Effects of the Ice Winter 1995/96

Abstract

Investigations in the framework of ELAWAT showed that cold winters with ice formation lead to mass mortalities of cold sensitive species in all parts of the Wadden Sea ecosystem and to an increased mobility of organisms. While in the phytoplankton cold resistant species became dominant due to their increased growth, changes in dominance in the benthos were due to mortality of coldsensitive species. Inspite of the massive die-off of large individuals of the macrofauna, no strong increase in TOC (total organic carbon) was measured in the sediment.

In the following regeneration, the seasonal development in the water column differed markedly from the preceding years, which could be explained by extraordinarily calm weather, low water temperatures and competition between algal species. In the zooplankton the meroplankton showed severe changes in the temporal sequence of appearance and density of species when compared to 1994 and 1995.

During a “black area event” (end of May/beginning of June 1996) a short term reduction in depth of the oxidized sediment layer was measured together with a small increase in TOC and bacterial activity. Besides this event, regeneration of the benthic community by successful recruitment of several species was undisturbed. Due to the fact that the “black area event” and recruitment (larvae were already in the water column, settlement occurred after redevelopment of an oxidized layer) were temporally disconnected, a fast recovery of even strongly affected areas was possible.

Among the macrozoobenthos the trophic groups of the suspension-feeders and epibenthic predators were most negatively affected by the ice winter. The disappearance of suspension-feeding species such as Mytilus edulis, Cerastoderma edule or Lanice conchilega was partly outbalanced by very good recruitment of single species (M. edulis, My a arenaria). Predation pressure by epibenthic predators was low and thus not a limiting factor for the recruitment of prey species.

Most parts of the macrobenthic association recovered fast from the effects of the ice winter (< 1 year), with the exception of populations of the more longliving cold sensitive species which presumably take 3-4 years to recover completely. Comparing the ice winter effects observed in the backbarrier system of Spiekeroog (East Frisian Wadden Sea) and Sylt (North Frisian Wadden Sea) the duration of the ice cover was longer in the Sylt area, but had in principle the same effects on the pelagic and benthic system (modification of the species composition towards cold adapted/cold tolerant species). The largest differences between the areas were observed in the temporal development of the pelagic system (dominance of different species, different temporal course in the meroplankton). There are indications that additional differences occurred subtidally. As the source populations for the recolonization of the intertidal by cold sensitive species lie subtidally, future studies on recruitment after ice winters should include both the intertidal and subtidal benthic systems.