|Rekrutierungsmuster ausgewählter Wattfauna nach unterschiedlich strengen Wintern = Recruitment patterns of selected Wadden Sea fauna after winters of differing severity|
Strasser, M. (2000). Rekrutierungsmuster ausgewählter Wattfauna nach unterschiedlich strengen Wintern = Recruitment patterns of selected Wadden Sea fauna after winters of differing severity. Ber. Polarforsch. Meeresforsch. 377: 3-127
In: Berichte zur Polar- und Meeresforschung = Reports on Polar and Marine Research. Alfred-Wegener-Institut für Polar- und Meeresforschung: Bremerhaven. ISSN 1618-3193, more
Severe winters have significant effects on the ecosystem of the Wadden Sea. On the one hand, intertidal subpopulations of cold-sensitive species such as the cockle Cerastoderma edule and the polychaete Lanice conchilega are often eliminated. On the other hand, some species reach unusually high abundances in the following summer. This phenomenon has in the past been regularly observed in juveniles of the Intertidal bivalves Cerastoderma edule, Macoma balthica and Mya arenaria. The main objective of this study was to determine which factors are primarily responsible for such high bivalve recruitment after severe winters. Other benthic species of the Wadden Sea demonstrate low recruitment after severe winters. The polychaete Lanice conchilega was chosen as a model organism to determine which factors govern the recruitment pattern here. Using the severe winter of 1995/96 as a natural large-scale experiment, recruitment was studied in the years 1996 to 1998. The effects of the severe winter of 1995/96 were compared with the effects of a moderate winter (1996/97) and a mild winter (1997/1998). This study contributes to a better understanding of the general problem of high recruitment variability in benthic marine organisms. The results enhance our knowlegde of the long-term dynamics of coastal marine invertebrates and reveal potential opportunities in their sustainable harvest. The expectation of unusually high bivalve recruitment after the severe winter was confirmed for the intertidal and subtidal of the Sylt-Rømø Bight for Cerastoderma but not for Macoma and Mya. In the fall after the severe winter, average abundances of Cerastoderma amounted to 900 ind m-² in the intertidal (30 stations) and even 3600 ind m-² in the subtidal (4 transects). In the fall after the moderate and the mild winter average abundances did not exceed 100 ind m-². Abundances of Macoma and Mya remained below 150 ind m-²in the fall of all years studied. One hypothesis to explain the high bivalve recruitment after severe winters is enhanced larvalsupply. This hypothesis was rejected based on the results of daily plankton sampling. The total number of all sampled bivalve larvae was three times lower after the severe winter than after the mild winter. In selected samples determination of larvae down to species level also showed the highest abundances after the mild winter for Cerastoderma and Mya. For Macoma, abundances after the severe winter were in the same order of magnitude as after the moderate winter. Another hypothesis states that high bivalve recruitment after severe winters is caused by reduced epibenthic predation. To test this hypothesis predator exclusion experiments were conducted after the three different winters, both in sandy and muddy sediment. Using cages, the juvenile bivalves were protected from epibenthic predators for 10 to 12 days several times during each growing season. In all three bivalve species epibenthic predation was lower after the severe than after the moderate and the mild winter. While there was no significant predation effect in any cage experiment after the severe winter, abundances of juvenile bivalves after the moderate and the mild winter were, in some experiments, significantly higher inside the cages than on the experimental plots which allowed access for predators. It is assumed that the shore crab Carcinus maenas plays an important role in the observed pattern since the results of the predator exclusion experiments coincide well with the temporal appearance of Carcinus in the plankton: after the severe winter the larvae of Carcinus appeared 6 to 8 weeks later and in lower numbers than after the moderate and the mild winter. Although epibenthic predation after the severe winter was reduced for all three bivalve species, only Cerastoderma established a strong year class until the end of the season. By October of 1996 juvenile Cerastoderma had reached an average size of 8 mm. At the same time only few individuals of Macoma and Mya grew larger than 3 mm. These results support the hypothesis that juvenile bivalves have to reach a critical size to be protected against epibenthic predators. However, the juvenile cockle population was already considerably diminished - most likely by a cold spell of three weeks - after the following moderate winter. Predator exclusion experiments in winter indicated furthermore that birds might locally eliminate juvenile cockles. In contrast to the immediate recolonization of the intertidal by Cerastoderma, the recruitment of Lanice was very poor in the first two years after the severe winter. High recruitment in the third year then resulted in full recovery of the intertidal subpopulation of Lanice with a similar density and distribution pattern as before the extinction. Since larval abundances of Lanice were also much lower in the first two years than in the third year, a close relationship between larval supply and recruitment can be assumed. After the elimination of the intertidal subpopulations of the cockle Cerastoderma and the polychaete Lanice by the severe winter of 1995/96 recolonization was governed mainly by two different processes. With Lanice the pre-settlement process of high larval supply in the third year was the main determinant while with Cerastoderma the post-settlement process of reduced epibenthic predation in the first year was most important. While severe winters may be regarded as advantageous for the cockle population because they result in rapid rejuvenation of the overall population, the Lanice population is primarily reduced and exhibits only slow recovery.