Local human impacts interact with geography to drive benthic community depth zonation on contemporary coral reefs

Changes in biophysical conditions and energetic resource supply across
depths are predicted to promote or limit the abundance of different coral
reef benthic groups. However, the degree to which regional differences in
biophysical processes govern and local human activities might alter
naturally occurring depth zonation patterns remains unclear. Here we used
2,239 reef surveys conducted between 0-30 m depth around 33 islands (18
unpopulated and 15 populated) across the Pacific Ocean to quantify the
percentage cover change of seven broad benthic groups. We tested whether
natural depth zonation patterns differed across geographies (using six
ecoregions), and whether and how local human impacts might disrupt these
natural zonation patterns. We found benthic community depth zonation did
not always occur. At the three ecoregions where depth zonation existed,
there was no universal ‘natural’ zonation pattern and the benthic groups
most responsible for driving patterns of depth zonation differed across
geographies. We also found evidence of human-disrupted changes to benthic
community depth zonation; patterns were inversed across depths and less
distinct at populated compared to unpopulated islands within two
ecoregions. We show coral reef communities are naturally highly variable,
and that human activities can disrupt natural patterns of ecological
organisation in contemporary ecosystems.