[1] 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 O 2 m −2 d −1 ) tended to be significantly higher than the corresponding respiration (R) rates (mean ± SE = 187.6 ± 10.1 mmol O 2 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 O 2 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 O 2 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 km 2 and 41.47 to 101.39 Tg C yr −1 when a high estimate of global seagrass area of 600,000 km 2 was considered. The global loss of 29% of the seagrass area represents, therefore, a major loss of intense natural carbon sinks in the biosphere.

Stable carbon-isotope ratios of bacterial biomarkers were studied in Mediterranean seagrass meadows and macroalgae communities to identify the sources of organic carbon used by the sediment bacteria. Bacteria δ 13 C ratios in pristine sediments vegetated by the seagrass Posidonia oceanica were either similar to the seagrass signal or slightly enriched, suggesting that seagrass detritus was of major importance as a bacterial carbon source. There was a shift in bacterial carbon sources in anthropogenic impacted P. oceanica meadows towards seston and macroalgae. The net primary productivity was reduced in these meadows, whereas the rates of mineralization as measured by sulfate reduction rates were enhanced in the sediments. This effect on mineralization was probably due to the input of less refractory organic matter compared to seagrass detritus, which enhances the bacterial decomposition of organic matter. In the fast growing seagrass Cymodocea nodosa meadow, the bacterial carbon sources consisted of a mixture of seagrass detritus and seston, and the mineralization rates were much higher compared to the P. oceanica meadows, indicating that these carbon sources were more labile and easily decomposed by the bacteria. A similar pattern was found in the macroalgae bed with Caulerpa prolifera, where the rates of mineralization were high, similar to findings in organic impacted fish farm sediments. Here C. prolifera detritus was the most important carbon source and accounted for an increase in sediment organic content. The possible impacts of a shift in bacterial carbon sources due to nutrient loading are discussed in relation to the performance of P. oceanica in carbonate sediments.

We examined annual nutrient changes (nitrate, phosphate and ammonium) and the net dissolved organic carbon (DOC), nitrogen (DON) and phosphorus (DOP) release using in situ benthic incubations in a Posidonia oceanica meadow and in unvegetated sediments of Magalluf Bay (Mallorca Island, Spain) at monthly intervals. We also examined the role of the P. oceanica meadow in the C:N:P ratio transformations by comparing the ratios between the inorganic and dissolved organic fluxes in the seagrass ecosystem and those in the plant material. Our results indicate that P. oceanica communities enhance DOC fluxes relative to adjacent unvegetated sediments. The net DOC release from the P. oceanica meadow represents 71% of the net community production. The C:N:P ratio in the flux of dissolved organic material (DOM) from the P. oceanica community (1450:69:1) exceeded the C:N:P ratio in the DOM flux of unvegetated sediment communities. The high flux of DOC release from this seagrass community might be enriched in carbon compounds, as these plants produce nonstructural carbohydrates in excess.

We measured monthly dissolved oxygen (DO) changes in situ benthic incubations from March 2001 to October 2002 in a Posidonia oceanica meadow and unvegetated sediments of Magalluf Bay (Mallorca Island, Spain) to determine gross primary production (GPP), community respiration (R), and net community production (NCP). From June 2001 to October 2002, we also measured fluxes of dissolved inorganic carbon (DIC) and total alkalinity (TAlk). The yearly integrated metabolic rates based on DO changes show that the P. oceanica communities are net autotrophic while the metabolic rates in the unvegetated benthic communities are nearly balanced. Higher calcium carbonate (CaCO 3 ) cycling, both in terms of production and dissolution, was observed in P. oceanica communities than in unvegetated benthic communities. In the P. oceanica meadow, the annual release of CO 2 from net CaCO 3 production corresponds to almost half of the CO 2 uptake by NCP based on DIC incubations. In unvegetated benthic communities, the annual uptake of CO 2 from net CaCO 3 dissolution almost fully compensates the CO 2 release by NCP based on DIC incubations. CaCO 3 dynamics is potentially a major factor in CO 2 benthic fluxes in seagrass and carbonate-rich temperate coastal ecosystems.

Estimates of dissolved organic carbon (DOC) release by marine macrophyte communities (seagrass meadows and macroalgal beds) based on in situ benthic chambers from published and unpublished are compiled in this study. The effect of temperature and light availability on DOC release by macrophyte communities was examined. Almost 85% of the seagrass communities and all of macroalgal communities examined acted as net sources of DOC. Net DOC fluxes in seagrass communities increase positively with water temperature. In macroalgal communities net DOC fluxes under light exceeded those under dark condition, however, this trend was weaker in seagrass communities. Shading of a mixed seagrass meadow in The Philippines led to a significant reduction on the net DOC release when shading was maintained for 6 days compared to only 2 days of shading. Net DOC fluxes increased with increasing community respiration, but were independent of primary production or net community production. The estimated global net DOC flux, and hence export, from marine macrophytes is about 0.158 ± 0.055 or 0.175 ± 0.056 Pg C year −1 depending on the global extent of seagrass meadows considered.