After the Storm: Fate and Leaching of Particulate Nitrogen (PN) in the Fluvial Network and the Influence of Watershed Sources and Moisture Conditions

Large storms can erode, transport, and deposit substantial amounts of particulate nitrogen (PN) in the fluvial network. The fate of this input and its consequence for water quality is poorly understood. This study investigated the transformation and leaching of PN using a 56-day incubation experiment with five PN sources: forest floor humus, upland mineral A horizon, stream bank, storm deposits, and stream bed. Experiments were subjected to two moisture regimes: continuously moist and dry–wet cycles. Sediment and porewater samples were collected through the incubation and analyzed for N and C species, as well as the quantification of nitrifying and denitrifying genes (amoA, nirS, nirK). C- and N-rich watershed sources experienced decomposition, mineralization, and nitrification and released large amounts of dissolved N, but the amount of N released varied depending on the PN source and moisture regime. Drying and rewetting stimulated nitrification and suppressed denitrification in most PN sources. Storm deposits released large amounts of porewater N regardless of the moisture conditions, indicating that they could readily act as N sources under a variety of conditions. The inputs, processing, and leaching of large, storm-driven PN inputs become increasingly important as the frequency and intensity of large storms is predicted to increase with global climate change.

Sedimentary Nitrogen Uptake & Assimilation in the Temperate Zooxanthellate Anemone Anthopleura aureoradiata

<p>This study investigated the potential for, and efficiency of, particulate nitrogen uptake from the sediment and subsequent consequences of this on the nutrient status of endosymbiotic dinoflagellates (zooxanthellae) in the temperate zooxanthellate anemone Anthopleura aureoradiata. Sediment was collected from a mudflat and labelled with (15NH4)2SO4 before being provided to A. aureoradiata at low (5 g dry weight) and high sediment (20 g dry weight) loads for 6 hours. While no discernible change in the isotopic content of the sediment could be detected, analysis of the host and algal symbionts revealed that 15N had been taken up. Uptake by the host was similar at both high and low sediment loads, but the algal symbionts acquired more nitrogen at the lower load (1.13 versus 0.93 atom % 15N in the low and high loads, respectively). Evaluation of this particulate nitrogen uptake from the sediment was further examined by measuring the nitrogen status of the zooxanthellae. This was determined by measuring the extent to which ammonium (40 muM NH+4) enhanced the rate of zooxanthellar dark carbon fixation above that seen in filtered seawater (FSW) alone; the enhancement ratio was expressed as [dark NH+4 rate/dark FSW rate]. VD'/VL, a further index of nitrogen status, was also calculated where VD' = [dark NH+4 rate - dark FSW rate] and VL = rate of carbon fixation in the light. When anemones were starved for 2-8 weeks, zooxanthellar nitrogen deficiency became apparent at greater than or equal to 4 weeks, with NH+4/FSW and VD'/VL averaging up to 2.90 and 0.11, respectively. In comparison, when anemones were fed 5 times per week for 8 weeks the addition of ammonium had little effect, indicating nitrogen sufficiency; NH+4/FSW and VD'/VL values were 1.03 and -1.0 x 10-3, respectively. The nitrogen status of zooxanthellae from anemones starved and incubated with and without sediment was examined with no apparent difference between sediment and no sediment treatments; zooxanthellar nitrogen deficiency became apparent at greater than or equal to 4 weeks in both treatments, with NH+4/FSW and VD'/VL averaging up to 3.73 and 0.17 for the sediment treatment and 2.74 and 0.15 for the no sediment treatment, respectively. The nitrogen status of zooxanthellae from anemones found on a mudflat (Pauatahanui Inlet) and a rocky intertidal site (Kau Bay) was different. Zooxanthellae from mudflat anemones were nitrogen sufficient with NH+4/FSW and VD'/VL values averaging up to 1.26 and -6.0 x 10-3, respectively. Nitrogen deficient zooxanthellae were present in anemones from the rocky intertidal. Anemones from tide pools in the upper littoral zone had NH+4/FSW and VD'/VL values of 2.99 and 0.11, respectively, while anemones from the mid littoral zone had NH+4/FSW and VD'/VL of 2.90 and 0.13, respectively; there was no significant difference in nitrogen status between zooxanthellae from high shore tide pool anemones and aerially exposed mid-littoral anemones. These results suggest that while particulate nitrogen can be taken up from the sediment by this species, dissolved inorganic nitrogen such as ammonium in the seawater, and especially the interstitial water surrounding infaunal anemones on mudflats, may be a more important source of nitrogen in the field.</p>

Sedimentary Nitrogen Uptake & Assimilation in the Temperate Zooxanthellate Anemone Anthopleura aureoradiata

<p>This study investigated the potential for, and efficiency of, particulate nitrogen uptake from the sediment and subsequent consequences of this on the nutrient status of endosymbiotic dinoflagellates (zooxanthellae) in the temperate zooxanthellate anemone Anthopleura aureoradiata. Sediment was collected from a mudflat and labelled with (15NH4)2SO4 before being provided to A. aureoradiata at low (5 g dry weight) and high sediment (20 g dry weight) loads for 6 hours. While no discernible change in the isotopic content of the sediment could be detected, analysis of the host and algal symbionts revealed that 15N had been taken up. Uptake by the host was similar at both high and low sediment loads, but the algal symbionts acquired more nitrogen at the lower load (1.13 versus 0.93 atom % 15N in the low and high loads, respectively). Evaluation of this particulate nitrogen uptake from the sediment was further examined by measuring the nitrogen status of the zooxanthellae. This was determined by measuring the extent to which ammonium (40 muM NH+4) enhanced the rate of zooxanthellar dark carbon fixation above that seen in filtered seawater (FSW) alone; the enhancement ratio was expressed as [dark NH+4 rate/dark FSW rate]. VD'/VL, a further index of nitrogen status, was also calculated where VD' = [dark NH+4 rate - dark FSW rate] and VL = rate of carbon fixation in the light. When anemones were starved for 2-8 weeks, zooxanthellar nitrogen deficiency became apparent at greater than or equal to 4 weeks, with NH+4/FSW and VD'/VL averaging up to 2.90 and 0.11, respectively. In comparison, when anemones were fed 5 times per week for 8 weeks the addition of ammonium had little effect, indicating nitrogen sufficiency; NH+4/FSW and VD'/VL values were 1.03 and -1.0 x 10-3, respectively. The nitrogen status of zooxanthellae from anemones starved and incubated with and without sediment was examined with no apparent difference between sediment and no sediment treatments; zooxanthellar nitrogen deficiency became apparent at greater than or equal to 4 weeks in both treatments, with NH+4/FSW and VD'/VL averaging up to 3.73 and 0.17 for the sediment treatment and 2.74 and 0.15 for the no sediment treatment, respectively. The nitrogen status of zooxanthellae from anemones found on a mudflat (Pauatahanui Inlet) and a rocky intertidal site (Kau Bay) was different. Zooxanthellae from mudflat anemones were nitrogen sufficient with NH+4/FSW and VD'/VL values averaging up to 1.26 and -6.0 x 10-3, respectively. Nitrogen deficient zooxanthellae were present in anemones from the rocky intertidal. Anemones from tide pools in the upper littoral zone had NH+4/FSW and VD'/VL values of 2.99 and 0.11, respectively, while anemones from the mid littoral zone had NH+4/FSW and VD'/VL of 2.90 and 0.13, respectively; there was no significant difference in nitrogen status between zooxanthellae from high shore tide pool anemones and aerially exposed mid-littoral anemones. These results suggest that while particulate nitrogen can be taken up from the sediment by this species, dissolved inorganic nitrogen such as ammonium in the seawater, and especially the interstitial water surrounding infaunal anemones on mudflats, may be a more important source of nitrogen in the field.</p>

210Po and 210Pb as Tracers of Particle Cycling and Export in the Western Arctic Ocean

The distribution and vertical fluxes of particulate organic carbon and other key elements in the Arctic Ocean are primarily governed by the spatial and seasonal changes in primary productivity, areal extent of ice cover, and lateral exchange between the shelves and interior basins. The Arctic Ocean has undergone rapid increase in primary productivity and drastic decrease in the areal extent of seasonal sea ice in the last two decades. These changes can greatly influence the biological pump as well as associated carbon export and key element fluxes. Here, we report the export of particulate organic and inorganic carbon, particulate nitrogen and biogenic silica using 210Po and 210Pb as tracers for the seasonal vertical fluxes. Samples were collected as a part of US GEOTRACES Arctic transect from western Arctic Basin in 2015. The total activities of 210Po and 210Pb in the upper 300 m water column ranged from 0.46 to 16.6 dpm 100L–1 and 1.17 to 32.5 dpm 100L–1, respectively. The 210Pb and 210Po fluxes varied between 5.04–6.20 dpm m–2 d–1 and 8.26–21.02 dpm m–2 d–1, respectively. The corresponding particulate organic carbon (POC) and particulate nitrogen (PN) fluxes ranged between 0.75–7.43 mg C m–2 d–1 and 0.08–0.78 mg N m–2 d–1, respectively, with highest fluxes observed in the northern ice-covered stations. The particulate inorganic carbon (PIC) and biogenic silica (bSi) fluxes were extremely low ranging from 0 to 0.14 mg C m–2 d–1 and 0.14 to 2.88 mg Si m–2 d–1, respectively, at all stations suggesting absence of ballast elements in facilitating the biological pump. The variability in POC fluxes with depth suggest prominent influence of lateral transport to downward fluxes across the region. The results provide a better understanding of the spatial variability in the vertical fluxes POC, PN, bSi, and PIC in the western Arctic which is currently undergoing dramatic changes.

Origin of the Particulate Organic Matter in a Monsoon-Controlled Bay in Southern China

In this study, the isotopic composition (δ13C and δ15N), total organic carbon content, total nitrogen content, and C/N ratios of suspended particulate organic matter (POM) in Zhanjiang Bay, which is a semi-enclosed bay with concentrated artificial activities in Southern China, were analyzed in order to investigate the seasonal variations in the principal POM sources in the monsoon region. In summer, the δ13C and δ15N values showed a weak correlation with the chlorophyll a (Chl a), suggesting that terrigenous sources were dominant. However, in winter, the particulate organic carbon and particulate nitrogen values were correlated with the Chl a in the middle bay and bay mouth. Moreover, the δ13C values showed a significant correlation with Chl a during the winter, indicating that the contribution of the in situ phytoplankton was relatively important and was affected by the monsoon in winter. Compared with the corresponding δ13C values, the δ15N values exhibited a complex spatial distribution. By using a Bayesian mixing model, in the upper bay, the source of POM was mainly from marine organic matter (49%) in summer, and almost an equilibrated contribution of all sources in winter. In the middle bay and bay mouth, the POM contribution mainly originated from marine organic matter (53%) during the winter. In contrast, the POM source was mainly soil organic matter (63%) in summer, suggesting that the POM was sourced from the runoff from the upstream basin. Our results suggest that the seasonal shifts of the source of POM should be taken into account when estimating C or N mass balance in the monsoon-controlled bay.

Massive export of diazotrophs across the tropical south Pacific Ocean

Diazotrophs are widespread microorganisms that regulate marine productivity in 60% of our oceans by alleviating nitrogen limitation. Yet, their contribution to organic carbon and nitrogen export fluxes has never been quantified, making an assessment of their impact on the biological carbon pump impossible. Here, we examine species-specific fates of several groups of globally-distributed unicellular (UCYN) and filamentous diazotrophs in the mesopelagic ocean. We used an innovative approach consisting of the combined deployment of surface-tethered drifting sediment traps, Marine Snow Catcher, and Bottle-net, in which we performed nifH sequencing and quantitative PCR on major diazotroph groups across the subtropical South Pacific Ocean. nifH sequencing data from sediment traps deployed at 170 m, 270 m and 1000 m provide clear evidence that cyanobacterial and non-cyanobacterial diazotrophs are systematically present in sinking particles down to 1000 m, with export fluxes being the highest for the UCYN-A1 symbiosis, followed by UCYN-B or Trichodesmium (depending on station and depth), Gamma A and UCYN-C. Specific export turnover rates (a metric similar to the export efficiency adapted to organisms) point to a more efficient export of UCYN groups relative to the filamentous Trichodesmium. This is further confirmed by Marine Snow catcher data showing that the proportion of sinking cells was significantly higher for UCYN compared to Trichodesmium. Phycoerythrin-containing UCYN-B and UCYN-C-like cells were indeed recurrently found embedded in large (> 50 micrometers) seemingly organic aggregates, or organized into clusters of tens to hundreds of cells linked by an extracellular matrix, facilitating the export. Overall, diazotrophs accounted for 6-13% (170 m) to 45-100% (1000 m) of the total particulate nitrogen export fluxes in our study. We thus conclude that diazotrophs are important contributors to carbon sequestration in the subtropical South Pacific Ocean and need to be considered in future studies to improve the accuracy of current regional and global estimates of export.

Insights into nitrogen fixation below the euphotic zone: trials in an oligotrophic marginal sea and global compilation

Nitrogen (N2) fixation, the energetically expensive conversion of N2 to ammonia, plays an important role in balancing the global nitrogen budget. Defying historic paradigms, recent studies have detected non-cyanobacterial N2 fixation in deep, dark oceanic waters. Even low volumetric rates can be significant considering the large volume of these waters. However, measuring aphotic N2 fixation is an analytical challenge due to the low particulate nitrogen (PN) concentrations. Here, we investigated N2 fixation rates in aphotic waters in the South China Sea (SCS). To increase the sensitivity of N2 fixation rate measurements, we applied a novel approach requiring only 0.28 μg N for measuring the isotopic composition of particulate nitrogen. We conducted parallel 15N2-enriched incubations in ambient seawater, seawater amended with amino acids and poisoned (HgCl2) controls, along with incubations that received no tracer additions to distinguish biological N2 fixation. Experimental treatments differed significantly from our two types of controls, those receiving no additions and killed controls. Amino acid additions masked N2 fixation signals due to the uptake of added 14N-amino acid. Results show that the maximum dark N2 fixation rates (1.28 ± 0.85 nmol N L−1 d−1) occurred within upper 200 m, while rates below 200 m were mostly lower than 0.1 nmol N L−1 d−1. Nevertheless, N2 fixation rates between 200 and 1000 m accounted for 39 ± 32 % of depth-integrated dark N2 fixation rates in the upper 1000 m, which is comparable to the areal nitrogen inputs via atmospheric deposition. Globally, we found that aphotic N2 fixation studies conducted in oxygenated environments yielded rates similar to those from the SCS (< 1 nmol N L−1 d−1), regardless of methods, while higher rates were occasionally observed in low-oxygen (< 62 µM) regions. Regression analysis suggests that particulate nitrogen concentrations could be a predictive proxy for detectable aphotic N2 fixation in the SCS and eastern tropical south Pacific. Our results provide the first insight into aphotic N2 fixation in SCS and support the importance of the aphotic zone as a globally-important source of new nitrogen to the ocean.

Seasonal variations in the transport and biogeochemical turnover of mainly dissolved organic nitrogen from the Lena Delta to the nearshore Laptev Sea

&lt;p&gt;Pan-arctic rivers transport a huge amount of nitrogen to the Arctic Ocean. The permafrost-affected soils around the Arctic Ocean containe a large reservoir of organic matter including carbon and nitrogen, which partly reach the river after permafrost thaw and erosion.&lt;/p&gt;&lt;p&gt;Our study aims to estimate the load of nitrogen supplied from terrestrial sources into the Arctic Ocean. Therefore, water, suspended particulate matter (SPM) and sediment samples were collected in the Lena Delta along a (~200 km) transect from the center of the Lena Delta to the open Laptev Sea in late winter (April) and in summer (August) 2019. In winter, 21 sample from 13 stations and in summer, 51 samples from 18 stations were taken. 9 of these sampling stations in the outer delta region were sampled in both seasons.&lt;/p&gt;&lt;p&gt;We measured organic and inorganic nitrogen and the &lt;sup&gt;15&lt;/sup&gt;N stable isotopes composition of all three sample types to determine sources, sinks and processes of nitrogen transformation during transport.&lt;/p&gt;&lt;p&gt;In winter, the nitrogen transported from the delta to the Laptev Sea were mainly dissolved organic nitrogen (DON) and nitrate, which occur in similar amounts. The load of nitrate increased slightly in the delta, while no changes to the isotope values of DON and nitrate were observe indicating a lack of biological activity in the winter season. However, lateral transport from soils was a likely source. In summer, nitrogen was mainly transported as DON and particulate nitrogen in the SPM fraction, including phytoplankton.&lt;/p&gt;&lt;p&gt;The nitrogen stable isotope values of the different nitrogen components ranges between 0.5 and 4.5 &amp;#8240;, and were subsequently enriched from the soils via SPM/sediment and DON to nitrate. This indicates that nitrogen in the soils mainly originates from nitrogen fixation from the atmosphere. During transport and remineralisation, biogeochemical recycling via nitrification and assimilation by phytoplankton led to an isotopic enrichment in summer from organic to inorganic components. In the coastal waters of the Laptev Sea, the river waters are slowly mixed with marine nitrate containing waters from the Arctic Ocean, and a part of the riverine organic nitrogen is buried in the sediments.&lt;/p&gt;&lt;p&gt;We assume that the ongoing permafrost thawing and erosion will intensify and increase the transport of reactive nitrogen to coastal waters and will affect the biogeochemical cycling, e.g. the primary production.&lt;/p&gt;