454
W. Schramm
German coasts of Kiel Bay (western Baltic), for example, the formerly
abundant perennial Fucus communities, still growing down to 6-8 m
depth in the 1950s, had completely disappeared below 2 m depth by
1988/1989. The total biomass decreased by more than 90% during this
period (Vogt and Schramm 1991).
Another indirect effect combined with feeback is that increasing
blooms of fast-growing, short-lived phytoplankton and macroalgae and
their subsequent breakdown are coupled with increasing input of organic
materials to the bottom systems, enhancing oxygen-consuming heterotrophic activities. Resulting oxygen depletion, anoxia and finally hydrogen sulphide development in the sediments lead to further inpairment
and mortality of the benthic communities. Reducing conditions under
anoxia accelerate nutrient release from the sediments which in turn
raises the eutrophication level.
The reduction of seagrasses, large seaweeds, but also of the benthic
micro algal films, involves changes in the hydrodynamic conditions in the
bottom layer and in the sediment stability. This results in enhanced res uspension of sediments and transfer of sediment nutrients into the water
column which further increases turbidity and nutrient levels, respectively.
Other feedback effects, both negative and positive, are related to the
activities of herbivores, which prefer to graze on microalgae and epiphytes (Fig. 4). The fact that epiphytic macro algae at low nutrient loadings
do not flourish and outcompete the slower growing perennials may be
the result of grazing on epiphytes which under nutrient limitation cannot
grow fast enough to compensate for grazing losses (Borum 1987; SandJensen and Borum 1991; Neckles et al. 1993; Cebrian and Duarte 1994).
Enhanced growth of epiphytic micro- and macroalgae or phytoplankton
due to eutrophication, on the other hand, supports the development of
associated grazer and filter-feeder populations which in turn mayaccelerate the eutrophication process through regeneration of nutrients (e.g.
Kautsky and Wallentinus 1980).
In summary, the changes in benthic vegetation due to eutrophication
are a reaction chain of direct and indirect effects, feedback mechanisms
and self-accelerating processes, which are difficult to control once initiated. Duarte (1995) uses the term "domino effect", meaning that the
eutrophication process once started is maintained and amplified until the
final phase of eutrophication is reached, i.e. the extinction of the macrophytobenthic communities.
The importance of the eutrophication induced changes derives not
only from changes in the primary producer compartment, but also from
changes in the structure and function of the whole ecosystem. Duarte
(1995, p. 99) discusses eutrophication effects on the ecosystem level and
W. Schramm
German coasts of Kiel Bay (western Baltic), for example, the formerly
abundant perennial Fucus communities, still growing down to 6-8 m
depth in the 1950s, had completely disappeared below 2 m depth by
1988/1989. The total biomass decreased by more than 90% during this
period (Vogt and Schramm 1991).
Another indirect effect combined with feeback is that increasing
blooms of fast-growing, short-lived phytoplankton and macroalgae and
their subsequent breakdown are coupled with increasing input of organic
materials to the bottom systems, enhancing oxygen-consuming heterotrophic activities. Resulting oxygen depletion, anoxia and finally hydrogen sulphide development in the sediments lead to further inpairment
and mortality of the benthic communities. Reducing conditions under
anoxia accelerate nutrient release from the sediments which in turn
raises the eutrophication level.
The reduction of seagrasses, large seaweeds, but also of the benthic
micro algal films, involves changes in the hydrodynamic conditions in the
bottom layer and in the sediment stability. This results in enhanced res uspension of sediments and transfer of sediment nutrients into the water
column which further increases turbidity and nutrient levels, respectively.
Other feedback effects, both negative and positive, are related to the
activities of herbivores, which prefer to graze on microalgae and epiphytes (Fig. 4). The fact that epiphytic macro algae at low nutrient loadings
do not flourish and outcompete the slower growing perennials may be
the result of grazing on epiphytes which under nutrient limitation cannot
grow fast enough to compensate for grazing losses (Borum 1987; SandJensen and Borum 1991; Neckles et al. 1993; Cebrian and Duarte 1994).
Enhanced growth of epiphytic micro- and macroalgae or phytoplankton
due to eutrophication, on the other hand, supports the development of
associated grazer and filter-feeder populations which in turn mayaccelerate the eutrophication process through regeneration of nutrients (e.g.
Kautsky and Wallentinus 1980).
In summary, the changes in benthic vegetation due to eutrophication
are a reaction chain of direct and indirect effects, feedback mechanisms
and self-accelerating processes, which are difficult to control once initiated. Duarte (1995) uses the term "domino effect", meaning that the
eutrophication process once started is maintained and amplified until the
final phase of eutrophication is reached, i.e. the extinction of the macrophytobenthic communities.
The importance of the eutrophication induced changes derives not
only from changes in the primary producer compartment, but also from
changes in the structure and function of the whole ecosystem. Duarte
(1995, p. 99) discusses eutrophication effects on the ecosystem level and