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Background  The chemically-rich seaweed  Galaxaura  is not only highly competitive with corals, but also provides substrate for other macroalgae. Its ecology and associated epiphytes remain largely unexplored. To fill this knowledge gap, we undertook an ecological assessment to explore the spatial variation, temporal dynamics, and diversity of epiphytic macroalgae of  Galaxaura divaricata  on patch reefs in the lagoon of Dongsha Atoll, a shallow coral reef ecosystem in the northern South China Sea that has been repeatedly impacted by mass coral bleaching events.    Methods  Twelve spatially independent patch reefs in the Dongsha lagoon were first surveyed to assess benthic composition in April 2016, and then revisited to determine  G .  divaricata  cover in September 2017, with one additional  Galaxaura -dominated reef (site 9). Four surveys over a period of 17 months were then carried out on a degraded patch reef site to assess the temporal variation in  G .  divaricata  cover. Epiphytic macroalgae associated with  G .  divaricata  were quantified and identified through the aid of DNA barcoding at this degraded site.    Results  Patch reefs in the Dongsha lagoon were degraded, exhibiting relatively low coral cover (5–43%), but high proportions of macroalgae (13–58%) and other substrate (rubble and dead corals; 23–69%). The distribution of  G .  divaricata  was heterogeneous across the lagoon, with highest abundance (16–41%) in the southeast area. Temporal surveys showed consistently high covers (mean ± SD = 16.9 ± 1.21%) of  G .  divaricata  for 17 months. Additional photographic evidence suggested that overgrowth of  G .  divaricata  can persist for 3.5 years. Yet,  G .  divaricata  provides substrate to other macroalgae (e.g.,  Lobophora  sp.) that also limit the growth of corals.    Conclusions  Our study demonstrates that an allelopathic seaweed, such as  G .  divaricata , can overgrow degraded coral reefs for extended periods of time. By providing habitat for other harmful macroalgae, a prolonged  Galaxaura  overgrowth could further enhance the spread of macroalgae, and strengthen negative feedback loops, decreasing the recovery potential of degraded reefs.

Filamentous calcareous alga provides substrate for coral-competitive macroalgae in the degraded lagoon of Dongsha Atoll, Taiwan