Heidi Holmroos

Resuspension-mediated temporal variation in phosphorus concentrations and internal loading.

Sediment resuspension is an important factor for nutrient cycling in shallow lakes. Temporal variation in sediment resuspension and its influence on P concentrations and internal loading were studied in the shallow and eutrophic Kirkkojärvi basin. Gross sedimentation and sediment resuspension were estimated with sediment traps during three exposure periods in spring, midsummer, and autumn. The effects of resuspension on the concentrations of suspended solids, total phosphorus (TP), soluble reactive P, and chlorophyll a were followed. The level of sediment resuspension was greatest in midsummer (July-August 63.9 g dw m(-2) d(-1)), due to strong phytoplankton blooms modifying the surface film of the sediment more prone to resuspension. The dependence of high total P concentrations (226-385 microg L(-1)) on sediment resuspension and suspended solids was significant at the time. The concentrations of soluble reactive P were also high in midsummer (52-91 microg L(-1)), but had a negative dependence on suspended solids. This dependence was due to algal assimilation as was proved by a negative effect of chlorophyll a on soluble reactive P. Consequently, we suggest that the increase in the total P concentrations caused by sediment resuspension in midsummer was possibly intensified by strong algal blooms occurring at the time because strong assimilation resulting in high pH (pH 7.3 in May, 9.3-10.2 in July-August, 7.6 in October) may have increased the desorption of P from the suspended inorganic particles and intensified the P transfer into the phytoplankton biomass in midsummer.

The actual role of oxygen deficit in the linkage of the water quality and benthic phosphorus release: Potential implications for lake restoration

Human activities in watersheds have resulted in huge accumulations of phosphorus (P) in sediments that have subsequently hindered restoration efforts of lake water quality managers worldwide. Much controversy exists about the factors that control the release of P from sediments (internal P loading). One of the main debates concerns the role of oxygen deficit (anoxia) in the regulation of water quality. Our results based on a comprehensive set of lakes worldwide demonstrate that internal P loading (IPtot) plays a significant role in water quality regulation. Internal P loading due to anoxia (IPanox) contributes significantly to the IPtot. However, this contribution is insufficient to significantly increase the chlorophyll a (Chl a) concentration in stratifying lakes. In the lakes of the north temperate and boreal zone, this is because the IPanox reaches surface water layer in the end of the growing season. Observed water quality implications of IPtot are most likely caused by the sedimentary P that actually originates from the shallow areas. These findings suggest limitations for the use of aeration (improvement of the oxygen conditions in the hypolimnion) in lake water quality restoration. Moreover, lake ecosystem managers can benefit from our model that enables to predict anoxia triggered sedimentary P release from the combination of lake characteristics. The final decision on the use of aeration is indeed unique to each lake, and lake specific targets should be considered.

Persistency of artificial aeration at hypertrophic Lake Tuusulanjärvi: A sociohistorical analysis

Abstract With present-day scientific evidence challenging the efficiency of artificial aeration as an effective restoration method for eutrophicated lakes, our sociohistorical investigation traces the reasons for the persistent support for this method in Finland, where about one hundred lakes are subject to this treatment. Our study employed the concepts of technological path and aeration frame to analyze the extensive restoration and aeration history of the hypertrophic Tuusulanjärvi in southern Finland. Continuously aerated since 1972, it has the longest history of aeration in Finland. Qualitative analysis of documentary and archival sources revealed that the longstanding preference for aeration in the context of increasing scientific controversy was based on its functional versatility and seemingly unproblematic applicability in regard to shifting emphasis and goal setting of restoration. Additionally, the stability of the aeration frame has been supported by the practical and emotional attachment of local residents to lake restoration, particularly aeration, and finally the problems and contradicting interests related to alternative restoration methods.

Aeration-Induced Changes in Temperature and Nitrogen Dynamics in a Dimictic Lake.

Low levels of oxygen (O) in the hypolimnion layer of lakes are harmful to benthic animals and fish; they may also adversely affect nutrient cycles. Artificial aeration is often used in lake management to counteract these problems, but the effects of aeration on nitrogen (N) cycling are not known. We studied the effects of hypolimnetic aeration on N dynamics and temperature in a eutrophic lake by comparing continuous and pulsed aeration with a nonaerated station. Aeration decreased the accumulation of NH-N deep in the lake (20-33 m) by supplying O for nitrification, which in turn provided substrate for denitrification and promoted N removal. Aeration also increased the temperature in the hypolimnion. Denitrification rate was highest in the nonaerated deep areas (average, 7.62 mg N m d) due to very high rates during spring turnover of the water column, demonstrating that natural turnover provides O for nitrification. During stratification, denitrification was highest at the continuously aerated station (4.06 mg N m d) and lowest at the nonaerated station (3.02 mg N m d). At the periodically aerated station, aeration pauses did not restrict the increase in temperature but resulted in accumulation of NH-N and decreased the contribution of denitrification as a nitrate reduction process. Our findings demonstrate that hypolimnetic aeration can substantially affect N cycling in lakes and that the effect depends on the aeration strategy. Because N is one of the main nutrients controlling eutrophication, the effects of aeration methods on N removal should be considered as part of strategies to manage water quality in lakes.