Magnuson, J. J., Benson, B. J. & Kratz, T. K. 2004: Patterns of coherent dynamics within and between lake districts at local to intercontinental scales. Boreal Env. Res. 9: 359–369.

Spatial patterns of coherent inter-annual dynamics between lakes occur within and between lake districts at regional, continental, and even intercontinental scales. Climatic variability and change are external drivers of lake dynamics but individual lake ecosystems differentially filter these signals and alter their expression. When related to landscape position spatial patterns in coherence can be uniform, unstructured, or structured. A structured pattern emerges for chemical responses to drought in a lake district dominated by groundwater while a more uniform pattern emerges in a stream-flow dominated lake district. Near-surface water temperatures and ice dates have a uniform pattern within and even between some lake districts; near-bottom water temperatures have an unstructured pattern; water levels have a complex pattern. Coherence in ice dates declines with latitudinal distance between lakes, but some coherence persists even at intercontinental scales that appears related to long-term climate change and common large-scale climate drivers.

Blenckner, T., Järvinen, M. & Weyhenmeyer, G. A. 2004: Atmospheric circulation and its impact on ice phenology in Scandinavia. Boreal Env. Res. 9: 371–380.

Atmospheric circulation is important in affecting surface climate and ecosystems. In this study, we compared the impact of north-atlantic and regional atmospheric circulation, as represented by the North Atlantic Oscillation (NAO) index and a set of regional circulation indices, on ice phenology of 50 Scandinavian lakes. Both ice freeze and ice break-up dates were coherent over the whole region and were significantly correlated with both types of circulation indices. Correlations were especially strong for regional circulation indices. The application of regional indices, here for the first time related with ice data over a large area, allowed the determination of the type (i.e. meridional/zonal wind and cyclonic/anticyclonic conditions) of atmospheric circulation influencing the ice phenology. The results suggest that regional circulation indices are very useful tools, in addition to global circulation, to improve the understanding of the interaction between ecosystem processes and climate.

Arvola, L., Räike, A., Kortelainen, P. & Järvinen, M. 2004: The effect of climate and landuse on TOC concentrations and loads in Finnish rivers. Boreal Env. Res. 9: 381–387.

The variability of the concentration and export of the total organic carbon (TOC) was analysed in 16 rivers in Finland (northern Europe). The river basins were categorized by major landuse settings, and their geographical location ranged from the southern boreal to sub-arctic. In the long-run (1975–2000), the concentration of the TOC had a statistically significant (?p < 0.05) decreasing trend in nine rivers at least once during March, May, August and October. The estimated annual load of the TOC was statistically significantly higher (?p < 0.03) in the basins characterized by peatlands than in those dominated by fields, being on average 5.4 and 4.1 g C m^–2 a^–1, respectively. The winter (January–March) North Atlantic Oscillation (NAO) index predicted rather well (mean of r² = 0.48 and p = 0.04125) the TOC load in March in the eight northernmost rivers.

George, D. G., Järvinen, M. & Arvola, L. 2004: The influence of the North Atlantic Oscillation on the winter characteristics of Windermere (UK) and Pääjärvi (Finland). Boreal Env. Res. 9: 389–399.

The atmospheric pressure gradient known as the North Atlantic Oscillation (NAO) has been shown to influence the dynamics of lakes in a number of European regions. Here, we compare the impact of the NAO on the winter dynamics of Windermere, a lake located in the north-west of England, and Pääjärvi, a lake situated in southern Finland. At both sites, the variations in the NAO index had a significant effect on the winter weather and the physical and chemical characteristics of the lakes. At Windermere, the strongest correlations were those noted for the air temperature, precipitation, the number of days when ice was recorded on the lake and the de-trended concentration of nitrate. At Pääjärvi the strongest correlations were with the air temperature and the de-trended concentrations of nitrate. One striking difference between the sites was the sign of the correlations noted between the NAO index and the de-trended nitrate. At Windermere, this correlation was negative and appeared to be driven by the enhanced terrestrial uptake of nitrate in mild winters. At Pääjärvi, the correlation was positive and was associated with the earlier `flush' of melt water from the catchment. The results are discussed in relation to the regional effects of climate change and the sensitivity of boreal lakes to subtle changes in the `centre of action' of the NAO.

Nõges, T. 2004: Reflection of the changes of the North Atlantic Oscillation Index and the Gulf Stream Position Index in the hydrology and phytoplankton of Võrtsjärv, a large, shallow lake in Estonia. Boreal Env. Res. 9: 401–407.

The North Atlantic Oscillation Index (NAO) was correlated with winter and spring air temperature in Estonia while wind speed dynamics were mediated by the position of the Gulf Stream. In the low-NAO years, the large (270 km²), shallow (mean depth 2.8 m), polymictic lake, Võrtsjärv, was significantly shallower than in the high-NAO years. Phytoplankton biomass was higher in the springs after high-NAO winters, and it was inversely related with the depth of the lake in summer and autumn but not in spring. In Võrtsjärv, the North Atlantic Oscillation directly influences spring phytoplankton via its effects on the ice cover dynamics. The climate signal `recorded' in the water levels results in lower summer/autumn phytoplankton biomass in the years when the water level remains high throughout the year. The effects of light and phosphorus limitation on phytoplankton at higher water levels are discussed.

Weyhenmeyer, G. A., Willén, E. & Sonesten, L. 2004: Effects of an extreme precipitation event on water chemistry and phytoplankton in the Swedish Lake Mälaren. Boreal Env. Res. 9: 409–420.

Extreme events can often be ecologically more relevant than fluctuations in the mean climate. We studied the effects of an exceptionally rainy period in 2000 on 13 physical and chemical variables and on the phytoplankton biomass and composition in Lake Mälaren, the third largest lake in Sweden. The rainy period caused a distinct increase in the chemical loading, in particular the loading of organic carbon, into Lake Mälaren. As a consequence water color, measured as light absorption of filtered water, was up to 3.4 times higher in 2001 than ever recorded since 1965. In addition, reactive silica concentrations doubled, particle concentrations were the highest and conductivity was the lowest registered. The changes in physical and chemical conditions of Lake Mälaren in 2001 showed temporal and spatial differences depending on runoff processes in the catchment, water residence times and the type of variable. It is suggested that especially the increase in water color was responsible for the observed doubling in the cryptophycean biomass in Lake Mälaren in May 2001. The increase in water color and cryptophycean biomass was also observed in a nearby lake. Such increases required alterations in the treatment process of raw water from Lake Mälaren for the drinking water supply of Stockholm city.

Huttunen, J. T., Hammar, T., Manninen, P., Servomaa, K. & Martikainen, P. J. 2004: Potential springtime greenhouse gas emissions from a small southern boreal lake (Keihäsjärvi, Finland). Boreal Env. Res. 9: 421–427.

Concentrations of dissolved greenhouse gases (GHGs) methane (CH₄), carbon dioxide (CO₂) and nitrous oxide (N₂O) were investigated in a small southern boreal lake during the late winter ice-cover in 2000, 2001 and 2002. Potential emissions of these gases to the atmosphere at ice melt were estimated from their concentration profiles in the water column under ice during winter. The concentrations of CH₄, CO₂ and N₂O increased with the depth, but the year-to-year variations were low. Carbon dioxide contributed to 99% of the global warming potential (GWP) of the total springtime GHG emissions which ranged from 103 to 128 g CO₂-equivalents m^–2. The results indicated that these kinds of northern mesotrophic lakes are not important sources of CH₄ and N₂O, but are probably significant sources of CO₂.

Reinart, A., Paavel, B., Pierson, D. & Strömbeck, N. 2004: Inherent and apparent optical properties of Lake Peipsi, Estonia. Boreal Env. Res. 9: 429–445.

Lake Peipsi on the border of Estonia and Russia is a large, shallow, biologically very productive lake. It is bordered by many wetland areas. On the basis of underwater radiation measurements and analyses of water samples we have shown that the optical properties of L. Peipsi are very variable both spatially and temporally, covering a large range of water types previously classified in Estonian and Finnish lakes. Inherent and apparent optical properties are compared with those of the two other large lakes, Vänern and Vättern in Sweden. Lake Peipsi has higher concentrations of optically active substances, which are known to influence satellite measurements and underwater light field. Therefore, the lake complements the continuum between L. Vänern and L. Vättern in terms of water optical properties and adds new data into the database about optical properties of turbid and humic lakes. Results obtained here can be used in bio-optical modeling to develop algorithms for remote sensing and calculations of underwater light field.

Sipelgas, L., Arst, H., Raudsepp, U., Kõuts, T. & Lindfors, A. 2004: Optical properties of coastal waters of northwestern Estonia: in situ measurements. Boreal Env. Res. 9: 447–456.

A field study of optical properties of the coastal waters of northwestern Estonia was performed from 29 July to 3 August 2002. It was the relaxation period of extensive cyanobacterial bloom in the Gulf of Finland. The concentrations of optically active substances (chlorophyll a, coloured dissolved organic matter, suspended matter) and spectrometric light attenuation coefficients were determined from water samples. In addition, continuous measurements of attenuation and absorption coefficients were performed using a flow-through system based on the ac-9 device. The results show that variations of the optical properties of the water masses in the coastal sea of northwestern Estonia are mainly due to changes in the concentration of coloured dissolved organic matter. Water turbidity is well characterized by the slope factor of the scattering coefficient spectrum that is approximated by the power law.