Spatial distribution of benthic algae in the South China Sea: responses to gradually changing environmental factors and ecological impacts on coral communities

Aim: In this study, we investigated whether environmental factors can
effectively control the spatial distribution of various benthic algae, and
examined the critical ecological impacts of algae on corals across the
South China Sea (SCS). Relationships between benthic algae and
environmental factors were assessed, and potential ecological impacts of
algae on coral communities were evaluated across spatial scales. Location:
A total of 104 sites at 12 coral reefs in four coral reef regions (CRRs)
of the SCS (latitude range of 9–22°). Taxa: Turf algae, macroalgae
(including fleshy macroalgae and Halimeda), crustose coralline algae
(CCA), coral, and juvenile coral. Methods: Using diver-based surveys (2015
to 2018), we investigated four CRRs with different reef distance to
mainland (RDM) across the SCS. We obtained field data on benthic algal
composition and cover, coral cover and diversity, and juvenile coral
density. We also measured and collected the environmental factors
(including seawater environmental parameters and reef fish diversity).
Results: The cover of turf algae and CCA gradually decreased and
increased, respectively, with an increase in RDM, which had a strong
relationship with the gradually changing environmental factors. Random
forest models suggested that nutrients, reef fish diversity, seawater
transparency and temperature were the most important factors for
predicting turf algae and CCA cover. Linear regression analyses showed a
significant relationship between the turf algae, macroalgae, and CCA
covers, and juvenile coral density. Main conclusions: Our results showed
that gradually changing environmental factors were correlated with the
spatial distributions of turf algae and CCA. However, the natural
biophysical relationships between macroalgae and environmental factors may
be disrupted by turf algal overgrowth and nutrient subsidies. Increases in
all benthic algal groups significantly impacted coral recruitment,
highlighting the critical role of benthic algae in determining recovery
trajectory of the Indo-Pacific reefs that are threatened by human
activities.