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Turning a spotlight on grazing

Posted by mmaheigan

· Thursday, July 23rd, 2020

Microscopic plankton in the surface ocean make planet Earth habitable by generating oxygen and forming the basis of marine food webs, yielding harvestable protein. For over 100 years, oceanographers have tried to ascertain the physical, chemical, and biological processes governing phytoplankton blooms. Zooplankton grazing of phytoplankton is the single largest loss process for primary production, but empirical grazing data are sparse and thus poorly constrained in modeling frameworks, including assessments of global elemental cycles, cross-ecosystem comparisons, and predictive efforts anticipating future ocean ecosystem function. As sunlight decays exponentially with depth, upper-ocean mixing creates dynamic light environments with predictable effects on phytoplankton growth but unknown consequences for grazing.

Figure caption: Rates (d⁻¹) of phytoplankton growth (μ), grazing mortality (g), and biomass accumulation (r) under four mixed layer scenarios simulated using light as a proxy of (a) sustained deep mixing, (b) rapid shoaling, (c) sustained shallow mixing, and (d) rapid mixed layer deepening. Error bars represent one standard deviation of the mean of duplicate experiments. Grazing was measured but not detected in the sustained deep mixing and rapid shoaling conditions, denoted with x.

Using data from a spring cruise in the North Atlantic, authors of a recent study published in Limnology & Oceanography compared the influences of microzooplankton predation and fluctuations in light availability—representative of a mixing water column—on phytoplankton standing stock. Data from at-sea incubations and light manipulation experiments provide evidence that phytoplankton’s instantaneous and zooplankton’s delayed responses to light fluctuations are key modulators of the balance between phytoplankton growth and grazing rates (Figure 1). These results suggest that light is a potential, remotely retrievable predictor of when and where in the ocean zooplankton grazing may represent an important loss term of phytoplankton production. If broadly verified, this approach could be used to systematically assess sparsely measured grazing across spatial and temporal gradients in representative regions of the ocean. Such data will be essential for enhancing our predictive capacity of ocean food web function, global biogeochemical cycles and the many derived processes, including fisheries production and the flow of carbon through the oceans.

Authors:
Françoise Morison (University of Rhode Island)
Gayantonia Franzè (University of Rhode Island, currently Institute of Marine Research, Norway)
Elizabeth Harvey (University of Georgia, currently University of New Hampshire)
Susanne Menden-Deuer (University of Rhode Island)

Modern OMZ copepod dynamics provide analog for future oceans

Posted by mmaheigan

· Thursday, July 23rd, 2020

Global warming increases ocean deoxygenation and expands the oxygen minimum zone (OMZ), which has implications for major zooplankton groups like copepods. Reduced oxygen levels may impact individual copepod species abundance, vertical distribution, and life history strategy, which is likely to perturb intricate oceanic food webs and export processes. In a study recently published in Biogeosciences, authors conducted vertically-stratified day and night MOCNESS tows (0-1000 m) during four cruises (2007-2017) in the Eastern Tropical North Pacific, sampling hydrography and copepod distributions in four locations with different water column oxygen profiles and OMZ intensity (i.e. lowest oxygen concentration and its vertical extent in a profile). Each copepod species exhibited a different vertical distribution strategy and physiology associated with oxygen profile variability. The study identified sets of species that (1) changed their vertical distributions and maximum abundance depth associated with the depth and intensity of the OMZ and its oxycline inflection points, (2) shifted their diapause depth, (3) adjusted their diel vertical migration, especially the nighttime upper depth, or (4) expanded or contracted their depth range within the mixed layer and upper part of the thermocline in association with the thickness of the aerobic epipelagic zone (habitat compression concept) (Figure 1). Distribution depths for some species shifted by 10’s to 100’s of meters in different situations, which also had metabolic (and carbon flow) implications because temperature decreased with depth. This observed present-day variability may provide an important window into how future marine ecosystems will respond to deoxygenation.

Figure caption: Schematic diagram showing how future OMZ expansion may affect zooplankton distributions, based on present-day responses to OMZ variability. The dashed line indicates diel vertical migration (DVM) and highlights the shoaling of the nighttime depth as the aerobic habitat is compressed. The lower oxycline community and the diapause layer for some species, associated with a specific oxygen concentration, may deepen as the OMZ expands.

Authors:
Karen F. Wishner (University of Rhode Island)
Brad Seibel (University of South Florida)
Dawn Outram (University of Rhode Island)

Can phytoplankton help us determine ocean iron bioavailability?

Posted by mmaheigan

· Wednesday, March 11th, 2020

Iron (Fe) is a key element to sustaining life, but it is present at extremely low concentrations in seawater. This scarcity limits phytoplankton growth in large swaths of the global ocean, with implications for marine food webs and carbon cycling. The acquisition of Fe by phytoplankton is an important process that mediates the movement of carbon to the deep ocean and across trophic levels. It is a challenge to evaluate the ability of marine phytoplankton to obtain Fe from seawater since it is bound by a variety of poorly defined organic complexes.

Figure 1: Schematic representation of the reactions governing dissolved Fe (dFe) bioavailability to phytoplankton (a) Bioavailability of dFe in seawater collected from various basins and depth and probed with different iron-limited phytoplankton species under dim laboratory light and sunlight (b) (See paper for further details on samples and species)

A recent study in The ISME Journal proposes a new approach for evaluating seawater dissolved Fe (dFe) bioavailability based on its uptake rate constant by Fe-limited cultured phytoplankton. The authors collected samples from distinct regions across the global ocean, measured the properties of organic complexation, loaded these complexes with a radioactive Fe isotope, and then tracked the internalization rates from these forms to a diverse set of Fe-limited phytoplankton species. Regardless of origin, all of the phytoplankton acquired natural organic complexes at similar rates (accounting for cell surface area). This confirms that multiple Fe-limited phytoplankton species can be used to probe dFe bioavailability in seawater. Among water types, dFe bioavailability varied by ~4-fold and did not clearly correlate with Fe concentrations or any of the measured Fe speciation parameters. This new approach provides a novel way to determine Fe bioavailability in samples from across the oceans and enables modeling of in situ Fe uptake rates by phytoplankton based simply on measured Fe concentrations.

Authors:
Yeala Shaked (Hebrew University of Jerusalem)
Kristen N. Buck (University of South Florida)
Travis Mellett (University of South Florida)
Maria. T. Maldonado (University of British Columbia)

Pumped up by the cold: Increased elemental density in polar diatoms

Posted by mmaheigan

· Monday, October 28th, 2019

Large diatoms are common in polar phytoplankton blooms, contributing significantly to food webs and carbon export, but relatively little is known about their elemental biogeochemistry. A recent study in Frontiers in Marine Science showed that the size-dependent increase in cell nutrient content for polar diatoms was similar to published values for temperate diatoms, whereas the elemental density (mass per unit volume) of polar diatoms was substantially greater for all elements measured (carbon, nitrogen, silicon and phosphorus). Furthermore, at near freezing culture temperatures, there was a positive relationship between diatom size and realized growth rates near their theoretical maximum (Figure 1). Because of the differences in elemental density between carbon and silica, these diatoms exhibited particulate C:Si ratios that are commonly interpreted as a sign of iron limitation; yet these cultures were trace metal-replete. The observed elemental composition differences suggest that it may be important for polar biogeochemical models to include different representations of diatom biogeochemistry by accounting for the functions of size and near freezing temperature.

Figure 1. Left: Cellular carbon content for polar diatoms across four orders of magnitude in biovolume compared to the same relationship for a wide range of non-polar diatoms (MD&L = Menden-Deuer & Lessard, 2000). The y-intercept is the estimate of the baseline carbon density in these polar diatoms, and is significantly higher than the literature values reviewed in MD&L (2000). Right: Growth rate of the same polar diatoms expressed as a percent of their calculated maximum growth rate at 2°C. Error bars represent the range of values observed in the experiments. Maximum growth rate was estimated by 1) applying the growth rate/biovolume relationships published by Chisholm (1992) and Edwards et al. (2012) to the observed biovolume for each culture, and 2) scaling this growth rate to 2°C growth temperature using the relationship of Eppley (1972).

Authors:
Michael Lomas (Bigelow Laboratory for Ocean Sciences)
Steven Baer (Maine Maritime Academy)
Sydney Acton (Dauphin Island Sea Lab)
Jeffrey Krause (Dauphin Island Sea Lab and University of South Alabama)

The ecology of the biological carbon pump

Posted by mmaheigan

· Tuesday, October 15th, 2019

Plankton in the surface ocean convert CO₂ into organic biomass thereby fueling marine food webs. Part of this organic biomass sinks down into the deep ocean, where the surface-derived organic carbon, or respired CO₂, is locked in for decades to millennia. Without the biological carbon pump, atmospheric CO₂ would be ~200 ppm higher than it is today. We know that ecological processes in the surface ocean plankton communities have a paramount importance on the efficiency of the biological carbon pump. Unfortunately, however, the mechanisms how ecology determines sinking fluxes are poorly understood.

A recent study in Global Biogeochemical Cycles used large-scale in situ mesocosms to explore how the ecological interplay within plankton communities affects the downward flux of organic material. Organic biomass tends to sink faster when produced by smaller organisms because the sinking material they generate forms dense aggregates. Conversely, larger organisms produce relatively porous particles that sink more slowly.

Figure: Flow chart illustrating how plankton community structure affects the properties of sinking organic particles and ultimately the strength and efficiency of the biological carbon pump. The thick arrows at the bottom indicate that flux attenuation depends on the properties of particulate matter formed in the surface ocean. For example, slow-sinking porous aggregates containing large amounts of easily degradable organic substances will decay faster (right side) than dense aggregates of more refractory organic matter (left side).

The key finding of this study was the unexpectedly large influence that plankton community composition has on the degradation rate of sinking organic biomass. In fact, degradation rates changed maximally 15-fold over the course of the study while sinking speed changed only 3-fold. Degradation rate of sinking material, measured in oxygen consumption assays, was quite variable and tended to be higher for more easily degradable fresh organic matter. The rate was lower during harmful algal blooms, which produce toxic substances that inhibit organisms that feed on aggregates thereby reducing degradation rates. These findings are an important step forward as they show that our predictive understanding of the biological carbon pump could be improved substantially when linking degradation rates of sinking material with ecological processes in surface ocean plankton communities.

Authors:
L. T. Bach (University of Tasmania)
P. Stange, J. Taucher, E. P. Achterberg, M. Esposito, U. Riebesell (GEOMAR)
M. Algueró‐Muñiz (Alfred-Wegener-Institut Helmholtz)
H. Horn (NIOZ and Utrecht University)

Where the primary production goes determines whether you catch tuna or cod

Posted by mmaheigan

· Friday, September 6th, 2019

Fishes are incredibly diverse, fill various roles in their ecosystems, and are an important resource—economically, socially, and nutritionally. The relationship between primary productivity and fish catches is not straightforward; fisheries oceanographers and managers have long struggled to predict abundances and fully understand the controls of cross-ecosystem differences in fish abundances and assemblages. A recent study in Progress in Oceanography modeled the relationships between fish abundances and assemblages and ecosystem factors such as physical properties and plankton productivity.

The mechanistic model simulated feeding, growth, reproduction, and mortality of small pelagic forage fish, large pelagic fish, and demersal (bottom-dwelling) fish in the global ocean using plankton food web estimates and ocean conditions from a high-resolution earth system model of the 1990s. Modeled fish assemblages were more related to the separation of secondary production into pelagic zooplankton or benthic fauna secondary production than to primary productivity. Specifically, the ratio of pelagic to benthic production drove spatial differences in dominance by large pelagic fish or by demersal fish. Similarly, demersal fish abundance was highly sensitive to the efficiency of energy transfer from exported surface production to benthic fauna.

The model results offer a systematic understanding of how marine fish communities are structured by spatially varying environmental conditions. With global climate change, the expected decrease in exported primary production would lead to fewer demersal fish around the world. This model provides a framework for testing the effect of changing conditions on fish communities at a global scale, which can also help inform managers of potential impacts on economic, social, and nutritional resources worldwide.

Figure 1: (A) Sample food web with three fish types, two habitats, two prey categories, and feeding interactions (arrows). Dashed arrow denotes feeding only occurs in shelf regions with depth <200 m. (B) Fraction of large pelagic vs. demersal fishes (LP/(LP+D)) as a function of the ratio of zooplankton production lost to higher predation (Zoop) to detritus flux to the seafloor (Bent) averaged over large marine ecosystems. Solid line: predicted linear model response, dashed lines: standard error. (Lower panels) Circles=mean biomasses (g m^-2) and lines=fluxes of biomass (g m^-2 d^-1) through the pelagic (top 100m) and benthic components of the food webs at two test locations, (C) Peruvian Upwelling (PUP) ecosystem and (D) Eastern Bering Sea (EBS) shelf ecosystem. Circles and lines scale with the modeled biomasses and fluxes. Circle color key: Gray=net primary productivity (NPP); yellow=medium and large zooplankton; red=forage fish; blue=large pelagic fish; brown=benthos; green=demersal fish.

Authors:
Colleen M. Petrik (Princeton University, Texas A&M University)
Charles A. Stock (NOAA Geophysical Fluid Dynamics Laboratory)
Ken H. Andersen (Technical University of Denmark)
P. Daniël van Denderen (Technical University of Denmark)
James R. Watson (Oregon State University)

Can microzooplankton shape the depth distribution of phytoplankton?

Posted by mmaheigan

· Tuesday, July 23rd, 2019

Photosynthetic, single-celled phytoplankton form the base of many marine and lacustrine (lake) food webs. These microscopic algae typically occur in the sunlit surface layer, but in many ecosystems, there are also sub-surface peaks in phytoplankton and chlorophyll-a, their key photosynthetic pigment. Historically, scientists have explained deep chlorophyll maximum (DCM) formation by invoking “bottom-up” processes such as nutrient and light co-limitation, while less attention has been paid to “top-down” controls such as predation.

A recent study in Nature Communications challenges this conventional wisdom by arguing that microzooplankton (top-down control) can cause the formation of DCMs by preferentially consuming phytoplankton near the surface. This can occur when microzooplankton exhibit light-dependent grazing—a known but not well-understood phenomenon in which prey consumption rates increase with increasing light intensity. By incorporating this phenomenon into mathematical models, the authors showed that this can create a “spatial refuge” for phytoplankton in deeper, darker parts of the water column, where there is enough sunlight to photosynthesize, but too little for efficient microzooplankton predation. Furthermore, when light-dependent grazing is incorporated into a global ocean biogeochemistry model (COBALT: Carbon, Ocean Biogeochemistry and Lower Trophics – planktonic ecosystem model), DCMs that are already present due to bottom-up controls deepen, improving agreement between model predictions, satellite data, and in situ observations.

Figure legend: Global comparison of annual mean deep chlorophyll maxima (DCM) depths (A) predicted by the unmodified COBALT model, (B) predicted by the COBALT model modified to include light-dependent microzooplankton grazing, and (C) estimated based on satellite data. Incorporating light-dependent grazing deepens the DCM, especially in oligotrophic gyres, and improves agreement with observational data.

These findings highlight the importance of higher trophic levels in regulating aquatic primary productivity. The model predictions suggest that not only can microzooplankton suppress primary production near the surface, but by shifting phytoplankton abundances deeper, they may increase carbon export via the biological pump. Future field tests of this hypothesis—i.e. detailed grazing measurements in stratified water columns with DCMs—can elucidate the extent to which light-dependent grazing shapes phytoplankton distribution in real biological systems.

Authors:
Holly Moeller (University of California Santa Barbara)
Charlotte Laufkötter (University of Bern and Princeton University)
Edward Sweeney (Sea Education Association and Santa Barbara Museum of Natural History)
Matthew Johnson (Woods Hole Oceanographic Institution)

Ocean color offers early warning signal of climate change’s impact on marine phytoplankton

Posted by mmaheigan

· Monday, April 15th, 2019

Marine phytoplankton form the foundation of the marine food web and play a crucial role in the earth’s carbon cycle. Typically, satellite-derived Chlorophyll a (Chl a) is used to evaluate trends in phytoplankton. However, it may be many decades (or longer) before we see a statistically significant signature of climate change in Chl a due to its inherently large natural variability. In a recent study in Nature Communications, authors explored how other metrics, in particular the color of the ocean, may show earlier and stronger signals of climate change at the base of the marine food web.

Figure 1. Computer model results indicating the year in which the signature of climate change impact is larger than the natural variability for (a) Chl a, and (b) remotely sensed reflectance in the blue-green waveband. White areas indicate where there is not a statistically significant change by 2100, or for regions that are currently ice-covered.

In this study, the authors use a unique marine physical-biogeochemical and ecosystem model that also captures how light penetrates the ocean and is reflected upward. The model shows that over the course of the 21^st century, remote sensing reflectance (R_RS, the ratio of upwelling radiance to the downwelling irradiance at the ocean’s surface) in the blue-green portions of the light spectrum is likely to have an earlier, more spatially extensive climate change-driven signal than Chl a (Figure 1). This is because R_RS integrates not only changes to Chl a, but also alterations in other optically important water constituents. In particular, R_RS also captures changes in phytoplankton community structure, which strongly affects ocean optics and is likely to be altered over the 21^st century. Monitoring the response of marine phytoplankton to climate change is important for predicting changes at higher trophic levels, including commercial fisheries. Our study emphasizes the importance of 1) maintaining ocean color sensor compatibility and long-term stability, particularly in the blue-green wavebands; 2) maintaining long-term in situ time-series of plankton communities – e.g., the Continuous Plankton Recorder survey and repeat stations (e.g., HOT, BATS); and 3) reducing uncertainties in satellite-derived phytoplankton community structure estimates.

Authors:
Stephanie Dutkiewicz, Oliver Jahn (Massachusetts Institute of Technology)
Anna E. Hickman (University of Southampton)
Stephanie Henson (National Oceanography Centre Southampton)
Claudie Beaulieu (University of California, Santa Cruz)
Erwan Monier (University of California, Davis)

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)

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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