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 CaCO 3 cm −2 yr −1 , while gross calcification determined using growth metrics (just newly deposited CaCO 3 ) was 0.43 to 0.59 g CaCO 3 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.

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The presence of banding in the skeleton of coralline algae has been reported in many species, primarily from temperate and polar regions. Similar to tree rings, skeletal banding can provide information on growth rate, age, and longevity; as well as records of past environmental conditions and the coralline alga’s growth responses to such changes. The aim of this study was to explore the presence and characterise the nature of banding in the tropical coralline alga Porolithon onkodes, an abundant and key reef-building species on the Great Barrier Reef (GBR) Australia, and the Indo-Pacific in general. To achieve this we employed various methods including X-ray diffraction (XRD) to determine seasonal mol% magnesium (Mg), mineralogy mapping to investigate changes in dominant mineral phases, scanning electron microscopy–electron dispersive spectroscopy (SEM-EDS), and micro-computed tomography (micro-CT) scanning to examine changes in cell size and density banding, and UV light to examine reproductive (conceptacle) banding. Seasonal variation in the Mg content of the skeleton did occur and followed previously recorded variations with the highest mol% MgCO3 in summer and lowest in winter, confirming the positive relationship between seawater temperature and mol% MgCO3. Rows of conceptacles viewed under UV light provided easily distinguishable bands that could be used to measure vertical growth rate (1.4 mm year-1) and age of the organism. Micro-CT scanning showed obvious banding patterns in relation to skeletal density, and mineralogical mapping revealed patterns of banding created by changes in Mg content. Thus, we present new evidence for seasonal banding patterns in the tropical coralline alga P. onkodes. This banding in the P. onkodes skeleton can provide valuable information into the present and past life history of this important reef-building species, and is essential to assess and predict the response of these organisms to future climate and environmental changes.