Seasonal responses of a fringing reef community to eutrophication (Reunion Island, Western Indian Ocean)

Seasonal changes in benthic community composition were investigated on a degraded fringing reef subject to nutrient-enriched submarine groundwater discharge. From December to February (hot season), algae dominate the benthic commuruty, overgrowing scleractinians: 19 % of hard corals died in the studied area, of which 16 % were overgrown by algae (turfs mostly composed of Ceramiales, Dictyosphaeria setchellii and encrusting coralline algae) and 3 % were bleached and subsequently died. In a qualitative assessment, the rhodophyte Gracilaria crassa appeared to be a major factor in the overgrowth and mortality of corals on the reef flat. Algal components displayed strong seasonal variation in abundance and dominance. Changes in the algal community are highly correlated with 2 environmental factors : (1) early onset of intermittent trade winds in March, correlated with replacement of large fleshy algal facies by lithophytic algal facies and (2) calm conditions in November, associated with warming of the waters, correlated with an increase in large fleshy algae capable of overgrowing many substrata. Thus, during the hot season, hydrological conditions cause eutrophic conditions and enhance algal growth while reducing coral vitality. Therefore, this is the most critical period for a coral community growing in eutrophic conditions.

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