DOI: 10.5061/dryad.w3r2280pw
title: Spatial distribution of benthic algae in the South China Sea: responses to gradually changing environmental factors and ecological impacts on coral communities

description: 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.
description: This field dataset was conducted on 12 coral reefs in four coral
 reef regions (CRRs) in the South China Sea during summer seasons from 2015
 to 2018. Photoquadrat method was used to determine the benthic community
 composition and cover in the surveyed reefs. At each coral
 reef, 4–12 transects (50 m long) were deployed at a depth of 2–6 m in
 coastal reefs and Weizhou Island and of 2–15 m in Xisha Islands and Nansha
 Islands. At each transect (or site), a 50 m fiberglass measuring tape was
 fixed to the reef substrate, and sampling quadrats of 0.5 m × 0.5 m were
 randomly set along the tape. SCUBA divers used OLYMPUS TG-4 cameras to
 take over 20 photographs per quadrat in a vertical line from the reef
 substrate (about 0.6 m) for each sampling quadrat, and we set up 6 to 24
 quadrats on each transect. A total of 1,450 photoquadrats from 104 sites
 (one transect was done per site) across 12 reefs were used to calculate
 the benthic biotic and abiotic cover using Coral Point Count with Excel
 extensions (CPCe version 4.1) software.
 Briefly, this software was used to place 50 simple-random points on each
 photoquadrat, and benthos and substrate were identified beneath each
 point. Benthic communities were mainly defined as hard corals, benthic
 algae (turf algae, macroalgae, and CCA functional groups), and other
 benthic groups such as sponges, Milleporina, Helioporacea, Gorgonians, and
 soft corals. Macroalgae were differentiated as fleshy macroalgae (e.g.,
 <i>Dictyota</i>, <i>Lobophora</i>),
 <i>Halimeda</i>, and other calcified macroalgae, while this
 study did not specially focus on the other calcified macroalgae. An
 abiotic substrate was defined as one consisting of rubble, sand, sediment,
 bare rock, and dead coral. Hard coral and macroalgae genera were
 identified, and turf algae and CCA were identified to the level of
 functional groups.