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Baseline Study for the restoration of a formerly oligotrophic, presently eutrophicated lake in northern Germany

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Islam, Z. (2009): Baseline Study for the restoration of a formerly oligotrophic, presently eutrophicated lake in northern Germany , Master thesis, University of Bremen.
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Abstract:

Silver Lake (―Silbersee‖) is situated in the sandy Saalian moraine landscape east of Bremerhaven (Northern Germany), is about 8.0 m deep, and covers approximately 6.5 hectares. The lake is a nature reserve but, a part is nevertheless used for bathing and angling. In order to understand the water and sediment chemistry of the lake water samples were collected monthly from three different sites, while sediment samples were collected once from three other sites in the central deep area of the lake. During the summer, surface water is warmer than the bottom water, thus a thermocline with steep temperature changes is created. Water analysis indicated that P and N concentrations are too high to classify the lake as ―oligotrophic‖ as it was in former times. This is further supported by the oxygen depletion in the deep water body during summer. The pH of the lake water is weakly acidic (about 6.5). The source of the water is only precipitation, and almost no water comes from outside the lake. Even in shallow waters the brownish colorations of the water and its increased turbidity by phytoplankton provide light conditions that are disadvantageous to rare primary plants of the Littorelletea-community with the exclusive occurrence of Isoetes lacustris in Lower Saxonia (northwestern Germany). These plants are further endangered by the accelerated eutrophication process reflected in the increased competition of the emergent vegetation as well the coverage (overgrowth) by filamentous algae. Sediment analyses indicate that C, N and P contents are high in the near-the-surface layer but decline downwards in the sediment. The sediments are to be regarded as potential sources especially of P for the water nutrient regime. To evaluate the effect of filter-feeding bivalves on water quality, measurements were taken to estimate the respiration and ingestion rates of the swan mussel Anodonta cygnea, which is now abundant in Silver Lake. The result of the respiration rate experiment had a low, stable rate at about 3.07 mg O2 d-1 per average individual at a temperature of 10°C and ingestion rates were calculated at 47.5 μg Chl-a d-1. They can ingest only 1.9% of the assumed total Chl-a present in Silver Lake, which is not significant amount. So, the study presumes that filter feeding effects have not the sufficient potential for reducing the Chl-a from the Silver Lake Lake restorations are attempted to improve water quality and life conditions, aesthetic and recreational needs. This study advises i) to remove macrophytes, ii) taking out sediments, iii) to control of runoff from adjacent farm land, iv) to cut down the deciduous trees from the bank area and to remove live and dead material from the water edge v) some regulations for bathing and angling people as well as vi) mechanically ventilate the deep water of the lake during summer. Future research should however be undertaken to get insights in the production, the fluxes of nutrients and practibality of production control by fish as well as the possible dystrophication influences from the adjacent raised bog and the emergent vegetation before a very strong effort is put into restoration measures.

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