Long-term seasonal trends in the prey community of delta smelt within the Sacramento-San Joaquin Delta, California

Species introductions can alter food webs, disrupt life cycles, change life history expressions and the temporal scale of population dynamics in zooplankton communities. We examined physical, trophic, and zooplankton community dynamics were examined in the San Francisco Estuary across a 43-year dataset (1972-2014). Following the introduction of the Asian clam (Potamocorbula amurensis) in 1986, we observed a shift in peak phytoplankton bloom timing, with peak productivity now occurring in May compared to June prior to the invasion. Peak abundance of several zooplankton taxa — Eurytemora affinis, Pseudodiaptomus, other calanoids, and non-copepods — also shifted their appearances to earlier in the year. The most striking shift in phenology occurred for Eurytemora, with mean timing of peak abundance shifting to three months earlier across time. Overall abundance also declined significantly for Eurytemora, while Pseudodiaptomus abundance simultaneously rose. Although the well-documented long-term decline in phytoplankton biomass and primary productivity since the overbite clam invasion is believed to be a major driver of the decline in zooplankton and delta smelt, which preys on it, a simultaneous shift in the timing of prey presence for larval and juvenile delta smelt may have exacerbated the effects of food limitation. Temperature and salinity have been shown to control Eurytemora vital rates, including egg production and development, and swimming rate.  However, these factors did not change directionally over time. It appears more likely that the influence of introduced species, including the Asian clam, on the food web drove changes in abundance of Eurytemora through the year. The study underscores the disruptive effects of introduced species on the estuarine food web and highlights the need to continue measures to protect the estuary against further introductions of non-native species.  

Merz JA, Bergman PS, Simonis JL, Delany D, Pierson J, Anders P. 2016. Long-term seasonal trends in the prey community of delta smelt (Hypomesus transpacificus) within the Sacramento-San Joaquin Delta, California. Estuaries and Coasts 39:1526-1536.