136
A.C. Srnaal . A. van der Hoek
response to the Delta project in terms of biological productivity can be characterized
as maintenance of functional stability (homeostasis), owing to adaptive and structural
responses in the ecosystems.
12.2.1.3
Transformation Capacity
A descriptive model analysis comparing nitrogen loads and internal cycling for the
four saline/brackish ecosystems (Lakes Veere and Grevelingen and Oosterschelde and
Westerschelde) has revealed that the systems differ clearly in their transformation capacity, i.e. the potential for denitrification (de Vries et al. 1995,1996). Nitrogen input in
Lake Grevelingen has been balanced by removal of nitrogen by denitrification and retention in refractory detritus. The Oosterschelde shows more exchange with the North
Sea, and denitrification balances the input by 60%. In these cases, denitrification is a
function of organic matter cycling, which could be expressed as a cycling index (the
ratio of uptake by primary producers and external input) of 7.1 for Lake Grevelingen
and 3.7 for the Oosterschelde. The eutrophic Lake Veere has compensated input for 45%
and the Westerschelde for only 25%; their cycling indices are 2.1 and 0.03 respectively.
In these eutrophic systems denitrification is promoted by anaerobic conditions rather
than organic matter cycling. The researchers concluded that the transformation capacity is underexploited in the water systems in the Delta area (de Vries et ai. 1996).
12.2.1.4
Resilience
Resilience as a response parameter, defined as the ability to return to equilibrium after
disturbance, can best be evaluated for the stagnant Lake Grevelingen. After the link
with the sea was cut off in 1971,20% of the total area was colonized by eelgrass (Zostera
marina) which then unexpectedly declined sharply after 1989 and has almost vanished.
The most probable cause of this dramatic decline is the increased salinity resulting
from changes in the water management of this 'manipulated' lake, in combination with
warm and dry summers and a reduction in silicate loads brought about by the diversion of water discharges (de Vries et ai. 1996). Unexpected fluctuations have also been
observed in other populations, particularly Mytilus eduiis, Ostrea eduiis and Nassarius
reticuiatis, (Lambeck 1982, 1985). This is evidence of a loss of resilience or robustness,
i.e. increased vulnerability to external perturbation.
12.2.2
Socio-economy
The Delta project has changed the socio-economic functions of the water systems. As
explained above, it was originally envisaged that all the estuaries except the Western
ScheIdt would be closed and freshwater lakes would be created. The overriding management aim was protection against storm floods; the freshwater lakes were intended
to benefit agriculture. When the campaign to protect the area's outstanding natural
resources and shellfish industry began in the 1970s, the sea walls closing the Grevelingen
and Veerse Gat estuaries had already been completed, but work on the Oosterschelde
A.C. Srnaal . A. van der Hoek
response to the Delta project in terms of biological productivity can be characterized
as maintenance of functional stability (homeostasis), owing to adaptive and structural
responses in the ecosystems.
12.2.1.3
Transformation Capacity
A descriptive model analysis comparing nitrogen loads and internal cycling for the
four saline/brackish ecosystems (Lakes Veere and Grevelingen and Oosterschelde and
Westerschelde) has revealed that the systems differ clearly in their transformation capacity, i.e. the potential for denitrification (de Vries et al. 1995,1996). Nitrogen input in
Lake Grevelingen has been balanced by removal of nitrogen by denitrification and retention in refractory detritus. The Oosterschelde shows more exchange with the North
Sea, and denitrification balances the input by 60%. In these cases, denitrification is a
function of organic matter cycling, which could be expressed as a cycling index (the
ratio of uptake by primary producers and external input) of 7.1 for Lake Grevelingen
and 3.7 for the Oosterschelde. The eutrophic Lake Veere has compensated input for 45%
and the Westerschelde for only 25%; their cycling indices are 2.1 and 0.03 respectively.
In these eutrophic systems denitrification is promoted by anaerobic conditions rather
than organic matter cycling. The researchers concluded that the transformation capacity is underexploited in the water systems in the Delta area (de Vries et ai. 1996).
12.2.1.4
Resilience
Resilience as a response parameter, defined as the ability to return to equilibrium after
disturbance, can best be evaluated for the stagnant Lake Grevelingen. After the link
with the sea was cut off in 1971,20% of the total area was colonized by eelgrass (Zostera
marina) which then unexpectedly declined sharply after 1989 and has almost vanished.
The most probable cause of this dramatic decline is the increased salinity resulting
from changes in the water management of this 'manipulated' lake, in combination with
warm and dry summers and a reduction in silicate loads brought about by the diversion of water discharges (de Vries et ai. 1996). Unexpected fluctuations have also been
observed in other populations, particularly Mytilus eduiis, Ostrea eduiis and Nassarius
reticuiatis, (Lambeck 1982, 1985). This is evidence of a loss of resilience or robustness,
i.e. increased vulnerability to external perturbation.
12.2.2
Socio-economy
The Delta project has changed the socio-economic functions of the water systems. As
explained above, it was originally envisaged that all the estuaries except the Western
ScheIdt would be closed and freshwater lakes would be created. The overriding management aim was protection against storm floods; the freshwater lakes were intended
to benefit agriculture. When the campaign to protect the area's outstanding natural
resources and shellfish industry began in the 1970s, the sea walls closing the Grevelingen
and Veerse Gat estuaries had already been completed, but work on the Oosterschelde