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Percentage of all corals considered bleached in plots with Sargassum canopy removed (Removal) or left intact (Control), shown separately for reefs at Goold Island, north of Hinchinbrook Island, and Cannon Bay, on Great Palm Island.

Sargassum canopy decreases coral bleaching on inshore reefs.

Jamal Jompa1,2and Laurence McCook1

1 Australian Institute of Marine Science ‘ CRC: Reef Research
2 Department of Marine Biology, JCU

Widespread bleaching of reef corals has recently been reported from inshore reefs in the Townsville region, apparently as a result of exceptional weather conditions and flooding during January. The likely causes of this bleaching include low salinity, high temperature, and high UV light intensity. We here report a surprising increase in coral bleaching in plots from which the normally abundant canopy of seaweeds had been experimentally removed.

Many inshore reefs of the GBR have abundant beds of large brown seaweeds or macroalgae on the reef flat, often dominated by species of Sargassum . The abundance of these seaweeds have been suggested to be a result or even a cause of reef degradation, as increased terrestrial runoff causes makes reef waters less suitable for corals and more suitable for algae. However, there is remarkably little direct evidence for effects of macroalgae on corals. It may be that these conditions allow algae to outcompete corals, causing reef decline. In order to test for such effects, we have established and maintained large (5 x 5 m) plots from which we have been removing the Sargassum for nearly 18 months, at several sites on two inshore reefs, one at Goold Island, north of Hinchinbrook Island, and the other at Cannon Bay, on Great Palm Island. We also have control plots, in which the Sargassum has been left in place, forming a thick canopy often 1-2 metres high, with 100% cover. Despite this Sargassum canopy, the plots had quite high cover of live corals (up to 50%).

In the middle of February 1998, more than a month after the major flooding events, we noticed considerable bleaching of corals at both reefs. We consequently surveyed the amount and types of bleached corals in both Sargassum canopy (control) and removal plots. There are two sites at each reef and each site includes two plots of each treatment ( Sargassum removal and control). Four 5 m line intercept transects within the plots were used to measure coral cover (in cm). Corals were recorded at genus level and the condition of each coral scored in one of 4 categories: bleached (0), pale/mostly bleached (1), slightly bleached (2), and healthy/no bleaching (3). In this report, we consider categories 0 and 1 as 'bleached corals' and categories 2 and 3 as 'healthy corals'.

The percentage of corals which were bleached in removal and control treatments is presented in Fig. 1. At both reefs, the average percentage of corals bleached was significantly higher in plots which had had the Sargassum canopy removed than in plots with an intact canopy of the macroalgae (P<0.05). Overall, 19.6% of corals were bleached under "normal" conditions for these reefs, but 36.4% were bleached when the Sargassum canopy had been experimentally removed.

Species of Acropora seem to be the corals most affected, especially at Great Palm Island, where almost 100 % of Acropora were bleached. At Goold Reef, corals of the genera Acropora , Porites , Montipora and Favites were most affected by the bleaching event. However, bleaching was common among all taxa noted. Further, it seems that the protection afforded by the Sargassum canopy was not limited to particular coral species.

It seems likely that the seaweed canopy reduces damage to the corals by decreasing exposure to high temperatures, high UV light intensities, or perhaps by reducing mixing of low-salinity waters. Evidence is available for similar effects of algal canopies from temperate areas: such canopies can dramatically reduce thermal stress and water movement. The significance of this result is considerable, since it raises the possibility that algal canopies could actually provide protection to corals, instead of, or as well as, competing with them. Although the results by no means disprove the possibilities that corals are inhibited by macroalgae, they certainly provide further evidence that abundant macroalgae should not be assumed to be detrimental to inshore reefs without much more information.