Lake Ringsjon Research Articles

Reduction of benthic macroinvertebrates due to waterfowl foraging on submerged vegetation during autumn migration

If present in large numbers, as during migration, herbivorous waterfowlmay reduce the amount of submerged vegetation. Because the vegetation is a keyfactor in shallow eutrophic lakes, removal of the green biomass can be expectedto affect also other biota that depend on the vegetation. We conducted anexperiment to determine how the abundance of chironomids andPisidium sp. were affected by intense foraging ofwaterfowlon the submerged plant Potamogeton pectinatus. This wasdone in Lake Ringsjon in southern Sweden, during the autumn migration ofthe birds. Three treatments, replicated six times, were used: (i) closed cagesthat excluded all waterfowl, (ii) semi-open cages that excluded only largewaterfowl (geese and swans), and (iii) open plots where all waterfowl couldfreely enter. Waterfowl densities were monitored during the experiment. Theresults suggest that the foraging of large waterfowl (swans) had a clearlynegative effect on macroinvertebrate abundance and aboveground biomass ofP. pectinatus. At the end of the experiment, the densityofchironomids was about 46% lower in the open than in the closed cages. Ingeneral, the density of Pisidium sp. tended to be lower inthe open plots. Small waterfowl alone did not seem to affect either thevegetation or macroinvertebrates. We suggest that thePisidium sp. was influenced at an early stage of grazing,when waterfowl foraged on aboveground biomass, whereas chironomids wereaffectedat a later stage, when swans were digging for below-ground tubers.

Nitrification and dentrification in a eutrophic lake sediment bioturbated by oligochaetes

The influence on nitrification and denitrification of oligochaete Limnodrilus sp. and Tubifex tubifex bioturbation was determined in eutrophic lake sediment from the Basin of Lake Ringsjon in southern Sweden. Nitrification and denitrification activity was compared in sieved and in undisturbed sediment cores in laboratory mesocosms at 2 different concentrations of nitrate. Nitrification and total denitrification rates were positively correlated to oligochaete biomass between 0 and 4 g dry weight m(-2) at both 33 and 268 muM nitrate in the inflowing water. Denitrification (d(w)) was relatively more stimulated by the oligochaetes at high concentrations of nitrate in the overlying water than at low nitrate concentrations. Though nitrification was enhanced by oligochaetes, this tended to reach a maximum between 3 and 5 g of oligochaete dry weight m(-2). Comparison of the enhancement of denitrification by oligochaetes with other similar studies of denitrification in eutrophic sediment bioturbated by tube-dwelling chironomids indicates that at a similar biomass oligochaetes are less effective at mobilizing nitrate to deeper sediment layers. This is explained by the different ways oligochaetes and tube-dwelling chironomids bioturbate the sediment. Sieving and homogenising the sediment had no pronounced effect on rates of denitrification and nitrification compared to undisturbed sediment. (Less)

The development of submerged macrophytes in Lake Ringsjön after biomanipulation

The maximum water depth of submerged vegetation and the number of sites colonized by submerged macrophytes in Lake Ringsjon were studied in 1992, 1993 and 1996, and compared with data from 1947 and 1988, in order to investigate the development of submerged macrophytes after the biomanipulation of the lake (completed 1992). The submerged vegetation has declined considerably since 1947, both in species number and outer water depth. The submerged macrophytes in Lake Ringsjon did not show any clear improvement in outer water depth or number of sites colonized after biomanipulation. The lack of any larger increase in Secchi depth after biomanipulation along with a shortage of suitable habitats (i.e. substrate), waterfowl grazing and species composition of the macrophyte populations are discussed as possible reasons for the poor development of submerged macrophytes in Lake Ringsjon.

Internal phosphorus loading in Lake Ringsjön

Lake Ringsjon did not respond with decreased algal production following a substantial reduction in external phosphorus loading. This is typical of many shallow lakes which for decades have received excessive amounts of nutrients. The inertia is due to large amounts of phosphorus (P) stored in sediments and biota, causing internal phosphorus loading. Much of this phosphorus is thought to be released from the organic-rich profundal sediments. In Lake Ringsjon, only one third of the total bottom area is covered by such sediments, the rest being dominated by sand and silt. In the profundal sediments bulk P content was not exceptionally high (approximately 2 mg P·g DW−1), while the pore water phosphate concentrations, especially in Satofta Basin, were very high, indicating large potential for phosphorus release to the water. This is also indicated by the large proportion of Fe- and Al-bound P in the sediments of Satofta Basin. Although there are no direct quantifications of phosphorus release from the sediments in Lake Ringsjon, measurements of phosphorus concentrations in the water mass as well as budget calculations for the three basins clearly show a high capacity for internal loading. Phosphorus concentrations generally increase during summer, when external additions are minimal. Until 1980, the annual external phosphorus addition to Lake Ringsjon greatly exceeded the output, showing that the lake was an efficient phosphorus trap. Since then, input and output have been balanced, but in recent years signs that the lake is once again retaining phosphorus on an annual basis are evident. There are marked differences between the three basins, with Western Basin generally retaining phosphorus, while the upstream Eastern Basin and Satofta Basin during the 1980s often exported phosphorus. It is not possible to evaluate the effects of the fish biomanipulation on the internal loading of phosphorus from the sediment.

Juvenile fish expansion following biomanipulation and its effect on zooplankton

In Lake Ringsjon, southern Sweden, we estimated yearly variations in both 0+ and older fish abundances, following the fish reductions in 1988–1992. Between 1987 and 1994, the total planktivore biomass decreased fourfold whereas the abundance of 0+ fish increased twenty times. The resulting change in predation pressure from planktivorous fish on zooplankton increased up to 2–3 times, after the fish reduction. We conclude that after an initial decrease, the predation pressure on zooplankton increased considerably due to the increase in 0+ fish abundance. The expansion in juvenile fish abundance may therefore have been an important factor behind the apparent lack of top-down effect on the zooplankton community following biomanipulation of Lake Ringsjon. The horizontal distribution of 0+ fish in Lake Ringsjon is also discussed.

Autumn waterfowl abundance in Lake Ringsjön, 1968–1996

Lake Ringsjon has been subject to a severe eutrophication process followed by various counter measures, such as nutrient reduction and fish manipulation. We have quantified staging waterfowl populations from 1968, before the eutrophication process reached its maximum, to 1996 when both the nutrient and fish reduction programmes were performed. The general pattern is that high numbers staged in Lake Ringsjon in 1968–1972. In the following 15 years, few waterfowl used the lake as staging area, but from about 1989, the number of birds again began to increase. The likely explanation of this pattern is an improved food availability within the lake, rather than large-scale, national, increases in bird population sizes, a notion strengthened by comparisons with standardized national waterfowl counts. We conclude that there are strong relations between limnological conditions and waterfowl abundances, and that the measures undertaken in Lake Ringsjon improved the lake as a staging area for waterfowl.

Synthesis of theoretical and empirical experiences from nutrient and cyprinid reductions in Lake Ringsjön

The reduction in external phosphorus load to Lake Ringsjon during the 1980s, did not result in improved water transparency during the following ten-year period. Furthermore, a fish-kill in the Eastern Basin of the lake, in addition to a cyprinid reduction programme (biomanipulation; 1988–1992), in contrast to theory, did not lead to any increase in zooplankton biomass or size. This absence of response in the pelagic food chain may have been attributed to the increase in abundance of YOY (0+) fish, following the fish reduction programme. Despite the lack of effect on zooplankton, there was a decrease in phytoplankton biomass, a change in species composition and an increase in water transparency following biomanipulation. In 1989, one year after the fish-kill in Eastern Basin, the Secchi depth (summer mean) increased from 60 cm to 110 cm. In the following years, water transparency increased further, despite an increase in phosphorus loading. An unexpected effect of the biomanipulation was an increase in benthic invertebrate and staging waterfowl abundances, which occurred 2–4 years after fish reduction. Hence, the response in the benthic community following biomanipulation was considerably stronger than in the pelagic community. A likely explanation is that reduction in abundance of the benthic feeding fish species bream (Abramis brama), strongly affected the benthic invertebrate fauna. In this paper, we present what we believe happened in Lake Ringsjon, and which processes are likely to have been important at various stages of the restoration process.

Planning and execution of the fish reduction in Lake Ringsjön

Lake Ringsjon, a naturally productive lake in southern Sweden, was subjected to high external phosphorus loading during the 20th century and became highly productive. In spite of effective waste water treatment which reduced external phosphorus loading, the improvements in lake water quality were moderate and proceeded at low rate. Consequently cyprinid reduction, which was expected to affect water clarity via several intertrophic mechanisms, was implemented as a measure for restoring the lake. The fish removal was performed using a board trawler equipped with a herring trawl modified for bottom trawling. The bottom rope of the trawl was 11 m and the total length of the trawl was 20 m, the cod end mesh size was 11 mm (knot to knot). In the beginning the catch amounted to 1 ton of fish in less than 2 hours, logistic problems bringing the catch to shore and replacing all piscivorous fish into the lake reduced the initial daily catch to 2–3 tons. A grand total of 240 tons of fish were removed by trawling Lake Ringsjon over a 3 year period. The catch was either fermented with myric acid or deep-frozen before being used as mink and pelican food respectively. Some of the catch was also used for gas production. The cost for boats and equipment was approximately 60 000 US$. The three persons needed for fishing and sorting as well as the costs of taking care of the catch on land are not included in this sum.

Lake Ringsjön: its catchment area, its history and its importance

The aim of this chapter is to present a general description of Lake Ringsjon and its catchment area, serving as a background for the following, more specific chapters. We briefly describe human activities and land use, as well as the human demographics in the catchment area. Moreover, we present important historical events behind the development and present status of Lake Ringsjon. Finally, we describe the importance of the lake as a production unit for ‘ecological services’, including not only production of fish and drinking water, but also in terms of recreational value, such as swimming, fishing and bird-watching. Although these ‘services’ are nowadays of less importance in Lake Ringsjon than 40 years ago, due to the eutrophication problems, the lake has the potential to once again become a centre for human activities.

The effects of cyprinid reduction on the fish community

Lake Ringsjon in southern Sweden was restored by biomanipulation by means of a reduction in zooplanktivorous cyprinids. The fish community in the three different Basins of Lake Ringsjon were followed between 1987 and 1994. In 1990, c. 100 tons (60% of the community) were removed from Satofta Basin, in 1988, c. 500 tons (80%) were removed from Eastern Basin and in 1992, c. 120 tons (50%) of cyprinids were removed from Western Basin. In all basins, the fish biomass was much lower after fish reduction than before, but only in Eastern Basin has the proportion of piscivores increased substantially (from 10% to 25–40%). Due to very high abundance of young-of-the-year following the fish reductions, the fish abundance has increased in all basins. The abundance of the young-of-the-year was negatively correlated to the abundance of piscivore perch at the 3 m depth. Although the young-of-the-year abundance was very high after the fish reduction, there are no clear evidences that this affects recruitment to the adult populations. The results are discussed in relation to critical fish biomasses and critical proportions of planktivory and piscivory to ensure a long lasting effect of fish reduction.

Simulating the effects of biomanipulation on the food web of Lake Ringsjön

A dynamic, process-oriented, deterministic and phosphorus-based model was developed to simulate the food web dynamics of Lake Ringsjon, in particular the long-term effects of biomanipulation in terms of reduction of omnivorous fish. The model contains 14 state variables, each with a differential equation describing sources and sinks of phosphorus. The state variables encompass piscivorous and omnivorous fish, zooplankton, phytoplankton, sediment and lake water. The model simulates densities of fish and phytoplankton adequately, both before and after biomanipulation, although the actual lake phytoplankton density varied more year-to-year compared to the model predictions. According to the model, a biomanipulation will cause an increase in zooplankton biomass. This prediction contradicts available field data from the lake which do not indicate any significant change in zooplankton biomass resulting from the performed biomanipulation. This discrepancy may partly be attributed to structural uncertainties in the model, related to the size structure of predators on zooplankton, i.e. the omnivorous fish community. The simulations suggest that phosphorus was routed along the pelagic food chain to a larger extent after omnivorous fish were removed, whereas the amount of phosphorus routed via the sediment and benthivorous fish decreased following fish removal. Accordingly, translocation of phosphorus from sediment to water by benthivorous fish is predicted to be substantially reduced by biomanipulation, resulting in an overall reduction in the release of new phosphorus to phytoplankton. Irrespective of simulated fishing effort (reduction of ≤0.5% d−1 for two years), the model predicts that P-release from the sediment and the external load will remain sufficiently high to force the system back to its previous state within a decade. Thus, recurrent biomanipulations and/or combined abatement strategies may be necessary to maintain low phytoplankton density. Known structural model uncertainties may however affect the robustness of such detailed predictions about the system resilience.

Qualitative and quantitative investigations of phytoplankton in Lake Ringsjön, Scania, Sweden

The algal record from Lake Ringsjon covers a period of more than 100 years. Towards the end of the 19th century, the lake had a well-functioning commercial fishery, when a brownish mess began to appear in the water, clogging-up the fishing nets and making fishing difficult. This was the first record of algal problems in the lake. Following complaints from the fishermen, the algal flora was investigated and the mess was stated to be a mass development of diatoms belonging to the genus Melosira (Aulacoseira). Diatom maxima then appeared regularly in spring and autumn, whereas blue-green algae only occurred occasionally and in low abundances. Between 1900 and 1950, nutrient concentrations slowly increased in the lake. Algal blooms of Anabaena lemmermannii and Gloeotrichia echinulata began to appear in summer. Between 1960 and 1980, the lake developed into a hypertrophie status with extensive blooms of blue-green algae from May to October, including high biomasses of mainly Microcystis spp. This severe pollution started with increased usage of the lake, increased tourism and recreation, intensified farming with the introduction of artificial fertilization, and also the diversion of sewage water from a sewage treatment plant. In 1968, cattle death was reported and in 1981, algal toxicity was verified by mouse bioassay. After a period of extremely poor water quality, the lake began to show signs of improvement as a result of the nutrient and fish reduction programmes. The very heavy blooms of blue-green algae decreased and the duration of blooms became shorter. The Microcystis species were to a certain extent replaced by Anabaena and Aphanizomenon species, and biodiversity of algae increased. With respect to the algal community, the lake has improved considerably, but is still eutrophic and suffers summer blooms of toxic blue-green algae.