Eutrophication science: moving into the future

Abstract

We were impressed by the timely review of the effects of eutrophication in coastal marine systems by Smith and Schindler [1]. We agree that although there has been substantial work towards identifying the causes of regime shifts in coastal systems, our understanding of the drivers is still far from satisfactory. Nonetheless, we feel that a critical point was not addressed in their review; the effects of eutrophication are likely to be substantially altered under future climate conditions. There is a pressing need to understand how local eutrophication and global climate stressors will interact. Although the effects of combined climate stressors are increasingly well studied in marine systems (e.g. CO2 and temperature; [2] and [3]), it has only recently been recognized that local and global stressors are likely to interact in unpredicted ways [4]. For example, the historical and continuing deforestation of algal canopies in favour of small, fast-growing turfs across the temperate coastlines of the world is a focus of considerable research [5]. Developing theory explains these shifts as a function of altered water quality that enables the cover of ephemeral turfs to expand spatially and persist beyond normal seasonal limits. However, our recent work shows that elevated nutrient and CO2 levels can interact to have positive synergistic effects on algal turfs [6] and suggests that future conditions may exacerbate shifts from canopy to turf domination. Indeed, understanding the degree to which these global and local stressors combine to accelerate and expand ecosystem shifts is a key concern. It is noteworthy, however, that the negative effects of future CO2 levels might be substantially reduced in the absence of higher nutrient levels [6] and effective local management might help to mitigate the effects of climate change. For example, preventing the harvest of herbivorous fish might suppress algal growth and increase the resilience of coral reefs to climate-induced bleaching events [7]. Likewise, we argue that reducing the input of nutrient-rich wastewater into coastal marine systems might increase their resilience to phase shifts. In South Australia, the government is implementing a policy to recycle 45% of the Adelaide wastewater. By removing nutrient stress in near-shore marine systems, this recycling program will not only act as a long-term recovery experiment that builds on a 30-year data set documenting the large-scale decline of kelp forests [8] (filling a scientific gap identified by Smith and Schindler [1]), but will also assess the potential for enhancing the resilience of these systems to the deleterious effects of future climates.