18 Sep 2023
 | 18 Sep 2023
Status: this preprint is currently under review for the journal BG.

Unusual Hemiaulus Bloom Influences Ocean Productivity in Northeast U.S. Shelf Waters

S. Alejandra Casillo Cieza, Rachel H. R. Stanley, Pierre Marrec, Diana N. Fontaine, E. Taylor Crockford, Dennis J. McGillicuddy, Arshia Mehta, Susanne Menden-Deuer, Emily E. Peacock, Tatiana A. Rynearson, Zoe O. Sandwith, Weifeng Zhang, and Heidi M. Sosik

Abstract. Ocean production and trophic transfer rates, including Net Community Production (NCP), Net Primary Production (NPP), Gross Oxygen Production (GOP), and microzooplankton grazing rates are key metrics for understanding marine ecosystem dynamics and impacts on biogeochemical cycles. Because of its temperate location and high dynamic range of environmental conditions and long-term human utilization, the long-term ecological research site in the coastal Northeastern U.S. Shelf (NES) of the Northwestern Atlantic Ocean offers an ideal opportunity to understand how productivity shifts in response to changes in planktonic community composition. While small phytoplankton usually dominate in the NES waters during the summer, a bloom of the large diatom genus Hemiaulus, with N2 fixing symbionts, was observed in the mid-shelf region during the summer of 2019. NCP was 2.5 to 9 times higher when Hemiaulus dominated compared to NCP throughout the same geographic area during the summers of 2020–2022. The Hemiaulus bloom in summer 2019 also coincided with higher trophic transfer efficiency from phytoplankton to microzooplankton, higher GOP and NPP, and higher sea surface temperatures than summers 2020–2022. This study shows that the presence of an atypical phytoplankton community that alters the typical size distribution of the primary producers can greatly influence productivity and trophic transfer, highlighting the dynamic nature of the coastal ocean. Notably, summer 2018 NCP levels were also high although no atypical phytoplankton community was present. A better understanding of the dynamics of the NES in terms of biological productivity is of primary importance, especially in the context of changing environmental conditions due to climate processes.

S. Alejandra Casillo Cieza et al.

Status: open (until 30 Oct 2023)

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S. Alejandra Casillo Cieza et al.

S. Alejandra Casillo Cieza et al.


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Short summary
The coastal ocean in northeastern USA provides many services, including fisheries and habitat for threatened species. In summer 2019, a bloom occurred of an unusual large phytoplankton, the diatom Hemiaulus with nitrogen fixing symbionts. This led to vast changes in productivity and grazing rates in the ecosystem. This work shows that the emergence of one species can have profound effects on ecosystem function. Such changes maybe become more prevalent as the ocean warms due to climate change.