Carmen-Pia Günther

Larval supply of predator and prey: temporal mismatch between crabs and bivalves after a severe winter in the Wadden Sea

Enhanced bivalve recruitment after severe winters is a well-known phenomenon in the coastal North Sea. By comparing the bivalve larval abundances in the northern Wadden Sea after a severe (1995/96), a moderate (1996/97) and a mild (1997/98) winter we found no evidence for the hypothesis that high bivalve recruitment after severe winters is caused by enhanced larval supply. Total and peak abundances of all bivalve larvae as well as of each of four separate species Ensis americanus, Mytilus edulis, Cerastoderma edule and Mya arenaria were three to six times lower after the severe than after the mild winter. In Macoma balthica total and peak abundances after the severe winter were only slightly higher than after the moderate winter. The larvae of the epibenthic predator Carcinus maenas appeared in lower numbers and six to eight weeks later after the severe winter than after the moderate and the mild winter. Since the bivalve larvae appeared without, or with less, delay after the severe winter, there was a temporal mismatch between Carcinus and the bivalves, supporting the hypothesis that reduced epibenthic predation is an important factor in high bivalve recruitment after severe winters.

How predictable is high bivalve recruitment in the Wadden Sea after a severe winter

Higher than average recruitment among bivalves on the intertidal flats of the Wadden Sea was observed many times after severe winters in the period 1940 - 1995. The occurrence of another severe winter in 1995/96 prompted us to test the hypothesis of severe winters leading to universally high bivalve recruitment on a large geographic scale (500 km coastline) in temperate shallow waters. We analysed data sets on bivalve abundance from seven areas in the Dutch, German and Danish Wadden Sea. The longer data sets showed generally higher bivalve recruitment in the 1970Žs and 1980Žs than in the 1990Žs which may be related to the near absence of severe winters since 1987. Considering the period 1988 onwards (the longest possible period in which 1995/96 was the only severe winter), recruitment of Cerastoderma edule was in 1996 - in agreement with our hypothesis - above average at all seven investigated areas. In contrast, recruitment of Macoma balthica and Mya arenaria was for the same period above average only in the southern Wadden Sea (south-west of Jade Bay) but not in the northern Wadden Sea (north of Eiderstedt peninsula). These regional differences may be related to (i) the different topography of the northern Wadden Sea (with barrier islands westwards to the mainland) compared to the southern Wadden Sea (with barrier islands northwards to the mainland) and subsequent differential effects of wind induced currents on bivalve recruitment, (ii) differences in biotic factors such as standing stocks, larval supply or epibenthic predation or (iii) changes in environmental conditions. Our results demonstrate that large-scale comparisons along coasts are an indispensable addition to insights derived from local studies alone.

Settlement and recruitment of Mya arenaria L. in the Wadden Sea

Abstract Four stations (B1–B4) on an intertidal sandflat south of the island of Borokum (German Bight) were investigated in 1986 for the settlement of the soft shell clam Mya arenaria . The abundance of larvae in the plankton as well as of early postlarval stages in the benthos was recorded. Settlement of M. arenoria was restricted to those two stations with the lowest tidal elevations (B1, B2). In the upper part of the flat no early benthic stages were found. At that station where most of the larvae settled, recruitment in terms of abundance, growth, mortality and production was analysed using two sampling methods. The results suggest that size-specific peibenthic predation not only caused mortality but also affected growth and production.

Observations of a mass occurrence of Macoma balthica larvae in midsummer

Abstract In 1995 the seasonal development of concentrations of both phytoplankton and larvae of the bivalve Macoma balthica was studied in the coastal zone behind the back-barrier island of Spiekeroog (German Wadden Sea). In July/August larvae reached maximum concentrations of about 1000 to 4200 ind. m −3 (depending on the sampling site and tidal period), probably in relation to a phytoplankton bloom in July. This observation of an unusually late maximum of Macoma larvae is discussed in detail in connection with the recent literature available about spawning of M. balthica in the Wadden Sea.

Seasonal changes in the bivalve larval plankton of the White Sea

In 1994 and 1995 the abundance of bivalve larvae was studied in the subarctic region of Chupa Bay, White Sea. From June until October sampling was carried out in time intervals of 1–2 weeks. At each sampling date 3 parallel samples were taken at low water. For collection of the material an Apsteinnet was used (diameter of aperture 10 cm; mesh size 125 µm).Among the identified 10 species and 2 genera mytilid larvae (Mytilus edulis andModiolus modiolus combined) andHiatella “byssifera” were the most abundant. On species level larval densities not only varied interannually to a high degree but also showed a high temporal variability with shifts of 3–4 weeks in the occurrence of maximum abundances between years. These results are discussed with respect to the geographic origin of the species as well as to the seasonal patterns of phytoplankton blooms in the White Sea.

Variation in genetic traits of the Baltic clam Macoma balthica from a tidal gradient in the subarctic

Abstract In a subarctic tidal gradient, strong heterogeneity in genetic traits of the Baltic clam Macoma balthica was found. The heterogeneity was stronger within the intertidal gradient, over a distance of only about 60 m, than along a horizontal gradient over a distance of 1200 km in clams from the west European coast. For the locus Idh1 and the average heterozygosity, a tidal cline was found. The frequency of allele Idh1-B decreased with tidal level, whereas the frequency of allele Idh1-C, as well as the average heterozygosity, increased. The possibility is discussed that the strong genetic heterogeneity and tidal clines are caused by differential selection related to the (subarctic) temperatures to which the higher tidal zones are more exposed.

Concepts for offshore nature reserves in the southeastern North Sea

The authors summarize general criteria for selection of marine areas for protection, discuss intentions of the European Council initiative to create a coherent European network of nature reserves, point to increasing pressure on marine life by new offshore activities and installations, and arrive at preliminary proposals for offshore marine protected areas (MPAs) in the southeastern North Sea. To protect benthic communities, the minimum MPA size should not be less than 100 km2; distances of such MPAs, serving as ‘stepping-stones’ and refuges, should not exceed 100 km to guarantee coherence. Some shortcomings of the European Habitats Directive are identified and a few proposals made to consider additional marine habitat types and functional aspects within future protection initiatives.