Long-Term Abundance and Distribution Trends of the CladoceranPenilia avirostrisin the US Northeast Shelf Ecosystem

The average spatial distribution and interannual abundance patterns of the cladoceran Penilia avirostris are described from samples collected in the waters of the US Northeast continental shelf ecosystem. Integrated water column samples were collected with a bongo net from 1977–2009 on broad scale surveys of the shelf conducted approximately every two months. Collections were also made with a Continuous Plankton Recorder towed at 10 m depth along two routes; one that crossed the Gulf of Maine (1961–2009), and the other from New York City south-eastward toward Bermuda (1976–2009). P. avirostris was found to have a strong seasonal cycle in the southern half of the ecosystem. High coastal concentrations suddenly appear there in July–August, forming distinct onshore-offshore abundance gradients. The population slowly declines in the autumn months, essentially disappears from the zooplankton community during winter, and does not return again until its abrupt appearance in summer. Only low abundances in isolated patches were found in the northern half of the ecosystem and in the slope waters sampled by the CPR. Both samplers showed that annual abundance levels were variable with no long-term trend evident, nor were environmental measures or broad scale climate indices correlated to these variations. The cladoceransʼ summer population surge was significantly correlated with surface temperature and water column stratification indices. This is the first study to record the presence of P. avirostris in the Gulf of Maine region.