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Phytoplankton form the base of the pelagic food web in inland waters. Unlike rooted plants with access to nutrients in the sediment, phytoplankton depend on the open water as their sole direct source of minerals. Phytoplankton comprise cyanobacteria and phylogenetically diverse eukaryotic algae that convert light energy and mineral nutrients into organic matter. Many species also exploit the elements and energy within dissolved organic compounds and particles produced in the catchment or within the water. Here, we describe the nutrient requirements of phytoplankton, their different modes of nutrition, the mechanisms they employ to acquire nutrients and the ecological consequences of their varying ability to exploit an often scarce and spatially and temporally variable resource. When nutrients are abundant, often as a result of human disruption of nutrient cycles, phytoplankton productivity, and often biomass, increases to the point that it causes a range of ecological consequences that reduce the value of the water resource for mankind.

Climate change and other anthropogenic stressors have led to long-term changes in the thermal structure, including surface temperatures, deepwater temperatures, and vertical thermal gradients, in many lakes around the world. Though many studies highlight warming of surface water temperatures in lakes worldwide, less is known about long-term trends in full vertical thermal structure and deepwater temperatures, which have been changing less consistently in both direction and magnitude. Here, we present a globally-expansive data set of summertime in-situ vertical temperature profiles from 153 lakes, with one time series beginning as early as 1894. We also compiled lake geographic, morphometric, and water quality variables that can influence vertical thermal structure through a variety of potential mechanisms in these lakes. These long-term time series of vertical temperature profiles and corresponding lake characteristics serve as valuable data to help understand changes and drivers of lake thermal structure in a time of rapid global and ecological change.

Unlike in land plants, photosynthesis in many aquatic plants relies on bicarbonate in addition to carbon dioxide (CO₂) to compensate for the low diffusivity and potential depletion of CO₂ in water. Concentrations of bicarbonate and CO₂ vary greatly with catchment geology. In this study, we investigate whether there is a link between these concentrations and the frequency of freshwater plants possessing the bicarbonate use trait. We show, globally, that the frequency of plant species with this trait increases with bicarbonate concentration. Regionally, however, the frequency of bicarbonate use is reduced at sites where the CO₂ concentration is substantially above the air equilibrium, consistent with this trait being an adaptation to carbon limitation. Future anthropogenic changes of bicarbonate and CO₂ concentrations may alter the species compositions of freshwater plant communities.

Phytoplankton organisms are, by definition, photosynthetic, and include cyanobacteria as well as algae. Most of the phytoplankton in inland waters obtain all of their energy from light, and their chemical requirements from dissolved inorganic solutes, i.e. are photolithotrophs. Growth rate limitation by resource supply in photolithotrophic phytoplankton can involve light, phosphate, combined nitrogen (except in cyanobacterial nitrogen-fixers), inorganic carbon and iron. Some phytoplankton organisms are mixotrophs, i.e. can obtain energy and carbon from organic compounds in the environment as well as from light and inorganic carbon. This category includes sapromixotrophs (using dissolved organic carbon) and phagomixotrophs (using particulate organic matter). Sapromixotrophy in phytoplankton is rarer in the natural environment that is the loss of organic matter by photolithotrophs. Phagomixotrophy also occurs in the natural environment, and can relate to the acquisition of phosphorus, iron and nitrogen as well as of organic carbon. Even in photolithotrophic phytoplankton cells phosphorus and combined nitrogen can be obtained from dissolved organic phosphate and organic nitrogen.

1. The motivation for this report was the documented recent deterioration in the water quality in the two basins of Windermere since around 2000. The aim was to analyse the long-term records to assess the likely causes of this deterioration. 2. Records were analysed between 1950 and 2007, where available, including data on lake physics (annual mean water temperature at the surface, water temperature at depth, duration of stratification), nutrient availability (mean winter concentration of total phosphorus, soluble reactive phosphorus and nitrate); responding chemical variables (minimum concentration of nitrate, minimum concentration of silica, maximum pH and minimum oxygen concentration at depth); phytoplankton chlorophyll a (annual mean, spring maximum and summer maximum); zooplankton grazing (spring mean and summer mean abundance); and the fish predation on zooplankton (hydroacoustic data, perch recruitment and Arctic charr abundance, the latter only available in the North Basin). 3. Changes in the period upto around 1991 are largely the result of increasing nutrient load to the lake. This led to increasing winter concentrations of soluble reactive phosphorus, nitrate and total phosphorus (latter significant in South Basin only). This caused increased concentrations of phytoplankton chlorophyll a as an annual mean and as spring and summer maxima in both basins. This in turn caused a greater depletion of silica in the late spring, increased pH in the summer (significant in North Basin only) and reduction in the minimum oxygen concentration at depth. 4. Since around 2000 there has been a significant reduction in water quality in both basins: summer phytoplankton chlorophyll a has increased, as has maximum pH and the minimum concentration of oxygen (only significant in the North Basin). Summer Secchi depth has declined in both basins. 5. Annual phosphorus load from the two wastewater treatment works (WwTWs) that discharge directly to the lake has fallen by about 50%

1. The input from the wastewater treatment works (WwTW) that discharge directly into Windermere at Ambleside and Tower Wood contribute a large amount of the total phosphorus load to the lake. In 1992 this load was reduced by establishment of tertiary treatment at each works. This elicited a rapid improvement in water quality but this started to deteriorate subsequently. This report analyses the output phosphorus loads from the two WwTW to establish to what extent the deterioration might be caused by any increases from the WwTW. 2. Data were available from 1978 with, on average, weekly sampling or better from the two WwTW. Before 1987 there were infrequent measurements of P-load. Since 1995 there have been one or two load measurements per week from each plant. 3. The average seasonal pattern of flow is relatively even, but higher values in winter suggest input from non-domestic sources. There are large seasonal changes in the concentration of phosphorus leaving the WwTWs, with highest concentrations in summer, possibly as a result of higher tourist numbers and slightly lower flow. The load of phosphorus also varies seasonally: monthly load is highest in summer. 4. In 1991 the average concentration of phosphorus leaving the WwTW was 3.2 and 3.8 mg L-1 at Ambleside and Windermere Tower Wood respectively. There was a step-reduction in the concentration of phosphorus leaving the two WwTW in 1992 and no evidence for a return to the higher values of pre 1992, apart from transiently higher values at Ambleside in 1999 and 2000 (around 2.5 mg L-1). Between 1993 and 2007 the annual average concentration of phosphorus leaving the WwTWs was 1.48 mg L-1 and 1.26 mg L-1 for Ambleside and Windermere Tower Wood respectively. 5. Annual phosphorus load has fallen by about 50% in 1993-2007 compared to 1978-1991 at both WwTWs. At Ambleside it has fallen from 2.2 to 1.09 Mg y-1 and at Windermere Tower Wood it has fallen from 5.85 to 2.82 Mg y-1. Loads from storm overflows are not include

1. The results of biological, chemical and physical analyses undertaken on samples collected monthly during 2007 from the North and South Basin of Windermere, Grasmere, Derwent Water and Bassenthwaite Lake are presented and interpreted in this report. This work continues long-term measurements, of many years duration, although the reduction in sampling frequency from fortnightly to monthly has meant that some comparisons with historic data are difficult. 2. In many of the lakes, surface water temperature was higher than usual in April and May during a period of unseasonably hot weather, and cooler than usual during summer as a result of particularly cool and wet weather. As a result of the different volumes and depths of the lakes, temperature at depth was substantially different among lakes. Temperature at depth in the North Basin of Windermere was about 7ºC all summer while in Bassenthwaite Lake it reached 16.1ºC due to much weaker and intermittent stratification. 3. Oxygen depletion at depth was most marked in the productive and strongly stratified Grasmere with several months of anoxic water at the sediment surface. The weakly stratified Bassenthwaite Lake showed little oxygen depletion as did the relatively unproductive North Basin of Windermere. 4. Alkalinity varied seasonally with the concentration building up during the summer. Based on annual means, and using the categories of the EU Water Framework Directive, Derwent Water, Grasmere and Bassenthwaite Lake would be classified as having low alkalinity (less than 10 g CaCO3 m-3, although Bassenthwaite Lake is very close to the upper boundary) while the two basins of Windermere would be classified as having moderate alkalinity (between 10 and 50 g CaCO3 m-3). There was variation in pH across the lakes because of varying alkalinity. Superimposed on this were episodes of high pH, particularly in the South Basin of Windermere, resulting from high rates of phytoplankton carbon uptake during photosynthesis. 5. Phos

The Over Water SSSI was first notified because of the biodiversity of the aquatic macrophytes and marginal vegetation at the site, as well as the comparative eutrophy of the lake compared to surrounding water bodies. Recent aquatic macrophyte surveys suggest that the site has deteriorated, and has lost “characteristic” species for which the site was originally notified. Reduced water levels and nutrient enrichment have been suggested as causes of change. Over the study period there is no evidence of a trend in water levels or in the volume of water abstracted from Over Water. The length and severity of drawdown were greatest in 1995, 1996 and 2003 and abstraction volume was also high. In “dry” years, abstraction could potentially account for much of the variation in water level. There is considerable variation in water level that cannot be explained by abstraction. This indicates that there are unquantified losses of water from the lake and/or errors associated with the known components of the water balance. The present evidence suggests that high water level fluctuation, especially in dry years with high abstraction, could have an adverse effect on the macrophyte community. However, it is also possible that the presence of Elodea nuttallii may be responsible for the loss of native deeper-water species. Water chemistry data from the Environment Agency for the period April 1985 - December 1989 were analysed. These data suggest that Over Water had a moderate alkalinity and productivity in the late 1980s. The concentrations and seasonal patterns are what would be expected for this lake type apart from the moderately high concentrations of orthophosphate throughout the year. However, simple modelling suggests that phosphorus limits phytoplankton growth in the lake. Estimates of compliance with the Water Framework Directive, based on current understanding of the procedures, suggests that the average phytoplankton chlorophyll a concentration would give a ‘moderate’ status

This report continues a sequence of annual reviews of water quality in Windermere and Bassenthwaite Lake that are subject to the provisions of the Urban Waste Water Treatment Directive. Grasmere, which feeds into Windermere is also included because of concerns over a deterioration in water quality over the last, approximately 30 years and Derwent Water is included, partly as a comparison with Bassenthwaite Lake and partly because it is the sole refuge of a healthy population of the vendace, Coregonus albulus.

Ecosystem resistance to a single stressor relies on tolerant species that can compensate for sensitive competitors and maintain ecosystem processes, such as primary production. We hypothesize that resistance to additional stressors depends increasingly on species tolerances being positively correlated (i.e. positive species co-tolerance). Initial exposure to a stressor combined with positive species co-tolerance should reduce the impacts of other stressors, which we term stress-induced community tolerance. In contrast, negative species co-tolerance is expected to result in additional stressors having pronounced additive or synergistic impacts on biologically impoverished functional groups, which we term stress-induced community sensitivity. Therefore, the sign and strength of the correlation between species sensitivities to multiple stressors must be considered when predicting the impacts of global change on ecosystem functioning as mediated by changes in biodiversity.