Patch dynamics of the Mediterranean seagrass Posidonia oceanica: Implications for recolonisation process

Patch dynamics of the Mediterranean slow-growing seagrass Posidonia oceanica was studied in two shallow sites (3-10 m) of the Balearic Archipelago (Spain) through repeated censuses (1-2 year^-1). In the sheltered site of Es Port Bay (Cabrera Island), initial patch density (October 2001) was low: 0.05 patches m^-2, and the patch size (number of shoots) distribution was bimodal: most of the patches had less than 6 shoots or between 20 and 50 shoots. Mean patch recruitment in Es Port Bay (0.006 ± 0.002 patches m^-2 year^-1) exceeded mean patch loss (0.001 ± 0.001 patches m^-2 year^-1), yielding positive net patch recruitment (0.004 ± 0.003 patches m^-2 year^-1) and a slightly increased patch density 3 years later (July 2004, 0.06 patches m^-2). In the exposed site of S'Estanyol, the initial patch density was higher (1.38 patches m^-2, August 2003), and patch size frequency decreased exponentially with size. Patch recruitment (0.26 patches m^-2 year^-1) and loss (0.24 patches m^-2 year^-1) were high, yielding a slightly increased patch density in the area 1 year later (October 2004, 1.40 patches m^-2). Most recruited patches consisted of rooting vegetative fragments of 1-2 shoots. Seedling recruitment was observed in Summer 2004 at both sites. Episodic, seedling recruitment comprised 30% and 25% of total patch recruitment in Es Port Bay and S'Estanyol, respectively. Patch survival increased with patch size and no direct removal was observed among patches of 5 shoots or more. Most patches grew along the study, shifting patch distribution towards larger sizes. Within the size range studied (1-150 shoots), absolute shoot recruitment (shoots year^-1) increased linearly with patch size (R² = 0.64, p < 4 × 10^-5, N = 125), while specific shoot recruitment was constant (about 0.25 ± 0.05 year^-1), although its variance was large for small patches. Given the slow growth rate and the high survival of patches with 5 or more shoots, even the low patch recruitment rates reported here could play a significant role in the colonisation process of P. oceanica.