BUILDUP OF MICROBIAL BIOMASS DURING DEEP WINTER MIXING IN A MEDITERRANEAN WARM-CORE EDDY

The Cyprus Eddy, a warm-core eddy southeast of Cyprus, was sampled tow ards the end of an exceptionally cold winter in early March 1992, with in 4 d of a storm and within 24 h of an intrusion of cold air. Depth p rofiles of temperature, salinity and dissolved nutrients showed an act ive deep mixed layer from the surface to ca 500 m at the core of the e ddy, while at the eddy boundaries the mixed layer extended only to 150 m. Microbial populations were evenly distributed over the entire uppe r 500 m at the core station, as indicated by chlorophyll and high perf ormance liquid chromatography (HPLC)-determined pigment composition, b y flow-cytometric analysis of the ultraphytoplankton, by direct counts of 4',6-diamidino 2-phenylindole (DAPI)-stained bacteria and H-3-thym idine measurements of bacterial activity. As far as we know, this is t he first detailed description of the microbial populations in a warm-c ore eddy during the bloom season. The integrated water column chloroph yll content, 59 mg m(-2) at the core and 45.5 mg m(2) at the boundary, was more than double the typical late autumn values, suggesting a blo om was occurring. Noticeably, this bloom was not delayed until the est ablishment of summer stratification as has been observed previously in warm-core eddies. While theoretical considerations based on the calcu lated critical depth at the core of about 300 m suggested that a bloom should not have occurred, our data jointly with previous data from th e Cyprus Eddy support the hypothesis that interim periods of quiescenc e between mixing events enable bloom development even when the mixing depth is greater than the critical depth. Added nutrients and dilution of grazers, both resulting from the deep mixing, probably contributed jointly to the enhanced productivity. Based on phytoplankton light-sh ade adaptation features and cellular chlorophyll fluorescence per cell , we calculated that the rate of Vertical mixing in the core was at le ast 30 m h(-1).