7.l The Seasonal Development 1996 (Regeneration)
197
sensitivity of the adults to cold water temperatures, e.g. larvae of L. conchi/ega and
Magelona papillicornis were found in low densities only.
In contrast to preceding years, larvae of all bivalve species appeared with
maximum densities at the same time (June 1996) in the plankton. The larval densities of all species, except for Macoma balthica and M. edulis, were comparable to
results from 1994 and 1995. For M. halthica, the lowest larval densities of all three
years of investigation were observed in 1996. This is not in accordance with the
mechanism described by Honkoop & van der Meer (1997) and must probably be
related to the unusual late spawning of this species in 1995 (Gunther et al. 1998).
The density of M. edulis larvae was increased after the ice winter 1995/1996.
Some copepod species apparently took advantage from the intracellular oil produced by C. concinnus. Already at the beginning of the algal bloom they had incorporated small oil droplets, which is normally observed at the beginning of
autumn. Highest densities of copepodid stages were recorded as early as April.
indicating that reproduction had already taken place early in the year.
7.3.2
The Benthic System
Directly after the ice melt, the oxidized layer of the sediment had the same depth
as in the previous years. With the temperature related rise of remineralization in
April/May the RPD-layer reached closer to the sediment surface. In summer the
oxidized sediment layer of the sandtlat at the Groninger Plate (GP3, Fig. 3.1.4) had
a depth of 0.5-1 cm, which was lower than in the previous years (> I cm). The
lowest depth was measured in June and coincided with the appearance of black
areas at the sediment surface (see below).
With the loss of mussel beds on the tidal flat area Swinnplate, biodeposition of
fine sediment ceased. This caused a situation in early 1996 in which the morphodynamics of the area was characterized by erosion instead of sediment accumulation. In the sediment, the protein and fatty acid contents were lower than in the
preceding years, indicating that the organic material was of lower nutritional quality (Chap. 5.2.1). Only in the few mud patches left, higher concentrations of these
compounds were measured. In the sediments of the site were mussel beds existed
in previous years, maximum numbers of bacteria were only half as high, but the
enzyme activities (glucosidase and phosphatase) were higher.
At the Groninger Plate. the seasonal development of the total amount of bacteria
resembled the normal pattern. Bacterial density strongly increased from 29th of
May until II th of June. caused by an increase in sediment temperature and by the
input of organic matter derived from the sedimentation of the blooms of
C. concinnus and P. poucheffi. This was also followed by the occurrence of bacterial methanococci. Merismopedia and other cyanobacteria in the sediment.
During the course of the summer 1996 the Chl.-a concentration in the sediment
was 3-4 times lower when compared to 1994 and 1995. In the period of the "black
area event", a short-term decline in the density of benthic diatoms was observed.
The number of microphytobenthic species was comparable to that of preceding
years, but included three new species not observed in the benthos until then. From
March onwards high numbers of the pelagic diatom C. concinnus were found in
197
sensitivity of the adults to cold water temperatures, e.g. larvae of L. conchi/ega and
Magelona papillicornis were found in low densities only.
In contrast to preceding years, larvae of all bivalve species appeared with
maximum densities at the same time (June 1996) in the plankton. The larval densities of all species, except for Macoma balthica and M. edulis, were comparable to
results from 1994 and 1995. For M. halthica, the lowest larval densities of all three
years of investigation were observed in 1996. This is not in accordance with the
mechanism described by Honkoop & van der Meer (1997) and must probably be
related to the unusual late spawning of this species in 1995 (Gunther et al. 1998).
The density of M. edulis larvae was increased after the ice winter 1995/1996.
Some copepod species apparently took advantage from the intracellular oil produced by C. concinnus. Already at the beginning of the algal bloom they had incorporated small oil droplets, which is normally observed at the beginning of
autumn. Highest densities of copepodid stages were recorded as early as April.
indicating that reproduction had already taken place early in the year.
7.3.2
The Benthic System
Directly after the ice melt, the oxidized layer of the sediment had the same depth
as in the previous years. With the temperature related rise of remineralization in
April/May the RPD-layer reached closer to the sediment surface. In summer the
oxidized sediment layer of the sandtlat at the Groninger Plate (GP3, Fig. 3.1.4) had
a depth of 0.5-1 cm, which was lower than in the previous years (> I cm). The
lowest depth was measured in June and coincided with the appearance of black
areas at the sediment surface (see below).
With the loss of mussel beds on the tidal flat area Swinnplate, biodeposition of
fine sediment ceased. This caused a situation in early 1996 in which the morphodynamics of the area was characterized by erosion instead of sediment accumulation. In the sediment, the protein and fatty acid contents were lower than in the
preceding years, indicating that the organic material was of lower nutritional quality (Chap. 5.2.1). Only in the few mud patches left, higher concentrations of these
compounds were measured. In the sediments of the site were mussel beds existed
in previous years, maximum numbers of bacteria were only half as high, but the
enzyme activities (glucosidase and phosphatase) were higher.
At the Groninger Plate. the seasonal development of the total amount of bacteria
resembled the normal pattern. Bacterial density strongly increased from 29th of
May until II th of June. caused by an increase in sediment temperature and by the
input of organic matter derived from the sedimentation of the blooms of
C. concinnus and P. poucheffi. This was also followed by the occurrence of bacterial methanococci. Merismopedia and other cyanobacteria in the sediment.
During the course of the summer 1996 the Chl.-a concentration in the sediment
was 3-4 times lower when compared to 1994 and 1995. In the period of the "black
area event", a short-term decline in the density of benthic diatoms was observed.
The number of microphytobenthic species was comparable to that of preceding
years, but included three new species not observed in the benthos until then. From
March onwards high numbers of the pelagic diatom C. concinnus were found in