Revisiting the distribution of oceanic N₂ fixation and estimating diazotrophic contribution to marine production

Marine N2 fixation supports a significant portion of oceanic primary production by making N2 bioavailable to planktonic communities, in the process influencing atmosphere-ocean carbon fluxes and our global climate. However, the geographical distribution and controlling factors of marine N2 fixation remain elusive largely due to sparse observations. Here we present unprecedented high-resolution underway N2 fixation estimates across over 6000 kilometers of the western North Atlantic. Unexpectedly, we find increasing N2 fixation rates from the oligotrophic Sargasso Sea to North America coastal waters, driven primarily by cyanobacterial diazotrophs. N2 fixation is best correlated to phosphorus availability and chlorophyll-a concentration. Globally, intense N2 fixation activity in the coastal oceans is validated by a meta-analysis of published observations and we estimate the annual coastal N2 fixation flux to be 16.7 Tg N. This study broadens the biogeography of N2 fixation, highlights the interplay of regulating factors, and reveals thriving diazotrophic communities in coastal waters with potential significance to the global nitrogen and carbon cycles

Iron biogeochemistry across marine systems progress from the past decade

Based on an international workshop (Gothenburg, 14–16 May 2008), this review article aims to combine interdisciplinary knowledge from coastal and open ocean research on iron biogeochemistry. The major scientific findings of the past decade are structured into sections on natural and artificial iron fertilization, iron inputs into coastal and estuarine systems, colloidal iron and organic matter, and biological processes. Potential effects of global climate change, particularly ocean acidification, on iron biogeochemistry are discussed. The findings are synthesized into recommendations for future research areas

The unaccounted dissolved iron (II) sink: Insights from dFe(II) concentrations in the deep Atlantic Ocean

Hydrothermal vent sites found along mid-ocean ridges are sources of numerous reduced chemical species and trace elements. To establish dissolved iron (II) (dFe(II)) variability along the Mid Atlantic Ridge (between 39.5°N and 26°N), dFe(II) concentrations were measured above six hydrothermal vent sites, as well as at stations with no active hydrothermal activity. The dFe(II) concentrations ranged from 0.00 to 0.12 nmol L−1 (detection limit = 0.02 ± 0.02 nmol L−1) in non-hydrothermally affected regions to values as high as 12.8 nmol L−1 within hydrothermal plumes. Iron (II) in seawater is oxidised over a period of minutes to hours, which is on average two times faster than the time required to collect the sample from the deep ocean and its analysis in the onboard laboratory. A multiparametric equation was used to estimate the original dFe(II) concentration in the deep ocean. The in-situ temperature, pH, salinity and delay between sample collection and its analysis were considered. The results showed that dFe(II) plays a more significant role in the iron pool than previously accounted for, constituting a fraction >20 % of the dissolved iron pool, in contrast to <10 % of the iron pool formerly reported. This discrepancy is caused by Fe(II) loss during sampling when between 35 and 90 % of the dFe(II) gets oxidised. In-situ dFe(II) concentrations are therefore significantly higher than values reported in sedimentary and hydrothermal settings where Fe is added to the ocean in its reduced form. Consequently, the high dynamism of dFe(II) in hydrothermal environments masks the magnitude of dFe(II) sourced within the deep ocean

Betriebliche Bewirtschaftungsindikatoren für Biodiversität im Ökologischen Landbau und in extensiven Anbausystemen in Europa

Farming practices are the key to maintaining and restoring farmland biodiversity. Selected farm management indicators, regarded as scientifically sound, practicable and attractive to stakeholders, were tested against species indicators in various farm types in 12 case studies across Europe. A set of eight farm management indicators is recommended, reflecting the pressure on biodiversity by farm management via energy and nutrient input, mechanical operations, pesticide use and livestock

Biodiversity indicators in organic and conventional farming systems: main results from the European project BIOBIO

In the framework of the European project BIOBIO, we compared between countries habitat and cumulated species richnesses of plants, wild bees, spiders and earthworms, measured in 169 conventional and organic farms belonging to 10 case studies in 10 European countries. For the French case study (Gascony Valleys and Hills), correlations between direct (habitat and taxonomic richnesses) and indirect (agricultural practices) indicators of biodiversity within 8 conventional and 8 organic farms, were calculated. Results showed that the main driver of biodiversity at the farm level was the number of cultivated and above all semi-natural habitats, inthe French case study region as well as inthe other regions. This factor partially explained the highest biodiversity level of the French case study region. However, farming practices, specific or not to the organic and conventional systems, most often drove biodiversity parameters at the habitat level. In fine, the project proposed the BIOBIO method for monitoring biodiversity in farms

Particulate barium tracing of significant mesopelagic carbon remineralisation in the North Atlantic

The remineralisation of sinking particles by prokaryotic heterotrophic activity is important for controlling oceanic carbon sequestration. Here, we report mesopelagic particulate organic carbon (POC) remineralisation fluxes in the North Atlantic along the GEOTRACES-GA01 section (GEOVIDE cruise; May-June 2014) using the particulate biogenic barium (excess barium; Baxs/ proxy. Important mesopelagic (100-1000 m) Baxs differences were observed along the transect depending on the intensity of past blooms, the phytoplankton community structure, and the physical forcing, including downwelling. The subpolar province was characterized by the highest mesopelagic Baxs content (up to 727 pmol L-1/, which was attributed to an intense bloom averaging 6 mg chl a m-3 between January and June 2014 and by an intense 1500m deep convection in the central Labrador Sea during the winter preceding the sampling. This downwelling could have promoted a deepening of the prokaryotic heterotrophic activity, increasing the Baxs content. In comparison, the temperate province, characterized by the lowest Baxs content (391 pmol L-1/, was sampled during the bloom period and phytoplankton appear to be dominated by small and calcifying species, such as coccolithophorids. The Baxs content, related to oxygen consumption, was converted into a remineralisation flux using an updated relationship, proposed for the first time in the North Atlantic. The estimated fluxes were of the same order of magnitude as other fluxes obtained using independent methods (moored sediment traps, incubations) in the North Atlantic. Interestingly, in the subpolar and subtropical provinces, mesopelagic POC remineralisation fluxes (up to 13 and 4.6 mmol Cm-2 d-1, respectively) were equalling and occasionally even exceeding upper-ocean POC export fluxes, deduced using the 234Th method. These results highlight the important impact of the mesopelagic remineralisation on the biological carbon pump of the studied area with a near-zero, deep (> 1000 m) carbon sequestration efficiency in spring 2014

Iron, silicate, and light co-limitation of three Southern Ocean diatom species

The effect of combined iron, silicate, and light co-limitation was investigated in the three diatom species Actinocyclus sp. Ehrenberg, Chaetoceros dichaeta Ehrenberg, and Chaetoceros debilis Cleve, isolated from the Southern Ocean (SO). Growth of all species was co-limited by iron and silicate, reflected in a significant increase in the number of cell divisions compared to the control. Lowest relative Si uptake and drastic frustule malformation was found under iron and silicate co-limitation in C. dichaeta, while Si limitation in general caused cell elongation in both Chaetoceros species. Higher light intensities similar to SO surface conditions showed a negative impact on growth of C. dichaeta and Actinocyclus sp. and no effect on C. debilis. This is in contrast to the assumed light limitation of SO diatoms due to deep wind driven mixing. Our results suggest that growth and species composition of Southern Ocean diatoms is influenced by a sensitive interaction of the abiotic factors, iron, silicate, and light

Revisiting the distribution of oceanic N 2 fixation and estimating diazotrophic contribution to marine production

Marine N 2 fixation supports a significant portion of oceanic primary production by making N 2 bioavailable to planktonic communities, in the process influencing atmosphere-ocean carbon fluxes and our global climate. However, the geographical distribution and controlling factors of marine N 2 fixation remain elusive largely due to sparse observations. Here we present unprecedented high-resolution underway N 2 fixation estimates across over 6000 kilometers of the western North Atlantic. Unexpectedly, we find increasing N 2 fixation rates from the oligotrophic Sargasso Sea to North America coastal waters, driven primarily by cyanobacterial diazotrophs. N 2 fixation is best correlated to phosphorus availability and chlorophyll-a concentration. Globally, intense N 2 fixation activity in the coastal oceans is validated by a meta-analysis of published observations and we estimate the annual coastal N 2 fixation flux to be 16.7 Tg N. This study broadens the biogeography of N 2 fixation, highlights the interplay of regulating factors, and reveals thriving diazotrophic communities in coastal waters with potential significance to the global nitrogen and carbon cycles

An increase in food production in Europe could dramatically affect farmland biodiversity

Conversion of semi-natural habitats, such as field margins, fallows, hedgerows, grassland, woodlots and forests, to agricultural land could increase agricultural production and help meet rising global food demand. Yet, the extent to which such habitat loss would impact biodiversity and wild species is unknown. Here we survey species richness for four taxa (vascular plants, earthworms, spiders, wild bees) and agricultural yield across a range of arable, grassland, mixed, horticulture, permanent crop, for organic and non-organic agricultural land on 169 farms across 10 European regions. We find that semi-natural habitats currently constitute 23% of land area with 49% of species unique to these habitats. We estimate that conversion of semi-natural land that achieves a 10% increase in agricultural production will have the greatest impact on biodiversity in arable systems and the least impact in grassland systems, with organic practices having better species retention than non-organic practices. Our findings will help inform sustainable agricultural development

Seasonal Depletion of the Dissolved Iron Reservoirs in the Sub-Antarctic zone of the Southern Atlantic Ocean

Seasonal progression of dissolved iron (DFe) concentrations in the upper water column were examined during four occupations in the Atlantic sector of the Southern Ocean. DFe inventories from euphotic and aphotic reservoirs decreased progressively from July to February, while dissolved inorganic nitrogen (DIN) decreased from July to January with no significant change between January and February. Results suggest that between July and January, DFe loss from both euphotic and aphotic reservoirs were predominantly in support of phytoplankton growth (Iron to carbon (Fe:C) uptake ratio of 16±3 μmol mol‐1) highlighting the importance of the “winter DFe‐reservoir” for biological uptake. During January to February, excess loss of DFe relative to DIN (Fe:C uptake ratio of 44±8 μmol mol‐1 and aphotic DFe loss rate of 0.34±0.06 μmol m‐2 d‐1) suggests that scavenging is the dominant removal mechanism of DFe from the aphotic, while continued production is likely supported by recycled nutrients. Plain Language Summary Trace metal iron is one of the limiting nutrients for primary productivity in the Southern Ocean; however the relative importance of seasonal iron supply and sinks remains poorly understood, due to sparse data coverage across the seasonal cycle and lack of high‐resolution dissolved iron (DFe) measurements. Here, we present four “snap‐shots” of DFe measurements at a single station in the south‐east Southern Atlantic Ocean (one in winter and three in late spring‐summer), to address the seasonal evolution of DFe and dissolved inorganic nitrogen (DIN) concentrations within the biologically active sunlit and subsurface reservoirs. We observed a seasonal depletion of DFe inventories from July‐February, while DIN inventories decreases from July‐January with no concomitant changes between January‐February. This suggests that, in addition to biological uptake in the sunlit layer, the observed decrease in DFe inventories below this (relative to DIN) is driven by aggregation and incorporation of iron particles into larger "marine snow" sinking particles, while nutrient recycling is driving the observed continuation of primary productivity during late summer. Our results provide insight into seasonal change of DFe availability in different reservoirs where interplay between removal and supply processes are controlling its distributions and bioavailability to support upper surface primary production