Conventional Ponds Research Articles

Comparative study of the effect of depth and season on the performance of the reverse-side absorber-plate shallow solar pond in the Nsukka climate

The ‘reverse-side absorber-plate shallow solar pond’ (RASSP), consisting of a semi-cylindrical reflector attached to the exposed base of a conventional shallow solar pond, has been conceived in two variants, viz., one with double glass covers and the other with insulation over the top. The thermal performances of these RASSPs have also been studied analytically to determine the effect of depth variation and time of the year on the maximum water temperatures and overall daily efficiencies they are able to attain. The results of the study, here presented, show that attainable water temperatures decrease while overall daily efficiencies increase as pond depth increases. And that, for a fixed pond depth, the daily peak water temperatures attainable are strongly dependent on the time of the year, while overall daily efficiencies are not. For all seasons and depths, the RASSP with double glass covers is predicted to consistently deliver higher water temperatures but lower overall daily efficiencies compared to the RASSP with top insulation.

Design and operation of an eco-system for municipal wastewater treatment and utilization

An eco-system consisting of integrated ponds and constructed wetland systems is employed in Dongying City, Shandong Province for the treatment and utilization of municipal wastewater with design capacity of 100,000 m(3)/d. The total capital cost of this system is 680 Yuan (RMB) or US dollars 82/m(3)/d, or about half that of the conventional system based on activated sludge process, and the O/M cost is 0.1 Yuan (RMB) or US dollars 0.012/m(3), only one fifth that of conventional treatment systems. The performance of the wastewater treatment and utilization eco-system is quite good with a final effluent COD, BOD, SS, NH3-N and TP of 45-65 mg/l, 7-32 mg/l, 12-35 mg/l, 2-13 mg/l and 0.2-1.8mg/l respectively and the annual average removals of COD, BOD, SS, NH3-N and TP are 69.1%, 78.3%, 76.4%, 62.1% and 52.9%o respectively, which is much better than that of conventional pond system or constructed wetland used separately and illustrates that the artificial and integrated eco-system is more effective and efficient than the simple natural eco-system.

Performance of very shallow ponds treating effluents from UASB reactors

Polishing ponds are units conceived for the post-treatment of the effluents from anaerobic reactors, are designed as maturation ponds, and aim at a further removal of organic matter and a high removal of pathogenic organisms. The paper investigates the performance of four very shallow (H = 0.40 m) polishing ponds in series, with very low detention times (1.4-2.5 days in each pond), treating anaerobic effluent from the city of Belo Horizonte, Brazil. The system was able to achieve excellent results in terms of BOD and E. coli removal, and good results in terms of ammonia removal, allowing compliance with European standards for urban wastewater and WHO guidelines for unrestricted irrigation. The paper presents the values of BOD and E. coli removal coefficients, which were much higher than those found in conventional pond systems. No statistically significant difference was found in the effluent E. coli concentrations from a pond with low depth and low detention time, and another pond in parallel, with double the depth and approximately double the detention time. The results endorse the applicability of the system composed by UASB reactors followed by very shallow ponds in series, with low detention times.

Intensification of pond aquaculture and high rate photosynthetic systems

Aquaculture production systems may range from tanks and raceways, in which water quality is controlled by water dilution and discharge to the environment to captive water systems, in which water quality is controlled by microbial reactions within the tank or pond. Attempts at intensification of pond aquaculture beyond the commonplace practice of supplemental aeration may be classified into categories of physical/chemical techniques and a broad range of microbial techniques. Most of these techniques are directed at raising the ‘ceiling’ of the system ammonia detoxification rate. Physical–chemical techniques for intensification of pond aquaculture have included use of in-pond cages and raceways, water blending and shading of the algal community, as well as, direct flocculation and removal of algal and bacteria biomass from ponds. A variety of microbial processes can be used to reduce ammonia levels in a conventional pond. These processes include nitrification/denitrification, photosynthesis, and heterotrophic bacterial re-growth. In this paper, simplified microbial growth fundamentals, and elemental mass balances are used to analyze and compare the various aquaculture intensification techniques and, in particular, to compare conventional and heterotrophic techniques to the use of high rate photosynthetic systems. Direct or indirect photosynthetic systems include enhanced algal systems (with water mixing), polyculture, hydroponics, wetlands, and terrestrial irrigation/fertilization. The development of Clemson University's Partitioned Aquaculture System (PAS) constitutes an attempt to combine a number of the various physical, chemical, and microbial intensification techniques into a single integrated system. The PAS represents an adaptation of high rate microalgal culture to produce a sustainable, minimal discharge, high yield, and more controllable fish production process. The PAS combines the advantages of process control of recirculating tank aquaculture with the lower costs of earthen pond aquaculture. Central to the economic success of the PAS is the use of low speed (1–3 r.p.m.) paddlewheels as an energy efficient means of establishing a uniform water velocity field within an aquaculture pond. The PAS represents a redesign of the conventional aquaculture pond culture technology providing a spectrum of applications ranging from moderate yield (6700–11 200 kg/ha) ‘engineered ecosystems’ to high yield (16 800–33 600 kg/ha) controlled ‘production processes’. This high rate photosynthetic system offers the potential for a 90% reduction in total water usage per unit of fish produced. The modular nature of the PAS, the increased productivity per unit area, reduced water requirement, and reduced environmental impact offers the potential for fish culture systems to be installed at sites not currently suitable for conventional aquaculture.

Nutrient and microbial dynamics in high-intensity, zero-exchange shrimp ponds in Belize

Microbial and phytoplankton processes, and their effect on water quality were examined over a 3-week period in five high-intensity (120 animals m −2) shrimp ( Litopenaeus vannamei) ponds of varying crop ages at Belize Aquaculture Ltd., (BAL) in Central America. These ponds were characterized by zero water exchange throughout the crop, plastic lining and high aeration rates. Nitrogen (N) and organic carbon (C) inputs, in the form of fishmeal-based feed, grain-based feed and molasses, resulted in high concentrations of dissolved organic and inorganic N (2.29–5.56 and 0.17–10.66 mg l −1, respectively) and dissolved organic C (14.20–48.10 mg l −1). Phosphate levels were also high, ranging from 0.07 to 1.17 mg l −1. The high nutrient concentrations promoted the growth of bacteria, phytoplankton (mostly autotrophic flagellates) and protozoa. Up to 40% of the bacteria were associated with flocculated matter. However, bacterial numbers and oxygen (O 2) consumption in the water column did not appear to increase with crop age. This may be due to a reduction in the C/N ratio below the optimum for bacterial growth. Up to 22% of the O 2 consumption was due to nitrification and there was some indication of lowering of total ammoniacal N (TAN) concentrations and an increase in nitrite and nitrate levels in older crops. Both phytoplankton and bacteria were responsible for high rates of ammonium uptake. In ponds with high nitrate concentrations, nitrate uptake rates were also high. Phytoplankton productivity remained high irrespective of crop age and ponds fluctuated between net O 2 production (autotrophy) and net O 2 consumption (heterotrophy) irrespective of crop age. This reflected the highly dynamic nature of the bacterial and phytoplankton populations with frequent blooms and crashes of individual phytoplankton species. The high mixing rates resulted in phytoplankton and other detritus remaining suspended in the water column. However, a small area of sludge (<2% of pond area) did accumulate containing a high N and C content, and high pore water TAN. This study showed that despite what is generally considered as poor water quality in the ponds, i.e. high nutrient concentrations, high and unstable phytoplankton numbers, and high bacterial numbers, shrimp production was high relative to conventional ponds. There appeared to be scope for increasing bacterial production in these systems by increasing the C/N ratio, and hence C availability for bacterial growth. However, it remains to be established which microbial processes are likely to be promoted, and if the benefits of this outweigh the costs.

Differences in Survival and Physiology between Coho Salmon Reared in Seminatural and Conventional Ponds

Abstract Concern that hatchery fish may not transition well upon release into the wild has resulted in research directed at producing fish with more wild characteristics. To explore the possibility of generating more adaptable fish with alternative rearing strategies at a production-level hatchery, we modified an existing dirt-bottomed rearing pond at the Elochoman Hatchery, Washington, into a seminatural rearing pond (SNP) by adding large woody debris and pit-run rock. The pond was stocked with Type-N lower Columbia River coho salmon Oncorhynchus kisutch from three consecutive broods (1995–1997) at densities about 5% of those that are typical for hatchery rearing. Fish were allowed to migrate volitionally from treatment and control ponds (CP) and were sampled weekly to determine size, condition factor (105;th·;thweight/length3), and gill Na+, K+-ATPase enzyme activity. Each group was coded-wire-tagged, and smolt-to-adult survival was estimated. In 2 of the 3 years, fish from the SNP were larger at migrat...

Effect of anaerobic pretreatment on environmental and physicochemical characteristics of duckweed based stabilization ponds

Duckweed based stabilization ponds, an alternative for wastewater treatment, are attracting a growing interest from researchers because they are basically a low cost technology, easy to built and operate, and produce tertiary quality effluents. Besides, this technology offers the possibility of resource recovery by producing high quality duckweed protein, which can be of further use. Since the technology is rather new, there are many aspects to be studied before its full-scale implementation. It is necessary to gain sound knowledge of the basic principles of the complex processes occurring in the system, as well as of the practical aspects of design and operation. The presence of a layer of duckweed on the surface is expected to produce different environmental and physicochemical conditions in the water from those found in conventional stabilization ponds. These environmental and physicochemical conditions affect both plant growth and biological treatment processes in the system, therefore it is important to determine their behavior in a duckweed system and how they can be affected by an anaerobic pretreatment. Continuous flow pilot plants composed of seven ponds in series were operated with artificial substrate under two different conditions: with anaerobic pretreatment and without anaerobic pretreatment. The flow was kept constant during the operation. Conditions such as pH, temperature, dissolved oxygen, alkalinity, conductivity, chemical oxygen demand, biochemical oxygen demand, total and ammonium nitrogen, nitrites and nitrates, and phosphorus were evaluated in the system under steady state conditions. The main conclusions from the study include the following: pH, temperature and oxygen profiles are more stable in duckweed ponds than in conventional stabilization ponds; anaerobic pretreatment has a significant effect on the oxygen concentration in the system and on the organic matter removal but not on the nutrient removal.

STUDI KESESUAIAN LAHAN PESISIR UNTUK TAMBAK DI KECAMATAN NGOMBOL KABUPATEN PURWOREJO

The objective of this study was to know the feasibility of the coastal area for shrimp and fish pond based on physical, chemical and biological parameters of soil and water as well as topographycal situation.The results of the study indicated that : .i). the coastal area in Ngombol district Purworejo regency was suitable for pond, .ii). the area have two kinds of land units, namely clay alluvial and sandy land units, iii). the former one was suitable for conventional pond constraction, fertile and acid soil, .iv). the latter was unsuitable for conventional pond constraction, unfertile and slightly-netral soil, .v). the water for shrimp and fish culture might be supplied from Cokroyasan and Lereng rivers (fresh-brackiswater), well (freshwater), and directy from intestidal zone for seawater.vi). in general, the water quality were suitable for shrimp and fish culture.This study suggested that in the clay alluvial soil, extensive and intensive ponds might be developed. However, intensive pond should be developed in the area with sandy soil.

Distribution of bacteria in tropical freshwater fish and ponds

Bacteriological examination of the various organs (liver, kidney, intestine and inner muscle) of four freshwater fish species belonging to the family cyprininae reared in experimental ponds were compared to those reared in conventional pond. A total of 16 bacterial species were recovered from the water samples and the various organs of the fish. The intestines of all the fish species harboured the most number of different bacterial species. No bacteria was found in the muscle of any of the fish. In general, the bacterial species isolated from the intestine were also found in the water samples from the ponds. The common species found in the intestine of the fish belonged to seven Gram negative bacterial species: Aeromonas hydrophila, Citrobacter freundii, Escherichia coli, Enterobacter aerogenes, Klebsiella sp., Pseudomonas sp, Vibrio anguillarum and three Gram positive bacterial species: Bacillus sp., Listeria and Staphylococcus. The presence of the former group of bacteria which are members of the family Enterobacteriaceae and Vibrio-Aeromonas is a cause for concern as these organisms are potential enteropathogens, and any mishandling of the fish can lead to the transmission of the pathogen to humans. The type of bacterial species present in the water of the conventional ponds were similar to those found in the experimental ponds.

The design, management and production of a recirculating raceway system for the production of marine shrimp

Despite continuing problems with disease outbreaks and environmental concerns over effluent pollution and land usage, world shrimp farming continues to expand. Although traditional pond production systems will continue to be the dominant driving force in aquaculture expansion, there is continued interest in alternate production systems. The use of high density, water reuse systems is one alternative to conventional pond production systems which addresses restrictions associated with environmental regulations and user conflicts of coastal land and water usage. This paper reports on techniques which have been developed for the production of marine shrimp in recirculating raceway systems and typical results which have been observed over a 6-year production period. Both bait shrimp ( Penaeus setiferus) and food shrimp ( P. vannamei) have been produced with final biomass loads as high as 10 kg/m 3 utilizing 100–120-day and 160–175-day production cycles, respectively. Results from multi-phase growout are most promising and indicate that this may be a suitable mechanism to optimize biomass loading of the culture system. Although the economic viability of recirculating production systems for bait and food size shrimp have yet to be proven, the consistent results of production, low water usage and ease of waste management are encouraging and warrant further economic and marketing evaluations.

Experimental study of high rate pond system treating piggery wastewater

The results of an experimental study conducted in a full-scale high rate pond system treating piggery wastewater at Jianfengshan Piggery, Panyu City, Guandong Province, are presented. The system consists of two advanced anaerobic ponds (AAP) in parallel, followed by an anaerobic transformation pond (ATP) and a five-cell algae-bacterial pond (ABP). The mechanism of the AAP is described and the hydraulic flow pattern analyzed. Fermentation pits (FP) built on the bottom performed very efficiently, operating like UASB in principle. A new concept of ATP is advanced, based on its ability to transform poorly degradable materials to more easily degradable ones. It was found in the study that the HRP system was more efficient, more reliable and saved 40% land area compared with a conventional pond system. Economic analyses of both the energy consumption and the benefit to the pond system of fish farming are also included in the paper.

Experimental study of high rate pond system treating piggery wastewater

The results of an experimental study conducted in a full-scale high rate pond system treating piggery wastewater at Jianfengshan Piggery, Panyu City, Guandong Province, are presented. The system consists of two advanced anaerobic ponds (AAP) in parallel, followed by an anaerobic transformation pond (ATP) and a five-cell algae-bacterial pond (ABP). The mechanism of the AAP is described and the hydraulic flow pattern analyzed. Fermentation pits (FP) built on the bottom performed very efficiently, operating like UASB in principle. A new concept of ATP is advanced, based on its ability to transform poorly degradable materials to more easily degradable ones. It was found in the study that the HRP system was more efficient, more reliable and saved 40% land area compared with a conventional pond system. Economic analyses of both the energy consumption and the benefit to the pond system of fish farming are also included in the paper.

Methane fermentation, submerged gas collection, and the fate of carbon in advanced integrated wastewater pond systems

There are several basic reasons for concern regarding the fate of carbonaceous material in waste stabilization ponds: accumulation of solids; performance and useful life of the pond system; and, the control of methane emissions. In conventional ponds methane fermentation is minimal, and carbon-rich organic matter is integrated by bacteria and microalgae which grow and settle. The integration of carbon decreases pond volume and treatment capacity and causes the ponds to age prematurely, to produce odor, and to require frequent sludge removal; and, any methane produced escapes to the atmosphere. However, if carbon-rich organics are efficiently converted to methane or to harvested microalgae, the pond system will continue to treat wastewater effectively for an extended period of time. Advanced Integrated Wastewater Pond Systems (AIWPSs) developed at the University of California fully utilize methane fermentation and microalgal cultivation to treat wastewater and to reclaim energy and nutrients. First generation AIWPSs have provided reliable municipal sewage treatment at St. Helena and Hollister, California, for 28 and 16 years, respectively, without the need for sludge removal. However, these first generation systems lack the facilities to recover and utilize the carbon-rich treatment byproducts of methane and algal biomass. The recovery of methane using a submerged gas collector was demonstrated using a second generation AIWPS prototype at the University of California, Berkeley, and the optimization of in-pond methane fermentation, the growth of microalgae in High Rate Ponds, and the harvest of microalgae by sedimentation and dissolved air flotation were studied. Preliminary data are presented to quantify the fate of carbon in the second generation AIWPS prototype and to estimate the fate of carbon in a full-scale, 200 MLD second generation AIWPS treating municipal sewage. In the experimental system, 17% of the influent organic carbon was recovered as methane, and an average of 6 g C/m2/d were assimilated into harvestable algal biomass. In a full-scale second generation AIWPS in a climate comparable to Richmond, California, located at 37° N latitude, these values would be significantly higher--as much as 30% of the influent organic carbon would be recovered as methane and as much as 10 g C/m2/d would be assimilated by microalgae. These efficiencies would increase further in warmer climates with more abundant sunlight.

Methane fermentation, submerged gas collection, and the fate of carbon in advanced integrated wastewater pond systems

There are several basic reasons for concern regarding the fate of carbonaceous material in waste stabilization ponds: accumulation of solids; performance and useful life of the pond system; and, the control of methane emissions. In conventional ponds methane fermentation is minimal, and carbon-rich organic matter is integrated by bacteria and microalgae which grow and settle. The integration of carbon decreases pond volume and treatment capacity and causes the ponds to age prematurely, to produce odor, and to require frequent sludge removal; and, any methane produced escapes to the atmosphere. However, if carbon-rich organics are efficiently converted to methane or to harvested microalgae, the pond system will continue to treat wastewater effectively for an extended period of time. Advanced Integrated Wastewater Pond Systems (AIWPSs) developed at the University of California fully utilize methane fermentation and microalgal cultivation to treat wastewater and to reclaim energy and nutrients. First generation AIWPSs have provided reliable municipal sewage treatment at St. Helena and Hollister, California, for 28 and 16 years, respectively, without the need for sludge removal. However, these first generation systems lack the facilities to recover and utilize the carbon-rich treatment byproducts of methane and algal biomass. The recovery of methane using a submerged gas collector was demonstrated using a second generation AIWPS prototype at the University of California, Berkeley, and the optimization of in-pond methane fermentation, the growth of microalgae in High Rate Ponds, and the harvest of microalgae by sedimentation and dissolved air flotation were studied. Preliminary data are presented to quantify the fate of carbon in the second generation AIWPS prototype and to estimate the fate of carbon in a full-scale, 200 MLD second generation AIWPS treating municipal sewage. In the experimental system, 17% of the influent organic carbon was recovered as methane, and an average of 6 g C/m2/d were assimilated into harvestable algal biomass. In a full-scale second generation AIWPS in a climate comparable to Richmond, California located at 37° N latitude, these values would be significantly higher — as much as 30% of the influent organic carbon would be recovered as methane and as much as 10 g C/m2/d would be assimilated by microalgae. These efficiencies would increase further in warmer climates with more abundant sunlight.

Performance of methane fermentation pits in advanced integrated wastewater pond systems

Advanced Integrated Wastewater Pond Systems (AIWPSs) involve a series consisting of Advanced Facultative Ponds with internally located fermentation pits; secondary ponds with either photosynthetic oxygenation or mechanical aeration; tertiary ponds for sedimentation of either algae or aeration solids; and, quaternary ponds for controlled discharge, irrigation storage, aquaculture, or other beneficial uses of reclaimed wastewater. This paper deals mainly with design and performance of Advanced Facultative Ponds containing internally located fermentation pits. Experiences with a 1,894 m3 day−1 (0.5 MGD) AIWPS and a 7,576 m3 day−1 (2.0 MGD) AIWPS indicate that primary facultative ponds with internal fermentation pits require less land than do conventional anaerobic ponds and that sludge removal is postponed for many years. New, more detailed, and controlled scientific studies on a 133 m3 day−1 (0.035 MGD) demonstration AIWPS at the University of California, Berkeley, Environmental Engineering and Health Sciences Laboratory in Richmond, California provide evidence that these simple pits remove suspended solids and biochemical oxygen demand more effectively than do comparably loaded conventional anaerobic ponds and produce much less odor. In addition they improve removal of parasites, bacteria, viruses, heavy metals, and halogenated hydrocarbons. The reliability and cost effectiveness of AIWPS is compared with more conventional ponds and with mechanical wastewater treatment systems.

Site selection and regulation issues for trout and carp farming in Germany

In most industrialized countries very few natural freshwater habitats are available for the establishment of new fish farms. These limits require regulatory measures, largely depending on local conditions. Regulations on volume and type of water to be extracted from groundwater and/or surface resources exist in Germany on the «Lander» federal level (contained in the «Water Acts» («Wasserhaushaltsgesetz»). These regulations also set water quality criteria for effluent discharge into receiving waters. Site selection criteria are largely determined by the type of aquaculture considered: in extensive, conventional pond farming systems, criteria on natural factors include water resource availability, space and geomorphological (e.g. slope) and geochemical factors (e.g. soils). The appropriate positioning and levelling of ponds will decide on whether excessive energy costs can be avoided. Different site selection criteria will have to be employed for trout and carp culture. For carp, availability of large areas with suitable soil characteristics is essential. Comparably low yields per unit area will be achievable (productivity of the water determines stocking density and yield). Construction costs need to be kept at a minimium, otherwise the operation will never become economically feasible. Exceptions are special ponds for overwintering, or spawning or other special purposes. Trout culture in earthern ponds require different site selection criteria, depending on the volume of water supply, because yield does not depend on area specific productivity of the water body. Water quality management criteria become an overriding issue for this type of culture system. Stocking density will be determined by the permit for extractable water volumes per unit time. (set by the water authorities). Intensification methods for trout farming exist within the range of regulated effluent standards by using specifically designed system components such as concrete ponds, raceways, circular tanks, aeration, oxygenation, etc. The effluents, however, are considered an industrial waste and their release into receiving waters will be under the water management law (7). If standards are not met within the set limits effluents will be charged as the release of any other industrial wastewater

Analysis of an open type flat plate collector for tannery effluent treatment

In this paper, analysis of an open type flat plate collector for tannery effluent treatment is presented. The effects of parameters like orientation of collector, film thickness, insolation, ambient temperature and relative humidity on the effluent evaporation rate are studied. It is found that the collector facing south at an inclination of 12.68° and having a solution film thickness of 5 mm gives maximum year round performance. The performance of this open type flat plate collector is compared with that of the conventional pond.

Estimated Costs and Returns for Catfish Farms With Recirculating Ponds Along the Upper Texas Coast

AbstractCost, returns, and economies of scale for small, medium and large catfish farms with recirculating ponds are presented for the upper Texas coast. Internal rates of return are 0.150, 0.183 and 0.219, respectively. Total investment is higher than farms with static ponds but investment per unit production capacity is 7 percent to 16 percent lower. Average total cost per pound is between $0,565 and $0,541, (11 percent-20 percent lower than farms using current technology). These results have implications for regional comparative advantage of catfish production as well as incentive for adoption of new technology in conventional ponds.

Intensification of a Pond System by Fibrous Carriers

A study was carried out on the intensification of a pond system, consisting of an anaerobic pond, a facultative pond and a polishing pond, in Anda City, Heilongjiang Province, by fibrous carriers packed in the anaerobic pond, by means of which the performance of the pond system was remarkably improved with higher removal capacities and efficiencies for SS, BOD5, COD, TN, TP and bacteria, compared with the conventional system. The mechanism for the intensified effect was found to be ascribed to the increase of biomass in the form of biofilm attached to the surface of the fibrous carriers and more even distribution of the biomass in the pond. A mathematical model based on the complete mixed flow pattern was developed both for the intensified and conventional anaerobic pond, and the organic degradation rate constant value K in the former was found to be much greater than that in the latter. The microbial communities in both the ponds were also observed.

Nitrification pattern in a fluctuating anaerobic-aerobic pond environment

Nitrification does not occur in conventional fish ponds or in oxidation ponds. Yet, intensive nitrification occurs in continually aerated and mixed fish ponds and in activated sludge systems. In a combined fish pond system, water from a conventional pond is pumped through a continually aerated and mixed pond, with a 5 h average hydraulic residence time. Unlike other aerated ponds, nitrification did not occur in this case. The reasons for nitrification inhibition were studied in laboratory and field experiments. Inhibited nitrification was not due to any factor associated with water composition and persisted even when the water was replaced. In addition, it was not limited by a lack of inoculum. Nitrification in a laboratory assay started immediately in samples taken from a continually aerated pond, while it started only following a 24–48 h lag period in water sampled from a conventional pond or from the combined pond system. Nitrifying bacteria have to adapt to the partial anoxic conditions in the conventional pond. It is assumed that the lag period is needed by the reverse process, adaptation to oxic conditions. The ecological and practical implications of this phenomenon are discussed. The lag period is needed when nitrifiers are transferred from anoxic to oxic conditions as a protective mechanism to ensure that nitrification will take place only if continuous or long-term oxic conditions prevail.