Have coral calcification rates slowed in the last twenty years? Peter V Ridd a, ⁎, Eduardo Teixeira da Silva a , Thomas Stieglitz a,b a Marine Geophysical Laboratory, School of Engineering and Physical Science, James Cook University, Townsville 4811, Australia b Laboratoire des sciences de l'environnement marin CNRS UMR 6539, Institut Universitaire Européen de la Mer, 29280 Plouzané, France abstract article info Article history: Received 14 February 2012 Received in revised form 30 July 2013 Accepted 3 September 2013 Available online 14 September 2013 Communicated by G.J. de Lange Keywords: coral calcification Great Barrier Reef ocean acidification ocean pH This paper reports a reanalysis of calcification rates of 328 Porites cores from the Great Barrier Reef from which previous workers have concluded that a 14% reduction in calcification rates has occurred between 1990 and 2005. In this reanalysis it is shown that the apparent reduction in the Porites spp. calcification rate in the last two decades is at least partly due to a combination of (a) ontogenetic effects (disregarded in the previous analysis), combined with a highly variable age distribution of the coral growth bands with time, and (b) a systematic data bias clearly evident in the last growth band of each core. When the outermost growth band in addition to bands which have record age less than 20 years was excluded from the analysis, the dramatic fall in calcification after 1990 was no longer evident. © 2013 Elsevier B.V. All rights reserved. 1. Introduction There is widespread concern that a hitherto unperceived con- sequence of global carbon dioxide emissions is a decrease in ocean pH which will have dire consequences for the calcification of calcareous marine organisms such as corals. A recent analysis (De'ath et al., 2009) of coral calcification data extracted from 328 Porites corals collected from 69 reefs over the Great Barrier Reef (GBR) that span the last 400 years has indicated that there has been a dramatic decline in coral calcification by as much as 14% between 1990 and 2005. It was suggested that a tipping point was reached in 1990 when declining ocean pH due to in- creased atmospheric CO 2 combined with increasing temperature stress caused rapid reductions in calcification. The 14% decline in calcification rate between 1990 and 2005 (De'ath et al., 2009) is prima facie a surprising result because a previous comprehensive study (Lough and Barnes, 2000), using a subset of the data used in De'ath et al. (2009), demonstrated a statistically significant 4% increase in GBR coral growth over the 20th century. In addition, it is notable that a more recent paper on calcification rates on Australia's north western coastline does not indicate any significant decline in calcification rates after 1990 (Cooper et al., 2012). However, laboratory experiments show that calcification decreases under increasing pH for a variety of reef organisms (Hoegh-Guldberg et al., 2007), suggesting that modern coral reefs may be facing major challenges due to en- vironmental change (Carpenter et al., 2008). Clearly the precise na- ture of the trend in coral calcification is of considerable importance for scientists, managers and policy makers alike. The study of De'ath et al. (2009) used a linear mixed effect (LME) model, an analysis technique that aims to compensate for the many challenges involved with analysing complex data such as coral growth time series. The different number of corals from each location and a large latitudinal sampling range, which implies varying environmental factors leading to variable calcification rates, are examples of confounding variables that can be accounted for by LME. However there are additional important aspects of the data set that were not taken into account in the analysis of De'ath et al. (2009). These include (a) the use of three different types of coral samples (long cores, short cores and colonies) which produce samples of different lengths and therefore record ages (record age is defined as the time between a particular yearly calcification re- cord and the first data record in the series), (b) a strong temporal variation in the average age of the corals over the sampling period, and (c) the likelihood of systematic sampling problems. These three confounding variables intrinsic to the data set are discussed in this paper. At the centre of the analysis by De'ath et al. (2009) is the as- sumption that calcification rates for a particular coral do not change with the age of the coral (if environmental conditions remain con- stant), i.e., there are no ontogenetic effects. Here we reanalyse the coral calcification data from the Great Barrier Reef and show that the apparent decline over the last two decades may be the result of a combination of ontogenetic effects and measurement artefacts Marine Geology 346 (2013) 392–399 ⁎ Corresponding author. Tel.: +61 747814978; fax: +61 747815880. E-mail address: peter.ridd@jcu.edu.au (P.V. Ridd). 0025-3227/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.margeo.2013.09.002 Contents lists available at ScienceDirect Marine Geology journal homepage: www.elsevier.com/locate/margeo