More Like this

1. Resilience of plankton trophic structure to an eddy-stimulated diatom bloom in the North Pacific Subtropical Gyre

   https://doi.org/10.3354/meps13333  

   Décima, M; Landry, MR (June 2020, Marine Ecology Progress Series)

   null (Ed.)

   We investigated the response of an open-ocean plankton food web to a major ecosystem perturbation event, the Hawaiian lee cyclonic eddy Opal, using compound-specific isotopic analyses of amino acids (CSIA-AA) of individual zooplankton taxa. We hypothesized that the massive diatom bloom that characterized Opal would lead to a shorter food chain. Using CSIA-AA, we differentiated trophic position (TP) changes that arose from altered transfers through protistan microzooplankton, versus metazoan carnivory, and assessed the variability at the base of the food web. Contrary to expectation, zooplankton TPs were higher in the eddy than in ambient control waters (up to 0.8 trophic level), particularly for suspension feeders close to the food-web base. Most of the effect was due to increased trophic transfers through protistan consumers, indicating a general shift up, not down, of grazing and remineralization in the microbial food web. Eucalanus sp., which was 15-fold more abundant inside compared to outside of the eddy, was the only taxon observed to be a true herbivore (TP = 2.0), consistent with a high phenylalanine (Phe) δ 15 N value indicating feeding on nitrate-fueled diatoms in the lower euphotic zone. Oncaea sp., an aggregate-associated copepod, had the largest (1.5) TP difference, and lowest Phe δ 15 N, suggesting that detrital particles were local hot spots of enhanced microbial activity. Rapid growth rates and trophic flexibility of protistan microzooplankton apparently allow the microbial community to reorganize to bloom perturbations, as microzooplankton remain the primary phytoplankton grazers—despite the dominance of large diatoms—and are heavily preyed on by the mesozooplankton. 

   more » « less
2. Plankton food webs in the oligotrophic Gulf of Mexico spawning grounds of Atlantic bluefin tuna

   https://doi.org/10.1093/plankt/fbab023  

   Stukel, Michael R; Gerard, Trika; Kelly, Thomas B; Knapp, Angela N; Laiz-Carrión, Raúl; Lamkin, John T; Landry, Michael R; Malca, Estrella; Selph, Karen E; Shiroza, Akihiro; et al (April 2021, Journal of Plankton Research)

   null (Ed.)

   We used linear inverse ecosystem modeling techniques to assimilate data from extensive Lagrangian field experiments into a mass-balance constrained food web for the Gulf of Mexico open-ocean ecosystem. This region is highly oligotrophic, yet Atlantic bluefin tuna (ABT) travel long distances from feeding grounds in the North Atlantic to spawn there. Our results show extensive nutrient regeneration fueling primary productivity (mostly by cyanobacteria and other picophytoplankton) in the upper euphotic zone. The food web is dominated by the microbial loop (>70% of net primary productivity is respired by heterotrophic bacteria and protists that feed on them). By contrast, herbivorous food web pathways from phytoplankton to metazoan zooplankton process <10% of the net primary production in the mixed layer. Nevertheless, ABT larvae feed preferentially on podonid cladocerans and other suspension-feeding zooplankton, which in turn derive much of their nutrition from nano- and micro-phytoplankton (mixotrophic flagellates, and to a lesser extent, diatoms). This allows ABT larvae to maintain a comparatively low trophic level (~4.2 for preflexion and postflexion larvae), which increases trophic transfer from phytoplankton to larval fish. 

   more » « less
3. A Bayesian model for assessing organic matter supply in complex marine food webs using amino acid stable isotope analysis

   https://doi.org/10.7717/peerj.20220  

   Shea, Connor HH; Drazen, Jeffrey C; Popp, Brian N (November 2025, PeerJ)

   While several software packages have been developed to solve stable isotope mixing models, none are currently equipped to trace the flow of organic matter through the lower trophic levels of planktonic food webs. To address this gap, we have developed a new Bayesian mixing model tailored for use with δ¹⁵N values of individual amino acids. This model simultaneously estimates trophic relationships between consumers and organic matter sources at the base of the food web, determines the relative contributions of these basal organic matter sources to consumers, and accounts for trophic discrimination affecting amino acid δ¹⁵N values during protozoan and metazoan trophic steps. This “Organic Matter Supply Model” is uniquely suited for applications where food web structure is unknown and trophic intermediaries, such as protozoan and metazoan grazers with distinct amino acid trophic discrimination factors, play a critical role in nutrient transfer. In this paper, we describe the model’s basic structure, outline key considerations for adapting it to specific applications, evaluate its performance using simulated zooplankton data, discuss its strengths and limitations, and offer recommendations for its further development. By testing the model on simulated zooplankton amino acid δ¹⁵N data, we demonstrate that the Organic Matter Supply Model can enhance our understanding of the roles of small particles and diel vertical migration in deep-sea organic matter supply pathways. Furthermore, it provides a new framework for exploring the foundational role of heterotrophic protists in marine ecosystems. We find specific subsets of amino acids to be most useful as markers of trophic ecology (in this case including glutamic acid and proline) and to identify supply from basal organic matter sources (phenylalanine, lysine, and threonine). Other amino acids may be more ideal source tracers in other settings, although amino acids with inconsistent or poorly constrained isotope fractionation behavior (e.g., isoleucine, valine) should be excluded to optimize model reliability. 

   more » « less
4. Size‐specific grazing and competitive interactions between large salps and protistan grazers

   https://doi.org/10.1002/lno.11770  

   Stukel, Michael R.; Décima, Moira; Selph, Karen E.; Gutiérrez‐Rodríguez, Andres (January 2021, Limnology and Oceanography)

   null (Ed.)

   We investigated competition between Salpa thompsoni and protistan grazers during Lagrangian experiments near the Subtropical Front in the southwest Pacific sector of the Southern Ocean. Over a month, the salp community shifted from dominance by large (>100-mm) oozooids and small (<20-mm) blastozooids to large (~60-mm) blastozooids. Phytoplankton biomass was consistently dominated by nano- and microphytoplankton (>2 µm cells). Using bead-calibrated flow-cytometry light scatter to estimate phytoplankton size, we quantified size-specific salp and protistan zooplankton grazing pressure. Salps were able to feed at a >10,000:1 predator:prey size (linear-dimension) ratio. Small blastozooids efficiently retained cells >1.4-μm (high end of picoplankton size, 0.6-2 µm cells) and also obtained substantial nutrition from smaller bacteria-sized cells. Larger salps could only feed efficiently on >5.9-μm cells and were largely incapable of feeding on picoplankton. Due to the high biomass of nano- and microphytoplankton, however, all salps derived most of their (phytoplankton-based) nutrition from these larger autotrophs. Phagotrophic protists were the dominant competitors for these prey items and consumed approximately 50% of the biomass of all phytoplankton size classes each day. Using a Bayesian statistical framework, we developed an allometric-scaling equation for salp clearance rates as a function of salp and prey size: Clearance(ESD)=φ∙〖TL〗^ψ × min⁡((ESD⁄(θ×〖TL〗^γ ))^2/(0.16+(ESD⁄(θ×〖TL〗^γ )) ),1)× 〖Q_10〗^(((T-12°C))⁄10) where ESD is prey equivalent spherical diameter, TL is Salpa thompsoni total length, φ = 5.6×10-3 ± 3.6×10-4, ψ = 2.1 ± 0.13, θ = 0.58 ± 0.08, and γ = 0.46 ± 0.03. We discuss the biogeochemical and food-web implications of competitive interactions between salps, krill, and protozoans. 

   more » « less
5. Protistan grazing impacts microbial communities and carbon cycling at deep-sea hydrothermal vents

   https://doi.org/10.1073/pnas.2102674118  

   Hu, Sarah K.; Herrera, Erica L.; Smith, Amy R.; Pachiadaki, Maria G.; Edgcomb, Virginia P.; Sylva, Sean P.; Chan, Eric W.; Seewald, Jeffrey S.; German, Christopher R.; Huber, Julie A. (July 2021, Proceedings of the National Academy of Sciences)

   Microbial eukaryotes (or protists) in marine ecosystems are a link between primary producers and all higher trophic levels, and the rate at which heterotrophic protistan grazers consume microbial prey is a key mechanism for carbon transport and recycling in microbial food webs. At deep-sea hydrothermal vents, chemosynthetic bacteria and archaea form the base of a food web that functions in the absence of sunlight, but the role of protistan grazers in these highly productive ecosystems is largely unexplored. Here, we pair grazing experiments with a molecular survey to quantify protistan grazing and to characterize the composition of vent-associated protists in low-temperature diffuse venting fluids from Gorda Ridge in the northeast Pacific Ocean. Results reveal protists exert higher predation pressure at vents compared to the surrounding deep seawater environment and may account for consuming 28 to 62% of the daily stock of prokaryotic biomass within discharging hydrothermal vent fluids. The vent-associated protistan community was more species rich relative to the background deep sea, and patterns in the distribution and co-occurrence of vent microbes provide additional insights into potential predator–prey interactions. Ciliates, followed by dinoflagellates, Syndiniales, rhizaria, and stramenopiles, dominated the vent protistan community and included bacterivorous species, species known to host symbionts, and parasites. Our findings provide an estimate of protistan grazing pressure within hydrothermal vent food webs, highlighting the important role that diverse protistan communities play in deep-sea carbon cycling. 

   more » « less