Low Oxygen Zone Research Articles

Bacterial transformations of inorganic nitrogen in the oxygen-deficient waters of the Eastern Tropical South Pacific Ocean

Rates of transformations of inorganic nitrogen were measured in the low oxygen, subsurface waters (50–450 m) of the Eastern Tropical South Pacific during February 1985, using 15N tracer techniques. Oxygen concentrations over the entire region were in a range (O 2 < 2.5 μM) that allowed both oxidation and reduction of nitrogen to occur. A wide range of rates was observed for the lowest oxygen levels, indicating that observed oxygen concentration was not a primary factor regulating nitrogen metabolism. High values for subsurface metabolic rates correspond with high levels for surface primary production, both apparently associated with mesoscale features observed in satellite imagery and with mesoscale features of the current field. Measured rates of nitrate reduction and estimated rates of denitrification were sufficient to respire nearly all of the surface primary production that might be transported into the oxygen deficient zone. These results imply that the supply of labile organic material, especially from the surface, was more important than oxygen concentration in modulating the rates of nitrogen transformations within the low oxygen water mass of the Eastern Tropical South Pacific. The pattern of nitrite oxidation and nitrite reduction activities in the oxygen minimum zone supports the hypothesis ( Anderson et al., 1982, Deep-Sea Research, 29, 1113–1140) that nitrite, produced from nitrate reduction, can be recycled by oxidation at the interface between low and high oxygen waters. Rates for denitrification, estimated from nitrate reduction rates, were in harmony with previous estimates based on electron transport system (ETS) measurements and analysis of the nitrate deficit and water residence times. Assimilation rates of NH 4 + were substantial, providing evidence for heterotrophic bacterial growth in low oxygen waters. Ambient concentrations of ammonium were maintained at low values primarily by assimilation; ammonium oxidation was an important mechanism at the surface boundary of the low oxygen zone.

Algal and bacterial hydrocarbons in particulate matter and interfacial sediment of the Cariaco Trench

particulate matter samples (<53 μm and > 53 μm size fractions) and sediment from the sediment-water interface were collected in the Cariaco Trench, an anoxic marine basin on the continental shelf of Venezuela, and analyzed for hydrocarbons. Concentrations of hydrocarbons were highest (10–20 ng/1) in oxic surface waters (0–150 m) and at the top of the anoxic zone (350–600 m), and were low (3–4 ng/l) in the low oxygen zone above the oxic-anoxic interface (150–250 m) and anoxic bottom waters (600–1150 m). Branched C 25-trienes and -tetraenes dominated the hydrocarbon distributions in <53 μm particles in the oxic zone, while the isoprenoid C 25- and C 40-alkanes, 2,6,10,15,19-pentamethyleicosane and lycopane, dominated in the <53 μm particles in the anoxic waters; petrogenic C 17–C 40 n- alkanes dominated the > 53 μm particles. Particle size and depth distributions of the hydrocarbons demonstrate the importance of algal hydrocarbons in surface waters, coupled with increased abundances of bacterial hydrocarbons, especially in the <53 μm particles, in the anoxic zone. These observations illustrate the importance of oxic-anoxic interfaces, even in the oceanic water column, as zones of intense microbial alteration of organic matter and as sources of some of the organic compounds found in sediments.

Significance of low-oxygen zone for nitrogen cycling in a freshwater lake: Production of N2O by simultaneous denitrification and nitrification.

Denitrifying and nitrifying activities were determined to elucidate the mechanism of N2O accumulation in the oxygen depleted zone of Lake Kizaki. Both activities were detected without any substrate enrichments in a zone of low-oxygen content (0.03-0.14 ml·-1), where N2O and NO2-were accumulated. The determined nitrifying activity approximately coincided with the NH4+ utilization estimated from the NH4+ budget in this zone during the observation interval. On the basis of these activities and the change in N2O concentation, it was suggested that the observed N2O accumulation had dual origins of nitrification and denitirification and that these productions were in a dynamic equilibrium with further reduction of N2O to N2. The low-oxygen zone was found to be an active site for nitrogen cycling and was characterized by the presence of dissolved Mn and the absences of dissolved Fe and sulfide.

The oceanic non-sulfidic oxygen minimum zone: a habitat for graptolites?

The denitrified low oxygen zone in Early Paleozoic oceans is proposed as a potential habitat of planktic graptolites. Modern analogs of this zone are found in the eastern tropical Pacific (ETP) and in the north­ern Arabian Sea as shallow regions, up to a 100 meters thick, at the top of the pycnocline. There, oxygen is low or undetected and hydrogen sulfide has not been found. In modem oceans, denitrification regions are limited vertically and horizontally as oxygen is replenished from below by ventilated deep waters. In the Early Paleozoic ocean, the denitrification layer would be global due to poor deep ventilation. It would be transitional between oxygenated surface waters and toxic sulfide-rich water. Many branched graptolites could have evolved when the denitrified waters were in or close to the photic zone, feeding on the abun­dant phytoplankton attracted to both light and nutrients. As this zone sank below the photic zone, grapto­lites who developed planktic mode of life could have migrated daily toward the food supply, similar to euphausiids in the modern ETP. Thus the changes in graptolite rhabdosomes from pendent to scandent and from many branched to biserial and uniserial are suggested as adaptations to assist vertical migration and feeding. With the continued ventilation of the oceans and the shrinking of the denitrified layer, grap­tolite extinction could have resulted as a combination of reduction in food supply and living space, in­creased predation from the then evolving fish and ammonites, and competition in the zoop!ankton niche from smaller (less visible) and more motile forms.

The Reactions of Fish to Water of Low Oxygen Concentration

ABSTRACT This paper presents the results of about 100 experiments to test the reactions of small fish to water of low oxygen concentration, using an apparatus similar to that previously employed by the writer to study the reactions of fish to toxic solutions. At 13 ° C. the stickleback (Gasterosteos acoleatos) has no immediate appreciation of water of low oxygen concentration and will swim into it with no hesitation. If the fish remains in the poorly oxygenated water respiratory distress develops, accompanied by uneasiness and random movement. When this random movement takes the animal back into the well-oxygenated water the stimulus to swimming vanishes, the fish stops moving and quickly recovers. Over a wide concentration range at 13 ° C. the reaction takes 1 · 3 min. At 3 ° C. the reaction is very slow, taking 5-6 min. even when the water contains only 0 · 3 mg./l. of oxygen. At 20 ° C. and oxygen concentrations below 2 mg./l. the response is very prompt, respiratory distress developing so quickly that the fish usually will not swim into the poorly oxygenated water, but turns away from it or swims backwards; it may make repeated attempts to enter the low-oxygen zone, retreating each time as if violently irritated. Experiments with minnows (Phoxinus phoxinus’) and trout fry (Salmo truttà) gave generally similar results. Here again the essential factor appears to be the speed with which dyspnoea develops.