Abstract
Background Red calcareous seaweeds of the genus Galaxaura have been associated with coral reef degradation. Galaxaura has low nutritional value for most herbivorous fishes and produces allelopathic chemicals in competition with corals. In this study, the abundance of the filamentous Galaxaura divaricata was surveyed on 13 spatially independent patch reefs across the lagoon of Dongsha Atoll, an isolated coral reef ecosystem in the northern South China Sea. Variations in Galaxaura cover on a degraded reef were monitored over a period of 17 months. Epiphytic macroalgae associated with Galaxaura were quantified, and species identifications aided through DNA barcoding.
Results Patch reefs in the lagoon of Dongsha Atoll were degraded, exhibiting relatively low living coral cover (21 + 3%), but high cover of macroalgae (30 + 4%), and rubbles and dead coral (47 + 4%). The distribution of Galaxaura was heterogeneous across the lagoon, with highest abundance in the southeast area. In that area Galaxaura has persistently bloomed on degraded reef for at least four years, covering up to 41% of the substrate. Galaxaura provides substrate for various macroalgae, 15 of which were identified to the species level, four to the genus level, one to the order Gelidiales, and one to the phylum cyanobacteria. Some of these epiphytes are allelopathic and known to frequently overgrow corals. For instance, the brown alga Lobophora sp28, third most common epiphytic macroalga on Galaxaura, frequently overgrows corals across the lagoon of Dongsha Atoll.
Conclusions Our study demonstrated that an allelopathic and unpalatable seaweed, such as Galaxaura can bloom on degraded coral reefs for several years. The complex thallus-structure of Galaxaura provides suitable substrate for other macroalgae, some of which are noxious to corals. By increasing the substrate availability for macroalgae, Galaxaura could facilitate the abundance of macroalgae, and decrease the recovery potential of degraded coral reefs.