Increasing picocyanobacteria success in shelf waters contributes to long-term food web degradation

Continental margins are disproportionally important for global primary production, fisheries and CO2 uptake. However, across the Northeast Atlantic shelves, there has been an ongoing summertime decline of key biota—large diatoms, dinoflagellates and copepods—that traditionally fuel higher tropic levels such as fish, sea birds and marine mammals. Here, we combine multiple time series with in situ process studies to link these declines to summer nutrient stress and increasing proportions of picophytoplankton that can comprise up to 90% of the combined pico- and nanophytoplankton biomass in coastal areas. Among the pico-fraction, it is the cyanobacterium Synechococcus that flourishes when iron and nitrogen resupply to surface waters are diminished. Our field data show how traits beyond small size give Synechococcus a competitive edge over pico- and nanoeukaryotes. Key is their ability to grow at low irradiances near the nutricline, which is aided by their superior light-harvesting system and high affinity to iron. However, minute size and lack of essential biomolecules (e.g. omega-3 polyunsaturated fatty acids and sterols) render Synechococcus poor primary producers to sustain shelf sea food webs efficiently. The combination of earlier spring blooms and lower summer food quantity and quality creates an increasing period of suboptimal feeding conditions for zooplankton at a time of year when their metabolic demand is highest. We suggest that this nutrition-related mismatch has contributed to the widespread, ~50% decline in summer copepod abundance we observe over the last 60 years. With Synechococcus clades being prominent from the tropics to the Arctic and their abundances increasing worldwide, our study informs projections of future food web dynamics in coastal and shelf areas where droughts and stratification lead to increasing nutrient starvation of surface waters

Physical and biogeochemical controls on seasonal iron, manganese, and cobalt distributions in Northeast Atlantic shelf seas

Dissolved (<0.2 μm) trace metals (dTMs) including iron (Fe), manganese (Mn), and cobalt (Co) are micronutrients that (co-) limit phytoplankton growth in many ocean regions. Here, we present the spatial and seasonal distributions of dFe, dMn, and dCo on the Northeast Atlantic continental margin (Celtic Sea), along a transect across the shelf and two off-shelf transects along a canyon and a spur. Waters on the continental shelf showed much higher dTM concentrations (dFe 0.07–6.50 nmol L−1, average 1.41 ± 0.96 nmol L−1, n = 138; dMn 0.868–14.8 nmol L−1, 2.75 ± 2.37 nmol L−1, n = 148; dCo 54.8–217 pmol L−1, 109 ± 32 pmol L−1, n = 144) than on the slope (dFe 0.03–1.90 nmol L−1, 0.65 ± 0.43 nmol L−1, n = 454; dMn 0.223–1.14 nmol L−1, 0.58 ± 0.20 nmol L−1, n = 458; dCo 27.3–122 pmol L−1, 71.7 ± 11.7 pmol L−1, n = 441), attributed to strong dTM contributions from a low-salinity endmember, i.e., riverine discharge. Benthic sedimentary input via reductive dissolution (especially for dFe and dMn), delineated by short-lived radium (Ra) isotopic activities (223Raxs and 224Raxs), was only prominent at a station (Site A) characterized by fine sediments. On the continental slope, dMn levels at depth were mainly determined by the formation of insoluble Mn oxides and the intrusion of Mediterranean Outflow Waters. In contrast, dFe and dCo concentrations at depth were balanced by the regeneration from remineralization of sinking organic particles and scavenging removal. In addition, bottom and intermediate nepheloid layers along the slope illustrated both elevated dTM concentrations and Ra isotopic activities. The presence of nepheloid layers is especially significant along the canyon transect relative to the spur transect, demonstrating the importance of slope topography on the off-shelf transport of dTMs into the Northeast Atlantic Ocean. As a seasonal stratified shelf sea, dTMs and nutrients showed synchronized seasonal variations on the shelf, indicating the influence of biological processes in addition to source effects. Surface dFe and dCo were depleted in summer due to enhanced biological uptake, while sub-surface dFe and dCo were elevated in summer and autumn ascribed to the remineralization of sinking organic particles. In contrast, surface dMn levels were predominantly controlled by the seasonal variations in photoreduction, while sub-surface dMn concentrations were relatively constant throughout the year. The combined effects of fluvial and benthic sources, topographical controls, and biological processes shape the seasonal variations of dTM distributions. Such seasonal variations in dTMs and biological activities can affect the biological carbon pump on the Northeast Atlantic continental margin, and may further influence the carbon cycle in the Atlantic Ocean via the dynamic dTM exchange between continental margins and the open ocean

Equilibrium calculations of iron speciation and apparent iron solubility in the Celtic Sea at ambient seawater pH using the NICA-Donnan model

We used a combined ion pairing - organic matter speciation model (NICA-Donnan) to predict the organic complexation of iron (Fe) at ambient pH and temperature in the Celtic Sea. We optimized our model by direct comparison with Fe speciation determined by Adsorptive Cathodic Stripping Voltammetry using the added Fe-binding ligand 1-nitroso-2-naphthol (HNN) in the presence and absence of natural organic matter. We compared determined Fe speciation with simulated titrations obtained via application of the NICA-Donnan model with four different NICA parameter sets representing a range of binding site strengths and heterogeneities. We tested the assumption that binding sites scale to dissolved organic carbon (DOC) concentrations in marine waters. We found that a constant low DOC concentration resulted in an improved fit of our titration data to the simulated titrations, suggesting that inputs of autochthonous marine DOM may not increase the heterogeneity or concentrations of Fe binding sites. Using the optimal parameter set, we calculated pFe(III)´ (−log(∑Fe(OH)i3−i)) and apparent Fe(III) solubility (SFe(III)app) at ambient pH and temperature in the water column of the Celtic Sea. SFe(III)app was defined as the sum of aqueous inorganic Fe(III) species and Fe(III) bound to DOM formed at a free Fe (Fe3+) concentration equal to the limiting solubility of Fe hydroxide (Fe(OH)3(s)). SFe(III)app was within range of the determined dissolved Fe concentrations observed after winter mixing on the shelf and in waters >1500 m depth at our most offshore stations. Our study supports the hypothesis that the ocean dissolved Fe inventory is controlled by the interplay between Fe solubility and Fe binding by organic matter, although the overall number of metal binding sites in the marine environment may not be directly scalable to DOC concentrations

Ocean circulation and biological processes drive seasonal variations of dissolved Al, Cd, Ni, Cu, and Zn on the Northeast Atlantic continental margin

Nutrients and nutrient-like dissolved trace metals (dTMs) are essential for the functioning of marine organisms and therefore form an important part of ocean biogeochemical cycles. Here, we report on the seasonal distributions of dissolved zinc (dZn), nickel (dNi), copper (dCu), cadmium (dCd), aluminum (dAl), and nutrients on the Northeast Atlantic continental margin (Celtic Sea), which is representative for temperate shelf seas globally. Variations in surface water dTM and nutrient concentrations were mainly regulated by seasonal changes in biological processes. The stoichiometry of dTMs (especially for dCu and dZn) and nutrients on the continental shelf was additionally affected by fluvial inputs. Nutrients and dTMs at depth on the continental slope were determined by water mass mixing driven by ocean circulation, without an important role for local remineralization processes. The Mediterranean Outflow Waters are especially important for delivering Mediterranean-sourced dTMs to the Northeast Atlantic Ocean and drive dTM:nutrient kinks at a depth of ~1000 m. These results highlight the importance of riverine inputs, seasonality of primary production and ocean circulation on the distributions of nutrients and nutrient-like dTMs in temperate continental margin seas. Future climate related changes in the forcing factors may impact the availability of nutrients and dTMs to marine organisms in highly productive continental shelf regions and consequently the regional carbon cycle