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

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09:00 AM

Multi-trophic level responses to marine heatwave disturbances in the California Current Ecosystem (8880)

Primary Presenter: Tz-Chian Chen, Florida State University (tc22@fsu.edu)

Marine heatwaves (MHWs) caused by multiple phenomena with days to months duration are increasingly common disturbances in ocean ecosystems. We investigate the impacts of MHWs on pelagic communities using spatially resolved multi-decade time-series of multiple trophic levels (microbes to fish) from the Southern California Current Ecosystem. We show that most planktonic groups decrease in abundance during marine heatwaves, although results were statistically significant for more phytoplankton groups than zooplankton groups and some taxa (e.g., Prochlorococcus) were more abundant during heatwaves. We also test how marine heatwave characteristics (intensity, duration, and subsurface expression) shape the biotic response. Preliminary results suggest that indices of phytoplankton biomass are generally more sensitive to marine heatwave intensity than duration. In contrast, responses of most zooplankton taxa were not significantly correlated to either intensity or duration, although the total biomass (estimated by zooplankton displacement volume) was somewhat more strongly correlated with MHW duration than intensity. Mobile consumers (forage fish) showed more complex responses, with fish egg abundance declining during MHWs but not correlating with any MHW characteristics. Ongoing work will further explore how plankton communities respond to vertically integrated temperature anomalies, particularly whether these patterns can explain variability in metazoan response to these events.

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09:15 AM

Temperature Sensitivity of Size-fractionated Phytoplankton Growth and Net Primary Production Rates on the Northeast US Shelf (9721)

Primary Presenter: Zhengchen Zang, LUMCON (zzang@lsu.edu)

Understanding the spatiotemporal variations of phytoplankton growth and identifying its primary drivers is critical for assessing ocean biogeochemical cycles, productivity, and food web dynamics. In this study, we developed a data-driven, size-fractionated model to estimate phytoplankton growth rates on the Northeast US Shelf (NES). By utilizing comprehensive data collected during the NES Long-Term Ecological Research cruises, we applied both sequential model-based Bayesian optimization algorithm and Random Forest machine learning method to identify the primary stressor modulating phytoplankton growth. Our findings suggested temperature as the predominant limiting factor across the entire shelf, resulting in high/low growth rates in warm/cold season, respectively. Additionally, we conducted an attribution assessment using Taylor expansion to quantify the relative contribution of environmental drivers to growth rate variability. Our analysis revealed that the projected warming on the NES could impact the growth rates of small- and large-sized phytoplankton differently. Within the small-sized phytoplankton, biomass concentrations were relatively insensitive to environmental changes while growth rates were enhanced by higher water temperature, indicating these cells are likely to experience increased growth and net primary production rates as temperature rises. In contrast, the impacts of stratification-induced nutrient depletion on the growth and net primary production rates of large-sized phytoplankton were more dominant. This size-dependent phytoplankton seasonal dynamics provides valuable insights into the response of shelf ecosystem primary production in the context of global warming.

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09:30 AM

Persistent iron-limited water masses in the California Current Ecosystem (9424)

Primary Presenter: Kiefer Forsch, University of Southern California (forsch@usc.edu)

The California Current Ecosystem (CCE) is characterized by vertical and lateral gradients in inorganic nutrients and primary production, whereby vigorous stirring of the surface ocean transports phytoplankton-rich coastal waters offshore. During the spring and summer, diatom blooms are strongly limited by the availability of the micronutrient iron (Fe), leading to decoupling of silicate (Si) and nitrate (N) concentrations in upwelled waters. We leveraged at 25-year timeseries of coastal upwelling and offshore transport to understand how periodicity and anomalous extremes in regional climate impact the degree of Fe-limitation experienced in the CCE LTER domain through measurements of the geochemical proxy, Si_ex. Upwelling followed by retention is believed to be the primary physical mechanism favoring diatom blooms, and here we show a recent dramatic increase in the retention of Fe-limited water masses in the nearshore since 2014. These results could indicate that carbon export associated with Fe-limited diatom blooms is enhanced closer to shore and on continental shelves under recent upwelling conditions. Finally, the timeseries shows persistence of Fe-limitation in the CCE.

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09:45 AM

SEASONALITY OF PLANKTONIC PROKARYOTES FROM SURFACE WATERS TO THE SEAFLOOR IN THE NORTH PACIFIC SUBTROPICAL GYRE (8960)

Primary Presenter: Fuyan Li, C-MORE, University of Hawaii at Manoa (fuyanli@hawaii.edu)

Documenting the seasonal cycles as well as stochastic environmental responses of open ocean marine prokaryotes can assist in assessing how environmental change may impact microbial communities and ocean ecosystem processes. We report here on the seasonality of open ocean suspended prokaryotes in high resolution depth profiles (5 m to 4000 m) over 8.5 years in the North Pacific Subtropical Gyre at Station ALOHA. In the upper water column (>150 m), recurrent annual periodicity was observed across the entire microbial community, with 69.4% of individual bacterial and archaeal taxa exhibiting robust annual cycles.  Prochlorococcus variants dominated the annually periodic prokaryotes in surface waters in winter, while the majority of their annual maxima shifted to spring and summer in the subsurface of the photic zone. Surface water heterotrophs dominated the annually periodic prokaryote taxa that peaked in spring, summer and fall. Subsurface photic zone winter and fall maxima were dominated by heterotrophs and chemolithoautotrophs. This may be due to decreased photon fluxes in fall and winter, accompanied by decreased photoautotroph abundances and less competition for macronutrients in these seasons and depths.  Notably, although the number of taxa exhibiting annual cycles diminished at depths below 150 m, some bacteria and archaea at meso- and bathypelagic depths also displayed robust annual periodicities. Specifically, the percentage of total prokaryote taxa displaying significant annual periodicity at depths of 1000 m, 2000 m, 3000 m and 4000 m, were 11.6%, 7.5%, 11.1% and 2.8% respectively. Some annually periodic taxa recovered from deeper waters actually originated from the surface, presumably reflecting recurrent annual cycles of export. Most annually cycling taxa recovered in deep sea samples, however, were autochtonous in origin. Our study provides new perspective on the seasonality of suspended planktonic prokaryotes throughout the open ocean water column, and how these microbial communities respond to variable ocean ecosystem processes and climate change.

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