Biological studies upon Enchytraeus variatus Bouguenec & Giani 1987 in breeding cultures

The life history of an enchytraeid worm, Enchytraeus variatus, was studied under laboratory conditions at 18–22 °C. This species can reproduce simultaneously by asexual (architomy) or sexual reproduction. The number of ova per cocoon varies from 5 to 20 (x = 10.9). The generation period (from cocoon to next cocoon) varies from 14 to 39 days (x = 26.1) according to the period of the year. The number of generations per year is between 7.3 and 26.1 (x = 14). A mature worm can lay between 23.7 and 25.8 cocoons during its life (254 days as maximum observed) at a mean rate of 0.12 cocoon worm^–1 day^–1. Experimental cultures were carried out to determine the structure, density and biomass of the populations. A maximal density of 1 396 314 worms was recorded after 85 days of culture. Net production reached 21.48 g m^–2 day^–1 after 26 days in a culture initiated from cocoons.     

Contemporary state of the zooplankton in Lake Peipsi

L. Peipsi is one of the richest fish lakes in Europe. Planktivorous smelt dominates in the fish fauna. The abundance of zooplankton fluctuates between 43 600–2241 500 ind m^–3, with the average 974 000 ind m^–3, biomass ranges from 0,09–3,69 g m^–3, with the average 1,86 g m^–3. Since the 1960s the abundance of rotifers has risen considerably while the mean zooplankter weight (B/N) has decreased from 0.005 mg to 0.004 mg. Zooplankton production (herbivores 20.6, predators 1.8, whole zooplankton community 22.4 g C m^–2 per period between May and October) can be considered high. Predatory zooplankton eats on an average 50% of the production of herbivorous zooplankton; about 50% of the whole zooplankton production (P_{Filt + Pred}) reaches fishes. The production of herbivorous zooplankton constitutes 10.1% of primary production. This ratio indicates a direct relationship between zoo- and phytoplankton in the food chain; the detrital food chain seems of little importance. About 6% of phytoplankton energy reaches fishes. The transformation of energy in the food web is efficient. On the basis of zooplankton L. Peipsi can be considered a moderately eutrophic or meso-eutrophic lake.     

Populations and production of fish in two small tributaries of the Paraná River,Paraná, Brazil

Mean biomass (153-1) and production (P) of fish in two small tributaries of the Paraná River (Paraná, Brazil) were 61 kg ha^–1 and 48 kg ha^–1 yr^–1 in the Caracu River and 29 kg ha^–1 and 26 kg ha^–1 yr^–1 in the Agua do Rancho River, respectively. Matrix correlation analysis revealed high positive correlations of both 153-2 and P to maximum depth and hiding places and, at a lower level of significance, to mean depth, pH and oxygen level. Lower 153-3 and P values were found in the Agua do Rancho River, whose valley has retained a more natural character, rich canopy and scarcity of macrophytes, but also lower conductivity and nitrogen and phosphate levels than those in the Caracu River.Address for correspondence     

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.     

Annual production by a stream brook charr population and by its principal invertebrate food

Synopsis Annual production by the brook charr (Salvelinus fontinalis) population in Valley Creek, Minnesota, over the five-year period 1968–1972, was closely correlated to production by its main invertebrate food, Gammarus pseudolimnaeus, which had been reduced by siltation. Annual production was 163–191 kg ha^–1 (wet weight) in 1968–1969, respectively, and then dropped to a mininum of 79 kg ha^–1 in 1971, the year most seriously affected by decreased invertebrate production and siltation; year-class strength, standing stock, and total year-class (cohort) production followed approximately the same pattern. Total year-class production was highest for the 1968–1969 year classes at 211–178 kg ha^–1, respectively, and lowest for the 1971 year class at 76 kg ha^–1. Annual P/B ratios ranged from 1.0 to 1.9; cohort P/B ratios, for the 1968–1972 year classes, ranged from 5.6 to 7.2.Paper No. 11,384, Scientific Journal Series, Minnesota Agricultural Experiment Station, St. Paul, Minnesota 55108, U.S.A.     

Zooplankton feeding in the fish Trichogaster pectoralis Regan

The traditional culture method of Trichogaster pectoralis, using zooplankton produced from fermenting aquatic weeds as a source of food for the fry, leads to productions of ca 1 000 kg ha^–1 a^–1. If chicken manure is applied at a rate of ca 450 kg ha^–1 month^–1 instead, much more zooplankton is produced more rapidly, and fish production increases to ca 2 000 kg ha^–1 a^–1. This results from a higher survival rate of the fry, which feeds on rotifers, ciliates, copepods and cladocera until a size of ca 15.0 mm, when plant material also becomes important in the diet.     

Carbon balance of a tropical savanna of northern Australia

Through estimations of above- and below-ground standing biomass, annual biomass increment, fine root production and turnover, litterfall, canopy respiration and total soil CO₂ efflux, a carbon balance on seasonal and yearly time-scales is developed for a Eucalypt open-forest savanna in northern Australia. This carbon balance is compared to estimates of carbon fluxes derived from eddy covariance measurements conducted at the same site. The total carbon (C) stock of the savanna was 204±53 ton C ha^–1, with approximately 84% below-ground and 16% above-ground. Soil organic carbon content (0–1 m) was 151±33 ton C ha^–1, accounting for about 74% of the total carbon content in the ecosystem. Vegetation biomass was 53±20 ton C ha^–1, 39% of which was found in the root component and 61% in above-ground components (trees, shrubs, grasses). Annual gross primary production was 20.8 ton C ha^–1, of which 27% occurred in above-ground components and 73% below-ground components. Net primary production was 11 ton C ha^–1 year^–1, of which 8.0 ton C ha^–1 (73%) was contributed by below-ground net primary production and 3.0 ton C ha^–1 (27%) by above-ground net primary production. Annual soil carbon efflux was 14.3 ton C ha^–1 year^–1. Approximately three-quarters of the carbon flux (above-ground, below-ground and total ecosystem) occur during the 5–6 months of the wet season. This savanna site is a carbon sink during the wet season, but becomes a weak source during the dry season. Annual net ecosystem production was 3.8 ton C ha^–1 year^–1.     

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.     

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.     

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.     

Production rate ofEichhornia crassipes (Mart.) Solms in river Ganga water

The production rate ofEichhornia crassipes was stimulated by water of the river Ganga and by prevailing environmental conditions. It was highest in October (4.76 g.m^–2.d^–1) and was positively correlated with ammonia nitrogen and total phosphorus in the water but negatively correlated with total alkalinity and transparency. The average annual production of 14.13 t.ha^–1.a^–1 is equivalent to the average production of 0.067 t.ha^–1.a^–1 phosphorus and 0.40 t.ha^–1.a^–1 nitrogen. The concentrations of total nitrogen and total phosphorus of the plant varied seasonally. They decreased with increasing production rate in summer and monsoon.     

Biomass and primary-production of herbaceous plant communities in the Amazon floodplain

The increase in biomass of different aquatic and terrestrial herbaceous plant communities was measured during various growth periods in the Amazon floodplain near Manaus. Maximum biomass varied from 4–11.2 t ha^–1 dry weight in mixed annual terrestrial communities to 6–23 t ha^–1 in aquatic annual species (Paspalum repens, Oryza perennis, Luziola spruceana and Hymenachne amplexicaulis) and 15.6–57.6 t ha^–1 in communities of the perennial species Paspalum fasciculatum. Cumulative biomass of 3 successively growing annual species reached 30 t ha^–1 a^–1. Net primary production is considerably higher than maximum biomass. Paspalum fasciculatum reached 70 t during a growth period of 8 months. If one considers for annual species a monthly loss of 10–25% of the biomass, then net primary production in areas with three successive macrophyte communities and a cumulative maximum biomass of 30 t ha^–1 is estimated to reach up to 50 t ha^–1 a^–1. Annual P/B ratio may reach about 3.     

Nitrogen fixation by white clover in pastures grazed by dairy cows: Temporal variation and effects of nitrogen fertilization

Effects of rate of nitrogen (N) fertilizer and stocking rate on production and N₂ fixation by white clover (Trifolium repens L.) grown with perennial ryegrass (Lolium perenne L.) were determined over 5 years in farmlets near Hamilton, New Zealand. Three farmlets carried 3.3 dairy cows ha^–1 and received urea at 0, 200 or 400 kg N ha^–1 yr^–1 in 8–10 split applications. A fourth farmlet received 400 kg N ha^–1 yr^–1 and had 4.4 cows ha^–1.There was large variation in annual clover production and total N₂ fixation, which in the 0 N treatment ranged from 9 to 20% clover content in pasture and from 79 to 212 kg N fixed ha^–1 yr^–1. Despite this variation, total pasture production in the 0 N treatment remained at 75–85% of that in the 400 N treatments in all years, due in part to the moderating effect of carry-over of fixed N between years.Fertilizer N application decreased the average proportion of clover N derived from N₂ fixation (P_N; estimated by ¹⁵N dilution) from 77% in the 0 N treatment to 43–48% in the 400 N treatments. The corresponding average total N₂ fixation decreased from 154 kg N ha^–1 yr^–1 to 39–53 kg N ha^–1 yr^–1. This includes N₂ fixation in clover tissue below grazing height estimated at 70% of N₂ fixation in above grazing height tissue, based on associated measurements, and confirmed by field N balance calculations. Effects of N fertilizer on clover growth and N₂ fixation were greatest in spring and summer. In autumn, the 200 N treatment grew more clover than the 0 N treatment and N₂ fixation was the same. This was attributed to more severe grazing during summer in the 0 N treatment, resulting in higher surface soil temperatures and a deleterious effect on clover stolons.In the 400 N treatments, a 33% increase in cow stocking rate tended to decrease P_N from 48 to 43% due to more N cycling in excreta, but resulted in up to 2-fold more clover dry matter and N₂ fixation because lower pasture mass reduced grass competition, particularly during spring.     

Carbon Dioxide and Methane Production in Small Reservoirs Flooding Upland Boreal Forest

The FLooded Uplands Dynamics EXperiment (FLUDEX) was designed to assess the impact of reservoir creation on carbon cycling in boreal forests by (a) determining whether production of the greenhouse gases (GHG) carbon dioxide (CO₂) and methane (CH₄) in reservoirs is related to the amount of organic carbon (OC) stored in the flooded landscape, (b) examining temporal trends in GHG production during initial stages of flooding, and (c) considering the net difference between GHG fluxes before and after flooding to estimate the true effect of reservoir creation on atmospheric GHG levels. Three forested sites that varied in the amount of OC stored in soils and vegetation (30,870–45,860 kg C ha^–1) were experimentally flooded from June to September in 1999–2001. Throughout the study, net CO₂ and CH₄ production in all three reservoirs was not related to overall site OC storage. During the 1st flooding season, net CO₂ production in the three reservoirs was 703–797 kg C ha^–1, but it decreased during the 2nd and 3rd flooding seasons to between 408 and 479 kg C ha^–1. However, CH₄ production increased in all reservoirs with each flooding season, from about 3.2–4.6 kg C ha^–1 in 1999 to 12.8–24.9 kg C ha^–1 in 2000 and 29.7–35.2 kg C ha^–1 in 2001. Over the long term, effects of boreal reservoir creation on atmospheric GHG levels may be largely due to net changes in CH₄ cycling between the undisturbed and flooded ecosystems.     

Effect of torpor on the water economy of an arid-zone marsupial,the stripe-faced dunnart (<Emphasis Type="Italic">Sminthopsis macroura</Emphasis>)

Metabolic rate and evaporative water loss (EWL) were measured for a small, arid-zone marsupial, the stripe-faced dunnart (Sminthopsis macroura), when normothermic and torpid. Metabolic rate increased linearly with decreasing ambient temperature (T_a) for normothermic dunnarts, and calculated metabolic water production (MWP) ranged from 0.85±0.05 (T_a=30°C) to 3.13±0.22 mg H₂O g^–1 h^–1 (T_a=11°C). Torpor at T_a=11 and 16°C reduced MWP to 24–36% of normothermic values. EWL increased with decreasing T_a, and ranged from 1.81±0.37 (T_a=30°C) to 5.26±0.86 mg H₂O g^–1 h^–1 (T_a=11°C). Torpor significantly reduced absolute EWL to 23.5–42.3% of normothermic values, resulting in absolute water savings of 50–55 mg H₂O h^–1. The relative water economy (EWL/MWP) of the dunnarts was unfavourable, remaining >1 at all T_a investigated, and did not improve with torpor. Thus torpor in stripe-faced dunnarts results in absolute, but not relative, water savings.     

Effects of soil freezing disturbance on soil solution nitrogen, phosphorus, and carbon chemistry in a northern hardwood ecosystem

Reductions in snow cover undera warmer climate may cause soil freezing eventsto become more common in northern temperateecosystems. In this experiment, snow cover wasmanipulated to simulate the late development ofsnowpack and to induce soil freezing. Thismanipulation was used to examine the effects ofsoil freezing disturbance on soil solutionnitrogen (N), phosphorus (P), and carbon (C)chemistry in four experimental stands (twosugar maple and two yellow birch) at theHubbard Brook Experimental Forest (HBEF) in theWhite Mountains of New Hampshire. Soilfreezing enhanced soil solution Nconcentrations and transport from the forestfloor. Nitrate (NO₃ ^–) was thedominant N species mobilized in the forestfloor of sugar maple stands after soilfreezing, while ammonium (NH₄ ⁺) anddissolved organic nitrogen (DON) were thedominant forms of N leaching from the forestfloor of treated yellow birch stands. Rates ofN leaching at stands subjected to soil freezingranged from 490 to 4,600 mol ha^–1yr^–1, significant in comparison to wet Ndeposition (530 mol ha^–1 yr^–1) andstream NO₃ ^– export (25 mol ha^–1yr^–1) in this northern forest ecosystem. Soil solution fluxes of P_i from the forestfloor of sugar maple stands after soil freezingranged from 15 to 32 mol ha^–1 yr^–1;this elevated mobilization of P_i coincidedwith heightened NO₃ ^– leaching. Elevated leaching of P_i from the forestfloor was coupled with enhanced retention ofP_i in the mineral soil Bs horizon. Thequantities of P_i mobilized from the forestfloor were significant relative to theavailable P pool (22 mol ha^–1) as well asnet P mineralization rates in the forest floor(180 mol ha^–1 yr^–1). Increased fineroot mortality was likely an important sourceof mobile N and P_i from the forest floor,but other factors (decreased N and P uptake byroots and increased physical disruption of soilaggregates) may also have contributed to theenhanced leaching of nutrients. Microbialmortality did not contribute to the acceleratedN and P leaching after soil freezing. Resultssuggest that soil freezing events may increaserates of N and P loss, with potential effectson soil N and P availability, ecosystemproductivity, as well as surface wateracidification and eutrophication.     

Population dynamics,production and angling catch of brown trout,Salmo trutta,in a mature upland reservoir in mid-Wales

Synopsis The brown trout in Llyn Frongoch, a mature upland reservoir, and its nursery stream was sampled during 1983. The stream stock consisted largely of the 1983 and 1982 year classes, with fish reaching mean lengths of 7.0 and 11.6 cm at one and two years of age. The size and biomass of the stream stock at the beginning of 1983 and 1984 were estimated to be 120 and 125 (1.20 and 1.25 fish m^–2) and 1.41 and 0.69 kg (14.1 g m^–2 and 6.9 g m^–2) respectively. Annual stream production ranged from an estimated minimum of 2.49 kg (24.9 g m^–2) to an estimated maximum of 4.59 kg (45.9 g m^–2). Both downstream and upstream movements of 0+ juveniles were recorded. The adult spawning stock was estimated at 79 males and 32 females, a sex ratio of 2.5:1, with most spawners belonging to the 1980 yearclass. The average size of the lake stock over the year was estimated to be 1 650 (229 fish ha^–1) or 250.8 kg (34.8 kg ha^–1). The 1980 yearclass was predominant; there were few fish older than five years. Seasonal variations in netting catches suggested movements to and from the littoral region. Growth in the lake was moderately fast, with fish reaching mean lengths of 21.7 and 27.2 cm by three and four years of age. Fish entering the lake after one year appeared to grow faster than fish which remained in the stream for two years. Annual production in the lake was estimated at 136.7 kg (19.0 kg ha^–1). The total angling catch for the season was estimated to be 62.6 kg (8.7 kg ha^–1).     

Quantitative evaluation of atmospheric CO2 sink into forest soils from the tropics to the boreal zone during the past three decades

A model of soil carbon cycling in forest ecosystems was applied to predict the soil carbon balance in nine forest ecosystems from the tropics to the boreal zone during the past three decades (1965–95). The parameters of carbon flows and initial conditions of carbon pools were decided based on data obtained in each forest stand. Assumptions for model calculation were: (i) primary production (i.e. litterfall and root turnover rates) increased with increasing CO₂ concentrations in the atmosphere (10% per 40 p.p.m. CO₂); and (ii) temperature increased by 0.6°C per 100 years, but precipitation changed little. The simulation employed a daily time step and used daily air temperature and precipitation observed near each forest stand over an average year during the last decade. The model calculations suggest that the accumulation of total soil carbon increased 8.5–10.4 tC (ton of carbon) ha^–1 in broad-leaved forests from the tropics to the cool-temperate zone during the past three decades, but the amount of soil carbon (3.0–8.4 tC ha^–1) increased much less in needle forests from the subtropical to boreal zones during the same period. There is a linear relationship between the increasing rate of soil carbon stock during the past three decades (1965–95) in forest stands concerned (R_MS, % per 30 years) and annual mean temperature of their soils (T₀,°C), as: R_MS = 0.34T₀ + 4.1. Based on the data of carbon stock in forest soil in each climate zone reported, the global sink of atmospheric CO₂ into forest soil was roughly estimated to be 42 GtC (billion tons of carbon) per 30 years, which was 1.4 GtC year^–1 on average over the past three decades.     

Response of field-grown chickpea (Cicer arietinum L.) to phosphorus fertilization for yield and nitrogen fixation

Application of phosphorus at 40, 60, 80 and 100 kg P₂O₅ ha^–1 in the presence of a uniform dressing of nitrogen (N) and potash (K₂O) each applied at 20 and 24 kg ha^–1 to chickpea (CM-88) grown in sandy loam soil in a replicated field experiment improved the nodulation response of the crop, increased its grain yield (ka ha^–1) by 18, 59, 40 and 14 percent, biomass yield (ka ha^–1) by 32, 32, 54 and 14 percent, biomass N (kg ha^–1) by 31, 48, 49, 19 percent, and biomass P (kg ha^–1) by 26, 40, 41 and 11 percent, respectively. The effect of phosphorus on the nitrogenase activity of the excised roots of chickpea was, however, inconsistent.     

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.