Trends in vendace (Coregonus albula) biomass in Pyhäjärvi (SW Finland) relative to trophic state, climate change, and abundance of other fish species

Catches of vendace (Coregonus albula) from Pyhajarvi, a boreal lake in SW Finland, were exceptionally high for decades. Gradually, however, eutrophication and climate warming significantly changed the lake environment. From the 1970s to the 2010s, total phosphorus and chlorophyll a levels increased two- and threefold, respectively, while the average June-September surface temperature increased by 0.34 degrees C decade'. The highest population biomass of the young-of-the-year vendace in autumn, 6-18 (mean = 14) kg ha(-1). was recorded in 1973-1989, a period of sustainable fishery. Overfishing in 1990-1999 reduced autumn biomass of young-of-the-year vendace biomass to 3-11 (mean = 6) kg ha(-1), allowing the competing planktivores perch (Percafluviatills), roach (Rutilus rutilus), and smelt (Osmerus - eperlanus) to increase and consume a larger part of the plankton resources. Eutrophication and climate warming appear to have favoured these species, and the new resource division persisted even after the vendace population recovered

Two-year cycle of vendace (Coregonus albula) in Pyhäjärvi, SW Finland: evidence for asymmetric competition between adults and juveniles

Long-term monitoring of the planktivorous vendace (Coregonus albula) in Pyhajarvi, a lake in SW Finland, revealed periods characterised by different types of population regulation. In 1971-1989, the vendace stock was strong and exhibited a two-year cycle. In 1990-1999, extreme weather conditions and predation resulted in recruitment failures, after which overfishing kept year-classes small. From 2000 onwards, the two-year oscillations were re-established at a lower level. Here, we show that the two-year cyclicity prevalent in the 1980s was consistent with the hypothesis of asymmetric competition between adults and juveniles. Food consumption by juveniles of strong year-classes retarded growth and weakened condition of the co-occurring adults, resulting in less abundant year-classes. In 2000-2018, the role of intraspecific competition diminished due to interspecific competition from increased populations of other planktivorous fish, preventing vendace from attaining higher abundance. Elevated temperature probably confounded the effects of competition, but its direct role was masked by simultaneous gradual eutrophication and fish assemblage changes

Phylogenetic Study of Dioecious and Parthenogenetic Populations of Canthocamptus staphylinus (Crustacea, Copepoda, Harpacticoida)

The phylogenetic relationships of four dioecious populations and one parthenogenetic population of the harpacticoid Canthocamptus staphylinus Jurine were studied. Analysis of the mtCOI gene revealed two main clades as a phylogenetic tree and a network of haplotypes: a clade with Fennoscandian populations in Lake Pääjärvi (Finland) and Lake Vänern (Sweden), and a second clade with populations in Lake Võrtsjärv (Estonia), Orlov Pond in Saint Petersburg (Russia), and the type locality of the species in Lake Geneva (Switzerland).The parthenogenetic population of C. staphylinus showed the smallest nucleotide and haplotype polymorphisms and could have evolved as a reaction to the changing environmental conditions following the Last Glacial Maximum, 20K YBP.</p

Invasive submerged macrophytes complicate management of a shallow boreal lake: a 42-year history of monitoring and restoration attempts in Littoistenjärvi, SW Finland

The small, shallow lake Littoistenjärvi (SW Finland) experienced in 1978-2019 rapid fluctuations between extreme ecological states, initially associated with mass occurrences of the submerged macrophyte Elodea canadensis Michx. In collapse years following abundance peaks, water was turbid, in other years clear. Aeration prevented anoxia under ice-cover, but this favoured Elodea. Mechanical plant removal accelerated Elodea growth, and had to be abandoned. Recurrent cyanobacterial blooms started in 2000, and by 2006 phosphorus and chlorophyll reached new high levels because of increased internal loading. During this turbid state, internal loading showed significant positive correlation with maximum water temperature and pH. External loading was reduced in 2011 by one-third by diverting runoff from a former arable field transformed into a wetland. Precipitation of phosphorus with polyaluminium chloride in 2017 restored the clear-water state. The case of Littoistenjärvi shows that if internal loading has become the major factor controlling water quality, traditional restoration methods (external load reduction, aeration, removal fishing) provide limited possibilities to improve water quality. Instead, chemical precipitation of phosphorus seems a promising measure which can break the vicious circle of algal blooms and internal loading. Unfortunately, the uncontrollable growth of invasive submerged macrophytes may jeopardise the positive development

Modelling the impact of higher temperature on the phytoplankton of a boreal lake

We linked the models PROTECH and MyLake to test potential impacts of climate-changeinduced warming on the phytoplankton community of Pyhäjärvi, a lake in southwest Finland. First, we calibrated the models for the present conditions, which revealed an apparent high significance of internal nutrient loading for Pyhäjärvi. We then estimated the effect of two climate change scenarios on lake water temperatures and ice cover duration with MyLake. Finally, we used those outputs to drive PROTECH to predict the resultant phytoplankton community. It was evident that cyanobacteria will grow significantly better in warmer water, especially in the summer. Even if phosphorus and nitrogen loads to the lake remain the same and there is little change in the total chlorophyll a concentrations, a higher proportion of the phytoplankton community could be dominated by cyanobacteria. The model outputs provided no clear evidence that earlier ice break would advance the timing of the diatom spring bloom.peerReviewe

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

We quantified internal phosphorus (P) load for 26 years in the polymictic, large (155 km2) and shallow (mean depth 5.5 m) Lake S&auml;kyl&auml;n Pyh&auml;j&auml;rvi, 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&ndash;2 yr&ndash;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

Vesijärven kehitys neljän vuosikymmenen kunnostustoimenpiteiden aikana

The diversion of sewage inputs in the mid-1970s led to an order of magnitude reduction in nutrient loading to Lake Vesijarvi, southern Finland. After the diversion, nutrient concentrations declined, consistent with a simple dilution model, and by the mid-1990s the chlorophyll concentration was reduced by 80%. The favourable development was supported by a 5-year mass removal of planktivorous and benthivorous fish and the stocking of predatory pikeperch (Sander lucioperca(L.)), although the exact mechanisms behind their effects remain obscure. Starting in 2010, oxygen-rich water from the top of the water column was pumped to the deepest parts of the lake, resulting in high deepwater oxygen concentration in winter. In summer, hypoxic or even anoxic conditions could not be avoided, but the duration of the anoxic period was markedly shortened. Because nitrate was never depleted, leaching of total nitrogen from the sediment was reduced and the same was also true for total phosphorus, but only in winter. The oxygenation stabilized deepwater nutrient concentrations to a low level, but this was not reflected in the epilimnetic total nutrient concentration or in a further decrease in the chlorophyll concentration.Peer reviewe

Responses of zooplankton to long-term environmental changes in a small boreal lake

Peer reviewe

Spatial and seasonal variation in reproductive indices of the clupeids Limnothrissa miodon and Stolothrissa tanganicae in the Congolese waters of northern Lake Tanganyika

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