Short-term sedimentary evidence for increasing diatoms in Arctic fjords in a warming world.

Arctic fjords are hotspots of marine carbon burial, with diatoms playing an essential role in the biological carbon pump. Under the background of global warming, the proportion of diatoms in total phytoplankton communities has been declining in many high-latitude fjords due to increased turbidity and oligotrophication resulting from glacier melting. However, due to the habitat heterogeneity among Svalbard fjords, diatom responses to glacier melting are also expected to be complex, which will further lead to changes in the biological carbon pumping and carbon sequestration. To address the complexity, three short sediment cores were collected from three contrasting fjords in Svalbard (Krossfjorden, Kongsfjorden, Gronfjorden), recording the history of fjord changes in recent decades during significant glacier melting. The amino acid molecular indicators in cores K4 and KF1 suggested similar organic matter degradation states between these two sites. In contrast to the turbid Kongsfjorden and Gronfjorden, preserved fucoxanthin in Krossfjorden indicated a continuous increase in diatoms since the mid-1980s, corresponding to a 59 % increase in biological carbon pumping, as quantified by the δ13C of sedimentary organic carbon. The increasing biological carbon pumping in Krossfjorden is further attributed to its hard rock types in the glacier basin, compared to Kongsfjorden and Gronfjorden, which are instead covered by soft rocks, as confirmed by a one-dimensional model. Given the distribution of rock types among basins in Svalbard, we extrapolate our findings and propose that approximately one-fifth of Svalbard's fjords, especially those with hard rock basins and persistent marine-terminated glaciers, still have the potential for an increase in diatom fractions and efficient biological carbon pumping. Our findings reveal the complexity of fjord phytoplankton responses and biological carbon pumping to increasing glacier melting, and underscore the necessity of modifying Arctic marine carbon feedback to climate change based on results from fjords underlain by hard rocks.

Soil microorganisms and methane emissions in response to short-term warming field incubation in Svalbard.

Introduction: Global warming is caused by greenhouse gases (GHGs). It has been found that the release of methane (CH4) from Arctic permafrost, soil, ocean, and sediment is closely related to microbial composition and soil factors resulting from warming over several months or years. However, it is unclear for how long continuous warming due to global warming affects the microbial composition and GHG release from soils along Arctic glacial meltwater rivers. Methods: In this study, the soil upstream of the glacial meltwater river (GR) and the estuary (GR-0) in Svalbard, with strong soil heterogeneity, was subjected to short-term field incubation at 2°C (in situ temperature), 10°C, and 20°C. The incubation was carried out under anoxic conditions and lasted for few days. Bacterial composition and CH4 production potential were determined based on high-throughput sequencing and physiochemical property measurements. Results: Our results showed no significant differences in bacterial 16S rRNA gene copy number, bacterial composition, and methanogenic potential, as measured by mcrA gene copy number and CH4 concentration, during a 7- and 13-day warming field incubation with increasing temperatures, respectively. The CH4 concentration at the GR site was higher than that at the GR-0 site, while the mcrA gene was lower at the GR site than that at the GR-0 site. Discussion: Based on the warming field incubation, our results indicate that short-term warming, which is measured in days, affects soil microbial composition and CH4 concentration less than the spatial scale, highlighting the importance of warming time in influencing CH4 release from soil. In summary, our research implied that microbial composition and CH4 emissions in soil warming do not increase in the first several days, but site specificity is more important. However, emissions will gradually increase first and then decrease as warming time increases over the long term. These results are important for understanding and exploring the GHG emission fluxes of high-latitude ecosystems under global warming.

Reconstruction of the main phytoplankton population off the Changjiang Estuary in the East China Sea and its assemblage shift in recent decades: From observations to simulation.

Under eutrophication background, the increasing dinoflagellates blooms relative to diatoms blooms off the Changjiang Estuary has caused much concern. We have provided sediment evidence for the first time that the time window of diatoms-to-dinoflagellates shift off the Changjiang Estuary in the East China Sea is early 1990s. Investigations to the water column revealed different surface-bottom concentration matchup patterns between peridinin (dinoflagellates) and fucoxanthin (diatoms), which suggests that the diatoms-dinoflagellates shift recorded in the sediment may have come from more dinoflagellate blooms since 1990s. Physical-biogeochemical 3D numerical simulations for the past decades suggest that the effect of increasing spring sea surface temperature and increasing N/P ratio on the diatoms-dinoflagellates shift is dominant and recessive, respectively.

Increases in the seaward river flux of nutrients driven by human migration and land-use changes in the tide-influenced delta.

Most large megacities are located on areas adjacent to tide-influenced deltas. However, contribution of megacities to seaward nutrient fluxes in tide-influenced deltas are poorly quantified in regional and global levels. We analyzed nutrient concentrations and water current data for a large and tide-influenced delta, the Changjiang (Yangtze River) since 1980. Concentrations, species ratios and fluxes of nutrients in tide-influenced delta has been found to differ dramatically from those at upstream. Over the period 2004 to 2015, the seaward nutrient fluxes of dissolved inorganic nitrogen and dissolved silica increased by 5%-10%, but dissolved inorganic phosphorus increased by 15%-20%, in the tide-influenced delta of the Changjiang. Consequently, the DIP/DIN decreases by 11% and DIP/DSi increases by 14% at the river mouth relative to those farther upstream the tidal limit. The legacy and/or recycled contribution accounts for 10%-30% of this increased nutrient flux, hence additional sources are predominantly those involving anthropogenic land-use changes. These findings have implications not only for the Changjiang but also for other riverine systems with respect to management strategy. Nutrient dynamics in tide-influenced deltas near urban areas should receive increased research and policy attention. By not considering nutrient sources in tide-influenced deltas, knowledge of the seaward fluxes and species ratios of nutrients from land sources is incomplete and can be biased, to the point that assessments of their impacts on adjacent marine environments may be inaccurate and mitigation policies therefore ineffective.

Hypoxia off the Changjiang (Yangtze River) estuary and in the adjacent East China Sea: Quantitative approaches to estimating the tidal impact and nutrient regeneration.

Large areas of hypoxia have been reported off The Changjiang Estuary and in the East China Sea. Five cruises, covering winter, spring, and summer, were carried out from 2007 to 2013 in this region, and in August 2013 (summer), an extensive hypoxic event (11,150km2) was observed, which was characterized by an estimated bulk oxygen depletion of 5.1 million tons. A strong tidal impact was observed associated with the bottom oxygen depletion, with the periodicity of diel variations in dissolved oxygen being 12h (i.e., similar to the tidal cycle). A conservative estimate of nutrient regeneration suggested that during the hypoxic event of August 2013, the amount of regenerated nitrogen (as nitrate) and phosphorus (as dissolved inorganic phosphorus) was 27,000-30,000 tons and 1300-41,000tons, respectively. Estimates of the absolute (bulk) regenerated nutrient fluxes were much greater than the conservative estimates.

[Characteristics of dissolved oxygen and its affecting factors in the Yangtze Estuary].

The cruises on hypoxia research were carried out in the Yangtze Estuary and adjacent area of the East China Sea on June, August and October, 2006. DO, stability and the concentration of nitrate and phosphate in the section B were analyzed. The result indicates that there is distinct difference among the concentration of DO in the section B of the three cruises. The DO distribution is higher in surface water but lower in bottom. The oxygen minimum value (1.1 mg x L(-1)) is found at the bottom in August, and the apparent oxygen utilization (AOU) is higher than 2.79 mg x L(-1) in the section B. The strong halocline above the hypoxic zone, as a result of affluent water from the coastwise, Taiwan Warm Current, and the high concentration of organic substance are major factors causing the formation of the hypoxia. The stability of the seawater goes down, the vertical mixing affect the DO through its effect on stratification, in October. deltaDO is primarily affected by the stability and the concentration of deltaNO3-, deltaDIP.