Francis A. Richards

A model for nitrate distributions in oceanic oxygen minimum zones

Abstract The vertical distributions of nitrate and nitrate deficit (nitrate consumed during denitrification) in oxygen minimum zones are modeled using a recycling mechanism incorporating bacterially mediated reaction and diffusion. At the core of the oxygen minimum zone bacteria reduce nitrate to nitrite and part of the nitrte to molecular nitrogen (denitrification). The remaining nitrite diffuses out of the layer, is oxidized to nitrate by nitrifying bacteria, and diffuses back into the layer to complete the cycle. The peak nitrite and nitrate deficit concentrations and the amount of recycling depend on two parameters: θ, the ratio of the sum of the nitrate and nitrate reduction rates to the diffusion coefficient, and λ, the ratio of the nitrate reduction rate to the sum of the nitrate and nitrite reduction rates. We estimate from peak concentrations that for oxygen minimum zones in the Arabian Sea, the eastern tropical North Pacific, and the coastal waters of Peru that the nitrogen production rate is between 39 and 60% of the nitrate reduction rate, with the difference in rates equaling the recycling rate between the denitrifying and nitrifying layers. A dependency of θ and λ on organic substrates available to denitrifying bacteria is suggested from independent chemostat studies and the primary productivity overlying the oxygen minimum zones. The peak concentrations of nitrite and nitrate deficit are near the mid-depth of the denitrifying layer, and from this characteristic we estimate the thickness of the denitrifying layers to be between 30 and 70% of the thickness of the oxygen minimum zones.

The occurrence and distribution of methane in the marine environment

The distributions of methane dissolved in the sulfide-bearing waters of the Black Sea, the Cariaco Trench, and Lake Nitinat are reported. The gas was not detected in the oxygen-deficient water of the eastern tropical Pacific Ocean, in the Santa Barbara Basin, nor in the sulfide-bearing water of Saanich Inlet. Maximum concentrations observed were 70 μmole CH4/l., and the distribution tends to follow that of sulfide. The data indicate that methane is derived from organic compounds not containing nitrogen or phosphorus and that its formation is much slower than that of sulfide. The solubility coefficient in seawater (salinity = 40‰) is 0.0211. (S.T.P.)/l. at 30°C and increases to 0.041 at 5°C. The processes by which methane may be formed in the marine environment are discussed.

Distributions of dissolved oxygen, density, and nutrients off the Washington and Oregon coasts

Abstract The distributions of dissolved oxygen, density, and nutrients off the Washington and Oregon coasts are complicated by upwelling, entrainment of deep water, the presence of Columbia River water, biological activity, in-situ temperature changes, and possibly by anomalous surface exchanges associated with wave action. In much of the area there is a good statistical correlation between the dissolved oxygen and the density, except in the upper layers of the coastal region, where large negative oxygen anomalies were found. These anomalies were used to evaluate changes in the oxygen content resulting from biological activity and mixing associated with upwelling. An offshore, downward advection of water bearing excesses of oxygen photosynthetically produced in the surface layers near shore, is proposed to explain an oxygen maximum that occurs subsequent to the oxygens production and at depths that increase with distance from shore. Seasonal variations in this maximum are explained in terms of the balance between the rates of oxygen addition by photosynthesis and the oxygen flux associated with upwelling during the spring and summer, and diffusional erosion of the maximum during periods of more modest photosynthesis. Simultaneous observations of oxygen and nutrients can yield useful information regarding the biological activity in the surface waters. A first approximation at evaluating the biological changes in the oxygen can be made by assuming that a linear relationship exists between it and changes in nutrient concentrations. Since the nutrients will not be appreciably exchanged with the atmosphere, departures from this linear relationship are assumed to result from the effect of in-situ temperature changes on calculated oxygen solubilities and the escape of oxygen to the atmosphere. In areas of intense photosynthesis these departures are small, suggesting that in such regions oxygen production outstrips the escape rate. Under such conditions a good correlation is also found between the carbon-14 assimilation and the apparent oxygen production. On the other hand, low oxygen concentrations, compared to saturation values, coinciding with low nutrients indicate more modest photosynthesis.

Oxygen supersaturations in the Chukchi and East Siberian seas

Abstract Unusually high oxygen concentrations (sometimes greater than 150% of saturation values) are often observed in the Chukchi and East Siberian seas between Bering Strait and approximately 170°E. These high concentrations appear to arise primarily from in situ photosynthetic production. The highest concentrations were observed in stratified waters that probably left the surface in winter when the water was nearly saturated with air and contained high nutrient concentrations. These highly oxygenated strata generally were separated from the surface by shallow pycnoclines. High oxygen concentrations have also been observed in the relatively warm waters of the Bering Strait inflow. Nutrient enrichment of the surface layers by turbulence in the Strait may contribute to these high concentrations.

A note on the sources of excess alkalinity in anoxic waters

Abstract A model explaining the origin of the excess alkalinity of anoxic waters was tested in Lake Nitinat, a fjord in British Columbia, Canada. The excess alkalinity was nearly all accounted for by the formation of sulfide ions and the solution of alkaline earth metal carbonates. Calcium, magnesium, and strontium concentrations and chlorinity ratios are given.

Dissolved organic matter in an anoxic fjord, with special reference to the presence of mercaptans☆

Abstract Petroleum ether and ethyl acetate extraction of acidified sea water recovered some 40–60% of the dissolved organic carbon. Anoxic waters yielded 1·8–1·9 mg/l. of dissolved organic matter and oxygen-bearing water, 1·0–1·2 mg/l. Most of the polar organic constituents detected in the oxygen-bearing water were absent from the anoxic zone, and the carbon content of the extracted matter increased markedly with depth in the anoxic zone. On the basis of chromatographic behavior and spot tests the following classes of compounds were tentatively identified : sterols or sterol esters, choline-containing lipids, and ninhydrin-positive compounds. These were found in both oxygen-bearing and anoxic waters, while nonpolar and polar nonvolatile mercaptans were detected in the anoxic water.