N.J.P. Owens

The fate of nitrogen and phosphorus at the land-sea margin of the North Atlantic Ocean

Five large rivers that discharge on the western North Atlantic continental shelf carry about 45% of the nitrogen (N) and 70% of the phosphorus (P) that others estimate to be the total flux of these elements from the entire North Atlantic watershed, including North, Central and South America, Europe, and Northwest Africa. We estimate that 61 · 109 moles y−1 of N and 20 · 109 moles y−1 of P from the large rivers are buried with sediments in their deltas, and that an equal amount of N and P from the large rivers is lost to the shelf through burial of river sediments that are deposited directly on the continental slope. The effective transport of active N and P from land to the shelf through the very large rivers is thus reduced to 292 · 109 moles y−1 of N and 13 · 109 moles y−1 of P.The remaining riverine fluxes from land must pass through estuaries. An analysis of annual total N and total P budgets for various estuaries around the North Atlantic revealed that the net fractional transport of these nutrients through estuaries to the continental shelf is inversely correlated with the log mean residence time of water in the system. This is consistent with numerous observations of nutrient retention and loss in temperate lakes. Denitrification is the major process responsible for removing N in most estuaries, and the fraction of total N input that is denitrified appears to be directly proportional to the log mean water residence time. In general, we estimate that estuarine processes retain and remove 30–65% of the total N and 10–55% of the total P that would otherwise pass into the coastal ocean. The resulting transport through estuaries to the shelf amounts to 172–335 · 109 moles y−1 of N and 11–19 · 109 moles y−1 of P. These values are similar to the effective contribution from the large rivers that discharge directly on the shelf.For the North Atlantic shelf as a whole, N fluxes from major rivers and estuaries exceed atmospheric deposition by a factor of 3.5–4.7, but this varies widely among regions of the shelf. For example, on the U.S. Atlantic shelf and on the northwest European shelf, atmospheric deposition of N may exceed estuarine exports. Denitrification in shelf sediments exceeds the combined N input from land and atmosphere by a factor of 1.4–2.2. This deficit must be met by a flux of N from the deeper ocean. Burial of organic matter fixed on the shelf removes only a small fraction of the total N and P input (2–12% of N from land and atmosphere; 1–17% of P), but it may be a significant loss for P in the North Sea and some other regions. The removal of N and P in fisheries landings is very small. The gross exchange of N and P between the shelf and the open ocean is much larger than inputs from land and, for the North Atlantic shelf as a whole, it may be much larger than the N and P removed through denitrification, burial, and fisheries. Overall, the North Atlantic continental shelf appears to remove some 700–950· 109 moles of N each year from the deep ocean and to transport somewhere between 18 and 30 · 109 moles of P to the open sea. If the N and P associated with riverine sediments deposited on the continental slope are included in the total balance, the net flux of N to the shelf is reduced by 60 · 109 moles y−1 and the P flux to the ocean is increased by 20 · 109 moles y−1. These conclusions are quite tentative, however, because of large uncertainties in our estimates of some important terms in the shelf mass balance.

Natural Variations in 15N in the Marine Environment

Publisher Summary Since nitrogen is a key element in the biosphere, it was natural that as techniques became generally available for work with isotopes, those of nitrogen would be utilized. Approximately, a decade passed before nitrogen isotope techniques became fully established in marine research, but most studies have employed the stable isotope, rather than the radioactive isotopes. 13N is used currently in a small number of experiments but its short half-life restricts its use. 15N can be employed in two ways; first, as a tracer in which reactions involving nitrogen are followed after addition of quantities of 15N in various chemical forms. The second, and the subject of this review, is the investigation of the variations in the concentration of 15N in natural materials. The study of the natural variations in 15N can provide information in a wide variety of investigations, including biogeochemical, physical, physiological, and biochemical investigations. However, despite the wide variety of applications, variations in 15N are invariably employed as a natural tracer— that is, to track the sources and sinks of nitrogen under natural conditions. This chapter covers aspects of the natural variations in 15N in a wide range of subject areas, for example, dissolved gases in the deep oceans to the dietary relations of laboratory grown animals. It attempts to draw together many of the studies carried out in the marine environment.

Microzooplankton grazing and selectivity of phytoplankton in coastal waters

Microzooplankton grazing activity in the Celtic Sea and Carmarthen Bay in summer 1983 and autumn 1984 was investigated by applying a dilution technique to high-performance liquid chromatographic (HPLC) analysis of photosynthetic pigments in phytoplankton present within natural microplankton communities. Specific grazing rates on phytoplankton, as measured by the utilisation of chlorophyll a, were high and varied seasonally. In surface waters during the autumn, grazing varied between 0.4 d-1 in the bay and 1.0 d-1 in the Celtic Sea, indicating that 30 and 65% of the algal standing stocks, respectively, were grazed daily. Grazing rates by microzooplankton within the thermocline in summer suggest that 13 to 42% of the crop was grazed each day. Microzooplankton showed selection for algae containing chlorophyll b, in spite of a predominance of chlorophyll c within the phytoplankton community. Changes in taxon-specific carotenoids indicated strong selection for peridinin, lutein and alloxanthin and selection against fucoxanthin and diadinoxanthin. This indicates a trophic preference by microzooplankton for dinoflagellates, cryptophytes, chlorophytes and prasinophytes and selection against diatoms, even when the latter group forms the largest crop within the phytoplankton. Interestingly, those algal taxa preferentially grazed also showed the highest specific growth-rates, suggesting a dynamic feed-back between microzooplankton and phytoplankton. Conversion of grazing rates on each pigment into chlorophyll a equivalents suggests firstly, that in only one experiment could all the grazed chlorophyll a be accounted for by the attrition of other chlorophylls and carotenoids, and secondly that in spite of negative selection, a greater mass of diatoms could be grazed by microzooplankton than any other algal taxon. The former may be due either to a fundamental difference in the break-down rates of chlorophyll a compared to other pigments, or to cyanobacteria forming a significant food source for microzooplankton. In either case, chlorophyll a is considered to be a good measure of grazing activity by microzooplankton.

Methane in the southern North Sea: Low‐salinity inputs, estuarine removal, and atmospheric flux

Dissolved CH4was measured in coastal waters of the southern North Sea, in two adjacent U.K. estuaries with well-defined turbidity maxima (Humber and Tyne) and in their associated river catchments, during a series of campaigns covering the period 1993–1999. In general, samples from all three environments were significantly to highly CH4 enriched relative to atmospheric air. Observed river water concentrations, ∼ 33–152 nmol L−1 (940–4305% saturation) for the Humber river catchment and ∼ 3–62 nmol L−1 (86–1754% saturation) in the river Tyne, were within but toward the low end of the range of CH4 concentrations in river waters world wide. In sea waters from the outer Wash estuary (U.K. coast) and adjacent to the Dutch coast, CH4 was highly but nonlinearly correlated with salinity, consistent with strong CH4 removal from river and/or estuarine CH4 sources influencing these locations. In transects along the Humber and Tyne estuaries, CH4 was highly negatively nonconservative, confirming the estuarine removal hypothesis. For both estuaries, highest CH4 concentrations, ∼190–670 nmol L−1 (6000–21,000% saturation) in the Humber and ∼650 nmol L−1(21,800% saturation) in the Tyne, were observed at very low salinity in the vicinity of the turbidity maximum. Importantly, these concentrations greatly exceeded measured river water values, implying for both situations the existence of a large in situ CH4 supply associated with high turbidity. Time series measurements at two locations in the upper Tyne subsequently confirmed the strong correspondence of dissolved CH4 and turbidity in the vicinity of the turbidity maximum. CH4removal estimated for the Humber, Tyne, Wash, and Rhine-Scheldt estuaries was ∼ 90% of the low-salinity CH4 input. On the basis of this and river discharge data, −7.I×108 mol CH4 may be removed annually in estuaries bordering the southern North Sea. Of this, ∼6.6×108mol may be lost by air-sea exchange. This represents an additional atmospheric CH4 flux from the North Sea unaccounted for in previous work, which may have, consequently, underestimated this source by ∼50%. Upward scaling of this estimate based on the mean of reported river water CH4 concentrations implies a previously unaccounted for ∼6.3–24×109 mol (i.e., ∼ 0.1–0.4 × 1012 g) CH4 yr−1 which may be lost globally to gas exchange in estuaries, increasing previous such estimates by ∼8–50%. However, as it is based on data that exclude the possibility of elevated CH4 levels at estuarine turbidity maxima, even this revision is likely to be conservative. Detailed studies of CH4 distributions in major world estuaries would now be required in order to successfully reevaluate the CH4 budget of the coastal marine atmosphere.

Detection of novel marine methanotrophs using phylogenetic and functional gene probes after methane enrichment

A major limitation of rRNA-targeted group-specific probes is that they may cross-react with organisms of other physiological, or even phylogenetic groups when applied to environmental samples containing unknown sequences. We have exploited the restricted physiology of methane-oxidizing bacteria to assess the specificity and efficiency of probes for this physiological type which target the 16S rRNA or genes involved in methanotroph physiology. Seawater samples were enriched for methanotrophs by addition of methane and essential nutrients. The changes in composition of the bacterial population were monitored by analysis of 16S rRNA gene libraries. Methanotroph group-specific probes failed to give a signal with samples from these enrichments even though a methanol dehydrogenase structural gene was detected. A 16S rDNA sequence that was abundant only after methane addition was recovered and found to show a close phylogenetic relationship to Methylomonas. Organisms containing this sequence were observed in enrichments by in situ hybridization. The combination of enrichment on methane and screening with the broad specificity methanol dehydrogenase probe allowed detection of novel methanotrophs that were not detected with the original suite of methanotroph group-specific probes.

Nitrogen biogeochemical cycling in the northwestern Indian Ocean

Abstract The vertical distribution and fine scale structure of nitrate (NO 3 ), nitrite (NO 2 ), nitrous Oxide (N 2 O), phosphate (PO 4 ), oxygen (O 2 ) and chlorophyll α (chl α) were determined in the North Western Indian Ocean (NWIO) along a meridional section (67°E) from the Equator to the Gulf of Oman using an Autoanalyser for micromolar levels of nutrients, and chemiluminescence and gas chromatographic methods for nanomolar levels of NO 3 and NO 2 and N 2 O respectively. Three biogeochimically contrasting regimes were investigated: (1) the highly oligotrophic nutrient-depleted subtropical gyre; (2) the nonsoonal upwelling of nutrient-rich intermediate waters of the southeastern Arabian Coast; and (3) the denitrifying O 2 -depleted zone (ODZ; ca 150–1200 m depth) in the Arabian Sea. Concentrations of NO 3 and NO 2 were severely depleted in surface oligotrophic waters from the equator (average 43 and 3.6 nM respectively) to the subtropical gyre (12–15°N; average 13.3 and 2.0 nM respectively) with similar levels in the more stratified Gulf of Oman. Upwelling waters off Southern Arabia had three orders of magnitude higher NO 3 levels, and throughout the NWIO, the calculated NO 3 -fuelled primary production appeared to be regulated by NO 3 concentration. Existing Redfield ΔO 2 /ΔNO 3 regeneration ratios (=9.1) previously derived for the deep Indian Ocean were confirmed (= 9.35) within the oxic upper layers of the NWIO. The “NO”-potential temperature relationship ( Broecker , 1974 Earth and Planetary Science Letters , 23 , 100–107) needed for the derivation of expected NO 3 and NO 3 -deficits within the denitrifying ODZ were refined using an isopycnal, binary mixing model along the σ θ = 26.6%, density layer to take into account the inflowing contribution of NO 3 -depleted Persian Gulf Water. Vertically integrated NO 3 -deficits increased northwards from 0.8 mol NO 3 -N m −2 at Sta. 2 (04°N), up to 6.49 mol NO 3 -N m −2 at Sta. 9, at the mouth of the Gulf of Oman, then decreased to 4.10 moles NO 3 -N m −2 toward Sta. 11, near the Straits of Hormuz. When averaged for the denitrification area of the Arabian Sea, this corresponds to a deficit of 118 Tg NO 3 -N. Adopting a recent Freon-11 based estimate of water residence time of 10 years ( Olson et al. , 1993, Deep-Sea Research II , 40 , 673–685) for the O 2 -depleted layer, we calculate an annual net denitrification flux of 11.9 Tg N to the atmosphere or approximately 10% of the global water column denitrification rates. Supersaturated N 2 O concentrations were found in both surface oxic and upwelling waters (up to 246%) and peaked at the base of the ODZ (up to 1264%) in the northern Arabian Sea. Both nitrification in oxic waters and denitrification in hypoxic layers can be invoked as sources of N 2 O. The inventory of excess N 2 O amounted to 2.55 ± 1.3 Tg N 2 O-N, corresponding to annual production of 0.26 ± 0.13 Tg from denitrification. This is comparable to earlier ( Law and Owens , 1990, Nature , 346 , 826–828) estimates of the ventilation flux of N 2 O (0.22–0.39 Tg yr −1 ) from the upwelling region of the Arabian Sea. The decadal response times for circulation, deoxygenation, denitrification and ventilation of the ODZ-derived N 2 O and CO 2 greenhouse gases and their monsoonal coupling implies the Arabian Sea is a sensitive oceanic recorder of global climate change.

A simple and versatile micro-computer program for the determination of ‘most probable number’

Abstract A micro-computer programis described which will calculate the estimate of most probable number and an approximate confidence interval for any combination of dilution levels, numbers of replicates and sample volumes. The use of the program frees the experimenter from the constraints in experimental design often imposed by tables.

An assessment of nitrogen fixation as a source of nitrogen to the North Atlantic Ocean

The role of nitrogen fixation in the nitrogen cycle of the North Atlantic basin was re-evaluated because recent estimates had indicated a far higher rate than previous reports. Examination of the available data on nitrogen fixation rates and abundance ofTrichodesmium, the major nitrogen fixing organism, leads to the conclusion that rates might be as high as 1.09 × 1012 mol N yr−1. Several geochemical arguments are reviewed that each require a large nitrogen source that is consistent with nitrogen fixation, but the current data, although limited, do not support a sufficiently high rate. However, recent measurements of the fixation rates per colony are higher than the historical average, suggesting that improved methodology may require a re-evaluation through further measurements. The paucity of temporally resolved data on both rates and abundance for the major areal extent of the tropical Atlantic, where aeolian inputs of iron may foster high fixation rates, represents another major gap.

Determination of nitrogen-15 at sub-microgram levels of nitrogen using automated continuous-flow isotope ratio mass spectrometry

Recent developments in continuous-flow isotope ratio mass spectrometry have considerably enhanced the ability to make rapid and precise measurements of the stable isotope 15N. Although the amounts of nitrogen required for precise analyses have been reduced considerably, the levels required remain a limitation for many applications. A simple manipulation of the use of commercially available equipment is described that allows the simultaneous determination of nitrogen and 15N at levels as low as 10 nmol of total nitrogen.

Synechococcus And Its Importance To The Microbial Foodweb Of The Northwestern Indian-Ocean

Abstract The abundance, distribution, size, biomass, growth and grazing-induced mortality of phycoerythrin (PE) rich chrococcoid cyanobacteria were studied during September-October 1986 in the Arabian Sea, the Gulf of Oman and the monsoonal upwelling region off the South East Arabian coast. Cyanobacteria were abundant (>107 cells 1−1) through the region and particularly so (>108 cells 1−1) in oligotrophic waters where they exhbited distinct subsurface concentration maxima that were situated above, but related to the depth of the chlorophyll maxima. Cell diameter increased from 0.7 μm in surface waters to 1.2 μm at depth. Standing stocks of cyanobacteria ranged up to 50μgC 1−1, and accounted for up to 40% of the POC in oligotrophic stations indicating that Synechococcus constitutes an important trophic resource. Experimental investigations showed that cyanobacterial populations were growing fast, with specific growth rates of 0.5–1.0 day−1, while simultaneously experiencing high mortality due to microzooplankton grazing. Grazing rates varied between 0.3 and 1.2 day−1, indicating that 31–71% of the cyanobacteria were predated daily. Grazing and cyanobacterial growth were correlated, suggesting that Synechococcus production and its fate by microbial grazing activity were tightly coupled. Cyanobacteria are clearly a major component of a dynamic but well-balanced microbial foodweb present in oligotrophic regions of the northwest Indian Ocean.