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Factors structuring the Mytilus- and Zostera-community in the western Baltic : an experimental approach = Strukturbestimmende Faktoren für die Mytilus- und Zostera-Gemeinschaft in der westlichen Ostsee : ein experimenteller Ansatz
No full textThe principal objectives of this dissertation were (1) to explain the ecologicalscale distribution of blue mussels (Mytilus edulis) in Kiel Fjord (2) to examine why, in shallow water (1-3 m), mussels occur more often in association with eelgrass (Zostera marina) than as pure stands and (3) to examine whether mussels have an effect on Zostera growth, shoot density and vegetative propagation. Field observations and experiments were conducted in Friedrichsort (FO) and Moeltenort (MOE) by means of SCUBA diving. Triplicate passive mussel spat collectors were deployed during the main settlement period to assess the contribution of spatial variations in settlement to mussel distribution. They revealed that on a scale of decimeters to meters spatial variation was generally low and that spat densities were not significantly different in the two water depths studied (2 and 6 m). The densities of juvenile mussels were compared among the principal substrata found in shallow water (mussel bed, Zostera meadow, Zostera/Mytilus mixed stands and bare sand) over a period of 15 mo. On pure mussel beds, recruit abundances were only 50% of those found on both substrata with a Zostera canopy. I also regularly sampled density and feeding performance of seastars (Asterias rubens) and shore crabs (Carcinus maenas) on these substratum types. Whereas Carcinus densities were always low compared to those of seastars, and crabs were totally absent during winter months, Asterias individuals were present throughout the year with densities between 5 and 35 ind/0.25 m2. They strongly preferred juvenile mussels (<1 yr old) over adult individuals. The predation impact of Asterias on juvenile mussels was found to be similar among the 3 substratum types: adult mussel bed (a) with and (b) without eelgrass, and (c) pure eelgrass. Length/frequency distributions of mussel recruits also suggest that juveniles suffered a similar and high predation mortality on these substrata. Therefore I suggest that the modification of the hydrodynamic regime inside the Zostera canopy (with and without mussel understory) led to a higher recruitment via secondary settlement compared to pure mussel beds. According to the frequency distributions settlement continued from August 1993 until January 1994. On substratum type pure sand, recruit densities were always markedly lower compared to the other substrata except for autumn 1992 when densities
comparable to those in Zostera stands were attained. According to the spat collector data, this occurred after an approximately 6-fold higher settlement intensity than in 1993..
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