Primary productivity of reef-building crustose coralline algae

The primary productivity of four species of crustose coralline algae was measured as a function of depth (0–18 m) and irradiance on samples collected from and growing upon the windward coral reef at Lizard Island, northern Great Barrier Reef, Australia. Significantly higher productivities were measured in the field than in the laboratory. Maximum gross oxygen production in situ varied from 12.8 to 22.8 mmol m22 h21; dark respiration consumed between 2.7 and 4.5 mmol O2 m22 h21. Integration of photosynthesis–irradiance models with half sine curve approximations of whole-day irradiance yielded estimated in situ net productivities of 15–132 mmol O2 m22 d21. When multiplied by previously determined photosynthetic quotients, in situ net carbon fixation was estimated to vary from 0.2 to 1.3 g m22 d21. Multiplying these rates by measured surface relief factors of 3.1 for the reef crest and 5.0 for the windward slope yielded estimated contributions to reef organic production of ;0.9–5 g C (net) planar m22 d21 over the depth interval 0–18 m, given 100% cover. These data suggest that crustose coralline algae make a larger contribution to organic production on coral reefs than has been thought to this time. A curvilinear model is presented that enables their primary productivity to be estimated from measurements of in situ irradiance at the solar zenith. Although it is recognized that crustose coralline algae may make a significant overall contribution to coral reef primary production by virtue of their high abundance, they are regarded as low rate producers of organic carbon (Larkum 1983). If this is true, it is surprising that some species are able to calcify their tissues at rates of up to 9.1 g CaCO3 m22 d21 (Chisholm 2000), since this would require significant concomitant production of organic carbon. Given that specific rates of photosynthesis by crustose coralline algae have seldom been measured in the field and only once in a high-energy environment where conditions favor their development (Chisholm et al. 1990), it is possible that their organic productivity has been underestimated. The organic productivity of crustose coralline algae is no less important to the maintenance of coral reefs than is their inorganic production, as the latter depends on the former. Calcification occurs within the cells walls of coralline algae and not externally, as in corals and other invertebrates; thus, photosynthesis creates the organic environment in which the calcite crystals are deposited. The rate at which coralline algae are able to bind loose substrata and provide protective barriers to erosion is therefore a function of their primary productivity. Coralline algae are ubiquitous and often dominant components of coral reef communities (e.g., Littler 1973a; Stearn et al. 1977; Glynn et al. 1996; Keats et al. 1997), and their abundance in cryptic and shaded environments can be 1 Present address: 22A Rose Street, North Ward, Queensland 4810, Australia ([email protected]).