The effect of sewage-enriched river Ganga water on the biomass production of theAzolla-Anabaena complex

The biomass formation ofAzolla was greatly enhanced by water of the River Ganga and by prevailing environmental conditions. It increased gradually from January to April (first maximum 2.409 g.m^–2.day^–1), declined during June (1.185 g.m^–2.day^–1), and reached a second its maximum during September (2.629 g.m^–2.day^–1). The biomass formation was related to the nutrient availability in the medium in a particular season (measured were: nitrate-N, available phosphorus, total suspended solids, and conductivity). The average annual production of 6.73 ton.ha^–1.yr^–1 is equivalent to the average production of 0.025 ton.ha^–1.yr^–1 phosphorus, 0.252 ton.ha^–1.yr^–1 nitrogen, and 1.57 ton.ha^–1.yr^–1 crude protein.     

Distribution and production ofChironomus salinarius (Diptera: Chironomidae) in a shallow coastal lagoon in the Bay of Cádiz

The abundance, generation time and production ofChironomus salinarius larvae in a lagoon fish-pond system in the Bay of Cádiz were studied by taking monthly samples at 3 sites during 1991 and 1992. Numerical abundance and biomass of larvae showed considerable spatial, seasonal and interannual variation (ANCOVAs,P<0.001). The maximum mean annual density was 7048 larvae m^–2, and corresponded to a biomass of 3.08 g dry weight (DW) m^–2. It was recorded at the site with the lowest rate of water renewal. Seasonal patterns were similar at all sites, with main annual peaks of abundance and biomass in autumn-early winter. Chironomid density was positively related to the biomass of benthic macroalgae (P<0.001). The population studied was multivoltine with a probable average of five generations per year, with overlapping cohorts and a predominance of third- and fourth-instar larvae. Estimates of annual production ranged between 72.2 g DW m^–2 yr^–1 at the site with the lowest rate of water renewal in 1991 and 0.1 g DW m^–2 yr^–1 at the site with the highest rate of water renewal in 1992. Mean annual production and the production/biomass ratio for the system was estimated to be 16.8 g DW m^–2 yr^–1 and 12.7, respectively. Possible factors leading to the observed density fluctuations are discussed, as well as possible sources of error in production estimates.     

The cell composition of Nannochloropsis sp. changes under different irradiances in semicontinuous culture

Nannochloropsis sp. was grown semicontinuously with a rate of daily renewal of the culture media of 40% of the volume of the culture under different irradiances (40, 60, 80, 220 and 480 mol quanta m^–2 s^–1). Under the conditions tested, light saturation was achieved at 220 mol quanta m^–2 s^–1 with no significant increase in steady-state cell density or of dry weight productivity with higher irradiance, reaching values of 115 × 10⁶ cells ml^–1 and 375 mg l^–1 day^–1 respectively. C/N ratios clearly indicated the point of light saturation, decreasing with increasing irradiance for light-limited conditions and increasing for light-saturated conditions. Under light-limited conditions, an increase in the irradiance produced an increase in the protein percentage of the organic fraction to the detriment of lipids and carbohydrates, while small changes were recorded under light-saturated conditions. The degree of unsaturation of fatty acids was lower with increasing irradiance, with a three-fold decrease of the percentage of total n–3 fatty acids, from 29 to 8% of total fatty acids, caused mainly by a decrease of eicosapentaenoic acid (EPA) (20:5n–3). The microalga reached its maximal value of dry weight productivity (375 mg l^–1 day^–1), EPA productivity (3.2 mg l^–1 day^–1) and maximal protein content (36% of the organic content) at the point at which light saturation was achieved. Results demonstrate the efficiency of the use of the irradiance for the modification of the biochemical composition of Nannochloropsis sp.     

Studies on mixed mass cultivation of Anabaena spp. (nitrogen-fixing blue-green algae, cyanobacteria) on a large scale

Studies on mixed mass cultivation of Anabaena spp. on a large scale (5170m²) were conducted continuously for 3 years. Under the continental monsoon climate in northern subtropics (30°N, 115°E), 7–11 g dry weight m⁻² day⁻¹ of microalgal biomass on average was harvested in simple plastic greenhouses in the effective growth days during the warmer seasons. The maximum productivity was 22 g m⁻² day⁻¹ in the middle of summer. Observations on the productive properties of strains of Anabaena spp. indicated that they were different from and could compensate for each other in their productivities and adaptations to the seasonal changes. With different lining materials (PVC sheets, concrete, sand and soil) in the culture ponds, no significant variation of productivity was found, but bubbling with biogas in the middle of the day and the application of some growth regulating substances (2,4-D, NaHSO₃ and extracts of oyster mushroom spawn) was able to improve the production. The cost of microalgal biomass in this way was around 0·75–1·0 US dollar(s) per kilogram.     

Preliminary study of the aquatic Oligochaeta of the Changjiang (Yangtze) River

The aquatic oligochaetes in mainstream of the Changjiang River have not been studied previously. Since 1983, benthic samples have been collected mainly from mid-upper reach of the river. Altogether 19 species belonging to 4 families of the Oligochaeta were found. Among them, Nais inflata is the commonest species throughout the river, whereas Nais sp., Arcteonais lomondi, Limnodrilus silvani, Rhyacodrilus riabuschinskii and Telmatodrilus sp. ae recorded from China for the first time.Quantitatively, the aquatic oligochaetes constitute the majority of zoobenthos in the mainstream. In 1984, the standing crop of the Tubificidae was 231–343 ind·m^-2 in density and 0.73–0.97 g·m^-2 (wet weight) in biomass, and that of the Naididae was 21–4982 ind·m^-2 and 0.003–1.75 g·m^-2 (wet weight) respectively.     

Influence of environmental factors on the production of Potamothrix alatus paravanicus Poddubnaya & Pataridze (Tubificidae) in different areas of Lake Sevan

A modification of the Hynes' method was used to estimate production of Potamothrix alatus paravanicus Poddubnaya & Pataridze (Tubificidae), the dominant species of benthic invertebrate in Lake Sevan. Maximum production (5–24 g m^t\-2 wet weight) occurred in the sublittoral, while maximum biomass was found (8–17 g m^–2 wet weight) in the profundal. The turnover ratio of the average cohort varied between 2.4 and 5.0. The annual turnover ratio (P/B) decreased from 2.1–3.1 in the littoral to 0.1–0.4 in the profundal zone.A logarithmic correlation was found between P/B and temperature and oxygen regime, and depth and average size of clitellate specimens.     

Phytoplankton productivity and chlorophyll-A concentration of Oguta Lake in Southeastern Nigeria

The phytoplankton productivity and chlorphyll-a concentration of Oguta Lake, the largest natural lake in south-eastern Nigeria, are presented (Dec. 1983. Nov. 1984). The gross productivity ranged from 1.3 to 3.77 g C.m^–2.day^–1 for the water column, dropped during the period of heavy rainfall and varied with depth. The chlorophyll-a concentration had monthly means ranging from 2.31 to 4.00 mg.m^–3, with a drop during the rains, but little depth variation. Both productivity and chlorophyll-a showed non-significant correlation with the physico-chemical features of the water. The values of the biological parameters showed the lake as mesotrophic. The values are compared with those of other African lakes.     

Ulva rigida (Ulvales,Chlorophyta) tank culture as biofilters for dissolved inorganic nitrogen from fishpond effluents

Ulva rigida was cultivated in 7501 tanks at different densities with direct and continuous inflow (at 2, 4, 8 and 12 volumes d^–1) of the effluents from a commercial marine fishpond (40 metric tonnes, Tm, of Sparus aurata, water exchange rate of 16 m³ Tm^–1) in order to assess the maximum and optimum dissolved inorganic nitrogen (DIN) uptake rate and the annual stability of the Ulva tank biofiltering system. Maximum yields (40 g DW m^–2 d^–1) were obtained at a density of 2.5 g FW 1^–1 and at a DIN inflow rate of 1.7 g DIN m^–2 d^–1. Maximum DIN uptake rates were obtained during summer (2.2 g DIN M^–2 d^–1), and minimum in winter (1.1 g DIN m^–2 d^–1) with a yearly average DIN uptake rate of 1.77 g DIN m^–2 d^–1 At yearly average DIN removal efficiency (2.0 g DIN m^–2 d^–1, if winter period is excluded), 153 m² of Ulva tank surface would be needed to recover 100% of the DIN produced by 1 Tm of fish.Abbreviations DIN= dissolved inorganic nitrogen (NH _inf4 ^sup+ + NO _inf3 ^sup– + NO _inf2 ^sup– ); - FW= fresh weight; - DW= dry weight; - PFD= photon flux density; - V= DIN uptake rate     

Response on scallop culture to enhanced nutrient supply by experimental fertilisation of a landlocked bay

The effect of an enhanced nutrient supply to coastal waters of a landlocked bay, Hopavågen in Central Norway, on the phytoplankton production and biomass, and on growth of scallops (Pecten maximus) was studied in 1997–1999. Nitrogen, silicon and phosphorous (N:Si:P = 16:8:1, atomic) were added daily between May and October in 1998 at a level of 0.4 mg P m^–3 day^–1. The concentration of nutrient addition was doubled in 1999 during the same period. High addition of nutrients (1999) resulted in a significantly higher phytoplankton biomass in the summer period, expressed as chlorophyll a content, than without nutrient (1997) and low nutrient (1998). The respective mean chlorophyll a levels were 2.4 in 1999, 1.6 in 1998 and 1.2 g l^–1 in 1997. The mean primary production during the summers generally increased with the addition of nutrients from an average level of 320 mg carbon m^–2 day^–1 in 1997 to 1200 mg carbon m^–2 day^–1 in 1999. Scallops placed at 10 m depth in Hopavågen showed an increase in growth rate of the outer scallop shell in the period July–September from 0.16% day^–1 in 1997 to 0.53% day^–1 in 1998. Scallops grown in an unfertilised control station in the fjord outside Hopavågen had a significantly lower growth rate than those grown in the fertilised water of Hopavågen. The results showed decreased growth rate with increasing shell sizes. However, for all size groups studied a higher growth rate of the scallops was observed when nutrients were added to the bay. The tissue dry weight content of scallops grown in Hopavågen was 2–4 times higher than in the control scallops.     

Primary productivity and related parameters in three different types of inland waters in Sri Lanka

Gross and net primary production together with chlorophyll-a biomass were investigated with respect to depth and diurnal changes in three categories of inland waters (reservoirs, temporary ponds, brackish water lagoons) in Sri Lanka. Ten field sites, in both the dry and wet zones of the island, were investigated. Bimodal productivity profiles were recorded in two of the three reservoirs studied. The diel pattern of net photosynthetic rate varied between sites although peak photosynthetic efficiency occurred at solar noon. Surface photoinhibition was characteristic of the reservoirs and brackish water lagoons but not of the temporary ponds. Mean gross primary production was 3.02 g C m^–2 d^–1 but was higher in the temporary ponds than in the reservoirs. The gross primary production in the brackish water Koggala Lagoon at 0.08 g C m^–2 d^–1 is a record low for tropical lagoons and was 2.5 times less than the two other lagoons investigated. Variability in net primary production between sites was similar to the variation in gross production with a relatively low mean value for tropical inland waters of 0.495 C m^–2 d^–1. Mean maximum photosynthetic rate was 0.30 mg C m^–3 h^–1 but was lower in the reservoirs than in the temporary ponds and lagoons.     

Biomass and litter dynamics in a Melaleuca forest on a seasonally inundated floodplain in tropical,northern Australia

Litterfall from a Melaleuca forest was investigated as part of chemical cycling studies on the Magela Creek floodplain in tropical, northern Australia. The forest contained two species of tree, Melaleuca cajaputi and Melaleuca viridiflora, with a combined average density of 294 trees ha^–1. The M. viridiflora trees had diameter breast height measurements ranging from 11.8 to 62.0 cm, median class 25.1–30.0cm and a mean value of 29.2±1.0 cm, compared to 13.0 to 66.3 cm, 30.1–35.0cm and 33.5±1.0cm for M. cajaputi trees. A regression model between tree height, diameter breast height and fresh weight was determined and used to calculate average tree weights of 775±1.6kg for M. viridiflora and 1009±1.6kg for M. cajaputi, and a total above-ground fresh weight of 263±0.3t ha^–1. The weight of litter recorded each month on the ground beneath the tree canopy ranged from 582±103 to 2176±376 g m^–2 with a monthly mean value of 1105±51 g m^–2. The coefficient of variation of 52% on this mean indicates the large spatial and temporal variability in litter distribution over the study site. This variability was greatly affected by the pattern of water flow and litter transport during the Wet season. Litterfall from the trees was evaluated using two techniques - nets and trays. The results from these techniques were not significantly different with annual litterfall collected in the nets being 705 ± 25 g m^–2 and in the trays 716±49 g m^–2. The maximum monthly amount of litterfall, 108 ±55g m^–2, occurred during the Dry season months of June–July. Leaf material comprised 70% of the total annual weight of litter, 480±29 g m^–2 in the nets and 495 ± 21 g m^–2 in the trays. The tree density and weight of litter suggest that the Melaleuca forests are highly productive and contribute a large amount of material to the detrital/debris turnover cycle on the floodplain.     

Cultivation and carrageenan yield and quality ofKappaphycus alvarezii in the waters of Vietnam

The carrageenan-producing red algaKappaphycus alvarezii (Doty) Doty was brought to Vietnam from Japan in 1993. Branch fragments of this species were cultivated in a pond, lagoon, inlet and offshore in Vietnam for the first time. The best daily growth rate (DGR) of plants grown in the lagoon area attained 9–11 % day^–1 in May to June (cold season). The water temperature and salinity in this area ranged from 27.2–32.4 °C and 31.4–33.7 °C, respectively. DGR of plants grown in the inlet ranged from 7 to 9% day^–1 in June. Grazing by fish has been observed to occur in this area. The DGR of plants grown in the pond ranged from 5–6% in January–July, but decreased to less than 4% day^–1 in August (hot season). K. alvarezii in Vietnam showed a carrageenan yield of 18.8–24.6% and gel strength of 1566–1712 g cm^–2. These values are similar ones obtained fromK. alvarezii cultivated in the Philippines and Indonesia.     

Ammonia-nitrogen excretion in Daphnia pulex

Ammonia-nitrogen excretion rates were measured in natural summer and cultured populations of Daphnia pulex from Silver Lake, Clay County, Minnesota, USA during 1973. The mean rate of ammonia-nitrogen excretion for the summer populations was 0.20 µg N animal^–1 day^–1 or 5.11 µg N mg^–1 dry body weight day^–1 (N = 80) measured at 15°, 20°, and 25°C. These rates appear to be temperature and weight dependent, but they are probably affected by factors other than temperature and dry body weight. Ammonia-nitrogen excretion rates of Daphnia pulex cultured on Chlamydomonas reinhardi yielded the following relationship with temperature: Log₁₀E = (0.061) T 1.773, where E is µg N animal^–1 day^–1 and T is temperature °C. The ammonia-nitrogen excretion on a mg^–1 dry body weight day^–1 basis was related to temperature according to the following similar expression Log₁₀E = (0.043) T + 0.153, where E is µg N mg^–1 dry body weight day^–1, and T is temperature °C. The length-weight relationship of Daphnia pulex for the summer populations (N = 1583) was log₁₀W = (0.526) Log₁₀L + 1.357, where W is weight in µg and L is length in mm.     

Consequences of depletion of stratospheric ozone for terrestrial Antarctic ecosystems: the response of Deschampsia antarctica to enhanced UV-B radiation in a controlled environment

Mini UV lamps were installed over antarctic plants at Léonie Island, Antarctic peninsula, and shoot length measurements of Deschampsia antarctica were performed during the austral summer January–February 1999.We studied the response of the antarctic hairgrass, Deschampsia antarctica to enhanced UV-B. In a climate room experiment we exposed tillers of Deschampsia antarctica, collected at Léonie Island, Antarctic peninsula, to ambient and enhanced levels of UV-B radiation. In this climate room experiment with 0, 2.5 and 5 kJ m^–2 day^–1 UV-B_BE treatments we observed that length growth of shoots at 2.5 and 5 kJ m^–2 day^–1 UV-B_BE was markedly reduced compared to 0 kJ m^–2 day^–1 UV-B_BE. In addition, there was an increased number of shoots and increased leaf thickness with enhanced UV-B. The Relative Growth Rate (RGR) was not affected by UV-B, possibly because reduced shoot length growth by enhanced UV-B was compensated by increased tillering. Light response curves of net leaf photosynthesis of plants exposed to 5 kJ m^–2 day^–1 UV-B_BE did not differ from those exposed to 0 kJ m^–2 day^–1 UV-B_BE. The content of UV-B absorbing compounds of plants exposed to increasing UV-B did not significantly change.Mini UV-B lamp systems were installed in the field, to expose the terrestrial antarctic vegetation at Léonie Island to enhanced solar UV-B. In that study, the increment of shoot length of tagged plants of Deschampsia antarctica during the January-February 1999 at Léonie Island, was recorded and compared to shoot length growth under controlled conditions.The consequences of enhanced UV-B radiation as a result of ozone depletion for the terrestrial antarctic ecosytems are discussed.     

Comparison of growth rate in culture,dry matter content,agar content and agar quality of two New Zealand red seaweeds,Gracilaria chilensis Bird,McLachlanet Oliveira andGracilaria truncata Kraft

Plant growth rates and agar characteristics were compared for two agarophytes,Gracilaria chilensis (Gracilariales, Rhodophyta) andG. truncata, so that the suitability ofG. truncata for mariculture could be assessed.G. chilensis plants grew steadily in the laboratory culture system at rates of 5–8% day^-1 (mean Relative Growth Rate) throughout the 6 week experimental period, with no decline in health.G. truncata plants grew at rates of 2–4% day^-1 for 5 weeks, but during the sixth week their apical tips became necrotic and growth rates fell to zero. There was no significant difference in the dry matter content (as a percentage of their wet weight) between the two species ofGracilaria, with values falling in the range of 16–19%. Slightly higher agar yields were obtained from alkali-treatedG. chilensis (17–20% dry matter) than from untreatedG. truncata (16–18%) although the agar fromG. truncata formed softer gels from which it was more difficult to recover. The gel strength of untreated agar extracted fromG. chilensis was very low (ca. 100 g cm^-2 for a 1% gel) but when an alkali treatment step was included in the extraction process, it increased to 520 g cm^-2 for a 1% gel. Contrary to an earlier report, untreatedG. truncata agar also had a very low gel strength (ca. 100 g cm^-2 for a 1.5% gel), but it rose to only 167 g cm^-2 after alkaline treatment.     

Larval abundance of Chironomus circumdatus in relation to biotic and abiotic factors

Larval abundance of Chironomus circumdatus in sewage canal and pond systems was studied during 1988–1990. Monthly changes in the morphometric features of the pond revealed that both total and littoral areas progressively decreased from 1063 and 107 m² in December 1988 to 151 and 43 m² in May '89; the decrease during the year 1989–1990 was from 1116 and 92 m² in October to 109 and 31 m² in May. A significant negative correlation (r= – 0.52) was obtained for the relation between littoral area and larval density in the pond. Larval density and biomass depended generally on the nature of the substrate and quantity of organic matter. Larval density of Ch. circumdatus was positively correlated with O₂, bacterial count and organic matter content, but negatively correlated with CO₂ level. Daily removal of organic matter by the larvae ranged from 20 to 31 % of the available organic matter in the sewage canal and from 3 to 11 % in the pond.     

Population dynamics of Potamothrix alatus paravanicus Poddubnaya & Pataridze (Tubificidae) in different areas of Lake Sevan

Potamothrix alatus paravanicus Poddubnaya & Pataridze (Tubificidae) is the most abundant species of benthic invertebrate in Lake Sevan and the only species in the deep profundal. Differences in annual temperature, length of the stratification period (low oxygen content) and bottom sediment characteristics influence the population of P. a. paravanicus in different depth zones of Lake Sevan. From shallow to deep water a regular increase of the following indices was observed in 1984–1987: annual biomass from 1.2–2.0 to 8.2–17.0 g m^–2 wet weight; mean individual wet weight of clitellate specimens from 2.0–3.8 to 8.3–16.6 mg; total life span from one and a half to more than 20 years; the length of breeding season from 2–3 to 12 months.An important ecological distinction between P. a. paravanicus and allied Potamothrix hammoniensis (Michaelsen) is the much lower fecundity of the former species.     

Removal of municipal solid waste COD and NH4–N by phyto-reduction: A laboratory–scale comparison of terrestrial and aquatic species at different organic loads

Laboratory scale tests on phytodepuration of raw and pre-treated leachate from municipal sanitary waste were carried out with four vegetable aquatic and terrestrial species at different organic loads. We used the terrestrial species Stenotaphrum secundatum and the free-floating aquatic species Lemna minor, Eichhornia crassipes and Myriophyllum verticellatum to purify leachate from municipal solid waste. The organic load characterized by COD varied from 2–30 g m⁻² day⁻¹. Blanks using tap water served as controls. Duration of the experiments varied from 9–90 days. Maximum concentrations in the experiments were 1600 mg l⁻¹ COD and 300 mg l⁻¹ NH₄–N for S. secundatum. Best results in terms of COD, BOD, and ammonia removal were obtained for raw leachate with COD=2 g m⁻² day⁻¹ in free water surface (FWS) wetlands, and with 2 and 5 g m⁻² day⁻¹ in subsurface flow (SSF) wetlands. Results show that for pretreated leachate (labeled c) low in BOD and NH₄–N, the aquatic species showed low removal and stress even at the lowest load of COD=2 g m⁻² day⁻¹. We cannot say if this is due to the pretreatment itself or the chemical or microbial composition of this leachate. The Stenotaphrum system operated well with this load of leachate c. For untreated leachate (type a and b) the removal and plant growing conditions seemed good at COD=2 g m⁻² day⁻¹. For S. secundatum a load of COD=5 g m⁻² day⁻¹ operated well. All loads above COD=5 g m⁻² day⁻¹ caused low removal and stress, and the green parts of the plants disappeared. Oxygen was, however, consumed throughout the experimental period. For pretreated leachate (type c), the removal of COD were low (−24 to 17%) but good for NH₄–N (52–91%). This leachate also experienced high ammonia removal from the beginning of the experiments, probably due to existing consortia of nitrifying bacteria in it. Statistical analysis shows that the S. secundatum and L. minor systems maintained higher oxygen levels than the M. verticellatum and E. crassipes systems, when operated with tap water. For Lemna minor, this may be due to a better capacity for transporting oxygen into the water. With leachate all S. secundatum systems have higher oxygen levels than the aquatic systems, basically because the water content of the soil has been kept well below saturation. S. secundatum shows a significantly lower removal of COD than did the aquatic systems at a loading of COD=2 g m⁻² day⁻¹ of raw leachate. There is no significant difference between the systems in the removal of NH₄–N at a loading of COD=2 g m⁻² day⁻¹ of both types of leachate. E. crassipes has a lower removal of NH₄–N than M. verticellatum and S. secundatum at a loading of 5 g m⁻² day⁻¹ of COD of both types of leachate. In our experiments, it appears that the amount of free ammonia explains the toxicity of the leachate to the plants. This, however, does not exclude other possible toxic factors.     

Submerged Macrophyte-bed Effects on Water-Column Phosphorus,Chlorophyll <Emphasis Type="Italic">a</Emphasis>, and Bacterial Production

Submerged macrophytes are a major component of freshwater ecosystems, yet their net effect on water column phosphorus (P), algae, and bacterioplankton is not well understood. A 4-month mass-balance study during the summer quantified the net effect of a large (5.5 ha) undisturbed macrophyte bed on these water-column properties. The bed is located in a slow-flowing (0.05–0.1 cm s^–1) channel between two lakes, allowing for the quantification of inputs and outputs. The P budget for the study period showed that, despite considerable short-term variation, the macrophyte bed was a negligible net sink for P (0.06 mg m^–2 day^–1, range from –0.76 to +0.79 mg m^–2 day^–1), demonstrating that loading and uptake processes in the weedbed roughly balance over the summer. Chlorophyll a was disproportionately retained relative to particulate organic carbon (POC), indicating that the algal component of the POC was preferentially trapped. However, the principal contribution of the weedbed to the open water was a consistent positive influence on bacterioplankton production over the summer. Conservative extrapolations based on measured August specific exports (m^–2 day^–1) of P and bacterial production exiting the weedbed applied to five regional lakes varying in lake morphometry and macrophyte cover suggest that even in the most macrophyte dominated of lakes (66% cover), P loading from submerged weedbeds never exceeds 1% day^–1 of standing epilimnetic P levels, whereas subsidization of bacterioplankton production can reach upward of 20% day^–1. The presence of submerged macrophytes therefore differentially modifies algae and bacteria in the water column, while modestly altering P dynamics over the summer.     

Retention of nutrients in river basins

In Denmark, as in many other European countries, the diffuse losses of nitrogen (N) and phosphorus (P) from the rural landscape are the major causes of surface water eutrophication and groundwater pollution. The export of total N and total P from the Gjern river basin amounted to 18.2 kg ha^–1 and 0.63 kg P ha^–1 during June 1994 to May 1995. Diffuse losses of N and P from agricultural areas were the main nutrient source in the river basin contributing 76% and 51%, respectively, of the total export.Investigations of nutrient cycling in the Gjern river basin have revealed the importance of permanent nutrient sinks (denitrification and overbank sedimentation) and temporary nutrient storage in watercourses. Temporary retention of N and P in the watercourses thus amounted to 7.2–16.1 g N m^–2 yr^–1 and 3.7–8.3 g P m^–2 yr–1 during low-flow periods. Deposition of P on temporarily flooded riparian areas amounted from 0.16 to 6.50 g P m^–2 during single irrigation and overbank flood events, whereas denitrification of nitrate amounted on average to 7.96 kg N yr^–1 per running metre watercourse in a minerotrophic fen and 1.53 kg N yr^–1 per linear metre watercourse in a wet meadow. On average, annual retention of N and P in 18 Danish shallow lakes amounted to 32.5 g N m^–2 yr^–1 and 0.30 g P m^–2 yr^–1, respectively, during the period 1989–1995.The results indicate that permanent nutrient sinks and temporary nutrient storage in river systems represent an important component of river basin nutrient budgets. Model estimates of the natural retention potential of the Gjern river basin revealed an increase from 38.8 to 81.4 tonnes yr^–1 and that P-retention increased from –0.80 to 0.90 tonnes yr^–1 following restoration of the water courses, riparian areas and a shallow lake. Catchment management measures such as nature restoration at the river basin scale can thus help to combat diffuse nutrient pollution.