Dissolved Iron, Aluminium, and Dissolved Inorganic Phosphorus in the (Sub-)Tropical Atlantic Surface Ocean

Inorganic nitrogen depletion restricts productivity in much of the low-latitude oceans, generating a selective advantage for diazotrophic organisms capable of fixing atmospheric dinitrogen (N2). However, the abundance and activity of diazotrophs can in turn be controlled by the availability of other potentially limiting nutrients, including phosphorus (P) and iron (Fe). Here we present high-resolution data (~0.3°) for dissolved iron, aluminum, and inorganic phosphorus that confirm the existence of a sharp north-south biogeochemical boundary in the surface nutrient concentrations of the (sub)tropical Atlantic Ocean. Combining satellite-based precipitation data with results from a previous study, we here demonstrate that wet deposition in the region of the intertropical convergence zone acts as the major dissolved iron source to surface waters. Moreover, corresponding observations of N2 fixation and the distribution of diazotrophic Trichodesmium spp. indicate that movement in the region of elevated dissolved iron as a result of the seasonal migration of the intertropical convergence zone drives a shift in the latitudinal distribution of diazotrophy and corresponding dissolved inorganic phosphorus depletion. These conclusions are consistent with the results of an idealized numerical model of the system. The boundary between the distinct biogeochemical systems of the (sub)tropical Atlantic thus appears to be defined by the diazotrophic response to spatial-temporal variability in external Fe inputs. Consequently, in addition to demonstrating a unique seasonal cycle forced by atmospheric nutrient inputs, we suggest that the underlying biogeochemical mechanisms would likely characterize the response of oligotrophic systems to altered environmental forcing over longer timescales.

Combined effects of ocean acidification and light or nitrogen availabilities on 13C fractionation in marine dinoflagellates

Along with increasing oceanic CO2 concentrations, enhanced stratification constrains phytoplankton to shallower upper mixed layers with altered light regimes and nutrient concentrations. Here, we investigate the effects of elevated pCO2 in combination with light or nitrogen-limitation on 13C fractionation (epsilon p) in four dinoflagellate species. We cultured Gonyaulax spinifera and Protoceratium reticulatum in dilute batches under low-light (LL) and high-light (HL) conditions, and grew Alexandrium fundyense and Scrippsiella trochoidea in nitrogen-limited continuous cultures (LN) and nitrogen-replete batches (HN). The observed CO2-dependency of epsilon p remained unaffected by the availability of light for both G. spinifera and P. reticulatum, though at HL epsilon p was consistently lower by about 2.7 per mil over the tested CO2 range for P. reticulatum. This may reflect increased uptake of (13C-enriched) bicarbonate fueled by increased ATP production under HL conditions. The observed CO2-dependency of epsilon p disappeared under LN conditions in both A. fundyense and S. trochoidea. The generally higher epsilon p under LN may be associated with lower organic carbon production rates and/or higher ATP:NADPH ratios. CO2-dependent epsilon p under non-limiting conditions has been observed in several dinoflagellate species, showing potential for a new CO2-proxy. Our results however demonstrate that light- and nitrogen-limitation also affect epsilon p, thereby illustrating the need to carefully consider prevailing environmental conditions.

(Table 2) Lipid fluxes derived from sediment trap samples

Phytoplankton is a sentinel of marine ecosystem change. Composed by many species with different life-history strategies, it rapidly responds to environment changes. An analysis of the abundance of 54 phytoplankton species in Galicia (NW Spain) between 1989 and 2008 to determine the main components of temporal variability in relation to climate and upwelling showed that most of this variability was stochastic, as seasonality and long term trends contributed to relatively small fractions of the series. In general, trends appeared as non linear, and species clustered in 4 groups according to the trend pattern but there was no defined pattern for diatoms, dinoflagellates or other groups. While, in general, total abundance increased, no clear trend was found for 23 species, 14 species decreased, 4 species increased during the early 1990s, and only 13 species showed a general increase through the series. In contrast, series of local environmental conditions (temperature, stratification, nutrients) and climate-related variables (atmospheric pressure indices, upwelling winds) showed a high fraction of their variability in deterministic seasonality and trends. As a result, each species responded independently to environmental and climate variability, measured by generalized additive models. Most species showed a positive relationship with nutrient concentrations but only a few showed a direct relationship with stratification and upwelling. Climate variables had only measurable effects on some species but no common response emerged. Because its adaptation to frequent disturbances, phytoplankton communities in upwelling ecosystems appear less sensitive to changes in regional climate than other communities characterized by short and well defined productive periods.

Integrated Data Sets of the EU FP5 Research Project ORFOIS: Origin and fate of biogenic particle fluxes in the ocean and their interactions with atmospheric CO2 concentrations as well as the marine sediment

For a reliable simulation of the time and space dependent CO2 redistribution between ocean and atmosphere an appropriate time dependent simulation of particle dynamics processes is essential but has not been carried out so far. The major difficulties were the lack of suitable modules for particle dynamics and early diagenesis (in order to close the carbon and nutrient budget) in ocean general circulation models, and the lack of an understanding of biogeochemical processes, such as the partial dissolution of calcareous particles in oversaturated water. The main target of ORFOIS was to fill in this gap in our knowledge and prediction capability infrastructure. This goal has been achieved step by step. At first comprehensive data bases (already existing data) of observations of relevance for the three major types of biogenic particles, organic carbon (POC), calcium carbonate (CaCO3), and biogenic silica (BSi or opal), as well as for refractory particles of terrestrial origin were collated and made publicly available.

Effects of increased CO2 concentration on nutrient limited coastal summer plankton depend on temperature

Increasing seawater temperature and CO2 concentrations both are expected to increase coastal phytoplankton biomass and carbon to nutrient ratios in nutrient limited seasonally stratified summer conditions. This is because temperature enhances phytoplankton growth while grazing is suggested to be reduced during such bottom-up controlled situations. In addition, enhanced CO2 concentrations potentially favor phytoplankton species, that otherwise depend on costly carbon concentrating mechanisms (CCM). The trophic consequences for consumers under such conditions, however, remain little understood. We set out to experimentally explore the combined effects of increasing temperature and CO2 concentration for phytoplankton biomass and stoichiometry and the consequences for trophic transfer (here for copepods) on a natural nutrient limited Baltic Sea summer plankton community. The results show, that warming effects were translated to the next trophic level by switching the system from a bottom-up controlled to a mainly top-down controlled one. This was reflected in significantly down-grazed phytoplankton and increased zooplankton abundance in the warm temperature treatment (22.5°C). Additionally, at low temperature (16.5°C) rising CO2 concentrations significantly increased phytoplankton biomass. The latter effect however, was due to direct negative impact of CO2 on copepod nauplii which released phytoplankton from grazing in the cold but not in the warm treatments. Our results suggest that future seawater warming has the potential to switch trophic relations between phytoplankton and their grazers under nutrient limited conditions with the consequence of potentially disguising CO2 effects on coastal phytoplankton biomass.

Phosphorus concentrations and accumulation rates of ODP Leg 130 sites

Little is known about the fluxes to and from the ocean during the Cenozoic of phosphorus (P), a limiting nutrient for oceanic primary productivity and organic carbon burial on geologic timescales. Previous studies have concluded that dissolved river fluxes increased worldwide during the Cenozoic and that organic carbon burial decreased relative to calcium carbonate burial and perhaps in absolute terms as well. To examine the apparent contradiction between increased river fluxes of P (assuming P fluxes behave like the others) expected to drive increased organic carbon burial and observations indicating decreased organic carbon burial, we determined P accumulation rates for equatorial Pacific sediments from Ocean Drilling Program leg 138 sites in the eastern equatorial Pacific and leg 130 sites on the Ontong Java Plateau in the western equatorial Pacific. Although there are site specific and depth dependent effects on P accumulation rates, there are important features common to the records at all sites. P accumulation rates declined from 50 to 20 Ma, showed some variability from 20 to 10 Ma, and had a substantial peak from 9 to 3 Ma centered at 5-6 Ma. These changes in P accumulation rates for the equatorial Pacific are equivalent to substantial changes in the P mass balance. However, the pattern resembles neither that of weathering flux indicators (87Sr/86Sr and Ge/Si ratios) nor that of the carbon isotope record reflecting changes in organic carbon burial rates. Although these P accumulation rate patterns need confirmation from other regions with sediment burial significant in global mass balances (e.g., the North Pacific and Southern Ocean), it appears that P weathering inputs to the ocean are decoupled from those of other elements and that further exploration is needed of the relationship between P burial and net organic carbon burial.

Benthic carbon turnover, sediment pigment concentrations and sinking fluxes of POC in the southeastern Beaufort Sea during CCGS Amundsen cruise 2008

The effects of changing ice and atmospheric conditions on the upwelling of deep nutrient-laden waters and biological productivity in the coastal Beaufort Sea were quantified using a unique combination of in situ and remote-sensing approaches. Repeated instances of ice ablation and upwelling during fall 2007 and summer 2008 multiplied the production of ice algae, phytoplankton, zooplankton and benthos by 2 to 6 fold. Strong wind forcing failed to induce upward shifts in the biological productivity of stratified waters off the shelf.

Organic carbon concentrations and Cd/Ca ratios of sediment core RC13-229

A comparison of cadmium/calcium (Cd/Ca) records of benthic foraminifera from a deep Cape Basin and a deep eastern equatorial Pacific core suggests that over the past 400,000 years, the nutrient concentration of Circumpolar Deep Water (CPDW) has always been lower than that of the deep Pacific. The data further suggest that at the 100,000- and 23,000-year orbital periods, the contribution of North Atlantic Deep Water to CPDW is at a maximum during periods of ice growth and at a minimum during periods of ice decay. These results are not in agreement with results based on carbon isotope records of benthic foraminifera, which suggest intervals of CPDW nutrient enrichment relative to the deep Pacific and an approximately in-phase relationship between CPDW nutrient concentration and ice volume. Resolution of the apparent conflict between delta13C and Cd/Ca data may provide important constraints on past deep-ocean circulation and nutrient variability.

Concentrations of organic carbon, nitrogen and phosphorus on vertical profiles in waters of the tropical North Atlantic in March 2002

In the literature, an inconsistency exists between estimates of biotically-effected carbon export inferred from large-scale geochemical studies (Jenkins 1982; 47 gC m-2 a-1) and local measurements of turbulent nutrient supply (Lewis et al. 1986; 4 gC m-2 a-1) in the eastern subtropical North Atlantic. Nutrient supply to the upper ocean by turbulent mixing is reexamined using local standard oceanographic measurements and high-resolution vertical profiles of nutrients averaged over a large region directly comparable to that investigated by Jenkins (1982). Turbulent fluxes induced by internal waves and salt fingering, respectively, are separated according to Gregg (1989) and Zhang et al. (1998). Nutrient transport into the nutrient-consuming surface layer by salt fingering is more than fivefold higher than transport due to internal-wave induced turbulence. Still, this cannot resolve the above- mentioned apparent inconsistency, even if additional physical transport mechanisms such as eddy pumping, advection and horizontal diffusion are accounted for. Estimated nitrate fluxes due to vertical turbulent diffusion are 0.05-0.15 mol m-2 a-1, corresponding to 4-11 gC m-2 a-1. Observed NO3/PO4 turbulent flux ratios of up to 23 are interpreted as the imprint of N2 fixation.

Major, trace and rare earth element concentrations of sediments from ODP Leg 207 sites

The Cretaceous and Paleogene sediments recovered during Ocean Drilling Program Leg 207 can be divided into three broad modes of deposition: synrift clastics (lithologic Unit V), organic matter-rich, laminated black shales (Unit IV), and open-marine chalk and calcareous claystones (Units III-I). The aim of this study is to provide a quantitative geochemical characterization of sediments representing these five lithologic units. For this work we used the residues (squeeze cakes) obtained from pore water sampling. Samples were analyzed for bulk parameters (total inorganic carbon, total organic carbon, and S) and by X-ray fluorescence for major (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) and selected minor (As, Ba, Co, Cr, Cu, Mo, Ni, Pb, Rb, Sr, U, V, Y, Zn, and Zr) elements. Inductively coupled plasma-mass spectrometry analyses for rare earth elements (REEs) were performed on acid digestions of the squeeze cake samples from Site 1258. The major element composition is governed by the mixture of a terrigenous detrital component of roughly average shale (AS) composition with biogenous carbonate and silica. The composition of the terrigenous detritus is close to AS in Units II-IV. For Unit I, a more weathered terrigenous source is suggested. Carbonate contents reach >60 wt% on average in chalks and calcareous claystones of Units II-IV. The SiO2 contribution in excess of the normal terrigenous-detrital background indicates the presence of biogenous silica, with highest amounts in Units II and III. The contents of coarse-grained material (quartz) are enhanced in Unit V, where Ti and Zr contents are also high. This indicates a high-energy depositional environment. REE patterns are generally similar to AS. A more pronounced negative Ce anomaly in Unit IV may indicate low-oxygen conditions in the water column. The Cretaceous black shales of Unit IV are clearly enriched in redox-sensitive and stable sulfide-forming elements (Mo, V, Zn, and As). High phosphate contents point toward enhanced nutrient supply and high bioproductivity. Ba/Al ratios are rather high throughout Unit IV despite the absence of sulfate in the pore water, indicating elevated primary production. Manganese contents are extremely low for most of the interval studied. Such an Mn depletion is only possible in an environment where Mn was mobilized and transported into an expanded oxygen minimum zone ("open system"). The sulfur contents show a complete sulfidation of the reactive iron of Unit IV and a significant excess of sulfur relative to that of iron, which indicates that part of the sulfur was incorporated into organic matter. We suppose extreme paleoenvironmental conditions during black shale deposition: high bioproductivity like in recent coastal upwelling settings together with severe oxygen depletion if not presence of hydrogen sulfide in the water column.

Nutrients and oxygen concentrations measured in water samples from the North Atlantic during the James Cook cruise JC087 in March 2013

Analysis for micro-molar concentrations of nitrate and nitrite, nitrite, phosphate, silicate and ammonia was undertaken on a SEAL Analytical UK Ltd, AA3 segmented flow autoanalyser following methods described by Kirkwood (1996). Samples were drawn from Niskin bottles on the CTD into 15ml polycarbonate centrifuge tubes and kept refrigerated at approximately 4oC until analysis, which generally commenced within 30 minutes. Overall 23 runs with 597 samples were analysed. This is a total of 502 CTD samples, 69 underway samples and 26 from other sources. An artificial seawater matrix (ASW) of 40g/litre sodium chloride was used as the inter-sample wash and standard matrix. The nutrient free status of this solution was checked by running Ocean Scientific International (OSI) low nutrient seawater (LNS) on every run. A single set of mixed standards were made up by diluting 5mM solutions made from weighed dried salts in 1litre of ASW into plastic 250ml volumetric flasks that had been cleaned by washing in MilliQ water (MQ). Data processing was undertaken using SEAL Analytical UK Ltd proprietary software (AACE 6.07) and was performed within a few hours of the run being finished. The sample time was 60 seconds and the wash time was 30 seconds. The lines were washed daily with wash solutions specific for each chemistry, but comprised of MQ, MQ and SDS, MQ and Triton-X, or MQ and Brij-35. Three times during the cruise the phosphate and silicate channels were washed with a weak sodium hypochlorite solution.

Otolith microstructure and trace elemental concentrations in juvenile Pacific bluefin tuna (Thunnus orientalis) caught off of Southern California in 2012

We combined longitudinal analyses of otolith microstructure and trace elemental composition in ~ age 1-2 Pacific bluefin tuna (PBT, n = 24) for inferring the arrival of individuals in the California Current Large Marine Ecosystem (CCLME). Element:Ca ratios in transverse otolith sections (9-12 rows, triplicate ablations from coreprimordium to edge, ø50 µm) were quantified for eight elements: Li, Mg, Mn, Co, Cu, Zn, Sr, and Ba, which was followed by microstructure analysis to provide age estimates corresponding to each ablation spot. Age estimates from otoliths ranged from 328 to 498 days post hatch. The combined elemental signatures of four elements (Ba, Mg, Co, Cu) showed a significant increase at the otolith edge in approximately half of the individuals (30-60 days prior to catch). Given the different oceanographic properties of oligotrophic open Pacific vs. high nutrient, upwelling CCLME waters, this signal is consistent with the entry of the fish into the CCLME, which was estimated to occur primarily in July after a transoceanic migration of ~1.5-2.0 months.

Organic carbon and nitrogen concentrations in surface waters of the North Atlantic along 20°E in July-August 1996

Redfield stoichiometry has proved a robust paradigm for the understanding of biological production and export in the ocean on a long-term and a large-scale basis. However, deviations of carbon and nitrogen uptake ratios from the Redfield ratio have been reported. A comprehensive data set including all carbon and nitrogen pools relevant to biological production in the surface ocean (DIC, DIN, DOC, DON, POC, PON) was used to calculate seasonal new production based on carbon and nitrogen uptake in summer along 20°W in the northeast Atlantic Ocean. The 20°W transect between 30 and 60°N covers different trophic states and seasonal stages of the productive surface layer, including early bloom, bloom, post-bloom and non-bloom situations. The spatial pattern has elements of a seasonal progression. We also calculated exported production, i.e., that part of seasonal new production not accumulated in particulate and dissolved pools, again separately for carbon and nitrogen. The pairs of estimates of 'seasonal new production' and 'exported production' allowed us to calculate the C : N ratios of these quantities. While suspended particulate matter in the mixed layer largely conforms to Redfield stoichiometry, marked deviations were observed in carbon and nitrogen uptake and export with progressing season or nutrient depletion. The spring system was characterized by nitrogen overconsumption and the oligotrophic summer system by a marked carbon overconsumption. The C : N ratios of seasonal new as well as exported production increase from early bloom values of 5-6 to values of 10-16 in the post-bloom/oligotrophic system. The summertime accumulation of nitrogen-poor dissolved organic matter can explain only part of this shift.