7.2 The "[)Isturhing" Effects of the Icc Winter 1995/96
195
Besides the mechanical destruction of epibenthic structures by the ice movement, they were also frozen onto the ice and subsequently transported with the ice
and tides. Freezing onto the ice was also seen as a reason for the density reduction
of mussels observed after ice winters (Strasser pers. comm.). Both effects may
occur simultaneously, depending on the different locations of mussel beds within
the intertidal.
Not only epibenthic structures were frozen onto the ice at low tide, but also entire sediment layers, containing, according to ELA W AT results, living nematodes
and other meiofaunal taxa.
7.2.2
Direct Effects of Low Temperatures
The ice winter of 1995/96 caused changes in the species composition not only in
the benthic realm as already observed by Beukema (1990, 1992), but also in the
pelagic realm of the Spiekeroog backbarrier system.
In the pelagic system of the German Bight a Coscinodiscus wailesii bloom in
February and March was followed by a bloom of the closely related cold adapted
species C. concinnus (Henke 1997). In February no cells of Coscinodiscus spp.
were observed in the coastal zone between the island of Spiekeroog and the mainland. Here, the C. concinnus bloom started at the beginning of March (Niesel
1997).
Measurements of the bacterial enzyme activity showed that microorganisms
were active over winter. This activity was most likely the reason that the TOe of
the sediment did not increase during and after the ice winter although a mass mortality of some macrofauna species was recorded in the sediment
The benthic fauna showed pronounced changes after the ice winter. Locally
some cold sensitive nematode species disappeared, whereas other species found
refuge in deeper sediment layers. In the intertidal and nearby subtidal apparently
all L conchilega died off, as well as all cockles (Cerastoderma edule only intertidal observations). Reichert & Dorjes (1980) assumed that the low abundance of
more mobile species such as Nephtys hombergii, which was also observed after the
ice winter 1995/96, may be explained by mortality as well as migration into the
subtidal
Marine organisms living in the higher tidal flats were affected by long periods
of low temperatures in combination with the lack of inundation caused by easterly
winds. Here, cold sensitive species disappeared earlier than at periodically flooded
sites. As a long-term effect an increased mortality was observed even in cold tolerant species (Scoloplos armiger, Mya arenaria), presumably resulting from oxygen
deficiency (Reichert & Dbrjes 1980; Beukema 1989). Thus, mortality in cold winters may be site-specific (compare also Beukema 1985, 1990).
In February 1996 the total amount of resting waders in the entire East Frisian
Wadden Sea area was reduced by 35% compared to mean values for the same
months of the preceding years (Exo, Ketzenberg, unpubL). But exchange rates of
local bird stocks had increased, as many birds left the area while others, e.g. oystercatchers, wigeons and common gulls came in from even more unfavourable
regions. In MarchI April 1996 no differences to the preceding years could be detected concerning total bird numbers and species composition.
195
Besides the mechanical destruction of epibenthic structures by the ice movement, they were also frozen onto the ice and subsequently transported with the ice
and tides. Freezing onto the ice was also seen as a reason for the density reduction
of mussels observed after ice winters (Strasser pers. comm.). Both effects may
occur simultaneously, depending on the different locations of mussel beds within
the intertidal.
Not only epibenthic structures were frozen onto the ice at low tide, but also entire sediment layers, containing, according to ELA W AT results, living nematodes
and other meiofaunal taxa.
7.2.2
Direct Effects of Low Temperatures
The ice winter of 1995/96 caused changes in the species composition not only in
the benthic realm as already observed by Beukema (1990, 1992), but also in the
pelagic realm of the Spiekeroog backbarrier system.
In the pelagic system of the German Bight a Coscinodiscus wailesii bloom in
February and March was followed by a bloom of the closely related cold adapted
species C. concinnus (Henke 1997). In February no cells of Coscinodiscus spp.
were observed in the coastal zone between the island of Spiekeroog and the mainland. Here, the C. concinnus bloom started at the beginning of March (Niesel
1997).
Measurements of the bacterial enzyme activity showed that microorganisms
were active over winter. This activity was most likely the reason that the TOe of
the sediment did not increase during and after the ice winter although a mass mortality of some macrofauna species was recorded in the sediment
The benthic fauna showed pronounced changes after the ice winter. Locally
some cold sensitive nematode species disappeared, whereas other species found
refuge in deeper sediment layers. In the intertidal and nearby subtidal apparently
all L conchilega died off, as well as all cockles (Cerastoderma edule only intertidal observations). Reichert & Dorjes (1980) assumed that the low abundance of
more mobile species such as Nephtys hombergii, which was also observed after the
ice winter 1995/96, may be explained by mortality as well as migration into the
subtidal
Marine organisms living in the higher tidal flats were affected by long periods
of low temperatures in combination with the lack of inundation caused by easterly
winds. Here, cold sensitive species disappeared earlier than at periodically flooded
sites. As a long-term effect an increased mortality was observed even in cold tolerant species (Scoloplos armiger, Mya arenaria), presumably resulting from oxygen
deficiency (Reichert & Dbrjes 1980; Beukema 1989). Thus, mortality in cold winters may be site-specific (compare also Beukema 1985, 1990).
In February 1996 the total amount of resting waders in the entire East Frisian
Wadden Sea area was reduced by 35% compared to mean values for the same
months of the preceding years (Exo, Ketzenberg, unpubL). But exchange rates of
local bird stocks had increased, as many birds left the area while others, e.g. oystercatchers, wigeons and common gulls came in from even more unfavourable
regions. In MarchI April 1996 no differences to the preceding years could be detected concerning total bird numbers and species composition.