Interannual variability of the planktonic cnidarian community in a high heterogeneous area; the Catalan coast (NW Mediterranean)

Abstract

One of the most important weakness in the analyses of the interannual mesoscale shifts in planktonic communities is that the studies have rarely been done at the same spatial scale, have not covered the same area or have not used the same sampling methodology. To solve this problem, the Institute of Marine Sciences (ICM-CSIC) have been conducting campaigns for 40 years with the same oceanographic sampling strategy and covering the same sampling area. In this study we present the first results on the observed changes in gelatinous zooplankton populations. This zooplankton group has created a recent controversy about their wide spread proliferations all around the world oceans. This controversy is largely due to the lack of comparable mesoscale spatial data. In this context, the abundance and distribution of pelagic cnidarian populations have been analyzed comparing two cruises in July 2003 and 2004, covering the same regular grid of stations over the whole continental shelf of the Catalan coast. In both years, a marked surface temperature front of nearly 2°C, perpendicular to the coastline, was detected in the northern part of the area, around 41° 30'. However, the summer 2003 was warmer that 2004 with SST reaching values of 27.5 ºC in the southern part of the area. Higher mean density of planktonic cnidarians was observed in July 2004 being always Siphonophores more abundant than Hydromedusae and Scyphomedusae. Only Hydromedusae were slightly more abundant in July 2003. Nevertheless, higher species richness was found in July 2003 (Siphonophorae: 16, Hydromedusae: 31 and Scyphomedusae: 3) than in 2004 (Siphonophorae: 16, Hydromedusae: 26 and Scyphomedusae: 2). Shifts in biodiversity between years were observed in Anthomedusae and Leptomedusae. Possible long-term shifts in diversity and abundance could be detected when comparing this information with data obtained in similar cruises carried out in the area 30 years ago