Archive for nitrate

Mixotrophs in the northern North Atlantic

Posted by mmaheigan 
· Tuesday, April 16th, 2024 

Mixotrophs (or mixoplankton) are now accepted as a third group of plankton alongside phytoplankton and zooplankton. Our knowledge of mixotrophs lags far behind that of the other two groups. We currently have only a limited understanding of mixotrophs’ biogeographical distribution across ocean basins, and what environmental factors are associated with their distribution.

The authors of a study recently published in Frontiers in Marine Science reviewed nearly 230,000 individual microplankton samples collected by the North Atlantic Continuous Plankton Recorder program between 1958 and 2015 and calculated the proportion of organisms that are considered mixotrophs in each sample. They classified protist species in the dataset as phytoplankton, mixotrophs, or microzooplankton (heterotrophs), based on existing literature. Taken together across seasonsin shelf waters (depth ≤ 300m), mixotrophs comprise a greater proportion of the microplankton community when nitrate is high and photosynthetically available radiation (PAR) is low (e.g. during the late fall and winter), or when nitrate is low and PAR is moderate to high (e.g. during the summer and early fall). When both nitrate and PAR are high, mixotrophs comprise less of the community compared to phytoplankton. The same pattern was found in offshore waters (depth > 300m), but the key macronutrient was phosphate rather than nitrate. The annual average proportion of mixotrophs in microplankton samples compared to phytoplankton has increased since 1958 in the offshore portion of the study region, with a notable changepoint in 1993; this increasing trend is strongest in the winter season.

This paper is useful for aquatic ecologists who want to integrate mixotrophic plankton into their understanding of marine food webs and biogeochemical cycles. Understanding mixotroph temporal and spatial distributions, as well as the environmental conditions under which they flourish, is imperative to understanding their impact on trophic transfer and biogeochemical cycling.

Authors
Karen Stamieszkin (Bigelow Laboratory for Ocean Sciences)
Nicole Millette (Virginia Institute of Marine Science)
Jessica Luo (NOAA Geophysical Fluid Dynamics Laboratory)
Elizabeth Follett (University of Liverpool)
Nick Record (Bigelow Laboratory of Ocean Science)
David Johns (Marine Biological Association)

 

Backstory
This work and the collaboration that made it possible was catalyzed by the Eco-DAS XII symposium, attended by Karen Stamieszkin, Nicole Millette, Jessica Luo, and Elizabeth Follett in 2016. Nicole had an idea for an analysis but lacked collaborators, just as she was ready to give up on it, Karen, Jessica, and Elizabeth expressed interest in the project. Karen, Jessica, and Elizabeth each brought a unique perspective that helped make Nicole’s original idea more practical and ensured that the analysis would come to life.

The collaboration that began with this paper lead to the OCB Mixotrophs & Mixotrophy Working Group led by Karen, Jessica, and Nicole, and a successful grant proposal to study mixotrophy awarded to Nicole and Karen by NSF’s Biological Oceanography program. This story shows the importance and power of programs that connect researchers across disciplines, especially in the early stages of their careers.

The role of nutrient trapping in promoting shelf hypoxia in the southern Benguela upwelling system

Posted by mmaheigan 
· Thursday, September 3rd, 2020 

The southern Benguela upwelling system (SBUS) off southwest Africa is an exceptionally fertile ocean region that supports valuable commercial fisheries. The productivity of this system derives from the upwelling of nutrient-rich Subantarctic Mode Water, and from the concurrent entrainment of nutrients regenerated proximately on the expansive continental shelf. The SBUS is prone to severe seasonal hypoxic events that decimate regional fisheries, occurrences of which are inextricably linked to the inherent nutrient dynamics. In a study recently published in JGR Oceans, the authors sought to understand the mechanisms sustaining elevated concentrations and seasonally-variable distributions of nutrients in the SBUS, in relation to the subsurface oxygen content. Inter-seasonal measurements of nutrients and nitrate isotope ratios across the SBUS in 2017 revealed that upwards of 48% (summer) and 63% (winter) of the on‐shelf nutrients derived from regeneration in situ.  The severity of hypoxia at the shelf bottom, in turn, correlated with the incidence of regenerated nutrients. The accrual of nutrients at the shelf bottom appears to be aided by hydrographic fronts that restrict offshore transport, trapping regenerated nutrients on the SBUS shelf and increasing the pool of nutrients available for upwelling – ultimately contributing to hypoxic events. This study underscores the need – if we are to develop a mechanistic and predictive understanding of hypoxia in the SBUS and elsewhere – to elucidate the role of shelf circulation in promoting the accrual of regenerated nutrients on the continental shelf. The next step is to combine new and existing observations with quantitative simulations to further interrogate the coupled physical-biogeochemical mechanisms that modulate the intensity of hypoxia.

Figure caption: Schematic of proposed nutrient-trapping mechanism: Deep nutrient-rich Subantarctic Mode Water (SAMW) acquires more nutrients as it passes over the shelf sediments from the regeneration of exported particulate organic material (POM). The production of this POM is fueled by nutrients stripped from the surface waters advecting back off-shore. The thickness of the arrows represents nutrient concentrations. Triangles indicate the positions of the Shelf Break Front (SBF) and Columbine Front (CF), coincident with an observed subduction of the Ekman layer and downwelling at the inner front boundary.

Authors
Raquel Flynn (University of Cape Town)
Julie Granger (University of Connecticut)
Jennifer Veitch (South African Environmental Observation Network)
Samantha Siedlecki (University of Connecticut)
Jessica Burger (University of Cape Town)
Keshnee Pillay (South Africa Department of Environment, Forestry and Fisheries)
Sarah Fawcett (University of Cape Town)

Profiling floats reveal fate of Southern Ocean phytoplankton stocks

Posted by mmaheigan 
· Tuesday, September 1st, 2020 

More observations are needed to constrain the relative roles of physical (advection), biogeochemical (downward export), and ecological (grazing and biological losses) processes in driving the fate of phytoplankton blooms in Southern Ocean waters. In a recent paper published in Nature Communications, authors used seven Biogeochemical Argo (BGC-Argo) floats that vertically profiled the upper ocean every ten days as they drifted for three years across the remote Sea Ice Zone of the Southern Ocean. Using the floats’ biogeochemical sensors (chlorophyll, nitrate, and backscattering) and regional ratios of nitrate consumption:chlorophyll synthesis, the authors developed a new approach to remotely estimate the fate of the phytoplankton stocks, enabling calculations of herbivory and of downward carbon export. The study revealed that the major fate of phytoplankton biomass in this region is grazing, which consumes ~90% of stocks. The remaining 10% is exported to depth. This pattern was consistent throughout the entire sea ice zone where the floats drifted, from 60°-69° South.

Figure Caption: Southern Ocean Chlorophyll a climatology and floats’ trajectories (top panel). Total losses of Chlorophyll a (including grazing and phytodetritus export, left panel). Phytodetritus export (right panel).

 

This study region comprises two of the three major krill growth and development areas—the eastern Weddell and King Haakon VII Seas and Prydz Bay and the Kerguelen Plateau—so the observed grazing was probably due to Antarctic krill, underscoring their pivotal importance in this ecosystem. Building upon the greater understanding of ocean ecosystems via satellite ocean colour development in the 1990s, BGC-Argo floats and this new approach will allow remote monitoring of the different fates of phytoplankton stocks and insights into the status of the ecosystem.

 

Authors:
Sebastien Moreau (Norwegian Polar Institute, Tromsø, Norway)
Philip Boyd (Institute for Marine and Antarctic Studies, Hobart, Australia)
Peter Strutton (Institute for Marine and Antarctic Studies, Hobart, Australia)

Physics vs. biology in Southern Ocean nutrient gradients

Posted by mmaheigan 
· Tuesday, June 16th, 2020 

In the Southern Ocean, surface water silicate (SiO4) concentrations decline very quickly relative to nitrate concentrations along a northward gradient toward mode water formation regions on the northern edge (Figure 1a, b). These mode waters play a critical role in driving global nutrient concentrations, setting the biogeochemistry of low- and mid-latitude regions around the globe after they upwell further north. To explain this latitudinal surface gradient, most hypotheses have implicated diatoms, which take up and export silicon as well as nitrogen: (1) Diatoms, including highly-silicified species such as Fragilariopsis kerguelensis, are more abundant in the Southern Ocean than elsewhere; (2) Iron limitation, which is prevalent in the Southern Ocean, elevates the Si:N ratio of diatoms; (3) Mass export of empty diatom frustules pumps silicate but not nitrate to deeper waters.

Figure 1: (a) and (b) nitrate and silicate concentrations in surface waters of the Southern Ocean (GLODAPv2_2019 data). (c) Model results of a standard run (black diamonds), a run without biology (red diamonds) and a run without mixing (blue diamonds).

In a recent paper published in Biogeosciences, the authors use an idealized model to explore the relative roles of biological vs. physical processes in driving the observed latitudinal surface nutrient gradients. Over timescales of a few years, removing the effects of biology (no SiO4 uptake or export) from the model elevates silicate concentrations slightly over the entire latitudinal range, but does not remove the strong latitudinal gradient (Figure 1c). However, if the effects of vertical mixing processes such as upwelling and entrainment are removed from the model by eliminating the observed deep [SiO4] gradient, the observed surface nutrient gradient is greatly altered (Figure 1c). These model results suggest that, over short timescales, physics is more important than biology in driving the observed surface water gradient in SiO4:NO3 ratios and forcing silicate depletion of mode waters leaving the Southern Ocean. These findings add to our understanding of Southern Ocean dynamics and the downstream effects on other oceans.

 

Authors:
P. Demuynck (University of Southampton)
T. Tyrrell (University of Southampton)
A.C. Naveira Garabato (University of Southampton)
C.M. Moore (University of Southampton)
A.P. Martin (National Oceanography Centre)

Nitrate enrichment may threaten coastal wetland carbon storage

Posted by mmaheigan 
· Thursday, February 27th, 2020 

With their high primary productivity and slow decomposition in anoxic soils, salt marshes and other coastal wetlands can store carbon more efficiently than terrestrial uplands. These wetlands also provide critical ecosystem services such as interception of land-derived nutrients before they can enter the coastal ocean. Therefore, it is important to understand how anthropogenic supplies of nitrate (NO3–) affect marsh sustainability and carbon storage.

In marsh sediment studies, the most common form of experimental nitrogen enrichment uses pelletized fertilizer composed of ammonium, urea, or other organic based fertilizers. Authors of a recent study published in Global Change Biology hypothesized that when nutrients were instead added in the form of nitrate (NO3–), the most common form of nitrogen enrichment in coastal waters, it would stimulate microbial decomposition of organic matter by serving as an electron acceptor for microbial respiration in anoxic salt marsh sediments. Furthermore, decomposition would vary with sediment depth, with decreased decomposition at greater depths, where less biologically available organic matter accumulated over time.

Figure 1: DIC production as a proxy for microbial respiration in salt marsh sediments from three distinct depth horizons (shallow 0-5cm, mid 10-15cm, deep 20-25cm) that span a range of biological availability. The addition of NO3- (green) stimulated DIC production relative to unenriched sediments, regardless of sediment depth. All samples were run under anoxic conditions (without the presence of oxygen), closely matching that of normal salt marsh sediments.

Surprisingly, NO3– addition stimulated decomposition of organic matter at all depths, with the highest decomposition rates in the surface sediments. This suggests that there is a pool of “NO3–-labile” organic matter in marsh sediments that microbes can decompose under high-NO3– conditions that would otherwise remain stable. As human activities continue to enrich our coastal waters with NO3– through agricultural runoff, septic systems, and other pathways, it could inadvertently decrease coastal wetlands’ carbon storage capacity, with negative consequences for both blue carbon offsets and marsh sustainability in the face of sea level rise.

 

Authors:
Jennifer Bowen (Northeastern University)
Ashley Bulseco (MBL/WHOI)
Anne Giblin (MBL)

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