Plankton Recorder (CPR)-based diatom, dinoflagellate and coccolithophore relative-abundances; North Atlantic [chlorophyll-α] reconstruction from Boyce et al. (2010, Nature); ice core-based [MSA] PC1 productivity index. The “Industrial Onset” range shows the estimated initiation of declining subarctic Atlantic productivity; reconstructed (Rahmstorf et al., 2015, Nat. Clim. Change) and observed sea-surface temperature-based Atlantic Meridional Overturning Circulation (i.e., AMOC) index, alongside 5-year averaged subarctic Atlantic freshwater storage anomalies (relative to A.D. 1955) from Curry and Mauritzen (2005; Science).

Authors of a new study published in Nature used a high-resolution signal of marine biogenic aerosol emissions (methanesulfonic acid, or “MSA”) preserved within twelve Greenland ice cores to reconstruct a ~250-year record of marine productivity variations across the subarctic Atlantic basin, one of the most biologically productive and climatically sensitive regions on Earth. These results provide the most continuous proxy-based reconstruction of basin-scale productivity to date in this region, illuminating the following major findings: (1) subarctic Atlantic marine productivity has declined over the industrial era by as much as 10 ± 7%; (2) the early 19th century onset of declining productivity coincides with the regional onset of industrial-era surface warming, and also strongly covaries with regional sea surface temperatures and basin-scale gyre circulation strength; (3) there is strong decadal- to centennial-scale coherence between northern Atlantic productivity variability and declining Atlantic Meridional Overturning Circulation (AMOC) strength, as predicted by prior model-based studies.

Future atmospheric warming is predicted to contribute to accelerating Greenland Ice Sheet runoff, ocean-surface freshening, and AMOC slowdown, suggesting the potential for continued declines in productivity across this dynamic and climatically important region. Such declines will, in turn, have important implications for future maritime economies, global food security, and drawdown of atmospheric carbon dioxide.

Authors:
Matthew Osman (Massachusetts Institute of Technology)
Sarah Das (Woods Hole Oceanographic Institution)
Luke Trusel (Rowan University)
Matthew Evans (Wheaton College)
Hubertus Fischer (University of Bern)
Mackenzie Griemann (University of California, Irvine)
Sepp Kipfstuhl (Alfred-Wegener-Institute)
Joseph McConnell (Desert Research Institute)
Eric Saltzman (University of California, Irvine)

Figure references:
Boyce, D. G., Lewis, M. R. & Worm, B. (2010) Global phytoplankton decline over the past century. Nature 466, 591–596.

Curry, R. & Mauritzen, C. (2005) Dilution of the northern North Atlantic Ocean in recent decades. Science 308, 1772–1774.

Rahmstorf, S. et al. (2015) Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation. Nat. Clim. Change 5, 475–480.

Ocean microbes drive fluctuating nutrient loss

Posted by mmaheigan

· Tuesday, May 28th, 2019

The removal of bioavailable nitrogen (N) by anaerobic microbes in the ocean’s oxygen deficient zones (ODZs) is thought to vary over time primarily as a result of climate impacts on ocean circulation and primary production. However, a recent study in PNAS using a data-constrained model of the microbial ecosystem in the world’s largest ODZ revealed that internal species oscillations cause local- to basin-scale fluctuations in the rate of N loss, even in a completely stable physical environment. Such ecosystem oscillations have been hypothesized for nearly a century in idealized models, but are rarely shown to persist in a three-dimensional ocean circulation model.

Figure caption. Ecological variability in the basin-scale rate of nitrogen loss over time (left) and in the local-scale contribution of autotrophic anammox to total N loss (right) in a model with unchanging ocean circulation. In the left panel, colors represent model simulations with different biological parameters. In the right panel, colors represent distinct locations within the ODZ in the standard model simulation.

These emergent ecosystem dynamics arise at the oxic-anoxic interface from O₂-dependent resource competition between aerobic and anaerobic microbes, and leave a unique geochemical fingerprint: infrequent spikes in ammonium that are observable in nutrient measurements from the ODZ. Non-equilibrium ecosystem behavior driven by competition among aerobic nitrifiers, anaerobic denitrifiers, and anammox bacteria also generates fluctuations in the balance of autotrophic versus heterotrophic N loss pathways that help reconcile conflicting field observations.

These internally driven fluctuations in microbial community structure partially obscure a direct correspondence between the chemical environment and microbial rates, a universal assumption in biogeochemical models. Because of the fundamental nature of the underlying mechanism, similar dynamics are hypothesized to occur across wide-ranging microbial communities in diverse habitats.

Authors:
Justin L. Penn (University of Washington)
Thomas Weber (University of Rochester),
Bonnie X. Chang (University of Washington, NOAA)
Curtis Deutsch (University of Washington)

See also the OCB2019 plenary session: Anthropogenic changes in ocean oxygen: Coastal and open ocean perspectives (Monday, June 24, 2019)

A half century perspective: Seasonal productivity and particulates in the Ross Sea

Posted by mmaheigan

· Tuesday, April 2nd, 2019

Studies of cruise observations in the Ross Sea are typically biased to a single or a few year(s), and long-term trends have predominantly come from satellites. Consequently, the in situ climatological patterns of nutrients and particulate matter have remained vague and unclear. What are the typical patterns of nutrients and particulate matter concentrations in the Ross Sea in spring and summer? How do these concentrations affect annual productivity estimates?

Patterns of nutrient and particulate matter in the Ross Sea can play a wide-ranging role in a productive region like the Ross Sea. Smith and Kaufman (2018) recently synthesized austral spring and summer (November to February) observations from 42 Ross Sea research cruises (1967-2016) to analyze broad biogeochemical patterns. The resulting climatologies revealed interesting seasonal patterns of nutrient uptake and particulate organic carbon (POC) to chlorophyll (chl) ratios (POC:chl). Temporal patterns in the nitrate and phosphate climatologies confirm the role of early spring haptophyte (Phaeocystis antarctica) growth, followed by limited nitrogen and phosphorus removal in summer. However, a notable increase in POC occurred later in summer that was largely independent of chlorophyll changes, resulting in a dramatic increase in POC:chl. A gradual decline in silicic acid concentrations throughout the summer, along with an increased occurrence of biogenic silica during this time suggest that diatoms may be responsible for this later POC spike. Revised estimates of primary productivity based on these observed climatological POC:chl ratios suggests that summer blooms may be a significant contributor to seasonal productivity, and that estimates of productivity based on satellite pigments underestimate annual production by at least 70% (Figure 1).

Figure 1. Bio-optical estimates of mean productivity using a constant POC:chl ratio (black dots and lines) and modified estimates of productivity using the monthly climatological POC:chl ratios (red dots and lines), in a) the Ross Sea polynya region and b) the western Ross Sea region.

By clarifying typical seasonal patterns of nutrient uptake and POC:chl, these climatologies underscore the biogeochemical importance of both spring haptophyte growth and previously underestimated summer diatom growth in the Ross Sea. Further investigation of the causes and consequences of elevated summer ratios is needed, as assessments of regional food webs and biogeochemical cycles depend on more accurate understanding of primary productivity patterns. Likewise, these results highlight the need for continued efforts to constrain satellite productivity estimates in the Ross Sea using in situ constituent ratios.

For other relevant work on seasonal biogeochemical patterns in the Ross Sea, please see https://doi.org/10.1016/j.dsr2.2003.07.010. And for intra-seasonal estimates of particulate organic carbon to chlorophyll using gliders, please see: https://doi.org/10.1016/j.dsr.2014.06.011.

Authors:
Walker O. Smith Jr. (VIMS, College of William and Mary)
Daniel E. Kaufman (VIMS, College of William and Mary; now at Chesapeake Research Consortium)

New BioGEOTRACES data sets: Connecting pieces of the microbial biogeochemical puzzle

Posted by mmaheigan

· Wednesday, December 19th, 2018

Microorganisms play a central role in the transfer of matter and energy in the marine food web. Microbes depend on micronutrients (e.g. iron, cobalt, zinc, and a host of other trace metals) to catalyze key biogeochemical reactions, and their metabolisms, in turn, directly affect the cycling, speciation, and bioavailability of these compounds. One might therefore expect that marine microbial community structure and the functions encoded within their genomes might be related to trace metal availability in the ocean. The overall productivity of marine ecosystems—i.e. the amount of carbon fixed through photosynthesis—could in turn be influenced by trace metal concentrations.

For over a decade, the international GEOTRACES program has been mapping the distribution and speciation of trace metals across vast ocean regions. Given the important relationship between trace metals and the function of marine ecosystems, biological oceanographers collaborate with GEOTRACES scientists to simultaneously probe the biotic communities at some sampling sites, allowing these biological data to be interpreted in the context of detailed chemical and physical measurements.

Figure 1. Locations and depths of samples. (a) Global map of sample locations. Single cell genomes are represented by miniaturized stacked dot-plots (each dot represents one single cell genome), with organism group indicated by color, and cells categorized as “undetermined” if robust placement within known phylogenetic groups failed due to low assembly completeness/quality or missing close references. Larger points correspond to stations on associated GEOTRACES sections where metagenomes were also collected. (b) Depth distribution of metagenome samples along each of the four GEOTRACES sections. Transect distances are calculated relative to the first station sampled in the indicated orientation. For clarity, the depth distribution of samples collected below 250 m are not shown to scale (ranging from 281–5601 m). Adapted from Berube et al. (2018) Sci. Data 5:180154 and Biller et al. (2018) Sci. Data 5:180176.

Two recent papers published in Scientific Data describes two new, large-scale biological data sets that will facilitate studies aimed at understanding how microbes and metals relate to one another. Collected on four different sets of GEOTRACES cruises (Figure 1), these papers report the public availability of hundreds of single cell genomes and microbial community metagenomes from the Pacific and Atlantic Oceans. The single cell genomes focus on the marine photosynthetic bacteria Prochlorococcus and Synechococcus and how they and other community members vary in different regions of the ocean. The metagenomic sequences provide snapshots of the entire microbial community found in each of these samples, yielding a broad overview of which microbes—and which genes, including those important for understanding nutrient cycling—are found in each sample. These two datasets are complementary and further enhanced by the wealth of chemical and physical data collected by GEOTRACES scientists on the same water samples. In particular, iron is of key interest, since it often limits primary productivity. These data sets can directly link iron availability with microbial community structure and gene content across ocean basins.

With these data, researchers can now ask questions such as how microbes have evolved in response to the availability or limitation of key nutrients and explore which organisms may be contributing to biogeochemical cycles in different parts of the global ocean. The extensive suite of chemical and physical measurements associated with these sequence data underscore their potential to reveal important relationships between trace metals and the microbial communities that drive biogeochemical cycles. These data sets also encourage cross-disciplinary collaborations and provide baseline information as society faces the challenges and uncertainties of a changing climate.

Authors:
Paul M. Berube (Massachusetts Institute of Technology)
Steven J. Biller (Massachusetts Institute of Technology; current affiliation: Wellesley College)
Sallie W. Chisholm (Massachusetts Institute of Technology)

Dramatic Increase in Chlorophyll-a Concentrations in Response to Spring Asian Dust Events in the Western North Pacific

Posted by mmaheigan

· Tuesday, October 23rd, 2018

According to Martin’s iron hypothesis, input of aeolian dust into the ocean environment temporarily relieves iron limitation that suppresses primary productivity. Asian dust events that originate in the Taklimakan and Gobi Deserts occur primarily in the spring and represent the second largest global source of dust to the oceans. The western North Pacific, where productivity is co-limited by nitrogen and iron, is located directly downwind of these source regions and is therefore an ideal location for determining the response of open water primary productivity to these dust input events.

Figure 1. Daily aerosol index values (black squares) and chlorophyll-a concentrations (mg m-3, circles) during the spring (a) 2010 (weak dust event), (b) 1998 (strong dust event) in the western North Pacific. Color scale represents difference between mixed layer depth (MLD) and isolume depth (Z0.054) that indicates conditions for typical spring blooms; water column structures of MLD and isolume were identical in the spring of 1998 and 2010. Dramatic increases in chlorophyll-a (pink shading, maximum of 5.3 mg m-3) occurred in spring 1998 with a lag time of ~10 days after the strong dust event (aerosol index >2.5) on approximately April 20 compared to constant chlorophyll-a values (<2 mg m-3) in the spring of 2010.

A recent study in Geophysical Research Letters included an analysis of the spatial dynamics of spring Asian dust events, from the source regions to the western North Pacific, and their impacts on ocean primary productivity from 1998 to 2014 (except for 2002–2004) using long-term satellite observations (daily aerosol index data and chlorophyll-a). Geographical aerosol index distributions revealed three different transport pathways supported by the westerly wind system: 1) Dust moving predominantly over the Siberian continent (>50°N); 2) Dust passing across the northern East/Japan Sea (40°N‒50°N); and 3) Dust moving over the entire East/Japan Sea (35°N‒55°N). The authors observed that strong dust events could increase ocean primary productivity by more than 70% (>2-fold increase in chlorophyll-a concentrations, Figure 1) compared to weak/non-dust conditions. This result suggests that spring Asian dust events, though episodic, may play a significant role in driving the biological pump, thus sequestering atmospheric CO₂ in the western North Pacific.

Another recent study reported that anthropogenic nitrogen deposition in the western North Pacific has significantly increased over the last three decades (i.e. relieving nitrogen limitation), whereas this study indicated a recent decreasing trend in the frequency of spring Asian dust events (i.e. enhancing iron limitation). Further investigation is required to fully understand the effects of contrasting behavior of iron (i.e., decreasing trend) and nitrogen (i.e., increasing trend) inputs on the ocean primary productivity in the western North Pacific, paying attention on how the marine ecosystem and biogeochemistry will respond to the changes.

Authors:
Joo-Eun Yoon (Incheon National University)
Il-Nam Kim (Incheon National University)
Alison M. Macdonald (Woods Hole Oceanographic Institution)

When marine-terminating glaciers ‘pump’ the ocean

Posted by mmaheigan

· Wednesday, October 10th, 2018

How will increasing meltwater from Greenland affect the biogeochemistry of the ocean? Release of meltwater into the ocean has physical and biogeochemical effects on the local water column. With respect to nutrient availability, meltwater supplies the bioessential nutrients iron and silicic acid but is deficient in nitrate and phosphate. However, despite very low meltwater nitrate and phosphate concentrations, pronounced summertime phytoplankton blooms are observed in many, though not all, of Greenland’s large fjord systems. These unusual summertime blooms are associated with meltwater from marine-terminating glaciers. So if the meltwater itself is not supplying nitrate and phosphate that these blooms require, what is the source of the nutrients that support these blooms?

An illustration of how changing the depth of a glacier affects downstream productivity

A recent study published in Nature Communications shows that when meltwater is released below sea level under large marine-terminating glaciers, it rises rapidly towards the surface in buoyant discharge plumes. As these plumes rise, they entrain large quantities of deep, nutrient-rich seawater. This vertical transport, or ‘pumping’, of these nutrients to the surface sustains unusually high summertime productivity in Greenland’s fjords. Conversely, when meltwater is released at the ocean surface, primary production is reduced because the meltwater itself lacks the nitrate and phosphate required to fuel phytoplankton blooms. Consequently, the inland retreat of Greenland’s large marine-terminating glaciers is ultimately bad news for summertime marine phytoplankton communities. As the depth of the marine-terminating glaciers shoals, their associated nutrient ‘pumps’ collapse, which will likely have negative effects on primary production and associated inshore fisheries.

Authors:
M.J. Hopwood (GEOMAR)
D. Carroll (Jet Propulsion Laboratory)
T.J. Browning (GEOMAR)
L. Meire (Royal Netherlands Institute for Sea Research and Greenland Climate Research Centre)
J. Mortensen (Greenland Climate Research Centre)
S. Krisch (GEOMAR)
E.P. Achterberg (GEOMAR)

Primary productivity à la mode

Posted by mmaheigan

· Wednesday, October 10th, 2018

The presence of large-scale Ekman downwelling is the textbook explanation for low nutrient concentrations, and hence low productivity, in subtropical gyres. However, recent research has suggested that mesoscale eddies oppose and substantially reduce this downwelling, a process known as eddy cancellation (Doddridge et al, 2016). Eddy cancellation represents a substantial alteration to the widely accepted notion of large-scale Ekman downwelling in subtropical gyres, and motivates our study of the processes that determine nutrient concentration within subtropical gyres.

Figure 1: Sensitivity experiments for mode water thickness (h_mode) with two values of residual Ekman pumping. a) With no residual Ekman pumping, phosphate concentration responds strongly to mode water thickness. b) When Ekman pumping is strong, phosphate concentration does not depend on mode water thickness. The dashed lines represent transects of climatological phosphate concentration in the euphotic zone of the North Atlantic subtropical gyre (Garcia et al., 2013).

A recent paper published in the Journal of Geophysical Research: Oceans and featured in an MIT News article describes an idealized model for nutrient concentration in subtropical gyres that can account for this reduction in Ekman pumping. The model predicts that surface productivity is sensitive to the thickness of the underlying subtropical mode water layer, provided that the residual Ekman pumping is small (Figure 1). Comparison of this prediction with observations from the Bermuda Atlantic Time series Study (BATS) shows that surface productivity increases as the thickness of the underlying mode water increases (Figure 2), as predicted by the idealized model in the absence of substantial Ekman pumping.

Figure 2: Annually averaged primary productivity and mode water thickness from the BATS dataset. The linear fit between mode water thickness and primary productivity is statistically significant (p ≈ 0.027) and explains 19.5% of the variance in primary productivity.

The observed relationship between productivity and mode water thickness at BATS is consistent with a small residual Ekman pumping, indicating highly effective eddy cancellation in the subtropical North Atlantic. Previous research (Palter et al., 2005) has suggested that as the subtropical mode water layer thickens, it blocks nutrient entrainment from below, resulting in lower productivity in the euphotic zone. However, this study suggests that a thicker subtropical mode water layer actually increases the surface nutrient concentrations by promoting more effective recycling of nutrients within the gyre. With a thicker mode water layer, more of the nutrients in the particulate flux are remineralized before they pass through the thermocline and become isolated from the surface ocean. This means that a thicker mode water layer leads to higher nutrient concentrations and supports primary productivity in subtropical gyres. This represents a fundamental change in our understanding of how nutrients are supplied to the surface waters of subtropical gyres.

Authors:
Edward Doddridge (Department of Earth, Atmospheric and Planetary Sciences, MIT)
David Marshall (Atmospheric, Oceanic & Planetary Physics, University of Oxford)

See the Eos spotlight on this research

When it comes to carbon export, the mesoscale matters

Posted by hbenway

· Tuesday, September 11th, 2018

Figure 1. Difference in annual mean carbon export (ΔPOC flux) between a high resolution (0.1º, Hi-res) and standard resolution (1º, Analog) global climate model simulation using the CESM model. Highlighted regions show areas where vertical (purple boxes) and horizontal (red boxes) changes in nutrient transport drive increases or decreases in export, respectively.

Most Earth System models (ESMs) that are used to study global climate and the carbon cycle do not resolve the most energetic scales in the ocean, the mesoscale (10-100 km), encompassing eddies, coastal jets, and other dynamic features strongly affecting nutrient delivery, productivity, and carbon export. This prompts the question: What are we missing in climate models by not resolving the mesoscale?

Authors of a recent study published in Global Biogeochemical Cycles conducted a comparative analysis of the importance of mesoscale features in biological production and associated carbon export using standard resolution (1°) and mesoscale-resolving (0.1°) ESM simulations. The mesoscale-resolving ESM yielded only a ~2% reduction in globally integrated export production relative to the standard resolution ESM. However, a closer look at the local processes driving export in different basins revealed much larger, compensating differences (Fig. 1). For example, in regions where biological production is driven by natural iron fertilization from shelf sediment sources (Fig. 2), improved representation of coastal jets in the higher-resolution ESM reduces the cross-shelf iron delivery that fuels production (red boxes in Fig. 1). Resolving mesoscale turbulence further reduces the spatial extent of blooms and associated export, yielding a more patchy distribution than in the coarse resolution models. Together, these processes lead to a reduction in export in the Argentine Basin, one of the most productive regions on the planet, of locally up to 50%. In contrast, resolving the mesoscale results in enhanced export production in the Subantarctic (purple box in Fig. 1), where the mesoscale model resolves deeper, narrower mixed layer depths that support stronger nutrient entrainment, in turn enhancing local productivity and export.

Figure 2. An iron-driven plankton bloom structured by mesoscale features in the South Atlantic. Left is simulated dissolved iron (Fe), the limiting nutrient for this region, and right is iron in all phytoplankton classes, a proxy for biomass (phytoFe, shown in log10 scale), on January 11, the height of the bloom. Plankton blooms in the Subantarctic Atlantic are fueled by horizontal iron transport off coastal and island shelves and vertical injection from seamounts, whereas farther south in the Southern Ocean, winter vertical mixing is the primary driver of iron delivery. Mesoscale circulation, largely an unstructured mix of interacting jets and vortices, strongly affects the location and timing of carbon production and export. Click here for an animation.

In regions with very short productivity seasons like the North Pacific and Subantarctic, internally generated mesoscale variability (captured in the higher resolution ESM) yields significant interannual variation in local carbon export. In these regions, a few eddies, filaments or more amorphous mesoscale features can structure the entire production and export pattern for the short bloom season. These findings document the importance of resolving mesoscale features in ESMs to more accurately quantify carbon export, and the different roles mesoscale variability can play in different oceanographic settings.

Determining how to best sample these mesoscale turbulence-dominated blooms and scale up these measurements to regional and longer time means, is an outstanding joint challenge for modelers and observationalists. A key piece is obtaining the high temporal and spatial resolution data sets needed for validating modeled carbon export in bloom regions strongly impacted by mesoscale dynamics, which represent a large portion of the global carbon export.

Authors
Cheryl Harrison (NCAR, University of Colorado Boulder)
Matthew Long (NCAR)
Nicole Lovenduski (University of Colorado Boulder)
J. Keith Moore (University of California Irvine)

Marine Snowfall at the Equator

Posted by mmaheigan

· Thursday, July 19th, 2018

The continual flow of organic particles such as dead organisms and fecal material towards the deep sea is called “marine snow,” and it plays an important role in the ocean carbon cycle and climate-related processes. This snowfall is most intense where high primary production can be observed near the surface. This is the case along the equator in the Pacific and Atlantic Oceans. However, it is not well known how particles are distributed at depth and which processes influence this distribution. A recent study published in Nature Geoscience involved the use of high-resolution particle density data using the Underwater Vision Profiler (UVP) from the equatorial Atlantic and Pacific Oceans down to a depth of 5,000 meters, revealing that several previously accepted ideas on the downward flux of particles into the deep sea should be revisited.

Figure 1. The Underwater Vision Profiler (UVP) during a trial in the Kiel Fjord. The UVP provided crucial data for the new study. Photo: Rainer Kiko, GEOMAR

It is typically assumed that the largest particle density can be found close to the surface and that density attenuates continuously with depth. However, high-resolution particle data show that density increases again in the 300-600-meter depth range. The authors attribute this observation to the daily migratory behavior of organisms such as zooplankton that retreat to these depths during the day, contributing to the particle load via defecation and mortality.

Another surprising result is the observation of many small particles below 1,000 meters depth that contribute a large fraction of the bathypelagic particle flux. This observation counters the general assumption, especially in many biogeochemical models, that particle flux at depth comprises fast sinking particles such as fecal pellets. Diminished remineralization rates of small particles or increased disaggregation of larger particles may contribute to the elevated small particle fluxes at this depth.

Figure 2. Zonal current velocity and Particulate Organic Carbon (POC) content across the equatorial Atlantic at 23˚W as observed in November 2012. From left to right: Zonal current velocity, POC content in small particle fraction and POC content in large particle fraction (adapted from Kiko et al. 2017).

This study highlights the importance of coupled biological and physical processes in understanding and quantifying the biological carbon pump. Further work on this important topic can now also be submitted to the new Frontiers in Marine Science research topic “Zooplankton and Nekton: Gatekeepers of the Biological Pump” (https://www.frontiersin.org/research-topics/8114/zooplankton-and-nekton-gatekeepers-of-the-biological-pump; Co-editors R. Kiko, M. Iversen, A. Maas, H. Hauss and D. Bianchi). The research topic welcomes a broad range of contributions, from individual-based process studies, to local and global field observations, to modeling approaches to better characterize the role of zooplankton and nekton for the biological pump.

Authors:
R. Kiko (GEOMAR)
A. Biastoch (GEOMAR)
P. Brandt (GEOMAR, University of Kiel)
S. Cravatte (LEGOS, University of Toulouse)
H. Hauss (GEOMAR)
R. Hummels (GEOMAR)
I. Kriest (GEOMAR)
F. Marin (LEGOS, University of Toulouse)
A. M. P. McDonnell (University of Alaska Fairbanks)
A. Oschlies (GEOMAR)
M. Picheral (Laboratoire d’Océanographie de Villefranche-sur-Mer, Observatoire Océanologique)
F. U. Schwarzkopf (GEOMAR)
A. M. Thurnherr (Lamont-Doherty Earth Observatory,)
L. Stemmann (Sorbonne Universités, Observatoire Océanologique)

Hotspots of biological production: Submesoscale changes in respiration and production

Posted by mmaheigan

· Thursday, April 26th, 2018

The biological pump is complex and variable. To better understand it, scientists have often focused on variations in biological parameters such as fluorescence and community structure, and have less often observed variations in rates of production. Production rates can be estimated using oxygen as a tracer, since photosynthesis produces oxygen and respiration consumes it. In a recent article in Deep Sea Research Part I, the authors presented high-resolution maps of oxygen in the upper 140 m of the ocean in the subtropical and tropical Atlantic, produced from a towed undulating instrument. This provides a synoptic, high-resolution view of oxygen anomalies in the surface ocean. These data reveal remarkable hotspots of biological production and respiration co-located with areas of elevated fluorescence. These hotspots are often several kilometers wide (horizontal) and ~10 m long (vertical). They are preferentially associated with edges of eddies, but not all edges sampled contained hotspots. Although this study captures only two-dimensional glimpses of these hotspots, precluding formal calculations of production rates, likely estimates of source water suggest that many of these hotspots may actually be areas of enhanced respiration rather than enhanced photosynthesis. The paper describes a conceptual model of nutrients, new production, respiration, fluorescence, and oxygen during the formation and decline of these hotspots. These data raise intriguing questions–if the hotspots do indeed have substantially different rates of production and respiration than surrounding waters, then they could lead to significant changes in estimates of production in the upper ocean. Additionally, understanding the mechanisms that produce these hotspots could be critical for predicting the effects of climate change on the magnitude of the biological pump.

(a) Oxygen concentrations and (b) fluorescence at ~1 km resolution over 300 km from 15.13°N, 57.47°W to 12.30°N, 56.42° W, as measured by sensors attached to the (c) Video Plankton Recorder II. Note that no contouring was used for this plot – every pixel represents an actual data point. Figure modified from Stanley et al., 2017. VPR image photograph by Phil Alatalo.

Authors:
Rachel H. R. Stanley (Wellesley College)
Dennis J. McGillicuddy Jr. (WHOI)
Zoe O. Sandwith (WHOI)
Haley Pleskow (Wellesley College)

Funding for the Ocean Carbon & Biogeochemistry Project Office is provided by the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA). The OCB Project Office is housed at the Woods Hole Oceanographic Institution.

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