Baseline Study for the restoration of a formerly oligotrophic, presently eutrophicated lake in northern Germany
Year: 2009
Type: dissertation
Abstract: Silver Lake (―Silbersee‖) is situated in the sandy Saalian moraine landscape east \nof Bremerhaven (Northern Germany), is about 8.0 m deep, and covers \napproximately 6.5 hectares. The lake is a nature reserve but, a part is \nnevertheless used for bathing and angling. In order to understand the water and \nsediment chemistry of the lake water samples were collected monthly from three \ndifferent sites, while sediment samples were collected once from three other sites \nin the central deep area of the lake. During the summer, surface water is warmer \nthan the bottom water, thus a thermocline with steep temperature changes is \ncreated. Water analysis indicated that P and N concentrations are too high to \nclassify the lake as ―oligotrophic‖ as it was in former times. This is further \nsupported by the oxygen depletion in the deep water body during summer. The \npH of the lake water is weakly acidic (about 6.5). The source of the water is only \nprecipitation, and almost no water comes from outside the lake. Even in shallow \nwaters the brownish colorations of the water and its increased turbidity by \nphytoplankton provide light conditions that are disadvantageous to rare primary \nplants of the Littorelletea-community with the exclusive occurrence of Isoetes \nlacustris in Lower Saxonia (northwestern Germany). These plants are further \nendangered by the accelerated eutrophication process reflected in the increased \ncompetition of the emergent vegetation as well the coverage (overgrowth) by \nfilamentous algae. Sediment analyses indicate that C, N and P contents are high \nin 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 \nnutrient regime. \n To evaluate the effect of filter-feeding bivalves on water quality, \nmeasurements were taken to estimate the respiration and ingestion rates of the \nswan mussel Anodonta cygnea, which is now abundant in Silver Lake. The result \nof the respiration rate experiment had a low, stable rate at about 3.07 mg O2 d-1 \nper average individual at a temperature of 10°C and ingestion rates were \ncalculated at 47.5 μg Chl-a d-1. They can ingest only 1.9% of the assumed total \nChl-a present in Silver Lake, which is not significant amount. So, the study \npresumes that filter feeding effects have not the sufficient potential for reducing \nthe Chl-a from the Silver Lake \n Lake restorations are attempted to improve water quality and life \nconditions, aesthetic and recreational needs. This study advises i) to remove \nmacrophytes, ii) taking out sediments, iii) to control of runoff from adjacent farm \nland, iv) to cut down the deciduous trees from the bank area and to remove live \nand dead material from the water edge v) some regulations for bathing and \nangling people as well as vi) mechanically ventilate the deep water of the lake \nduring summer. \nFuture research should however be undertaken to get insights in the \nproduction, the fluxes of nutrients and practibality of production control by fish as \nwell as the possible dystrophication influences from the adjacent raised bog and \nthe emergent vegetation before a very strong effort is put into restoration \nmeasures.
Author: Zahidul Islam
Language: English
Subfield: Environmental Chemistry
Field: Environmental Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W203516263