Reduced calcification in Western Australian corals during anomalously high summer water temperatures

Here we report the seasonal response of calcification rates over two years, for three coral species (Acropora pulchra, Pocillopora damicornis and Goniastrea aspera), to anomalously warm summer water temperatures (2012 and 2013), following the most severe marine heat wave on record (2010-2011). The study sites at Coral Bay (Ningaloo Reef), the Houtman Abrolhos Islands and Marmion Reef (Perth) spanned over 10° of latitude and ranged from tropical to temperate habitats. Calcification rates were determined using repeated measurements of buoyant weight normalized to surface area and rates were monitored over two winters and two summers from 2011 to 2013. Average growth rates followed predicted latitudinal trends, with the fastest growth in the tropics (Coral Bay) and slowest growth rates in the temperate zone (Marmion). However, calcification rates did not show the expected seasonality (higher in summer, lower in winter), even at high latitude locations. Instead, there was either little difference between summer and winter growth or growth was slower in the summer (on average, a reduction of ~ 40% across locations and species). Additionally, differences in species growth rates did not follow expected trends, with usually fast growing branching corals not calcifying much faster than the normally slower growing massive corals (A. pulchra, grew ~ 40% faster than G. aspera, which grew ~ 6% faster than P. damicornis across all latitudes and seasons). Survival rates were also reduced in the summer months, while among species, P. damicornis had the lowest survival and G. aspera the highest. We conclude that high temperature stress through the summers of 2011 to 2013 was the most likely cause for the lack of seasonality in calcification rates, the similarity in calcification rates among species and the increased mortality of susceptible species in the summer months. The effect of prolonged elevated temperature anomalies (1.5 to 3 °C) on the growth and survival of colonies over consecutive years was often greater than the fundamental influences of season and species, highlighting the extent to which climate change could now be re-structuring the life histories of corals on Western Australian reefs.