Rask, M., Arvola, L., Forsius & Vuorenmaa, J. 2014: Preface to the Special Issue "Integrated Monitoring in the Valkea-Kotinen Catchment during 1990–2009: Abiotic and Biotic Responses to Changes in Air Pollution and Climate". 1–3.
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Jylhä, K., Laapas, M., Ruosteenoja, K., Arvola, L., Drebs, A., Kersalo, J., Saku, S., Gregow, H., Hannula, H.-R. & Pirinen, P. 2014: Climate variability and trends in the Valkea-Kotinen region, southern Finland: comparisons between the past, current and projected climates. Boreal Env. Res. 19 (suppl. A): 4–30.
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Ruoho-Airola, T., Hatakka, T., Kyllönen, K., Makkonen, U. & Porvari, P. 2014: Temporal trends in the bulk deposition and atmospheric concentration of acidifying compounds and trace elements in the Finnish Integrated Monitoring catchment Valkea-Kotinen during 1988–2011. Boreal Env. Res. 19 (suppl. A): 31–46.
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Vuorenmaa, J., Salonen, K., Arvola, L., Mannio, J., Rask, M. & Horppila, P. 2014: Water quality of a small headwater lake reflects long-term variations in deposition, climate and in-lake processes. Boreal Env. Res. 19 (suppl. A): 47–65.
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Holmberg, M., Futter, M. N., Kotamäki, N., Fronzek, S., Forsius, M., Kiuru, P., Pirttioja, N., Rasmus, K., Starr, M. & Vuorenmaa, J. 2014: Effects of changing climate on the hydrology of a boreal catchment and lake DOC — probabilistic assessment of a dynamic model chain. Boreal Env. Res. 19 (suppl. A): 66–82.
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Arvola, L., Salonen, K., Keskitalo, J., Tulonen, T., Järvinen, M. & Huotari, J. 2014: Plankton metabolism and sedimentation in a small boreal lake — a long-term perspective. Boreal Env. Res. 19 (suppl. A): 83–96.
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Lehtovaara, A., Arvola, L., Keskitalo, J., Olin, M., Rask, M., Salonen, K., Sarvala, J., Tulonen, T. & Vuorenmaa, J. 2014: Responses of zooplankton to long-term environmental changes in a small boreal lake. Boreal Env. Res. 19 (suppl. A): 97–111.
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Rask, M., Sairanen, S., Vesala, S., Arvola, L., Estlander, S. & Olin, M. 2014: Population dynamics and growth of perch in a small, humic lake over a 20-year period — importance of abiotic and biotic factors. Boreal Env. Res. 19 (suppl. A): 112–123.
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Jylhä, K., Laapas, M., Ruosteenoja, K., Arvola, L., Drebs, A., Kersalo, J., Saku, S., Gregow, H., Hannula, H.-R. & Pirinen, P. 2014: Climate variability and trends in the Valkea-Kotinen region, southern Finland: comparisons between the past, current and projected climates. Boreal Env. Res. 19 (suppl. A): 4–30.

Climate variability and temporal trends in the period 1990–2010 around the Valkea-Kotinen environmental monitoring area were compared both with earlier observations and with climate model projections for the 2040s and 2080s. The focus was on climatic variables relevant for aquatic and terrestrial ecosystems, i.e., air temperature, precipitation, snow depth, lake-ice cover, wind speed and solar radiation. Changes in these variables were categorized depending on how rapidly they are likely to emerge from the background of recently-experienced inter-annual variability. The statistically significant increases in annual and spring mean temperatures of about 0.4 °C per decade, observed during the period 1964–2011, are projected to continue in the future, accompanied by corresponding changes in snow depth and lake-ice duration. Based on the multi-model mean estimates, increases in annual mean precipitation and decreases in winter solar radiation would become statistically significant by the 2040s. In addition to the recorded inter-annual variability, inter-model uncertainty intervals were considered.

Ruoho-Airola, T., Hatakka, T., Kyllönen, K., Makkonen, U. & Porvari, P. 2014: Temporal trends in the bulk deposition and atmospheric concentration of acidifying compounds and trace elements in the Finnish Integrated Monitoring catchment Valkea-Kotinen during 1988–2011. Boreal Env. Res. 19 (suppl. A): 31–46.

The precipitation chemistry of nearly 30 acidifying compounds, base cations and several trace elements was monitored in the Valkea-Kotinen catchment in southern Finland during 1988–2011. Measurements of the atmospheric concentration of SO₂, NO₂ and O₃ covered a shorter period. Temporal trends in the components are presented. A notable finding was an indication of a possible increase in the bulk deposition of Cd and NO₃^–. The effect of the changing nitrate deposition on the hydrological continuum of groundwater is also briefly discussed. The transport of air masses to the Valkea-Kotinen catchment was estimated and binds all the results presented in this issue to the development in the emission patterns in Europe.

Vuorenmaa, J., Salonen, K., Arvola, L., Mannio, J., Rask, M. & Horppila, P. 2014: Water quality of a small headwater lake reflects long-term variations in deposition, climate and in-lake processes. Boreal Env. Res. 19 (suppl. A): 47–65.

Small headwater lakes reflect the changes in atmospheric pollution, and are also indicators of climate change. This study examined water chemistry changes in a pristine small lake in southern Finland during the period 1990–2009. Recovery from acidification was indicated, while no major changes in trace metal concentrations were found. In the course of the study, total phosphorus concentration was fluctuating in deteriorated oxygen conditions in the hypolimnion. However, total and organic nitrogen remained stable, but inorganic nitrogen concentration increased during the period, and was more dependent on the hydrology and in-lake processes than nitrogen deposition. Dissolved organic and inorganic carbon concentrations also increased, presumably due to decreasing acidity and changes in runoff, and increased alkalinity and decreased phytoplankton production, respectively. The results emphasise the importance of integrated intensive long-term monitoring of physical, chemical and biological variables for detecting impacts of changing environmental conditions.

Holmberg, M., Futter, M. N., Kotamäki, N., Fronzek, S., Forsius, M., Kiuru, P., Pirttioja, N., Rasmus, K., Starr, M. & Vuorenmaa, J. 2014: Effects of changing climate on the hydrology of a boreal catchment and lake DOC — probabilistic assessment of a dynamic model chain. Boreal Env. Res. 19 (suppl. A): 66–82.

Uncertainty in future climate projections is widely recognised, yet few impact model studies explore the implications of this uncertainty on catchment hydrological and biogeochemical responses. Here, we report a novel model chain using HBV, INCA-C and MyLake to simulate runoff, snow dynamics, ice cover, soil moisture, lake thermal stratification and in-lake DOC concentrations in Valkea-Kotinen, a headwater lake in a forest-covered boreal catchment. Impact response surfaces (IRSs) were constructed with 63 combinations of changes in temperature (–2 °C to +14 °C) and precipitation (–10% to +50%). Superimposing probabilistic projections of climate change onto the IRSs, we illustrate the uncertainty in impacts under projected climate change. Projected climate warming is likely to result in drier soils, shorter snow and ice periods, as well as earlier onset and longer duration of lake stratification. In contrast to earlier results, in-lake DOC concentrations are projected to decrease slightly (–6%), primarily due to decreased terrestrial runoff.

Arvola, L., Salonen, K., Keskitalo, J., Tulonen, T., Järvinen, M. & Huotari, J. 2014: Plankton metabolism and sedimentation in a small boreal lake — a long-term perspective. Boreal Env. Res. 19 (suppl. A): 83–96.

We used weekly or biweekly data from the years 1990–2009 to analyse variation in primary production (PP), chlorophyll a (Chl) and respiration (R) of plankton, and organic matter sedimentation (S) in a small boreal lake with intense water colour. PP and S had a decreasing trend over the study period while no trend was found in Chl and R. PP and R were in balance except during the last five years when R > PP. The results indicated that light limited PP, while occasionally phosphorus, nitrogen and/or dissolved inorganic carbon could also act as limiting factors. The decrease in S coincided with that of PP although during the last five years allochthonous organic matter load seemed to compensate for the decrease in PP. Weather and hydrological conditions were the key drivers in modifying the properties of the lake while the metabolic processes were intercorrelated, and R correlated strongly with water temperature.

Lehtovaara, A., Arvola, L., Keskitalo, J., Olin, M., Rask, M., Salonen, K., Sarvala, J., Tulonen, T. & Vuorenmaa, J. 2014: Responses of zooplankton to long-term environmental changes in a small boreal lake. Boreal Env. Res. 19 (suppl. A): 97–111.

Zooplankton dynamics were examined in a small boreal lake over a 20-year period and interpreted in relation to climate change, brownification and recovery from acidification. Significant changes were recorded in the abundance of dominating crustacean species but not of rotifer species. According to redundancy analysis (RDA), the long-term pattern in crustacean zooplankton was mainly associated with abiotic factors like water colour, alkalinity and total phosphorus. Primary production of phytoplankton was the most important biological parameter whereas planktivorous perch and Chaoborus larvae had a marginal contribution. Biological factors were relatively more important for rotifers than for crustaceans, primary production being the most powerful explanatory parameter, followed by alkalinity, total phosphorus and colour. The changes in the zooplankton community were mainly related to increased organic carbon load and recovery from acidification. Within the food web, bottom-up regulation seemed to exceed the importance of top-down control.

Rask, M., Sairanen, S., Vesala, S., Arvola, L., Estlander, S. & Olin, M. 2014: Population dynamics and growth of perch in a small, humic lake over a 20-year period — importance of abiotic and biotic factors. Boreal Env. Res. 19 (suppl. A): 112–123.

Population dynamics and growth of perch (Perca fluviatilis) were studied over a 20-year period in a small, humic, boreal lake (Valkea-Kotinen). The annual estimated total population size of perch (> 8 cm, total length) varied from 2700–13400 and the biomass from 16.1–44.2 kg ha^–1. Strong year-classes were born at four-year intervals between 1991 and 2003. A decreasing trend was recorded in the growth of 1- and 2-year-old perch, whereas a slight increase appeared in the yearly growth of 4- and 5-year-old fish. Our view is that the decrease in the early growth was caused by negative effects of increased organic carbon loads on water colour and hence on light conditions and the general productivity of the plankton community. These changes outweighed the expected positive effect of increasing temperature for young perch but not for the older fish.