The impact of effluents of Pitkäranta pulp mill on the water quality of Lake Ladoga: a model study

The aim of this study is to show the effects of the Pitkäranta pulp mill on the water quality of Northern Ladoga by using water quality models. The effluent loading of the pulp mill with its full production capacity is as follows: Water flow 85 000 m³ d^–1 BOD₅ 2.4 t d^–1 Suspended solids 4.1 t d^–1 tot-N 330 kg d^–1 tot-P 68 kg d^–1 COD(Cr) 14.4 t d^–1 Org. C 6 t d^–1 Lignosulphonates 9.4 t d^–1 Loadings of lignosulphonates and organic C are estimations. Lignosulphonate concentrations of 10.5 mg 1^–1 have been reported in the region of Pitkäranta. The study area of northern Ladoga near the pulp mill was divided into three zones (I, II and III). The mean depth of each of them is considered as 10 m, their respective areas 5, 10 and 50 km², and volumes 50, 100 and 500 million m³. The estimated discharges of the zones are 10, 20 and 50 m³ s^–1, respectively. With the aid of simple water quality models the effects of the pulp mill effluents on the concentrations of oxygen, total phosphorus, lignin, COD(Mn) and Secchi disk depths in each of the zones were estimated. Estimations were made during full production capacity and half production capacity of the pulp mill. The modelling results were compared with the preliminary water analysis results of the Finnish-Russian joint research expedition into Lake Ladoga in August 1993. The results show that near the pulp mill (zone I) phosphorus concentrations are high even with half production capacity. Also lignin and COD(Mn) contents have increased, and oxygen concentrations are low both in the summer and during wintertime. Farther away in the open water (zone III) the pollution effects are low. Eutrophication, indicated by high total phosphorus concentrations, is the main effect of the pulp mill effluents.     

Population Dynamics of the Pink Salmon Oncorhynchus gorbuschain the Basin of the Sea of Okhotsk in the 1990s

In the 1990s, an extensive body of data was gathered on the size of the Oncorhynchus gorbuschapink salmon populations of the Sea of Okhotsk at all the main developmental stages. A significant increase in numbers was found for juvenile pink salmon migrating into the offshore regions of the Sea of Okhotsk and the Pacific waters around the Kurils: from 250–450 million in 1990–1991 to 807–1570 million fish in 1993–1999. The overall number of migrating pink salmon in even years sharply increased in 1994 up to 215 million fish. After 1994, this estimate exceeded the number of migrating pink salmon in preceding odd years. Ocean survival of juvenile pink salmon gradually declined. This may be related to changes in the North Pacific pelagic ecosystems.     

Thermal, mixing, and oxygen regimes of the Salton Sea, California, 1997–1999

The Salton Sea is a shallow (mean depth = 8 m; maximum depth = 15 m), saline (41–45 g l^–1), intermittently mixing, 57 km long, 980 km² lake located in the arid southwestern United States. The Sea is a wind driven system, with predominant winds paralleling the long axis of the lake, being strongest in spring and weakest in summer and fall. The Sea mixed daily or nearly daily between September and January. During this cooling period, moderate to high levels of dissolved oxygen (3–11 mg l^–1) were found throughout the water column. Mean water column temperature ranged from a minimum of 13–14 °C in early January to a maximum of 30–34 °C in July–September. During most of this warming period, the Sea was thermally stratified but subject to periodic wind driven mixing events. Winds were stronger in spring 1998 than in 1997 or 1999, causing more rapid heating of the lake that year and also delaying onset of anoxic conditions in bottom waters. During summer months, mid-lake surface waters were sometimes supersatured with oxygen, and bottom waters were hypoxic or anoxic with sulfide concentrations > 5 mg l^–1. Oxic conditions (> 1 mg O₂ l^–1) often extended a few meters deeper nearshore than they did well offshore as a consequence of greater mixing nearshore. Mixing events in late summer deoxygenated the entire water column for a period of days. Consumption of oxygen by sulfide oxidation likely was the principal mechanism for these deoxygenation events. Sulfide concentrations in surface waters were 0.5–1 mg l^–1 approximately 3 days after one mixing event in mid-August 1999. These mixing events were associated with population crashes of phytoplankters and zooplankters and with large fish kills. In the southern basin, freshwater inflows tended to move out over the surface of the Sea mixing with saline lake water as a function of wind conditions. Salinity gradients often contributed more to water column stability than did thermal gradients in the southeasternmost portion of the lake.     

Pulp and paper mill waste pollution in the Swale,a tidal channel on the east coast of England

The effect of pulp and paper mill waste discharges in the Swale, a tidal channel on the southeast coast of England, has been studied. A pulsing tidal movement in the Swale results in effluent being trapped within the estuary for up to 20 days. This has resulted in adverse effects on the quality of the water and sediment along a substantial part of the estuary with the greatest effect occurring east of the mill in the direction of the residual current. A reduction in the dissolved oxygen concentration occurred along a 13-km stretch of the estuary with mean values falling to 53 % saturation near the mill. High levels of organic matter in the sediment reflect the deposition of material from the mill effluent stream and loss on ignition values of up to 12 % were found east of the mill, falling to 5 % within 4 km northwest of the mill. The macrofauna of the intertidal mudflats was typical of a stressed environment with a low number of species and with a few being represented by large numbers of individuals. The dominant species was the sabellid polychaeteManayunkia aestuarina which reached peak numbers of over 1.5 million m⁻², 1.5 km east of the mill. An increase in the total biomass near the mill is thought to be associated with organic enrichment from the mill wastes.     

Farmland treatment and utilization of straw pulp mill effluents

Straw pulp production accounts for 74% of the total raw pulp production in China. The pulping waste, containing high contents of silicate and high pH, is difficult to treat. Instead of the conventional alkaline process, ammonia sulfite pulping discharges neutral effluent with nitrogen, phosphorus, potassium, sulfur and organic matter which are potential fertilizers and water resources for agriculture. The effect of effluent from ammonia sulfite pulping on rice growth and yield, was studied in Baigezhuang farm, Hebei province. Diluted rice straw digester waste or composite waste were irrigated on a paddy field. In plot tests, rice output increased by 16.2–25.3% with 0.7–6% pulp digester waste and by 22.9–37.9% with 2.7–5.4% composite waste. In field tests, rice output increased by 8.6–15.9% with 2.7–4.1% composite waste. Due to clean water dilution and paddy purification, the removal efficiency of biochemical oxygen demand (BOD), chemical oxygen demand (COD), suspended solid (SS), sulfide and phenol can reach over 97%. Rice quality also tested had higher protein content than the control. The study suggests that the effluents from ammonia sulfite pulping can be widely used in the farming industry. The ammonia sulfite process is a good approach to overcome pulp pollution in rural areas.     

Changes in water quality in Lake Päijänne following decrease of effluent load from the pulp and paper industry in 1969–1989

Lake Päijänne, the second largest lake in Finland, has been seriously polluted since the 1960s due to the effluents from pulp and paper mills, notably the Äänekoski sulphate and sulphite pulp mills situated about 50 km north of the lake, and the sulphite pulp mill and paper mills of Jämsänkoski and Kaipola on Central Lake Päijänne.A sulphite lye evaporation and combustion plant installed at the Jämsänkoski sulphite pulp mill in 1969 reduced the organic pollution of Central Päijänne. Lignin concentration decreased and the oxygen balance improved. The sulphite pulp mill was closed in 1981 and replaced by a thermo-mechanical pulp mill. Eutrophication is the main threat to water quality in Central Lake Päijänne at present.A significant improvement in the water quality, especially in oxygen balance, was achieved in the watercourse of Äänekoski and in Northern Päijänne after replacement of the old sulphite and sulphate pulp mills at Äänekoski by a large sulphate pulp mill with a biological purification plant employing the activated sludge method. The BOD₇-loading dropped from 46 to 3–4 t d^–1, but the nutrient loading has not decreased sufficiently, and the Äänekoski watercourse and Northern Päijänne are still eutrophic.     

Nutrient enrichment of northern rivers in response to pulp mill and municipal discharges

To evaluate the effects of pulp mill and municipal sewageeffluents on the nitrogen (N) and phosphorus (P) status of northernCanadian rivers, the Northern River Basins Study required an integratedresearch and assessment program consisting of field observation andexperimentation. Analysis of monitoring data collected over 3–13 yshowed that on an annual basis, pulp mills contributed 22% of theP and 20% of the N load discharged from the Wapiti to the Smokyriver, and 6 to 16% of the P load and 4 to 10% of the Nload in the Athabasca River. Despite these low contributions, N and Pconcentrations were elevated below pulp mill discharges on all threerivers during the low discharge period of September to April. Insitu experiments conducted with nutrient diffusing substrata showedthat periphyton biomass was maintained at low levels by insufficient Pin the upper reaches of the Athabasca River and insufficient N+P inthe Wapiti River upstream of point-source discharges. In contrast,effluent loading from pulp mill and sewage inputs alleviated nutrientlimitation downstream of major discharges on both rivers. Experimentsconducted in artificial streams located beside the Athabasca Riverupstream of the first pulp mill showed that P addition increasedperiphyton biomass (expressed as chlorophyll a content) suchthat biomass increased sharply at low P concentrations (2–5µg/L soluble reactive P) and approached saturation at 35µg/L soluble reactive P. As a result of recommendations fromthis integrated program of river monitoring and experimentation,nutrient loading has been reduced to the most affected river(Wapiti-Smoky) and federal and provincial departments of environment arereviewing loading limits for other river reaches.     

Responses to hypoxia of plaice,Pleuronectes platessa,and dab,Limanda limanda,in the south-east Kattegat: distribution and growth

Synopsis Plaice, Pleuronectes platessa, and dab, Limanda limanda, were sampled with a Glommen lobster trawl at 25 to 40 m depth in the SE Kattegat during spring and autumn of 1984 to 1990. During autumn, hypoxia (O₂-concentration < 3 mg I^–1) occurred in the bottom water below the halocline for four to ten weeks every year, except in 1984 when moderate hypoxia (O₂-concentration 3–5 mg 1^–1) occurred. Biomass of both species was shown to be negatively correlated with oxygen concentration during autumn. Further, a decrease in population mean total length was observed during the study period in both spring and autumn samples. Laboratory studies of growth of juvenile plaice and dab, at 15° C and 30–34%, showed that growth is reduced at 50 and 30% O₂-saturation for both species during a 20 d period. There was some adaptation to hypoxia resulting in less reduction of growth during the second half of the experiment. The frequency of fish eating was reduced in plaice at 30% O₂-saturation. Reduced mean total length of the plaice and dab population of the SE Kattegat is discussed in view of sublethal effects of oxygen deficiency.     

Below-halocline oxygen consumption in the Kattegat

Sediment and seston oxygen consumption rates below the sharp halocline in the south-eastern part of the shallow Kattegat were measured and compared to calculated rates of carbon addition through the halocline. The mean rate of decrease in deep-water oxygen concentrations between March and September 1988 was 1.0 ml O₂ M^–3 h^–1. Measurements of benthic oxygen uptake using laboratory-incubated sediment cores from depths 30 m gave a mean value of 7.8 ml O₂ m^–2 h^–1. Below-halocline water (from 20 m, 30 m and 1 m above bottom) incubated in bottles showed oxygen consumption rates varying from 0.5 ml O₂ m ^–3 h^–1 in March to 2.8 ml O₂ M^–3 h^-1 in late August. The sum of benthic and deep-water oxygen consumption was equivalent to a mean oxygen decrease rate of 1.7 ml O₂ m^–3 h^–1 below the halocline. Of the total oxygen consumption below the halocline 65% was due to oxygen up-take in the water and 35% was due to benthic oxygen consumption. The sum of oxygen consumption measured in sediment cores and in bottles corresponds to a carbon utilisation of 80.1 g C m^–2 (respiratory quotient (RQ), assumed 1.0 and 1.4 for water and sediment, respectively), while the decrease in deep-water oxygen concentration was equivalent to 43.0 g C m^–2 (RQ assumed = 1.0). Using published values for the external N loading (including deep-water supply), ¹⁵NO₃-uptake, ¹⁴CO₂-uptake in combination with % ¹⁵NO₃-uptake of total ¹⁵N-uptake (nitrate, ammonia and urea) and a Redfield C/N ratio of 6.6, rates of carbon addition (new or export production) through the halocline were calculated to 31.9, 46.7 and 36.3 g C m^–2, respectively, with a mean value of 38.3 g C m^–2 for the 8 month period March–September. This is somewhat less than the value (50.5 g C m^–2) calculated from a published empirical relationship between total and export production. The fact that the calculated carbon addition through the halocline was appreciably less than the carbon equivalent of the measured below-halocline respiration may be an effect of sediment focusing (horizontal transport of sedimenting material to deeper areas), since the bottom area below the halocline is much smaller than the total area of the Kattegat. A lower observed decrease in the oxygen concentration below the halocline compared to the sum of measured sediment and deep-water oxygen consumption on the other hand indicates oxygen supply to below-halocline waters through advection and/or vertical entrainment.     

Behavioural approaches to demonstrate the ecological significance of exposure of juvenile Pacific salmon (genus Oncorhynchus) to the antisapstain fungicide TCMTB

Simulated stream conditions were used to expose underyearling Pacific salmon (genus Oncorhynchus) to sublethal doses of TCMTB (2-thiocyanomethylthio) benzothiazole; a fungicide used by the lumber industry to protect against discolouration by sapstain fungi. Observed changes in schooling, swimming, and respiration were studied in more detail utilizing progressively more complex and ecologically relevant experimental designs. Laboratory troughs (600 litre) were used to quantify innate shelter-seeking behaviour and susceptibility to predation during salinity stress. Swimming speed, schooling behaviour and responses to salinity and hypoxia were studied in a 4500-litre Water Column Simulator under vertically stratified conditions (fresh water overlying sea water). Finally a similarly stratified 15,500-litre outdoor tank was designed to compare susceptibility to predation during a volitional salinity challenge. Relative to controls, TCMTB-exposed underyearling salmon exhibited a reduction in cover response, were slower to seek shelter from bright light, were more erratic in their swimming behaviour and more likely to be eaten by marine predators. Videotape analyses confirmed that TCMTB reduced exploratory behaviour and swimming speed under both normoxic and hypoxic conditions in the WCS. The ecological consequences of these overt behavioural changes were experimentally linked to a clear increase in risk from predation by yellowtail rockfish (Sebastes flavidus) inhabiting the marine zone below the halocline of the 15,500-litre tank. These techniques illustrate our most recent efforts to correlate cumulative sublethal physiological stress with ecologically meaningful behavioural dysfunction.     

Aquatic surface respiration,an adaptive response to hypoxia in the guppy,Poecilia reticulata (Pisces,Poeciliidae)

Synopsis Aquatic respiration at the air-water interface, herein termed aquatic surface respiration (ASR), is used by the guppy,Poecilia reticulata (Poeciliidae) to meet oxygen demand in hypoxid water. A specific position in which the head contacts the surface and the jaws open just beneath the surface is adopted. ASR is initiated at a P0₂ of about 50 torr and the percent time spent rises in a steep, linear fashion as PO₂ decreases. Below 4 torr more than 90% of the animal's time is spent in ASR. Males spend less time in ASR than do females. The percent time in ASR increases with increasing size of female guppies, but decreases with increasing size of male guppies. At low oxygen (18 torr) laboratory-born guppies derived from stocks likely to experience deoxygenation spend less time in ASR than do guppies derived from stocks less likely to experience deoxygenation. The percent time in ASR increases with temperature when PO₂ is held constant. Acclimation to low oxygen decreases the percent time in ASR.Guppies not permitted ASR die in 6–41 h at 14–17 torr and 10–15 min at 1–4 torr. Guppies performing ASR survive the duration of experiments at 13–35 torr (13 days), 14–17 torr (96 h), and 1–4 torr (9.5 h). Activity, courtship, and dive duration in response to a shadow stimulus are all reduced by low oxygen. ASR is an effective alternative to aerial respiration as an adaptation to hypoxic waters, but is probably energetically and temporally more costly.     

Dissolved oxygen decline in ice-covered rivers of northern Alberta and its effects on aquatic biota

To determine whether existing dissolved oxygen (DO) regulationswere adequate to protect riverine biota, the Northern River Basins Studyrequired a research and assessment program to establish the effect ofpulp mill and municipal sewage discharges on under-ice DO concentrationsand aquatic biota in the Athabasca, Wapiti and Smoky rivers of northernAlberta, Canada. Analysis of monitoring data collected over >30 yearsshowed that pulp mill and municipal effluents have caused sags in DObelow their discharges and contributed to an increased rate of declinein DO over 10s of kilometers in the Wapiti-Smoky rivers or 100s ofkilometers in the Athabasca River. To permit forecasting of DO declinesin response to changes in river discharge or effluent biochemical oxygendemand (BOD), a one-dimensional steady-state river water quality modelwas implemented that successfully (r² =0.74–0.92) predicted DO concentrations over an 820 km reach of theAthabasca River for the 1990–1994 winters. Experiments on nativefish and benthic macroinvertebrate species showed that exposure to lowDO and low temperatures (2–3°C) caused delays in hatch ofmountain whitefish eggs (at 6.5 mg/L DO), reduced mass of bull troutalevins post-hatch (at 5.0 mg/L DO), extended the spawning period ofburbot (at 6.0 mg/L DO), and depressed feeding rates and loweredsurvival of the mayfly Baetis tricaudatus (at 5 mg/L DO).As a result of recommendations from this integrated program of rivermonitoring, modelling and experimentation, effluent BOD loading has beenreduced to the Athabasca and Wapiti rivers and the provincial DOguideline for protection of aquatic life has been increased from 5.0 to6.5 mg/L.     

The effects of turbidity and vegetation on the risk of juvenile salmonids, Oncorhynchus spp., to predation by adult cutthroat trout, O. clarkii

Synopsis During their seaward migration, juvenile salmonids encounter structural and visual cover which varies between and within watersheds. In this study, the effects of two types of cover (turbidity and artificial vegetation) on the predation mortality of juvenile salmonids exposed to fish piscivores was investigated in outdoor concrete ponds. During experiments, adult coastal cutthroat trout, Oncorhynchus clarkii clarkii, were allowed to feed on juvenile salmonid prey — chinook salmon, O. tshawytscha, chum salmon, O. keta, sockeye salmon, O. nerka, and cutthroat trout — in separate trials. Daily instantaneous per capita predation rate was determined for each turbidity and vegetation treatment, within each trial. Mean predation rates varied between 1% and 76% daily. In the presence of cover, mean daily predation rates were 10–75% lower than those in controls (no vegetation and clear water), depending on prey species. Predation rates were significantly lower in the presence of vegetation cover and did not covary with prey size or species. The effects of turbidity were generally not significant and were not additive with the effects of vegetation. However, turbidity appeared to significantly reduce the effectiveness of vegetation as cover for juvenile chinook and sockeye salmon. We suggest that these two forms of cover do not affect risk of predation by fish piscivores to juvenile salmonids via the same mechanism.     

The effect of dissolved oxygen,sediment, and sewage treatment plant discharges upon growth,survival and density of Asiatic clams

The biology of Corbicula fluminea, the Asiatic clam, in the Vermilion River, Louisiana, as affected by sediment, dissolved oxygen (DO) levels, and sewage treatment plant (STP) effluents was investigated. A point source of high DO water to the Vermilion River established a gradient of DO that decreased as the river moved towards the Gulf of Mexico. Lowering DO levels were exacerbated by municipal sewage treatment plant discharges in the 20 km reach studied. Low dissolved oxygen was associated with reduced Corbicula density in the river and 30-day in-stream growth studies (weight and length) demonstrated that low DO inhibited growth. Generally, if DO was < 1.0 mg l^–1 in sediment pore water and/or < 3.0 mg l^–1 at the sediment-water interface, growth was significantly impaired (p < 0.05). Corbicula experienced substantial mortality near the STP discharges (up to 70% in 30 days) and laboratory toxicity tests with Ceriodaphnia dubia, a sensitive cladoceran, also strongly suggested discharges were chronically toxic at 6.25–25.0% effluent. Respiration experiments along with environmental measurements of DO, temperature, and STP discharge chemistry support a hypothesis that clam populations are adversely affected by the suite of environmental conditions present in the Vermilion River. Further, growth studies were consistent with observed population densities in situ.     

Reaction of estuarine ecosystems to effluent from pulp and paper industry

A major problem associated with the discharge of pulp or paper-mill effluent into shallow estuarine ecosystems, such as the upper L'Etang in Canada or the Don in Scotland, is the development of anoxic conditions in water column and sediment. The effluent modifies the balance, within the ecosystem, between the supply of, and demand for, oxygen. The lignosulphonate component of pulpmill effluent blocks the transmission of light energy to photosynthetic green plants. The oxygen demand is increased by the microbial degration of the additional organic matter in water and sediment. The addition of cellulosic pulp fibre to the sediment also results in an increase in the concentration of their metabolic by-product, sulphide. The oxidation of sulphide exerts an additional oxygen demand on the system, which is an important factor in the development and maintenance of anoxic conditions. It is suggested that temperature is an important factor in controlling the release of sulphide from the sediment into the overlying water and in modifying the toxicity of sulphide towards the bacterial populations and, therefore, the capacity of the ecosystem to cope with the effluent.     

Methane and oxygen dynamics in a shallow floodplain lake: the significance of periodic stratification

Temperature, dissolved oxygen and dissolved methane profiles were measured during autumn and summer, in a shallow floodplain lake in south-eastern Australia to determine the effects of water-column stability on methane and oxygen dynamics. The water column was well mixed in autumn. Strong thermal stratification developed in the late afternoon in summer, with top-to-bottom temperature differences of up to 6 °C. Methane concentrations in surface waters varied over a daily cycle by an 18-fold range in summer, but only by a 2-fold range in autumn. The implication of short-term temporal variation is that static chambers deployed on the water surface for short times (less than a day) in summer will significantly underestimate the diffusive component of methane emissions across the water–atmosphere interface. There was a marked diel variation in dissolved oxygen concentrations in summer, with the highest oxygen values (commonly 5–8 mg l^–1) occurring in the surface waters in late afternoon; the bottom waters were then devoid of oxygen (< 0.2 mg l^–1). Because of high respiratory demands, even the surface water layers could be nearly anoxic by morning in summer. The concentration of dissolved oxygen in the surface waters was always less than the equilibrium value. When the water column became thermally stratified in summer, the dissolved oxygen and methane maxima were spatially separated, and planktonic methanotrophy would be limited to a moving zone, at variable depth, in the water column. In summer the whole-wetland rates of oxygen production and respiration, calculated from long-term (5 h) shifts in dissolved oxygen concentrations over a diel period, were approximately 6–10 and 3–6 mmol m^–3 h^–1, respectively. These values correspond to net and gross primary production rates of 0.7–1.2 and 1.0–1.9 g C m^–3 day^–1, respectively.     

The hydrography of the southern Canada Basin, 2002

Physical and geochemical data collected in 2002 during a multi-disciplinary expedition across the southern Canada Basin are interpreted here to provide context for the biological program that included observations from the entire water column. The mixed layer was shallow (15–20 m) and depleted of nitrate, indicating it was the limiting nutrient. Below the mixed layer a strong, multi-layered halocline complex, consisting of Pacific-origin summer and winter waters and Atlantic-origin lower halocline water, occupied the water column down to ~200 m. Pacific and Atlantic-origin waters were shallower by 25–50 m in comparison to data collected in 1997/1998, suggesting a progressive decrease in Pacific-origin water extent. Fram Strait Branch waters were warmer everywhere in the southern Canada Basin, indicating the arrival of the pulse of anomalously warm Atlantic-origin water observed in the three upstream basins during the 1990s. The highest temperatures (1.13°C) were observed along the flank of the Northwind Ridge in the core of the topographically steered boundary current. Comparison with 1997/1998 observations showed that this pulse of anomalously warm water advanced ~430 km in 4 years with an apparent speed of ~0.34 cm/s. Observation of high temperatures at a station in the Canada Abyssal Plain suggest that the boundary current bifurcates north of the Northwind Ridge, with one branch continuing eastward into the basin and the other turning southward, following the ridge.     

Advection-induced oxygen variability in the North Sea-Baltic Sea transition

Instances of strong oxygen variations are described for two shallow water stations in the Kattegat, situated at the fluctuating frontal zone between outflowing surface water from the Baltic and inflowing bottom water from the Skagerrak/North Sea.The events consist of both a rapid emergence and a rapid disappearance of oxygen-depletion. Changes in oxygen concentration amounted to more than 20 g m^–2 d^–1 for the total water columns. Such high rates of change can not be explained by net local bottom oxygen consumption (0.6 g m^–2 d^–1) or net local water oxygen consumption (1.6 g m^–2 d^–1). The oxygen variations were influenced by the local and regional meteorological conditions. The observed instance of shallow water oxygen-depletion was connected to upward movement of the pycnocline and associated advective transport of oxygen-depleted Kattegat bottom waters to a shallow water area. Similarly, rapid disappearance of the bottom water oxygen deficit in a shallow water area was found to depend more on pycnocline lowering in connection with advective transport, than on the effect of local wind driven mixing.     

Development of dependence on aerial respiration in Polypterus senegalus (Cuvier)

The respiratory behaviour and partitioning of O₂ uptake between air and water were investigated in Polypterus genegalus using continuous-flow and two-phase respirometers and lung gas replacement techniques P. senegalus rarely resorts to aerial respiration under normal conditions. Partitioning of O₂ consumption depends on the activity and age of fish and the availability of aquatic oxygen. Immature fish (12–22 g) cannot utilize aerial O₂ but older fish exhibit age-dependent reliance on aerial respiration in hypoxic and hypercarbic waters. Pulmonary respiration accounts for 50% of the total requirement at aquatic O₂ concentrations of about 3.5 mg · l^–1 (or CO₂ of about 5%) and fish rely exclusively on aerial respiration at O₂ concentrations of less than 2.5 mg · l^–1. Branchial respiration is initially stimulated by hypercarbia (CO₂: 0.5–0.8%) but increased hypercarbia (CO₂ – 1%) greatly depresses (by over 90%) brancial respiration and initiates (CO₂: 0.5%) and sustains pulmonary respiration.     

An evaluation of the quality of fall-fingerling Atlantic salmon (Salmo salar L.) released to natural lacustrine nursery areas in Newfoundland,Canada

Results of experiments on controlled releases of fall-fingerling Atlantic salmon to lacustrine habitats in Newfoundland are reviewed in terms of survival during freshwater residence and smolt yield per unit of standing water habitat. Average survival from parr to smolt in three consecutive annual releases was 12.9% and we conclude that the quality of fall-fingerling salmon, produced from our semi-natural rearing techniques, was satisfactory. Year-class survival decreased progressively with consecutive releases. Rate of increase in juvenile salmon biomass remained stable over the duration of the study, suggesting that the average annual biomass of 1.6 kg ha^–1 of juvenile salmon released did not exceed habitat carrying capacity. Precocity did not appear to represent a significant deterrent to fall-fingerling survival. Annual smolt yield (by year-class) from stocking ranged from 53 to 130 ha^–1 (mean = 77 smolts ha^–1).