AME 22:69-78 (2000)  -  doi:10.3354/ame022069

ABSTRACT: Globally, north-south gradients in diversity are well known, and in the world ocean they have been related to north-south gradients in water column structure. Here diversity in planktonic ciliates, both taxonomic and morphological, is explored in the Mediterranean Sea, which displays west-east gradients in the water column characteristics of chlorophyll concentrations and distributions. Tintinnids, which are loricate planktonic ciliates, were enumerated and identified in samples from 23 stations between 2 and 32°E, obtained from the Catalan Sea in June 1993 and from a cross-Mediterranean transect in May-June 1996. From west to east, concentrations of tintinnids showed little variability, while numbers of species and genera, as well as diversity indices (H¹values), increased. These parameters were positively related to depths of the chlorophyll maximum layer and negatively correlated with chlorophyll concentrations. Corresponding to the west to east increases in diversity were increases in community averages of lorica oral diameter and overall length, again positively correlated to deepening of chlorophyll maximum depths, and negatively related to average chlorophyll concentrations. 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 depth of the chlorophyll maximum layer. Trophic specialization as a dominant mechanism underlying diversity was not supported by data on lorica oral diameters.

KEY WORDS: Biodiversity · Plankton · Latitudinal gradient