204
P.H. Nienhuis
significantly lower than in normal years, implying excess N uptake by
macroalgae during specific summers (Kolbe et al. 1995). Until 1980, the
tidal flats of the German Wadden Sea were regarded as not endangered
by oxygen deficiency. During benthos surveys around 1984, however,
attention was drawn to small black spots occurring in sandy sediments
without any visible traces of decaying plant or animal material. Within
these spots the oxidized surface layer has disappeared, exposing the
black reduced sediment layer. Surveys from 1987 onwards revealed that
the number and size of the black spots increased dramatically on the
tidal flats between Ems and Elbe (Michaelis et al. 1992). The presence of
these black spots, which were recently also found in the Dutch Wadden
Sea, is connected to eutrophication. An increased loading of the sediments
with organic matter can easily lead to a flow of anaerobic seepage water
from sandy, elevated spots in intertidal areas to the sediment surface
(Philippart and Brinkman 1992).
The situation in the German Wadden Sea can be summarized as
follows:
1. There is a decrease of seagrass distribution and abundance during
and after the "wasting disease" in the 1930s, but to a far lesser extent
than in the Dutch Wadden Sea. Some littoral populations of Zostera
marina re-established themselves, but all sublittoral populations disappeared, owing to increased turbidity and eutrophication.
2. There is an increase of green macro algal mats (mainly Enteromorpha
prolifera) from the late 1970s onwards. From 1989 onwards, the
increase becomes alarming, especially between the outlets of the rivers
Ems, Weser and Elbe. Quantitative data to substantiate this 'increase',
however, is scarce, but a monitoring system has been started recently
(Kolbe et al. 1995).
3. There are changes in the infauna composition and abundance. An
increased abundance of polychaetes in intertidal and subtidal sandy
bottoms is manifest.
4. Konigshafen is the well-studied northernmost locality of the German
Wadden Sea. Seagrasses decreased in the 1930s but were rather
common again in the late 1980s. The sublittoral Zostera marina
populations disappeared, and a dramatic increase in green algal mats
has recently been observed.
7.2.3 The Netherlands
Microphytobenthos (mainly benthic diatoms) and phytoplankton are the
main primary producers of the Wadden Sea (Fig. 7.6), providing a net
P.H. Nienhuis
significantly lower than in normal years, implying excess N uptake by
macroalgae during specific summers (Kolbe et al. 1995). Until 1980, the
tidal flats of the German Wadden Sea were regarded as not endangered
by oxygen deficiency. During benthos surveys around 1984, however,
attention was drawn to small black spots occurring in sandy sediments
without any visible traces of decaying plant or animal material. Within
these spots the oxidized surface layer has disappeared, exposing the
black reduced sediment layer. Surveys from 1987 onwards revealed that
the number and size of the black spots increased dramatically on the
tidal flats between Ems and Elbe (Michaelis et al. 1992). The presence of
these black spots, which were recently also found in the Dutch Wadden
Sea, is connected to eutrophication. An increased loading of the sediments
with organic matter can easily lead to a flow of anaerobic seepage water
from sandy, elevated spots in intertidal areas to the sediment surface
(Philippart and Brinkman 1992).
The situation in the German Wadden Sea can be summarized as
follows:
1. There is a decrease of seagrass distribution and abundance during
and after the "wasting disease" in the 1930s, but to a far lesser extent
than in the Dutch Wadden Sea. Some littoral populations of Zostera
marina re-established themselves, but all sublittoral populations disappeared, owing to increased turbidity and eutrophication.
2. There is an increase of green macro algal mats (mainly Enteromorpha
prolifera) from the late 1970s onwards. From 1989 onwards, the
increase becomes alarming, especially between the outlets of the rivers
Ems, Weser and Elbe. Quantitative data to substantiate this 'increase',
however, is scarce, but a monitoring system has been started recently
(Kolbe et al. 1995).
3. There are changes in the infauna composition and abundance. An
increased abundance of polychaetes in intertidal and subtidal sandy
bottoms is manifest.
4. Konigshafen is the well-studied northernmost locality of the German
Wadden Sea. Seagrasses decreased in the 1930s but were rather
common again in the late 1980s. The sublittoral Zostera marina
populations disappeared, and a dramatic increase in green algal mats
has recently been observed.
7.2.3 The Netherlands
Microphytobenthos (mainly benthic diatoms) and phytoplankton are the
main primary producers of the Wadden Sea (Fig. 7.6), providing a net