Laboratory feasibility tests improve decision support for wetland eco-engineering; Lake Bartkow Lug (Poland) as test case

摘要

Many lakes worldwide are exposed to high anthropogenic impacts including the influx of polluted surface water and groundwater, fish production and nutrient run-off from agriculture in the catchment. We selected the shallow Lake Bartk6w tug in Poland, receiving eutrophic River Chodelka water and currently used for fish farming as a case study for decision support with respect to inexpensive ways to improve water quality and facilitate higher biodiversity after fish stock removal. We selected four scenarios: (1) complete isolation from eutrophic water, (2) inflow of P-stripped surface water, (3) temporary water table drawdown, and (4) in-lake P sequestration. For financial reasons, dredging was not an option. The study started with an ecohydrological field survey to investigate the first scenario: isolation from river water inflow and restoration of the discharge of nutrient-poor, Fe-rich groundwater. Although groundwater appeared to be much less eutrophic, it contained low concentrations of Fe and high concentrations of SO_4~(2-), which would lead to higher risks of phosphate mobilization from the sediment. Moreover, local hydrology showed almost no discharge of groundwater with the exception of spring. To reduce the risk of failure and avoid high costs without benefits, we pre-tested the other three scenarios using small-scale laboratory experiments. The first experiment, which tested the effects of the influx of water stripped of P (scenario 2), showed that water quality would not improve due to high P-mobilization rates from the sediment to the water layer (internal eutrophication). We therefore investigated two alternative mitigation measures: temporary water table drawdown (scenario 3) and the addition of Fe (scenario 4), both in order to increase P-sorption in the sediment. Temporary lowering of the water table indeed decreased P concentrations in pore water, but not in surface water because sediment Fe concentrations were insufficient. The addition of Fe to the sediment strongly improved water quality by P sequestration leading to almost complete disappearance of P in both pore water and surface water. For the studied lake, successful improvement of the water quality and biodiversity can therefore only be achieved by scenario 4, internal P fixation, after the cessation of fish production. The removal of fish only will not lead to improvement, because of the strong mobilization of P in the sediment. We show that the combination of field research and small-scale laboratory feasibility tests strongly reduces the risks of failure, and avoids less efficient and more costly approaches based on trial and error in the field. We therefore recommend the inclusion of such tests in decision support systems for ecological engineering.