Spatial patterns in the cover and composition of macroalgal assemblages on fringing and nearshore coral reefs

Abstract

Context Although increases in macroalgal cover on coral reefs are often reported alongside declines in coral, the composition of algal assemblages and their spatial dynamics are not commonly investigated. Aims To quantify changes in macroalgal assemblage composition over two spatial environmental gradients, depth and distance from shore, within a nearshore reef system in Kimbe Bay, Papua New Guinea, where coral cover has declined. Methods Benthic cover was quantified at three depths (reef flat, 10 and 15 m) on the windward reef slopes of six reefs located three distances from shore (fringing reefs, and platform reefs 100–200 m and 0.7−1 km offshore). Key results Macroalgal cover was highest on the reef flat, and assemblage composition varied among depths and distances from shore. Macroalgal cover was not correlated with coral cover except where macroalgal cover was greater than 20%, where a negative correlation occurred. There was no correlation between macroalgal cover and turf algal cover. All three benthic groups were negatively correlated with the combined total cover of sand and gravel. Conclusions These results indicated a fine-scale spatial structure of macroalgal assemblages on coral reefs over a narrow depth range and short distance from shore and highlighted the importance of a solid substratum. Implications It is likely that the ecological interactions between corals and macroalgae vary considerably over narrow spatial gradients.