Antton Keto

The effect of water level regulation on species richness and abundance of aquatic macrophytes in Finnish lakes

More than 300 lakes, including most of Finlands large lakes, are under artificial water level regulation d ue to hydro power production or flood protection. In Finland, several research projects related to regulated lakes have shown the central ro le o f water level regulation as a source of hydro-morphological disturbance ( e.g. MARTTUI\u.; e t al. 200 l). Changes in water level fluctuation may have harmful impacts on the littoral zone and especially on aquatic macrophytes (R0RSLETT 1985, 1989, HELLSTE\; 2001, 2002, Hill et al. 1998, and NrLSSO\; et al. 1997). The main aim o f this study was to compare the species richness an d abundance o f aquatic macrophytes between regulated and unregulated lakes and distinguish major environmental factors affecting them.

The relationship between water level fluctuation and distribution of emergent aquatic macrophytes in large, mildly regulated lakes in the Finnish Lake District

Many emergent aquatic macrophytes have a wide distribution over 1arge areas ofthe world (ScULTHORPE 1967). Of these, he1ophyte species usua11y form the most notab1e part oflake vegetation and the zonation ofthe 1ittora1 (LoHAMMAR 1965). In the northem European countries, a 1arge proportion of the tota1 lake area is regulated, main1y for hydropower production purposes, and this determines eco1ogical conditions o f the lake vegetation (NILSSON 1981, R0RSLETT 1988, HELLSTEN et al. 1996). The vegetation is distributed into zones along the summer water level on a specific shore based on several environmental properties, such as exposure, substratum, light attenuation and 1ife form (QUENNERSTEDT 1958, HEJNY 1971, MÃKIRINTA 1978). In regu1ated lakes the effects of water leve1 regu1ation are comp1icated, vary from 1ake to 1ake and are a1ways dependent on frequency, amplitude and the water leve1 in certain time sequences (HEJNY 1971, R0RSLETT 1984). Ear1ier studies have shown the connection between water level fluctuation and the vertica1 zones o f helophyte vegetation (HELLSTEN 200 l). This relationship has 1ed to a number of app1ications used in hydropower assessment studies in Fin1and (HELLSTEN et al. 2002a). Despite wide use of this model, verification with historica1 data has not been app1ied, and speciesspecific responses to water level fluctuation have been shown only vertically. The present study aims to combine the modeled vertical changes of the most frequent helophytes, Phragmites australis, Equisetum jluviatile and Carex sp., with the observed changes of their cover areas by using comparisons with aerial photos over a period o f approximately 50 years.

Long-term changes in the aquatic vegetation of Lake Päijänne, Southern Finland

The environmenta1 changes of Lake Pãijãnne represent typical examp1es of human induced changes in the 1ake district of Fin1and. Heavy point source pollution and water Ievel regulation during the I 960s have caused significant changes in the ecological status ofthe 1ake, which is characterised by several subareas with different limnological properties (MERILÃINEN et al. 200 l). Intensive water protection measures during the 1980s have drastically improved water quality, but the overgrowth of aquatic vegetation, especially the expansion of common reed (Phragmites australis Trin ex. Steudl), is still significant (HELLSTEN 2000, KETO et al. 2002). In addition to ecological status, the increase in aquatic vegetation has caused significant changes to the recreational use ofthe lake. Increased demand for restoration measures an d awareness o f environmental issues brought about an interest to study the factors causing this change. The main aim o f this study is to compare the development of aquatic vegetation with aerial photos and species composition from different decades before and after major human impact. The main factors affecting the vegetation are also described by comparing the vegetation of Lake Pãijãnne to reference Lake Keitele and three smaller lakes, Saraavesi, Kuhnamo and Vatia, with higher nutrient status but a relatively natural water level fluctuation regime.