Glider-Observed Seasonal and Spatial Distributions of Zooplankton in the Mid-Atlantic Bight

As secondary producers, zooplankton are essential in the energy flow within marine ecosystems, acting as a trophic link between photosynthetic primary producers and predatory organisms such as migratory fishes and marine mammals, including the critically endangered North Atlantic right whale (Eubalaena glacialis). However, the distribution of zooplankton, and drivers of those distributions, are not well studied in the highly productive Mid-Atlantic Bight coastal shelf ecosystem. This region exhibits strong variability that occurs over multiple time scales, from seasons to years to decades, and is located within the broader U.S. Northeast shelf that is rapidly warming and is susceptible to ongoing ocean acidification. Furthermore, offshore wind construction is scheduled to begin in New Jersey coastal shelf waters within the next few years, and potential impacts of offshore wind construction and operation on the oceanography and local ecology are currently unknown. Therefore, establishing a baseline dataset of oceanographic and ecological parameters is crucial to inform not only future studies focused on determining trends in zooplankton distribution but also the offshore wind planning process toward responsible development. Autonomous underwater vehicles (AUVs) called gliders can reliably collect high-resolution data over a wider depth range and often at a lower cost compared to vessel-based sampling. Active acoustic approaches using multi-frequency echosounders make it possible for AUVs to observe marine pelagic species' distributions more directly, and when paired with other oceanographic and ecological sensors, provide insight into how seasonal changes in ocean conditions overlap with the distribution of fish, marine mammals, and their prey. In this study, gliders were used to collect a suite of oceanographic and ecological variables covering three distinct seasons (Spring 2023, Fall 2023, Winter 2024). Variables measured and included in this analysis were temperature, salinity, depth, chlorophyll-a, pH, colored dissolved organic matter, and zooplankton abundance and biomass. From integrated glider-based acoustic and discrete tow data, small copepods were the most abundant taxa, while large copepods dominated total zooplankton biomass for all seasons. Zooplankton abundance and biomass were lowest in the spring across all depth bins. In fall, the highest depth-integrated biomass and abundance values were in the mid- and outer-shelf waters, while in the spring and winter seasons, highest values were nearshore. Average ocean temperatures were observed to be highest in the fall and lowest in the winter, while salinity was the highest in the outer-shelf waters during the spring. No statistically significant correlations were found between zooplankton abundance and biomass values and measured oceanographic variables. Future research should be directed to determine other potential physical or biological drivers of zooplankton distributions that were outside the scope of this study. Data produced here will assist in developing predictive models that could inform “hot spots” of prey distributions and respective predator feeding locations and provide a baseline from which to analyze potential impacts of offshore wind on zooplankton distributions.