APPENDICULARIAN ASSEMBLAGES IN A SHELF AREA AND THEIR RELATIONSHIP WITH TEMPERATURE

The structure of the community of appendicularians was described by multivariate analyses throughout a seasonal cycle on the central Cantabrian coast- It is shown by correlation and principal components analysis that the appendicularian species may be arranged in a successional sequence in relative abundance that is closely coupled to a temperature gradient. This sequence starts with Fritillaria borealis, which exhibits highest relative abundance during January, being sequentially followed by Oikopleura dioica, Fritillaria pellucida. Oikopleura fusiformis and Oikopleura longicauda. This species numerically dominated the community from September to December. Sea surface temperature and the temperature at the depth of the chlorophyll maximum were both reliable predictors of the species composition of the community. However, only the latter provided an adequate explanation for the persistence of cryophilic communities in stratified oceanic environments and the dominance of thermophilic communities after the autumn mixing period. Under stratified conditions, surface temperatures are high (up to 21-degrees-C), but temperatures at the depth of the chlorophyll maximum are low (< 15-degrees-C). These differences disappear after the autumn mixing, when the water column exhibits a uniform temperature profile (16-degrees-C). Critically, however, although there is a sharp decline in surface temperature, water at the depth of the chlorophyll maximum is warmer than during stratification. A simple conceptual model is proposed to account for these features and predictions are made regarding the vertical distribution of appendicularians during stratification. The relevance of non-anthropomorphic temperature measures. such as the temperature at the depth of the chlorophyll maximum, for other zooplankton groups is also discussed.