Decoupled shifts of dominant and rarer fish species as a response to warming and extreme events in a large estuary

Disentangling community responses to multiple stressors in a warming context is one of the most challenging tasks ecologists face. Taking advantage of a long-term (1989–2017) and intensive fish monitoring survey (N = ~900K from 804 seines-day), we present a comprehensive analysis on the dynamics of coastal fish communities in Jamaica Bay, New York, by addressing multiple dimensions of community change that although closely related are rarely considered in a single work. Specifically, we tested hypotheses about changes in composition, composition variability and community functional attributes, and the role of environmental drivers acting at different temporal scales. Our analyses suggest two decoupled community dynamics triggered by two different extreme events. First, the 1999 Atlantic Multidecadal Oscillation phase switch caused an abrupt shift from a single-species dominance to intermittent co-dominance, with species preferring higher temperatures, a shift that has persisted in time but that may be cyclical at a multidecadal scale. Second, the 2012 heat wave promoted an abrupt collapse of rare and cold-water species, and the sudden arrival of warmer-water species in a portion of the community that was already increasingly variable due to long-term warming. Functionally, the entire community subtly shifted toward species with faster turnover and lower trophic levels. Our work sheds light on the complex responses of biological communities to warming in terms of the impacts on composition, its temporal variability, and its functional dimension, by disentangling the interplay of long-term environmental trends such as warming, multidecadal cycles, and extreme events on different portions (i.e., dominant and rare thermal species) of the community.