MARINE ECOLOGY - PROGRESS SERIES Mar. Ecol. Prog. Ser. Published November 8 Plankton distributions and processes across a front in the open Baltic Sea M. Kahrul J. Elkenl I. Kotta2, M. Simm2 and K. Vilbaste3 * Department of the Baltic Sea, Institute of Thermophysics and Electrophysics, Paldiski St. 1, Tallinn 200031, USSR Tallinn Department of the Baltic Sea Fishery Research Institute, Apteegi St. 1, Tallinn 200001, USSR Institute of Zoology and Botany, Vanemuise St. 21, Tartu 202400, USSR ABSTRACT: Sections of the temperature-salinity structure in the southeastern Gotland Basin, central Baltic Sea, revealed the existence of a pronounced salinity front. A water mass with anomalously low salinity extended vertically across the horizontally uniform thermocline. Plankton distribution a n d primary productivity showed consistent features with the frontal structure: the near-surface primary productivity increased ?-fold adjacent to the front and levelled down farther away from it; the column chlorophyll had a broader and less distinct maximum on the high-salinity side of the front; the zooplankton community was similar in composition but its biomass more than doubled in the higher- salinity water. It is suggested that upwelling of nutrient-rich water in the frontal zone had triggered the productivity peak, whereas the subsequent advection and diffusion were instrumental for the broader chlorophyll maximum. Owing to the lower phytoplankton/zooplankton ratio, the nutrient cycling had to be faster in the higher-salinity water mass. INTRODUCTION The existence of thermohaline fronts, i.e. boundaries between horizontally juxtaposed water masses, is probably a characteristic feature of the Baltic Sea. This has recently gained support from in situ CTD surveys (Aitsam et al., 1982b) as well as satellite imagery (Horstmann, 1983). The existence of fronts is to be expected as the brackish Baltic waters result from complicated mixing processes - lateral and vertical - between water masses with different characteristics. Oceanic fronts are known to have important implica- tions for biological productivity, distribution of pollu- tants, mixing, circulation, etc. (Bowman and Esaias, 1978).Due to the notorious difficulties in making high- resolution measurements in the conceivably ubiqui- tous but relatively ephemeral and highly dynamic fronts in the Baltic Sea, there are only single observa- tional studies, apart from scattered occasional observa- tions. From recent studies of the northwest European shelf (Floodgate et al., 1981; Holligan, 1981; Pingree et al., 1982), it has increasingly become apparent that fronts - tidal, coastal, and shelf-break - are regions of elevated phytoplankton standing crop and biological productiv- ity. The location of these fronts is controlled, among @ Inter-Research/Printed in F. R. Germany other factors, by the intensity of tidal stirring (Simpson and Hunter, 1974). Although the Baltic Sea fronts are evidently of different nature, they may have similar biological implications. In 1980 we made a number of spatial chlorophyll/ CTD surveys of a stationary area in the central Baltic Sea, south-east from Gotland (Kahm et al., 1981, 1982). The striking mesoscale variability of the chlorophyll standing crop that was revealed was sometimes clearly associated with patterns in the thermohaline layering. This was interpreted as an indication of localized verti- cal fluxes of nutrients due to certain hydrographic features (shoaling of the halocline, narrowing of the intermediate layer). In 1981 in a near-by area we observed a synoptic-scale eddy carrying a cold and fresh water anomaly with a 2 to 3-fold reduced chlorophyll a content (Aitsam et al., 1982a). Here we report a series of measurements made in 1982 in the same area as in 1980, but including more biological detail. The space interval was reduced from 5 nautical miles (n.mi) to 2.5 n.mi, thus providing bet- ter resolution and more confidence in the observed patterns. As the low-frequency variability of the Baltic Sea is to a great extent controlled by the bottom topo- graphy (Aitsam et al., 1983),we extended our observa- tions across the whole trough, the locally dominant