Role of organophosphates in adaptations of an obligate water-breathing teleost (Tilapia nilotica L.) to hypoxia

Dry matter, total carbon (C), nitrogen (N) and phosphorus (P) content of mature bream from Lake Balaton were investigated and the quantities of N and P stored in the bream population and their possible removal by fishery were estimated. Carbon made up 43.3–44.8% of dry weight, N made up on average 10.6% of the dry weight of bream and P accounted for a further 2.7%. About 3.3 kg N ha^–1 and 0.9 kg P ha^–l are stored in the bream population. Approximately 0.5 kg N ha^–1 and 0.1 kg P ha^–1 are removed from the lake by bream harvest. Taking into account the total fish yield, the N removal is 2.1% and P removal 3.4% of the amount entering the lake.     

Trophic relationships between primary producers and fish yields in Lake Balaton

Relationships between chlorophyll a content of the water, the shoreline-length: water area ratio and the annual total fish yield as catch per unit effort (CUE: kg ha^–1 100 h^–1 as annual mean values) have been calculated by multivariable regression. The determination coefficient (r ² = 0.913) showed a significant dependence of fish yield on morphometry of different lake areas. Accordingly, fish carrying capacity of the open water areas, calculated from chlorophyll a content and S/A, ranged from 12 to 34%, but that of the littoral zone between 66 and 88%. These findings have also been supported by echo-sounding records of the horizontal distribution of fish.Bream (Abramis brama L.) contributes the majority (70–80%) of fish stock and yield. Its food mainly consists of zooplankton and benthic invertebrates in ratios that are widely variable with season and depend on the age of fish. Average daily food consumption of individuals (age group 3 + and over) varies between 2 and 5 g. Bream consumes two- to three-times more food in the SW basin than in the NE one. This means that the present stocks inhabiting areas from NE to SW consume annually 13249–20085 t yr^–1 of food. According to estimated calorific values, the annual energy consumption of local populations along the longitudinal axis of the lake varies between 93 and 141 kJ m^–2 yr^–1. The efficiency of energy transfer from primary producers to fish is low and varies from 0.04 to 0.1%.     

Phosphorus,nitrogen, and carbon cycling by fish populations in two small lowland rivers in Poland

Amounts of C, P, and N consumed by all fish populations were estimated at 9 sites in two small lowland rivers. They mainly depended on fish density and were: 151.8 (27.9–453.3) kgC ha^–1a^–1, 3.1(0.5–8.8) kgP ha^–1 a^–1, and 30.3 (5.3–89.9) kg N ha^–1 a^–1. To build one kg of each of these elements into their body the fish consumed 7.9 ± 1.7 (% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabeiEayaara% aaaa!3912!\[{\text{\bar x}}\] ± S.D.) kg of C, 3.1 ± 0.8 kg of P, and 6.6 ± 1.3 kg of N. Thus, phosphorus was assimilated twice more efficiently than carbon and nitrogen. Pools of the three elements, calculated as mean biomass, are: 12.7 (1.2–42.1) kg C ha^–1, 0.7(0.1–2.2) kgP ha^–1, and 3.0 (0.3–9.7) kgN ha^–1 The elements were assimilated especially effectively by young stages of fish.     

Particulate organic matter in a mountain stream in the south-western Cape,South Africa

The quality and quantity of allochthonous inputs and of benthic organic matter were investigated in a second-order, perennial mountain stream in the south-west Cape, South Africa, between April 1983 and January 1986. Although the endemic, riparian vegetation is sclerophyllous, low and evergreen, inputs of allochthonous detritus to the stream (434 to 500 g m^–2y^–1) were similar to those recorded for riparian communities worldwide, as were calorific values of these inputs (9548 to 10 032 KJ m^–2y^–1). Leaf fall of the riparian vegetation is seasonal, occurring in spring (November) as discharge decreases, resulting in retention of benthic organic matter (BOM) on the stream bed during summer and early autumn (maximum 224 g m^–2). Early winter rains (May) scoured the stream almost clean of benthic detritus (winter minimum 8 g m^–2). Therefore, BOM was predictably plentiful for about half of each year and predictably scarce for the other half. Coarse BOM (CBOM) and fine BOM (FBOM) constituted 46–64% of BOM standing stock, ultra-fine BOM (UBOM) 16–33% and leaf packs 13–24%. The mean annual calorific value of total BOM standing stock was 1709 KJ m^–2. Both standing stocks and total calorific values of BOM were lower than those reported for streams in other biogeographical regions. Values of C:N ratios decreased with decrease in BOM particle size (CBOM 27–100; FBOM 25–27; UBOM 13–19) with no seasonal trends. The stream is erosive with a poor ability to retain organic detritus. Its character appears to be dictated by abiotic factors, the most important of which is winter spates.     

Abundance and distribution of a brown trout (Salmo trutta, L.) population in a remote high mountain lake

Brown trout (Salmo trutta) population of a remote high mountain lake (Lake Redó, Pyrenees, 2240 m above sea level) were studied during autumn using hydroacoustic techniques. This acoustic technique was for the first time used on fish at such high altitude in Spain. Sampling using multimesh nets fish catches and echosounding recording were carried out in September 1998. Mean density of fish was estimated to be 1.82 fish per 1000 m³ (597 fish ha^–1). The results exhibited mainly a littoral habitat, with the brown trout being preferentially in the 10–25 m deep layer, where the water was warmer and the richness and diversity of macroinvertebrates higher. The sampling by hydroacoustic technique found a length frequency range of fish higher than the multimesh gill nets but both of them estimated similar fish proportion for the common length range. The more frequent target strength (TS) for the population was –38 dB (TS range –37 to –39 dB). Good correlation was found between TS frequency distribution obtained by echosounding and that predicted by means of a model based on the log of the fish total length from multimesh gill nets captures.     

Allochthonous input and retention in a small mountain stream,South Africa

Allochthonous input and benthic coarse particulate organic matter (CPOM) standing stocks were investigated in a first-order stream in South Africa between May 1984 and April 1985. Monthly falls into the stream of all litter types (total) ranged from 11 (September) to 79 g m^–2 (March). Total annual litter fall was 426 g dry weight, which corresponds to 1.2 g m^–2 d^–1. Flowers, fruits and seeds contributed 37 g m^–2, woody debris, 122 g m^–2, and leaves 267 g m^–2 to this total. Leaf fall from native trees, which accounted for approximately 57% of total litter input (244 g m^–2 a^–1), was significantly higher in summer than in winter. The summer peak in leaf fall recorded is far smaller and more protracted than the autumnal peak recorded for many Northern Hemisphere streams.Monthly total standing stocks of CPOM ranged from 14 g dry weight m^–2 in January to 69 g m^–2 in August, and a mean total CPOM standing stock of 41 g m^–2 mth^–1 was estimated. This comprised 18 g m^–2 mth^–1 soft litter, and 23 g m^–2 mth^–1 hard litter. CPOM standing stocks showed no seasonal trends, and with the exception of two species, standing stocks of endemic leaf species reflected their contributions to the total litter fall. Contrary to earlier reports for streams in the Fynbos Biome, Window Stream has CPOM standing stocks well within the ranges reported for low-order streams worldwide.     

Net primary productivity of two mangrove forest stands on the northwestern coast of Sri Lanka

Productivity studies were carried out from September, 1985 to August, 1987 in two mangrove stands, i.e. estuarine and island fringing, in Dutch bay, a lagoon situated on the northwestern coast of Sri Lanka. Net above-ground primary productivity was measured by monitoring litterfall and above-ground biomass increment. The average annual rate of litterfall in the estuarine and island-fringing mangrove stands are 588.14 g m^–2 (approximately 6 t ha^–1) and 407.33 g m^–2 (approximately 4 t ha^–1) respectively. The average annual rates of above ground woody growth are 614.74 g m^–2 (approximately 6 t ha^–1) in the estuarine stands and 286.8 g m^–2 (approximately 3 t ha^–1) in the island-fringing mangrove stands. Hence estuarine mangrove stands record a higher annual rate of above-ground net primary production (NPP; 1207.88 g m^–2 or approximately 12 t ha^–1) than the fringing mangrove stands (694.22 g m^–2); approximately 7 t ha^–1). The annual rate of NPP in the water front zones of the stands (1300.47 g m^–2 in the estuarine stands and 874.56 g m^–2 in the fringing stands) are greater than those in the back-mangrove zones (115.28 g m^–2 in the estuarine stands and 513.88 g m^–2 in the island-fringing stands). These variations may be attributed to the differences in tidal flushing and influence of freshwater in the two localities.     

The diversity,biomass and production of zooplankton in Lake Inarijärvi

About 650 zooplankton samples were collected from Lake Inarijärvi in 1977–1979 from the littoral and pelagial zones of the lake. One hundred and twenty-three zooplankton taxa were found and most of them can be considered euplanktonic.The most important species were Holopedium gibberum, Daphnia cristata, Cyclops spp. and Eudiaptomus spp. Mean pelagial zooplankton biomass was 0.29 g m^–3 in the 0–5 m depth zone, 0.17 g m^–3 in 5–10 m and 0.11 g m^–3 in 10–20 m.The zooplankton biomass at a sandy shore was about 0.09 g m^–3, at a stony shore 0.05 g m^–3 and at a vegetated shore 0.76 g m^–3. About 70% of the whole zooplankton production consisted of crustaceans.The sum of herbivore and carnivore zooplankton production in the pelagial area during the summer was 210–330 kg ha^–1 × 3 months.     

Impacts of Reservoir Creation on the Biogeochemical Cycling of Methyl Mercury and Total Mercury in Boreal Upland Forests

The FLooded Upland Dynamics Experiment (FLUDEX) at the Experimental Lakes Area (ELA) in northwest Ontario was designed to test the hypothesis that methylmercury (MeHg) production in reservoirs is related to the amount, and subsequent decomposition, of flooded organic matter. Three upland forest sites that varied in the amounts of organic carbon stored in vegetation and soils (Low C, 30,870 kg C ha⁻¹; Medium C, 34,930 kg C ha⁻¹; and High C, 45,860 kg C ha⁻¹) were flooded annually from May to September with low-organic carbon, low-MeHg water pumped from a nearby lake. Within five weeks of flooding, MeHg concentrations in the reservoir outflows exceeded those in reservoir inflows and remained elevated for the duration of the experiment, peaking at 1.60 ng L⁻¹ in the Medium C reservoir. We estimated the net production of MeHg in each reservoir by calculating annual changes in pools of MeHg stored in flooded soils, periphyton, zooplankton, and fish. Overall, there was an initial pulse of MeHg production (range = 120–1590 ng m⁻² day⁻¹) in all FLUDEX reservoirs that lasted for 2 years, after which time net demethylation (range = 360–1230 ng MeHg degraded m⁻² day⁻¹) began to reduce the pools of MeHg in the reservoirs, but not back to levels found prior to flooding. Rates of MeHg production were generally related to the total amount of organic carbon flooded to create the reservoirs. Large increases in MeHg stores in soils compared to those in water and biota indicate that flooded soils were the main sites of MeHg production. This study should assist hydroelectric utilities and government agencies in making informed decisions about selecting sites for future reservoir development to reduce MeHg contamination of the reservoir fisheries.     

Fish production in the Warta River,Poland: a preimpoundment study

The total mean biomass and production of fish populations were estimated at 80–116 kg ha^–1 and 135–164 kg ha^–1 yr^–1, respectively, at two sites of a large alluvial river (the Warta River, Poland). In comparison with data for two large, temperate zone rivers (Thames, Pilica) the present values are intermediate, as is the degree of eutrophication in the rivers' water.     

The potential of fish production based on periphyton

Periphyton is composed of attached plant andanimal organisms embedded in amucopolysaccharide matrix. This reviewsummarizes research on periphyton-based fishproduction and on periphyton productivity andingestion by fish, and explores the potentialof developing periphyton-based aquaculture.Important systems with periphyton arebrush-parks in lagoon areas and freshwaterponds with maximum extrapolated fish productionof 8 t ha^–1 y^–1 and 7 t ha^–1y^–1, respectively. Experiments with avariety of substrates and fish species havebeen done, sometimes with supplemental feeding.In most experiments, fish production wasgreater with additional substrates compared tocontrols without substrates. Colonization ofsubstrates starts with the deposition oforganic substances and attraction of bacteria,followed by algae and invertebrates. Afterinitial colonization, biomass density increasesto a maximum when competition for light andnutrients prevents a further increase. Often,more than 50% of the periphyton ash-free drymatter is of non-algal origin. Highest biomass(dm) in natural systems ranges from 0 to 700g m^–2 and in aquaculture experiments wasaround 100 g m^–2. Highest productivity wasfound on bamboo in brush-parks (7.9 gC m^–2 d^–1) and on coral reefs (3 gC m^–2 d^–1). Inorganic and organicnutrients stimulate periphyton production.Grazing is the main factor determiningperiphyton density, while substrate type alsoaffects productivity and biomass. Better growthwas observed on natural (tree branches andbamboo) than on artifical materials (plasticand PVC). Many herbivorous and omnivorous fishcan utilize periphyton. Estimates of periphytoningestion by fish range from 0.24 to 112 mg dm(g fish)^–1 d^–1. Ingestion rates areinfluenced by temperature, fish size, fishspecies and the nutritional quality of theperiphyton. Periphyton composition is generallysimilar to that of natural feeds in fishponds,with a higher ash content due to the entrapmentof sand particles and formation of carbonates.Protein/Metabolizable Energy (P/ME) ratios ofperiphyton vary from 10 to 40 kJ g^–1.Overall assimilation efficiency of fish growingon periphyton was 20–50%. The limited work onfeed conversion ratios resulted in valuesbetween 2 and 3. A simple simulation model ofperiphyton-based fish production estimates fishproduction at approximately 2.8 t ha^–1y^–1. Together with other food resources infishponds, total fish production with thecurrent technology level is estimated at about5 t ha^–1 y^–1. Because grazingpressure is determined by fish stocking rates,productivity of periphyton is currently themain factor limiting fish production. Weconclude that periphyton can increase theproductivity and efficiency of aquaculturesystems, but more research is needed foroptimization. Areas for attention include theimplementation and control of periphytonproduction (nutrient levels, substate types andconformations), the ratio of fish to periphytonbiomass, options for utilizing periphyton inintensive aquaculture systems and with marinefish, and possibilities for periphyton-basedshrimp culture.     

Seasonal variation in phytoplankton primary production in relation to light and nutrients in Lake Awasa,Ethiopia

The biomass and primary production of phytoplankton in Lake Awasa, Ethiopia was measured over a 14 month period, November 1983 to March 1985. The lake had a mean phytoplankton biomass of 34 mg chl a m^–3 (n = 14). The seasonal variation in phytoplankton biomass of the euphotic zone (mg chl a m^–2 h^–1) was muted with a CV (standard deviation/mean) of 31%. The vertical distribution of photosynthetic activity was of a typical pattern for phytoplankton with light inhibition on all but overcast days. The maximum specific rates of photosynthesis or photosynthetic capacity (Ø_max) for the lake approached 19 mg O₂ (mg chl a)^–1 h^–1, with high values during periods of low phytoplankton biomass. Areal rates of photosynthesis ranged between 0.30 to 0.73 g O₂ m^–2 h^–1 and 3.3 to 7.8 g O₂ m^–2 d^–1. The efficiency of utilisation of PhAR incident on the lake surface varied from 2.4 to 4.1 mmol E^–1 with the highest efficiency observed corresponding to the lowest surface radiation. Calculated on a caloric basis, the efficiency ranged between 1.7 and 2.9%. The temporal pattern of primary production by phytoplankton showed limited variability (CV = 21 %).     

Feeding of Burbot, Lota lota, at Different Temperatures

Daily food intake of adult burbot, Lota lota, fed on vendace, Coregonus albula, were estimated experimentally at four different water temperatures (2.4, 5.1, 10.8 and 23.4°C). Mean daily food intake (MDI; g d^–1) and relative daily food intake (RDI; g g^–1 d^–1) increased with temperature from 2.4 to 10.8°C and decreased at 23.4°C. Temperatures of maximum daily food intake values were 13.6°C for MDI and 14.4°C for RDI. No correlation between food intake values and burbot weight was observed. RDI values were used to estimate annual food consumption of burbot population. Annual food consumption estimates were 9.7kg ha^–1 and 24.3kg ha^–1 when burbot biomass was 2.0 or 5.0kg ha^–1, respectively.     

Predator-induced bottom-up effects in oligotrophic systems

Five treatments (replication n=2) were applied to mesocosms in an oligotrophic lake (TP=6–10 µg 1^-1) to assess the effects of fish on planktonic communities. The treatments were: (1) high fish (30 kg ha^–1 Lepomis auritus, Linnaeus), (2) low fish (10 kg ha^–1), (3) high removal of zooplankton, (4) low removal of zooplankton and (5) control. Total phosphorus, chlorophyll a, zooplankton biomass, and species richness decreased from high fish > low fish > control > low removal > high removal treatments. The fish treatments were dominated by crustacean zooplankton, while rotifers outnumbered the other zooplankters in the removal treatments. Calculations of zooplankton grazing rates suggested that clearance rates seldom exceeded 2% of the enclosure volume d^–1 and were unlikely to have had much influence on phytoplankton biomass. Calculations from a phosphorus bioenergetics model revealed that when fish were present, their excretion rates were higher than the rates ascribed to zooplankton. Diet analysis showed that the fish derived most of their energy from the benthos and periphyton, and that fish excretion and egestion made significant contributions to the very oligotrophic pelagic phosphorus pool. In the absence of fish, zooplankton excretion was highest in the control treatments and lowest in the zooplankton removal treatments. Our results suggest that in oligotrophic systems, planktivorous fish can be significant sources of phosphorus and that fish and zooplankton induced nutrient cycling have significant impacts on planktonic community structure.     

Impacts of bleak (Alburnus alburnus) and roach (Rutilus rutilus) on water quality,sedimentation and internal nutrient loading

Lake Vesijärvi, southern Finland, suffered sewere eutrophication by sewage effluent from the city of Lahti during the 1960's and the early 1970's. The municipal sewage loading was diverted from the lake in 1976 and the lake started to recover. However, in the 1980's blue-green algal blooms increased again and the recovery of the lake faded. Enclosure experiments demonstrated that high roach (Rutilus rutilus) biomass is one of the key factors in the fading recovery of the lake. In this study, the influence of roach and another cyprinid fish species (bleak, Alburnus alburnus) to planktonic algal productivity and biomass in Lake Vesijärvi was examined. Enclosure experiments in the field showed the impacts of planktivorous bleak on water quality; in an enclosure with a density of 1 fish m^–2 average daily algal production (1370 mg C m^–2) and chlorophyll-a concentration (50–90 µg 1^–1) were more than twice that in an enclosure without fish. Laboratory experiments showed that the availability of planktonic food affects the foraging behaviour of roach and consequently the internal nutrient loading from the sediment into the water. Roach caused the highest phosphorus loading and turbidity when there was no zooplanktonic food available in the water. The possible interactions between planktivorous and omnivorous fish species are discussed.     

Fish stock assessments in Lake Konnevesi

In 1977, the average fish yield in the oligotrophic and oligohumic lake Konnevesi was 6.4 kg ha^–1 (2.6 kg vendace, 1.3 kg perch, 0.7 kg whitefish, 0.5 kg roach, 0.3 kg pike and 0.3 kg burbot). In that year the stock of vendace was exceptionally poor. A study was made to determine the optimum strategy for exploiting the stocks. The field work included test fishing (1970–78), sampling of catches (1977–80) and collection of catch per unit effort data (1978–80), and catch statistics (1969, 1970, 1977). The stocks were assessed by the Beverton and Holt method. Whitefish is exploited at a proper level. Fishing of vendace and roach can be increased, and the yield of pike would be greater, if the recruiting age were higher than at present.     

Growth,biomass, mortality,production and potential yield of the West African clam,Egeria radiata (Lamarck) (Lamellibranchia,Donacidae) in the Cross River system,Nigeria

An important fishery for the West African hemi-estuarine clam, Egeria radiata exists in the lower reaches of the Cross River (CR) system. The stock is heavily exploited but its productivity and potential yield have not been studied. This paper discusses the distribution and ecology of, and presents data on growth, abundance, mortality, biological production and potential yield of the CR stock of E. radiata based principally on length structured relative age. Growth was exponential and conformed to the von Bertalanffy's growth model (VBGM), with the parameters L = 9.3 cm, K = 0.36 and T _o = –0.10 year. The instantaneous total mortality coefficient (Z) by the catch curve method was 0.82 yr^–1; natural mortality coefficient (M) was estimated at 0.32 yr^–1. Total mean biomass computed from data obtained in two month's grab sample surveys in the area of occurrence of E. radiata was 1923 kg ha^–1 for total (i.e. flesh + shell) and 628 kg ha^–1 for flesh (i.e. shell-free) production units. The production-to-mean-biomass (or P/B ratio, R) were 0.31 and 0.52 for total and shell-free production units respectively. The maximum sustainable yield (Y _max) was estimated at 398.4 kg ha^–1 for total and 112.2 kg ha^–1 for shell-free production units. With an exploitation ratio (E) of 0.61 and an estimated present catch rate of 1970 tonnes(t) yr^–1 total weight (or about 82% of Y _max) the Egeria stock of the CR is currently being fished above the level of its optimum yield. Much of the fishing pressure is on the very young clams, thus lowering the age at first capture (T _c) to 1 year. This is an unhealthy situation for a species with limited area of distribution and low (< 1.0) P/B ratio; it can lead to rapid extinction of the stock if for some reason there is a substantial reduction in annual recruitment. It is recommended that in order to conserve this valuable resource and keep it at a level of sustained optimum yield, there should be a regulation prohibiting the landing of young clams (< 6.0 cm shell length). Furthermore the fishermen should learn to cultivate the animal by transplanting the young clams to new beds upstream to give them time to grow larger before harvesting. This type of management has been practised in Ghana for a long time and has helped to conserve the Egeria stock of the Volta River. The need for a continuous monitoring of the effluents from the Nigerian Newsprint Manufacturing Company's factory located at the bank of the CR in the centre of the clam industry is stressed; such monitoring would ensure that dangerous toxic chemicals are not discharged into the river to damage the Egeria industry.     

Respiration, nutrient excretion and filtration rate of tropical freshwater mussels and their contribution to production and energy flow in Lake Kariba, Zimbabwe

The productivity and ecological role of benthos in man-made Lake Kariba was assessed through the use of P/B-ratios and by measuring the metabolism (respiration, N and P excretion) of the most abundant mussel species (Aspatharia wahlbergi, Corbicula africana and Caelatura mossambicensis) in laboratory experiments. For A. wahlbergi also filtration rate was estimated.The annual production of benthos for the populated 0–12 m interval was estimated at 11.0 g m ^–2 yr^–1 (shellfree dry weight) of which mussels contributed for 8.81 g (80%), snails 2.16 g (20%) and insects 0.03 g (0.3%) respectively. The most important mussel species in the lake were Caelatura mossambicensis (4.97 g m^–2 yr^–1) and Corbicula africana (3.33 g). The dominant snail species was Melanoides tuberculata (1.63 g). For the total lake, also including deeper unpopulated bottoms, the annual production of benthos was 2.70 g m^–2 yr^–1 (shell-free dry weight).Respiration and excretion varied with temperature displaying a bell-shaped relationship. Metabolic rates in Aspatharia wahlbergi increased about 5× between 16.5 °C and the maximum at 34.0 °C and then decreased again at 39.0 °C, when the mussels showed signs of severe stress. Metabolism in Corbicula africana had a lower optimum with fairly constant activity between 18.6 and 29.2 °C, rapidly decreasing above this temperature.The average respiration, nutrient excretion and water filtration rates for mussels in Lake Kariba at 25.2 °C were estimated to about 0.6 mg O₂ 85 µg NH₄–N, 1.5 µg PO₄–P and 0.51 water filtered h^–1 g^–1 shellfree dry weight. This gives that a volume corresponding to about the total epilimnion of the lake is filtered by the mussels annually. Further, mussels can be estimated to remineralise % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaGymaiaac+% cacaaI0aaaaa!3A2B!\[1/4\] of the total load of phosphate, and 8 times the total load of nitrogen every year. The population needs 3.5 × 10⁴ tons of organic carbon for its maintainance, which indicates that about 5% of the annual phytoplankton production is channeled through mussels. We conclude that the mussels, rather than being an important food source for fish, seem to play a large role in the nutrient dynamics of Lake Kariba.     

A quantitative study of seasonal foraging by the grass harvesting termite,Trinervitermes geminatus (Wasmann), (Isoptera,Nasutitermitinae) in Southern Guinea savanna,Mokwa, Nigeria

Summary Trinervitermes geminatus (Wasmann) harvests standing grass tussocks which are cut into pieces and carried back to the nest. During this activity a certain proportion is left on the soil surface as litter whilst some is ingested. Foraging is mainly accomplished during the dry season (mid-October to the end of April) for periods of 2–4 h daily, either early in the morning, in the evening, or occasionally during both morning and evening. The duration and daily pattern of foraging is partly dependent on temperature, with a lower temperature threshold of 20° C and an upper threshold of 35° C below or above which foraging is restricted.The quantity of grass cut down and carried back to the nest by a population of 737 m^-2 (3.08 g m^-2) was estimated at 60.3 kg ha^-1a^-1 with approximately 18 kg ha^-1a^-1 cut and left as litter. Estimated consumption whilst foraging was approximately 20.4 kg ha^-1a^-1, giving a total consumption of about 81 kg ha^-1a^-1. Compared with a total estimated grass production of 3157 kg ha^-1a^-1 and consumption by cattle of 1404 kg ha^-1a^-1, the quantity of grass removed by T. geminatus, amounting to only 3.1% of the net primary production, did not appear to be economically significant in this locality.     

Soil N mineralization and nitrification in relation to nitrogen solution chemistry in a small forested watershed

Spatial variations in soil processes regulating mineral N losses to streams were studied in a small watershed near Toronto, Ontario. Annual net N mineralization in the 0–8 cm soil was measured in adjacent upland and riparian forest stands using in situ soil incubations from April 1985 to 1987. Mean annual rates of soil N mineralization and nitrification were higher in a maple soil (93.8 and 87.0 kg.ha^–1) than in a pine soil (23.3 and 8.2 kg.ha^–1 ). Very low mean rates of mineralization (3.3 kg.ha^–1) and nitrification (3.4 kg.ha^–1) were found in a riparian hemlock stand. Average NO₃-N concentrations in soil solutions were 0.3–1.0 mg.L^–1 in the maple stand and >0.06mg.L^–1 in the pine stand. Concentrations of NO₃–N in shallow ground water and stream water were 3–4× greater in a maple subwatershed than in a pine subwatershed. Rapid N uptake by vegetation was an important mechanism reducing solution losses of NO₃–N in the maple stand. Low rates of nitrification were mainly responsible for negligible NO₃–N solution losses in the pine stand.