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 (39u30.259N, 2u32.599E), performing direct monthly- bimonthly censuses from June 2001 to November 2004, in 12 permanent plots. Initially, there were 568 ± 54 (mean ± SE) shoots m^-2. 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% 6 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 (r_r = 0.34 ± 0.05 SE yr^-1) than the global population (r_N0 = -0.16 ± 0.01 SE yr^-1). Shoot probability to survive to the next age class (p_x) increased with shoot age. Our results suggest that the general decline of P. oceanica meadows could be enhanced by the decadal Mediterranean warming trend.