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.
In order to establish the potential role of Antarctic krill ( Euphausia superba ) in the recycling of bioactive elements, we have quantified the release of iron, phosphate, and ammonia by these organisms along the Antarctic Peninsula sector of the Southern Ocean. The experimental results suggested that the presence of krill has a significant impact on ambient iron concentrations, as large amounts of this trace element were released by the krill (22–689 nmol Fe g Dry Weight −1 h −1 , equivalent to 0.2 to 4.3 nmol Fe L −1 d −1 ). Half of this iron release occurred within the first hour of the experiment, and differences in iron and phosphate release rates (3.1 to 14.0 μ mol PO 4 3− g DW −1 h −1 ) seemed to reflect differences in food availability. These results identify krill as a major node in iron cycling in the Southern Ocean, potentially influencing iron residence time in the upper water column of this region.
The effect of the presence of sulfide on the survival of benthic organisms under hypoxia was tested using a meta‐analysis of published experimental results evaluating the effects of the presence of hydrogen sulfide on the median survival time of benthic macrofauna under hypoxia. The meta‐analysis confirmed that survival times under hypoxia are reduced by an average of 30% in marine benthic communities exposed to hydrogen sulfide. The effect of sulfide on survival was higher for egg forms than for juvenile or adult stages. The aggravation of the negative effects of spreading hypoxia in the presence of sulfide suggests that the threats derived from hypoxia to marine biodiversity are greater than anticipated on the basis of the direct effects of low oxygen concentration alone.
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.
