Biomass and productivity of tropical macroalgae on three nearshore fringing reefs in the central Great Barrier Reef, Australia

Recently, concerns about human disturbance on coral reef communities have focused attention on macroalgae of nearshore fringing coral reefs of the Great Barrier Reef, Australia (GBR). However, the scarcity of baseline information makes it difficult to establish whether nearshore reef communities in the central GBR are perturbed or in a 'natural' state. This study provides data on biomass and productivity of nearshore reef macroalgae to serve as background information for the detection of future community changes. Over a period of 15 months, we estimated: i) biomass of conspicuous macroalgae in transects at three nearshore fringing reefs and ii) in situ net growth rates and net production of Sargassum baccularia. In summer, biomass was significantly dominated by large Fucales, especially S. baccularia (up to 200 g ash free dry weight m(-2)). Ephemeral algae (species of Padina, Hydroclathrus, Colpomenia, Chnoospora, Laurencia) were most abundant in austral spring (up to 40 g AFDW m(-2)). Maximum growth rates of S. baccularia of 3-4% day(-1) (entire thalli) occurred during the summer, preceding the reproductive period. Subsequent shedding of lateral branches resulted in negative growth rates during autumn/winter. In situ growth of excised shoots showed seasonal patterns comparable to entire thalli, except that growth rates were always positive. Biomass specific net production of excised shoots was significantly higher from spring to autumn than during winter. Areal productivity of S. baccularia had a significant maximum of 3g C m(-2) day(-1) in spring, a value comparable to the productivity of coral-reef epilithic algal communities or temperate kelp forests. The high standing biomass, high productivity and a presumably rapid turnover of biomass via detrital pathways suggest that fucoid macroalgae are important contributors to the stock of organic compounds on these nearshore reefs.