Nutrient Enrichment and Its Effects on the Phytoplankton Populations of the Standing Freshwaters of the Shetland Islands

Biogeochemistry, Limnology, Microbiology, Ecology

Hennessy, Mary Margaret

University:University of Glasgow

关键词: Biogeochemistry, Limnology, Microbiology, Ecology;   

Others  :  http://theses.gla.ac.uk/75701/1/13818418.pdf

来源: University of Glasgow

【 摘 要 】

During the period 1989-1990, phytoplankton blooms were observed in three lochs on Mainland Shetland. Land improvement procedures were taking place in each of the three drainage basins involved. The standing freshwaters of the Shetland Islands constitute a limited resource, having amenity value in terms of brown trout fishing, potable water supply and nature conservation. There was therefore concern that deterioration in water quality might be occurring as a result of fertiliser usage within loch catchment areas. In order to investigate this hypothesis, studies of current water quality, phytoplankton and macrophyte community structure, soil and sediment characteristics were undertaken. Of the thirty one lochs examined, only six were oligotrophic in nature. This indicated that twenty five lochs were at risk of developing excessive algal growth, as once total phosphorus levels exceed 10 mug P L-1 this becomes increasingly likely. In addition, monitoring of loch inflow waters of five drainage basins indicated that in catchment areas incorporating improved grassland, cattle/dairy farming and septic tanks, P concentrations were higher than those of both the receiving waters and the inflows in other watersheds with little anthropogenic influence. Examination of data collected from thirty one lochs on phytoplankton taxa and numbers, combined with information on environmental parameters measured, indicated that certain phytoplankton were associated with particular water column conditions. Green algae were advantaged where enriched waters were relatively high in total phosphorus (TP) and total ammoniacal nitrogen (TAN). Of the blue green algae, Anabaena was found to be successful at the highest concentrations of TP and TAN, in association with low total oxidised nitrogen (TON) levels. Macrophyte community structure was not found to be a useful means to characterise water column trophic status in the sites investigated. Examination of the macrophyte communities was not therefore used in assessing the susceptibility of waters to development of high phytoplankton biomass. This lack of explanation was partly due to the direct influence of sediment quality on macrophyte community structure. Many sediments represent an accessible nutrient source, whilst other types of bottom deposit may either retain low levels of available nutrients, or inhibit macrophyte colonisation. The types of macrophyte which tend to grow in oligotrophic water bodies have little effect on nutrient cycling, whereas those associated with nutrient rich waters may act as nutrient "pumps" from sediment to water column. P adsorption tests indicated that soils with an organic content of >80% were likely to retain little of the P added to them. Low soil pH (pH <4.00) also had a deleterious effect on P retention capacity. P adsorption tests with sediment from four lochs indicated that P retention within the upper 5 cm was related to organic content and magnitude of previous P additions. Soils and sediments rich in Ca, Mg or Fe and low in organic content retain a higher proportion of the P applied to them, than those poor in cations and high in organic matter. The more organic soils and sediments also release P which has been added to them previously. After considering the literature on ameliorative measures in enriched loch systems, it was concluded that implementation of appropriate catchment management procedures in order to prevent development of nutrient enrichment difficulties was a more appropriate approach than attempting to improve water quality after enrichment has occurred. Using an established model, existing P loadings from five drainage basins were estimated. In the watershed with the highest proportion of the catchment undergoing pasture improvement, P export coefficients were found to be greatest. The combined data from inflow water quality, soil and sediment nutrient retention and estimates of nutrient runoff indicate that the fresh waters of Shetland are clearly susceptible to enrichment. The deleterious effects which result are those of excessive phytoplankton growth. In order to prevent further deterioration of water quality, caution must be exercised in the management of both point and non-point nutrient discharges in the future.