Title: FINDING A BIOLOGICAL UPPER LIMIT FOR THE MESOPHOTIC ZONE

Abstract: The lower boundary of the mesophotic zone on coral reefs is largely accepted as coinciding with the last photosynthetic hard corals. The upper limit is set at 30m. These two boundaries are fundamentally different in character. The lower bound is capable of moving, accommodating changes between sites in water clarity and local species capabilities. Such a mobile boundary allows the logical comparison of mesophotic reefs from different sites based on their biology. The upper bound, as currently defined, is a more limiting concept. Generally there is greater environmental variability in the shallows than at depth. This variability could cause the upper limit to vary from site to site. A fixed upper limit may lead to illogical comparisons between sites across reef zones. Here it is argued that we should try to reach a consensus on an upper limit of the mesophotic zone, rooted in biology, allowing the easy identification of mesophotic reefs by researchers. Potential defining characters may include irradiance levels, changes in physiology or taxonomic indicators. Here we propose using benthic community composition. Coral presence or absence as a signal integrates many environmental variables which may reveal the boundaries of mesophotic reefs. Identifying corals visually may also be faster and require less specialist equipment than other approaches. Data from Honduras is used to show vertical reef structure if we take a lead from community data. An in progress follow up project employing a systematic review to capture the global picture will also be explained.

Authors: Laverick JH, Rogers AD

Presentation: Oral

Session: 29

Date: 06/22/16

Time: 15:00

Location: 308 A/B

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