Seasonal growth and calcification of a reef-building crustose coralline alga on the Great Barrier Reef

Crustose coralline algae (CCA) play a central role in tropical reef ecology, helping to build and stabilise the reef framework and, due to their high Mg-calcite skeleton, are potential indicators for assessing the impacts of ocean acidification. However, basic information on CCA seasonal growth and calcification is relatively unknown on the Great Barrier Reef (GBR). This study provides seasonal baseline rates of vertical growth, marginal (horizontal) growth and calcification of the abundant CCA species Porolithon onkodes, together with the effect of reduced irradiance on these metrics to better understand the natural variability between CCA living in exposed and shaded areas. Seasonal variation was detected in each of the metrics, with maximum vertical growth and calcification observed in spring and marginal extension in autumn. Annual vertical growth rate was 1.45 mm yr(-1), absolute marginal growth rate (surface area) was 11.12 cm(2) yr(-1), and absolute marginal extension rate (diameter) was 24.66 mm yr(-1). Net calcification, determined using the buoyant weight method (includes secondary calcification deposits) was approximately 2.72 to 3.40 g CaCO3 cm(-2) yr(-1), while gross calcification determined using growth metrics (just newly deposited CaCO3) was 0.43 to 0.59 g CaCO3 cm(-2) yr(-1). Shaded treatments yielded higher growth and gross calcification rates compared to exposed in all seasons except summer. This data provides empirical information necessary to monitor the impacts of future climate change on the GBR and to better understand the response of CCA to manipulative experiments on ocean acidification and warming.