Response of the mesozooplankton community in the western Gulf of Maine to changing oceanographic conditions: the 2010 regime shift

The Gulf of Maine has experienced pronounced changes in recent decades, including rapid warming and changes in circulation. Notably, a shift in water masses entering the Gulf occurred around 2010. Concurrent declines in critically endangered North Atlantic right whales, lobster recruitment and abundance of the foundational, subarctic copepod, <i>Calanus finmarchicus</i>, have designated the 2010 event as a possible regime shift. We present results from two time series stations documenting change in the mesozooplankton biomass and community composition before and after 2010. We examine both seasonal and interannual variability to elucidate potential changes in phenological drivers of the mesozooplankton population in Wilkinson Basin. Abundances of smaller copepod species increased across all seasons between the two time periods, and significantly lower abundance of late-stage <i>C. finmarchicus</i> was observed in late summer through winter, resulting in a decrease in mesozooplankton biomass but increases in biodiversity indices post-2010. The results highlight the contribution of ecologically important increases in chlorophyll-<i>a</i> concentration and warmer temperatures as drivers of mesozooplankton growth and reproduction. An important ecological influence on food availability to smaller copepods may be reduced grazing competition by late-stage <i>C. finmarchicus</i>, a consequence of its declined abundance due to increased predation loss and reduced advective supply.