7.5 Comparison of Ice Winter Effects
201
7.5
Comparison of Ice Winter Effects in Different Parts of the
Wadden Sea
The comparison of results obtained in the East and North Frisian part of the Wadden Sea may elucidate effects of cold winters and subsequent regeneration on a
geographical scale.
With 66 days of ice cover the cold period prevailed longer in the North Frisian
Wadden Sea around the island of Sylt when compared to 48 days on the East
Frisian coast. In both systems salinity had increased, either caused by a lack of
precipitation or by an upwelling front. In the pelagic of both areas a bloom of cold
adapted agal species was observed: close to Sylt island it was produced by Odontella aurita, close to Spiekeroog at both sites by C. concinnus. Simultaneously low
concentrations of silicate were recorded, indicating that this nutrient became limiting soon, the same applied for nitrate. The intracellular oil production of
C. concinnus was not observed in the North Frisian part of the Wadden Sea.
Reactions of cold sensitive species varied between the two regions (Table 7.S.I). While L. conchilega disappeared in the entire Wadden Sea the intertidal
population of the cockle (c. edule) was reduced by 69 % in the northern Wadden
Sea. The local differences may be due to reinvasion of adult cockles surviving in
the subtidal. Another site-specific response was observed for N. hombergii. After
the winter 1995/96 this species disappeared almost completely from the intertidal
tlats at the island of Sylt, while it was regularly recorded in the backbarrier tidal
tlats of Spiekeroog Island, although in very low abundances. According to Beukema (1985) the likelihood for survival of N. hombergii is higher in areas close to
the low water level than in areas with higher tidal elevation. Thus, the differences
may result from different tidal elevations of the sampling sites in the two areas.
Following the ice winter, an extremely good recruitment of Nereis diversicolor
took place in the Spiekeroog backbarrier tidal area, but not of Phyllodoce mucosa
or Capitella capitata, as observed in tidal tlats at Sylt.
In both areas, the recruitment of bivalves was of special interest, as the existing
literature indicated a link between cold winters and recruitment success in intertidal bivalve species (e.g. Beukema 1979, 1982). With the exception of M. balthica
whose larval densities were low in the backbarrier area of Spiekeroog, but dominated the meroplankton in the waters around Sylt, the rank order of species by their
larval densities was similar in both regions: E. americanus, followed by a cluster
containing M. arenaria, M. edulis and C. edule. But the temporal pattern of occurrence differed clearly. In the Wadden Sea near Sylt, several maxima of bivalve
larvae were observed in the plankton, with each one clearly dominated by a specific species. In the backbarrier tidal tlats of Spiekeroog, nearly all species reached
maximum larval densities at the same time. In both areas the proportion of larval
densities was not reflected in the proportion of abundances in recruits (Table 7.5.1,
comparison meroplankton/recruitment strength).
201
7.5
Comparison of Ice Winter Effects in Different Parts of the
Wadden Sea
The comparison of results obtained in the East and North Frisian part of the Wadden Sea may elucidate effects of cold winters and subsequent regeneration on a
geographical scale.
With 66 days of ice cover the cold period prevailed longer in the North Frisian
Wadden Sea around the island of Sylt when compared to 48 days on the East
Frisian coast. In both systems salinity had increased, either caused by a lack of
precipitation or by an upwelling front. In the pelagic of both areas a bloom of cold
adapted agal species was observed: close to Sylt island it was produced by Odontella aurita, close to Spiekeroog at both sites by C. concinnus. Simultaneously low
concentrations of silicate were recorded, indicating that this nutrient became limiting soon, the same applied for nitrate. The intracellular oil production of
C. concinnus was not observed in the North Frisian part of the Wadden Sea.
Reactions of cold sensitive species varied between the two regions (Table 7.S.I). While L. conchilega disappeared in the entire Wadden Sea the intertidal
population of the cockle (c. edule) was reduced by 69 % in the northern Wadden
Sea. The local differences may be due to reinvasion of adult cockles surviving in
the subtidal. Another site-specific response was observed for N. hombergii. After
the winter 1995/96 this species disappeared almost completely from the intertidal
tlats at the island of Sylt, while it was regularly recorded in the backbarrier tidal
tlats of Spiekeroog Island, although in very low abundances. According to Beukema (1985) the likelihood for survival of N. hombergii is higher in areas close to
the low water level than in areas with higher tidal elevation. Thus, the differences
may result from different tidal elevations of the sampling sites in the two areas.
Following the ice winter, an extremely good recruitment of Nereis diversicolor
took place in the Spiekeroog backbarrier tidal area, but not of Phyllodoce mucosa
or Capitella capitata, as observed in tidal tlats at Sylt.
In both areas, the recruitment of bivalves was of special interest, as the existing
literature indicated a link between cold winters and recruitment success in intertidal bivalve species (e.g. Beukema 1979, 1982). With the exception of M. balthica
whose larval densities were low in the backbarrier area of Spiekeroog, but dominated the meroplankton in the waters around Sylt, the rank order of species by their
larval densities was similar in both regions: E. americanus, followed by a cluster
containing M. arenaria, M. edulis and C. edule. But the temporal pattern of occurrence differed clearly. In the Wadden Sea near Sylt, several maxima of bivalve
larvae were observed in the plankton, with each one clearly dominated by a specific species. In the backbarrier tidal tlats of Spiekeroog, nearly all species reached
maximum larval densities at the same time. In both areas the proportion of larval
densities was not reflected in the proportion of abundances in recruits (Table 7.5.1,
comparison meroplankton/recruitment strength).