Planktonic cnidarians in the neustonic layer: their diversity and abundance off the Catalan coast (NW Mediterranean)

Abstract

The surface layer of the oceans, the neuston, is a highly specialized habitat. Although it is a relatively known system, little is known about the presence and role of the gelatinous zooplankton in the neustonic community. This study aims to analyse the planktonic cnidarian populations of the neuston layer along the Catalan coast (NW Mediterranean) in July of two consecutive years of contrasting climatic conditions: the exceptionally warm summer of 2003 and the summer of 2004 with temperatures within the climatic average. The near surface layer (between 0 and 1.5 m depth) was sampled by means of a neustonic net (800 μm, 0.5 m x 1 m mouth). A total of 133 samples were analysed collected during day and night hours. A rich cnidarian community was detected (28 species), dominated by Siphonophorae (15 species). Some of them were species that inhabit along the water column but concentrate in superficial layers at night (e.g.: Chelophyes appendiculata), while others were typical inhabitants of this layer (e.g.: Sulculeolaria species and the free-floating hydrozoan colonies Porpita porpita and Velella velella). Differences between both years were principally found in the abundance but not in the specific composition. Three times lower abundance was observed in the warm year (2003) with respect to the standard one (2004). Siphonophores were the most representative group in both years (83 and 53 % of total abundance). The spatio-temporal distributions were related to different environmental factors and the potential trophic impact of carnivore gelatinous zooplankton on the neustonic community is discussed