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

Abstract

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 w~thin 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. Microb~al populations were evenly distributed over the entire upper 500 m at the core station, as iiidicated by chlorophy!! and high performance liquid chromatography (HPLC)-determined pigment composition, by flow-cytometric analysis of the ultraphytoplankton, by direct counts of 4',6-diam~dino-2-phenylindole (DAP1)-stained bacter~a and %-thymidine measurements of bacterial activity. As far as we know, thls is the first deta~led 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 mixlng 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 11ght-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-'

Plankton in the open Mediterranean Sea: a review

“…At the Ligurian Sea DYFAMED station, Synechococcus is dominant in the upper layers during stratification periods when, despite the pronounced oligotrophy, it is apparently responsible for maximum photosynthetic efficiency values, probably due to its capacity to cope with low nutrient conditions . By contrast, as in other oceans, prochlorophytes are most often found in deeper layers in stratified conditions (Yacobi et al, 1995;Li et al, 1993), with a sharp peak near the bottom of the euphotic zone (Zohary et al, 1998;Partensky et al, 1999;Christaki et al, 2001), while they become abundant at surface over the autumn/winter (Fig. 9, .…”

“…In the Otranto Straits, high concentrations of healthy Chaetoceros were found as deep as 500 m (Vilicic et al, 1989). Prymnesiophytes and prokaryotes instead of diatoms dominated in March in the Cyprus eddy, which is not a site of deep convection, with moderately high chl a concentrations (59 mg m −2 at the core and 45.5 mg m −2 at the boundary) (Zohary et al, 1998). Apparently, also in highly dynamic areas, very high biomass only accumulates when diatoms are the species involved.…”

“…Picoplankton is often dominant also in the DCM, both in the western basin, e.g., at DYFAMED and in the Aegean Sea . The prokaryotic fraction of the picoplankton, namely Synechococcus and Prochlorococcus, can reach abundance values of up to 10 4 cells ml −1 (Zohary et al, 1998;Christaki et al, 2001). At the Ligurian Sea DYFAMED station, Synechococcus is dominant in the upper layers during stratification periods when, despite the pronounced oligotrophy, it is apparently responsible for maximum photosynthetic efficiency values, probably due to its capacity to cope with low nutrient conditions .…”

“…9, and at several localities Claustre et al, 1994;Bustillos-Guzmán et al, 1995;Barlow et al, 1997;Zohary et al, 1998). Among them, the coccolithophores deserve a special mention, as they show a high diversity in the MS (Cros and Fortuno, 2002).…”

Flow cytometry as a tool for the study of phytoplankton

“…For that reason the development of flow cytometric techniques for particle size determination was advocated by Legendre and Le Fevre (1991). Shipboard based instruments allow fast assessment of abundance and biomass variations (Burkill, 1987;Borsheim et al, 1989;Zohary et al, 1998;Zubkov et al, 1998Zubkov et al, , 2000, to probe three dimensional patchiness dynamics such as in eddies (Tintore et al, 1998). Buck et al (1996) estimated living carbon biomass of microplankton, nanoplankton and picoplankton using a combination of dual beam flow cytometry and epifluorescence microscopy.…”