While the biogeochemical properties of the Israeli coastal shelf (ICS) are similar to adjacent pelagic waters, the external sources of inorganic nitrogen (N) are very different. The main source of ‘new’ N to the pelagic zone is deep winter mixing, with minor contributions from atmospheric deposition and eddy diffusion across the nutricline. For the ICS, major N sources include offshore water advection (260 × 10⁶ mol N y−¹), atmospheric input (115 × 10⁶ mol N y−¹), and riverine input (138 × 10⁶ mol N y−¹), which primarily consists of treated wastewater and stormwater runoff. Direct pollutant discharge from sewage outfalls and submarine groundwater discharge are relatively minor. Key N sinks are new production (420 × 10⁶ mol N y−¹) and sediment deposition and uptake (145 × 10⁶ mol N y−¹). Inputs of nitrate and ammonium were similar and dominant in winter. Unlike temperate shelves, where riverine input is dominant, here it was only slightly higher than atmospheric input, with net N advection onto the shelf being significant. External N inputs did not change net primary production (NPP) by more than ~30% or affect dominant pico and nanophytoplankton genera, except in localized patches. This study offers baseline values for future climate and environmental change assessments.

Environmental context Mineral dust is an important external source of trace metals to the offshore ocean. Dust exposure to acids is a significant driver of the release of dissolved trace elements. This study provides an analysis of mineral dust interaction with acid, as a proxy for atmospheric processes. An insight is given into the processes that may occur in the atmosphere where desert dust may add nutrient or toxic metals to oceans.Abstract Trace metal concentrations in oceans are influenced by several factors including biogeochemical cycling effects on distributions, concentrations and speciation. The major input of trace metals (and P) to the surface waters of the offshore ocean is mineral dust, predominantly from desert regions. This dust can be subject to acid processing in the atmosphere due to the presence of anthropogenic acidic gases (oxides of nitrogen and sulfur), potentially making trace metals more bioavailable when dust is deposited in the oceans. Here we present a study on the release of trace metals from a desert dust when exposed to a series of acid addition treatments. Al, Fe, Co, Cu, Zn and Pb are preferentially leached from the dust only when the calcite phase has been exhausted and the pH is no longer buffered at circumneutral values. Further acid additions quickly released the majority of leachable trace metals, although lower concentrations of most metals continue to be leached with further acid addition cycles. This contrasts with the behaviour of Ca and P, where in prior work it had been shown that dissolution mirrors closely the addition of protons to mineral surfaces demonstrating the related but contrasting processes for trace element dissolution.

The formation of green rust (GR; a mixed ferric/ferrous hydroxide) and vivianite (ferrous phosphate) are likely to have exerted a major control on phosphorus (P) cycling in ancient anoxic oceans. However, the factors that influence the formation of these minerals under different chemical conditions are poorly constrained, which limits understanding of the pathways that ultimately result in P drawdown and retention in anoxic sediments. This, in turn, limits understanding of P cycling in anoxic oceans and hence potential productivity feedbacks. Here we explore the effect of dissolved P concentration on the formation of sulfate GR (FeIII2FeII3(OH)12SO4) versus vivianite (FeII3(PO4)2·8H2O) under anoxic conditions. Our results show that at low dissolved P concentrations and with P:Fe(II) molar ratios <1:30, P drawdown is effectively controlled by interlayer anion exchange and adsorption onto GR species, via the formation of amorphous Fe-P precursors. Such precursors may delay the precipitation of crystalline GR, but vivianite was not detected under these conditions. At higher dissolved P concentrations and P:Fe(II) ratios, GR also forms. However, the GR formed under these conditions rapidly dissolves, likely forming amorphous ferric hydroxides together with dissolved Fe(II) and phosphate, with the dissolved species subsequently reacting to form crystalline vivianite. Our observations agree with studies showing the water column formation of GR in modern oligotrophic, anoxic Fe-rich (ferruginous) settings, and provide support for a major role for GR in controlling P cycling in ancient oligotrophic ferruginous oceans. By contrast, in more productive ancient anoxic settings, enhanced redox-controlled P recycling and/or increased weathering inputs would have led to higher dissolved P concentrations in the water column and sediments. Our observations show that such conditions ultimately promote the formation of vivianite, which would have exerted a limiting control on the extent of P recycling in ancient, more productive settings, via the long-term fixation of P in the sediments.

Coastal oceans, known as the major nitrous oxide (N2O) source to the atmosphere, are increasingly subject to eutrophication and concurrent near-bottom hypoxia. The natural nitrogen cycle is likely to be altered markedly in hypoxic coastal oceans. However, the processes responsible for N2O production and emission remain elusive because of lacking field rate measurements simultaneously conducted in the water column and sediment. Here, we quantified N2O production rates using a 15N-labeled technique in the water-column and surface sediments off the Changjiang (Yangtze) River estuary, the largest hypoxic zone in the Pacific margins. Our results showed that the estuarine surface sediments were the major source for N2O production, accounting for approximately 90% of the total water-column accumulation and consequent efflux of N2O in the hypoxic zone, whereas the water-column nitrification and denitrification combined only contributed <10%. More importantly, the coupling of nitrification and denitrification at the presence of abundant supply and remineralization of labile organic matter was the main driver of the N2O release from the sediment-water interface in this region. This study highlights the dominant role of benthic processes occurring at the sediment-water interface controlling the coastal N2O budget, as the anthropogenic eutrophication and hypoxia are expanding in coastal oceans.

Major storms, which are increasing in frequency due to climate change, flush pollutant nutrients, including phosphorus (P), from river catchments through estuaries to the coast. Changes in P speciation alter the potential for P removal in sediments. We measured suspended particle matter (SPM), dissolved and particulate phosphorus and other physicochemical parameters at two river outlets of the Jiulong River (SE China) and a fixed station in the estuary during an entire storm (June 2019). During the storm, riverine total particulate phosphorus (TPP) more than doubled to approximately 100 μg P L −1 mainly from pollutant sources, while increased soil erosion reduced the TPP:SPM ratio by 1/3. The riverine DIP increase during the storm was only moderate (approximately 25%). As the storm intensified, the fresh‐brackish water interface moved downstream. There was increased SPM and TPP flux (up to approximately 25,000 kg P d −1 ) from resuspended surficial sediment that had been deposited during normal flow in the adjacent tidal flats and mangrove areas. These sediments had acted as microbial incubators. Reduced Fe in the resuspended sediment was converted to labile Fe oxyhydroxides in the oxic water column, which adsorbed DIP (and probably also DOP) and increased labile TPP exported downstream. During the storm, the total flux of riverine dissolved nutrients increased while the TDN:TDP ratio decreased from 43:1 to 32:1. Our study showed that estuaries are locations for temporary deposition of labile TPP during normal flow, which are flushed out during major storms, likely resulting in increased eutrophication, including encouraging harmful algal blooms in coastal zones.

The Nile Basin drains about one-tenth of the African continent and contains the longest river channel system in the world. This chapter reviews the evolution of the river – with a particular focus on the Quaternary Period including the present-day hydrology and fluvial geomorphology. Fluctuations in the climate and hydrology of tropical Africa during the Quaternary exerted an important influence on the behaviour of the Nile sediment system. During the cold stages of the Pleistocene, low lake levels reflect a decrease in precipitation as the expanded ice sheets cooled the global oceans and monsoon intensity fell. The marine sedimentary record in the Eastern Mediterranean provides a valuable record of the long-term behaviour of the River Nile because long-term changes in the flux of water and sediment from various parts of the catchment have exerted an important influence on oceanographic and sedimentation dynamics. The chapter examines the contemporary suspended sediment budget of the delta complex.

The Eastern Mediterranean Sea (EMS) despite being an inland sea, has many of the characteristics of a P depleted oceanic gyre system such as the N.W. Atlantic gyre (e.g. BATS). As a result of its anti-estuarine circulation it is ultra-oligotrophic and picoplankton dominate the autotrophic community. Results from a series of monthly cruises, carried out using an ultra-sensitive nutrient analysis, showed a characteristic nutrient dynamic; In winter , which is also the time of the annual phytoplankton bloom, the photic zone was phosphate depleted while there was excess nitrate present at a concentration of 300-500nM. After the seasonal stratification was established in April, nitrate decreased until it became depleted in nitrate by mid-simmer into autumn. Dissolved organic phosphorus (DOP) was also unusually low (30-40nM) with only minor seasonal changes. This seasonal nutrient dynamic seems to be characteristic of P depleted oceanic systems. Nutrient limitation is a key parameter in controlling primary productivity and carbon uptake in such seasonally changing ocean systems. In this study we have developed and used a new method for determining the limiting nutrient. This method involves a 5 x 5 matrix of nutrient (N & P) additions and the determination of 2 endpoints simultaneously; Chlorophyll a is used as a proxy to define the limiting nutrient for the autotrophic community and an Alkaline Phosphatase activity (APA) endpoint in which the entire microbial community responds to the presence/absence of inorganic phosphate by producing or suppressing APA. Our results showed that in late summer, when the system was both phosphate and nitrate depleted, the autotrophs became N&P co-limited -> N limited while the

The Eastern Mediterranean Sea (EMS) is an ultra-oligotrophic, enclosed basin strongly impacted by regional and global anthropogenic stressors. Here, we describe the annual (2018-19) dynamics of phyto- and bacterioplankton (abundances, pigments and productivity) in relation to the physical and chemical conditions in the photic water column at the offshore EMS water (Station THEMO-2, ∼1,500m depth, 50km offshore). Annual patterns in phytoplankton biomass (as chlorophyll a), primary and bacterial productivity differed between the mixed winter (January-April) and the thermally stratified (May-December) periods. Prochlorococcus and Synechococcus numerically dominated the picophytoplankton populations, with each clade revealing different temporal and depth patterns, while picoeukaryotes (primarily haptophytes) were less abundant, yet likely contributed significant biomass. Integrated primary productivity (∼32 gC m-2 y-1) was lower compared with other well-studied oligotrophic locations, including the north Atlantic and Pacific (HOT and BATS observatories), the western Mediterranean (DYFAMED observatory) and the Red Sea, and was on-par with the ultra-oligotrophic South Pacific Gyre. In contrast, integrated bacterial production (∼11 gC m-2 y-1) was similar to other oligotrophic locations. Phytoplankton seasonal dynamics were reminiscent of those at BATS and the Red Sea, suggesting an observable effect of winter mixing in this ultra-oligotrophic location. These results highlight the ultra-oligotrophic conditions in the EMS and provide, for the first time in this region, a full-year baseline and context to ocean observatories in the region.Bacterioplankton dynamics were assessed monthly in the Eastern Mediterranean SeaSmall-sized picophytoplankton numerically dominated the phytoplankton communitySeasonal phytoplankton dynamics are similar to BATS and Red Sea, but not ALOHAAnnual primary productivity is among the lowest in the world’s oceansBacterial to primary production ratio is higher than most oligotrophic seas

Anthropogenic activities have greatly accelerated phosphorus (P) inputs from land to coastal seas. The increased P inputs from major rivers can cause adjacent coastal areas to experience seasonal hypoxia with the enhancing coastal eutrophication, which can subsequently increase P cycling and alter long term preservation. Analysis of sediment core measurements including SEDEX P speciation coupled with diagenetic kinetic models were performed on two cores in the coastal area under the Changjiang river plume, that experiences seasonal hypoxia. It was found that the benthic flux of dissolved reactive phosphate (DRP) in the Changjiang Estuary (CJE) was higher than that of adjacent areas of the Chinese coastal shelf. Sedimentary phosphorus transformations of Fe-bound P and organic P resulted in the in-situ formation of authigenic P (probably apatite), which was the major form of reactive P buried in the sediment. P burial efficiency (PBE) was lower than that of the oxic Chinese shelf but higher than that of other seasonally hypoxic areas in the world away from major river inputs. An exponential relationship between PBE and bottom water dissolved oxygen was developed, which suggested a positive feedback mechanism of increased hypoxia increasing P recycling, and hence intensifying eutrophication. The relatively high input of sediment including detrital P from the adjacent major river can explain many of the observed differences in P cycling from other seasonally hypoxic areas.

Low sulfate, euxinic water-column conditions were a common feature of many Precambrian and Phanerozoic periods of ocean anoxia. The cycling of phosphorus in anoxic marine environments exerts a fundamental control on primary productivity, organic carbon production and burial, and hence ultimately oxygen production, but the dynamics of the phosphorus cycle in low sulfate, euxinic settings are largely unknown. Here, we provide a detailed geochemical investigation of phosphorus cycling in the low sulfate, euxinic Lake Cadagno, Switzerland, which is considered a prime analogue for ancient euxinic oceans. We find evidence for extensive recycling of phosphorus from the sediments back to the water column, stimulated by the microbial release of phosphorus from organic matter and Fe (oxyhydr)oxide minerals. Consistent with previous studies of modern and ancient anoxic settings, this regenerated flux maintains high concentrations of phosphorus in the water column, thus promoting a positive productivity feedback. However, the low-sulfate condition of the overlying water column, combined with the rapid removal of sulfide (as pyrite) from porewaters, promotes formation of Fe(II) phosphate minerals (e.g., vivianite) close to the sediment-water interface. This, in turn, modulates the extent of phosphorus recycling back to the water column, and contrasts with modern fully marine euxinic settings, where the higher concentrations of dissolved sulfate promote sulfide formation to greater depths, thus limiting Fe(II) phosphate formation close to the sediment-water interface. The prevalence of low-sulfate conditions during past euxinic episodes suggests that the operation of this near-surface sedimentary trap for recycled phosphorus would have limited the positive P-driven productivity feedback, promoting only a moderate degree of P recycling. Furthermore, the precise magnitude of this recycled P flux would, on a global scale, have been dependent on changes in the size of Earth’s marine sulfate reservoir through time. Thus our findings have major implications for rates of P-driven productivity and organic carbon burial in ancient euxinic settings, which have not previously been factored into reconstructions of Earth’s oxygenation history.

We report the development of a modified method for evaluating different reservoirs of sedimentary phosphorus (P) in ancient marine sedimentary rocks and in modern Fe-rich sediments. Utilising the existing SEDEX scheme for P partitioning in modern sediments, we initially demonstrate limitations in the application of the original scheme to sediments and rocks containing crystalline hematite and magnetite. We tested additional extractions for these crystalline Fe phases, using both synthetic minerals, and modern and ancient sediments. The addition of 6 h oxalate and 6 h citrate-dithionate-acetate extractions considerably enhanced the total recovery of synthetic magnetite and hematite to 88.7 +/- 1.1% and 76.9 +/- 3.8%, respectively. In addition, application of the 6 h oxalate extraction to synthetic P-containing magnetite recovered 93.9 +/- 1.7% of the Fe present and 88.2 +/- 12.8% of the co-precipitated P. Based upon these results we developed a modified SEDEX extraction scheme. The modified scheme was applied to modern Fe-rich sediments from Golfo Dulce, Costa Rica, which resulted in 16% higher Fe-bound P recovery. Application of the scheme to a variety of ancient marine rocks increased the recovery of Fe-bound P by up to 22%. We also highlight the potential for authigenic carbonate fluorapatite to convert to more crystalline apatite in ancient rocks during deep burial and metamorphism. We suggest that in such systems minimum and maximum estimates of the total reactive P pool may be calculated with and without the inclusion of crystalline P. It is noted that the application of the revised method may have important implications for understanding the cycling of P in ancient marine environments.

The role of dust as a source of bioavailable phosphorus (Bio‐P) is quantified using a new parameterization for apatite dissolution in combination with global soil data maps and a global aerosol transport model. Mineral dust provides 31.2 Gg‐P/year of Bio‐P to the oceans, with 14.3 Gg‐P/year from labile P present in the dust, and an additional 16.9 Gg‐P/year from acid dissolution of apatite in the atmosphere, representing an increase of 120%. The North Atlantic, northwest Pacific, and Mediterranean Sea are identified as important sites of Bio‐P deposition from mineral dust. The acid dissolution process increases the fraction of total‐P that is bioavailable from ~10% globally from the labile pool to 18% in the Atlantic Ocean, 42% in the Pacific Ocean, and 20% in the Indian Ocean, with an ocean global mean value of 22%. Strong seasonal variations, especially in the North Pacific, northwest Atlantic, and Indian Ocean, are driven by large‐scale meteorology and pollution sources from industrial and biomass‐burning regions. Globally constant values of total‐P content and bioavailable fraction used previously do not capture the simulated variability. We find particular sensitivity to the representation of particle‐to‐particle variability of apatite, which supplies Bio‐P through acid‐dissolution, and calcium carbonate, which helps to buffer the dissolution process. A modest 10% external mixing results in an increase of Bio‐P deposition by 18%. The total Bio‐P calculated here (31.2 Gg‐P/year) represents a minimum compared to previous estimates due to the relatively low total‐P in the global soil map used.

Human activities have significantly modified the inputs of land-derived phosphorus (P) and nitrogen (N) to the Mediterranean Sea (MS). Here, we reconstruct the external inputs of reactive P and N to the Western Mediterranean Sea (WMS) and Eastern Mediterranean Sea (EMS) over the period 1950-2030. We estimate that during this period the land derived P and N loads increased by factors of 3 and 2 to the WMS and EMS, respectively, with reactive P inputs peaking in the 1980s but reactive N inputs increasing continuously from 1950 to 2030. The temporal variations in reactive P and N inputs are imposed in a coupled P and N mass balance model of the MS to simulate the accompanying changes in water column nutrient distributions and primary production with time. The key question we address is whether these changes are large enough to be distinguishable from variations caused by confounding factors, specifically the relatively large inter-annual variability in thermohaline circulation (THC) of the MS. Our analysis indicates that for the intermediate and deep water masses of the MS the magnitudes of changes in reactive P concentrations due to changes in anthropogenic inputs are relatively small and likely difficult to diagnose because of the noise created by the natural circulation variability. Anthropogenic N enrichment should be more readily detectable in time series concentration data for dissolved organic N (DON) after the 1970s, and for nitrate (NO3) after the 1990s. The DON concentrations in the EMS are predicted to exhibit the largest anthropogenic enrichment signature. Temporal variations in annual primary production over the 1950-2030 period are dominated by variations in deep-water formation rates, followed by changes in riverine P inputs for the WMS and atmospheric P deposition for the EMS. Overall, our analysis indicates that the detection of basin-wide anthropogenic nutrient concentration trends in the MS is rendered difficult due to: (1) the Atlantic Ocean contributing the largest reactive P and N inputs to the MS, hence diluting the anthropogenic nutrient signatures, (2) the anti-estuarine circulation removing at least 45% of the anthropogenic nutrients inputs added to both basins of the MS between 1950 and 2030, and (3) variations in intermediate and deep water formation rates that add high natural noise to the P and N concentration trajectories.

The effects of atmospheric deposition on plankton community structure were examined during a mesocosm experiment using water from the Cretan Sea (Eastern Mediterranean), an area with a high frequency of atmospheric aerosol deposition events. The experiment was carried out under spring-summer conditions (May 2012). The main objective was to study the changes induced from a single deposition event, on the autotrophic and heterotrophic surface microbial populations, from viruses to zooplankton. To this end, the effects of Saharan dust addition were compared to the effects of mixed aerosol deposition on the plankton community over 9 days. The effects of the dust addition seemed to propagate throughout the food-web, with changes observed in nearly all of the measured parameters up to copepods. The dust input stimulated increased productivity, both bacterial and primary. Picoplankton, both autotrophic and heterotrophic capitalized on the changes in nutrient availability and microzooplankton abundance also increased due to increased availability of prey. Five days after the simulated deposition, copepods also responded, with an increase in egg production. The results suggest that nutrients were transported up the food web through autotrophs, which were favored by the Nitrogen supplied through both treatments. Although, the effects of individual events are generally short lived, increased deposition frequency and magnitude of events is expected in the area, due to predicted reduction in rainfall and increase in temperature, which can lead to more persistent changes in plankton community structure. Here we demonstrate how a single dust deposition event leads to enhancement of phytoplankton and microzooplankton and can eventually, through copepods, transport more nutrients up the food web in the Eastern Mediterranean Sea.