Lake Pyhajarvi Research Articles

Phytoplankton assemblages respond differently to climate warming and eutrophication: A case study from Pyhäjärvi and Taihu

Abstract Long-term monitoring data from two lakes located at different latitudes were used to test the hypothesis that phytoplankton communities respond differently to environmental changes (e.g., global warming and anthropogenic activities, mainly eutrophication). Lake Pyhajarvi (temperate area) and Lake Taihu (subtropical area) are both shallow and productive lakes. Presence/absence data indicated that phytoplankton taxa present did not change significantly in the two lakes over the last two decades. However, biomass data showed that dominance relationships of species changed in both lakes. Results of assemblage ordination indicated that climate change played a vital role in mediating phytoplankton assemblage dynamics in Lake Pyhajarvi, while eutrophication was the primary factor driving the phytoplankton assemblage dynamics in Lake Taihu. In Lake Pyhajarvi, the variability of the climatic variables was greater than in Lake Taihu. In Lake Pyhajarvi, increasing wind speed decreased Secchi depth, promoting shade-tolerant taxa such as Planktothrix . At the same time, increased mixing reduced the loss rate of the heavy taxa such as Tabellaria . Increased nutrient concentrations and water temperature would also promote both Planktothrix and Tabellaria . In the warmer Lake Taihu, the biomass of the dominant Microcystis , which has high optimum temperature for growth, was strongly related to conductivity and less to temperature. Our results showed that climate change potentially enhanced cyanobacteria dominance in lakes Pyhajarvi and Taihu but by different mechanisms. In Lake Pyhajarvi, the former dominant species were replaced by cyanobacteria; in Lake Taihu, cyanobacterial dominance was accentuated by climate warming.

Phytoplankton community response to the increasing summer daily minimum temperature in Lake Pyhajarvi, Finland

Phytoplankton community response to the increasing summer daily minimum temperature in Lake Pyhajarvi, Finland

An approach to improve direct runoff estimates and reduce uncertainty in the calculated groundwater component in water balances of large lakes

Summary Groundwater is important in the overall water budget of a lake because it affects the quantity and quality of surface water and the ecological health of the lake. The water balance equation is frequently used to estimate the net groundwater flow for small lakes but is seldom used to determine net groundwater flow components for large lakes because: (1) errors accumulate in the calculated groundwater term, and (2) there is an inability to accurately quantify the direct runoff component. In this water balance study of Lake Pyhajarvi (155 km 2 ) in Finland, it was hypothesized a hydrograph separation model could be used to estimate direct runoff to the lake and, when combined with a rigorous uncertainty analyses, would provide reliable net groundwater flow estimates. The PART hydrograph separation model was used to estimate annual per unit area direct runoff values for the watershed of the inflowing Ylaneenjoki River (a subwatershed of the lake) which were then applied to other physically similar subwatersheds of the lake to estimate total direct runoff to the lake. The hydrograph separation method provided superior results and had lower uncertainty than the common approach of using a runoff coefficient based method. The average net groundwater flow into the lake was calculated to be +43 mm per year (+3.0% of average total inflow) for the 38 water years 1971–2008. It varied from −197 mm to 284 mm over that time, and had a magnitude greater than the uncertainty for 17 of the 38 years. The average indirect groundwater contribution to the lake (i.e., the groundwater part of the inflowing rivers) was 454 mm per year (+32% of average total inflow) and demonstrates the overall importance of groundwater. The techniques in this study are applicable to other large lakes and may allow small net groundwater flows to be reliably quantified in settings that might otherwise be unquantifiable or completely lost in large uncertainties.

Ecosystem change in the large and shallow Lake Säkylän Pyhäjärvi, Finland, during the past ~400 years: implications for management

Lake Sakylan Pyhajarvi has been an important fishing site and drinking water source for the local population for centuries. The lake has undergone significant changes: (1) the water level was lowered in the 1600s and in the 1850s; (2) planktivorous coregonid fish were successfully introduced in the early 1900s; (3) nutrient input from intensified agriculture has increased since the 1950s and (4) the effects of the current variable climate on the lake and its catchment have become more evident since the 1990s. We determined the phases of oligotrophication, eutrophication and recovery and elucidated the ecosystem changes by combining palaeolimnological records with detailed neolimnological data. The sedimentary diatom and cladoceran assemblages first showed a relatively eutrophic period followed by oligotrophic periods, linked with the artificial changes in water level and consequent shifts in macrophyte abundance. The oligotrophic period in the early 1900s is thought to represent the target trophic state for the lake. After the 1950s, introduction of vendace resulted in higher planktivory reflected by an increased relative abundance of small-bodied pelagic cladocerans. Signs of eutrophication occurred due to increased nutrient load. During the last 10 years, signs of recovery have been recorded. A complex history such as that of Lake Pyhajarvi illustrates the difficulties in selecting management targets, and the risk of setting false targets, for lakes based solely on monitoring data—both neolimnological and palaeolimnological approach are needed.

Sedimentation dynamics and metal loading in Lake Pyhäjärvi, Finland

Post-isolation (9,600 year BP) sedimentology in Lake Pyhajarvi, Finland, was studied using sub-bottom echo-sounding and sediment coring. The sediment sequence appeared in the echo-sounding profiles as two sections: (1) a lower section of homogenous, light grey gyttja clays, and (2) an upper section with dark layers of mainly clay gyttjas and gyttjas, here referred to as C_gyttja. We investigated the thickness and volume of sediments as well as their morphology and areas of accumulation. Approximately 77 % of the lake represents areas of accumulation, containing 34 and 1.3 million tons of dry matter and carbon, respectively. Spatial variability in sedimentological properties indicates that accumulation in the Kirkkoselka sub-basin was focused into the deepest areas since lake isolation. In the Isoselka sub-basin, however, accumulation was focused partially onto the flanks of the depression, whereas the deepest parts (23–27 m) of the sub-basin represent erosional areas. There appears to have also been sediment re-suspension and re-deposition, as indicated by erosion surfaces. This, combined with dune-like bottom morphology in the accumulation areas, provides evidence for the role of wind-driven bottom currents on sedimentation dynamics in the Isoselka sub-basin. Increases in Cu and Zn concentrations, driven by Pyhasalmi mining activities, were used as a geochemical marker for recent sedimentation (RS) between 1966 and 2008. As a consequence of wind-driven bottom currents, 60 % of the metals accumulated in the Kirkkoselka sub-basin and 40 % accumulated in the Isoselka sub-basin. There is a spatial correlation between amounts of C_gyttja and RS. In the middle of dune-like C_gyttja formations in the Isoselka sub-basin, the RS grain size is smaller than in other areas. Variations in RS are greatest along the borders of the accumulation area, likely a consequence of bottom currents in those areas.

Effects of mining on testate amoebae in a Finnish lake

Sediment geochemistry and testate amoebae (thecamoebians) were used to track the spatial extent and temporal evolution of mine loading and its impact on aquatic biota in Lake Pyhajarvi, central Finland. Pyhasalmi is an operating Zn-Cu–S mine, located by Lake Pyhajarvi. Mine loading to the lake comes mainly from wastewater. Highest metal loading occurred in the 1970s and 1980s. Present loading consists mainly of Ca and S that nearly saturate dense wastewaters with gypsum. This affects oxygen concentration in the lake’s northern basin, where effluent is presently discharged. Of 25 short sediment cores retrieved from Lake Pyhajarvi, 10 came from sites ‘downstream’ and 15 from ‘upstream’ of the mine. Three sample levels from each core were selected for further investigation after exploratory XRF metal analysis. Samples were chosen to represent lake conditions before the mine impact, during the peak impact phase, and at present. Results from pre-disturbance deposits show that the two lake basins are naturally different. The ‘downstream’ basin (Junttiselka) has lower oxygen and a higher sedimentation rate, which was revealed by concentrations of redox-sensitive and clastic-related elements, as well as faunal distributions. The peak mine impact is reflected in the high concentration of S and metals Cu, Zn, Cd, Pb, which spread widely in the lake and increased > 100% at sites near the tailings area. These concentrations have since decreased, but not to pre-disturbance levels. The faunal response to peak metal concentrations was weak, spatially limited and masked by larger responses to other environmental factors, including eutrophication, oxygen concentration, and increased clastic input generated by land-use changes. These environmental factors and changes in mine water composition probably account for the reason that faunal assemblages did not return to the pre-mine situation. Instead, the modern fauna reflects a third assemblage type that developed after the peak-loading phase. Geochemical and thecamoebian analyses show that the mine has affected the two basins of the lake differently during the lake history. Results suggest that mining impacts could be mitigated with good planning and management.

Spatiotemporal and long-term variation in phytoplankton communities in the oligotrophic Lake Pyhäjärvi on the Finnish-Russian border

As part of the joint Finnish-Russian research and development project “Assessment of the ecological state of the transboundary waters”, seasonal and spatial variations in the phytoplankton communities of Lake Pyhajarvi were studied in order to identify possible long-term (1963–2002) changes and to present recent data (since 2002) on phytoplankton biomass and species composition. Some changes in both phytoplankton biomass and species composition, particularly in the littoral zone and northern basins of the lake, were obvious at the end of the 1980s; in particular, the density of blue-greens increased during that period. Biomasses increased five-fold in 1980, decreased after 1990 and have remained low (0.14 mg l−l) to the present time. The effective water protection measures applied, especially reducing the phosphorus load from municipal wastewaters and airborne pollution, can now be seen to have improved the water quality. Although no significant temporal differences were found in phytoplankton biomass during the intensive biweekly sampling in 2002, considerable spatial variation was seen within the lake. An obvious change in the species composition of phytoplankton and an increase in biomass were seen even in the loaded parts of the lake. Despite the spatial differences in phytoplankton biomass and community structure, the changed species composition in the northern part of the lake indicated a clear recovery from the blue-green and Chrysophyceae dominated high biomass of the 1980s.

Sediment and phytoplankton records of the cyanobacterial genus Anabaena in boreal Lake Pyhäjärvi

Past occurrence and quantities of Anabaena cyanobacteria in Lake Pyhajarvi, SW Finland, were investigated using sediment and phytoplankton records. A short sediment core covering the past 20 years was examined for Anabaena resting spores (akinetes) in order to assess the utility of akinetes as a paleolimnological proxy. Sedimentary akinetes confirmed the past existence of Anabaena in water, but did not show a direct correlation with the amount of Anabaena spp. in water samples. The amount of planktonic Anabaena spp. correlated with total phosphorus and nitrogen concentrations in Lake Pyhajarvi, but the number of akinetes was considered to have a relationship with low nutrient concentrations and dominant bloom-forming cyanobacteria. Akinetes are probably suitable for low-resolution and long time scale paleolimnological investigations, where they provide information of past cyanobacteria that cannot be directly attained otherwise.

Horizontal distribution of vendace larvae in Lake Pyhäjärvi (SW Finland): effect of wind-induced currents

(2002). Horizontal distribution of vendace larvae in Lake Pyhajarvi (SW Finland): effect of wind-induced currents. SIL Proceedings, 1922-2010: Vol. 28, No. 2, pp. 726-730.

Relations between planktivorous fish abundance, zooplankton and phytoplankton in three lakes of differing productivity

Water chemistry, phytoplankton, zooplankton and fish populations were studied over several years in three shallow, non-stratified lakes with differing nutrient loadings and fish communities in southwest Finland. Lake Pyhajarvi was weakly mesotrophic in 1980–1996, Lake Koylionjarvi was highly eutrophic in 1991–1996, and Lake Littoistenjarvi was mesotrophic in 1993–1996 and eutrophic in 1992. In Lake Pyhajarvi, natural year-class fluctuations of vendace and smelt (range of combined biomass 5–28 kg ha−1) caused significant variation in planktivory. The very dense fish stocks of Lake Koylionjarvi (mainly roach, bream and smelt) were decimated from > 175 kg ha−1 in 1991 to about 50 kg ha−1 in 1996 by removal fishing. The roach stock of Lake Littoistenjarvi declined from about 71 kg ha−1 to about 28 kg ha−1 during 1993–1996. In Lake Pyhajarvi, strong stocks of planktivorous fish were accompanied with depressed crustacean zooplankton biomass, reduced role of calanoids and cladocerans, a low proportion of larger cladocerans (length > 0.5 mm), and a high chlorophyll level. In the lakes Littoistenjarvi and Koylionjarvi, zooplankton was dependent on both fish and phytoplankton: in spite of dense fish stocks, a high crustacean biomass developed in a phytoplankton peak year, but it was dominated by very small cladocerans. In Lake Pyhajarvi, late summer chlorophyll concentration was predictable from total phosphorus in water and cladoceran biomass (r2 = 0.68), both factors explaining roughly similar fraction of total variation. In combined data from all three lakes, chlorophyll was almost solely dependent on total phosphorus, while the cladocerans were regulated both from below by productivity and from above by fish. Our data from Pyhajarvi lend support to consumer regulation of late summer phytoplankton; low chlorophyll values prevailed when planktivorous fish biomass was below 15 kg ha−1. In large eutrophic lakes it may be difficult to reduce fish stocks to such a low level: in Lake Koylionjarvi, after six years of removal fishing, fish biomass still remained higher, and changes in plankton were accordingly small. Unexpectedly, in 1993–1996, phytoplankton biomass in Littoistenjarvi remained low in spite of low crustacean zooplankton biomass; submerged macrophytes probably regulated the water quality.

The response of sedimentation in three varved lacustrine sequences to air temperature, precipitation and human impact

Human activity and climatic forcing have influenced sedimentation in three of Finland's deepest lakes during the last centuries. High-resolution sediment sequences of Lake Paijanne, Lake Paajarvi and Lake Pyhajarvi represent records of the last 440 years, 839 years and 633 years, respectively. The accumulation rates of dry matter, organic carbon and biogenic silica refer to changes in human activity in the catchments. However, they also reveal the importance of climatic forcing on lake sediment deposition. A significant correlation was found between instrumentally measured records of temperature (163 years) and precipitation (148 years), and varve thickness. Warm winter months indicating a short ice-cover period have the strongest control on varve thickness. This shows that wind-driven resuspension of littoral material is the forcing mechanism of climate on lake sediments. The long-term human-induced erosion pulses observed may even have magnified the climatic signals in some cases. Nevertheless, increased anthropogenic field erosion in the catchment, and the associated leaching of bioavailable nutrients, hampers the observation of climate signals, especially during the last 50 years, in the lakes studied.

Recent history of sedimentation and biotic communities in Lake Pyh�j�rvi, SW Finland

The sedimentary record covering the last 150 years was studied in the productive clear water lake Pyhajarvi in SW Finland. The lake has faced significant human-induced changes: (1) the water level was lowered by almost 2 metres in the early 1850s; (2) planktivorous coregonid fishes were successively introduced, commencing in 1908; and (3) nutrient input from intensified agriculture has increased during this century.