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Nitrogen loads in coastal areas have increased dramatically, with detrimental consequences for coastal ecosystems. Shallow sediments and seagrass meadows are hotspots for denitrification, favoring N loss. However, atmospheric dinitrogen (N 2 ) fixation has been reported to support seagrass growth. Therefore, the role of coastal marine systems dominated by seagrasses in the net N 2 flux remains unclear. Here, we measured denitrification, anaerobic ammonium oxidation (anammox), and N 2 fixation in a tropical seagrass (Enhalus acoroides) meadow and the adjacent bare sediment in a coastal lagoon in the central Red Sea. We detected high annual mean rates of denitrification (34:9±10:3 and 31:6±8:9 mgNm -2 d -1 ) and anammox (12:4±3:4 and 19:8±4:4 mgNm 2 d -1 ) in vegetated and bare sediments. The annual mean N loss was higher (between 8 and 63-fold) than the N 2 fixed (annual meanD5:9±0:2 and 0:8± 0:3 mgNm -2 d -1 ) in the meadow and bare sediment, leading to a net flux of N 2 from sediments to the atmosphere. Despite the importance of this coastal lagoon in removing N from the system, N 2 fixation can contribute substantially to seagrass growth since N 2 fixation rates found here could contribute up to 36% of plant N requirements. In vegetated sediments, anammox rates decreased with increasing organic matter (OM) content, while N 2 fixation increased with OM content. Denitrification and anammox increased linearly with temperature, while N 2 fixation showed a maximum at intermediate temperatures. Therefore, the forecasted warming could further increase the N2 flux from sediments to the atmosphere, potentially impacting seagrass productivity and their capacity to mitigate climate change but also enhancing their potential N removal.

The Arctic is undergoing rapid environmental change, potentially affecting the physicochemical constraints of microbial communities that play a large role in both carbon and nutrient cycling in lacustrine environments. However, the microbial communities in such Arctic environments have seldom been studied, and the drivers of their composition are poorly characterized. To address these gaps, we surveyed the biologically active surface sediments in Lake Hazen, the largest lake by volume north of the Arctic Circle, and a small lake and shoreline pond in its watershed. High-throughput amplicon sequencing of the 16S rRNA gene uncovered a community dominated by Proteobacteria, Bacteroidetes, and Chloroflexi, similar to those found in other cold and oligotrophic lake sediments. We also show that the microbial community structure in this Arctic polar desert is shaped by pH and redox gradients. This study lays the groundwork for predicting how sediment microbial communities in the Arctic could respond as climate change proceeds to alter their physicochemical constraints.

Glacial runoff is predicted to increase in many parts of the Arctic with climate change, yet little is known about the biogeochemical impacts of meltwaters on downstream freshwater ecosystems. Here we document the contemporary limnology of the rapidly changing glacierized watershed of the world’s largest High Arctic lake (Lake Hazen), where warming since 2007 has increased delivery of glacial meltwaters to the lake by up to 10-times. Annually, glacial meltwaters accounted for 62–98% of dissolved nutrient inputs to the lake, depending on the chemical species and year. Lake Hazen was a strong sink for NO 3− -NO 2− , NH 4+ and DOC, but a source of DIC to its outflow the Ruggles River. Most nutrients entering Lake Hazen were, however, particle-bound and directly transported well below the photic zone via dense turbidity currents, thus reinforcing ultraoligotrophy in the lake rather than overcoming it. For the first time, we apply the land-to-ocean aquatic continuum framework in a large glacierized Arctic watershed, and provide a detailed and holistic description of the physical, chemical and biological limnology of the rapidly changing Lake Hazen watershed. Our findings highlight the sensitivity of freshwater ecosystems to the changing cryosphere, with implications for future water quality and productivity at high latitudes.