The microcosm of particles within seagrass Posidonia oceanica canopies

Comparison of the amount and nature of suspended material within Posidonia oceanica canopies, in 6 meadows in the Spanish Mediterranean coast differin g in extent and depth, with those in the overlying waters showed the canopi es to be significantly enriched in particulate organic carbon, nitrogen and phosphorus relative to the overlying waters (on average, 87, 34 and 54% mo re C, N and P, respectively). Biovolume of detritus (both angiosperm-derive d and plankton-derived) was large, particularly within seagrass canopies, w here it dominated the seston pool (about 5-fold greater biovolume than that of living particles), compared to a roughly equal biovolume of detrital an d living particles in the particle pools in the overlying waters. The domin ance of detrital particles was further reflected in the high C/N and C/P ra tios of the suspended materials (median atomic C:N:P ratios = 492:40.9:1 an d 596:45:1 of the materials suspended within the canopy and in the overlyin g waters, respectively), which were intermediate between those of Living pl ankton and P. oceanica. The relative enrichment of P. oceanica canopies by particles tended to be greatest when particle loads in the overlying waters were small, suggesting that the effect of seagrasses as traps of particles is enhanced in particle-poor waters. The results obtained support the hypo thesis that the water within seagrass canopies is enriched by (mostly detri tal) particles, particularly in particle-poor waters. This suggests that se agrasses not only contribute a substantial fraction of the particles themse lves, but also act as sinks of particles.