Marjo Tarvainen

Long-Term Field-Scale Experiment on Using Lime Filters in an Agricultural Catchment

The River Yläneenjoki catchment in southwest Finland is an area with a high agricultural nutrient load. We report here on the nutrient removal performance of three on-site lime-sand filters (F1, F2, and F3), established within or on the edge of the buffer zones. The filters contain burnt lime (CaO) or spent lime [CaO, Ca(OH), and CaCO]. Easily soluble lime results in a high pH level (>11) and leads to an efficient precipitation of soluble phosphorus (P) from the runoff. Water samples were taken from the inflow and outflow of each site in different hydrological situations. The length of the monitoring period was 4 yr for F1, 6 yr for F2, and 1.5 yr for F3. F1 and F2 significantly reduced the suspended solids (SS), total P (PTOT), and dissolved reactive P (DRP) in the treated water. The proportional reduction (%) varied but was usually clearly positive. Filter F3 was divided into two equal parts, one containing burnt lime and the other spent lime. Both filter parts removed PTOT and SS efficiently from the water; the burnt-lime part also removed DRP. The mixed-lime part removed DRP for a year, but then the efficiency decreased. The effect of filters on nitrogen compounds varied. We conclude that sand filters incorporating lime can be used together with buffer zones to reduce both P and SS load to watercourses.

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.

Internal phosphorus load estimation during biomanipulation in a large polymictic and mesotrophic lake

Abstract We quantified internal phosphorus (P) load for 26 years in the polymictic, large (155 km2) and shallow (mean depth 5.5 m) Lake Säkylän Pyhäjärvi, which was heavily biomanipulated by fish removal. Internal load was estimated as (1) partially net estimates from in situ P summer increases, (2) net estimates from P budgets (mass balance approach), and (3) gross estimates from predicted active sediment release area and sediment P release, dependent on August lake temperature. Long-term averages of these estimates were similar and large at about 60% of average external load (105 mg m−2 yr−1) and were larger than external load in years with high water temperature and low water load. Regression analysis revealed that external load is decreasing but internal load is increasing over time. Internal load was negatively correlated with annual water load and positively correlated with lake water temperature. Long-term average annual or summer P concentrations are adequately predicted by a P mass balance model that includes external load, internal load (Method 3), and sedimentation as independently predicted retention. Predictability was poor for individual years, however, partially due to the poor correlation of observed lake and outflow P concentrations, the variable abundance of planktivorous fish, and reflecting the violation of the steady state assumption when individual years are modeled. Scenario modeling shows that biomanipulation cancels out the effects of internal load and forecasts a rapid increase of internal load and P concentration due to climate change; therefore, measures that further decrease external and internal P load and strengthen biomanipulation are recommended.

Size-Fractionated δ 15 N and δ 13 C Isotope Ratios Elucidate the Role of the Microbial Food Web in the Pelagial of Lake Tanganyika

Food web structure of the pelagic community in Lake Tanganyika was studied using the stable nitrogen and carbon isotopes 15 N and 13 C. Size-fractionated seston, zooplankton, shrimps, medusae and fish were sampled in the northern part of Lake Tanganyika. Picoplankton fractions as well as cyanobacteria-dominated nano/microplankton fractions had very low nitrogen isotope signatures typical for nitrogen-fixing organisms. Fractions containing mainly dead organic matter (and associated bacteria) or nano/microalgae (chlorophytes and diatoms) had i 15 N 2 to 4 higher. The low i 15 N signatures of small cyclopoids and shrimps suggest they are feeding on nitrogen-fixing cyanobacteria (picoplankton or larger forms), while the higher i 15 N signature of larger copepods suggest mixed feeding on large algae and small zooplankton and/or cyanobacteria. Medusae were slightly enriched in i 15 N relative to large copepods. Among fish, the signatures of Stolothrissa and small Lates stappersi suggested feeding on large copepods, while Limnothrissa and larger Lates were slightly more enriched, indicating partial piscivory. The enrichment of 13 C between the putative trophic levels (2 to 3) was higher, while that of 15 N (2 to 3) was lower, than usual in isotope studies. Our results indicate that picocyanobacteria and possibly also larger cyanobacteria are important producers in the pelagic food web of Tanganyika.

Danish and other European experiences in managing shallow lakes

Abstract For a century eutrophication has been the most serious environmental threat to lakes in the densely populated or agricultural areas of Europe. During the last decades, however, major efforts have been used to reduce the external nutrient loading, not least from point sources. Despite these comprehensive efforts, lake eutrophication remains a major problem. Today, the highest pollution input is derived from diffuse sources mainly from agricultural land in lake catchments. We describe the actions taken to reduce the external nutrient loading and the lake responses to these actions as well as the use of additional methods to reinforce recovery, such as biomanipulation. We further discuss resilience and short and long-term responses. We highlight the Danish experiences, but add several examples from restoration measures taken elsewhere in Europe. We also briefly discuss how a potential change in climate may affect lake responses to diminished nutrient loading.

Unionid mussels as stable isotope baseline indicators for long-lived secondary consumers in pelagic food web comparisons

Stable isotope signatures of filter-feeding freshwater mussels have been established as a convenient baseline measure at the primary consumer level for comparisons of pelagic food webs. We studied this idea with four unionid mussel species Anodonta sp. (A. anatina), Pseudanodonta complanata, Uniopictorum and U. tumidus in four Finnish lakes. δ 15 N and δ 13 C signatures of mussels were compared with surface sediment and summer averages of their potential food sources and planktonic herbivores. Isotope compositions of Anodonta, Pseudanodonta and Unio differed significantly between lakes but were roughly similar within lakes, except in the smallest study lake. There was considerable deviation in the δ 13 C of mussels relative to that of particulate organic matter (POM) in water or sediment, suggesting selective use of food sources. The δ 13 C of POM also showed considerable temporal variation. Enrichment in δ 15 N mussels over that of POM was less than expected, but the mussel signatures matched closely with those of herbivorous Daphnia. We suggest that unionid mussels can provide a reasonable baseline estimate for pelagic food web comparisons with much less sampling effort than other suggested primary consumers.

Monitoring of spatial water quality in lakes by remote sensing and transect measurements

New tools, such as intensive measurements, together with advanced mathematical models, are increasingly needed in water management and environmental research. The new approaches are being developed at Pyhäjärvi, a large (155 km2) lake in southwest Finland. Pyhäjärvi is highly valuable in terms of water supply, fisheries and recreational use. The ecological state of Pyhäjärvi has been closely monitored for decades, particularly since eutrophication became a major concern in the late 1980s. Two relatively new research methods were used to assess the spatial water quality of Pyhäjärvi: (i) transect measurements from a moving boat; and (ii) remote sensing data based estimates. First, a flow-through method from a moving boat was successfully used to collect high resolution transect water quality data from the lake over six field campaigns. The method is relatively accurate but costly, and its use is mostly limited to special campaigns and intensive research, but not for long-term monitoring. Second, remote sensing methods were used to retrieve water quality information which was found consistent with the surface measurements from the boat. The estimation of parameters such as turbidity and humic substance concentration is possible with simple algorithms when using remote sensing (MERIS) data. The quantitative estimation of water quality by the methods used here requires concurrent in situ measurements for algorithm training. These methods will be further developed utilizing frequent on-line water quality and weather data from a recently installed lake float.

Predation by signal crayfish Pacifastacus leniusculus on fish eggs and its consequences for coregonid recruitment

The character and magnitude of predation by the invasive, ectothermic Pacifastacus leniusculus, a crayfish widely introduced to Europe and Japan from North America, on the eggs of coregonid fishes, vendace Coregonus albula and whitefish Coregonus lavaretus were examined by experimentation, modelling and field data. The present results showed that P. leniusculus has the potential to be very efficient predator of fish eggs under winter conditions, but the predation by P. leniusculus did not significantly decrease production of coregonid larvae during the years with a high P. leniusculus population in the study lake. Hence, the mortality caused by the novel invertebrate predator appeared to compensate for other yet unexplored mortality factors instead of having an additive effect on the present salmonids.

Responses of Aquatic Ecosystems to Environmental Changes in Finland and China

The concern for the state of global freshwater reservoirs has increased due to deterioration of the water quality during the last decades. This has prompted monitoring and restoration efforts such as the European Water Framework Directive and the national-scale 2nd-investigation and monitoring of the water quality, water volume and biota resources in China. The challenge so far has been the determination of the “natural” state (reference conditions) of freshwater ecosystems. We used the sediment archives of five lakes and one brackish water embayment in Finland and China to assess the impact of selected variables of climatology, hydrology, nutrients, and changes in human population on these ecosystems during the last few centuries. The study sites represent catchment areas with varying land use. Despite the long distance between the sites and their different land-use characteristics, the direction and timing of changes during the last few centuries are well comparable between the high latitudes of Finland and the mid-low latitudes of China. This study reinforces the sensitivity of aquatic ecosystems to environmental change and underlines the usefulness of the palaeolimnological approach as a tool for determining reference conditions.

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.