MEPS 522:33-48 (2015)  -  DOI: https://doi.org/10.3354/meps11118

Coral reef benthic regimes exhibit non-linear threshold responses to natural physical drivers

Jamison M. Gove1,6,*,**, Gareth J. Williams2,**, Margaret A. McManus3, Susan J. Clark1,4, Julia S. Ehses1,4, Lisa M. Wedding5

1Coral Reef Ecosystem Division, NOAA Pacific Islands Fisheries Science Center, Honolulu, Hawai'i 96818, USA
2Center for Marine Biodiversity and Conservation, Scripps Institution of Oceanography, La Jolla, California 92083, USA
3School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i 96822, USA
4Joint Institute for Marine and Atmospheric Research, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i 96822, USA
5Center for Ocean Solutions, Stanford University, Monterey, California 93940, USA
6Present address: Ecosystems and Oceanography Division, NOAA Pacific Islands Fisheries Science Center, Honolulu, Hawai‘i 96818, USA
  *Corresponding author: **These authors contributed equally to this work

ABSTRACT: We assessed the independent effects of natural physical drivers in structuring coral reef benthic communities at a remote oceanic atoll in the central equatorial Pacific with minimal local human impacts. High-resolution bathymetric data combined with in situ oceanographic measurements and a nearshore hydrodynamic model revealed complex intra-atoll gradients in geomorphic complexity, wave forcing, currents, and temperature. For example, maximum wave-driven bed shear stress spatially varied on the forereef (15-20 m depth) by over 2 orders of magnitude, peaking in areas exposed to multiple wave regimes. Benthic community composition, quantified via towed-diver imagery collected in a complete circumnavigation of the atoll (~40 km), also exhibited considerable spatial heterogeneity. Benthic competitors showed distinct, non-linear threshold-type responses to variations in physical drivers. For example, at a wave-driven bed shear stress threshold of 18 N m-2, calcifying crustose coralline algae lost relative dominance and were replaced by non-calcifying fleshy turf algae. Hard coral communities also demonstrated considerable flexibility in response to physical drivers, with distinct shifts in the relative dominance of different growth morphologies along gradients of wave forcing, presumably as a means of local adaptation. Our results highlight (1) the importance of natural gradients in physical drivers in determining dominant benthic regimes on coral reefs (e.g. calcifying vs. fleshy), (2) that non-linear thresholds (or tipping points) exist between key benthic competitors in response to key physical drivers, and (3) that coral assemblages show inherent flexibility and can reorganize in response to physical drivers rather than exhibit wholesale changes in overall cover.


KEY WORDS: Coral morphology · Regime shift · Reef-building organisms · Competition · Habitat complexity · Biophysical coupling · Waves · Bed shear stress · Tipping points


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Cite this article as: Gove JM, Williams GJ, McManus MA, Clark SJ, Ehses JS, Wedding LM (2015) Coral reef benthic regimes exhibit non-linear threshold responses to natural physical drivers. Mar Ecol Prog Ser 522:33-48. https://doi.org/10.3354/meps11118

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