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: The present paper reviews the literature related to the life cycle of the prymnesiophyte Phaeocystis and its controlling factors and proposes novel hypotheses based on unpublished observations in culture and in the field. We chiefly refer to P. globosa Scherffel as most of the observations concern this species. P. globosa exhibits a complex alternation between several types of free-living cells (non-motile, flagellates, microzoopores and possibly macrozoospores) and colonies for which neither forms nor pathways have been completely identified and described. The different types of Phaeocystis cells were reappraised on the basis of existing microscopic descriptions complemented by unpublished flow cytometric investigations. This analysis revealed the existence of at least three different types of free-living cells identified on the basis of a combination of size, motility and ploidy characteristics: non-motile cells, flagellates and microzoospores. Their respective function within Phaeocystis life cycle, and in particular their involvement in colony formation is not completely understood. Observational evidence shows that Phaeocystis colonies are initiated at the early stage of their bloom each by one free-living cell. The mechanisms controlling this cellular transformation are still uncertain due to the lack of information on the overwintering Phaeocystis fomms and on the cell type responsible for colony induction. The existence of haploid microzoospores released from senescent colonies gives however some support to sexuality involvement at some stages of colony formation. Once colonies are formed, at least two mechanisms were identified as responsible of the spreading of colony form: colony multiplication by colonial division or budding and induction of new colony from colonial cells released in the external medium after colony disruption. The latter mechanism was clearly identified, involving at least two successive cell differentiations in the following sequence: motility development, subsequent flagella loss and settlement to a surface, mucus secretion and colony formation, colonial cell division and colony growth. Aggregate formation, cell motility development and subsequent emigration from the colonies, release of non-motile cells after colony lysis on the other hand, were identified as characteristics for termination of Phaeocystis colony development. These pathways were shown to occur similarly in natural environments. In the early stages of the bloom however, many recently-formed colonies were found on the setae of Chaetoceros spp, suggesting this diatom could play a key-rôle in Phaeocystis bloom inception. Analysis of the possible environmental factors regulating the transition between the different phases of the life cycle, suggested that nutrient status and requirement of a substrate for attachment of free-living cells would be essential for initiation of the colonial form. Physical constraints obviously would be important in determining colony shape and fragmentation although autogenic factors cannot be excluded. Some evidence exists that nutrients regulate colony division, while temperature and nutrient stress would stimulate cell emigration from the colonies.

Description: Anhand von zwei käuflichen Präparaten wird die Extinktion von Chlorophyll a in verschiedenen Lösungsmitteln untersucht und daraus der Extinktionskoeffizient für frisch extrahiertes, ungetrocknetes Chlorophyll a in Methanol abgeleitet. Er beträgt für das rote Extinktionsmaximum bei 665 nm 75.0 l/g cm. Die Extinktionskoeffizienten von getrocknetem Chlorophyll a sind in den untersuchten Lösungsmitteln 10-25% niedriger als die von einer frisch extrahierten Chlorophyllösung (Tabelle 2). Die Messung des Chlorophyll a-Gehaltes kann mit hinreichender Genauigkeit in einem Filterphotometer (ELKO II, Zeiss) ausgeführt werden. Hierzu werden Eichkurven für drei verschiedene Interferenzfilter angegeben. From two purchasable preparations of crystalline chlorophyll a, the extinction coefficient of undried chlorophyll a in methanol has been calculated as 75.0 l/g cm at the maximum of extinction at 665 nm. The extinction coefficients of dried chlorophyll a in the different solvents tested are 10-25% lower than those of undried chlorophyll a (table 2). Chlorophyll a can be measured in a filter photometer (ELKO II, Zeiss) with sufficient precision. Standard curves for three different interference filters are given.

Description: Die Teilchengrößenanalyse van Detrituspartikeln mit dem Teilchengrößenanalysator TGZ 3 der Firma Zeiss wird ausführlich beschrieben. Nach dieser Methode wurden insgesamt 92 Wasserproben von 6 Stationen in der westlichen Ostsee untersucht und eine Gesamtverteilungskurve für alle Detritusteilchen gebildet. In dem untersuchten Größenbereich zwischen 8 und 60 µ wurden die folgenden Mittelwerte gefunden: Teilchengröße: 17.6 ± 6.5 µ., Teilchenzahl: 1.35 Mill./l, Teilchenvolumen: 5.68 mm3/l, Teilchenoberfläche: 14.91 cm2/l. Es wird gezeigt, daß unter gewissen Voraussetzungen aus der Teilchengrößenverteilung das Detritustrockengewicht einer Wasserprobe errechnet werden kann. Die gewonnenen Ergebnisse werden mit den Detrituswerten verglichen, die aus den gleichzeitig ausgeführten chemischen Analysendaten durch die Differenzbildung Sestongewicht - Planktongewicht (nach dem Eiweiß- und Chlorophyllgehalt) = Detritusgewicht erhalten wurden. The analysis of the size distribution of detritus particles by means of the 'Teilchengrößenanalysator TGZ 3, Fa. Zeiss' is described in detail. 92 water samples collected at 6 stations in the western Baltic were analysed by this method. A total size spectrum for all counted particles is formed. in the investigated size group ranging from 8 to 60 µ average values were found as follows: particle size: 17.6 ± 6.5 µ, particle number: 1.35 mill./l, particle volume: 5.68 mm3/l, particle surface: 14.91 cm2/l. It is shown, with some assumptions, that the dry weight of detritus particles present in a water sample can be calculated from their size frequency. The obtained results are compared with parallel values derived from chemical analysis of the same samples as follows: seston weight - plankton weight (derived from albumen and chlorophyll content) = detritus weight.