Regional change of seasonal temperature regimes do exceeds the prognosed global climate change effect in inshore Baltic ecosystems - implications for fisheries management

Today, spatial patterns of sea surface warming become increasingly evident in the context of climate change processes. Therefore, adjustment of knowledge on temperature variability is very important to predict and model biotic responses. The Greifswalder Bodden is the main spawning area of Western Baltic spring spawning herring. This shallow, semi-closed estuary is highly susceptible to be affected by a changing temperature regime. Time series of mean daily sea surface temperatures (SST), mean daily air temperatures and mean number of sunshine hours were compared in weekly intervals from 1976 to 2006. In addition, the effects of large scale oceanographic and atmospheric indices (NAO and BSI) on the regional climate development were analyzed over the same 30 yrs period. Results indicate that mean SST and mean air temperatures are highly correlated (R = 0.91). Air temperatures however, correlate with the number of sunshine hours only during the summer month. Overall, mean SST in 2006 was 1.1°C higher than in 1976. Additionally the seasonal flux of SST differed significantly between 1976 and 2006. Whereas over the decades only a minor increase was observed for winter temperatures, a maximum increase of more than 2.8°C was observed for spring and summer seasons with the exception of a short period in June showing a slight decrease of SST. Additionally we observed a lower mean SST in November during recent years. Although mean SST in the Greifswalder Bodden correlates with NAO and BSI during winter (R ~ 0.7), both indices do not explain the variability of summer air and water temperatures. This study clearly shows that strong increases of sea surface temperatures actually do occur in shallow inshore waters where high seasonal climate variability cannot directly be linked to global indices. Although geographically on a small scale, those regional increases of SST can have immense ecological and economical consequences, when they affect the reproduction center of an important fish stock. The regional climate change effects in this case might affect reproduction success of spring spawning herring and should be incorporated in future research and management actions.