Abstract: Final report on the biophysical modelling of Lake IJsselmeer
Lake IJsselmeer in the Netherlands provides fresh water supply, recreation, fisheries and is classified as a NATURA 2000 site. At present, the main environmental concerns regarding the lake are decreasing numbers and diversity of birds. It is suggested this is due to decrease in food biomass and a deterioration of the food quality. Climate change, nutrient reductions and direct human interference are expected to increase the stress on ecological functioning further.
An extended Delft3D was used to explore the effects of changes in climate and land use on fresh water biota in the lake . This model comprises not only the usual water quality (nutrients) and algal modelling (biomass and species composition), but also the modelling of mussel biomass and its effect on algae.
The scenarios simulated were based on climate change and land use change scenarios. The scenarios simulated were:
In summary, changes in climate and land use exert effects on the lake’s ecosystem via changed nutrient availability:
- If temperature only increases (climate change), primary production is increased.
- If nutrient loads are reduced (land use change) without climate change, primary production is decreased.
- A combination of increased temperatures and decreased nutrients lead to decreased primary production.
- In general, nutrient reductions exert larger effects on the fresh water biota as Lake IJsselmeer is a nutrient limited lake.
Changes in climate and land use exert effects on the lake’s ecosystem via changes in habitat suitability for species:
- The level of increased temperatures (climate change) poses stress on zebra mussels and smelt and lowers their habitat suitability.
- Decreased light extinction (land use change) lead to higher transparency, and affects the habitat suitability of smelt.
- The effects of temperature and/or nutrient reductions are species dependent. For instance, zebra mussels experience more effect from temperature increases and smelt from nutrient reductions.
- Birds experience via their food effects of climate change and land use change.
Thus Lake IJsselmeer is already under stress in terms of carrying capacity of specific birds (like the pochard, scaup, smew and black tern) and fish species (like the smelt). A further reduction in nutrients will increase this tension as primary production falls and therefore affects the food web. Temperature increase yields higher primary production which could increase the lakes’ productivity. However, the increased temperatures lead to less habitat suitability for the species that are used as bird food nowadays.
Nutrient reductions following land use changes improve the lake WFD status. Thus for maintaining or reaching a good WFD status for Lake IJsselmeer nothing should be done: Lake IJsselmeer is right on track. However, on a species level, climate change and land use change are likely to threaten the habitat suitability of some of the present day species. This again increases the stress on the lake’s ecological functioning and can lead to not reaching the Nature2000 goals.
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