Implication of forms and distribution of phosphorous in sediments of three Stavanger lakes and ponds of constructed wetland on the potential for sedimentary phosphorous release and future eutrophication.

Abstract

Excess of bioavailable phosphorus in fresh-water systems results in algal blooms and subsequent biogeochemical activities which in turn as positive feedback can escalate eutrophication and significantly reduce water quality. The forms of sedimentary phosphorus is major factor determining internal release of this nutrient. Knowledge about phosphorous forms and distribution can contribute in prediction of potential lake response to nutrient enrichment and other biochemical changes in the water systems, and it can guide in planning of measures to maintain good water quality. This study is focused on phosphorous forms and dynamics in sediments of three urban lakes: Mosvatnet, Hålandsvatnet and Stora Stokkavatnet, which are geographically close, but have different morphological and trophical features. The study has the aim to assess potential for sedimentary phosphorus release and future eutrophication development. The results showed differences in phosphorus distributions both vertically with sediment depth and laterally between locations in the lakes. Relations to a number of factors such as mixing regime, presence of oxygen, water temperature and pH level were observed. It was determined that the dominant forms of phosphorus in the all studied lakes sediments were bound with iron (oxyhydr)oxides which is a redox sensitive form and potentially mobile, and bound to organic matter which can be released by biological mineralization. Eutrophic lake Hålandsvatnet currently has the highest sedimentary phosphorous concentrations of the three lakes. Due to anoxic conditions phosphorous is released from deep-water sediment and circulates into epilimnion maintaining high productivity. Since phosphorous is depleted in lower sediment layers because of long-term diagenetic release, the sediment may be capable only to sustain high release rates over limited time periods, and faster response to reduction of external nutrient load may in this case be expected. Neighboring mesotrophic lake Stora Stokkavatnet seems to be highly vulnerable to increase in external phosphorus load, because it can contribute to develop anoxic conditions in hypolimnion and trigger similar internal anaerobic release of iron bound phosphorus as in Hålandsvatnet. Furthermore, phosphorous in the sediment of a wetland pond constructed to protect Stora Stokkavatnet is dominantly in potentially mobile form and is capable to be released and transported into the lake water. Removal of pond sediment can therefore be a recommendation to protect the water quality in Stora Stokkavatnet. Mosvatnet can also be expected as sensitive to increase in external phosphorus load. Other factors than anoxic conditions can facilitate subsequent internal phosphorous release in this lake, and any acceleration of internal water fertilization may have strong eutrophication impact due to very small water volume compared to the sediment area, and because of direct contact between these two compartments that implies little limitation of transport for released phosphorus to be available for algal uptake.