Summary 1.We analysed the development of submerged macrophytes in Lake Fure, Denmark, experiencing a 30-fold increase of phosphorus input from year 1900 to 1970 and a subsequent decline to twice the 1900 level in 2005. Nutrient enrichment stimulated phytoplankton growth and restricted macrophyte distribution by reducing water transparency from a summer mean of 5-6 m in the early 1900s to a minimum of 1.6 m at the peak of eutrophication, followed by recovery to a recent maximum of 4.1 m. 2. Macrophyte occurrence and abundance changed in accordance with altered environmental conditions and species' life-history traits. Small angiosperms, mosses and characeans disappeared in the 1970s to 1980s, along with all vegetation in deeper waters (5 -8 m), and have only partly recovered recently. Tall angiosperms became dominant while small species vanished. All 10 characeans originally present disappeared at the peak of eutrophication, but four reappeared. Mesotrophic macroalgae were replaced by hypertrophic species whose dominance has persisted. 3. Species richness decreased from 37 to 13 species at the peak of eutrophication, before returning to 25 species during the recent recovery. Species richness increased with transparency because deeper growth generates more niches. 4. Reduction of species distribution and richness has been reversible following nutrient reduction of the long eutrophied lake, whereas species composition and abundance have not. The historical legacy of community composition is strong, as reflected by closer correlations to time than to measures of nitrogen and phosphorus availability and water transparency. 5. Synthesis . Although phosphorus input may decline further, reassembly of the original macrophyte community will face difficulties. Oligotrophic freshwater species have become rare throughout Denmark, reducing the probability of recolonization. Species reaching Lake Fure may fail to establish because sediments have become richer in nutrients and organic matter and less consolidated, while shading and competition have increased from emergent reeds, tall submerged angiosperms and fast-growing macroalgae.

Summary 1. We examined the detailed temperature dependence (0–40 °C) of bacterial metabolism associated with fine sediment particles from three Danish lowland streams to test if temperature dependence varied between sites, seasons and quality of organic matter and to evaluate possible consequences of global warming. 2. A modified Arrhenius model with reversible denaturation at high temperatures could account for the temperature dependence of bacterial metabolism and the beginning of saturation above 35 °C and it was superior to the unmodified Arrhenius model. Both models overestimated respiration rates at very low temperatures (<5 °C), whereas Ratkowsky's model – the square root of respiration – provided an excellent linear fit between 0 and 30 °C. 3. There were no indications of differences in temperature dependence among samples dominated by slowly or easily degradable organic substrates. Optimum temperature, apparent minimum temperature, Q10‐values for 0–40 °C and activation energies of bacterial respiration were independent of season, stream site and degradability of organic matter. 4. Q10‐values of bacterial respiration declined significantly with temperature (e.g. 3.31 for 5–15 °C and 1.43 for 25–35 °C) and were independent of site and season. Q10‐values of bacterial production behaved similarly, but were significantly lower than Q10‐values of respiration implying that bacterial growth efficiency declined with temperature. 5. A regional warming scenario for 2071–2100 (IPCC A2) predicted that mean annual temperatures will increase by 3.5 °C in the air and 2.2–4.3 °C in the streams compared with the control scenario for 1961–1990. Temperature is expected to rise more in cool groundwater‐fed forest springs than in open, summer‐warm streams. Mean annual bacterial respiration is estimated to increase by 26–63% and production by 18–41% among streams assuming that established metabolism–temperature relationships and organic substrate availability remain the same. To improve predictions of future ecosystem behaviour, we further require coupled models of temperature, hydrology, organic production and decomposition.

Abstract:Continuous temperature measurements at 11 stream sites in small lowland streams of North Zealand, Denmark over a year showed much higher summer temperatures and lower winter temperatures along the course of the stream with artificial lakes than in the stream without lakes. The influence of lakes was even more prominent in the comparisons of colder lake inlets and warmer outlets and led to the decline of cold-water and oxygen-demanding brown trout. Seasonal and daily temperature variations were, as anticipated, dampened by forest cover, groundwater input, input from sewage plants and high downstream discharges. Seasonal variations in daily water temperature could be predicted with high accuracy at all sites by a linear air-water regression model (r 2 : 0Ð903-0Ð947). The predictions improved in all instances (r 2 : 0Ð927-0Ð964) by a non-linear logistic regression according to which water temperatures do not fall below freezing and they increase less steeply than air temperatures at high temperatures because of enhanced heat loss from the stream by evaporation and back radiation. The predictions improved slightly (r 2 : 0Ð933-0Ð969) by a multiple regression model which, in addition to air temperature as the main predictor, included solar radiation at un-shaded sites, relative humidity, precipitation and discharge. Application of the non-linear logistic model for a warming scenario of 4-5°C higher air temperatures in Denmark in 2070-2100 yielded predictions of temperatures rising 1Ð6-3Ð0°C during winter and summer and 4Ð4-6Ð0°C during spring in un-shaded streams with low groundwater input. Groundwater-fed springs are expected to follow the increase of mean air temperatures for the region. Great caution should be exercised in these temperature projections because global and regional climate scenarios remain open to discussion.

1. Temperature, organic carbon and oxygen consumption were measured over a year at 13 sites in four lowlands streams within the same region in North Zealand, Denmark with the objectives of determining: (i) spatial and seasonal differences between open streams, forest streams and streams with or without lakes, (ii) factors influencing the temperature dependence of oxygen consumption rate, (iii) consequences of higher temperature and organic content in lake outlets on oxygen consumption rate, and (iv) possible consequences of forecasted global warming on degradation of organic matter. 2. High concentrations of easily degradable dissolved (DOC) and particulate organic carbon (POC) were found in open streams downstream of plankton-rich lakes, while high concentrations of recalcitrant DOC were found in a forest brook draining a forest swamp. Concentrations of predominantly recalcitrant POC and DOC were low in a groundwaterfed forest spring. Overall, DOC concentration was two to 18 times higher than POC concentrations. 3. Oxygen consumption rate at 20°C was higher during summer than winter, higher in open than shaded streams and higher in lake outlets than inlets. Rate was closely related to concentrations of chlorophyll and POC but not to DOC. The ratio of oxygen consumption rate to total organic concentrations (DOC + POC), serving as a measure of organic degradability, was highest downstream of lakes, intermediate in open streams and lowest in forest streams. 4. Temperature coefficients describing the exponential increase of oxygen consumption rate between 4 and 20°C averaged 0.121°C )1 (Q 10 of 3.35) in 70 measurements and showed no significant variations between seasons and stream sites or correlations with ambient temperature and organic content. 5. Oxygen consumption rate was enhanced downstream of lakes during summer because of higher temperature and, more significantly, greater concentrations of degradable organic carbon. Oxygen consumption rates were up to seven times higher in the stream with three impoundments than in a neighbouring unshaded stream and 21 times higher than in the groundwater-fed forest spring. 6. A regional climate model has calculated a dramatic 4-5°C rise in air temperature over Denmark by 2070-2100. If this is realised, unshaded streams are estimated to become 2-3°C warmer in summer and winter and 5-7°C warmer in spring and, thereby, increase oxygen consumption rates at ambient temperature by 30-40% and 80-130%, respectively. Faster consumption of organic matter and dissolved oxygen downstream of point sources should increase the likelihood of oxygen stress of the stream biota and lead to the export of less organic matter but more mineralised nutrients to the coastal waters.