Linking pelagic community structure and production regimes: Exploring interannual variability at the Northeast U.S. Shelf Long-Term Ecological Research (NES-LTER) site

The northwest Atlantic, renowned for its fisheries, is experiencing faster-than-average warming and other climate-related impacts. Lack of systematic and detailed measurements over a sufficient length of time has hampered our ability to observe the responses of pelagic food webs to environmental perturbations and uncover the underlying causes and implications. To address this challenge, an NSF-supported Long-Term Ecological Research site was established on the Northeast U.S. Shelf (NES-LTER) in 2017. NES-LTER goals are to understand and predict how planktonic food webs change through space and time, and how those changes impact the productivity of higher trophic levels. Observations to date show seasonal patterns consistent with phytoplankton dominated by larger cells (especially diatoms) under low temperatures in winter, when we also find relatively high magnitude and variability in net community production (NCP) and high fraction of primary production consumed by protozoan grazers. Smaller phytoplankton cells are more important in warmer waters of summer, when NCP magnitude and grazing impact are both lower. Important interannual variability is emerging, providing opportunities to investigate drivers that may be important for responses to on-going climate change. For example, during summer 2019 we sampled an unexpected bloom of large diatoms of the genus Hemiaulus , which we found associated with nitrogen-fixing symbionts. This bloom occurred in highly stratified conditions during an ocean heat wave over the NES. In earlier years, herbivorous grazing pressure was absent or low during the summer, but under these circumstances grazing was coupled to phytoplankton growth, resembling `winter' conditions in terms of trophic transfer. Notably, NCP showed spatial variability tightly coupled to Hemiaulus biomass, but overall the amount of NCP was similar to the previous summer, suggesting that diversity in phytoplankton and their niches may act to buffer against drastic changes in productivity of the NES. Nonetheless, compositional impacts at higher trophic levels may occur in response to combined changes in food web structure and habitat.