Biological condition gradient: Applying a framework for determining the biological integrity of coral reefs

Background/Question/Methods

The goals of the U.S. Clean Water Act (CWA) are to restore and maintain the chemical, physical and biological integrity of water resources. Although clean water is a goal, another is to safeguard biological communities by defining levels of biological integrity to protect aquatic resources. One tool used for defining biological impairment thresholds is the biological condition gradient (BCG), a conceptual model that describes how biological attributes of aquatic ecosystems change in response to increasing levels of stressors. The BCG has been used successfully in freshwater systems. We are using the BGC as a framework for developing biological criteria for coral reefs, an ecosystem that is in rapid decline. The BCG is divided into multiple tiers of biological condition along a stressor-response curve, defining biological conditions found at no or low levels of stress to those at high levels of stressors. Expert panelists determine which biological attributes are important, such as features of community structure, organism condition, ecosystem function, and connectivity. In a 2012 pilot study, an expert panel evaluated coral data and visual media in a facilitated workshop. The panelists used the information to recommend which coral reef attributes should be considered in the coral reef BCG.

Results/Conclusions

After panelists evaluated photos and videos of reefs from recent monitoring surveys along the southern coast of Puerto Rico, all sites were ordered from best to worst condition and classified into three categories (good, fair or poor condition). The panelists agreed that an undisturbed site was not among their choices, therefore historic data will be used to establish reference condition. Next, a facilitated discussion identified which factors influenced their ordering process. These factors included present or absence, size class distributions, disease or lesions, diversity, richness, taxa or morphology and abundance of scleractinian corals, sponges, gorgonian octocorals, algae and fish. Other important factors were substrate quality, amount of sediment, water clarity, and reef surface heterogeneity or complexity. Future panel considerations will include numeric data such as total surface area of sessile organisms and biomass of fishes to select attributes and assign levels for developing narrative decision rules and descriptions for distinguishing among the levels. The unique feature of the BCG is it combines scientific knowledge with the practical experience and the needs of resource managers. This process is being considered to rank ecological integrity for ecosystem services in an ecological services gradient.