Growth and chronology of the rhodolith-forming, coralline red alga Sporolithon durum

We report extension rates and growth patterns of the coralline red alga Sporolithon durum (rhodolith form) from New Caledonia. Alizarin red S staining was used to mark 43 rho do lith branches and helped determine extension rates over the rhodoliths’ last living year. A combination of radiocarbon dating, major Mg/Ca ratio cycles and growth band determinations provided a chronological approach to characterise extension rates for 5 branches over the last 5 decades. A seasonal asymmetry in the extension rates was observed, with higher extension during the austral summer−fall−winter period, indicating that variations in ambient seawater temperature are not of major influence in the seasonal growth pattern of S. durum. At the sub-seasonal level, minor growth bands were observed with a maximum frequency of 14 d but were too variable to be considered as a reliable chronological tool. The concordance of the results from both the monitoring experiment over the year 2010 to 2011 and the chronological approach applied to the period 1968 to 2008 suggests that an extension rate (mean ± SD) of 0.6 ± 0.2 mm yr−1 is typical of the S. durum community at the study site. However, annual extension rates varied considerably among branches and individuals. Long-term trends still appeared, such as a decrease during the early 1970s attributed to the impact of mining activity. A slight but consistent decrease was observed in the annual extension rates throughout the record and may reflect an ontogenic effect potentially enhanced by reduced light penetration due to increasing suspended particulate matter in the water column.