Seasonal variability of copepods and cladocerans in Bahía de los Ángeles (Gulf of California) and importance of Acartia clausi as food for whale sharks

Abstract

Bahía de los Ángeles is located on the Baja Californian coast of the Gulf of California, adjacent to Ballenas Channel. The bay receives a dynamic tidal influence from the channel but is protected by numerous islands. It is a highly productive bay that harbors a high diversity of species, some of them protected such as the whale shark (Rhincodon typus). This study describes the seasonal variability of major zooplankton taxa and copepod and cladoceran species between May 2003 and October 2004. Copepods were dominant during winter and spring (83–99% of the zooplankton abundance), experiencing a dramatic decrease in autumn 2003 (37–66%) and summer 2004 (25–45%). Cladocerans were the second most abundant group, represented by Pseudevadne tergestina and Penilia avirostris. The former species occurred year-round with low abundance in winter, whereas the latter occurred only in summer and autumn. Other holozooplankton taxa contributed with less than 15% of the abundance in most of the samples, while merozooplankton contributed less than 8%, though in October of 2003 and 2004 there was a remarkable abundance of echinoderms and other larvae contrasting with their shortage in other seasons. In winter 2004, temperate-subtropical copepod species dominated, such as Calanus pacificus and Acartia tonsa. The high abundance of copepods during spring obeyed to Acartia clausi (median = 28,034 ind m^–3); however, the maximal zooplankton abundance (40,468 ind m^–3, 99.5% copepods) came from a sample collected south of Punta Arena, where two whale sharks were observed foraging. The shift in species composition during summer coincides with the predominance of easterly winds described in the literature inducing a reversed circulation and a probable transport of tropical species into the bay during summer and autumn (Centropages furcatus, Subeucalanus subcrassus, Acrocalanus longicornis, Temora discaudata, and Corycaeus amazonicus).