The metabolic rates of seagrass communities were synthesized on the basis of a data set on seagrass community metabolism containing 403 individual estimates derived from a total of 155 different sites. Gross primary production (GPP) rates (mean ± SE = 224.9 ± 11.1 mmol O₂ m⁻² d⁻¹) tended to be significantly higher than the corresponding respiration (R) rates (mean ± SE = 187.6 ± 10.1 mmol O₂ m⁻² d⁻¹), indicating that seagrass meadows tend to be autotrophic ecosystems, reflected in a positive mean net community production (NCP 27.2 ± 5.8 mmol O₂ m⁻² d⁻¹) and a mean P/R ratio above 1 (1.55 ± 0.13). Tropical seagrass meadows tended to support higher metabolic rates and somewhat lower NCP than temperate ones. The P/R ratio tended to increase with increasing GPP, exceeding, on average, the value of 1 indicative of metabolic balance for communities supporting a GPP greater than 186 mmol O₂ m⁻² d⁻¹, on average. The global NCP of seagrass meadows ranged (95% confidence limits of mean values) from 20.73 to 50.69 Tg C yr⁻¹ considering a low global seagrass area of 300,000 km² and 41.47 to 101.39 Tg C yr⁻¹ when a high estimate of global seagrass area of 600,000 km² was considered. The global loss of 29% of the seagrass area represents, therefore, a major loss of intense natural carbon sinks in the biosphere.

The annual depositional flux of carbon (C), nitrogen (N), and phosphorous (P) to the sediments under a northeast Spain Posidonia oceanica meadow was evaluated, and the sources and fate of the material deposited elucidated. The annual deposition of carbon represented 198 g C m⁻² yr⁻¹, 72% of which was derived from the seston and 28% from P. oceanica detritus. The depositional flux was poor in nitrogen (13.4 g N m⁻² yr⁻¹) and phosphorous (2.01 g P m⁻² yr⁻¹), although comparable to the nutrient inputs required to support the growth of P. oceanica. Remineralization in the sediment only returned 15.6 g C m⁻² yr⁻¹, yielding a net carbon accumulation of 182 g C m⁻² yr⁻¹. Our results show that in the Mediterranean littoral P. oceanica meadows are important sites of net organic carbon burial, derived from sedimented sestonic particles and seagrass detritus.