Abstract: The eutrophication of surface waters has become an endemic global problem. Nutrient loadings from agriculture are a major driver, but it remains very unclear what level of on-farm controls are necessary or can be justified to achieve water quality improvements. In this review article, we use the UK as an example of societiesâ multiple stressors on water quality to explore the uncertainties and challenges in achieving a sustainable balance between useable water resources, diverse aquatic ecosystems and a viable agriculture. Our analysis shows that nutrient loss from agriculture is a challenging issue if farm productivity and profitability is to be maintained and increased. Legacy stores of nitrogen (N) and phosphorus (P) in catchments may be sufficient to sustain algal blooms and murky waters for decades to come and more innovation is needed to drawdown and recover these nutrients. Agricultureâs impact on eutrophication risk may also be overestimated in many catchments, and more accurate accounting of sources, their bioavailabilities and lag times is needed to direct proportioned mitigation efforts more effectively. Best practice farms may still be leaky and incompatible with good water quality in high-risk areas requiring some prioritization of society goals. All sectors of society must clearly use N and P more efficiently to develop long-term sustainable solutions to this complex issue and nutrient reduction strategies should take account of the whole catchment-to-coast continuum. However, the right balance of local interventions (including additional biophysical controls) will need to be highly site specific and better informed by research that unravels the linkages between sustainable farming practices, patterns of nutrient delivery, biological response and recovery trajectories in different types of waterbodies.
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Lakes may have alternative states due to excessive phosphorus (P) input: a clear-water state and turbid one with high chlorophyll concentrations. Because shifts between these states have large ecosystem effects, and restoration after the shifts is costly or sometimes impossible, precise evaluation of the possibility of alternative states is needed for lake management. Yet the shifts are quite variable and seem to depend on many factors, including lake morphometry, temperature, and dominance of macrophytes. Here we evaluated the role of these factors using an empirically based model that included more mechanistic detail than earlier models of regime shifts in trophic state. Mean depth and temperature strongly influenced the susceptibility of lakes to regime shifts and lake restoration. The macrophyte effect of preventing P recycling from sediments was critical to the susceptibility of shallow lakes to regime shift. With warmer temperatures, eutrophication was more likely and restoration was less successful due to increased internal P recycling from the sediment. Lakes with intermediate depths were most susceptible to regime shifts and were least restorable. These lakes were too deep to be protected by macrophytes in their littoral zones and were too shallow to mitigate P recycling through hypolimnetic dilution. Our results illustrated the interplay of multiple physical, chemical, and biotic mechanisms in regime shifts, a complex type of causality that may arise in regime shifts of other types of ecosystems.
Eutrophication (the overenrichment of aquatic ecosystems with nutrients leading to algal blooms and anoxic events) is a persistent condition of surface waters and a widespread environmental problem. Some lakes have recovered after sources of nutrients were reduced. In others, recycling of phosphorus from sediments enriched by years of high nutrient inputs causes lakes to remain eutrophic even after external inputs of phosphorus are decreased. Slow flux of phosphorus from overfertilized soils may be even more important for maintaining eutrophication of lakes in agricultural regions. This type of eutrophication is not reversible unless there are substantial changes in soil management. Technologies for rapidly reducing phosphorus content of overenriched soils, or reducing erosion rates, are needed to improve water quality.
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