Population Ecology - Recruitment of benthic organisms
The processes involved in recruitment of soft-sediment fauna are studied using bivalves and polychaetes as model organisms. These studies aim to identify the developmental phase most critical to recruitment. This helps understanding the dynamics of the populations and sustains environmental management in finding meaningful spatial units in the coastal area
In the Wadden Sea, the planktonic larvae of polychaetes and bivalves are among the most abundant zooplankton groups. They are an important food source for planktivorous fish. The interannual and seasonal variability of larval distribution is studied to quantify the importance of larval supply and larval exchange between the North Sea and the Wadden Sea.
After their planktonic larval phase the juveniles settle to the sediment. Frequently, this occurs outside the adult habitats implicating the need for the half-grown worms to migrate to the adult sites. The mode, costs, and benefits of these migrations are analysed in the field. We assume that habitat change of age classes results in populations with a higher temporal stability. Because both polychaetes and bivalves are important trophic links in the food web this might even stabilise the entire Wadden Sea community.
Once in the sediment the juveniles are released from planktonic predation but face benthic and epibenthic predators and competitors. The interactions between primary settlers and the established fauna are quantified in field experiments.
Excluding epibenthic predators by cages resulted in high recruitment of juvenile cockles while recruitment was close to nothing below incomplete cages (one wall omitted) and in uncaged areas.
The importance of epibenthic predation on bivalve density in the Wadden Sea is also evident from time series data. After mild winters juvenile shore crabs arrive just in time to find bivalve prey of a fitting size class (lower half). However, after cold winters they arrive later on the tidal flats. Then the bivalves already grew too large to be consumed. As a result, bivalve recruitment is high after cold winters and low after mild ones.
Strasser, M., Günther, C.-P. (2001) Larval supply of predator and prey: temporal mismatch between crabs and bivalves after a severe winter in the Wadden Sea. J Sea Res 46:57-67
Strasser, M. (2002) Reduced epibenthic predation on intertidal bivalves after a severe winter in the European Wadden Sea. Marine Ecology Progress Series 241: 113-123