Food Competition between the Benthic Polychaete Hediste Diversicolor and the Semipelagic Mysid Neomysis Integer in the Northern Baltic Sea/Toidukonkurents Bentilise Hulkharjasussi Hediste Diversicolor'i Ja Semipelaagilise Musiidi Neomysis Integer'i Vahel Laanemere Pohjaosas

INTRODUCTION As the structure of benthic communities in the Baltic Sea is relatively simple, the area offers a unique opportunity to study the mechanisms and detect the driving forces of changes in biodiversity. Nevertheless, scientists are still debating about the relative contribution of biotic interactions and physical disturbance in determining the patterns of coastal communities in the northern Baltic Sea. Earlier studies demonstrated significant intra- and interspecific competitive links among several benthic algal and invertebrate species in the region (Kotta et a1., 2001, 2008; Orav-Kotta et al., 2009). Other studies, however, supported an alternative view that physical disturbance is the main mechanism of community dynamics in the northern Baltic Sea region (e.g. Kautsky & van der Maarel, 1990; Herkul et al., 2006). Earlier studies have focussed on the competitive links either between benthic or pelagic species. To date, there are virtually no studies where the competitive interactions between benthic and pelagic species are experimentally evaluated although such benthic--pelagic coupling might be relevant in many instances. Neomysis integer (Leach) is the most prevalent mysid species in the coastal areas of the northern Baltic Sea (Kotta & Kotta, 1999). In May-June the bulk of the population is concentrated in the shallow areas below the 10 m depth. In the less exposed bays the density of mysids may occasionally exceed 10 000 ind. [m.sup.2]. It is likely that in such shallow areas the density of plankton is low and the food should be primarily obtained from the upper layer of the sediment. Neomysis integer is considered to be omnivorous. It is known to feed besides phytoplankton and zooplankton on bottom detritus and filamentous algae (Mauchline, 1971). The polychaete Hediste diversicolor (O. F. Muller) is the key benthic invertebrate species in these shallow and isolated bays. The species is a deposit feeder at juvenile stages and a predator at the later stages of its development (Bonsdorff & Pearson, 1999). Hence, competition between the species is likely in these areas. The aim of this study was to investigate if the two species are competing for space and food during May-June when the densities of both species are very high in the shallow bays. We expect that at low food levels the competitive interactions reduce the growth and survival of the test organisms whereas at high food levels such negative effects do not occur. MATERIAL AND METHODS The in situ food competition between the prevalent mysid N. integer and the polychaete N. diversicolor was studied in a shallow (2 m) bay (59[degrees]50'N, 23[degrees]15'E) adjacent to the Tvarminne Zoological Station in the north-eastern Baltic Sea. The bay is moderately exposed to the sea and the prevailing sediment is silty sand. The test organisms and sediment were collected in the vicinity of the experimental site. The sediment was sieved through a 1 mm mesh and dried in the sun for 3 days to assure that it was free from living benthic invertebrates. To investigate interspecific interactions between the test organisms, 3 L plastic buckets ([empty set] 18 cm) were used. The buckets were filled with an 8 cm layer of sediment and a 7 cm layer of seawater and allowed to settle for 6 h. The test organisms were added to four replicate buckets per treatment (totalling 24 buckets). Four individuals of H. diversicolor and/or 15 individuals of N. integer were added to each bucket so that their densities corresponded to natural densities in the study area. The animals were fed by adding sediment rich in detritus and microphytobenthos. A more detailed setup of the experiment is given in Table 1. The buckets were closed by a mesh-net (0.2 mm mesh size) to minimize the risk of migration but at the same time to assure sufficient water exchange in the buckets. The experimental buckets were randomly placed on the seafloor at a depth of 0. …