Seasonal and interannual variability of zooplankton community in waters off Mallorca island (Balearic Sea, Western Mediterranean): 1994–1999

Abstract

During 1994–1999, the mesozooplankton community was sampled monthly in the upper sea layer (up to 100 m) along a cross-shelf transect of three stations in waters off Mallorca island. Copepods formed the most abundant group (54%) followed by appendicularians (17%), cladocerans and meroplankton larvae (13%). The abundance of all these groups decreased in the offshore direction but it was not the case for other zooplankton groups. A clear seasonal cycle was evident consisting of a general decrease of the abundance from the beginning to the end of the year. In addition to the general increase during late winter and spring, an absolute maximum was detected in May, particularly important at the neritic station and a relative maximum in March and October. During this annual cycle, the community structure was almost maintained. An interannual trend was also present, characterized by an overall decrease in the total zooplankton abundance from 1994 to 1999. Concurrent measurements of water and air temperature show that this trend was correlated with a warming of coastal waters of atmospherical origin. In particular, a marked minimum in zooplankton abundance was recorded during 1998, which was the warmest year registered. During 1997–1998, the warming was also associated with the presence of fresh, nutrient-poor southern waters of Atlantic origin.