Mass aggregations of the free-living marine nematode Pontonema vulgare (Oncholaimidae) in organically polluted fjords

Abstract

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.