Description: Standing stock and size composition of the zooplankton comunity (〉100 μm) were studied in four depth strata of the upper 200 m of the water column during a “Meteor” cruise to the central Red Sea and Gulf of Aden in spring 1987. The central Red Sea was divided into a northern area of higher salinity and a less saline southern part. Both areas exhibited significant differences in zooplankton abundance and standing stock. The latter increased by the ratio 1:2:3 from the northern central Red Sea to its southern part and further south to the Gulf of Aden. For size structure analysis samples were fractionated into three size classes (100 to 300, 300 to 500, 500 to 5000 μm). In the central Red Sea the smallest size was dominant whereas in the Gulf of Aden the largest size fraction played a greater relative role than in the central Red Sea. This shift in size structure of the zooplankton community from the Red Sea to the Gulf of Aden is apparently primarily related to ecosystem difference between both areas, leading to a change in species composition. In addition, size reduction of individual species common to both seas may be of some significance in the extreme environment of the Red Sea.

Description: Die Sestonbestimmung hat den Vorteil, eine einfache, vielseitig anwendbare Methode in der biologischen Meereskunde zu sein. Ihre Genauigkeit hängt in erster Linie von der Beschaffenheit des Filtermaterials, der Größe der Filter und den Wägebedingungen ab. In diesem Zusammenhang werden drei Filtersorten miteinander verglichen: Papierfilter (Schleicher & Schüll 575), Sartorius-Membranfilter (SM 11304) und Glasfaserfilter (Whatman GF/C). Die Glasfaserfilter wurden hierbei, soweit bekannt, zum ersten Mal für die Sestonbestimmung benutzt. In der Filtrationsleistung sind sie den Membranfiltern vergleichbar. Aus voneinander unabhängigen Doppelbestimmungen wird die Standardabweichung der Methode bei der Verwendung der verschiedenen Filtersorten ermittelt. Die größte Genauigkeit erreichten Whatman-Glasfaserfilter (⌀ 2,5 cm) mit ± 0,15 mg auf der 95% Wahrscheinlichkeitsebene. Zum Schluß wird eine neue Filtationsapparatur beschrieben und eine ausführliche Arbeitsvorschrift gegeben. Seston determination has the advantage of being a simple method with a wide range of application in biological oceanography. Its accuracy depends mainly on the quality of the filter material, the size of the filters and the weighing conditions. Under these aspects three filter types are compared: paper filters (Schleicher& Schüll 575), Sartorius membrane filters (SM 11304) and glassfibre filters (Whatman GF/C). The glassfibre filters have been used here, as far as is known, for the first time for seston determination. Their filtration efficiency is comparable to that of the membrane filters. By means of independent double determinations the standard deviation of the method shown by the three filter types tested is calculated. The highest accuracy was attained by Whatman glassfibre filters (⌀ 2.5 cm) with ± 0.15 mg on the 95% probability level. Finally, a new filtration apparatus is described and detailed directions for determining seston given.