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Colacium vesiculosum, a common epizoic freshwater euglenoid occasionally was found in the Western Baltic infesting planktonic copepods. Its occurrence coincided with low salinity, high temperature and large biomass of cope pods in the area. Infestation was most pronounced on Acartia sp. living in the upper part of the water column, but decreased in Centropages hamatus, Temora longicornis and Pseudocalanus elongatus (in that order), probably reflecting the vertical migrational behaviour of these copepods. Survival and egg production in the copepods were not affected by Colacium infestation, and as its biomass only amounted to 1.5% of phytoplankton biomass, it is concluded that C. vesiculosum only plays a minor role in the area.

The retention efficiency of 13 species of suspension feeding bivalves has been determined by analysing the particle size distribution in the inhalant and exhalant water currents using a Coulter electronic particle counter. The bivalves were held in aquaria under conditions very similar to their natural environment. It is shown that 11 species, Cardium echinatum, Cardium edule, Modiolus modiolus, Mytilus edulis, Musculus niger, Cultellus pellucidus, Arctica islandica, Ostera edulis, Hiatella striata, Mya arenaria and Venerupis pullastra, which all possess eu-latero-frontal cirri completely retain particles above 1 μm. Particles down to 1 /Lm were in most cases efficiently retained. The retention efficiency in Pecten opercularis and Pecten septemradiatus both lacking the eu-latero-frontal ciliary tract decreases below 7 μm and reaches only 20 % at 1 μm.

Mytilus edulis (average length 3.7 em) were fed mixtures of algae (10000 Phaeodactylum tricornutum cells · ml-1) and natural silt (0 to 55 mg · l-1), and the effects of silt concentration on filtration behaviour, food uptake and carbon budget were studied. Particle clearance averaged 66 ml · min-1 at 3.5 mg silt · l-1, and decreased at higher and lower concentrations. Increasing amounts of material were retained by the gills with increasing silt concentration, but an increasing proportion of this was rejected as pseudofaeces. Pseudofaeces was produced at silt concentrations above 1 mg · l-1, and the amount produced increased lineally with the amount of material retained (regression line: pseudofaeces produced (mg · min-1) = 0.98 x material retained (mg · min-1) - 0.057; r = 0.98). Dry matter ingestion increased with increasing concentration of silt. (Regression line: ingestion rate (μg · min-1) = 4.36 x In concentration (mg · l-1) + 31.98; r = 0.52). The mussels were able to select between algae ...

The influence of suspended bottom material (0–25 mg · l−1) in addition to algal cells (Phaeodactylum tricornutum) (0–500 µg dry org wt · l−1) on clearance, growth and energetics in Spisula subtruncata (da Costa) has been studied. Clearance and respiration rate were independent of concentrations of algae and suspended bottom material. Algal ingestion rate was proportional to algal concentration, and independent of the concentration of suspended bottom material. Growth rate increased with algal concentration, and was further increased by 10 to 110 % by the addition of suspended bottom material. The positive effect of suspended bottom material on growth is due to a higher efficiency of assimilation of the ingested algae and/or to the utilization of organic matter in the suspended bottom material. The efficiency of algal assimilation decreased from 76 % at a low (150 µg dry org wt · 1−1) to 33 % at a high (500 µg) algal concentration in specimens fed pure suspensions of algae. The water processing po...

A new sampling system for quantitative zooplankton studies is described. It consists of a centrifugal pump and a plankton net combined in a single submersible unit. The performance of the net-pump was compared to vertical hauls with a 100-µm WP-2 net on two occasions characterized by large differences in zooplankton biomass. The filtration efficiency of the WP-2 net decreased steadily, due to clogging, from ca. 100% at the onset of sampling (25 m) to about 50% at the surface. Clogging in the net-pump was not observed even after prolonged sampling. In vertical hauls, the capacity of the net-pump (420 1 · min-1) was sufficient to catch even adult Calanus finmarckicus quantitatively. Used in the stationary mode at fixed depths, most adult C. finmar chicus escaped from the pump intake. The volume filtered by the net-pump was only a fraction of the volume filtered by the WP-2 net. However, the total variation (variation among samples + subsample error) was identical (May) or markedly lower (October) f...

Ingestion rate, gut clearance rate and gut content were measured in the laboratory in adult females of the planktonic copepod Centropages hamatus fed the centric diatom Ditylum brightwelli. Ingestion rate increased with algal concentration approaching a maximum rate equal to 85 % body C · d−1 at 15 °C. Within the temperature interval studied (1–15 °C), maximum ingestion rate increased with temperature with a Q10 of 3.9. Gut clearance rate was estimated by following the exponential decrease in the gut content of plant pigments (measured fluoremetrically) with time after feeding, and showed approximately the same temperature dependence (Q10 = 3.3) as ingestion rate. The relationship between gut content and algal concentration was described by an equation similar to that relating ingestion rate to algal concentration. Gut content multiplied with gut clearance rate, which can be considered an estimate of ingestion rate, yielded values similar to ingestion rates actually measured. This implies that in...

Seasonal and diel variation in rate of algal grazing were estimated from measurements of gut content (plant pigments) and gut turnover in the copepods Centropages hamatus and Pseudocalanus sp. during spring (Januar–May) in the Oresund. Both species exhibited significant diel variation in gut content and ingestion rate at the three depths studied (5, 10 and 22 m), with the highest ingestion rates and gut contents during night. The variation was most pronounced in March, but almost insignificant in April. Since the copepods did not migrate vertically, the observed pattern is due to a variable feeding activity rather than caused by continuous feeding at varying food concentrations. We found a positive correlation between ambient algal concentration and algal ingestion rate and gut content for both species in weekly morning samples. The results indicated satiation of the ingestion rates at high algal concentrations. Maximum algal ingestion rates measured in the field were similar to maximum ingestion...