Juha Riihimäki

Retaining agricultural nutrients in constructed wetlands—experiences under boreal conditions

Constructed wetlands are among the eutrophication abatement measures for which farmers may receive public subsidies in Finland. To assess the performance of constructed wetlands under boreal conditions, we monitored the retention of total suspended solids (TSS), total P (TP), dissolved reactive phosphorus (DRP), total N (TN), nitrate+nitrate-N (NOx-N) and ammonium-N (NH4-N) for 15–26 months in three constructed wetlands located in southern Finland. Annual retentions were −5–72% for TSS, −6–67% for TP, −33–33% for DRP, −7–40% for TN, −8–38% for NOx-N and −50–57% for NH4-N. The constructed wetland with the longest water residence time (WRT) showed the best performance, retaining annually about 25 kg of TP and 300 kg of TN per hectare. In contrast, the constructed wetland with the shortest WRT functioned only occasionally and was a net source for DRP and NOx-N. In addition to the WRT, high P sorption capacity of constructed wetland soil and high input concentrations appeared to promote retention. Vegetation had a limited effect, input load being transported mostly outside the growing season. When carefully designed and located, constructed wetlands may efficiently reduce nutrient loading from agriculture.

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.

Regional and local determinants of macrophyte community compositions in high-latitude lakes of Finland

Species distributions are structured by regional and local determinants, which operate at multiple spatial and temporal scales. The purpose of our work was to distinguish the relative roles of local variables, climate, geographical location and post glaciation condition (i.e., delineation between supra- and subaquatic lakes during the post-glacial Ancylus Lake) in explaining variation in macrophyte community composition of all taxa, helophytes and hydrophytes. In addition, we investigated how these four explanatory variable groups affected macrophyte strategy groups based on Grime’s classification. Using partial linear regression and variation partitioning, we found that macrophyte communities are primarily filtered by local determinants together with regional characteristics at the studied spatial scale. We further evidenced that post glaciation condition indirectly influenced on local water quality variables, which in turn directly contributed to the macrophyte communities. We thus suggest that regional determinants interact with local-scale abiotic factors in explaining macrophyte community patterns and examining only regional or local factors is not sufficient for understanding how aquatic macrophyte communities are structured locally and regionally.

Experimental revegetation of the regulated lake Ontojärvi in northern Finland.

Water level regulation causes large-scale ecological changes in the littoral areas of lakes in northern Finland. If the summertime water level is raised, intensive erosion processes begin, causing a sudden decline in shore vegetation. The need for shore protection is obvious in areas of high recreational value. At lake Ontojärvi, planting experiments with littoral helophytes and bushes were carried out during the years 1990–92. All the experiments were carried out in the eroded sandy areas, which were partly protected by mechanical barriers. Several plant species were planted on the shore which had been treated with different peat mixtures, etc. The frequencies of the different species were followed monthly. After the first summer, the average survival rates were about 45 % due to the drying of seedlings. A rapid decrease in the survival rates took place during the high water level period in 1991 at which point only 20% of the planted individuals were alive. The best results were obtained for the helophyte Carex rostrata Stokes, of which 30% had survived erosion. Tall willows (Salix phylicifolia L.) were also erosion-resistant with a survival rate of 80%.