The U.S. Long Term Ecological Research (LTER) network expanded in 2017 to encompass four marine sites with a pelagic focus, complementing existing time-series maintained by NSF, NOAA, and multiple international funding agencies and enhancing our ability to document multidecadal ecosystem responses to climate variability and change across a global gradient. The four LTER sites span a range in terms of physical environment, geography, bathymetry, and terrestrial influence. Palmer LTER, on the western Antarctic Peninsula and extending 200 km offshore, is dominated by seasonal changes in sea ice cover and light availability. The California Current Ecosystem LTER is an eastern boundary upwelling regime and primarily an open ocean site with minor terrestrial inputs. The Northeast U.S. Shelf and Northern Gulf of Alaska LTER sites extend from the continental shelf into the open ocean, with significant terrestrial influences. All of the sites are associated with productive fisheries, and all are experiencing warming trends in surface air temperatures with associated impacts on mixed layer temperature and depth, and/or sea ice cover. This session will showcase ongoing studies of ecological responses at the LTER sites in terms of primary production, community composition and structure, carbon and nutrient cycling, and disturbance. We strongly encourage submissions related to these topics from other (non-LTER) national and international pelagic time-series sampling programs. Recent cross-LTER site synthesis efforts will also be highlighted.
Lead Organizer: Katherine Barbeau, Scripps Institution of Oceanography, UC San Diego (kbarbeau@ucsd.edu)
Co-organizers:
Russ Hopcroft, University of Alaska Fairbanks (rrhopcroft@alaska.edu)
Heidi Sosik, Woods Hole Oceanographic Institution (hsosik@whoi.edu)
Oscar Schofield, Rutgers University (oscar@marine.rutgers.edu)
Presentations
02:30 PM
THE SALPS OF SUMMER: GELATINOUS ZOOPLANKTON REPLACE HETEROTROPHIC PROTISTS AS MAIN PHYTOPLANKTON GRAZERS IN A TEMPERATE, COASTAL FOOD WEB (8966)
Primary Presenter: Susanne Menden-Deuer, University of Rhode Island (smenden@uri.edu)
Identification of trophic transfer pathways in marine pelagic food webs is foundational to quantifying ecosystem processes, including production. We measured the relative grazing rates of 3 types of phytoplankton consumers: herbivorous protists, crustacean and gelatinous zooplankton in summer 2022 in the Northwest Atlantic, part of the Northeast Shelf LTER (NES-LTER). The system was oligotrophic with low phytoplankton biomass dominated by pico- and nanoplankton; conditions typical for microbial loop systems. Heterotrophic protists, krill (Euphausia krohnii), and a mixed assemblage of copepods did occasionally exert substantive grazing pressure while on average, salps (Thalia democratica) removed about an order of magnitude more phytoplankton biomass than crustacean zooplankton (normalized to biomass). Consumption of three flowcytometrically identified pico- and nanophytoplankton groups yielded similar differences between grazer types, with salp grazing exceeding those of other zooplankton. It is unclear whether recent rapid warming plays a role in the observed food web structure. Our results identify trophic pathways, dominant grazer types, and quantify their relative grazing impacts. Consumption of micron sized phytoplankton by salps is a clear demonstration that canonical predator-prey size ratios are insufficient. Together these data are foundational to the utility of model simulations and conceptional frameworks that require realistic scenarios to assess organic matter production and its fate in a rapidly changing and heavily utilized coastal ecosystems.
02:45 PM
Vertical nitrate flux fuels new production over summertime Northeast U.S. Shelf (8934)
Primary Presenter: Bofu Zheng, Woods Hole Oceanographic Institution (bzheng@whoi.edu)
In aquatic ecosystems, allochthonous nutrient transport to the euphotic zone is an important process that fuels new production. Here, we use high-resolution physical and biogeochemical observations from five summers to estimate the mean vertical nitrate flux, and thus new production over the Northeast U.S. continental Shelf (NES). We find that the summertime nitrate field is primarily controlled by biological uptake and physical advection-diffusion processes, above and below the 1% light level depth, respectively. We estimate the vertical nitrate flux to be 8.2 ± 5.3 x 10-6 mmol N m-2 s-1 for the mid-shelf and 12.6 ± 8.6 x 10-6 mmol N m-2 s-1 for the outer shelf. Furthermore, we show that the new production to total primary production ratio (i.e., the f-ratio), consistently ranges between 10% and 15% under summer conditions on the NES. Two independent approaches - nitrate flux-based new production and O2/Ar-based net community production - corroborate the robustness of the f-ratio estimation. Since ~85% of the total primary production is fueled by recycled nutrients over sufficiently broad spatial and temporal scales, less than 15% of the organic matter produced in summer is available for export from the NES euphotic zone. Our direct quantification of new production not only provides more precise details about key processes for NES food webs and ecosystem function, but also demonstrates the potential of this approach to be applied to other similar datasets to understand nutrient and carbon cycling in the global ocean.
03:00 PM
SEASONAL VARIABILITY OF THE MICROZOOPLANKTON COMMUNITY STRUCTURE ON THE NORTHEAST US SHELF (8784)
Primary Presenter: Frankie Lopez, University of Rhode Island (frankie.lopez@uri.edu)
Phytoplankton grazing by microzooplankton is a critical step in making material and energy available to higher trophic levels, and therefore shapes marine food webs as well as biogeochemical cycles. Understanding the drivers of microzooplankton grazing requires understanding the key players through quantification and description of the microzooplankton biomass and community composition. This study leverages samples collected in boreal summer and winter from 2018-2023, at three stations spanning the Northeast US Shelf from coastal Rhode Island to the shelf break, resulting in a dataset of microzooplankton biomass that encompasses the seasonal, temporal, and spatial variability of this continental shelf ecosystem. Using semi-automated image analysis, we estimated biomass for two key microzooplankton groups, dinoflagellates and ciliates, collected in the context of the Northeast US Shelf Long Term Ecological Research Program. Results reveal a strong seasonal pattern in dinoflagellates, with large dinoflagellates contributing 61% of total microzooplankton biomass in winter, and 34% in summer, while small dinoflagellates contributed only 10% of total microzooplankton biomass in winter, and 42% in summer. Ciliate biomass did not demonstrate a strong seasonal signal. These results can inform our understanding of planktonic food webs by further constraining the drivers of phytoplankton community structure and energy transfer to higher trophic levels.
03:15 PM
Investigating the effects of marine heatwaves on primary productivity in the California Current Ecosystem (9165)
Primary Presenter: MARGARET BAKER, Rice University (mb203@rice.edu)
Marine heatwaves (MHWs) have become increasingly frequent and prolonged over the past decades. These events pose significant risks to marine ecosystems including lower trophic levels such as primary producers. Despite their importance, limited research has been conducted on how productivity and underlying physiological mechanisms respond to MHWs on short timescales. This study examined the effects of a simulated MHW in the California Current Ecosystem (CCE) during the P2402 CCE-LTER cruise, aiming to assess the resistance and resilience of the natural photoautotrophic communities. Using temperature-controlled deck-board incubators, both oligotrophic offshore and eutrophic onshore communities were subjected to MHW conditions for five consecutive days, followed by a one-day “recovery” period at ambient temperatures. Photophysiology and gross primary productivity (GPP) was assessed via Fast Repetition Rate Fluorometry (FRRf), net primary productivity (NPP) was measured through 14C incubations and community composition was assessed using flow cytometry. The results indicated that phytoplankton communities exhibited relatively stable base photophysiology but displayed instability in community composition, growth (measured by chlorophyll a), and carbon fixation rates in both onshore and offshore communities. These findings highlight the complex and variable responses of primary producers to MHWs, underscoring the need for further research to better understand their role in ecosystem resilience.
03:30 PM
Chemical characterization of metabolites across gradients of productivity in the California Current Ecosystem (9575)
Primary Presenter: Ralph Riley Torres, Scripps Institution of Oceanography (rrtorres@ucsd.edu)
Around half of ocean primary production is processed through the labile dissolved organic matter pool. The molecular composition of these metabolites may reveal the role of organic matter in driving microbial ecology and metabolic processes controlling global carbon and nutrient cycling. Mass spectrometry-based approaches characterizing marine organic matter are usually limited to elemental characteristics via molecular formula assignment. To understand metabolic processes associated with organic matter, however, a deeper exploration into the functional properties of metabolites is required. The California Current Ecosystem (CCE) region exhibits strong gradients in microbial community structure, nutrients, and primary production, providing an ideal location to observe the ecological interactions and structural changes in the organic matter pool. Here, we compare molecular signatures from three Lagrangian-based cruises in the CCE. Solid phase extraction via liquid chromatography mass spectrometry conjoined with in-silico tools (SIRIUS) and the chemical classification tool NPClassifier reveal biosynthetic origins and structural properties responsible for observed changes in metabolites. Hierarchical clustering followed by biogeochemical correlations (changes in nutrient, particulate organic matter, productivity, etc.) reveal specific molecules and molecular classes that fluctuate within this upwelling system. Further comparison of species composition via 16S and 18S sequencing reveals organisms that are disproportionately responsible for trends in our molecular analysis.
03:45 PM
QUANTITATIVE IMPROVEMENTS IN MARINE ZOOPLANKTON METABARCODING USING PCR BIAS CORRECTION (9072)
Primary Presenter: Dante Capone, Scripps Institution of Oceanography (dcapone@ucsd.edu)
Recent advances in DNA metabarcoding have revolutionized how we measure and monitor zooplankton. However, methodological biases such as those introduced during PCR amplification remain a critical challenge in our quantitative interpretation of molecular-based measurements. In this study, we applied a novel PCR-bias correction method to environmental samples of marine zooplankton. We employed variable PCR cycle experiments to calibrate and attempt to mitigate amplification differences across the mixed community. This approach, only trialed in microbiome studies, was adapted to marine zooplankton by pooling samples from a range of environmental conditions and estimating taxon-specific amplification efficiencies. Our study focuses on size-fractionated mesozooplankton from the California Current Ecosystem, where we examined the variability in PCR amplification for key zooplankton taxa, including copepods and euphausiids. By integrating metabarcoding with Zooscan imaging of the same samples, we assessed the improvements in quantifying zooplankton community composition and abundance following bias correction. The results demonstrated enhanced agreement between molecular and morphological estimates for abundant taxa such as calanoid copepods. This work represents the first application of PCR bias correction to marine zooplankton and highlights its potential for improving the quantitative reliability of metabarcoding data. This methodology can be a critical tool for ecologists aiming to integrate DNA-based approaches with traditional morphological methods.
SS27D - Long-term perspectives in marine pelagic ecosystem research
Description
Time: 2:30 PM
Date: 31/3/2025
Room: W207AB