Determining the effects of nutrient enrichment on macroalgae-dominated reefs : (observational, experimental and predictive capabilities)

The impacts caused by chronic nutrient enrichment on coastal habitats usually comprise a slow stepwise progression of chemical and biological changes which can be highly influenced by the physical environment. As a consequence, the process of change is dependent upon the unique environmental circumstances in each system and detection of impacts, particularly in the early stages, can be difficult. In addition, the influence of multiple anthropogenic nutrient sources interacting at a community or species level further complicates impact assessments. However, understanding the mechanism by which increasing changes in nutrient availability might affect the complex nature of the coastal reef ecology is essential for early detection, subsequent prevention and control of impacts. The capacity to predict how natural variability interacts with anthropogenic stressors is a challenge for both marine scientists and environmental managers. Natural selective forces in costal habitats (e.g., light levels, wave exposure, salinity, temperature and species interactions) will influence the degree to which reef systems respond to sources of nutrients. However, these forces can also promote the system's ability to cope with impacts (resistance) or to recover from a given disturbance (resilience), and will vary spatially and temporally throughout geographical gradients within the same system. Understanding the system characteristics and key biological responses may help to determine where potential impacts may or may not take place. This study outlines a field experiment, which measurably increased the nutrient availability in three reef systems to determine changes in macroalgal community composition, successional stages and classical indicators (i.e., fast-growing opportunistic species). In addition, this study examined how alternative indicators, including underlying indicators of impact such as physiological sensitivity of key macroalgal species), can provide better indication of the status of impact/ stress. Furthermore, location-specific variations of abiotic factors were monitored to test if physical drivers, the structuring forces of gradients in community structure, may improve our understanding of eutrophication. The results indicate that there was no evidence of major effects of nutrient enrichment on the overall community structure and that the observed responses of opportunistic species were not consistent. The abundance of opportunistic species differed between locations and showed significant effects of small-scale variability within each community. Physical drivers (variations in wave exposure and light, as well as differences in salinity and temperature) were correlated with fundamental differences in the established and the successional community structure throughout the study region. This suggests that fundamental and prevailing components of the community (e.g., Caulerpales/Fucales and/or Encrusting algae/Fucales-dominated reefs) and abiotic fluctuations may underpin opportunistic algae abundances and define the responses to enhancement of nutrients. This has important implications for our previous understanding of nutrient enrichment indicators and how this relationship might be applied to management (i.e., where the presence of fast-growing opportunistic species in abundance is indicative of an adverse environmental impact) - as in many cases this interaction may simply represent a difference resulting from prevailing environmental conditions. The spatial effects of diverse sources of nutrient (i.e. sewage treatment plants, marine farming, catchment and river inputs) could be differentiated by analysing the nutrient tissue composition (C, N, P) in key algal species, and was to a lesser extent evident in their photosynthetic performance, which may require further research given the limitation that the assessment of Chlorophyll-a fluorescence may have in macroalgae from nutrient limiting waters. These findings suggest that the physiological responses of key algal species may constitute a more reliable indicator of system status. Since some species are likely to reach their nutrient requirements at low/moderate nutrient concentrations (e.g., fucoids) they may be good at long-term storage of limiting nutrients that may otherwise mask species responses to nutrient variation and/ or addition (Pedersen 1995). For this reason it is also suggested that macroalgae on particular reefs may be pre-adapted to different nutrient inputs depending on the prevailing nutrient regime. In conclusion, these results highlight that in a real life scenario the usefulness of classical indicators of eutrophication may be constrained by location-specific gradients and small-scale spatial variability. Spatial patterns in the biophysical environment (e.g., wave exposure, light environment) indicated gradients in community structure and habitat attributes that in turn would appear to restrict eutrophication responses in macroalgal reefs. Inclusion of tissue nutrient levels and photosynthetic performance of key species in assessments of eutrophication appeared to better define the spatial role of physical drivers (like wave exposure and light), and sources of nutrient availability. Incorporating this information into ecosystem models may improve the analytical power. This study provides important data to improve spatial management in systems with macroalgal reefs and suggests that characterisation of the physical environment is essential to contextualise the system response and that evaluation of the sensitivity of key species at a physiological level may provide a more accurate and effective indication of the impact due to nutrient enrichment in coastal habitats than the determination of localised increases in opportunistic species.