Abstract
Globally, tropical coral reefs are being degraded by human activities, and as a result, reef-building corals have declined while macroalgae have increased. Recent work has focused on measuring macroalgal abundance in response to anthropogenic stressors. To accurately evaluate the effects of human impacts, however, it is necessary to understand the effects of natural processes on reef condition. To better understand how coral reef communities are influenced by natural processes, we investigated how spatial and seasonal changes in environmental conditions (temperature and PAR) influence benthic community structure, and the composition and frequency of coral-algal interactions across eight distinct zones and over a 23-month period at Heron reef on the southern Great Barrier Reef. Hard coral cover and macroalgal density showed distinct spatio-temporal variations, both within and between zones. Broad hard coral cover was significantly higher at the reef slope sites compared to the lagoon and was not significantly influenced by season. The composition and biomass of macroalgae increased in spring and declined in summer, with maximum macroalgal abundance corresponding with average temperatures of between 22 and 24°C and average 24 h PAR of 300-500 μmol qanta m s. Changes in macroalgal biomass further influenced the composition and frequency of coral-algal interactions, however the incidence of coral-algal contact was best explained by coral cover. The results presented here emphasize that natural levels of macroalgae and coral-algal interactions are context-specific, and vary not only with-in zones, but in somewhat predictable seasonal cycles. Further, these results emphasize that the frequency of coral-algal interactions is dependent on hard coral, not just macroalgal cover, and an increase in coral-algal interactions does not necessarily translate to degradation of coral reefs.
Highlights
Many coral reefs are shifting away from coral dominance to assemblages that include macroalgae (Pandolfi et al, 2003; McManus and Polsenberg, 2004; Hughes et al, 2007)
In order to understand the effects of human impacts, it is first necessary to understand how macroalgae respond to natural processes and to include more dynamic indicators of reef condition, such as coral-algal interactions (Connell et al, 2004; Bruno et al, 2014; Flower et al, 2017)
Broad hard coral cover was significantly higher at the reef slope sites compared to the lagoon and was not significantly influenced by season (Table 2)
Summary
Many coral reefs are shifting away from coral dominance to assemblages that include macroalgae (Pandolfi et al, 2003; McManus and Polsenberg, 2004; Hughes et al, 2007). In order to understand the effects of human impacts, it is first necessary to understand how macroalgae respond to natural processes and to include more dynamic indicators of reef condition, such as coral-algal interactions (Connell et al, 2004; Bruno et al, 2014; Flower et al, 2017). Very few studies have investigated natural drivers of the spatio-temporal dynamics of macroalgal biomass and its effect on coral-algal competition in the absence of anthropogenic influence (Bruno et al, 2014; Sangil and Guzman, 2016). Environmental factors do not operate independently, and investigations into how temperature and light interact to influence macroalgal seasonality have been less clear (Mathieson and Dawes, 1986; Fong and Zedler, 1993; Ferrari et al, 2012)
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