Harri Helminen

Long-term management of Pyhäjärvi (southwest Finland): eutrophication, restoration – recovery?

Abstract Pyhäjärvi, located in the centre of an intensive agricultural area in southwest Finland, is an example of a lake suffering from eutrophication. The lake has been intensively studied for decades and was the object of comprehensive restoration activities both in the catchment and in the lake since the 1990s. During the last 20 years the quality and general usability of water in Pyhäjärvi has deteriorated due to increased algal blooms but has shown some signs of recovery during recent years. These changes have been driven by both a variety of human activities and natural climate related factors such as dry years. Pyhäjärvi has been the object of intensive biomanipulation for decades, carried out by commercial fishermen, whose annual harvest rate approaches the total production of vendace (Coregonus albula), the main planktivore in Pyhäjärvi. The restoration project has also subsidized the harvest of commercially unwanted fish since 1995. In 2002–2006, the EU provided funds for this fishing, which was especially intensive in 2002–2004. The main goal of the future management is to maintain and ensure the current levels of moderately low algal biomass considered acceptable by the financiers and local users of the lake. Achieving the level of water quality during the 1980s is not currently realistic due to current intensive agricultural use of the catchment, lack of cost-effective tools for load reduction from the agriculture, and climate change threats.

Growth and food consumption of vendace (Coregonus albula (L.)) in Lake Pyhäjärvi, SW Finland: a bioenergetics modeling analysis

We used a bioenergetics model to estimate daily food consumption of young-of-the-year (YOY) and adult vendace in the mesotrophic Lake Pyhajarvi. Weekly samples of fish and zooplankton, plus temperature data, were collected from May to October 1987, to describe the growth and diet of vendace. The estimates of population size were based on spring abundance of YOY and catch statistics. Information on the physiological energetics of vendace was derived from literature sources. The YOY showed steady growth throughout the summer, but the adults did not grow at all from late July to late August. The calculated daily food consumption of the vendace population was the highest in August. After mid-June, the YOY were the dominant consumers and were responsible for 83% of the cumulative food consumption of the whole vendace population in June—October. This consumption, 22 g wet mass m−2, was only 13% of the total crustacean zooplankton production, but about twice the production of Bosmina or Eudiaptomus, the two most important single prey taxa. At certain times the food consumption of vendace was high enough to influence the population dynamics of its major prey species.

Restoration of Finnish lakes using fish removal: changes in the chlorophyll—phosphorus relationship indicate multiple controlling mechanisms

Lake restoration through biomanipulation, primarily through fish removal, has been widely tried as a complementary tool for the improvement of water quality in heavily eutrophicated lakes (Moss et al. 1997). Successful results obtained especially in northwestern Europe (e.g. ]EPPESEN et al. 1990, Moss et al. 1997, HANssON et al. 1998, MEIJER et al. 1999) have aroused interese in this method among lake manager~. However, fish removal has not always led to the deSired results, and there have been divergent opinions about the actual mechanisms effective in successful cases (e.g. HORPPILA et al. 1998). Examination of the chlorophyll-phosphorus relationship may help to distinguish between different mechanisms, and to evaluate the probability of success prior to biomanipulation. Between-lake scatter around the general relationship between phytoplankton chlorophyll and total phosphorus in water has been associated with the abundance of planktivorous fish and/or large herbivorous crustacean woplankton (MAz.UMDER 1994) for a given phosphorus concentration, lakes with abundant planktivorous ~sh, which usually ha~e reduced abundance o f largesized crustacean herb1vores, have higher chlorophyll than lakes with low fish biomass. However, as found in lake restoration through reduction of phosphorus loading (SAS 1989), such between-lake trends are not necessarily good predictors of temporal withinlake change. For several years now, we have been studying the potential of biomanipulation in a suite of lakes in southern Finland which have different levels of external loading and which are being restored in different ways. In one of these lakes, Lake Pyhiijiirvi (SW Finland), we showed earlier that between:year ~ariation in the chlorophyll-phosphorus relanonsh1p was associated with changes in the total biomass of planktivorous fish (HELMINEN & SARVALA 1997, SARVALA et al. 1998a). In the present paper, we analyse the chlorophyll-phosphorus relationship in several other Finnish lakes in which fish removal has been applied in order to improve water quality.

Eutrophication and fish biodiversity in Finnish lakes

Summary Eutrophication due to excessive nutrient loading is a major factor affecting Finnish aquatic ecosystems. In this work we compared the biodiversity of fish in lakes of different trophic status to determine what changes occur under eutrophication. Our work was based on data sets collected using standard multimesh gillnets. In eutrophicated waters cyprinids dominate but we found also that in hypereutrophic lakes even a short-term perturbation, such as oxygen deficiency, may lead to sudden changes in species diversity. In essence, eutrophication has a negative impact on lakes by decreasing their fish biodiversity. This work is part of the Finnish Biodiversity Research Programme FIBRE (1997–2000).

Impacts of nutrient enrichment and sediment on phytoplankton community structure in the northern Baltic Sea

A three-week mesocosm experiment was conducted in order to study the effects of bottom sediment and nutrient enrichment on phytoplankton and zooplankton community structure in the Archipelago Sea, northern Baltic Sea. The transparent polyethylene enclosures included the whole water column and varied in volume from 30 to 40 m3. There were two types of enclosures: some with natural sediment as a bottom and others with a plastic bottom. The experiment was a 2 × 2 factorial design with presence of sediment and nutrient enrichment as treatment factors. Both the sediment presence and nutrient enrichment significantly increased water nutrient concentrations and the rate of primary production. However, external nutrient enrichment and the presence of sediment stimulated the growth of different phytoplankton groups, indicating that the effect of sediment was not related to nutrient fluxes alone, but involved more complex interactions. External nutrient enrichment was primarily channelled to picoplanktonic cyanobacteria, the biomass of which increased four- to fivefold due to enrichment. The presence of sediment increased the biomass of cryptophytes, chrysophytes and prasinophytes, but decreased the biomass of N2-fixing cyanobacteria. Zooplankton biomass increased during the experiment, but was not affected by the treatments. The study shows that sediment plays a significant role in phytoplankton dynamics, underlining the importance of including sediment in shallow-water mesocosm experiments.

Selective removal fishing — water quality and practical viewpoints

Removal fishing can be an effective method for restoration of eutrophied lakes (J EPPESEN & SAMMALKORPI 2002, COOKE et al. 2005). Costs may become very high in large lakes, however, which makes it difficult to maintain efficient fishing effort to achieve permanent results. In large, mesotrophic Lake Pyhãjãrvi in southwest Finland (Table l), a profitable commercial fishery targeting planktivorous vendace ( Coregonus albula) has for decades acted as unplanned biomanipulation, and the existing fishery infrastructure has made it possible to develop large-scale intentional and intensive fish removal operations (VENTELÃ et al. 2007). As a specialized zooplankton feeder, vendace i s akeystone species in the pelagic system, controlling zooplankton an d indirectlyphytoplankton (HELMINEN & SARVALA 1997). In the 1980s and early 1990s there was a positive correlation in Pyhãjãrvi between vendace year-class strength and the Iate summer values of chlorophyll a concentration, while zooplankton biomass was negatively correlated with both vendace year-class and chlorophyll a concentration, indicating an intermediary role of zooplankton between vendace and phytoplankton (HELMINEN & SARVALA 1997). Fishing of vendace has been very efficient; nearly 90 % o f the youngest age group is fished out yearly (SARVALA et al. 1998). Fishing methods include mainly winter seines (usually 8 seining groups in the 1980s, but mostly 4 since 1995) and f)rke nets in spring and autumn. In the 1990s, water quality in Lake Pyhãjãrvi deteriorated in spite of continued commercial fishing and weak vendace stock: phosphorus concentrations and phytoplankton biomass increased, and cyanobacterial blooms became more frequent, harming the recreational uses of the lake and even causing taste problems in the fish (VENTELÃ et al. 2007). The vendace population collapsed because of failed recruitment in 2 successive years due to unfavourable weather conditions and excessive predation pressure and remained small due to temporary overfishing (SARVALA & HELMINEN 2002). As a result of competitive release, the other fish species became more abundant and caused adverse water quality effects (SARVALA et al. 1998). The eutrophication of Pyhãjãrvi is connected to high extemal nutrient load from the catchment. The lake received industria! and community waste waters Table l. Description ofLake Pyhãjãrvi. Nutrient and chlorophyll a values represent averages ± SD during the open water periods (May-Oct) during 2000-2005.