Studies on the meroplankton in the inner Oslofjord I. Composition of the plankton at Nakkholmen during a whole year

Abstract

The meroplankton at Nakkholmen in the inner Oslofjord has been studied during one annual cycle (December 1963–December 1964). Each of the 70 samples taken consisted of the larvae from 1000 litres of sea water delivered by a suction pump and filtered through a 125 μ plankton net. In total, these samples contained about 182,000 larvae belonging to 54 species (polychaetes: 27 species, bivalves: 16 species, gastropods: 6 species, echinoderms: 5 species). For almost the whole year the meroplankton was dominated by larvae of polychaetes which constituted 90.6 % of the total number of larvae. In particular Polydora ciliata and P. antennata were found in enormous numbers. Thus, about 78 % of the total number of larvae were identified as P. ciliata, followed in frequency by larvae of Mytilus edulis (8.3 %) and of Polydora antennata (6.7 %). The number of echinoderm and gastropod larvae were negligible. The observed composition of the larval plankton agrees well with that from the southern part of the Copenhagen harbour (Smidt, 1944) but it differs considerably from observations from other Scandinavian localities. However, the larvae of the dominating species were more numerous at Nakkholmen than in the southern part of the Copenhagen harbour, e. g., the larvae of Polydora ciliata were more than 15 times as numerous at Nakkholmen. The atypical meroplanktonic pattern at Nakkholmen is an effect of the heavy pollution from sewage in the fjord system. It is suggested that high numbers of Polydora larvae, and in particular of those of P. ciliata, indicate pollution.