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Seasonal dynamics of Posidonia oceanica in Magalluf Bay (Mallorca, Spain): Temperature effects on seagrass mortality

We recorded seasonal and interannual population dynamics of the endemic Mediterranean seagrass Posidonia oceanica in a shallow meadow, Magalluf Bay, Spain (39°30.25′N, 2°32.59′E), performing direct monthly‐bimonthly censuses from June 2001 to November 2004, in 12 permanent plots. Initially, there were 568 ± 54 (mean 6 SE) shoots m22. An average decline of ‐63 ± 10 SE shoots m−2 yr−1 was detected. Shoot mortality (median: 1.2% month−1; range: 0% to 12.1% month−1) increased in summer, enhanced by seawater temperature. There was also substantial interannual variability characterized by very high mortality in summer‐autumn 2001 and in autumn‐winter 2003–2004, coinciding with hot summers and meadow flowering in autumn (2001: 2.0% ± 1.4% of shoots; 2003: 53.8% ± 10.4% of shoots), but fruiting and seedling recruitment were only observed in 2004. Annual vegetative recruitment (0.034 ± 0.007 yr−1) was 53 times higher than annual sexual recruitment (0.0006 ± 0.0007 yr−1), and total recruitment (vegetative + sexual) was on average 4.5 times lower than mortality. Specific net population growth was always negative during the study. No seasonality was detected in shoot recruitment (median: 0.002 month−1; range: 0 to 0.021 month−1), which did not correlate with solar radiation or temperature. Shoots recruited during the study had a steeper exponential depletion rate (rr = ‐0.34 ± 0.05 SE yr−1) than the global population (rN0 = ‐0.16 ± 0.01 SE yr−1). Shoot probability to survive to the next age class (px) increased with shoot age. Our results suggest that the general decline of P. oceanica meadows could be enhanced by the decadal Mediterranean warming trend.