Role of aquatic vegetation coverage on hypoxia and sunfish abundance in bays of a eutrophic reservoir

We studied the relation between aquatic vegetation coverage, summer dissolved oxygen and density of sunfishes (Lepomis spp.) in 10 shallow bays of a eutrophic reservoir. The bays ranged 5.2–15.7 ha in area, 0.6–1.3 m mean depth and 6–91% vegetation area coverage. Over the 10 bays mean dissolved oxygen concentration ranged 5.8–9.0 mg l^-1 in open water at least 20 m away from the vegetation, 0.8–8.1 mg l^-1 at the vegetation-water edge and 0.5–7.7 mg l^-1 in dense vegetation. Dissolved oxygen concentrations were inversely related to vegetation coverage. In open water, dissolved oxygen levels were consistently higher than 8 mg l^-1 when vegetation coverage was <20% of the bay and decreased to about 6 mg l^-1 at coverages near 80%. At the vegetation-water edge and within dense vegetation, dissolved oxygen levels dropped rapidly as vegetation coverage increased to 20% of the bay; when vegetation reached about 50% coverage, dissolved oxygen remained near 1.5 mg l^-1at the vegetation-water edge, but oxygen dropped below 1 mg l^-1in dense vegetation. Scarce vegetation harbored high Lepomis relative abundance (fish per m² of vegetation) whereas extensive vegetation harbored low relative abundance, both contributing little to absolute abundance (total fish in all vegetation); however, intermediate coverage offered a combination of mid-level fish relative abundance that together with mid-level plant coverage translated into high absolute fish abundance. We suggest this response is related to hypoxia, and where aquatic vegetation is extensive, the effect of vegetation on hypoxia and water quality in general may influence fish populations in a way similar to that often attributed to reduced foraging efficiency and increased competitive interactions.     

Processing of leaf matter by lake-dwelling shredders at low oxygen concentrations

Organic sediments in freshwaters are regularly subject to low concentrations of oxygen. The ability of detritivores to sustain their feeding in such conditions should therefore be of importance for the decomposition process. In the present study, aquaria were used to determine processing rates of five lake-dwelling shredders at three different oxygen concentrations; normoxic (9 mg O₂ l^–1) and two levels of hypoxia (1 and 2 mg O₂ l^–1). Discs of alder leaves (Alnus glutinosa (L.)) were used as food. Four species of caddisfly larvae (Trichoptera Limnephilidae) and the isopod, Asellus aquaticus (L.) were compared in the experiments. Significant differences in processing rates per g animal biomass were found both at normoxia and 2 mg oxygen l^–1. At l mg O₂ l^–1 none of the invertebrates fed on leaf discs. The caddisfly larvae Halesus radiatus (Curtis), being one of the two most efficient shredders at normoxia, did not feed at 2 mg oxygen l^–1. The other species fed at rates 15–50 of that at normoxia. The least efficient shredder at normoxia, A. aquaticus was similar to two of the trichopterans at 2 mg O₂ l^–1. This study shows that the importance of specific shredder species may shift in case of hypoxia. Species-specific traits regarding oxygen sensitivity may also be influential for distribution patterns of shredder species both within and between lakes.     

Food consumption of Sarotherodon mossambicus (Trewaves) exposed to sublethal concentration of diammonium phosphate

S. mossambicus was exposed to toxic and sublethal concentrations of the fertilizer diammonium phosphate (0.2 to 1.0 g l^–1). Mortality, food utilization and growth were studied. At a concentration of 0.6 g l^–1 DAP, 100% mortality was observed within 96 h; no mortality occurred at 0.5 g l^–1; LC50 was 0.55 g l^–1. Rearing the fish in increasing sublethal concentrations of DAP, it was found that the feeding rate decreased from 25.4 mg g^–1 fish^–1 d^–1 (fish reared in DAP-free water) to 10.1 mg g^–1 d^–1 at the highest sublethal concentration (0.5 g l^–1). Growth rate was drastically reduced. At high sublethal concentrations of DAP, the fish lost reserve energy, in addition to the energy obtained from food intake for survival, as a result of increased swimming activity and opercular beats.     

Hypoxia-induced growth rate reduction in two juvenile estuary-dependent fishes

As eutrophication of coastal waters increases, water quality issues such as hypoxia have come to the forefront of environmental concerns for many estuarine systems. Chronic hypoxia during the summer has become a common occurrence in numerous estuaries, degrading nursery habitat and increasing the potential for exposure of juvenile fish to low levels of dissolved oxygen (DO).We conducted a laboratory study to investigate how hypoxic conditions and temperature affect growth rates of two juvenile estuary-dependent fish: the Atlantic menhaden (Brevoortia tyrannus) and spot (Leiostomus xanthurus). For a 2-week period, we exposed the fish to one of four constant DO levels (6.0, 4.0, 2.0 or 1.5 mg O₂ l⁻¹), at one of two temperatures (25 or 30 °C). A fifth DO treatment, included for spot at 30 °C, allowed DO to fluctuate from 10.0 mg O₂ l⁻¹ during the day, to 2.0 mg O₂ l⁻¹ at night. This diel fluctuation approximated the natural DO cycle in tidal estuarine creeks. Size measurements were recorded at the beginning, middle and end of experiments.Growth rates were generally unaffected by low DO until concentrations dropped to 1.5 mg O₂ l⁻¹, resulting in 31-89% growth reductions. Our results suggest that DO levels must be severely depressed, and in fact, approaching lethal limits, to negatively impact growth of juvenile spot and Atlantic menhaden.     

Effects of sublethal exposure to copper on diel activity of sea catfish,Arius felis

The effect of sublethal exposure to copper on the diel activity of sea catfish, Arius felis, was examined after 72 h static exposure to copper (0.0, 0.05, 0.1, or 0.2 mg l^–1 ). The locomotor behavior of 28 individual fish was electronically monitored in a 16-compartment rosette tank. Activity was quantified by the number of entries into compartments per hour. Controls and copper-exposed fish (0.05 mg l^–1) behaved similarly after treatment and displayed no significant effects of handling or copper exposure on diel activity. However, fish exposed to 0.1 and 0.2 mg l^–1 copper were hyperactive immediately following exposure; these fish had significantly greater numbers of entries into compartments per hour for entire 24-h monitoring periods (P < 0.01) and for the 12 h of photophase (P < 0.001) and of scotophase (P < 0.001) than fish in the other two experimental groups. The normal diel activity cycle was abolished. Additionally, exposure of fish to copper at 0.1 and 0.2 mg l^–1 resulted in significantly less variability in activity (P < 0.05) compared to control fish and those exposed to 0.05 mg Cu l^–1.     

Physiological tolerances of juvenile robust redhorse, Moxostoma robustum: conservation implications for an imperiled species

The robust redhorse, Moxostoma robustum (Teleostei: Catostomidae), is an imperiled sucker native to large rivers of the Atlantic slope of the southeastern United States. Juvenile M. robustum were tested for tolerances to temperature, salinity, pH, and hypoxia in order to evaluate basic early life-history requirements. Static (acute) tests resulted in estimates of mean lower temperature tolerances (5.3–19.4 °C) that varied with prior thermal acclimation and indicated no apparent difference in tolerance among fish 30, 60, and 90 days old. Fish acclimated to 20 °C and 30 °C had significantly different mean critical thermal maxima (34.9 °C and 37.2 °C, respectively) and exhibited pronounced increased opercular ventilation rates with elevated temperatures. Fish exposed to acute and chronic increases in salinity showed unusual patterns of mortality above the isosmotic point (9 ppt) that reflected possible differences in body mass and prior acclimation conditions (i.e., water ionic composition); small fish and those held in soft water were the least tolerant of increased salinity. Abrupt exposure to extreme pH values resulted in greater than 50% mortality at pH values below 4.3 and above 9.5 within a 96-hour period. Fish exposed to progressive hypoxia utilized aquatic surface respiration at a mean oxygen concentration of 0.72–0.80 mg O₂ l^-1 (20 °C and 30 °C acclimated fish, respectively), and lost equilibrium at 0.54–0.57 mg O₂ l^-1. Juvenile M. robustum are moderately tolerant of a wide range of ambient physicochemical parameters, but further research is needed to determine how both abiotic and biotic factors have contributed to population decline and extirpation of this species.     

The Methemoglobin Blood Level and Stability of Circulating Erythrocytes of the Rockfish Scorpaena porcus to Osmotic Shock under Conditions of Experimental Hypoxia

Under experimental conditions the oxygen content in water was reduced to 1.3–1.4 mg l^–1 (15% of the saturation) and 2.6–2.7 mg(l^–1 (30% of the saturation). The rockfish (Scorpaena porcus L. ) were kept under these conditions for 90 min. Analysis of samples has shown a decrease of methemoglobin level by 58.2% (p < 0.001) and an increase of erythrocyte stability to the osmotic shock. The cell lysis occurred at lower concentrations of NaCl. It is concluded that the process of natural oxygenation is one of the causes of the rise of the oxidized pigment concentration in the fish blood. This is due to low efficiency of erythrocytic NADH-diaphorases and is compensated by an increase of activity of enzymes of the cell antioxidant protection system. The latter prevents from development of peroxidation processes and lysis of cells during their circulation despite a comparatively high level of methemoglobin in blood.     

Overturn in a hypertrophic,warm, monomictic impoundment (Hartbeespoort Dam,South Africa)

A major decrease in air temperature with an increase in wind stress created thermal instability resulting in overturn. Surface oxygen values decreased from 12.6 (170% saturation) prior to overturn to 0.4 mg · l^–1 (5.7%) after overturn. Recovery of surface oxygen concentrations to supersaturation took approximately three months. Changes in the nitrogen balance after overturn indicated that extremely high rates of nitrification occurred. Little change in phosphorus concentration was observed. Surface chlorophyll concentrations decreased from 235 to approximately 2 mg · m^–3, resulting in very low primary production values (A_max = 12.4 mg C m^–3 h^–1). Reduced rates of primary production were maintained for several months after overturn, mainly owing to the low ratio of Z_eu/Z_m (0.17). No significant changes occurred in the total bacterial population other than a redistribution within the water column. Significant changes occurred in the distribution and density of the zooplankton population. No fish kills were observed although fish populations were oxygen-stressed.     

Uptake and accumulation of zinc in juvenile rainbow trout,Salmo gairdneri

Synopsis The toxicity of zinc to rainbow trout was determined and the 72 h median lethal concentration was found to be 2.00 mg l^–1 in freshwater, hardness 7.50 mg l^–1 as calcium. An insignificant increase in zinc concentration of internal tissues occurred in fish exposed to 1.52 mg l^–1 in freshwater for 72 h. However, there was a significant uptake of zinc by gills and the body surface. Fish exposed to 10 mg l^–1 zinc for 72 h in two-thirds sea water showed significant zinc uptake by liver, rectum and muscle, when compared to control fish. Drinking rate decreased from 1.43 to 0.26 ml kg^–1 h^–1 when zinc sulphate was added to freshwater. Trout adapted to two-thirds sea water showed no decrease in drinking, about 7 ml kg^–1 h^–1 when zinc was added to the water.     

Behavioral responses of red hake,Urophycis chuss,to decreasing concentrations of dissolved oxygen

Synopsis Laboratory experiments were conducted to examine changes in behavior of red hake,Urophycis chuss, under decreasing concentrations of dissolved oxygen (DO). Since the ecological requirements of this species change with age, responses were measured for three different groups: (1) age 0+, = 89 mm total length (TL); (2) age 1+, = 238 mm TL; and (3) age 2–3+, = 397 mm TL. As DO decreased from 8–10 mg l^-1 to < 0.5 mg l^-1, changes were evident in active time, water column activity, range of horizontal movement, food searching, and agonistic behavior. Age 0+ fish were most sensitive, moving up into the water column and swimming continuously as DO levels fell below 4.2 mg l^-1. Age 2–3+ fish were the least responsive, remaining on the substrate and increasing only their range of movement at concentrations below 3 mg l^-1. Responses of age 1 + fish were variable, possibly reflecting a transition stage between the younger and older fish. Common to all groups was the decrease and eventual cessation of food searching.     

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.     

Effect of microbial concentration on biodegradation rates of phenols

Summary Biodegradation rates of 12 phenols were measured with respect to acclimated microbial biomass ranging from 2.3×10⁴ to 2.3×10⁸ cells/l. Rates ranged between 0.02 mg l^–1 day^–1 for 1.6 mg/lp-bromophenol exposed to 2.3×10⁴ cells/l and 1.41 mg l^–1 day^–1 for 3.2 mg/lp-methylphenol exposed to 2.3×10⁸ cells/l. Generally, rates for all phenols were first-order in substrate concentration and zero-order in biomass concentration. Bromophenol biodegradation was preceded by lag periods of varying lengths and to a small extent the rate was dependent on microbial biomass. Results from this study suggest chemical biodegradation generally exhibits pseudo-first-and occasionally, second-order kinetics.     

Continuous acetone-butanol fermentation: influence of vitamins on the metabolic activity of Clostridium acetobutylicum

Summary A detailed investigation was undertaken to examine the influence of biotin and paminobenzoic acid (PABA) in chemostat cultures of Clostridium acetobutylicum ATCC 824. Initiation of chemostat cultures with a basic synthetic medium (biotin 0.01 mg l^–1; PABA 1.0 mg l^–1) have resulted in a low biomass together with a low specific rate of solvent production. A different picture emerged on elevating the concentration of both vitamins 8-fold: biomass and specific rates (solvent production, glucose consumption) were increased and a solvent productivity of 2.54 g l^–1 h^–1 at the solvent concentration of 13.1 g l^–1 was achieved. It has also been shown that PABA was the only limiting factor for the metabolism of Clostridium acetobutylicum in the basic synthetic medium and that the optimised concentration was 8 mg l^–1 in the chemostat cultures with the growth conditions employed.     

Respiratory responses and tolerance to hypoxia in two marine teleosts,Epinephelus akaara (Temminck & Schlegel) and Mylio macrocephalus (Basilewsky)

The respiratory responses and tolerance of hypoxia were studied in two marine teleosts, the red grouper (Epinephelus akaara, a sluggish species) and the black sea bream (Mylio macrocephalus, an active species). Neither species showed abnormal behaviour or mortality when exposed to 2 mg O₂ l^–1 for 7 h. The black sea bream was, however, comparatively more tolerant when exposed to 1 mg O₂ l^–1, but tolerance of both species became similar under extremely hypoxic conditions (i.e. 0.5 mg O₂ l^–1). In contrast to most other teleosts, both species showed a reduction in opercular beating rate during hypoxia, and oxygen conformity was found in the range of 0.5 to 7.0 mg O₂l ^–1. O₂ dissociation curves were constructed, and the P₅₀ value of the black sea breams (27 ± 5.6 mm Hg) was found to be much lower than that of the red groupers (50 ± 2.5 mm Hg). For both species, the general levels of venous PO₂ showed a direct relationship to ambient PO₂, and were markedly reduced after 1 h exposure to various levels of hypoxia. Compared with the red groupers, the black sea breams appeared to be more able to maintain its venous PO₂ levels during prolonged hypoxic exposure.     

Effects of food and silt on filtration,respiration and condition of the freshwater mussel Hyridella menziesi (Unionacea: Hyriidae): implications for bioaccumulation

The effect of exposure to different concentrations of food and suspended silt on filtration, respiration and condition were studied in the freshwater mussel Hyridella menziesi. Using a milk solids-based food and kaolin to simulate silt, mussels were maintained at different combinations of food and silt concentrations for 3 weeks. Between treatments mean filtration rates ranged from 0.97–1.66 l g^–1 h^–1, and respiration from 0.50–1.35 mg O₂ g^–1 h^–1. Silt (non-volatile suspended solids up to 35 mg l^–1) failed to have a significant effect on filtration rate or condition, but with increasing food levels (volatile suspended solids up to 35 mg l^–1) filtration rate was reduced, and condition was reduced at the lowest food concentration (<5 mg l^–1). Respiration showed a food × silt interaction between treatment blocks. When food was low respiration increased with increasing silt concentrations, and when silt was low (<5 mg l^–1) respiration increased with increasing food concentrations. The observed effects of food and silt on filtration, respiration and condition are discussed in terms of their potential for affecting contaminant bioaccumulation. In low-food situations (i.e., <5 mg l^–1), if mussels are pumping large volumes of water, contaminant uptake rates could be enhanced, whereas abundant food would result in lower pumping rates and lower uptake rates. Changes in metabolism with food concentration have implications for contaminant elimination, and changes in biochemical composition associated with changing condition could affect the tissue distribution and retention of contaminants.     

Relationships between water chemistry and respiration rate in several populations of lymnaea peregra Müller (Gastropoda: Mollusca)

Respiration rates of specimens of L. peregra obtained from waters with a wide range of ionic concentrations were measured using a constant volume respirometer. Multiple regression analysis showed that hardness of the water from which the snails were taken could significantly affect dry-wet weight relationships, oxygen consumption in relation to shell-free dry weight and oxygen consumption in relation to live wet weight. However there were also significant differences in respiration rate between populations from within water hardness categories (hard > 40 mg l^–1 Ca²⁺, medium 40 – 10 mg l^–1 Ca²⁺, soft < 10 mg l^–1 Ca²⁺). In experiments with constant anion but different cation concentrations, calcium and potassium concentrations were the most significant variables tested. There were no differences in respiration rates of animals taken from the same sites in different years.     

Physical and chemical post-dam alterations in the Jamari River, a hydroelectric-developed river of the Brazilian Amazon

The regulation of the Jamari River advanced peak floods by 1–2 months and increased dry-season discharges from 60 to 200 m³ s^–1, resulting in water levels approximately 1 m above those recorded before regulation. Daily variation in water level associated with fluctuations in electricity production by the dam propagated to the lower reaches of the tributary Candeias River. Dissolved oxygen (DO), temperature, pH, and conductivity measured over 1.5 years on three locations along the regulated and two free-flowing rivers indicated important alterations in the case of oxygen concentrations. DO levels shifted from saturated (7–8 mg l^–1) during the flood season (when the spillways were open releasing epilimnetic water) to hypoxic (1.5–3 mg l^–1) during the dry season (when the floodgates were closed and only hypolimnetic water used to run the turbines was feeding the river). Fluctuations in water level and variation in dissolved oxygen tended to be greater at the site closest to the dam, gradually attenuating downstream. Mitigation of the downstream effects of river regulation would require modifications in the operation of the dam.     

Comparative evaluation of the larvivorous fish Gambusia affinis and Aphanius dispar as mosquito control agents

Under laboratory conditions Aphanius was more successful than Gambusia in preying upon the 3rd, 4th and pupal stages of mosquitoes. The reverse was found for the first two instars. However. Aphanius consumed more 2nd instar larvae under the cover of vegetation when larger fish were able to penetrate shallow water and feed on the mosquito larvae.The two species showed a similar prey-size selection except for Aphanius of the medium size (31–35 mm) which ate larger larvae than Gambusia of the same size range.When provided access to the surface, neither fish species showed any adverse effect at oxygen levels as low as 0.5 mg l^-1 (6% saturation). When denied access to the surface, both species behaved normally at oxygen levels as low as 1.3 mg l^-1 (15% saturation).This study suggests that Gambusia affinis and Aphanius dispar can complement each other as mosquito control agents in different habitat conditions. We suggest that in mosquito infested situation which are characterized by high organic matter and low oxygen levels biological control could best be achieved by introduction of a range of sizes of both fish species. Repeated introductions of the fish, in large enough numbers, may be required for ad-hoc alleviation of a mosquito problem. Best results are thus to be expected in relatively small water bodies such as oxidation ponds.     

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.     

Sub‐lethal effects on fish provide insight into a biologically‐relevant threshold of hypoxia

Hypoxia (low dissolved oxygen) is a mounting concern for aquatic ecosystems as its prevalence increases with rising anthropogenic nutrient inputs. Hypoxia is most commonly defined as 2.0 mg l^–1 of dissolved oxygen, although this level varies widely across studies and agency regulations. Such definitions may be too conservative, as ecologically‐relevant non‐lethal effects (e.g. consumption and growth) of hypoxia on important aquatic species, such as fish, often occur at oxygen levels much higher than 2.0 mg l^–1. In addition, many mechanisms that regulate hypoxia tolerance in fish have been proposed, including temperature, habitat, location in the water column, and body size, but there is ongoing debate over which mechanisms are most important. Using a structured meta‐analysis of published studies, we showed consistent, significant negative effects on fish growth and consumption below 4.5 mg l^–1. While the total amount of variation explained was generally low, below 4.5 mg l^–1 of dissolved oxygen, phylogenetic relationships accounted for most of the explained variation in fish growth. Ecological factors including body size, location in the water column (pelagic, demersal, or benthopelagic), habitat (freshwater, marine, or diadromous), and temperature explained very little of the effect of hypoxia on fish growth and explained only a moderate level of variation in consumption. Our results suggest a dramatically higher threshold for sub‐lethal effects of hypoxia on fish than oxygen levels generally set for regulation purposes, and provide little support for accepted ecological mechanisms thought to influence hypoxia tolerance.