Multi-Decadal (1972-2019) Mnemiopsis Leidyi (Ctenophora) Abundance Patterns In Narragansett Bay, Rhode Island, Usa

The influence of a warming climate on patterns of abundance and seasonality of the lobate ctenophore Mnemiopsis leidyi is of interest worldwide, especially in regions where the species occurs at or near the limits of its thermal niche or is shifting its spatial distribution poleward. A 47-year (1972-2019) time series of weekly observations of M. leidyi abundance in Narragansett Bay, the northern limit of its distribution in the Northwest Atlantic, was analyzed to identify links between abundance and environmental factors such as temperature. During this entire period, the species exhibited a striking degree of interannual variability with a gradual decline during the most recent years. Temperature influenced ctenophore abundance, with warmer winter and spring temperatures supporting higher abundances and longer bloom durations. Two blooms were typically observed each year, and both the summer and autumn bloom shifted earlier over time. Chlorophyll was an important factor in the abundance and bloom dynamics of reproductive stage ctenophores, where high levels of summer chlorophyll indicated lower autumn ctenophore abundance, suggesting top-down effects by M. leidyi. These results suggest that multiple factors affect M. leidyi abundance and phenology but that ctenophores in both native and introduced ranges may benefit from continued ocean warming.