A stratified transect approach captures reef complexity with canopy-forming organisms

On the Great Barrier Reef (GBR), persistent changes to reef communities have begun to be documented, and on inshore reefs these shifts may favour the proliferation of macroalgae. Critical to understanding changes to reef community structure in response to anthropogenic impacts is developing effective methods to accurately document the abundance of different reef organisms. Effective monitoring must be time and cost efficient, replicable, and able to sufficiently and accurately detect disturbances to allow development of strategies to mitigate their impacts. Traditional techniques to document coral reef communities (i.e. photo-quadrats, benthic intercept transects) rely on planar views, which tend to either over- or under-represent canopy-forming organisms. As canopy-forming organisms are likely to be affected by anthropogenic influences (corals negatively, algae positively), it is essential for monitoring programs to implement methods sufficient to document changes to the vertical dimension of coral reefs. Here we build on previous work to document the canopy effect in coral-dominated ecosystems and propose a new survey approach suitable for implementation in algal-dominated systems. A vertically stratified transect, modified from a traditional point intercept transect, captures benthic and canopy-forming members of reef communities and provides information on three-dimensional complexity. To test the capability of the new method to detect changes in vertical reef structure, seaweed was removed from experimental quadrats and monitoring techniques were applied before and after four months of regrowth. A stratified method more accurately captured the three-dimensional change resulting from algal canopy growth, while resolving the over- and under-representation of algal biomass in two traditional techniques. We propose that a stratified transect method improves abundance estimates of canopy-forming organisms whilst maintaining data compatibility with traditional methods.

Automated summary

Persistent changes to reef communities have begun to be documented on the Great Barrier Reef, with shifts that may favor the proliferation of macroalgae on inshore reefs. Understanding these changes and accurately documenting the abundance of different reef organisms is crucial for developing strategies to mitigate their impacts. Traditional techniques for documenting coral reef communities may either over- or under-represent canopy-forming organisms, making it essential to implement methods that capture changes to the vertical dimension of coral reefs. The study proposes a vertically stratified transect method that accurately captures three-dimensional changes resulting from algal canopy growth, improving abundance estimates while maintaining compatibility with traditional methods.