Mass aggregations of the marine free-living nematode Pontonema vulgare containing thousands to milhons of individuals were observed in the summer half-year (May to Oct) above the surface of sublittoral (mainly 6 to 9 m depth) soft bottoms of the inner Flensburg and Kiel fjords. Sedlments were rich in organic matter, reduced, and black nearly to the surface. Oxygen content of the overlying water was very low (1 to 2 m1 O2 I-'). The aggregations occurred on dead and moribund macrofaunal animals, on beds of rothng Ulva sp. and Zostera manna, and near organic wastes from a trout farm. They marked sharply the transition zone between oxlc sites ( 2 4 m1 0, I-' in summer) inhabited by an intact macrofauna, and anoxlc sites (0 to 0.5 m1 O2 1 -I ln summer) lacking any macrofauna (including P. vulgare). It is concluded that these aggregations above (not withm) the surface of the sediment indicate dramatic mortality of the macrofauna livlng there due to decreasing oxygen content and increasing H,S content. These processes are known to occur especially in organically polluted areas. Previous taxonomic descriptions of P. vulgare are supplemented, and P. balticum (Schultz 1932) is suggested to be synonymous with P. vulgare.
In-situ investigations on the life of the common sea star (Asterias rubens L.) were carried out in 1976, employing the Underwater Laboratory "Helgoland" in Liibe& Bay (Western Baltic Sea). The abundance of A. rubens amounted to 2--31 m "2 on sediment (fine sand), and to 324-809 m -2 on mobile algal carpets drifting over the bottom. Actual population parameters (abundance, size class distribution) are influenced by both substrate quality and drilling. Stomach investigations revealed prey-size selectivity: Small sea stars feed mainly on the snail Hydrobia ulvae when living on the sediment, but on mussel brood (Mytilus edulis) in the phytal. The principal food items of larger sea stars are the sand-dwelling clam Macorna baltica and the phytaI-living isopod Idotea baltica respectively. A, rubens is very adaptive to the food available; the diversity of its diet corresponds to the species diversity found in its environment. A change of biotope during active or passive migrations causes switching. The sea star is able to catch motile animals and to dig out infaunaI clams. It exhibits a diurnal feeding pattern related to light periodicity; the activity decreases at night. The average frequency of feeding is highly dependent on predator body size; it declines with growth. Insitu experiments indicate an exponential relationship between the feeding duration upon M. baltlca and the quotient of clam size to logarithm of sea-star size. An approach is made toward a rough estimate of macrofauna consumption by A. rubens on sediment. The sea star seems to be an important predator and thus a competitor of demersal fishes on sol°c bottoms of the western Baltic Sea.
In-situ studies on sublittoral soft bottom macrofauna (depth: 14-16 m) employing the underwater laboratory (UWL) "Helgoland" were carried out. Sets of samples were compared for small-scale local and short-term changes in species richness, faunal abundance, numerical dominance, diversity, evenness, homogeneity, and similarity. It could be shown that minor differences in sediment quality can cause conspicuous heterogeneity within a small sampling area (diameter: ca. 100 m). Both spatfalt and mortality of benthic invertebrates can change the faunal structure within a short period (two months). The degree of change varies between species and thus at stations harbouring different faunal assemblages as welt.
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