Search

Summary 1. Spatial variation of methane (CH4) efflux from the littoral zone of a meso-eutrophic boreal lake was studied with a closed-chamber technique for three summer days in 22 vegetation stands, consisting of three emergent and three floating-leaved species. 2. Between-species differences in CH4 emission were significant. The highest emissions were measured from the emergent Phragmites australis stands (0.5–1.7 mmol m−2 h−1), followed by Schoenoplectus lacustris > Equisetum fluviatile > Nuphar lutea > Sparganium gramineum > Potamogeton natans. Within-species differences between stands were not significant. 3. In P. australis stands, the stand-specific mean CH4 emission was significantly correlated with solar radiation, probably indicating the role of effective pressurised ventilation on CH4 fluxes. The proportion of net primary production emitted as CH4 was significantly higher in P. australis stands (7.4%) than in stands of S. lacustris and E. fluviatile (both 0.5%). 4. In N. lutea stands, CH4 efflux was negatively correlated with the mean fetch and positively with the percentage cover of leaves on the water surface. There were no differences in CH4 efflux between intact N. lutea leaves and those grazed by coleopteran Galerucella nymphaeae. In S. graminaeum and P. natans stands, CH4 effluxes were not related to any of the measured environmental variables. 5. For all vegetation stands, the biomass above water level explained about 60% of the observed spatial variation in CH4 emission, indicating the important role of plants as gas conduits and producers of substrates for methanogens in the anoxic sediment.

Summary 1. Temperature and many other physical and chemical factors affecting CO2 production in lake sediments vary significantly both seasonally and spatially. The effects of temperature and sediment properties on benthic CO2 production were studied in in situ and in vitro experiments in the boreal oligotrophic Lake Paajarvi, southern Finland. 2. In in situ experiments, temperature of the water overlying the shallow littoral sediment varied seasonally between 0.5 and 15.7 °C, but in deep water (≥20 m) the range was only 1.1–6.6 °C. The same exponential model (r2 = 0.70) described the temperature dependence at 1.2, 10 and 20 m depths. At 2.5 and 5 m depths, however, the slopes of the two regression models (r2 = 0.94) were identical but the intercept values were different. Sediment properties (wet, dry, mineral and organic mass) varied seasonally and with depth, but they did not explain a significantly larger proportion of variation in the CO2 output rate than temperature. 3. In in vitro experiments, there was a clear and uniform exponential dependence of CO2 production on temperature, with a 2.7-fold increase per 10 °C temperature rise. The temperature response (slope of regression) was always the same, but the basic value of CO2 production (intercept) varied, indicating that other factors also contributed to the benthic CO2 output rate. 4. The annual CO2 production of the sediment in Lake Paajarvi averaged 62 g CO2 m−2, the shallow littoral at 0–3 m depth releasing 114 g CO2 m−2 and deep profundal (>15 m) 30 g CO2 m−2. On the whole lake basis, the shallow littoral at 0–3 m depth accounted for 53% and the sediment area in contact with the summer epilimnion (down to a depth c. 10 m) 75% of the estimated total annual CO2 output of the lake sediment, respectively. Of the annual production, 83% was released during the spring and summer. 5. Using the temperature-CO2 production equations and climate change scenarios we estimated that climatic warming might increase littoral benthic CO2 production in summer by nearly 30% from the period 1961–90 to the period 2071–2100.

In situ enclosure experiments were performed in a highly humic lake to examine the effects of acidification on phyto-, bacterio- and zooplankton. The acidity of three enclosures was adjusted with H2SO4 to pH 3.5, 4 and 5 and one enclosure and the lake served as controls. The diversity of plankton as well as the mean concentration of chlorophyll α, primary production and respiration of plankton decreased with increasing acidity. Bacterial density was slightly lower in the pH 3.5 enclosure than in the other enclosures and in the lake, but there were no differences in the morphological type or mean volume of the cells between different treatments. In the acidified enclosures Cryptomonas ovata and Chlamydomonas spp. were the dominant phytoplankters, while Dinobryon and Mallomonas species seemed to be most sensitive to acidity. Keratella cochlearis, Kellicottia longispina and Bosmina longispina were the most tolerant zooplankton to acidity.