Role of advection in the distribution and abundance of Pleurobrachia pileus in Kiel Bight

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Abundance of the ctenophore Pleurobrachia pileus and s h t y were studied at a station in the western Kiel Bight from January to July 1981. Results suggest that variations of the ctenophore stock were primarily caused by advection and did not reflect the biological population cycle of this species. Kiel Bight and adjacent areas connect the Kattegat with the relatively less saline waters of the Baltic proper. Strong oscillations in the hydrographic conditions of thls transition area occur due to the prevailing wind regime and the annual river runoff pattern (summarized by Lenz 1977a). The advection of either Kattegat or Baltic water into Kiel Bight can be monitored comparatively easily by studying changes in salinity (Kandler & Wattenberg 1940, Wattenberg 1949). Such hydrographic variations make biological investigations complex, because polyhaline and mesohaline waters exhibit dfferent plankton populations (Lenz 1977b) so plankton studies at a fixed station are affected by horizontal water movements. Such movements are most pronounced in Kiel Bight from spring to autumn (Lenz 1981). Advective processes may, on the one hand, transport plankton species into Kiel Bight which do not normally occur in this area ('indicator species'; Kandler & Wattenberg 1940) and, on the other hand, induce variation in abundance of indigenous species if generation times are greater than the residence time of the water in Kiel Bight. These variations cannot be explained by reproduction and mortality rates. Therefore, a closer look at the role of horizontal water movements on planktological studies is needed. The aim of this note is to demonstrate the effect of advection on the abundance of the ctenophore Pleurobrachia pileus during the first half of 1981. The investigation was carried out between January and July 1981 inclusive at the 'Boknis Eck' station in the western Kiel Bight (Fig. 1). Water depth at the O Inter-Research/Printed in F. R . Germany station was 26 to 27 m, and samples were taken every 2 wk. Ctenophores were collected with a 'Helgolander Larvennetz' (diameter 1 m, 300 pm mesh) by vertical hauls from about 2 m above the sea bottom to the surface. Individuals were counted alive in the laboratory and their diameters measured. For younger stages a dissecting microscope was used. Hydrographic conditions were recorded using an Electronic Switchgear TS-Probe. During winter, water was well mixed, with salinity values close to 20 560 (Fig. 2) . Due to advection of Baltic waters in spring, salinity decreased to minimum values at the end of April. This is a regular annual event (summarized by Babenerd 1980, 1986) caused by increase of river discharge due to melting snow and ice (Brogmus 1952) and increase of easterly winds (Schweimer 1978). In contrast, the summer situation is characterized by a well-developed halocline and salinFig. 1. Kiel Bight with the 'Boknis Eck' station indicated by a dot Mar. Ecol. Prog. Ser. 41: 99-102, 1987