Tintinnid ciliate diversity in the Mediterranean Sea: longitudinal patterns related to water column structure in late spring-early summer

Globally, north-south gradients in diversity are well known, and in the wor ld ocean they have been related to north-south gradients in water column st ructure. Here diversity in planktonic ciliates, both taxonomic and morpholo gical, is explored in the Mediterranean Sea, which displays west-east gradi ents in the water column characteristics of chlorophyll concentrations and distributions. Tintinnids, which are loricate planktonic ciliates, were enu merated and identified in samples from 23 stations between 2 and 32 degrees E, obtained from the Catalan Sea in June 1993 and from a cross-Mediterrane an transect in May-June 1996. From west to east, concentrations of tintinni ds showed Little variability, while numbers of species and genera, as well as diversity indices (H' values), increased. These parameters were positive ly related to depths of the chlorophyll maximum layer and negatively correl ated with chlorophyll concentrations. Corresponding to the west to east inc reases in diversity were increases in community averages of lorica oral dia meter and overall length, again positively correlated to deepening of chlor ophyll maximum depths, and negatively related to average chlorophyll concen trations. Morphological diversity was related to species diversity, not as variability in lorica oral diameters, but as variability in average lorica length. In the Mediterranean Sea, a gradient of taxonomic and morphological diversity is relatable to a specific feature of the water column, the dept h of the chlorophyll maximum layer. Trophic specialization as a dominant me chanism underlying diversity was not supported by data on lorica oral diame ters.