Zooplankton monitoring at fixed station time series records responses of the foundation species, <i>Calanus finmarchicus</i>, to seasonal and multiannual drivers in the western Gulf of Maine

Abstract Over the past two decades, a regional collaboration, now part of the US Marine Biodiversity Observation Network (MBON), has established the Wilkinson Basin Time Series (WBTS) and the Coastal Maine Time Series (CMTS) stations to observe change at subannual as well as multiannual scales in plankton of the western Gulf of Maine (GoM), USA. The stations are strategically located to monitor plankton in the Maine Coastal Current, a regional production driver, and in Wilkinson Basin, the primary deep basin in the western GoM. Here, we develop seasonal indices tracking change in mesozooplankton biomass and abundance of the planktonic copepod, Calanus finmarchicus, the energy-rich copepod that supports the regional ecosystem. The time series spans a shift in oceanographic conditions that occurred around 2010. In Wilkinson Basin, the abundance of C. finmarchicus varies by over an order of magnitude during its annual life cycle. At the WBTS station, the fall/winter late-stage abundance of C. finmarchicus has declined up to 80% between 2005 and 2023. The fall/winter abundance decline is likely related to a change in supply from the western Scotian Shelf after 2010 combined with increased seasonal predation mortality. However, in spring the abundance of C. finmarchicus remained steady, although initially increased after 2010. The trend in spring abundance corresponds to slight increases in chlorophyll a standing stock in late winter/early spring, favoring C. finmarchicus egg production. Similar trends in mesozooplankton biomass reflect the predominance of C. finmarchicus in the zooplankton community. We propose that the abundance cycles and trends in C. finmarchicus and biomass be reported regularly as seasonal indices, serving as a sentinel indicator of subarctic western GoM pelagic ecosystem function.