Impact of long-term alternations of discharge and spate on the chironomid community in the lowland Widawka River (Central Poland)

Spatial and temporal distribution, abundance and production of macroinvertebrate communities were estimated over two years in a fifth-order section of the Widawka River. Discharge of this river has been increased artificially by coal mine water inputs. Additionally, during the second year, one of the highest discharges of the current 20-year period was recorded. Chironomidae were co-dominant in macrobenthos, both in a straight reach (WIA) and in a meandering site (WIB). More mosaic habitats resulted in higher densities of midges, reaching 6215 ind.m^–2 in year 1 and 1141 ind.m^–2 in year 2 at WIA, while at WIB 896 densities were ind.m^–2 and 257 ind.m^–2, respectively. Flooding affected the distribution and abundance of the chironomid assemblages. Recolonization by psammophilous Polypedilum began after the various microhabitats were buried with sand. Chironomid production was estimated on a species-specific basis for the dominant taxa. In year 1 (mean annual water temperature 10.0° C) chironomid production was 12.4 g dry wt m^–2 yr^–1 1 at WIA and 1.9 g dry wt m^–2 yr^–1 at WIB. These values sharply decreased in year 2 (mean annual water temperature 9.8° C) reaching 1.9 g dry wt m^–2 yr^–1 at WIA and 0.4 g dry wt m^–2 yr^–1 at WIB, as effects of the high spate.     

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.     

Carbon dioxide consumption during soil development

Carbon is sequestered in soils by accumulation of recalcitrant organic matter and by bicarbonate weathering of silicate minerals. Carbon fixation by ecosystems helps drive weathering processes in soils and that in turn diverts carbon from annual photosynthesis-soil respiration cycling into the long-term geological carbon cycle. To quantify rates of carbon transfer during soil development in moist temperate grassland and desert scrubland ecosystems, we measured organic and inorganic residues derived from the interaction of soil biota and silicate mineral weathering for twenty-two soil profiles in arkosic sediments of differing ages. In moist temperate grasslands, net annual removal of carbon from the atmosphere by organic carbon accumulation and silicate weathering ranges from about 8.5 g m^–2 yr^–1 for young soils to 0.7 g M^–2 yr^–1 for old soils. In desert scrublands, net annual carbon removal is about 0.2 g m^–2 yr^–1 for young soils and 0.01 g m^–2 yr^–1 for old soils. In soils of both ecosystems, organic carbon accumulation exceeds CO₂ removal by weathering, however, as soils age, rates of CO₂ consumption by weathering accounts for greater amounts of carbon sequestration, increasing from 2% to 8% in the grassland soils and from 2% to 40% in the scrubland soils. In soils of desert scrublands, carbonate accumulation far outstrips organic carbon accumulation, but about 90% of this mass is derived from aerosolic sources that do not contribute to long-term sequestration of atmospheric carbon dioxide.     

Benithic-pelagic coupling of nutrient metabolism along an estuarine eutrophication gradient

The shallow, brackish (11–18% salinity) Roskilde Fjord represents a eutrophication gradient with annual averages of chlorophyll, ranging from 3 to 25 mg chl a m^–3. Nutrient loadings in 1985 were 11.3–62.4 g N m^–2 yr^–1 and 0.4–7.3 g P m^–2 yr^–1. A simple one-layer advection-diffusion model was used to calculate mass balances for 7 boxes in the fjord. Net loss rates varied from –32.2 to 17.9 g P m^–2 yr^–1 and from –3.3 to 66.8 g N m^–2, corresponding to 74% of the external P-loading and 88% of the external N-loading to the entire estuary.Gross sedimentation rates measured by sediment traps were between 7 and 52 g p m^–2 yr^–1 and 50 and 426 g N M^–2 yr^–1, respectively. Exchangeable sediment phosphorus varied in annual average between 2.0 and 4.8 g P m^–2 and exchangeable sediment nitrogen varied from 1.9 to 33.1 g N m–1. Amplitudes in the exchangeable pools followed sedimentation peaks with delays corresponding to settling rates of 0.3 m d^–1. Short term nutrient exchange experiments performed in the laboratory with simultaneous measurements of sediment oxygen uptake showed a release pattern following the oxygen uptake, the changes in the exchangeable pools and the sedimentation peaks.The close benthic-pelagic coupling also exists for the denitrification with maxima during spring of 5 to 20 mmol N m^–2 d–1. Denitrification during the nitrogen-limited summer period suggests dependence on nitrification. Comparisons with denitrification from other shallow estuaries indicate a maximum for denitrification in estuaries of about 250 µmol N m^–2 h^–2 achieved at loading rates of about 25–125 g N m^–2 yr^–1.     

Production estimates of the dominant taxa of Chironomidae (Diptera) in the modified,River Widawka and the natural,River Grabia,Central Poland

Production and community composition of selected taxa of Chironomidae were estimated in cross-sections of the lower course of two lowland rivers, the Widawka and the Grabia. At the selected site, the Grabia has not changed its morphometry for many years, whereas the discharge of the Widawka has increased (mainly due to inputs of coal mine water) by 75% in comparison with its hypothetical natural discharge. In the Widawka, the production value, for 76.7% of total Chironomidae abundance, amounted to 12.45 g dry wt m^–2 yr^–1, while in the Grabia it amounted to 11.91 g dry wt m^–2 yr^–1 The annual P : B ranged from 14.5 for large-sized species (Chironomus in the Widawka) to 82.0 for small-sized species (Eukiefferiella in the Grabia). The similar production values estimated for both rivers, despite a three times higher density of Chironomidae in the Grabia, is noteworthy.     

Nitrogen flows in Louisiana Gulf Coast salt marsh: Spatial considerations

Nitrogen flux data was synthesized in developing a nitrogen flow budget for a Louisiana Barataria BasinSpartina alterniflora salt marsh. Results demonstrate the importance of spatial consideration in developing a nitrogen budget for coastal marshes. Using a mass balance approach nitrogen inputs balanced nitrogen sinks or losses from a marsh soil-plant system with a specific rooting depth. However, per unit areas on a local scale, marshes serve as a large sink for nitrogen due to rapid accretion which removes 17.O g N m^–2yr^–1 through subsidence below the root zone. On a larger spatial scale (regional) it is shown that the marshes do not serve as a large nitrogen sink. The rapid marsh deterioration currently occurring in the rapidly subsiding marshes of the Mississippi River deltaic plain account for a net regional loss of 12.5 g N m^–2yr^–1. Thus, regionally the net sink is equivalent to only 5 g N m^–2yr^–1 as compared to 17.0 g N m^–2yr^–1 on a local scale.     

Respiration from coarse wood litter in central Amazon forests

Respiration from coarse litter (trunks and large branches >10 cm diameter) was studied in central Amazon forests. Respiration ratesvaried over almost two orders of magnitude (1.003–0.014 µg Cg^–1 C min^–1, n = 61), and weresignificantly correlated with wood density (r² _adj= 0.42), and moisture content (r² _adj= 0.39). Additional samples taken from a nearby pasture indicatedthat wood moisture content was the most important factor controllingrespiration rates across sites (r² _adj =0.65). Based on average coarse litter wood density and moisture content,the mean long-term carbon loss rate due to respiration was estimated tobe 0.13 yr^–1 (range of 95% prediction interval(PI) = 0.11–0.15 yr^–1). Comparing meanrespiration rate with mean mass loss (decomposition) rate from aprevious study, respiratory emissions to the atmosphere from coarselitter were predicted to be 76% (95% PI =65–88%) of total carbon loss, or about 1.9 (95% PI= 1.6–2.2) Mg C ha^–1yr^–1. Optimum respiration activity corresponded toabout 2.5 g H₂O g^–1 dry wood, and severelyrestricted respiration to < 0.5 g H₂O g^–1dry wood. Respiration from coarse litter in central Amazon forests iscomparable in magnitude to decomposing fine surface litter (e.g. leaves,twigs) and is an important carbon cycling component when characterizingheterotrophic respiration budgets and net ecosystem exchange(NEE).     

Production,Respiration, and Overall Carbon Balance in an Old-growth <Emphasis Type="Italic">Pseudotsuga</Emphasis>-<Emphasis Type="Italic">Tsuga</Emphasis> Forest Ecosystem

Ground-based measurements of stores, growth, mortality, litterfall, respiration, and decomposition were conducted in an old-growth forest at Wind River Experimental Forest, Washington, USA. These measurements were used to estimate gross primary production (GPP) and net primary production (NPP); autotrophic respiration (R_a) and heterotrophic (R_h) respiration; and net ecosystem production (NEP). Monte Carlo methods were used to calculate uncertainty (expressed as ± 2 standard deviations of 200–400 calculations). Live carbon (C) stores were 39,800 g C m^–2 (34,800–44,800 g C m^–2). The store of C in detritus and mineral soil was 22,092 g C m^–2 (20,600–23,600 g C m^–2), and the total C stores were 61,899 g C m^–2 (56,600–67,700 g C m^–2). Total NPP was 597 g C m^–2 y^–1 (453 to 741 g C m^–2 y^–1). R_a was 1309 g C m^–2 y^–1 (845–1773 g C m^–2 y^–1), indicating a GPP of 1906 g C m^–2 y^–1 (1444–2368 g C m^–2 y^–1). R_h, including the respiration of heart rots in tree boles, was 577 g C m^–2 y^–1 (479–675 g C m^–2 y^–1). Long-term NEP was estimated to be +20 g C m^–2 y^–1 (–116 to +156 g C m^–2 y^–1), indicating this stand might be a small sink. These estimates contrast with the larger sink estimated at the same site using eddy-flux methods. Several hypotheses to explain this discrepancy were explored, including (a) undetected biomass increases, (b) underestimates of NPP, (c) unmeasured losses, and (d) a temporal mismatch between the two sets of measurements. The last hypothesis appears the most likely.     

Hydrological and nutrient budgets of freshwater and estuarine wetlands of Taylor Slough in Southern Everglades, Florida (U.S.A.)

Hydrological restoration of the Southern Everglades will result in increased freshwater flow to the freshwater and estuarine wetlands bordering Florida Bay. We evaluated the contribution of surface freshwater runoff versus atmospheric deposition and ground water on the water and nutrient budgets of these wetlands. These estimates were used to assess the importance of hydrologic inputs and losses relative to sediment burial, denitrification, and nitrogen fixation. We calculated seasonal inputs and outputs of water, total phosphorus (TP) and total nitrogen (TN) from surface water, precipitation, and evapotranspiration in the Taylor Slough/C-111 basin wetlands for 1.5 years. Atmospheric deposition was the dominant source of water and TP for these oligotrophic, phosphorus-limited wetlands. Surface water was the major TN source of during the wet season, but on an annual basis was equal to the atmospheric TN deposition. We calculated a net annual import of 31.4 mg m^–2 yr^–1 P and 694 mg m^–2 yr^–1N into the wetland from hydrologic sources. Hydrologic import of P was within range of estimates of sediment P burial (33–70 mg m^–2 yr^–1 P), while sediment burial of N (1890–4027 mg m^–2 yr^–1 N) greatly exceeded estimated hydrologic N import. High nitrogen fixation rates or an underestimation of groundwater N flux may explain the discrepancy between estimates of hydrologic N import and sediment N burial rates.     

Mercury budget of an upland-peatland watershed

Inputs, outputs, and pool sizes oftotal mercury (Hg) were measured in a forested 10 hawatershed consisting of a 7 ha hardwood-dominatedupland surrounding a 3 ha conifer-dominatedpeatland. Hydrologic inputs via throughfall andstemflow, 13±0.4 g m^–2 yr^–1over the entire watershed, were about doubleprecipitation inputs in the open and weresignificantly higher in the peatland than in theupland (19.6 vs. 9.8 g m^–2 yr^–1). Inputs of Hg via litterfall were 12.3±0.7g m^–2 yr^–1, not different in thepeatland and upland (11.7 vs. 12.5 g m^–2yr^–1). Hydrologic outputs via streamflow were2.8±0.3 g m^–2 yr^–1 and thecontribution from the peatland was higher despiteits smaller area. The sum of Hg inputs were lessthan that in the overstory trees, 33±3 gm^–2 above-ground, and much less than eitherthat in the upland soil, 5250±520 gm^–2, or in the peat, 3900±100 gm^–2 in the upper 50 cm. The annual flux of Hgmeasured in streamflow and the calculated annualaccumulation in the peatland are consistent withvalues reported by others. A sink for Hg of about20 g m^–2 yr^–1 apparently exists inthe upland, and could be due to either or bothstorage in the soil or volatilization.     

The role of rock weathering in the phosphorus budget of terrestrial watersheds

Residual soils (saprolites) developed on crystalline rocks appear to form by an essentially isovolumetric process (i.e. without dilation or compaction). Isovolumetric geochemical analysis of a suite of saprolite samples developed on a common parent rock can be used to estimate the relative rates of long-term losses of P and Si during weathering. Using the export of dissolved Si in rivers as a weathering index, one can then estimate the rate of P release due to chemical weathering by means of the P-Si loss ratio in saprolite. For three basins where data are available (Liberty Hill, SC; Amazon River, Brazil: Rio Negro, Brazil) estimated P weathering release rates are 163, 457, and 242 moles P km^–2 yr^–1 respectively. These compare to precipitation inputs of 684, 700 and 630 moles P km^–2 yr^–1 and total river exports of 256, 4490 and 820 moles P km^–2 yr^–1, respectively. The Rio Negro shows a near perfect balance between the input of P via precipitation and chemical weathering and the riverine output of dissolved and suspended P. This system, however, raised the unsolved problem of the source that supports the atmospheric P input.     

The soil fauna of a beech forest on limestone: trophic structure and energy budget

Summary The soil fauna of a mull beech forest on lime-stone in southern Lower Saxony (West Germany) was sampled quantitatively. Biomass estimates, trophic characteristics, and measurement and calculation of the energetic parameters of the constituent animal populations were used to construct an energy budget of the total heterotrophic subsystem of the forest. Mean annual zoomass amounted to about 15 g d wt m^–2; earthworms (about 10 g d wt m^–2) and other groups of the macrofauna were dominant. Protozoa constituted about 1.5 g d wt m^–2. Relative distribution of zoomass among the trophic categories was 50% macrosaprophages, 30% microsaprophages, 12% microphytophages, and 4% zoophages. Total annual consumption rate of the saprophagous and microphytophagous soil fauna (6328 and 4096 kJ m^–2 yr^–1, respectively) was of the same order of magnitude as annual litter fall (canopy leaves 6124 kJ m^–2 yr^–1, flowers and fruits 944 kJ m^–2 yr^–1, herbs 1839 kJ m^–2 yr^–1, fine woody material 870 kJ m^–2 yr^–1, tree roots 3404 kJ m^–2 yr^–1, without coarse woody litter). Primary decomposers (macrosaprophages) were the key group for litter comminution and translocation onto and into the soil, thus contributing to the high decomposition rate (k=0.8) for leaf litter. Consumption rates of the other trophic groups were (values as kJ m^–2 yr^–1): bacteriophages 2954, micromycophages 416, zoophages 153. Grazing pressure of macrophytophages (including rhizophages) was low. Faeces input from the canopy layer was not significant. Grazing pressure on soil microflora almost equalled microbial biomass; hence, a large fraction of microbial production is channelled into the animal component. Predator pressure on soil animals is high, as a comparison between consumption rates by zoophages and production by potential prey — mainly microsaprophages, microphytophages and zoophages — demonstrated. Soil animals contributed only about 11% to heterotrophic respiration. However, there is evidence that animals are important driving variables for matter and energy transfer: key processes are the transformation of dead organic material and grazing on the microflora. It is hypothesized that the soil macrosaprophages are donor-limited.     

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.     

The organic carbon budget of a shallow arctic tundra lake on the Tuktoyaktuk Peninsula,N.W.T., Canada

The organic carbon cycle of a shallow, tundra lake (mean depth 1.45 m) was followed for 5 weeks of the open water period by examining CO₂ fluxes through benthic respiration and anaerobic decomposition, photosynthesis of benthic and phytoplankton communities and gas exchange at the air-water interface. Total photosynthesis (as consumption of carbon dioxide) was 37.5 mmole C m^–2 d^–1, 83% of which was benthic and macrophytic. By direct measurement benthic respiration exceeded benthic photosynthesis by 6.6 mmole C m^–2 d^–1. The lake lost 1.4 × 10⁶ moles C in two weeks after ice melted by degassing C0₂, and 6.8 mmole C m^–2 d^–1 (1.5 × 10⁶ moles) during the remainder of the open water period; 2.2 mmole C m² d^–1 of this was release Of CO₂ stored in the sediments by cryoconcentration the previous winter. Anaerobic microbial decomposition was only 4% of the benthic aerobic respiration rate of 38 mmole C m^–2 d^–1. An annual budget estimate for the lake indicated that 50% of the carbon was produced by the benthic community, 20% by phytoplankton, and 30% was allochthonous material. The relative contribution of allochthonous input was in accordance with measurement of the ¹⁵N of sedimented organic matter.     

Longitudinal Spatial Patterns of Bacterial Production and Respiration in a Large River–Estuary: Implications for Ecosystem Carbon Consumption

Rivers and estuaries transport organic carbon (C) from terrestrial and freshwater ecosystems to the marine environment. During this transit, bacteria actively utilize and transform organic C, but few studies have measured detailed spatial variation in rates of bacterial respiration (BR) and production (BP). We measured BP at 39 stations and BR at 12 stations at monthly intervals along a 200-km reach of the tidal Hudson River. We observed strong repeatable spatial patterns for both BP and BR, with rates declining in the downstream direction. Bacterial Production had much greater dynamic range of spatial variation than BR. We used the detailed seasonal and spatial data on BP and BR to measure the total C demand of bacteria at several scales. We calculated volumetric and areal rates for 12 sections of the Hudson, as well as the total C utilization. Volumetric BR averaged 20 g-C-m^–3 y^–1, but it was highest in the most upstream section at 30 g C m^–3 y^–1. Areal rates averaged over the entire river were 174 g C m^–2 y^–1, but they were 318 g C m^–2 y^–1 in the deepest section of the river, indicating the importance of morphometric variation. Total bacterial C demand increased downriver with increasing total volume. Overall, bacteria in the freshwater section of the river consumed approximately 18–25.5 × 10⁹ g C y^–1, about 20% of the total organic C load.     

The nitrogen cycle in lodgepole pine forests,southeastern Wyoming

Storage and flux of nitrogen were studied in several contrasting lodgepole pine (Pinus contorta spp.latifolia) forests in southeastern Wyoming. The mineral soil contained most of the N in these ecosystems (range of 315–860 g · m^–2), with aboveground detritus (37.5–48.8g · m^–2) and living biomass (19.5–24.0 g · m^–2) storing much smaller amounts. About 60–70% of the total N in vegetation was aboveground, and N concentrations in plant tissues were unusually low (foliage = 0.7% N), as were N input via wet precipitation (0.25 g · m^–2 · yr^–1), and biological fixation of atmospheric N (<0.03 g · m^–2 · yr^–1, except locally in some stands at low elevations where symbiotic fixation by the leguminous herbLupinus argenteus probably exceeded 0.1 g · m^–2 · yr^–1).Because of low concentrations in litterfall and limited opportunity for leaching, N accumulated in decaying leaves for 6–7 yr following leaf fall. This process represented an annual flux of about 0.5g · m^–2 to the 01 horizon. Only 20% of this flux was provided by throughfall, with the remaining 0.4g · m^–2 · yr^–1 apparently added from layers below. Low mineralization and small amounts of N uptake from the 02 are likely because of minimal rooting in the forest floor (as defined herein) and negligible mineral N (< 0.05 mg · L^–1) in 02 leachate. A critical transport process was solubilization of organic N, mostly fulvic acids. Most of the organic N from the forest floor was retained within the major tree rooting zone (0–40 cm), and mineralization of soil organic N provided NH₄ for tree uptake. Nitrate was at trace levels in soil solutions, and a long lag in nitrification was always observed under disturbed conditions. Total root nitrogen uptake was calculated to be 1.25 gN · m^–2 · yr^–1 with estimated root turnover of 0.37-gN · m^–2 · yr^–1, and the soil horizons appeared to be nearly in balance with respect to N. The high demand for mineralized N and the precipitation of fulvic acid in the mineral soil resulted in minimal deep leaching in most stands (< 0.02 g · m^–2 · yr^–1). These forests provide an extreme example of nitrogen behavior in dry, infertile forests.     

Phosphorus and nitrogen retention in five Precambrian shield wetlands

Phosphorus and nitrogen mass balances of five wetlands (two beaver ponds, two conifer-Sphagnum swamps and one sedge fen) situated in three catchments in central Ontario, Canada, were measured. Monthly and annual input-output budgets of total phosphorus (TP), total nitrogen (TN), total organic nitrogen (TON), total inorganic nitrogen (TIN), ammonium ion (NH₄ ⁺ -N), nitrate (NO ₃ ^– -N) and dissolved organic carbon (DOC) were estimated for the five wetlands during the 1982–83 and 1983–84 water years. Except for the deepest beaver pond (3.2 m) which had annual TP retention of –44% (–0.030 ± 0.015 g m^–2 yr^–1), the wetlands retained < 0.001 to 0.015 g M^–2 yr^–1 ; however, this wasless than 20% of the inputs and the estimated budget uncertainties were equal to or greater than the retention rates. Annual TN retentions ranged from –0.44 to 0.56 g m^–2 yr^–1 (–12 to 4%) but were not significantly different from zero. The wetlands transformed nitrogen by retaining TIN (16 to 80% RT) and exporting an equivalent amount as TON (–7 to 102% RT). The beaver ponds, however, retained NO ₃ ^– while NH ₄ ⁺ was passed through or the outputs exceeded the inputs. In contrast, the conifer swamps retained both NH ₄ ⁺ and NO ₃ ^– . DOC fluxes into and out of the beaver ponds were equal (–18 and 4% RT) but output from the conifer swamps exceeded input by > 90%. Marked seasonal trends in nutrient retention were observed. Nutrient retention coincided with low stream flow, increased evapotranspiration and biotic uptake during the summer. Net nutrient export occurred during the winter and spring when stream flows were highest and biotic uptake was low.     

Growth, production and losses of phytoplankton in the lowland River Spree: carbon balance

1. Phytoplankton carbon assimilation and losses (exudation, dark carbon losses) as well as oxygen release and dark community respiration were measured regularly for 2 years at four stations along the lower Spree (Germany). Carbon balance of river phytoplankton was estimated using measured assimilation, metabolic losses and variations in algal carbon along a stretch of river. 2. The light/dark bottle method was modified to simulate vertical mixing. 3. Waxing and waning of phytoplankton populations dominated the load of particulate organic carbon as well as the oxygen budget of the river. 4. Phytoplankton assimilated 310–358 g C m^?2 yr^?1. A mean value of 586 mg C m^?3 day^?1 was fixed in photosynthesis, with 16.7 mg C being exuded during the day and 20.1 mg lost at night. The measured dark respiration was equivalent to only 28% of the daily gross oxygen production of the plankton community. Phytoplankton washed from upstream lakes and reservoirs was not measurably damaged by turbulent transport. 5. In spring, 18–22% of assimilated carbon was used for net biosynthesis of phytoplankton along the river course. At this time, the carbon balance of this part of the Spree was dominated by autochthonous net production. During summer, however, total carbon losses exceeded the intensive carbon assimilation. The decline of algal biomass along the river course in summer was not explicable by measurable physiological losses. The importance of sedimentation and grazing losses is discussed.     

Predictive models for phosphorus retention in wetlands

The potential of wetlands to efficiently remove (i.e., act as a nutrient sink) or to transform nutrients like phosphorus under high nutrient loading has resulted in their consideration as a cost-effective means of treating wastewater on the landscape. Few predictive models exist which can accurately assess P retention capacity. An analysis of the north American data base (NADB) allowed us to develop a mass loading model that can be used to predict P storage and effluent concentrations from wetlands. Phosphorus storage in wetlands is proportional to P loadings but the output total phosphorus (TP) concentrations increase exponentially after a P loading threshold is reached. The threshold P assimilative capacity based on the NADB and a test site in the Everglades is approximately 1 g m^–2 yr^–1. We hypothesize that once loadings exceed 1 g m^–2 yr^–1 and short-term mechanisms are saturated, that the mechanisms controlling the uptake and storage of P in wetlands are exceeded and effluent concentrations of TP rise exponentially. We propose a One Gram Rule for freshwater wetlands and contend that this loading is near the assimilative capacity of wetlands. Our analysis further suggests that P loadings must be reduced to 1 g m^–2 yr^–1 or lower within the wetland if maintaining long-term low P output concentrations from the wetlands is the central goal. A carbon based phosphorus retention model developed for peatlands and tested in the Everglades of Florida provided further evidence of the proposed One Gram Rule for wetlands. This model is based on data from the Everglades areas impacted by agricultural runoff during the past 30 years. Preliminary estimates indicate that these wetlands store P primarily as humic organic-P, insoluble P, and Ca bound P at 0.44 g m^–2 yr^–1 on average. Areas loaded with 4.0 g m^–2 yr^–1 (at water concentrations>150 g·L^–1 TP) stored 0.8 to 0.6 g m^–2 yr^–1 P, areas loaded with 3.3 g m^–2 yr^–1 P retained 0.6 to 0.4 g m^–2 yr^–1 P, and areas receiving 0.6 g m^–2 yr^–1 P retained 0.3 to 0.2 g m^–2 yr^–1. The TP water concentrations in the wetland did not drop below 50 g·L^–1 until loadings were below 1 g m² yr^–1 P.     

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.