Eutrophication and restoration in temperate lakes

Eutrophication affects many lakes and reservoirs worldwide. It is caused by excessive amounts of nutrients entering waterbodies from their catchments, mainly due to human activity. The main sources of these nutrients are discharges from industry and wastewater treatment systems, and agricultural runoff. The water quality problems caused by eutrophication, such as harmful algal blooms, affect the sustainable use of lakes for agriculture, fisheries, recreation, tourism and water supply. They also degrade habitat quality and threaten biodiversity. A range of methods for improving lake water quality are explored, including catchment management and in-lake restoration measures. The potential impacts of these on lake biodiversity are explored, including species interactions and ecosystem feedbacks that may confound the recovery process. A particular challenge is the fact that achieving sustainable recovery may take many years, mainly due to the impact of legacy pollution problems. This must be taken into account when planning and implementing eutrophication management options, because these slow recovery periods can exceed the timescales that people are willing to accept. While this review focuses on the many well documented studies of restoration and recovery processes in temperate lakes, it also highlights the need for similar research on tropical and sub-tropical systems

Phytoplankton community responses in a shallow lake following lanthanum-bentonite application

The release of phosphorus (P) from bed sediments to the overlying water can delay the recovery of lakes for decades following reductions in catchment contributions, preventing water quality targets being met within timeframes set out by environmental legislation (e.g. EU Water Framework Directive: WFD). Therefore supplementary solutions for restoring lakes have been explored, including the capping of sediment P sources using a lanthanum (La)-modified bentonite clay to reduce internal P loading and enhance the recovery process. Here we present results from Loch Flemington where the first long-term field trial documenting responses of phytoplankton community structure and abundance, and the UK WFD phytoplankton metric to a La-bentonite application was performed. A Before-After-Control-Impact (BACI) analysis was used to distinguish natural variability from treatment effect and confirmed significant reductions in the magnitude of summer cyanobacterial blooms in Loch Flemington, relative to the control site, following La-bentonite application. However this initial cyanobacterial response was not sustained beyond two years after application, which implied that the reduction in internal P loading was short-lived; several possible explanations for this are discussed. One reason is that this ecological quality indicator is sensitive to inter-annual variability in weather patterns, particularly summer rainfall and water temperature. Over the monitoring period, the phytoplankton community structure of Loch Flemington became less dominated by cyanobacteria and more functionally diverse. This resulted in continual improvements in the phytoplankton compositional and abundance metrics, which were not observed at the control site, and may suggest an ecological response to the sustained reduction in filterable reactive phosphorus (FRP) concentration following La-bentonite application. Overall, phytoplankton classification indicated that the lake moved from poor to moderate ecological status but did not reach the proxy water quality target (i.e. WFD Good Ecological Status) within four years of the application. As for many other shallow lakes, the effective control of internal P loading in Loch Flemington will require further implementation of both in-lake and catchment-based measures. Our work emphasizes the need for appropriate experimental design and long-term monitoring programmes, to ascertain the efficacy of intervention measures in delivering environmental improvements at the field scale

Unified concepts for understanding and modelling turnover of dissolved organic matter from freshwaters to the ocean: the UniDOM model

The transport of dissolved organic matter (DOM) across the land-ocean-aquatic continuum (LOAC), from freshwater to the ocean, is an important yet poorly understood component of the global carbon budget. Exploring and quantifying this flux is a significant challenge given the complexities of DOM cycling across these contrasting environments. We developed a new model, UniDOM, that unifies concepts, state variables and parameterisations of DOM turnover across the LOAC. Terrigenous DOM is divided into two pools, T1 (strongly-UV-absorbing) and T2 (non- or weakly-UV-absorbing), that exhibit contrasting responses to microbial consumption, photooxidation and flocculation. Data are presented to show that these pools are amenable to routine measurement based on specific UV absorbance (SUVA). In addition, an autochtonous DOM pool is defined to account for aquatic DOM production. A novel aspect of UniDOM is that rates of photooxidation and microbial turnover are parameterised as an inverse function of DOM age. Model results, which indicate that ~5% of the DOM originating in streams may penetrate into the open ocean, are sensitive to this parameterisation, as well as rates assigned to turnover of freshly produced DOM. The predicted contribution of flocculation to DOM turnover is remarkably low, although a mechanistic representation of this process in UniDOM was considered unachievable because of the complexities involved. Our work highlights the need for ongoing research into the mechanistic understanding and rates of photooxidation, microbial consumption and flocculation of DOM across the different environments of the LOAC, along with the development of models based on unified concepts and parameterisations

Nutrient modelling and a nutrient budget for Llangorse Lake Final Report

Llangorse Lake is the largest natural lake in South Wales. There have been concerns about eutrophication problems here for many years. The problem is believed to have been caused by high nutrient loads entering the lake from the surrounding catchment. This study aimed to determine the size and main sources of those loads. The phosphorus (P) and nitrate (NO3-N) load to Llangorse Lake was found to be approximately 2 tonnes P y 1 and 74 tonnes NO3-N y 1 (which is equivalent to 1.5 g P m-2 y-1 and 53 g NO3 N m-2 y-1). Most of the P and NO3-N loads were found to be entering the lake from only two of the inflow streams (i.e. those draining to Sites 2 and 6). These accounted for 85 per cent of the annual P load and 82 per cent of the annual NO3-N load to the lake. The hydrology of the Llangorse catchment appears to be strongly affected by groundwater. This is best demonstrated at Site 6, where the surface water catchment upstream of the site accounts for only 36 per cent of the lake catchment but contributes 67 per cent of annual hydraulic load. This, and the fact that the flow at Site 6 is almost double that which can be accounted for by rainfall, suggests that there is considerable groundwater flow in this area. If the streams that flow into the lake have significant input from groundwater, this will affect their hydrology and chemistry. As this groundwater may enter the drainage system from an area beyond the boundary of the surface water catchment, this has serious implications for catchment management aimed at reducing the nutrient loads to the lake. Some of the inflow streams showed evidence of occasional point source pollution in very wet weather. This suggests that there is a need, in some places at least, to control point sources of pollution that leak or overflow during heavy rainfall. However, the evidence suggests that most sources of nutrients within the catchment are diffuse sources. Most of the published literature concerning the eutrophication and recovery of Llangorse Lake focuses on P. This is probably for historical reasons. This study shows that the water quality of the lake is determined by a delicate balance between P and N availability. So, it is important to consider both of these nutrients in developing a catchment management plan. One final consideration in developing a catchment management plan is the possible impact of occasional flooding of the meadows around the lake in terms of transporting animal waste into the lake. Although flooding was once encouraged by local farmers as a way of carrying plant food to the pastures “both in solution and in very finely divided particles” (Griffiths, 1939), in these days of intensive agriculture, flooding of the meadows is more likely to carry nutrients from the meadows to the lake

The ecology of freshwater epipelic algae: an update

Epipelic algae perform a range of ecosystem functions including biostabilisation of sediments, regulation of benthic–pelagic nutrient cycling, and primary production. There is a growing need to understand their ecological role in light of current and future alterations in sediment loading resulting from land-use change and land management practices. Although the majority of recent work on epipelic algal ecology has been conducted within estuarine ecosystems, significant advances have also been made in freshwaters. We review these recent studies in combination with more classical freshwater approaches to highlight the importance of freshwater epipelic algal ecology and to aid discussions regarding future research. A summary of benthic algal groups is given with particular emphasis on substratum preference and habitat boundaries. No standard methodology exists for sampling freshwater epipelon, and we discuss the advantages and disadvantages of a suite of currently employed methodologies. Spatial variability in epipelic community biodiversity is discussed from the micro-scale (i.e. vertical migration in the sediment surface) to the ecosystem scale (i.e. lake vs river habitats), and finally at the geographic scale (i.e. the ‘ubiquity’ of epipelic species). Factors regulating epipelon community composition and biomass (e.g. reproductive strategies, habitat disturbance, grazing pressures, resource limitation, resilience, symbiosis, and parasitism) are also reviewed. Finally, examples of specific epipelic ecosystem functions (e.g. primary production, biostabilisation, and regulation of biogeochemical cycling) are given and areas of research requiring particular focus in the future are outlined

Assessment of sediment phosphorus capping to control nutrient concentrations in English lakes - project summary

Project summary - full report available separately. This project investigated the effectiveness of a technique for stopping the release of phosphorus from lake sediments