Coastal and shelf ecosystems are the most productive regions of the world’s oceans. Broad, shallow shelf ecosystems like those in the west Atlantic basin also interact with ocean boundary currents, creating conditions that play important roles in biogeochemical cycles. Here we seek to “take the pulse” of shelf systems in the context of shifting boundary currents with predicted environmental change, which requires a baseline understanding of current status and concerted efforts across multiple disciplines. This session welcomes research on coast-to-boundary current processes encompassing physical, chemical, biological, and biogeochemical studies of cross-shelf systems, particularly in areas that intersect western boundary currents (e.g., Gulf Stream, Kuroshio, East Australia, Brazil). We seek to showcase research from process studies, time-series, computational modeling, or other ecosystem assessments that address questions aimed at understanding the function within any region across this coast to boundary current continuum at all levels of ecological organization, from genes to ecosystems. Interdisciplinary studies that integrate multiple ecosystem processes are particularly encouraged. Our goal for this session is to bring together the community of scientists across the globe focused on these comprehensive regional studies to combine efforts and enhance collaborative and comparative opportunities across our datasets.
Lead Organizer: Christian Briseño-Avena, University of North Carolina Wilmington (brisenoavenac@uncw.edu)
Co-organizers:
Matthew McLean, University of North Carolina Wilmington (mcleanm@uncw.edu)
Winifred Johnson, University of North Carolina Wilmington (johnsonwm@uncw.edu)
Bradley Tolar, University of North Carolina Wilmington (tolarb@uncw.edu)
Presentations
04:30 PM
Dynamics of upwelling on the West Florida Shelf slope (9705)
Primary Presenter: Siddhesh Tirodkar, Florida Atlantic University (stirodkar@fau.edu)
The oligotrophic West Florida shelf (WFS) frequently experiences strong red tides of a toxic dinoflagellate Karenia brevis, which impact the coastal and human health. Nutrients play an important role in shaping these blooms, which come from either land-based river runoff or subsurface waters over the outer shelf. In turn, slope upwelling over the west Florida slope is likely the main process bringing deep ocean nutrients to shelf edge. This process is yet to be well studied. Based on the Regional Ocean Modelling Systems (ROMS), a numerical model has been developed for the eastern Gulf of Mexico including the WFS with a horizontal resolution of 2 km. A two-year (2011-2012) simulation has been conducted and calibrated with available data including temperature, salinity, currents, and volume transport. An analysis of the model results indicates that the upwelling/downwelling along the WFS slope is strongly affected by the Loop Current (LC) transport and position. During the port-to-port mode (LC flowing into the Florida Straits directly), interactions of LC with the shelf slope typically drive a significant northward jet along the slope, which leads to significant downwelling due to the bottom Ekman transport. In contrast, during the extended mode, LC penetrates deep into the Gulf, and it is flowing southward along the southern part of the shelf slope, leading to strong slope upwelling. Other than seasonal cycle, this mechanism appears to be the dominant factor shaping the bottom temperature and nutrients along the west Florida shelf break.
04:45 PM
Upwelling Intrusions drive Colonial Radiolarian Aggregations and Associated Zooplankton Dynamics in a Shelf Ecosystem (9209)
Primary Presenter: Mary Mann, University of Georgia - Skidaway Institute of Oceanography (mary.mann@uga.edu)
Colonial radiolarians are ubiquitous marine protists capable of forming dense aggregations in oceanic and coastal waters worldwide. Their taxonomic diversity and mixotrophic abilities enable them to thrive across diverse ecological niches. While recent studies have focused on radiolarians in open oceans, their role in productive shelf environments remain largely unknown. As a result of their fragile structure and dense biomass in large aggregations, conventional sampling techniques have underestimated their abundance. To address these sampling limitations, three summer cruises (2021-2023) in the South Atlantic Bight used high vertical depth resolution (~1m) shadowgraph and color imaging systems and oceanographic sensors to link fine-scale conditions to plankton aggregations. Dense aggregations of colonial radiolarians were quantified along the outer edge of the mid-shelf (45m isobath), correlating with high chlorophyll-a (12-50 µg m-3 ) and cold bottom water upwelling intrusions. These aggregations also decreased downwelling light and influenced zooplankton community composition. Hydromedusae and siphonophores were often observed near these dense layers, potentially related to enhanced feeding opportunities. These findings suggest that radiolarian aggregations may represent a significant, previously unrecognized source of carbon flux, not only from their own population growth but interactions with other zooplankton groups. Understanding the drivers and associated organisms is crucial for revealing ecological functions and consequences of radiolarian blooms in shelf systems.
05:00 PM
Episodic winter deep chlorophyll events in the Southern Mid-Atlantic Bight revealed by autonomous glider deployments (8946)
Primary Presenter: Francesco Lane, University of North Carolina - Chapel Hill (flane@unc.edu)
The southern Mid-Atlantic Bight (sMAB) is likely a hotspot for the episodic export of carbon-rich shelf waters to the open ocean. Over a 2 year period, from March 2017 to May 2019, Spray gliders repeatedly occupied transects, along the slope and across the shelf, generating high-resolution chlorophyll fluorescence data in the sMAB. The resultant data resolved 2 full annual cycles of chlorophyll in a region previously undersampled, particularly in winter. The seasonal surface chlorophyll maximum was associated with the onset of stratification in early spring followed by a subsurface maximum persisting over summer. Counterintuitively, our data also revealed that depth integrated chlorophyll was highest prior to the onset of stratification, with relatively high chlorophyll concentrations observed below 100m, suggesting downward transport rather than local production. We defined “deep chlorophyll events” (DCEs) as profiles where maximum chlorophyll and/or depth integrated chlorophyll in the 100-300m range were in the 99th percentile of the dataset. We found that these episodic DCEs were key in driving the annual peak of depth integrated chlorophyll in late winter/early spring. We find that DCEs are generally driven either by sub-mesoscale exchange processes at the Gulf Stream boundary, or larger scale winter storm mixing events. Such events have likely been missed by previous ship-based surveys. Our results strongly suggest that carbon-rich shelf waters are exported from the sMAB shelf to depth below the Gulf Stream during episodic events, potentially contributing to cross-shelf exchange of particulate organic carbon.
05:15 PM
EXPLORING ENVIRONMENTAL DRIVERS OF PHYTOPLANKTON VERTICLE DISTRIBUTION PATTERNS COMPARING IN-SITU CHLA FLUORESCENCE AND HOLOGRAPHIC IMAGING AT FINE-SCALES (9295)
Primary Presenter: Jess Shearer, University of North Carolina Wilmington (js7102@uncw.edu)
Phytoplankton play a vital role in global carbon and nutrient cycling. However, accurately quantifying phytoplankton biomass, productivity, and community composition remains a challenge due to their small size and patchy distribution. A common technique for monitoring phytoplankton biomass is using chlorophyll a (Chla) fluorescence as a proxy. But this method has important limitations - Chla can be influenced by various physiological and environmental parameters, such as dissolved oxygen (DO) and temperature, which can significantly impact primary productivity, and does not provide information on phytoplankton size, shape, or taxonomy. These factors can lead to uncertainties in the relationship between fluorescence and phytoplankton biomass. Advances in in-situ holographic imaging have made it possible to identify and quantify phytoplankton assemblages at cm to meter scales. The integration of multiple monitoring techniques –Chla, DO, temperature, and in-situ holographic imaging–provides a more complete picture of phytoplankton distribution and productivity, as well as the complex interactions between these organisms and their physical-chemical environment. This is especially true for often overlooked benthic primary producers. Preliminary findings indicate notable fluorescence peaks at bottom depths, with holographic particle statistics suggesting this may be related to an abundance of benthic producers, primarily diatoms. Understanding these relationships aids in predicting how phytoplankton respond to climate change and other stressors.
05:30 PM
Pulses of Productivity, Patches, and Pelagic Tunicates: Fine-scale aggregations influence trophic structure (9276)
Primary Presenter: Patrick Duffy, University of Georgia (patrick.duffy@skio.uga.edu)
Pulses of productivity resulting from upwelling of deep, nutrient rich water occur periodically in boundary current systems. Pelagic tunicates (PT) are a filter-feeding group of gelatinous zooplankton that often form aggregations or blooms in response to boundary current upwelling, although the mechanisms driving upwelling differ among ecosystems. Aggregations of PT can have pronounced impacts on local trophic webs through consumption of small prey and production of detritus. While it has been established that oceanic-PT can serve as a mechanism of carbon export in upwelling zones through sinking of large detritus and carcasses, molecular and stable isotope techniques have suggested the roles of PT in coastal and shelf ecosystems involve greater feedback with the microbial loop. To better understand these dynamics, we conducted cruises in 3 sequential years in the South Atlantic Bight to capture cross-shelf gradients under different upwelling conditions. Using an in situ imaging vehicle, we observed PT populations which revealed aggregations over small (1-5 m) vertical spatial scales in response to upwelling. In layers with PT, we also found lowered bulk dissolved Oxygen concentrations despite elevated bulk Chl a. Finally, microbial respiration incubations from stations with PT aggregations revealed enhanced heterotrophic activity in the microbial loop. Our initial findings indicate that PT may be influential to the fate of primary productivity pulses on shelves, likely through aggregating labile carbon in the detrital pool.
05:45 PM
Ecosystem implications of Gulf Stream modes in the South Atlantic Bight (9418)
Primary Presenter: Sutara Suanda, University of North Carolina Wilmington (suandas@uncw.edu)
Poleward oceanic transport by the Gulf Stream is a prominent feature of outer shelf advection in the South Atlantic Bight. In the Carolina Capes region north of the Charleston Bump, the Gulf Stream alternates between two well-documented modes: a deflected seaward state with upper-slope recirculation (i.e., equatorward flows) and a shelf-slope attached state that produces smaller-scale frontal eddy activity. Although both states can impact shelf ecosystems, the distinct mechanisms of water mass mixing and export and the frequency of occurrence are not well-understood. As a precursor to an upcoming field program, we present a framework to decipher how alternating states can affect regional coastal ecosystems. This study also uses long-term satellite sea-surface temperature and Chlorophyll-a observations to empirically distinguish an ecosystem response and offer hypotheses related to circulation features within the two modes of variability. The frequency of occurrence, seasonal trends and frontal activity for the Gulf Stream modes are quantified. Their variation is then connected to measured phytoplankton blooms, the inferred flux of nutrients and seaward biomass export from the Carolina Capes continental shelf.
SS36 - Coast-to-boundary current systems and the ecological, biogeochemical, and physical processes within
Description
Time: 4:30 PM
Date: 31/3/2025
Room: W206A