456
W. Schramm
In his recent paper, Duarte (1995) emphasizes in his conclusions that
present knowledge of the response of marine vegetation to eutrophication and the associated changes in the structure and function of the
ecosystem is sufficient to make general qualitative predictions. However,
he also concluded that we are still far away from predictive quantitative
models, in particular from non-linear functional models which account
for the high complexity of the eutrophication process due to feedback
effects and self-acceleration. Such models which would enable us to define, for example, critical nutrient loadings at which the above-described
reaction chain is triggered are urgently needed for coastal management
and as a basis for environmental legislation for setting tolerable nutrient
loadings.
Compared to our knowledge of the effects of increased nutrient loadings on marine vegetation, there is very little information available about
the consequences of reduced nutrient loadings, or whether changes in
benthic vegetation due to eutrophication are reversible. Comprehensive
models that could predict the consequences of a reduction of nutrient
loadings as planned, for example, on a larger scale for the Baltic or the
North Sea (Helsinki and North Sea Convention), do not exist as far as we
know. Only recently, Duarte (1995) presented a simple simulation model
to show the time scales involved in the recovery of populations of different seagrasses in denuded areas. The model is based on the general
observation that under eutrophication slow-growing plants are replaced
by faster-growing ones in increasingly shorter recolonisation periods.
This implies an inverse process under de-eutrophication, i.e. replacement
of the fast-growing short-lived nutrient opportunists by slower-growing
long-lived macrophytes. The length of the lag period between nutrient
reduction and complete recovery of the system depends on the reproduction, recruitment and growth mode of the macrophytes. Re-colonization
by rooting vascular plants, for example seagrasses, depends primarily on
the supply of seeds from neighbouring fertile populations, formation of
new patches by seedlings and subsequent patch growth through peripheral extension of the rhizome system (Duarte and Sand-Jensen 1990a, b).
The few available reports on patch formation and patch growth reveal
considerable differences between theseagrass species. For example, net
patch formation ranges from 3 x 10- 4 patches m- 2 yeac 1 to 5 x 10- 4
patches m- 2 year- 1 for Posidonia ocenanica and Cymodocea nodosa,
respectively (Meinesz and Lefevre 1984; Duarte and Sand-Jensen 1990a),
whereas rhizome elongation rates may range from 1 to over 500 em
yeac 1 in the largest and smallest seagrasses, respectively (Duarte 1991).
U sing these figures to simulate recovery for a wide combination of patch
formation and patch growth, Duarte (1995) could show that the time
W. Schramm
In his recent paper, Duarte (1995) emphasizes in his conclusions that
present knowledge of the response of marine vegetation to eutrophication and the associated changes in the structure and function of the
ecosystem is sufficient to make general qualitative predictions. However,
he also concluded that we are still far away from predictive quantitative
models, in particular from non-linear functional models which account
for the high complexity of the eutrophication process due to feedback
effects and self-acceleration. Such models which would enable us to define, for example, critical nutrient loadings at which the above-described
reaction chain is triggered are urgently needed for coastal management
and as a basis for environmental legislation for setting tolerable nutrient
loadings.
Compared to our knowledge of the effects of increased nutrient loadings on marine vegetation, there is very little information available about
the consequences of reduced nutrient loadings, or whether changes in
benthic vegetation due to eutrophication are reversible. Comprehensive
models that could predict the consequences of a reduction of nutrient
loadings as planned, for example, on a larger scale for the Baltic or the
North Sea (Helsinki and North Sea Convention), do not exist as far as we
know. Only recently, Duarte (1995) presented a simple simulation model
to show the time scales involved in the recovery of populations of different seagrasses in denuded areas. The model is based on the general
observation that under eutrophication slow-growing plants are replaced
by faster-growing ones in increasingly shorter recolonisation periods.
This implies an inverse process under de-eutrophication, i.e. replacement
of the fast-growing short-lived nutrient opportunists by slower-growing
long-lived macrophytes. The length of the lag period between nutrient
reduction and complete recovery of the system depends on the reproduction, recruitment and growth mode of the macrophytes. Re-colonization
by rooting vascular plants, for example seagrasses, depends primarily on
the supply of seeds from neighbouring fertile populations, formation of
new patches by seedlings and subsequent patch growth through peripheral extension of the rhizome system (Duarte and Sand-Jensen 1990a, b).
The few available reports on patch formation and patch growth reveal
considerable differences between theseagrass species. For example, net
patch formation ranges from 3 x 10- 4 patches m- 2 yeac 1 to 5 x 10- 4
patches m- 2 year- 1 for Posidonia ocenanica and Cymodocea nodosa,
respectively (Meinesz and Lefevre 1984; Duarte and Sand-Jensen 1990a),
whereas rhizome elongation rates may range from 1 to over 500 em
yeac 1 in the largest and smallest seagrasses, respectively (Duarte 1991).
U sing these figures to simulate recovery for a wide combination of patch
formation and patch growth, Duarte (1995) could show that the time