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 differing in extent and depth, with those in the overlying waters showed the canopies to be significantly enriched in particulate organic carbon, nitrogen and phosphorus relative to the overlying waters (on average, 87, 34 and 54% more C, N and P, respectively). Biovolume of detritus (both angiosperm-derived and plankton-derived) was large, particularly within seagrass canopies, where it dominated the seston pool (about 5-fold greater biovolume than that of living particles), compared to a roughly equal biovolume of detrital and living particles in the particle pools in the overlying waters. The dominance of detrital particles was further reflected in the high C/N and C/P ratios of the suspended materials (median atomic C:N:P ratios = 492:40.9:1 and 596:45:1 of the materials suspended within the canopy and in the overlying waters, respectively), which were intermediate between those of Living plankton 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 hypothesis that the water within seagrass canopies is enriched by (mostly detrital) particles, particularly in particle-poor waters. This suggests that seagrasses not only contribute a substantial fraction of the particles themselves, but also act as sinks of particles.