Areal Oxygen Research Articles

Direct Oxidative Azo Coupling of Anilines Using a Self-Assembled Flower-like CuCo2O4 Material as a Catalyst under Aerobic Conditions.

Herein, we report the synthesis of a self-assembled flower-like CuCo2O4 material by the oxalate decomposition method. The crystalline structure and morphology of the material have been analyzed by powder X-ray diffraction, Raman spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray measurement techniques. The self-assembled flower-like CuCo2O4 material showed remarkable catalytic activity in the direct aerobic oxidative azo coupling of anilines under oxidant and other additive-free reaction conditions. The mechanistic insight of CuCo2O4 in the oxidative azo coupling reaction has been established by density functional theory calculations, which disclosed that the absorption and dissociation of areal oxygen preferentially take place at the Cu site and dissociation of aniline takes place at the Co site. Thus, the Cu and Co sites of CuCo2O4 exert a cooperative effect on the direct oxidative azo coupling reactions through the selective activation of anilines and aerobic oxygen. The CuCo2O4 material was recovered from the reaction mixture and reused for at least eight runs without appreciable loss of catalytic activity.

Oxygen consumption in seasonally stratified lakes decreases only below a marginal phosphorus threshold

Areal oxygen (O2) consumption in deeper layers of stratified lakes and reservoirs depends on the amount of settling organic matter. As phosphorus (P) limits primary production in most lakes, protective and remediation efforts often seek to reduce P input. However, lower P concentrations do not always lead to lower O2 consumption rates. This study used a large hydrochemical dataset to show that hypolimnetic O2 consumption rates in seasonally stratified European lakes remain consistently elevated within a narrow range (1.06 ± 0.08 g O2 m−2 d−1) as long as areal P supply (APS) exceeded 0.54 ± 0.06 g P m−2 during the productive season. APS consists of the sum of total P present in the productive top 15 m of the water column after winter mixing plus the load of total dissolved P imported during the stratified season, normalized to the lake area. Only when APS sank below this threshold, the areal hypolimnetic mineralization rate (AHM) decreased in proportion to APS. Sediment trap material showed increasing carbon:phosphorus (C:P) ratios in settling particulate matter when APS declined. This suggests that a decreasing P load results in lower P concentration but not necessarily in lower AHM rates because the phytoplankton community is able to maintain maximum biomass production by counteracting the decreasing P supply by a more efficient P utilization. In other words, in-lake organic matter production depends only on APS if the latter falls below the threshold of 0.54 g P m−2 and correspondingly, the atomic C:P ratio of the settling material exceeds ~155.

In situ, high-resolution time series of dissolved phosphate in Green Bay, Lake Michigan

Abstract In nearly every instance in which the environment has been sampled on a higher resolution in time or space, fundamental processes have come to light that were previously undetected or unobserved. In this study, an autonomous dissolved phosphate sensor was deployed at the Entrance Light station in lower Green Bay, Lake Michigan in 2012 and 2013. Hourly phosphate sensor measurements were compared with other real time sensor data to gain insights into the processes occurring at this site. Results showed that the water column at this location undergoes repeated stratification and turnover during the course of the summer. Often, the stratification results from intrusions of cold hypolimnetic bottom water from the north, while turnover is associated with significant northerly and/or easterly wind events. It was observed that, during calm periods, dissolved phosphate concentrations increased at a rate that was stoichiometrically consistent with the consumption of dissolved oxygen during the remineralization of organic matter; specifically, areal oxygen consumption rates ranged from 3.2 to 43 mmol m−2 d−1 and oxygen to phosphate ratios ranged from 120 to 210. At other times, the inverse relationship between dissolved oxygen and dissolved phosphate was not stoichiometrically linked; during these times, areal oxygen depletion rates ranged from 51 to 240 mmol m−2 d−1 and oxygen to phosphate ratios ranged from 260 to 2300. Future strategic deployment of multiple in situ dissolved phosphate and other nutrient sensors will enhance the understanding of nutrient cycling in this important aquatic system.

In situ analysis of oxygen consumption and diffusive transport in high‐temperature acidic iron‐oxide microbial mats

The role of dissolved oxygen as a principal electron acceptor for microbial metabolism was investigated within Fe(III)-oxide microbial mats that form in acidic geothermal springs of Yellowstone National Park (USA). Specific goals of the study were to measure and model dissolved oxygen profiles within high-temperature (65-75°C) acidic (pH = 2.7-3.8) Fe(III)-oxide microbial mats, and correlate the abundance of aerobic, iron-oxidizing Metallosphaera yellowstonensis organisms and mRNA gene expression levels to Fe(II)-oxidizing habitats shown to consume oxygen. In situ oxygen microprofiles were obtained perpendicular to the direction of convective flow across the aqueous phase/Fe(III)-oxide microbial mat interface using oxygen microsensors. Dissolved oxygen concentrations dropped from ∼ 50-60 μM in the bulk-fluid/mat surface to below detection (< 0.3 μM) at a depth of ∼ 700 μm (∼ 10% of the total mat depth). Net areal oxygen fluxes into the microbial mats were estimated to range from 1.4-1.6 × 10(-4) μmol cm(-2) s(-1) . Dimensionless parameters were used to model dissolved oxygen profiles and establish that mass transfer rates limit the oxygen consumption. A zone of higher dissolved oxygen at the mat surface promotes Fe(III)-oxide biomineralization, which was supported using molecular analysis of Metallosphaera yellowstonensis 16S rRNA gene copy numbers and mRNA expression of haem Cu oxidases (FoxA) associated with Fe(II)-oxidation.

Electrochemical performance of carbon gels with variable surface chemistry and physics

This study describes the electrochemical characterization of N-doped carbon xerogels in the form of microspheres and of carbon aerogels with varied porosities and surface oxygen complexes. The interfacial capacitance of N-doped carbon xerogels decreased with increased micropore surface area as determined by N2 adsorption at −196°C. The interfacial capacitance showed a good correlation with the areal NXPS concentration, and the best correlation with the areal concentration of pyrrolic or pyridonic nitrogen functionalities. The gravimetric capacitance decreased with greater xerogel microsphere diameter. The interfacial capacitance of carbon aerogels increased with higher percentage of porosity as determined from particle and true densities. The interfacial capacitance showed a linear relationship with the areal oxygen concentration and with the areal concentrations of CO- and CO2-evolving groups.

Empirical assessment of incorporating sediment quality triad data into a single index to distinguish dominant stressors between sites

Benthic infaunal community structure, sediment contamination, and sediment toxicity data (Sediment Quality Triad) were condensed into a single index based on the area of tri-axial plots, which were examined in relation to various habitat parameters. The purpose was to assess its utility for evaluating the relative impact of contaminants versus other stressors on benthic communities. The regression relationship between the areal index and the Effects Range-Median quotient (ERMq) was used to separate contaminant-impacted sites from sites impacted by hypoxia in Chesapeake Bay. Regression using the areal index and bottom oxygen confirm the utility of the approach. Data from Delaware, Galveston, and Biscayne Bays were also examined to determine if the approach may be effective in other estuaries.

Restoration of hypolimnetic dissolved oxygen through light irradiation‐induced periphyton production

AbstractLarge quantities of detritus can accumulate on the bottom sediments of eutrophic lakes, leading to depletion of dissolved oxygen (DO) in the hypolimnion. Decreased redox potential associated with low hypolimnetic DO concentrations can accelerate the remobilization of ammonia and phosphorus from the sediments back into the water column. As maintenance of aerobic conditions in the hypolimnion is an important consideration for successful lake management, this research focused on the use of periphyton production of DO to control hypolimnetic DO levels. To augment periphyton photosynthesis, light was irradiated to the bottom sediments via glass fibres, in order to examine the effects of light irradiation on DO restoration on bottom sediments in test chambers. Dissolved oxygen restoration by periphytic photosynthesis was confirmed in these experiments, with both temperature and light intensity having positive effects on the areal net oxygen production rate. The Q10 temperature coefficient for net oxygen production was estimated to be 2.24. In typical temperate‐zone lakes, the hypolimnetic water temperature is &lt;15 °C, resulting in an expected areal net oxygen production rate ranging from 100 to 200 mg O2 m−2 day−1 at a light irradiation of 50 μE m−2 s−1. Based on these experimental results, the condenser area required to achieve DO restoration under hypolimnetic light irradiation alone was estimated to be 3.2–4.1% of the hypolimnetic area. Thus, hypolimnetic light irradiation combined with hypolimnetic aeration can reduce energy consumption required for hypolimnetic aeration in DO restoration efforts.

Oxygen depletion in lake hypolimnia

The Livingstone and Imboden (1996) model for the oxygen depletion rate in lake hypolimnia was evaluated with field results in 32 lakes. The volumetric oxygen consumption rate (JV) was strongly related to lake trophic state, as represented by the growing season mean chlorophyll a (Chl a) and annual mean total phosphorus concentration, and might reach a maximum of 0.23‐0.24 g O2 m−3 d−1 in eutrophic lakes. The areal oxygen consumption rate (JA) was not related to trophic state and was lower than the value estimated by the sediment core method, particularly in eutrophic lakes; the mean was 0.0816 6 0.0150 g O2 m−2 whole sediment d−1. We modified the model and it was used to confirm that the field JA, the average for the hypolimnion for the deoxygenation period, is influenced by JV and the range of sediment area to water volume ratio, α(Z) (Z is water depth), in the hypolimnion, as well as the oxygen consumption rate in whole sediment (R). It was suggested that this is the reason for the difference between field and sediment core JA values. The R in 10 lakes varied from 7 to 460 g O2 m−3 whole sediment d−1. Overall, the Livingstone and Imboden model with constant JA and with a JV value derived from an empirical relationship with either measure of trophic state was considered to be a suitable condensed model for oxygen depletion in lake hypolimnia.

Bioturbation and bioirrigation extend the open exchange regions in permeable sediments

Large‐scale experimental exclusion of lugworms (Arenicola marina) from 400 m2 intertidal fine sand revealed significant effects of their bioturbation and bioirrigation on sediment characteristics, benthic infauna composition, and the dominant mineralization and benthic‐pelagic exchange processes in the sediment. Absence of lugworms resulted in sediment clogging with organic‐rich fine particles, an eightfold decrease in sediment permeability, low oxygen penetration depths, and accumulation of reduced mineralization products in the pore water. The sand flat inhabited by lugworms had low fine‐particle and chlorophyll contents and low sulfide and nutrient concentrations in the pore water. The effects were not limited to the vicinity of lugworm burrows but extended throughout the entire sediment down to ~20‐cm depth. Sediments with the lugworm shared the characteristic of low‐organic, advection‐driven permeable sand rather than of muddy, diffusion‐dominated sediments in the absence of lugworms. Areal oxygen uptake rates depended strongly on hydrodynamic conditions: under calm conditions, sedimentary oxygen uptake was slightly higher at the exclusion site. Experimental flushing using controlled hydrodynamic conditions showed that oxygen uptake at the lugworm site would be higher during more dynamic conditions (e.g., storms) due to significantly deeper oxygen penetration permitted by higher sediment permeability. Our results indicate an interactive effect of bioturbating organisms and hydrodynamics on water‐sediment exchange processes and highlight the importance of benthic infauna for sedimentary processes even in physically dominated systems.

Surficial and deep pore water circulation governs spatial and temporal scales of nutrient recycling in intertidal sand flat sediment

This study addresses organic matter decomposition in permeable sediment of a sloping intertidal sand flat (German Wadden Sea) affected by current-induced pore water exchange and pore fluid drainage. Seasonal and spatial scales of aerobic and anaerobic mineralization were investigated at 2 sites, one near the water line and one on the upper flat. Hydrodynamic forcing during inundation caused deeper oxygen penetration through flushing of the uppermost sediment layer. This flushing resulted in higher areal oxygen consumption rates and lower depth integrated sulfate reduction rates in the submerged flat compared to the rates measured during exposure. Mineralization rates in the top 15 cm of the sediment were similar between both study sites and ranged from 38 (winter) to 280 mmol C m–2 d–1 (summer), with sulfate reduction contributing 3 to 25% to total mineralization, depending on the season. At the upper flat, these seasonal differences were reflected in the pore water concentrations of nutrients, dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC). Near the low water line, however, pore water nutrient and DIC concentrations were independent of the season and up to 15 times higher compared to the values recorded in the upper flat. The differences in concentrations of metabolic products between the 2 sites resulted from a low tide drainage extending deep below the uppermost flushed layer and causing seepage of pore water near the low water line. Mineralization and nutrient release in these permeable intertidal sediments is affected by 2 circulation processes that work on distinctly different temporal and spatial scales: (1) rapid ‘skin circulation’ through the uppermost sediment layer during inundation that is characterized by short flow paths, low pore water residence time and immediate feedback to the ecosystem, and (2) slow ‘body circulation’ through deeper sediment layers during low tide that is characterized by long flow paths and pore water residence times, and acts as a buffered nutrient source to the ecosystem.

Effect of temperature on photosynthesis, oxygen consumption and sulfide production in an extremely hypersaline cyanobacterial mat

We used microsensors to study short-term temperature effects on gross photosynthesis, oxygen consumption and sulfate reduction in an extremely hypersaline (salinity 200‰) cyanobac- terial mat. In situ, the mat surface can reach high temperatures (ca. 50°C in summer), and experi- ences daily temperature fluctuations due to tidal inundation. A mat sample was incubated in the lab- oratory at 25, 35, 45, 50 and 60°C. Areal rates of gross and net photosynthesis increased with increasing temperature, and maximum rates were detected at 45°C. The photosynthetic zone (i.e. the depth zone in which oxygenic photosynthesis was detectable by the light-dark shift method) decreased from 3 to 1.75 mm with increasing temperature. Above 50°C, photosynthesis was com- pletely inhibited, probably due to high sulfide concentrations. This inhibition was confirmed by the pH profiles, where the peak in the top 2 to 4 mm at 25 to 45°C disappeared at 50 to 60°C. Between 25 and 45°C, areal oxygen consumption rates in the light did not change significantly but showed a decreasing trend in the photosynthetic zone, suggesting a decoupling between photosynthesis and oxygen consumption in the light. In both the light and dark, sulfide production was strongly en- hanced by increasing temperature to a maximum flux at 50°C, above which it became inhibited. At 50 to 60°C, the sulfide produced could not be completely removed by sulfide oxidation, resulting in sulfide leakage to the overlying water. Our data demonstrate that these mat ecosystems are well adapted to high ambient temperatures. Photosynthesis, oxygen consumption in the light and sulfide production are differentially controlled by temperature. However, the individual processes in this complex community are not only directly controlled by temperature, but also by the temperature- dependent response of other processes.

The responses of photosynthesis and oxygen consumption to short-term changes in temperature and irradiance in a cyanobacterial mat (Ebro Delta, Spain).

We have evaluated the effects of short-term changes in incident irradiance and temperature on oxygenic photosynthesis and oxygen consumption in a hypersaline cyanobacterial mat from the Ebro Delta, Spain, in which Microcoleus chthonoplastes was the dominant phototrophic organism. The mat was incubated in the laboratory at 15, 20, 25 and 30 degrees C at incident irradiances ranging from 0 to 1,000 micromol photons m(-2) s(-1). Oxygen microsensors were used to measure steady-state oxygen profiles and the rates of gross photosynthesis, which allowed the calculation of areal gross photosynthesis, areal net oxygen production, and oxygen consumption in the aphotic layer of the mat. The lowest surface irradiance that resulted in detectable rates of gross photosynthesis increased with increasing temperature from 50 micromol photons m(-2) s(-1) at 15 degrees C to 500 micromol photons m(-2) s(-1) at 30 degrees C. These threshold irradiances were also apparent from the areal rates of net oxygen production and point to the shift of M. chthonoplastes from anoxygenic to oxygenic photosynthesis and stimulation of sulphide production and oxidation rates at elevated temperatures. The rate of net oxygen production per unit area of mat at maximum irradiance, J0, did not change with temperature, whereas, JZphot, the flux of oxygen across the lower boundary of the euphotic zone increased linearly with temperature. The rate of oxygen consumption per volume of aphotic mat increased with temperature. This increase occurred in darkness, but was strongly enhanced at high irradiances, probably as a consequence of increased rates of photosynthate exudation, stimulating respiratory processes in the mat. The compensation irradiance (Ec) marking the change of the mat from a heterotrophic to an autotrophic community, increased exponentially in this range of temperatures.

Light-enhanced oxygen respiration in benthic phototrophic communities

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 139:193-203 (1996) - doi:10.3354/meps139193 Light-enhanced oxygen respiration in benthic phototrophic communities Epping EHG, Jørgensen BB Two microelectrode studies demonstrate the effect of light intensity and photosynthesis on areal oxygen respiration in a hypersaline mat at Guerrero Negro, Mexico, and in an intertidal sediment at Texel, The Netherlands. The hypersaline mat was studied in the laboratory at light intensities of 0, 10, 25, 50, 100, 200, and 500 µE m-2 s-1 using the light-dark shift technique to measure gross photosynthesis rates. Areal gross photosynthesis increased from 0 to 31.3 nmol O2 cm-2 min-1 and areal net photosynthesis increased from -3.9 to 16.7 nmol O2 cm-2 min-1 with increasing light intensities. Areal respiration, calculated from the difference between areal gross and areal net photosynthesis, increased from 3.9 to 14.4 nmol O2 cm-2 min-1 with increasing surface irradiance. This light-enhanced areal respiration was related to an increase in oxygen penetration depth from 0.2 to 2.0 mm, thus expanding the volume of sediment involved in oxygen respiration beneath the mat surface. The mean rate of oxygen respiration per volume of mat remained constant at a rate of ~100 nmol O2 cm-3 min-1. Oxygen profiles for the intertidal sediment were recorded in situ during the day at prevailing light intensities. A 1-dimensional diffusion-reaction model was used to estimate gross photosynthesis and oxygen respiration per volume of sediment, as well as the euphotic depth and the sediment-water interface concentration of oxygen. Areal gross photosynthesis ranged from 9.7 to 26.3 nmol O2 cm-2 min-1 and areal net photosynthesis ranged from 1.1 to 8.1 nmol O2 cm-2 min-1, both showing maximum values in the early afternoon. Areal respiration ranged from 8.1 to 18.2 nmol O2 cm-2 min-1 and showed a good positive linear correlation with areal gross photosynthesis (R = 0.97, n = 9). The enhanced areal respiration in the intertidal sediment was due to an increase in mean respiration per volume of sediment from 18.5 to 52 nmol O2 cm-3 min-1. The oxygen penetration depth only varied between 3.5 and 4.3 mm. Both studies thus indicate that areal respiration is enhanced significantly with increasing light intensities due to increased areal gross photosynthesis. By assuming that areal respiration in darkness is equal to areal respiration in the light, bell jar incubations would have underestimated areal respiration in the light 3.7-fold and areal gross photosynthesis 1.5-fold for the hypersaline mat and 10.7- and 2.6-fold for the intertidal sediment. Microbial mat · Microelectrode · Photosynthesis · Respiration · Bell jar Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 139. Publication date: August 29, 1996 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1996 Inter-Research.

Trophic State and Hypolimnetic Nitrogen Metabolism of Lake Hibara.

This is the first attempt to elucidate the seasonal changes in some physical and chemical parameters in a water column of Lake Hibara. The observation extended from spring to early winter of 1991.In the hypolimnion, nitrification started between 13 June and 10 July. The nitrification rate in this period was 0.21-0.44μg atom N⋅1-1⋅day-1. The dissolved oxygen (DO) level of about 0.6-0.7 mg⋅1-1 appeared critical in impeding efficient nitrification. The gross ammonia production rate was estimated to be 0.05-0.15 μg atom N⋅1-1⋅day-1. Denitrification seems to have taken place in the lower hypolimnion and/or on the surface of the bottom sediment when DO decreased to 2-3%(0.2-0.3 mg⋅1-1). The levels of TN, TP, Chl. a and transparency in the autumn circulation period indicate that the lake is on the boundary between oligo- and mesotrophic, though the epilimnetic water in summer was oligotrophic. The relative areal oxygen deficit in the hypolimnion (0.055-0.061mg⋅cm-2⋅day-1) was at the lower limit of eutrophy. The question of why, in spite of the lake's overlying oligotrophic epilimnion, the hypolimnetic DO consumption rate was at a eutrophic level remained to be solved. The hypolimnetic DO consumption rate seems to have been accelerating for the last 20 years. To elucidate the extent of the fluctuation of the hypolimnetic processes from year to year is important for future study.

Effects of oxygen transport on the areal hypolimnetic oxygen deficit

Using a modified flux gradient technique, transmetalimnetic oxygen transport was evaluated in Lakes Mendota and Delavan, two eutrophic Wisconsin basins featuring moderate depth (17 &lt; zmax &lt; 25 m), good wind exposure, and classic clinograde oxygen profiles. The biweekly fluxes intensified between mid‐May and late July 1972, mainly on account of seasonal steepening of the metalimnetic O2 gradients. Fluxes also increased sharply during windy intervals characterized by accelerated transmetalimnetic heat fluxes. The fluxes across the lower epilimnetic boundary exceed those across the lower metalimnetic boundary; this differential is consistent with the metalimnion's role as an oxidation zone in these turbid lakes. The vertical fluxes are a large but variable (over time) fraction of the net areal oxygen deficit in both lakes, thus complicating statistical analysis of seasonal and intergenerational trends in lake productivity using the areal hypolimnetic oxygen deficit method. Even after correcting for the effects of O2 transport, the deficit in Lake Mendota exceeds that in Delavan following the onset of hypolimnetic anoxia in the shallower lake. This difference likely results, in part, from transmetalimnetic fluxes of reducing substances (CH4, NH3, etc.) into the epilimnion.

Hypolimnetic Oxygen Consumption in Small Lakes

The hypolimnetic oxygen consumption rates of 28 Ontario lakes were calculated and compared with lake morphology, chemical concentrations, and productivity measures. In most cases, hypolimnia had upper zones where average light intensities were greater than 1% of surface light. In these layers, oxygen dynamics were highly variable from year to year and production rather than consumption was common. The ratio of areal oxygen consumption below the 1% light level to planktonic production corrected for retention was studied as a measure of percent available material decomposed. Using stepwise multiple regressions, we found this parameter to be related to mean thickness of the hypolimnion, lake organic carbon, and iron concentrations. The range of lake productivities in the data set was small and did not explain a significant portion of the variance in areal hypolimnetic depletion rates.

The contribution of nitrification in the water column and profundal sediments to the total oxygen deficit of the hypolimnion of a mesotrophic lake (Grasmere, English Lake District)

Estimates ofin situ nitrifying activity have been made in the hypolimnetic water column and surface 1.0 cm of profundal sediments at 2 sites in Grasmere, a mesotrophic lake in the English Lake District. Increases of nitrate concentrations were used to estimate nitrification in the water column whereas a mini-core technique, involving the use of a nitrification inhibitor (allylthiourea), was used to estimate the rate in surface sediments. The pattern of oxygen depletion in the water column was used to estimate the maximum depth to which sediments affect the overlying water. Nitrification in the sediment and in the water column made approximately equal contributions to the total areal oxygen deficit and, as a whole, nitrification accounted for 15-20% of the total oxygen depletion. There was no significant difference in oxygen depletion due to nitrification between the 2 sites. Attempts were made, using the nitrification potential technique, to determine the depth distribution of nitrifying activity in the surface 1.0 cm of sediment.

Photosynthetic activity and respiration in an equatorial African soda lake

SUMMARY. Photosynthetic activity and respiration in Lake Sonachi (Kenya), a meromictic soda lake lying in a volcanic crater, were measured through diel cycles during a 15‐month period. A pattern of thermal stratification in the morning and mixing in the afternoon and night occurred in the mixolimnion. Diel variations in dissolved oxygen at 50 cm were 2.2–7.5 mgO2 1‐11% of the incident photosynthetically available irradiance (PAR) reached a depth of 1.3–2.4 m and, as a consequence, the steepest thermal gradients and highest oxygen concentrations occurred in the top 1–2 m.Vertical profiles of dissolved oxygen were used in three ways to estimate photosynthetic and respiration rates. Changes in dissolved oxygen at the depth of maximal photosynthesis (c. 50 cm) during mid‐morning were corrected for vertical diffusion to determine net free water oxygen increases of 70‐1800 mg O2 m‐3 h‐1 Variations in areal oxygen content at successive intervals throughout the day and night were corrected for air‐water oxygen exchange to calculate net free water oxygen change per h. Maximal rates of increase (550–4850 mg O2 m‐2 h‐1) usually occurred in late morning or early afternoon; maximal rates of decrease (440–2600 mg O2 m‐2 h‐1) were common at sunset. The correction for air‐water exchange was usually small because of the low wind speeds and the nearness to saturation of the surface water. Summation of daytime and night‐time rates of oxygen change provide estimates of net (‐3.4–12 gO2 m ‐2) and gross (‐0.7‐18.7 g O2 m‐2) daily photosynthesis and respiration (0.8‐7.2 gO2 m‐2).Photosynthetic rates of bottled samples ranged from 150 to 870 mgO2m ‐2h ‐1 and 1.4 to 6.8 g O2, m‐2 day ‐1The efficiency of utilization of PAR incident on the lake surface varied from 1.0 to 7.2 mmol O2E‐1 periods with higher irradiance typically had lower efficiencies.Free water estimates of photosynthesis usually exceeded the rates measured in bottles. For example, net, free water changes per hour were 1.2–10 times higher than gross areal rates per hour in bottles.Photosynthetic activity in Lake Sonachi in 1973 and 1974 was modest when compared to other tropical African soda lakes.

Quantification of the Lake Trophic Typologies of Naumann (Surface Quality) and Thienemann (Oxygen) with Special Reference to the Great Lakes

Separate trophic scales and indices are developed for two of the most significant symptoms of eutrophication: surface water quality and hypolimnetic dissolved oxygen depletion. The scales are made comparable by expressing them in dimensionless form with a lower bound of zero and a mesotrophic range from 5 to 10. In this way, the two symptoms can be compared and their relative importance judged. This is done for the Great Lakes with the result that for both scales Lakes Superior, Huron, and Michigan are classified as oligotrophic. However, while central and eastern Lake Erie and Lake Ontario are classified as mesotrophic in terms of surface water quality, they range from eutrophic (central Lake Erie) to oligotrophic (Lake Ontario) on the oxygen scale. This is because, although these lakes are similar in surface water quality, their hypolimnion thicknesses range from approximately 4 m for central Erie to 70 m for Lake Ontario. Because of its shallowness, western Lake Erie does not have a persistent oxygen problem. In terms of surface quality it is classified as eutrophic. We have attempted to relate the two scales by correlating surface primary production and areal depletion rate. The results indicate that for lakes of similar primary production, areal oxygen depletion is directly proportional to hypolimnion thickness.

Hypolimnetic Oxygen Deficits: Their Prediction and Interpretation

Rates of hypolimnetic oxygen depletion can be predictedfrom a knowledge of a lake's phosphorus retention, the average hypolimnetic temperature, and the mean thickness of the hypolimnion. Areal oxygen deficits cannot be used to index lake trophic status because areal calculations do not eliminate the influence of hypo-limnetic morphometry.