Multi-level Effects of Severe Storms on an Offshore Temperate Reef System: Benthic Sediments, Macroalgae, and Implications for Fisheries

Abstract. Episodic events have been shown to strongly affect structure and function of marine benthic ecosystems. Severe storms can have profound effects on the distribution of marine sediments which could, in turn, influence the development of benthic communities. The rich and diverse epibenthic communities on the United States mid-Atlantic continental shelf owe their existence to the presence of a complex sequence of rocky outcrops. An unusually strong storm struck this shelf system in March 1993. Two of these carbonate platforms had been characterized by moderate sediment cover for at least the previous two years, but bottom water velocities generated by this storm removed considerable amounts of sediment from these upper flat hardbottom habitats. Macroalgal cover on these platforms dramatically increased between 1992 and 1993 with the increased exposure of hard substrate for attachment. The edges of the outcrops (scarps and rubble ramps), which are usually free of sediment, maintained their dense algal cover. Settlement blocks placed in various sub-habitats showed little variation in algal cover among flat hardbottom and scarp areas during both years, indicating that available hard substrate habitat may be the primary limiting factor for algal growth on the North Carolina continental shelf. Since macroalgal meadows provide food and shelter for juvenile fish, the increase in critical habitat following these storms may have implications for recruitment of economically important fish species. Thus, indirect effects of episodic storms, i.e ., redistribution of sand bodies leading to algal meadow development over large spatial scales, may have important consequences for benthic community development and persistence in temperate reef systems.     

Effects of Soft Corals on Scleractinian Coral Recruitment. II: Allelopathy, Spat Survivorship and Reef Community Structure

Abstract. Two field experiments were performed on the Great Barrier Reef, Australia, at Orpheus Island and Lizard Island, respectively, to investigate the effects of allelopathic soft corals on survivorship and community structure of scleractinian coral spat. Ceramic tiles were placed around the allelopathic soft corals Sinularia flexibilis (Quoy & Gaimard 1833) and Sarcophyton glaucum (Quoy & Gaimard 1833), and controls. One control consisted of settlement plates surrounding a scleractinian coral (non‐allelopathic planktivore); another control had no adult soft or scleractinian coral present. The experimental soft corals affected the recruitment of various taxonomic groups of coral spat differentially, as evidenced by the diversity of coral spat settling in treatments and controls. At Orpheus Island (O.I., n = 1038 spat) and Lizard Island (L.I., n = 7032 spat), there were significant differences between recruitment success of the two dominant coral taxa, Pocilloporidae (O.I., 61.4 %; L.I., 20.5 %) and Acroporidae (O.I., 33.7 %; L.I., 53 %). Settlement plates exposed to Sinularia flexibilis at either site had the lowest proportion of acroporid recruits. Diversity indices (Shannon‐Wiener Indices) varied significantly between treatments at both Orpheus and Lizard Islands. This appears to be due to selective inhibition of acroporid spat by Sinularia flexibilis at both sites. Growth of coral spat was higher on settlement plates in the presence of Sarcophyton at Lizard Island. Settlement of most associated epibiota was generally inhibited under these conditions. Coral spat survivorship was highest in the presence of Sinularia at Orpheus ­Island; at Lizard Island, this was the case with the Sarcophyton treatment. Higher survivorship, and in some cases growth, of coral spat near soft corals was apparently due to reduced competition for space between spat and associated epibiota. This hypothesis is supported by the results of a sister experiment where a coating of Sinularia flexibilis extract on settlement tiles significantly decreased fouling by sessile epibiota. Soft corals have an allelopathic effect on recruitment and early development of scleractinian corals and, consequently, on early coral reef community succession.