Biology subjects

Sosik, H. M.

Publications and source records attributed to Sosik, H. M..

5 recordsLinked to original sources

bioRxiv2025

Small phytoplankton community composition cycles annually with a coastal bloom

Small photosynthetic eukaryotes are a productive and dynamic component of marine planktonic communities. Here, we investigate how seasonal changes in abundance of these primary producers relate to changes in their community composition at a coastal site on the Northeast U.S. Shelf. We present a 9-yr time series of 18S rRNA sequencing data and identify gradual transitions within the pico- and nanoplankton community that occur repeatedly over the annual cycle. We compare these compositional changes to concurrent high-resolution in situ flow cytometry measurements of eukaryotic phytoplank-ton abundance and division rate. We find that the Chlorophyta contribute a large proportion of the sequences in our samples and drive much of the seasonal variability within the small phytoplankton community. Across the time series, Bathycoccus, Micromonas, and Picochlorum are the dominant genera, with the first being present year round, while Micromonas bravo and Picochlorum are representative of the summer community. We also find a strong winter Phaeocystis signal which might be leading to flow cytometry measurements of relatively large cells in the early spring. Our results provide fundamental knowledge of the taxonomic composition of the phytoplankton community on the Northeast U.S. Shelf, improving our understanding of the regions diversity and compositional variability over time.

ecology↗

bioRxiv2025

A nonequilibrium framework for community responses to pulse perturbations

Understanding responses of ecological communities to shocks that displace species abundances is of paramount importance given the increasing frequency of extreme climatic events. However, current theory on responses to such pulse perturbations focuses on equilibrium points and we lack a unified framework that accommodates other common, but more complicated, types of population fluctuations such as transients and cycles. Here we introduce this frame-work by deriving metrics that quantify the minimum, typical, and maximum amplification of perturbed abundances under nonequilibrium population dynamics. By simulating models under several nonequilibrium scenarios, we demonstrate that these metrics accurately characterize the full range of amplification of perturbed abundances in the short and long terms. Notably, we show that perturbation amplification depends strongly on community state in the short term, but this state dependency vanishes in the long term. Overall, our framework enables stability analysis for model and natural communities that do not exhibit equilibrium dynamics.

ecology↗

bioRxiv2025

Eukaryotic biodiversity and ecological networks from the surface to the mesopelagic in the Northwest Atlantic Slope Water

1.The diversity and interactions among mesopelagic organisms are difficult to study and as a result, are insufficiently unaccounted for in food web and biogeochemical models. This knowledge gap hinders our ability to model and forecast ecosystem function and formulate effective policies for conservation and management in the face of growing interest in exploiting midwater living resources. We used multi-marker metabarcoding of environmental DNA (eDNA) samples collected from Northwest Atlantic Slope Water to resolve patterns of eukaryotic community composition spanning taxonomically across protists (microbial eukaryotes), invertebrates, and vertebrates and vertically from the ocean surface to the base of the mesopelagic zone. With statistical network analyses, we explored cross-kingdom associations including putative trophic dynamics such as food web interactions and evaluated network robustness to biodiversity loss. We found depth-specific communities of distinct protist, invertebrate, and vertebrate assemblages. Ecological networks for the epipelagic, upper mesopelagic, and lower mesopelagic suggest that protists are keystone taxa and important mediators of trophic interactions; they increase network complexity and contribute to network stability. We also identified metazoans including copepods, gelatinous taxa (cnidarians, tunicates), and mesopelagic fish as important components of network interactions. Our study demonstrates a holistic approach to generate insights on mesopelagic biodiversity and implications for ecosystem resilience that can inform ocean governance.

ecology↗

bioRxiv2023

Toward a synthesis of phytoplankton community composition methods for global-scale application

The composition of the marine phytoplankton community has been shown to impact many biogeochemical processes and marine ecosystem services. A variety of methods exist to characterize phytoplankton community composition (PCC), with varying degrees of taxonomic resolution. Accordingly, the resulting PCC determinations are dependent on the method used. Here, we use surface ocean samples collected in the North Atlantic and North Pacific Oceans to compare high performance liquid chromatography (HPLC) pigment-based PCC to four other methods: quantitative cell imaging, flow cytometry, and 16S and 18S rRNA amplicon sequencing. These methods allow characterization of both prokaryotic and eukaryotic PCC across a wide range of size classes. PCC estimates of many taxa resolved at the class level (e.g., diatoms) show strong positive correlations across methods, while other groups (e.g., dinoflagellates) are not well captured by one or more methods. Since variations in phytoplankton pigment concentrations are related to changes in optical properties, this combined dataset expands the potential scope of ocean color remote sensing by associating PCC at the genus- and species-level with group- or class-level PCC from pigments. Quantifying the strengths and limitations of pigment-based PCC methods compared to PCC assessments from amplicon sequencing, imaging, and cytometry methods is the first step toward the robust validation of remote sensing approaches to quantify PCC from space.

ecology↗

bioRxiv2019

Preference and switching in the Kill-the-Winner functional response: Diversity, size structure, and synergistic grazing in plankton models

Grazing by zooplankton can maintain diversity in phytoplankton communities by allowing coexistence between competitors in situations that would otherwise lead to competitive exclusion. In mathematical models, grazing is represented by a functional response that describes the consumption rate by an individual zooplankter as a function of phytoplankton concentration. Since its initial description, the Kill-the-Winner functional response has been increasingly adopted for large-scale biogeochemical modeling. Here, we analyze how two properties of the Kill-the-Winner functional response--preference and switching--interact to promote coexistence and increase diversity in two simple models: a diamond-shaped nutrient-phytoplankton-zooplankton model and a size-structured phytoplankton community model. We found that, compared to preference, switching leads to coexistence and increased diversity over a much wider range of environmental conditions (nutrient supply and mixing rate). In the absence of switching, preference only allows for coexistence within the narrow range of environmental conditions where the preference is precisely balanced against the competitive difference between phytoplankton types. We also explored a counterintuitive aspect of the Kill-the-Winner functional response that we have termed "synergistic grazing". Synergistic grazing occurs when the grazing rate on one phytoplankton type increases as the biomass of an alternative phytoplankton type increases. This unrealistic effect is most evident when switching is strong and when zooplankton have a preference for the weaker competitor.

ecology↗