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Seagrasses cover about 0.1-0.2% of the global ocean, and develop highly productive ecosystems which fulfil a key role in the coastal ecosystem. Widespread seagrass loss results from direct human impacts, including mechanical damage (by dredging, fishing, and anchoring), eutrophication, aquaculture, siltation, effects of coastal constructions, and food web alterations; and indirect human impacts, including negative effects of climate change (erosion by rising sea level, increased storms, increased ultraviolet irradiance), as well as from natural causes, such as cyclones and floods. The present review summarizes such threats and trends and considers likely changes to the 2025 time horizon. Present losses are expected to accelerate, particularly in South-east Asia and the Caribbean, as human pressure on the coastal zone grows. Positive human effects include increased legislation to protect seagrass, increased protection of coastal ecosystems, and enhanced efforts to monitor and restore the marine ecosystem. However, these positive effects are unlikely to balance the negative impacts, which are expected to be particularly prominent in developing tropical regions, where the capacity to implement conservation policies is limited. Uncertainties as to the present loss rate, derived from the paucity of coherent monitoring programmes, and the present inability to formulate reliable predictions as to the future rate of loss, represent a major barrier to the formulation of global conservation policies. Three key actions are needed to ensure the effective conservation of seagrass ecosystems: (1) the development of a coherent worldwide monitoring network, (2) the development of quantitative models predicting the responses of seagrasses to disturbance, and (3) the education of the public on the functions of seagrass meadows and the impacts of human activity. ; Peer Reviewed

The distribution of dissolved organic carbon (DOC) concentration across coastal waters was characterized based on the compilation of 3510 individual estimates of DOC in coastal waters worldwide. We estimated the DOC concentration in the coastal waters that directly exchange with open ocean waters in two different ways, as the DOC concentration at the edge of the shelf break and as the DOC concentration in coastal waters with salinity close to the average salinity in the open ocean. Using these estimates of DOC concentration in the coastal waters that directly exchange with open ocean waters, the mean DOC concentration in the open ocean and the estimated volume of water annually exchanged between coastal and open ocean, we estimated a median ± SE (and average ± SE) global DOC export from coastal to open ocean waters ranging from 4.4 ± 1.0 Pg C yr to 27.0 ± 1.8 Pg C yr (7.0 ± 5.8 Pg C yr to 29.0 ± 8.0 Pg C yr) depending on the global hydrological exchange. These values correspond to a median and mean median (and average) range between 14.7 ± 3.3 to 90.0 ± 6.0 (23.3 ± 19.3 to 96.7 ± 26.7) Gg C yr per km of shelf break, which is consistent with the range between 1.4 to 66.1 Gg C yr per km of shelf break of available regional estimates of DOC export. The estimated global DOC export from coastal to open ocean waters is also consistent with independent estimates of the net metabolic balance of the coastal ocean. The DOC export from the coastal to the open ocean is likely to be a sizeable flux and is likely to be an important term in the carbon budget of the open ocean, potentially providing an important subsidy to support heterotrophic activity in the open ocean. ; This is a contribution to the >Malaspina 2010 Expedition> consolider project funded by the Spanish Ministry of Economy and Competitivity (reference CONSOLDER2008-00077). C.B. was funded by a scholarship from the government of the Balearic Islands, a Juan de la Cierva fellowship from the Spanish Ministry of Economy and Competitivity and a research fellow scholarship from the Australian Research Council --discovery projects--120101778 ; Peer Reviewed

The development, growth, and space occupation by the canopy and rhizosphere of a temperate Mediterranean seagrass meadow of Cymodocea nodosa, and the possible effects of meadow seasonal development on sediment redox conditions, were examined during the 1998 growth season. The meadow supported maximum biomass of leaves and rhizomes during July, and maximum root biomass in August. The meadow maintained 123.6 g dry wt m-2 of leaves and 94.4 g dry wt m-2 of rhizomes (July data), and 121.2 g dry wt m-2 of roots (August data) during peak biomass. On average, the root network contained 607 m of roots m-2, had 3.6 cm between neighbouring roots and comprised 1.7% of sediment volume. Half of the root biomass occupied the top 12.6 cm of sediment, although a few roots reached >35 cm sediment depth. The meadow developed 70, 62 and 50% of leaf, rhizome and root biomass during the growth season respectively, showing that the structure of the temperate seagrass rhizosphere is highly dynamic. C. nodosa produced leaves, rhizomes and roots at rates ranging between 1.17 and 3.98 g dry wt m-2 d-1, 0.01 and 0.75 g dry wt m-2 d-1, and 0.04 and 0.84 g dry wt m-2 d-1, respectively. C. nodosa meadow grew on sediments where redox conditions during the growth season varied from -74 to 396 mV, being between 21 and 112 mV more positive than adjacent unvegetated sediments from July to September. The magnitude of redox potential anomaly in seagrass rhizosphere tended to be coupled to the above and belowground meadow biomass, suggesting that C. nodosa metabolism alters sediment redox conditions. Structural change of seagrass meadows during the growth season, therefore, is expected to influence benthic biogeochemical processes. ; This work was funded by the project PhaSE (contract MAS3-CT96-0053) of the ELOISE programme of the European Union. N.M. was supported by a grant from the CIRIT (Government of Catalonia). ; Peer Reviewed

Coastal hypoxia is a problem that is predicted to increase rapidly in the future. At the same time, we are facing rising atmospheric CO2 concentrations, which are increasing the $\textit{p}$CO2 and acidity of coastal waters. These two drivers are well studied in isolation; however, the coupling of low O2 and pH is likely to provide a more significant respiratory challenge for slow moving and sessile invertebrates than is currently predicted. The Gullmar Fjord in Sweden is home to a range of habitats, such as sand and mud flats, seagrass beds, exposed and protected shorelines and rocky bottoms. Moreover, it has a history of both natural and anthropogenically enhanced hypoxia as well as North Sea upwelling, where salty water reaches the surface towards the end of summer and early autumn. A total of 11 species (Crustacean, Chordate, Echinoderm and Mollusc) of these ecosystems were exposed to four different treatments (high or low oxygen and low or high CO2; varying $\textit{p}$CO2 of 450 and 1300Ä '¬Î¼atm and O2 concentrations of 2-3.5 and 9-10Ä '¬mgÄ '¬LÄ '1) and respiration measured after 3 and 6 days, respectively. This allowed us to evaluate respiration responses of species of contrasting habitats to single and multiple stressors. Results show that respiratory responses were highly species specific as we observed both synergetic as well as antagonistic responses, and neither phylum nor habitat explained trends in respiratory responses. Management plans should avoid the generalized assumption that combined stressors will result in multiplicative effects and focus attention on alleviating hypoxia in the region. ; This research was funded by the project ASSEMBLE (grant agreement no. 227799; under the EU Research Infrastructure Action FP7) and the Estres-X project funded by the Spanish Ministry of Economy and Competitiveness (CTM2012-32603). Aisling Fontanini was funded by the School of Plant Biology at the University of Western Australia (grant 10300374) and Alexandra Steckbauer was funded by a fellowship from the Government of the Balearic Islands (Department on Education, Culture and Universities) and the EU (European Social Fund) as well as King Abdullah University of Science and Technology. Sam Dupont is funded by the Linnaeus Centre for Marine evolutionary Biology at the University of Gothenburg and supported by a Linnaeus grant from the Swedish research Councils VR and Formas. We thank Karen Chan, Pia Engström and Julia Dombrowski for their assistance.

We evaluated the photosynthetic performance of Posidonia oceanica during short-term laboratory exposures to ambient and elevated temperatures (24–25°C and 29–30°C) warming and pCO2 (380, 750 and 1000ppm pCO2) under normal and low light conditions (200 and 40μmol photons m−2s−1 respectively). Plant growth was measured at the low light regime and showed a negative response to warming. Light was a critical factor for photosynthetic performance, although we found no evidence of compensation of photosynthetic quantum efficiency in high light. Relative Electron Rate Transport (rETRmax) was higher in plants incubated in high light, but not affected by pCO2 or temperature. The saturation irradiance (Ik) was negatively affected by temperature. We conclude that elevated CO2 does not enhance photosynthetic activity and growth, in the short term for P. oceanica, while temperature has a direct negative effect on growth. Low light availability also negatively affected photosynthetic performance during the short experimental period examined here. Therefore increasing concentrations of CO2 may not compensate for predicted future conditions of warmer water and higher turbidity for seagrass meadows. ; This research was supported by the MedSeA project (www.medsea-project.eu, contract number 265103 of the Framework Program 7 of the European Union), and ESTRESX (ref. CTM2012-32603), funded by the Spanish Ministry of Economy and Competitiveness. I.E.H. was supported by a JAE-DOC fellowship (CSIC, Spain). YSO was funded by a Marie Curie IEF from the European Union Seventh Framework Programme (FP7/2007–2013) under grant agreement no. 254297: FP7-PEOPLE-2009-IEF.

Spanish Ministry of Economy and Competitiveness (projects ESTRESX CTM2012-32603 and CLIMPACT CGL2014-54246-C2-1-R). Ramón y Cajal contract (RYC-2013-14714) funded by the Spanish Ministry of Economy and Competitiveness and the Regional Government of the Balearic Islands. ; Peer Reviewed

Xabier Irigoien et al. ; With a current estimate of ~1,000 million tons, mesopelagic fishes likely dominate the world total fishes biomass. However, recent acoustic observations show that mesopelagic fishes biomass could be significantly larger than the current estimate. Here we combine modelling and a sensitivity analysis of the acoustic observations from the Malaspina 2010 Circumnavigation Expedition to show that the previous estimate needs to be revised to at least one order of magnitude higher. We show that there is a close relationship between the open ocean fishes biomass and primary production, and that the energy transfer efficiency from phytoplankton to mesopelagic fishes in the open ocean is higher than what is typically assumed. Our results indicate that the role of mesopelagic fishes in oceanic ecosystems and global ocean biogeochemical cycles needs to be revised as they may be respiring ~10% of the primary production in deep waters. ; This research was conducted by the Malaspina 2010 Expedition project, funded by the Spanish Ministry of Economy and Competitiveness (Consolider-Ingenio 2010, CSD2008-00077). Additional financial support was provided by the Basque Country Government and by AZTI-Tecnalia ; Peer Reviewed