MEPS 167:47-57 (1998)  -  doi:10.3354/meps167047

Buildup of microbial biomass during deep winter mixing in a Mediterranean warm-core eddy

Tamar Zohary1,*,**, Stephen Brenner2, Michael D. Krom3, Dror L. Angel4, Nurit Kress2, William K. W. Li5, Amir Neori4, Yosef Z. Yacobi1

1Israel Oceanographic and Limnological Research Ltd. (IOLR), Kinneret Limnological Laboratory, Box 345, Tiberias 14102, Israel 2IOLR, National Institute of Oceanography, Box 8030, Haifa 31080, Israel 3Department of Earth Sciences, Leeds University, Leeds LS2 9JT, United Kingdom 4IOLR, National Center for Mariculture, Box 1212, Eilat 88112, Israel
5Bedford Institute of Oceanography, Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada
**Order of author names alphabetical, except for first 3 names

The Cyprus Eddy, a warm-core eddy southeast of Cyprus, was sampled towards the end of an exceptionally cold winter in early March 1992, within 4 d of a storm and within 24 h of an intrusion of cold air. Depth profiles of temperature, salinity and dissolved nutrients showed an active deep mixed layer from the surface to ca 500 m at the core of the eddy, while at the eddy boundaries the mixed layer extended only to 150 m. Microbial populations were evenly distributed over the entire upper 500 m at the core station, as indicated by chlorophyll and high performance liquid chromatography (HPLC)-determined pigment composition, by flow-cytometric analysis of the ultraphytoplankton, by direct counts of 4',6-diamidino-2-phenylindole (DAPI)-stained bacteria and 3H-thymidine measurements of bacterial activity. As far as we know, this is the first detailed description of the microbial populations in a warm-core eddy during the bloom season. The integrated water column chlorophyll 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 bloom was occurring. Noticeably, this bloom was not delayed until the establishment of summer stratification as has been observed previously in warm-core eddies. While theoretical considerations based on the calculated critical depth at the core of about 300 m suggested that a bloom should not have occurred, our data jointly with previous data from the Cyprus Eddy support the hypothesis that interim periods of quiescence 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-shade adaptation features and cellular chlorophyll fluorescence per cell, we calculated that the rate of vertical mixing in the core was at least 30 m h-1.

Eastern Mediterranean · Warm-core eddy · Phytoplankton bloom · Vertical mixing rate · Critical depth · Light-shade adaptation

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