The large lakes of Sweden (Vänern, Vättern, Mälaren and Hjälmaren) have been monitored for more than four decades for water quality conditions to assess the impact of eutrophication from anthropogenic activities. Lake Vänern is basically an oligotrophic lake that showed signs of eutrophication, notably the emergence of algal blooms in the coastal areas (1967–1968). The lake was also contaminated, due to the discharge of pulp and paper effluents including metals such as mercury. However, ecosystem-based information is lacking for Lake Vänern. Consequently a symposium was organized in 2012 by the University of Gothenburg, Mötesplats Vänersborg and the Aquatic Ecosystem Health and Management Society to: assess the current status of the health of Lake Vänern's ecosystem, identify knowledge gaps and develop a road map for the future. In this regard, Lake Vänern was compared with the North American Great Lakes to learn from their extensive, long-term data sets. A special issue devoted to the “State of Lake Vänern Ecosystem” symposium was published in Aquatic Ecosystem Health and Management (AEHM, Vol. 17, no. 4) including keynotes and contributed papers. The conclusions shown in the appendix (available in the online supplementary information) summarize the authors' contributions. Most of the articles covered Lake Vänern, but some were directed towards monitoring and management of Great Lakes in general, and others addressed co-operation under the auspices of international agreements and directives. Based on the background information provided by the State of Lake Vänern Ecosystem symposium and its publication in the special issue, the conveners decided that a synthesis of the symposium with recommendations for the future would be useful in boosting ecosystem research in Lake Vänern.
Lake Vänern is a large lake (5650 km2) in the south of Sweden. It is also a young lake, created through land uplift after the last glacial period (10,000–8000 years ago) which separated it from the North Sea (Lambeck, 1999). The lake is divided into two basins. To the east is the larger, deeper one, Värmlandssjön, containing most of the inlets (75%) while the outlet through the Göta River is from the western basin, Dalbosjön (Figure 1).
Lake Vänern's drainage basin is more than eight times its size (46,800 km2) with the largest part in the north where the Klarälven River originates in the Norwegian and Swedish mountains. The northern drainage basin is dominated by forest, while agriculture dominates in the south. The total inflow to the lake varies between 300 and 700 m3 s−1 with the highest values observed during fall and spring and lowest in the summer. As a large lake, Lake Vänern has long been a source of pride for Swedes and is well known for its ecosystem services, especially fish, since the middle ages (Drotz et al., 2014). The mercury contamination of fish in Lake Vänern served, however, as an alarm during the environmental enlightenment in the 1960s which lead to the recognition of other types of problems such as eutrophication and changes in fish stocks. It was realized that knowledge of this important natural resource was too limited to manage sustainably. Consequently six agencies, the Environmental Protection Agency, the Fishery Board, the Meteorological Hydrological Institute and three regional boards, collaborated and established a special committee – Vänervattenkommittén (the committee for the water in Lake Vänern). They initiated a five-year research program with the ultimate goal of creating scientific models for predicting the future development of the lake for various uses, which resulted in a range of publications summarized in the report: “Vänern – en naturresurs” by the Swedish environmental protection agency, Statens Naturvårdsverk (SNV, 1978). In many respects this is the best source of knowledge since only a limited number of research projects about the basic conditions of the lake have been carried out since then. In addition some data have been collected by a variety of environmental monitoring programs to support the management of the lake and its tributaries as per Swedish law and the European Union Water Frame Directive (EU WFD; www.viss.lansstyrelsen.se).
Physical and chemical regimes
Data gathered during the 1970s program contained a comprehensive description of the lake's geology, hydrology, water quality, biology, pollution sources and fisheries. This resulted in the generation of new data about the lake and its general hydrographic conditions including a physical model of water movement. Important data dealing with hydrographic features emerged showing a counterclockwise circulation common in Värmlandssjön, as well as in Dalbosjön. During spring, a thermal bar develops that reduces the exchange of water between coastal areas and the pelagic region of the lake resulting in lower temperatures and lower nutrients in the offshore areas. Currents inside the bar are normally counterclockwise while they are clockwise outside of the bar. During summer a thermocline develops which reduces vertical exchanges and also reduces exchanges between the two basins, as shown in the differences of concentrations of nutrients and organic matter. These hydrographic features were described by Kvarnäs (2001). These general conclusions about water movement in the main basins were confirmed by the modeling of Dahl and Wilson (2004). It was concluded that the two basins should be treated separately, while also taking into consideration the stratified conditions of each basin (epilimnion vs. hypolimnion).
The Statens Naturvårdsverk report (SNV, 1978) also provided data about various types of sediments from the pre-industrial period indicating the natural background concentrations of different elements. Mercury contamination has a special history in Lake Vänern since high concentrations of mercury was one of the conditions which triggered the 1970s investigations. Mercury contamination originated from industries in the northern part of the lake, especially the chloralkali plant in Skoghall, which discharged 3000 kg y−1 in the 1960s (Lindeström, 2001) and even up to 1980 it was found to discharge over 10 kg Hg y−1. In the 1970s the concentration in surface sediments in Värmlandsjön ranged between 990–2070 ppb, with peaks of 7350 ppb. In Dalbosjön the concentration was lower, 240 ppb (SNV, 1978). Wihlborg and Danielsson (2006) analyzed sediment cores in different parts of the lake which described how the concentration in sediments decreased from a peak in the 1960s to the current levels. Currently the concentrations are still high but have stabilized since the late 1990s around 300–500 μg Hg kg−1 in the more contaminated parts of the lake (Sjölin, 2012). The high mercury concentration resulted in enhanced contamination of fish, which in Pike (Esox lucius) reached concentrations up to 1.5 mg kg−1 in some bays and consequently the fish in such areas were blacklisted for consumption until the early 1980s.
Lake Vänern is an oligotrophic lake with the exception of some coastal areas. Total phosphorus values in the pelagic areas during the summer are around 5 μg l−1 compared to 15 μg l−1 in the 1970s. Total organic carbon concentration also decreased from 10 mg l−1 in 1973 to 4 mg l−1 in the late 1990s but it has shown a small increase since then probably due to climate changes as well as increased land run-off. It is interesting to note that the reduction in the phosphorus values is not reflected in algal biomass measured as chlorophyll a, for which the concentration has been stable at around 2 μg l−1 since the early 1970s.
Studies of phytoplankton composition revealed a typical spring bloom dominated by diatoms in April–May while different groups of flagellates dominated during the rest of the year. In bays and coastal areas the situation can be very different as seen in a recent monitoring report where chlorophyll a concentrations over 20 μg l−1 were found in one of the four bays during 2009–2013 (Stål Delbanco and Olbers, 2014). Zooplankton biomass and species composition also represented oligotrophic conditions (SNV, 1978) and no recent trends were apparent (Peilot, 2014). The same holds true for the sediment fauna, which outside the archipelagos areas is poor both qualitatively and quantitatively. The biomass is dominated by Pontoporeia affinis (now Monoporeia affinis) while different oligochaetes have the highest number of individuals (SNV, 1978).
Lake Vänern has the highest number of reproducing fish species in Sweden. Perch (Perca fluviatilis) and Roach (Rutilus rutilus) dominate in open coasts areas while Roach and Bream (Abramis brama) dominate in coasts with vegetation. Fish like Zander (Sander lucioperca), Pike and Eel (Anguilla anguilla) are also found. In the deeper parts of the lake Burbot (Lota lota), Smelt (Osmerus erlangus), Lake Whitefish (Coregonus clupeaformis) and Ruffe (Gymnocephalus cernua) are also found with Burbot dominating. Pelagic fishes ranged from Vendace (Coregonus albula), Burbot, Zander, Pike, Salmon (Salmo salar) to Trout (Salmo trutta). All these species of fishes are affected by fishing. However, Salmon and Trout populations are supplemented by stocking programs. Migratory fish, especially, Salmon and Trout are strongly affected by water power generating plants. For both these species there are several stocks (populations) which are unique to specific rivers.
Since very little was known about Lake Vänern's ecosystem compared to other large lakes, an international symposium on the “State of Lake Vänern Ecosystem – Past, present and future (SOLVE),” was organized in Vänersborg, Sweden from 11–14 June 2012. The symposium was divided into 10 sessions including a poster session. The scope of the presentations was not restricted to Lake Vänern which was in line with one of the objectives of the symposium to study the lake from a broader perspective. With these objectives in mind, three keynotes were invited who had long term experience in the North American Great Lakes to provide some advice and guidance (Minns, 2014; Nalepa, 2014; Munawar et al., 2014). The program and the power point presentations from the symposium can be found at the website: http://www2.dpes.gue/project/vanern2012/.
Another objective of the SOLVE symposium was to publish papers resulting from some of the presentations in a special issue of Aquatic Ecosystem Health and Management (AEHM). This was accomplished in issue 17(4); the conclusions and recommendations from these articles are summarized in the appendix (available in the online supplementary information). The subject of the articles has been divided into three parts: (1) State of Lake Vänern Ecosystem, (2) State of Great Lakes Ecosystems and (3) International Management and Agreements. The conclusions shown are essentially those made by the authors of the articles (AEHM, 2014) which are summarized in the appendix.
Future: Recommendations and suggestions
A review of the published literature indicates that Swedish freshwater research programs were one of the first to focus on pollution remediation and phosphorus abatement (Willén, 2001a,b; Wilander and Persson, 2001). An excellent example of lakewide water quality research was the Lake Mälaren Project that began in 1964 which examined both physico-chemical and biological characteristics (Willén, 2001a). However there are some information gaps especially for lower trophic levels which need attention in future projects in Lake Vänern as well as other Swedish lakes. A glance at the present issue shows that only two articles dealt with the lower trophic levels (i.e. one each on phytoplankton and zooplankton). The current synthesis therefore attempts to highlight gaps of knowledge which need attention in future investigations.
The design of a research and monitoring program is an extremely difficult task that needs experience and thorough planning. The bi-national research program of the Laurentian Great Lakes which has evolved over nearly 50 years could serve as an excellent example for developing a Lake Vänern research plan in the future. The special issue (AEHM, Vol. 17, no. 4) includes articles by Dave and Munawar; Munawar et al.; Minns; and Nalepa, which offer some lessons. Similarly, the article by Philips offers a timely review of the European Union's guidelines for monitoring water quality. Based on these and other studies, a multi-pronged assessment strategy is outlined below:
Integrated, science-based research
An integrated science-based strategy is proposed that includes both spatially and temporally intensive sampling; monitoring of various physical and chemical parameters as well as structural and functional assessments of the biological community (Figure 2) based on the Great Lakes program. This may be a good example to adopt for Lake Vänern in some form or fashion to fill in the data gaps.
Lake-wide and intensive sampling
Lake wide (spatial) sampling and temporally intensive sampling of selected stations are suggested to complement the research on Lake Vänern (AEHM, 2014; Ambio, 2001). A lake-wide spatial program at seasonal intervals, ideally spring, summer and fall adopting the Great Lakes pattern, is proposed. A North-South transect sampling plan is recommended for Lake Vänern (Figure 1, dashed lines) with six sampling stations per transect based on bathymetry. Also an intensive weekly or biweekly sampling program is suggested to be conducted at selected index stations within the seasonal transects, thereby increasing sampling frequency (Willen, 2001b) and generating robust and publishable data (Willén, 2001a). This type of temporally intensive sampling has been carried out continuously for over 40 years in the Bay of Quinte, Lake Ontario (Minns et al., 2011; Munawar et al., 2011, 2012).
Physico-chemical and biological parameters
It is important that various physical and chemical parameters continue to be measured including: temperature, dissolved oxygen, transparency, total phosphorus, soluble reactive phosphorus, nitrite + nitrate, silica and chlorophyll a, among others, that form the backbone of most monitoring programs and provide readily comparable data with other aquatic environments. Newer technologies including temperature and oxygen sensors with data loggers have created opportunities for continuous monitoring. In addition, lake-specific pollutants and pollutants of general concern (such as those listed in the European Union's Water Framework Directive), also need regular monitoring.
Structural assessment of lower trophic levels
A comprehensive structural assessment includes the biomass and composition of the microbial food web (bacteria, autotrophic picoplankton, heterotrophic nanoflagellates, and ciliates), phytoplankton, zooplankton and benthos. The taxonomic expertise for lower trophic levels, particularly phytoplankton and microbial loop, are in short supply and there is a great need for individuals who can analyze species and size composition accurately and consistently (Willén, 2001b; Munawar et al., 2014). We believe that robust data and a long-term data base is extremely important for assessing the status and ecosystem health of the lower trophic levels to provide early warning indicators of food web alterations and disruptions that will affect the management of sustainable fisheries
Functional assessment of lower trophic levels
Size fractionated primary productivity experiments (picoplankton <2 μm, nanoplankton 2–20 μm and net plankton >20 μm) using Carbon-14 techniques have been conducted for a long time. Such community metabolism data have been extremely useful for investigating phytoplankton dynamics as a means to understanding ecosystem health (Munawar and Munawar, 2014). Similarly an estimate of bacterial growth rates is also suggested for determining the heterotrophic community metabolism and improving the understanding of the role of heterotrophs in carbon dynamics (Heath et al., 2003).
Higher trophic levels, fish and fishers
The upper trophic levels including fish and fishers (birds, mammals) should not be forgotten, and several articles have dealt with fish and fishery in the Laurentian Great Lakes and also in Lake Vänern. An ecosystem without fish is unrealistic and an assessment of ecosystem health without considering fish is incomplete. Some factors that need to be considered include: stock recruitment; assessment of stocks using appropriate gear types, and management strategies that span ecosystem as well as political boundaries.
Don't forget the Pressures
Monitoring requires knowledge of Pressure (P) like water regulation, traffic, discharges to water and air, fishery etc., which is part of the parameters used in the DPSIR (Driving forces, Pressures, States, Impacts, Responses) model proposed by the EEA (European Environment Agency). The ultimate societal goal is to Respond to environmental Impacts (I) by measures (Responses, R) that will correct the Driving forces (D). In this context Ecosystem Health concerns the effects of the Pressures (P) on the State (S) and the Impact (I) that can be seen in the ecosystem. All of us who work with this understand the scientific difficulties involved in understanding causality in a large lake and making predictions on what might happen in the future, but we must also be aware of the fact that we need to co-operate with others to make it happen.
Lake Vänern continues to be an oligotrophic lake and has good prospects for its health despite past and present anthropogenic stressors. However it will not be possible to properly manage its ecosystem resources and future health without accurate, holistic data, both current and long-term. Researchers and managers of the lake are aware of the need for such information and made strides towards developing research strategies for the lake at the “State of Lake Vänern: Past, Present and Future” (SOLVE) symposium, held at Vanersborg, Sweden, in June 2012. In this article, information gaps are identified and a research and monitoring plan, based on North American Great Lakes experience, is suggested.
We would like to thank Jennifer Lorimer, Mark Fitzpatrick, Susan Blunt and Robin Rozon for technical editing and figure preparation.
Supplemental data for this article can be accessed on the publisher's website.