Abstract. The Mediterranean Sea is warming faster than the global Ocean, with important consequences for organisms and biogeochemical cycles. Warming is a major stressor for key marine benthic macrophytes. However, the effect of warming on marine N2 fixation remains unknown, despite the high productivity of macrophytes in oligotrophic waters is partially sustained by the input of new nitrogen (N) into the system by N2 fixation. Here, we assess the impact of warming on N2 fixation rates on three key marine macrophytes: Posidonia oceanica, Cymodocea nodosa, and Caulerpa prolifera. We experimentally measured N2 fixation rates in vegetated and bare sediments at temperatures encompassing current summer mean, projected summer mean and projected summer maximum seawater surface temperatures (SST) by the end of the century under a scenario of moderate greenhouse-gas emissions. We found that N2 fixation rates in vegetated sediments were 2.8-fold higher than in bare sediments at current summer mean SST, with no differences among macrophytes. Currently, the contribution of N2 fixation to macrophytes productivity could account for up to 7 %, 13.8 % and 1.8 % of N requirements for P. oceanica, C. nodosa, and C. prolifera, respectively. We show the temperature dependence of sediment N2 fixation rates. However, the thermal response differed for vegetated sediments, where rates showed an optimum at 31 °C followed by a sharp decrease at 33 °C, and bare sediments, where rates increased along the range of experimental temperatures tested here. The activation energy and Q10 were lower in vegetated sediments than in bare sediments. The projected warming is expected to increase the contribution of N2 fixation to Mediterranean macrophytes' productivity. Therefore, the thermal dependence of N2 fixation might have important consequences for primary production in coastal ecosystems in the context of warming.
Abstract. Vegetated coastal habitats, including seagrass and macroalgal beds, mangrove forests and salt-marshes, form highly productive ecosystems, but their contribution to the global carbon budget remains overlooked and these forests remain “hidden” in representations of the global carbon budget. Despite being confined to a narrow belt around the shoreline of the world’s oceans, where they cover less than 7 million km2, vegetated coastal habitats support about 10 % of the global marine net primary production, and generate a large organic carbon surplus, of about 40 % of their NPP, which is either buried in sediments within these habitats or exported away. Large, 10-fold uncertainties in the area covered by vegetated coastal habitats, along with variability about carbon flux estimates, result in a 10-fold bracket around the estimates of their contribution to organic carbon sequestration in sediments and the deep sea from 73 Tg C year−1 to 866 Tg C year−1 , representing up to 1/3 of the biological CO2 removal by marine biota. Up to 1/2 of this carbon sequestration occurs in sink reservoirs (sediments or the deep sea) beyond these habitats. The organic carbon exported that does not reach depositional sites subsidizes the metabolism of heterotrophic organisms. In addition to their significant contribution to organic carbon production and sequestration, vegetated coastal habitats contribute as much to carbonate accumulation as coral reefs do. Whereas globally-relevant, the magnitude of global carbon fluxes supported by salt-marsh, mangrove, seagrass and macroalgal habitats is declining due to rapid habitat loss, contributing to loss of CO2 sequestration, storage capacity and carbon subsidies. Incorporating the carbon fluxes vegetated coastal habitats support into depictions of the carbon budget of the global ocean and its perturbations will improve current representations of the carbon budget of the global ocean.
Antarctic krill are known to release large amounts of inorganic and organic nutrients to the water column. Here we test the role of krill excretion of dissolved products in stimulating heterotrophic bacteria on the basis of three experiments where ammonium and organic excretory products released by krill were added to bacterial assemblages, free of grazers. Our results demonstrate that the addition of krill excretion products (but not of ammonium alone), at levels expected in krill swarms, greatly stimulates bacteria resulting in an order-of-magnitude increase in growth and production. Furthermore, they suggest that bacterial growth rate in the Southern Ocean is suppressed well below their potential by resource limitation. Enhanced bacterial activity in the presence of krill, which are major sources of DOC in the Southern Ocean, would further increase recycling processes associated with krill activity, resulting in highly efficient krill-bacterial recycling that should be conducive to stimulating periods of high primary productivity in the Southern Ocean.
Codlurn bursa, a balloon-llke chloroph~cean macroalga, a c h ~e v e s cons~derable biomass (300 g DW m-') In the NW Mediterranean httoral C bursa grows as a hollow sphere ranging in slze up to 40 cm In d~ameter, where the t h ~c k algal thallus (-0 5 cm) encloses a lumen fllled w ~t h seawater that receives substantial amounts of organlc carbon from the thallus The water enclosed within C bursa supports an active mcroheterotroph~c (bactena flagellates and clllates) community, about 2-fold denser than that in the ambient water where metazoans are absent Protlstan grazing removed the entlre bactenal populabon dally (121 7 % d-l) Oxygen concentratlon withln the enclosed water showed large die1 fluctuations, reaching concentrations close to saturat~on (-90%) dunng day time, and concentratlon close to anoxia (after 12 h) In the dark as a result of the comblned n ~e t a b o l ~s m of the alga and the microheterotrophs The relat~ve actlvlty and abundance of mlcroheterotrophs decreased as C bursa slze Increased, consistent wlth the reduct~on In the ratlo of macroalgal w e ~g h t Internal water volume That the carbon supplied by the algal enclosure becomes increasingly diluted as C bursa grows was also indicated by reduced exoproteolytic actlv~ty w t h dechne of mlcloheterotrophs abundance dunng C bursa growth The hlgh heterotroph~c actlvity w ~t h i n C bursa balloons resulted in nutnent concentrabon 5to 15-fold hlgher than those in the surround~ng seawater, w h ~c h could partially support algal production
Hulme points out that observed rates of range expansion by invasive alien species are higher than the median speed of isotherm movement over the past 50 years, which in turn has outpaced the rates of climate-associated range changes of marine and terrestrial species ([ 1 ][1], [ 2 ][2]). This is not
