Northern Hemisphere Glaciation during the Globally Warm Early Late Pliocene

The early Late Pliocene (3.6 to ∼3.0 million years ago) is the last extended interval in Earth''s history when atmospheric CO₂ concentrations were comparable to today''s and global climate was warmer. Yet a severe global glaciation during marine isotope stage (MIS) M2 interrupted this phase of global warmth ∼3.30 million years ago, and is seen as a premature attempt of the climate system to establish an ice-age world. Here we propose a conceptual model for the glaciation and deglaciation of MIS M2 based on geochemical and palynological records from five marine sediment cores along a Caribbean to eastern North Atlantic transect. Our records show that increased Pacific-to-Atlantic flow via the Central American Seaway weakened the North Atlantic Current and attendant northward heat transport prior to MIS M2. The consequent cooling of the northern high latitude oceans permitted expansion of the continental ice sheets during MIS M2, despite near-modern atmospheric CO₂ concentrations. Sea level drop during this glaciation halted the inflow of Pacific water to the Atlantic via the Central American Seaway, allowing the build-up of a Caribbean Warm Pool. Once this warm pool was large enough, the Gulf Stream–North Atlantic Current system was reinvigorated, leading to significant northward heat transport that terminated the glaciation. Before and after MIS M2, heat transport via the North Atlantic Current was crucial in maintaining warm climates comparable to those predicted for the end of this century.     

Prevalence and microdiversity of Alteromonas macleodii-like microorganisms in different oceanic regions

The presence, prevalence and variability of microorganisms related to the species Alteromonas macleodii, a well known culturable gamma-Proteobacterium, has been studied in different seawater samples from diverse geographical locations, in both the Northern and Southern hemispheres, and tested with two molecular techniques (rRNA hybridization and gene cloning and sequencing). Results show that A. macleodii-like microorganisms are present in high proportions in North Atlantic and, especially, Mediterranean waters, being higher at deep samples and particle-associated fractions, in agreement with previous findings. In contrast, Southern samples (all from very cold areas near Antarctica) presented no significant hybridization signals. The analysis of the ribosomal ITS (16S-23S internal transcribed spacers) revealed that A. macleodii-like microorganisms from Mediterranean, North Atlantic, Caribbean and Red Sea waters differed in both size and sequence, mostly depending on their geographical origin, with Mediterranean and North Atlantic clones clustering into two main groups whereas Caribbean and Red Sea clones appeared separated.     

Mitochondrial DNA genome evidence for the existence of a third divergent lineage in the western Atlantic Ocean for the bull shark (Carcharhinus leucas)

We report for the first time a highly divergent lineage in the Caribbean Sea for the bull shark (Carcharhinus leucas) based on the analysis of 51 mitochondrial DNA genomes of individuals collected in the western North Atlantic. When comparing the mtDNA control region obtained from the mitogenomes to sequences reported previously for Brazil, the Caribbean lineage remained highly divergent. These results support the existence of a discrete population in Central America due to a phylogeographic break separating the Caribbean Sea from the western North Atlantic, Gulf of Mexico and South America.     

Dynamic Model for the Adsorption in a Multibed Three‐Phase Fluidization Column Applied to Wastewater Biological Treatment

It is proposed a dynamic model for adsorption of NH₄⁺ ions from ammonia waters on volcanic tuff in a 10‐bed three‐phase (air – ammonia waters – volcanic tuff) fluidization column. The model consists in the nonstationary material balance differential equations. For each layer the ideal well‐mixing conditions are considered. The effluent ammonia ion concentrations, corresponding to each layer, have been measured at several time values in a laboratory‐scale column. The absolute relative mean error between the calculated and measured values of ammonia ion concentrations into liquid phase for all layers and times is 6.65 %, being in the order of magnitude of experimental errors.     

The biological production of marine suspended barite and the barium cycle in the Western Mediterranean Sea

Suspended particulate barium was measured in the Western Mediterranean along 4 profiles sampled during the PHYCEMED 1 cruise in 1981. The non-terrigenous fraction of particulate barium (i.e. excess Ba; Ba_xs) can account for up to 96% of total barium. This fraction follows the barite settling and dissolution rate model we described earlier for the Atlantic Ocean, confirming barite as the carrier of excess barium. We propose that these Ba_xs concentrations, which are higher in the Western Mediterranean than in the overall Atlantic, may represent an adaptation of the microplankton to the high densities and density gradients of surface Mediterranean waters.A mass-balance for Ba in the Mediterranean Sea shows: (1) that the atmospheric source must be much more important than previously reported: (2) that contrary to the Atlantic situation, the internal recycling of barium is of little importance compared to the flow of barium through the system.     

Modeling the effects of snowpack on heterotrophic respiration across northern temperate and high latitude regions: Comparison with measurements of atmospheric carbon dioxide in high latitudes

Simulations by global terrestrial biogeochemical models (TBMs) consistently underestimate the concentration of atmospheric carbon dioxide (CO₂ at high latitude monitoring stations during the non-growing season. We hypothesized that heterotrophic respiration is underestimated during the nongrowing season primarily because TBMs do not generally consider the insulative effects of snowpack on soil temperature. To evaluate this hypothesis, we compared the performance of baseline and modified versions of three TBMs in simulating the seasonal cycle of atmospheric CO₂ at high latitude CO₂ monitoring stations; the modified version maintained soil temperature at 0 °C when modeled snowpack was present. The three TBMs include the Carnegie-Ames-Stanford Approach (CASA), Century, and the Terrestrial Ecosystem Model (TEM). In comparison with the baseline simulation of each model, the snowpack simulations caused higher releases of CO₂ between November and March and greater uptake of CO₂ between June and August for latitudes north of 30° N. We coupled the monthly estimates of CO₂ exchange, the seasonal carbon dioxide flux fields generated by the HAMOCC3 seasonal ocean carbon cycle model, and fossil fuel source fields derived from standard sources to the three-dimensional atmospheric transport model TM2 forced by observed winds to simulate the seasonal cycle of atmospheric CO₂ at each of seven high latitude monitoring stations. In comparison to the CO₂ concentrations simulated with the baseline fluxes of each TBM, concentrations simulated using the snowpack fluxes are generally in better agreement with observed concentrations between August and March at each of the monitoring stations. Thus, representation of the insulative effects of snowpack in TBMs generally improves simulation of atmospheric CO₂ concentrations in high latitudes during both the late growing season and nongrowing season. These simulations highlight the global importance of biogeochemical processes during the nongrowing season in estimating carbon balance of ecosystems in northern high and temperate latitudes.     

Spawning ofConger oceanicus andConger triporiceps (Congridae) in the Sargasso Sea and subsequent distribution of leptocephali

Synopsis Distribution of leptocephali ofConger in the Western North Atlantic Ocean was studied using specimens from our collections, specimens from other collections, and various existing collection records. The presence of leptocephali ofConger oceanicus andConger triporiceps < 30 mm long over deep water in the southwestern Sargasso Sea in autumn and winter implies a protracted spawning period there. The subtropical convergence zone, meandering east-west across the Sargasso Sea, is probably the northern limit of spawning of both species. Spawning may also occur close to the Bahamas and Antilles.C. triporiceps may spawn also in the Caribbean Sea judging by the capture of small leptocephali in the western Caribbean and of the more southerly continental distribution of its juveniles. The claim of Johannes Schmidt in 1931 that the EuropeanC. conger spawns across the North Atlantic into the western Sargasso Sea is probably incorrect, because leptocephali ofConger are rare in the eastern Sargasso Sea and becauseC. triporiceps, with myomere numbers overlapping those ofC. conger, was recently described in the western North Atlantic. With increasing size, leptocephali ofC. oceanicus and a portion ofC. triporiceps spread westward and northward in the Florida Current and Gulf Stream, but larger leptocephali especially ofC. triporiceps are found also in the Caribbean and Gulf of Mexico. Spawning ofC. oceanicus in the Sargasso Sea indicates that adults cross the Florida Current-Gulf Stream, and successful leptocephali cross the current in the opposite direction to colonize juvenile habitat on the continental shelf, a migratory pattern similar to that of the American eelAnguilla rostrata (Anguillidae).     

Microsatellite and mitochondrial DNA analyses of the genetic structure of blacktip shark (Carcharhinus limbatus) nurseries in the northwestern Atlantic, Gulf of Mexico, and Caribbean Sea

Abstract We investigated the genetic structure of blacktip shark (Carcharhinus limbatus) continental nurseries in the northwestern Atlantic Ocean, Gulf of Mexico, and Caribbean Sea using mitochondrial DNA control region sequences and eight nuclear microsatellite loci scored in neonate and young-of-the-year sharks. Significant structure was detected with both markers among nine nurseries (mitochondrial PhiST = 0.350, P < 0.001; nuclear PhiST = 0.007, P < 0.001) and sharks from the northwestern Atlantic, eastern Gulf of Mexico, western Gulf of Mexico, northern Yucatan, and Belize possessed significantly different mitochondrial DNA haplotype frequencies. Microsatellite differentiation was limited to comparisons involving northern Yucatan and Belize sharks with nuclear genetic homogeneity throughout the eastern Gulf of Mexico, western Gulf of Mexico, and northwestern Atlantic. Differences in the magnitude of maternal vs. biparental genetic differentiation support female philopatry to northwestern Atlantic, Gulf of Mexico, and Caribbean Sea natal nursery regions with higher levels of male-mediated gene flow. Philopatry has produced multiple reproductive stocks of this commercially important shark species throughout the range of this study.     

Bioturbation determines the response of benthic ammonia-oxidizing microorganisms to ocean acidification

Ocean acidification (OA), caused by the dissolution of increasing concentrations of atmospheric carbon dioxide (CO₂) in seawater, is projected to cause significant changes to marine ecology and biogeochemistry. Potential impacts on the microbially driven cycling of nitrogen are of particular concern. Specifically, under seawater pH levels approximating future OA scenarios, rates of ammonia oxidation (the rate-limiting first step of the nitrification pathway) have been shown to dramatically decrease in seawater, but not in underlying sediments. However, no prior study has considered the interactive effects of microbial ammonia oxidation and macrofaunal bioturbation activity, which can enhance nitrogen transformation rates. Using experimental mesocosms, we investigated the responses to OA of ammonia oxidizing microorganisms inhabiting surface sediments and sediments within burrow walls of the mud shrimp Upogebia deltaura. Seawater was acidified to one of four target pH values (pH_T 7.90, 7.70, 7.35 and 6.80) in comparison with a control (pH_T 8.10). At pH_T 8.10, ammonia oxidation rates in burrow wall sediments were, on average, fivefold greater than in surface sediments. However, at all acidified pH values (pH ≤ 7.90), ammonia oxidation rates in burrow sediments were significantly inhibited (by 79–97%; p < 0.01), whereas rates in surface sediments were unaffected. Both bacterial and archaeal abundances increased significantly as pH_T declined; by contrast, relative abundances of bacterial and archaeal ammonia oxidation (amoA) genes did not vary. This research suggests that OA could cause substantial reductions in total benthic ammonia oxidation rates in coastal bioturbated sediments, leading to corresponding changes in coupled nitrogen cycling between the benthic and pelagic realms.     

Interannual variability of carbon dioxide drawdown by subantarctic surface water near New Zealand

The ocean-atmosphere flux of carbon dioxide in subantarctic surface water (SASW) east of New Zealand has been determined using data from bi-monthly cruises on a time series transect for 8 years. The 60 km long transect extends from the coast (45.770°S 170.720°E) to a station at 45.833°S 171.500°E. Sea surface temperature, salinity, nutrient concentrations and pCO₂ have been measured at a frequency of about once every 2 months from January 1998 until December 2005. Measured pCO₂ exhibits a seasonal cycle with a maximum in late winter/spring, and a minimum in late summer/autumn, a mean 356 ??atm, and an amplitude of 9 ??atm. The magnitude of ??pCO₂ (the air-sea concentration gradient) has increased over the 8 years, primarily due to the increase in atmospheric CO₂ concentration. The air-sea flux of CO₂ was determined from wind speed data and ??pCO₂. The uptake of atmospheric CO₂ by SASW in the study area changed from + 1 and +82 mmol m^?2 in 1998 and 1999 respectively (ocean as source) to ?870 and ?510 mmol m^?2 in 2004 and 2005 (ocean as sink). These values are substantially less in magnitude than the value obtained from the Takahashi et al. (Deep-Sea Res II, 2009) flux climatology.     

Regional nitrogen budgets and riverine N & P fluxes for the drainages to the North Atlantic Ocean: Natural and human influences

We present estimates of total nitrogen and total phosphorus fluxes in rivers to the North Atlantic Ocean from 14 regions in North America, South America, Europe, and Africa which collectively comprise the drainage basins to the North Atlantic. The Amazon basin dominates the overall phosphorus flux and has the highest phosphorus flux per area. The total nitrogen flux from the Amazon is also large, contributing 3.3 Tg yr^–1 out of a total for the entire North Atlantic region of 13.1 Tg yr^–1 . On a per area basis, however, the largest nitrogen fluxes are found in the highly disturbed watersheds around the North Sea, in northwestern Europe, and in the northeastern U.S., all of which have riverine nitrogen fluxes greater than 1,000 kg N km^–2 yr^–1.Non-point sources of nitrogen dominate riverine fluxes to the coast in all regions. River fluxes of total nitrogen from the temperate regions of the North Atlantic basin are correlated with population density, as has been observed previously for fluxes of nitrate in the world's major rivers. However, more striking is a strong linear correlation between river fluxes of total nitrogen and the sum of anthropogenically-derived nitrogen inputs to the temperate regions (fertilizer application, human-induced increases in atmospheric deposition of oxidized forms of nitrogen, fixation by leguminous crops, and the import/export of nitrogen in agricultural products). On average, regional nitrogen fluxes in rivers are only 25% of these anthropogenically derived nitrogen inputs. Denitrification in wetlands and aquatic ecosystems is probably the dominant sink, with storage in forests perhaps also of importance. Storage of nitrogen in groundwater, although of importance in some localities, is a very small sink for nitrogen inputs in all regions. Agricultural sources of nitrogen dominate inputs in many regions, particularly the Mississippi basin and the North Sea drainages. Deposition of oxidized nitrogen, primarily of industrial origin, is the major control over river nitrogen export in some regions such as the northeastern U.S.Using data from relatively pristine areas as an index of change, we estimate that riverine nitrogen fluxes in many of the temperate regions have increased from pre-industrial times by 2 to 20 fold, although some regions such as northern Canada are relatively unchanged. Fluxes from the most disturbed region, the North Sea drainages, have increased by 6 to 20 fold. Fluxes from the Amazon basin are also at least 2 to 5 fold greater than estimated fluxes from undisturbed temperate-zone regions, despite low population density and low inputs of anthropogenic nitrogen to the region. This suggests that natural riverine nitrogen fluxes in the tropics may be significantly greater than in the temperate zone. However, deforestation may be contributing to the tropical fluxes. In either case, projected increases in fertilizer use and atmospheric deposition in the coming decades are likely to cause dramatic increases in nitrogen loading to many tropical river systems.     

Areas of shallow water in the North Atlantic

We report here estimates of the areas of water that are between 1 and 100 m and between 1 and 200 m deep in the North Atlantic, including the Caribbean Sea, the Gulf of Mexico, Hudson Bay and the North Sea, but excluding estuaries. The total areas within these depths, from the equator to 70° N, are 3.91 × 10⁶ km² and 5.66 × 10⁶ km², respectively. We also report the respective areas by selected geographic regions.     

On the hydrographic and ice conditions in the northern North Atlantic during different phases of a glaciation cycle

A theoretical investigation of the hydrographic conditions in the northern North Atlantic (N of the Greenland—Scotland Ridge) for different forcing conditions is undertaken. First a simple dynamical model for the “mediterranean” circulation in the Greenland and Norwegian seas (the Subpolar Sea) is developed. The calculated contemporary rate of deepwater formation (about 2.5 × 10⁶ m³/s) seems to be quite realistic. It is concluded that the mediterranean circulation is possible as long as the cyclonal atmospheric circulation in the Subpolar Sea is strong enough to prevent the fresher water in the coastal currents along the eastern and western boundaries from spreading over the interior.Utilizing the Arctic Ocean model in Stigebrandt (1981) the hydrographic and ice conditions in the Subpolar Sea and the Arctic Ocean during different phases of a glaciation are, finally, investigated. As so many parameters are involved the derivations are, by necessity, rather uncertain. However, it seems possible that the hydrographic state in the Subpolar Sea changes between mediterranean and estuarine circulation several times during a glacial cycle.     

Reconstruction of pollutant lead invasion into the tropical North Atlantic during the twentieth century

In this study we reconstruct pollutant lead transport and input into the Caribbean basin in relation to the meteorological changes and past industrial activities. These reconstructions are based on the geochemical data obtained from massive corals collected in 1998 in Mona Island. Lead concentration profile mimics the general atmospheric lead trend obtained from the previous investigations on ice cores, carbonate skeletons and in sediments from the North Atlantic. This similarity corroborates the significance of anthropogenic impact on all reservoirs of the North Atlantic as associated with the proliferation of the automobile and use of leaded gasoline during the twentieth century. Our high-resolution record reveals a 2–4 years shift between the maximum pollutant lead input to Mona Island and leaded gasoline consumption peaks in North America, suggesting a possible longer secondary oceanic transport of US lead within the Subtropical North Atlantic Gyre system. A striking relationship is found between lead isotopic imprints and the winter North Atlantic oscillation (NAO) index for two specific periods (1914–1929 and 1965–1997). These periods are characterized by distinct isotopic signatures from the main pollutant lead emitter to the North Atlantic troposphere, North America and Western Europe. This involves a noteworthy influence of the NAO on pollutant input and distribution in the North Atlantic that strengthens the usefulness of lead isotopes as tracers of paleo-atmospheric circulation and changes in marine circulation pathways.     

Hexavalent chromium is cytotoxic and genotoxic to the North Atlantic right whale (Eubalaena glacialis) lung and testes fibroblasts

Although hexavalent chromium is a known genotoxic agent in human and terrestrial mammals and is present in seawater and air, its effects on marine mammals including the endangered North Atlantic right whale are unknown and untested. The present study investigated the cytotoxic and genotoxic effects of hexavalent chromium in primary cultured North Atlantic right whale lung and testes fibroblasts and levels of total chromium in skin biopsies from North Atlantic right whales. Cytotoxicity was measured by clonogenic survival assay. Genotoxicity was measured as production of chromosome aberrations. Tissue chromium levels were determined from skin biopsies of healthy free-ranging whales in the Bay of Fundy using inductively coupled plasma optical emission spectroscopy. Hexavalent chromium-induced concentration-dependent increases in right whale lung and testes fibroblast cytotoxicity with the testes more sensitive to the cytotoxic effects. It also induced concentration-dependent increases in chromosomal aberrations in both cell types with no significant difference in sensitivity. Skin biopsy data indicate that North Atlantic right whales are exposed to chromium and accumulate a range of 4.9-10 microg Cr/g tissue with a mean of 7.1 microg/g. Hexavalent chromium is cytotoxic and genotoxic to North Atlantic right whale cells. The whales have tissue chromium levels that are concerning. These data support a hypothesis that chromium may be a concern for the health of the North Atlantic right whales. Considering these data with chromium chemistry, whale physiology and atmospheric chromium levels further suggest that inhalation may be an important exposure route.     

Effect of eutrophication on air–sea CO2 fluxes in the coastal Southern North Sea: a model study of the past 50 years

The RIVERSTRAHLER model, an idealized biogeochemical model of the river system, has been coupled to MIRO‐CO₂, a complex biogeochemical model describing diatom and Phaeocystis blooms and carbon and nutrient cycles in the marine domain, to assess the dual role of changing nutrient loads and increasing atmospheric CO₂ as drivers of air–sea CO₂ exchanges in the Southern North Sea with a focus on the Belgian coastal zone (BCZ). The whole area, submitted to the influence of two main rivers (Seine and Scheldt), is characterized by variable diatom and Phaeocystis colonies blooms which impact on the trophic status and air–sea CO₂ fluxes of the coastal ecosystem. For this application, the MIRO‐CO₂ model is implemented in a 0D multibox frame covering the eutrophied Eastern English Channel and Southern North Sea and receiving loads from the rivers Seine and Scheldt. Model simulations are performed for the period between 1951 and 1998 using real forcing fields for sea surface temperature, wind speed and atmospheric CO₂ and RIVERSTRAHLER simulations for river carbon and nutrient loads. Model results suggest that the BCZ shifted from a source of CO₂ before 1970 (low eutrophication) towards a sink during the 1970–1990 period when anthropogenic DIN and P loads increased, stimulating C fixation by autotrophs. In agreement, a shift from net annual heterotrophy towards autotrophy in BCZ is simulated from 1980. The period after 1990 is characterized by a progressive decrease of P loads concomitant with a decrease of primary production and of the CO₂ sink in the BCZ. At the end of the simulation period, the BCZ ecosystem is again net heterotroph and acts as a source of CO₂ to the atmosphere. R‐MIRO‐CO₂ scenarios testing the relative impact of temperature, wind speed, atmospheric CO₂ and river loads variability on the simulated air–sea CO₂ fluxes suggest that the trend in air–sea CO₂ fluxes simulated between 1951 and 1998 in the BCZ was mainly controlled by the magnitude and the ratio of inorganic nutrient river loads. Quantitative nutrient changes control the level of primary production while qualitative changes modulate the relative contribution of diatoms and Phaeocystis to this flux and hence the sequestration of atmospheric CO₂.     

Estimation of the load of cyclic organochlorines in North Sea zooplankton

During a North Sea survey in late spring 1986, zooplankton biomass (g dry weight/m²) as well as pollutant concentrations in zooplankton (ng/g dry weight) were determined at 127 stations. On the basis of these data, the load of several cyclic organochlorines in zooplankton (ng/m²) was estimated. It appeared that the highest loads of these pollutants were incorporated in the large zooplankton stocks of the northern and central North Sea, north of 56°N, whereas the highest concentrations were found in zooplankton of the southern North Sea. An explanation for the accumulation of pollutants in sediments as well as in benthos organisms of the central and northern North Sea as found by several authors is discussed in connection with the assumption of a considerably high vertical flux of organic material in these areas. The calculation of total zooplankton content in the North Sea during the late spring survey amounted to about 5 million tons dry weight. This biomass was estimated to contain about 2.6 t PCBs, 19.3 kg p,p′-DDE, 12 kg τ-HCH, 6.4 kg HCB und 5.6 kg α-HCH. These results agree with those on other compartments of the North Sea ecosystem (benthos, fish) found in recent literature. The estimated annual turnover of PCBs in zooplankton (15.2 t) is of the same order of magnitude as the estimated amount transported yearly into the North Sea by rivers and the atmosphere (6–13 t).     

A latitudinal gradient of seasonal temperature variation recorded in oyster shells from the coastal waters of France and The Netherlands

Cathodoluminescence (CL) microscopy of the foliated calcite shell hinge sections of live-collected oyster Crassostrea gigas collected at seven locations along a latitudinal gradient from the Netherlands in the North Sea to the Atlantic coast of France, revealed variations in luminescence that were attributable to seasonal variations in calcification of the hinge. Photomicrographs of hinge sections and luminescence profiles were analyzed to define a micro-sampling strategy that was adopted to drill the hinge samples to determine their isotopic composition. Reconstructed seasonal seawater temperatures determined from the stable oxygen isotope (δ¹⁸O) composition along growth profiles from 32 oyster shell hinges showed distinct seasonal isotopic cycles that were compared with in situ measured seawater temperatures and salinities at each location. Comparison of the amplitude of the (δ¹⁸O) signal and the annual maximum and minimum seawater temperatures demonstrated that C. gigas shells from several locations provided a reliable record of seasonal seawater temperature variation. The exception to this was oysters from the Netherlands and northern France where winter growth rates at low temperatures were slow so that insufficient shell was deposited to allow adequate spatial resolution of sampling and this resulted in time-averaging of the reconstructed seawater temperatures and an overestimation of winter seawater temperature. A potential variability in δ¹⁸O_w–salinity relationship at low salinities could also explain the high difference between measured and predicted seawater temperatures in Dutch areas. The finding that latitudinal differences in oyster hinge growth rates and/or changes in the δ¹⁸O_w–salinity relationship can result in bias of the seawater temperature deduced from the stable isotopic composition of the hinge should be taken into account when reconstructing latitudinal gradients in seawater temperature.     

Kinetics of ammonia oxidation by a marine nitrifying bacterium: Methane as a substrate analogue

In pure culture, the marine ammonia oxidizer,Nitrosococcus oceanus, exhibits normal Michaelis Menten kinetics with respect to its primary substrate, ammonia.N. oceanus also exhibits a kinetic response to methane. In the absence of methane, oxidation of ammonia is first order with respect to ammonia concentration under atmospheric oxygen concentrations at seawater pH. In the presence of methane, ammonia oxidation is inhibited, and the amount of inhibition is related to the relative concentrations of methane and ammonia. Using semicontinuous batch cultures as a source of organisms for short-term kinetic experiments, I investigated the relationship between ammonia and methane oxidation inN. oceanus by varying the absolute and relative concentration of both substrates. Methane appeared to act as a substrate analogue, and its effect on ammonia oxidation was modeled as a permutation of competitive inhibition involving a cooperative enzyme system. Methane was oxidized byN. oceanus, even in the absence of measurable ammonia oxidation, but the process was inhibited at increasing methane concentrations. Of the two product pools analyzed, an average of 37% of methane oxidized was detected in particulate (cell) material and the remainder was detected in¹⁴CO₂. The contribution of methane to total carbon assimilation varied with the ratio [CH₄]/[NH₃] and may be significant under substrate concentrations typical of a dilute aquatic environment.     

Inputs,losses and transformations of nitrogen and phosphorus in the pelagic North Atlantic Ocean

The North Atlantic Ocean receives the largest allochthonous supplies of nitrogen of any ocean basin because of the close proximity of industrialized nations. In this paper, we describe the major standing stocks, fluxes and transformations of nitrogen (N) and phosphorus (P) in the pelagic regions of the North Atlantic, as one part of a larger effort to understand the entire N and P budgets in the North Atlantic Ocean, its watersheds and overlying atmosphere. The primary focus is on nitrogen, however, we consider both nitrogen and phosphorus because of the close inter-relationship between the N and P cycles in the ocean. The oceanic standing stocks of N and P are orders of magnitude larger than the annual amount transported off continents or deposited from the atmosphere. Atmospheric deposition can have an impact on oceanic nitrogen cycling at locations near the coasts where atmospheric sources are large, or in the centers of the highly stratified gyres where little nitrate is supplied to the surface by vertical mixing of the ocean. All of the reactive nitrogen transported to the coasts in rivers is denitrified or buried in the estuaries or on the continental shelves and an oceanic source of nitrate of 0.7–0.95 × 10¹² moles NO ₃ ^–1 y^–1 is required to supply the remainder of the shelf denitrification (Nixon et al., this volume). The horizontal fluxes of nitrate caused by the ocean circulation are both large and uncertain. Even the sign of the transport across the equator is uncertain and this precludes a conclusion on whether the North Atlantic Ocean as a whole is a net source or sink of nitrate. We identify a source of nitrate of 3.7–6.4 × 10¹² moles NO ₃ ^– y^–1 within the main thermocline of the Sargasso Sea that we infer is caused by nitrogen fixation. This nitrate source may explain the nitrate divergence observed by Rintoul & Wunsch (1991) in the mid-latitude gyre. The magnitude of nitrogen fixation inferred from this nitrate source would exceed previous estimates of global nitrogen fixation. Nitrogen fixation requires substantial quantities of iron as a micro-nutrient and the calculated iron requirement is comparable to the rates supplied by the deposition of iron associated with Saharan dust. Interannual variability in dust inputs is large and could cause comparable signals in the nitrogen fixation rate. The balance of the fluxes across the basin boundaries suggest that the total stocks of nitrate and phosphate in the North Atlantic may be increasing on time-scales of centuries. Some of the imbalance is related to the inferred nitrogen fixation in the gyre and the atmospheric deposition of nitrogen, both of which may be influenced by human activities. However, the fluxes of dissolved organic nutrients are almost completely unknown and they have the potential to alter our perception of the overall mass balance of the North Atlantic Ocean.