7.4 The "Black Area Event" 1996
199
experimental data (e.g. Andre & Rosenberg 1991). The quantitati ve importance
of suspension-feeders on recruitment cannot be estimated as the populations are
usually not quantified on a system level.
3. The decline of epibenthic predators esp. Crangon crangon and Carcinus maenas
due to the cold winter, positively affects the survival rate of newly settled macrofauna. According to Kruse (pers. comm.) young of the year 1996 of cockles
outgrew the prey size range of their epibenthic predators. Due to the combination of a high growth rate with a low mortality rate, recruitment after the winter
was good. In the frame of ELA W AT length frequency measurements of the
species Arenicola marina, C. edule, M. halthica and M. arenaria also indicated
good growth of juveniles after the cold winter. As for the predator, the intertidal
population of the shore crab C. maenas was drastically reduced. Until autumn
1996 neither mean abundance nor age structure of their population had recovered. This effect probably persisted into the following years as the recruitment
of shore crabs was low in 1996.
The three factors limiting recruitment mentioned above work after each cold winter. But they do not explain why different species show good recruitment after
different cold winters or why recruitment of M. halthica was poor in 1996.
It seems that in addition to the abundance of adults the seasonal development in
the preceding year has to be considered. For example in years of bad feeding conditions adults probably produce only small amounts of gametes. In case of a successive cold winter, even a high survival rate of larvae and juveniles may not result
in good recruitment. The temporal development is of special importance if a series
of cold winters occurs as in the period of 1985-1987 (Beukema 1992). First results
of a model on effects of cold winters on the population dynamics of L. conchilega
indicate long term disappearance of the intertidal population in case of successive
cold winters.
At both study areas, the Swinnplate and Groninger Plate, highest numbers of
species were recorded in July 1996, as a consequence of the temporal delay of
recruitment.
A potential indirect effect of the ice winter on the bird fauna might be the lack
of food due to the mortality of some preferred prey species such as L. (one hi/ega.
The example of the grey plover showed that the range of potential prey organisms
is wide enough for the birds to switch to those which are still available (Ketzenberg, Exo, unpubl. data).
7.4
The "Black Area Event" 1996
During the regeneration phasc aftcr the ice winter 1995/96 the occurrence of extended intertidal areas with a black sediment surface were observed. The macrofauna from these areas protruded from the sediment and died. The occurrence of
such black areas and the reaction of the macrofauna varied within the different
backbarrier systems. Most affected was the backbarrier system behind the island of
Baltrum.
199
experimental data (e.g. Andre & Rosenberg 1991). The quantitati ve importance
of suspension-feeders on recruitment cannot be estimated as the populations are
usually not quantified on a system level.
3. The decline of epibenthic predators esp. Crangon crangon and Carcinus maenas
due to the cold winter, positively affects the survival rate of newly settled macrofauna. According to Kruse (pers. comm.) young of the year 1996 of cockles
outgrew the prey size range of their epibenthic predators. Due to the combination of a high growth rate with a low mortality rate, recruitment after the winter
was good. In the frame of ELA W AT length frequency measurements of the
species Arenicola marina, C. edule, M. halthica and M. arenaria also indicated
good growth of juveniles after the cold winter. As for the predator, the intertidal
population of the shore crab C. maenas was drastically reduced. Until autumn
1996 neither mean abundance nor age structure of their population had recovered. This effect probably persisted into the following years as the recruitment
of shore crabs was low in 1996.
The three factors limiting recruitment mentioned above work after each cold winter. But they do not explain why different species show good recruitment after
different cold winters or why recruitment of M. halthica was poor in 1996.
It seems that in addition to the abundance of adults the seasonal development in
the preceding year has to be considered. For example in years of bad feeding conditions adults probably produce only small amounts of gametes. In case of a successive cold winter, even a high survival rate of larvae and juveniles may not result
in good recruitment. The temporal development is of special importance if a series
of cold winters occurs as in the period of 1985-1987 (Beukema 1992). First results
of a model on effects of cold winters on the population dynamics of L. conchilega
indicate long term disappearance of the intertidal population in case of successive
cold winters.
At both study areas, the Swinnplate and Groninger Plate, highest numbers of
species were recorded in July 1996, as a consequence of the temporal delay of
recruitment.
A potential indirect effect of the ice winter on the bird fauna might be the lack
of food due to the mortality of some preferred prey species such as L. (one hi/ega.
The example of the grey plover showed that the range of potential prey organisms
is wide enough for the birds to switch to those which are still available (Ketzenberg, Exo, unpubl. data).
7.4
The "Black Area Event" 1996
During the regeneration phasc aftcr the ice winter 1995/96 the occurrence of extended intertidal areas with a black sediment surface were observed. The macrofauna from these areas protruded from the sediment and died. The occurrence of
such black areas and the reaction of the macrofauna varied within the different
backbarrier systems. Most affected was the backbarrier system behind the island of
Baltrum.