Retention of nitrogen and phosphorus in a Danish lowland river system: implications for the export from the watershed

In a Danish lowland river system intensive measurements were made, in four 80 m reaches, of the nitrogen (N) and phosphorus (P) stored in the stream sediment. The results were used for calculation of the total retention in the river system during two summers (June to August). In addition, the mobilization of nutrients from the stream bottom in autumn 1987 was compared with the export from the watershed.During the study period (June 1987 to September 1988) the amounts of N and P stored in stream reaches were determined fortnightly using a core-sample technique. In reaches dominated by submersed macrophytes, 25–40 g N m^–2 and 20–30 g P m^–2 were stored during two summers, against only 10–15g N and P m^–2 for sandy and gravely reaches. In riparian zones with emergent macrophytes the retention was even higher than in the submersed macrophytes. Gross retention exceeded net retention by a factor of two to three.Net retention of P in the river system during the summer of 1987 was equal to the summer export from the watershed. On an annual basis, retention in the summer constituted 20% of the P export. In contrast, retention in the summer of 1988 amounted to 60% of the total P export during the same period (38% reduction) and 22% in comparison with the annual export. The corresponding figures for N were lower, showing reductions of 16% and 12% of the export of total N in the two summer periods, and about 1% of the annual exports.In September 1987 6.4% of the total N export and 65% of the total P export from the watershed consisted of resuspended material. In 1987 the N and P retained during the summer was almost completely resuspended during storm events during September to November.     

Phosphorus Deposition in a Low-Phosphorus Landscape: Sources,Accuracy and Contribution to Declines in Surface Water P

Atmospheric deposition can be an important source of phosphorus (P) to watersheds in geologically low-P landscapes like the Precambrian Shield, in south-central Ontario. Recent observations in this region of declining total P (TP) concentrations in streams and lakes, as well as P limitation in vegetation, prompted this investigation into the potential contributing role of atmospheric deposition. Long-term (1980–2014) measurements of bulk TP deposition at four monitoring stations, as well as wet-only deposition at one of the sites (2012–2014) and pollen measurements in 2014, were used to assess the potential contribution of biogenic sources to TP deposition in the region. Ratios of phosphorus to potassium (P:K) in historical bulk deposition and variability in P deposition among the four sites indicated that continuously open, bulk deposition collectors are enriched in ‘local’ biogenic sources of P, particularly in the summer, fall and spring months. Furthermore, in 2014 pollen could account for as much as 30% of bulk TP deposition, and pine trees (Pinus strobus) were the most important source of pollen (>75% of collected pollen). In contrast, winter bulk deposition was the least affected by local, biogenic sources, and likely best reflects net P inputs to this region from long-range (that is, external to the watershed) sources. Catchment input–output budgets for P that use measured annual bulk deposition totals as inputs are generally strongly positive (indicating net retention/accumulation of P within watersheds), whereas annual budgets calculated using extrapolated winter deposition are in approximate balance. Balanced P budgets are more consistent with regional observations of declining surface water TP and P limitation in terrestrial vegetation. Importantly, there was no significant decline in winter TP deposition over the period of record (1980–2014), which suggests that changes in regional deposition are not responsible for declines in surface water TP. Although we recommend extrapolating winter bulk deposition measurements to approximate annual atmospheric TP input to terrestrial ecosystems in non-dusty, oligotrophic landscapes such as this, local sources of P, particularly pollen, are likely important seasonal sources of P nutrition to lakes.     

The influence of water level and salinity on plant assemblages of a seasonally flooded Mediterranean wetland

We studied the key environmental variables shaping plant assemblages in Mediterranean abandoned ricefields with contrasting freshwater inputs over saline sediments. Plant species cover, water levels and soil variables were studied following a stratified random sampling design. Multivariate analysis identified water regime, particularly summer and autumn irrigation, as the most important environmental variable associated with vegetation composition. Distribution of annual and emergent macrophytes was not associated to salinity as found at the study site (0.57–4.1 mS/cm). Increased soil salinity, caused by summer irrigation near the soil surface did affect shallow-marsh assemblage distribution. These key environmental characteristics allowed us to identify six main assemblages. Annual macrophytes (such as Zannichellia palustris) were defined by high (over 10 cm) annual mean water level (MWL) and early successional conditions; emergent macrophytes (such as Typha spp., Scirpus lacustris) by annual MWL of 10 to − 25 cm and continuous shallow flooding in summer and autumn (MWL of 0–10 cm). The shallow-marsh group, correlated with annual MWL − 25 to − 100 cm, separated into two subgroups by salinity: grassland (including Paspalum distichum) with summer and autumn MWL below − 25 cm and brackish (with Juncus subulatus or Agrostis stolonifera) with summer and autumn MWL just below the soil surface (0 to − 25 cm). Water levels for the grassland subgroup may equate with a salinity ‘refuge’ for P. distichum. Time was a further determinant of variation in the full data set. Abundance of a large group of agricultural annuals (such as Sonchus tenerrimus) and damp ground annuals (including ricefield weeds such as Ammania robusta) decreased with time as bare ground disappeared. Maintenance of spatial vegetation heterogeneity in abandoned ricefields is contingent on continued water regime management.     

Regulation of submersed macrophyte biomass in a temperate lowland river: Interactions between shading by bank vegetation, epiphyton and water turbidity

Growth of aquatic vegetation is often controlled by light supply, which is potentially decreased by bank vegetation, water turbidity and epiphytic biofilm. To understand the relative importance of these shading factors and the interactions between them we analysed the seasonal course of macrophyte biomass, shading by bank vegetation, turbidity of the water column and epiphytic light absorption in shaded and sunny sections of a temperate eutrophic lowland river. At a shaded site, bank vegetation decreased the light supply by 79%, 0.5 m water column by 45% and 2-week-old epiphyton by 28% during the vegetation period. Growth of submersed macrophytes, but not of epiphyton, was light-limited in the shaded sections. We found a saturation-type correlation between light supply and macrophyte biomass. Therefore, the additional light absorption of the water column or epiphyton only shortened the period of optimum light supply at the sunny site, but was crucial for macrophyte development at the shaded site. Light absorption of phytoplankton was most important in spring and that of epiphyton in late summer. Submersed macrophytes effectively retained particles and thus improved light supply of downstream stands, but this positive feedback effect was only relevant for shaded sections in summer.     

A northern Italian shallow lake as a case study for eutrophication control

Lake Varese (northern Italy) has shown deterioration in water quality since the 1960s and, as a result of the long duration of direct discharge of untreated sewage into the lake, it was classified as being hypertrophic. To recover the lake water quality, a series of externally and internally remedial actions were implemented in subsequent years. The applied sewage collecting system induced a reduction of the external P loads from 50 t P year⁻¹ to 16t P year⁻¹ and the weighted mean annual TP concentration decreased from 352 μg P l⁻¹ to 85 μg P l⁻¹, typical of eutrophic conditions. The hypolimnetic water withdrawals, adopted in the years 2000–2003, allowed a reduction of the internal P loads of about 3–5 t P. In the same years, 500t O₂ were injected at depths of 4.5–8 m during the summer months. In spite of these internal remedial actions, no significant reduction of the weighted mean annual concentration of the TP could be observed, and during the summer stratification period no significant reduction of the volumes of anoxic water and of the duration of the anoxia were detected. The anoxic conditions are still the prevailing force driving the lake P-budget, maintaining the lake in eutrophic status.     

Importance of sediment deposition and denitrification for nutrient retention in floodplain wetlands

Questions: Various floodplain communities may differ in their relative abilities to influence water quality through nutrient retention and denitrification. Our main questions were: (1) what is the importance of sediment deposition and denitrification for plant productivity and nutrient retention in floodplains; (2) will rehabilitation of natural floodplain communities (semi‐natural grassland, reedbed, woodland, pond) from agricultural grassland affect nutrient retention? Location: Floodplains of two Rhine distributaries (rivers Ussel and Waal), The Netherlands. Methods: Net sedimentation was measured using mats, denitrification in soil cores by acetylene inhibition and bio‐mass production by clipping above‐ground vegetation in winter and summer. Results: Sediment deposition was a major source of N and P in all floodplain communities. Highest deposition rates were found where water velocity was reduced by vegetation structure (reedbeds) or by a drop in surface elevation (pond). Sediment deposition was not higher in woodlands than in grassland types. Denitrification rates were low in winter but significantly higher in summer. Highest denitrification rates were found in an agricultural grassland (winter and summer) and in the ponds (summer). Plant productivity and nutrient uptake were high in reedbeds, intermediate in agricultural grasslands, ponds and semi‐natural grasslands and very low in woodlands (only understorey). All wetlands were N‐limited, which could be explained by low N:P ratios in sediment. Conclusions: Considering Rhine water quality: only substantial P‐retention is expected because, relative to the annual nutrient loads in the river, the floodplains are important sinks for P, but much less for N. Rehabilitation of agricultural grasslands into ponds or reedbeds will probably be more beneficial for downstream water quality (lower P‐concentrations) than into woodlands or semi‐natural grasslands.     

Lake responses to reduced nutrient loading – an analysis of contemporary long-term data from 35 case studies

1. This synthesis examines 35 long‐term (5–35 years, mean: 16 years) lake re‐oligotrophication studies. It covers lakes ranging from shallow (mean depth <5 m and/or polymictic) to deep (mean depth up to 177 m), oligotrophic to hypertrophic (summer mean total phosphorus concentration from 7.5 to 3500 μg L^?1 before loading reduction), subtropical to temperate (latitude: 28–65°), and lowland to upland (altitude: 0–481 m). Shallow north‐temperate lakes were most abundant. 2. Reduction of external total phosphorus (TP) loading resulted in lower in‐lake TP concentration, lower chlorophyll a (chl a) concentration and higher Secchi depth in most lakes. Internal loading delayed the recovery, but in most lakes a new equilibrium for TP was reached after 10–15 years, which was only marginally influenced by the hydraulic retention time of the lakes. With decreasing TP concentration, the concentration of soluble reactive phosphorus (SRP) also declined substantially. 3. Decreases (if any) in total nitrogen (TN) loading were lower than for TP in most lakes. As a result, the TN : TP ratio in lake water increased in 80% of the lakes. In lakes where the TN loading was reduced, the annual mean in‐lake TN concentration responded rapidly. Concentrations largely followed predictions derived from an empirical model developed earlier for Danish lakes, which includes external TN loading, hydraulic retention time and mean depth as explanatory variables. 4. Phytoplankton clearly responded to reduced nutrient loading, mainly reflecting declining TP concentrations. Declines in phytoplankton biomass were accompanied by shifts in community structure. In deep lakes, chrysophytes and dinophytes assumed greater importance at the expense of cyanobacteria. Diatoms, cryptophytes and chrysophytes became more dominant in shallow lakes, while no significant change was seen for cyanobacteria. 5. The observed declines in phytoplankton biomass and chl a may have been further augmented by enhanced zooplankton grazing, as indicated by increases in the zooplankton : phytoplankton biomass ratio and declines in the chl a : TP ratio at a summer mean TP concentration of <100–150 μg L^?1. This effect was strongest in shallow lakes. This implies potentially higher rates of zooplankton grazing and may be ascribed to the observed large changes in fish community structure and biomass with decreasing TP contribution. In 82% of the lakes for which data on fish are available, fish biomass declined with TP. The percentage of piscivores increased in 80% of those lakes and often a shift occurred towards dominance by fish species characteristic of less eutrophic waters. 6. Data on macrophytes were available only for a small subsample of lakes. In several of those lakes, abundance, coverage, plant volume inhabited or depth distribution of submerged macrophytes increased during oligotrophication, but in others no changes were observed despite greater water clarity. 7. Recovery of lakes after nutrient loading reduction may be confounded by concomitant environmental changes such as global warming. However, effects of global change are likely to run counter to reductions in nutrient loading rather than reinforcing re‐oligotrophication.     

Love Handles in Aquatic Ecosystems: The Role of Dissolved Organic Carbon Drawdown, Resuspended Sediments, and Terrigenous Inputs in the Carbon Balance of Lake Michigan

During the unstratified (winter) and stratified (summer) periods of 1999 and 2000, we examined carbon (C) dynamics in the upper water column of southern Lake Michigan. We found that (a) bacterial respiration (BR) and planktonic respiration (PR) were major sinks for C, (b) C flux through bacteria (CFTB) was diminished in winter because of reduced bacterial production (BP) and increased bacterial growth efficiency (BGE) at colder temperatures, and (c) PR exceeded primary production (PP) during the spring–summer transition. Drawdown of dissolved organic C (DOC), resuspended organic matter from the lake floor, and riverine organic matter likely provided organic C to compensate for this temporal deficit. DOC in the water column decreased between winter and summer (29–91 mg C m² d⁻¹) and accounted for 20%–53% of CFTB and 11%–33% of PR. Sediment resuspension events supported elevated winter heterotrophy in the years that they occurred with greatest intensities (1998 and 2000) and may be important to interannual variability in C dynamics. Further, riverine discharge, containing elevated DOC (5×) and dissolved P (10×) relative to lake water, peaked in the winter–spring season in southern Lake Michigan. Collectively, terrigenous inputs (river, stream, and groundwater discharges; storm water runoff; and atmospheric precipitation) may support approximately 10%–20% of annual in-lake heterotrophy as well as autotrophy. Terrestrial subsidies likely play a key role in the C balance of even very large lakes, representing a critical linkage between terrestrial and aquatic ecosystems. Received 11 June 2001; Accepted 14 December 2001.     

The distribution of macroinvertebrate assemblages in a reach of the River Allier (France), in relation to riverbed characteristics

Investigations on large canalised rivers, for example the Danube, have shown that transported particulate matter, which is typically inorganic, is predominantly deposited in waters near the river’s main channel. This investigation deals with the lower section of the River Havel (NE Germany), a canalised lowland river with a very flat floodplain. This river is highly polluted by nutrients from urban areas (Berlin) and a long chain of river lakes produces high concentrations of phytoplankton. Due to the high proportion of planktogenic detritus, it was hypothesised that greater quantities of nutrient-rich fine particulate organic matter (FPOM) would be deposited in floodplain waters located further from the main channel than has been reported for large rivers. The total nutrient, P-binding metal (Fe, Al, and Mn), organic and inorganic carbon (TOC, TIC) contents of the upper organic sediment layer (0–4 cm) were analysed in samples collected from 48 floodplain water and river sites. The sediment bulk density, calculated on the basis of dry mass content and loss on ignition, was used to characterise the waters according to the impact of the river current. The results showed that the variability of total phosphorus (TP) was best explained by the variability of total iron (TFe, R² = 0.52). The floodplain water sediments could clearly be separated into two groups on the basis of the sediment particle size composition, and of the element ratios TOC:TP, TN:TP, primarily TFe:TP. The sediments from impounded river sections and from mouth sections of backwaters (approx. 100–200 m) were characterised by a high proportion particles from the 0.1 to 0.5 mm size fraction and by homogeneous, low TFe:TP, TOC:TP and TN:TP ratios. Sediments from distal sections of backwaters and of oxbow lakes tended to exhibit high element ratios with much higher variability. These results were interpreted as a spatially limited impact of the river on the floodplain water sediments. Contrary to expectation, the phosphorus bound in river seston was predominantly and very homogeneously deposited in the impounded river and mouth sections of backwaters. This implies that the inundation of the floodplain waters during spring floods seems to have no important material impact on the sediments in waters of low hydrological connectivity with the River Havel.     

A new method to assess water trophy and organic pollution – the Macrophyte Biological Index for Rivers (IBMR): its application to different types of river and pollution

The paper presents a new index for assessing water trophy and organic pollution. It is based on only true aquatic macrophytes – being calculated on species score, coefficient of ecological amplitude and degree of cover. The method was tested in an acidic lowland river and an alkaline mountain river, and is shown to be validated by bio-indication scales based on macrophyte communities. The practical interest is discussed regarding the Water Framework Directive. Electronic Supplementary Material Electronic supplementary material is available for this article atand accessible for authorised users.     

Long-term response of a shallow, moderately flushed lake to reduced external phosphorus and nitrogen loading

1. The responses of nutrient concentrations, plankton, macrophytes and macrozoobenthos to a reduction in external nutrient loading and to contemporary climatic change were studied in the shallow, moderately flushed Lake Müggelsee (Berlin, Germany). Weekly to biweekly data from 1979 to 2003 were compared with less frequently collected historical data. 2. A reduction of more than 50% in both total phosphorus (TP) and total nitrogen (TN) loading from the hypertrophic (1979–90) to the eutrophic period (1997–2003) was followed by an immediate decline in TN concentrations in the lake. TP concentrations only declined during winter and spring. During summer, phosphorus (P) release from the sediments was favoured by a drastic reduction in nitrate import. Therefore, Müggelsee acted as a net P source for 6 years after the external load reduction despite a mean water retention time of only 0.1–0.16 years. 3. Because of the likely limitation by P in spring and nitrogen (N) in summer, phytoplankton biovolume declined immediately after nutrient loading was reduced. The formerly dominant cyanobacteria (Oscillatoriales) Limnothrix redekei and Planktothrix agardhii disappeared, but the mean biovolume of the N₂‐fixing species Aphanizomenon flos‐aquae remained constant. 4. The abundance of Daphnia spp. in summer decreased by half, while that of cyclopoid copepod species increased. Abundances of benthic macroinvertebrates (mainly chironomids) decreased by about 80%. A resource control of both phytoplankton and zooplankton is indicated by significant positive correlations between nutrient concentrations and phytoplankton biovolume and between phytoplankton and zooplankton biomass. 5. Water transparency in spring increased after nutrient reduction and resulted in re‐colonisation of the lake by Potamogeton pectinatus. However, this process was severely hampered by periphyton shading and grazing by waterfowl and fish. 6. Water temperatures in Müggelsee have increased in winter, early spring and summer since 1979. The earlier development of the phytoplankton spring bloom was associated with shorter periods with ice cover, while direct temperature effects were responsible for the earlier development of the daphnid maximum in spring.     

Origin and succession of phytoplankton in a river-lake system (Spree,Germany)

The River Spree (Germany) flows through an impoundment and several shallow lakes in its middle and lower course. In this river-lake system, the seasonal and longitudinal dynamics of dominant phytoplankton populations were studied in relation to retention time of water, mixing conditions and nutrient supply from 1988–92. Some phytoplankton species populated the same river section for weeks or months each year at their season. Such stable populations have to origin from river zones functioning like mixed reactors. In the Spree system, centric diatoms originated from an impoundment and filamentous cyanobacteria from a flushed lake with longer retention time of water. Downstream, biomass and composition of phytoplankton altered nearly simultaneously along the system.The fate of planktonic organisms washed from mixed reactors into the flow depended on the conditions at the zones of origin. During spring, populations dominating phytoplankton communities of the well-mixed lakes grew further under river conditions. However the biomass of summer species, adapted to intermittent stratification, was halved along the river course. These seasonal differences were probably caused by lower maximum growth rates of summer species and enhanced losses (photorespiration, sedimentation or grazing of benthic filter feeders, but not of zooplankton) of algal populations under river conditions in summer.Phytoplankton assimilation, settlement of diatoms, or denitrification caused declining (probably growth limiting) concentrations of dissolved inorganic phosphorus (spring), silicon (early summer) or nitrogen (summer) along the river course, respectively. The minimum content of DRP was often followed by a clear-water phase. Reduced DSi supply selected against diatoms and additional DIN shortage favoured N₂-fixing cyanobacteria in the last lake of the system.R-strategists (sensu Reynolds) were selected in both the flushed, shallow lakes and the lowland river. In general, the biomass of cyanobacteria increased within the lakes and declined along the river course. Some diatom populations grew in the river, but were grazed or settled down in the lakes. Beside this general picture, different populations from the same phylogenetic group did not necessarily perform in similar ways.     

基于氮磷比解析太湖苕溪水体营养现状及应对策略

生态化学计量学是评价水体营养状态的重要手段,利用其氮磷比指标探讨了我国太湖主要入湖河流苕溪的营养状态。野外监测结果显示,苕溪水体氮素超标严重,磷素污染轻度,硝酸盐、颗粒态磷为氮磷的主要赋存形态,且氮磷浓度呈现相似的季节变化规律,表明苕溪主要受农业面源污染影响。氮磷比分析表明,苕溪水体春、秋季处于磷素限制状态,夏季适合藻类生长,冬季低温条件下不利于藻类的大量繁殖;苕溪生物量增长受磷素限制,线性拟合亦显示其氮磷比主要受磷素波动的调控;苕溪干流大面积暴发蓝藻水华的风险较部分支流及死水区低,苕溪水入湖后,特别是夏季其暴发风险将显著提高。针对苕溪水体的富营养化现状,提出若干条水质改善应对策略。     

Canonical variables of aquatic bryophyte combinations for predicting water trophic level

Concentrations of soluble reactive phosphorus (SRP), nitrate, and soluble reactive silicon (SRSi) were monitored in 12 streams draining small catchments (<10 km2) in the English Lake District. The catchments varied with respect to underlying geology, soil type and land cover. Average concentrations of SRP were in the range 0.5–11.2 μg P l-1, and estimated loads ranged from 0.01 to 0.14 kg P ha-1 a-1. The higher concentrations and loads were associated with catchments containing improved pasture. Mean streamwater concentrations of nitrate varied from 55 to 660 μg N l-1, while loads were in the range 0.8–9.6 kg N ha-1 a-1; no general dependence on catchment properties was discerned. Concentrations of SRSi were similar in all the streams (0.8–2 mg Si l-1), and annual loads were in the range 10–26 kg Si ha-1 a-1. Loads of all three nutrients were greatest during the winter, because of higher discharges, but in some catchments containing improved pasture, considerable transport of P also took place during the summer. Concentrations of nitrate in streams draining unimproved moorland catchments are approximately twice those reported for samples taken from similar streams in 1973 and 1974, possibly because of increased atmospheric deposition of inorganic nitrogen (ammonium and nitrate). Concentrations of SRP in such streams were similar to those reported for the earlier samples. Comparisons of stream loads of SRP and nitrate with estimated inputs suggest that catchment soils retain substantial amounts of these nutrients. Implications for surface water eutrophication of changes in P retention by soils are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Abrupt shift from clear to turbid state in a shallow eutrophic, biomanipulated lake

Monitoring data were used to assess causes behind a recent shift from a clear-water to a turbid-water state in Lake Major, a 10 ha shallow lake in Hungary. In 1999–2000, fish manipulation was conducted in this hypertrophic lake. Reduced fish stock resulted in clearing water and the development of a dense (>80% coverage) submerged vegetation in 2005. During the recent abrupt shift, which occurred in 2007, submerged vegetation subsequently declined after a two-year period of clear water and abundant vegetation. An intense decay of macrophytes within the lake produced a rapid transition between the clear- and turbid-water states. During the clear-water state in 2005–2006, the most important variables predominantly correlating with macrophyte cover were Secchi transparency, temperature and TN, while TN, temperature, Secchi depth and chlorophyll-a were the most significant variables during the turbid-water state in 2007. Nitrogen may play a significant role in the cover of submerged macrophytes when TP is moderate. We argue that several factors in concert are necessary to initiate a shift. Water temperature likely has contributed to triggering shift through inter-year-dependent changes in cover of macrophytes, with fish recruitment having key roles in the dynamics of shallow lakes. Handling editor: Luigi Naselli-Flores     

Nutrients and chlorophyll-a dynamics in a temperate reservoir influenced by Asian monsoon along with in situ nutrient enrichment bioassays

Long-term nutrients and chlorophyll-a dynamics during 1993–2000 were analyzed in a temperate reservoir influenced by the Asian monsoon. Nonparametric Mann–Kendall tests and seasonal trend analyses indicated that there were no long-term annual increasing or decreasing trends in major trophic parameters over 8 years, but the monsoon seasonality was evident. Seasonality in chlorophyll (CHL) and total phosphorus (TP) showed a mono-modal pattern, which was closely associated with the monsoon season of July–August, and the magnitude of the mono-modal peak was greater in the headwater zone than in the downlake zone. Such temporal patterns fluctuated interannually over the study period, and the magnitude of the variation was directly controlled by the intensity of the monsoon rain. Empirical models of annual mean CHL–TP were developed supporting the view that phytoplankton in lentic ecosystems responds to P enrichment and that annual mean TP may provide a reliable basis for predicting the average algal abundance. Ambient nutrient analyses, N:P ratios and in situ nutrient enrichment bioassay experiments (NEBs) in premonsoon and postmonsoon supported the P limitation for phytoplankton growth. Ambient nutrients and non-volatile suspended solid (NVSS) data on CHL in the intense monsoon year, however, showed the possibility of light limitation, even though the NEBs did not show the direct evidence. These findings were confirmed by two-dimensional graphic approaches of trophic state index deviations (TSIDs).     

Estimation of long-term variation in nutrient loads from the Hii River by comparing the change in observed and calculated loads in the catchments

To study the long-term change in nutrient loads from the Hii River to Lake Shinji, water samples were taken repeatedly over a year in 1983/1984 and again in 2001/2002. Annual total nitrogen (TN) loads, estimated from observations of water quality and river flow, increased from 860 to 920 t with a corresponding increase in NO₃–N concentration during the cool season. In contrast, total phosphorus (TP) loads decreased from 96 to 62 t. Annual TN and TP loads, calculated using emission factors and annual statistics for the catchments, showed a tendency to decline from 1986 to 2002. No source could be identified which would result in the increase in TN in the catchments, therefore, the increase in observed TN loads was considered to originate in other areas. Atmospheric nitrogen deposition transported from long distances has elevated the sum of NH₄–N and NO₃–N concentration in rainwater in the cool season. Therefore, it was considered that this resulted in the increase in TN loads in the Hii River.     

The recovery of a very shallow eutrophic lake, 20 years after the control of effluent derived phosphorus

1. Monitoring at fortnightly to monthly intervals of a very shallow, lowland lake over 24 years has enabled the time course of recovery from nutrient enrichment to be investigated after high external P loading of the lake (>10 g P m^?2 year^?1) was reduced between 1977 and 1980. 2. The lake showed a relatively rapid response during the spring and early summer, with a reduction in phytoplankton biomass occurring after 5 years when soluble reactive phosphorus concentration was <10 μg L^?1. 3. However, during the later summer the response was delayed for 15 years because of sustained remobilisation of phosphorus from the sediment. The greater water clarity in spring and a gradual shift from planktonic to benthic algal growth may be related to the reduction in internal loading after 15 years. 4. Changes in the phytoplankton community composition were also observed. Centric diatoms became less dominant in the spring, and the summer cyanobacteria populations originally dominated by non‐heterocystous species (Limnothrix/Planktothrix spp.) almost disappeared. Heterocystous species (Anabaena spp. and Aphanizomenon flos‐aquae) were slower to decline, but after 20 years the phytoplankton community was no longer dominated by cyanobacteria. 5. There were no substantial changes in food web structure following re‐oligotrophication. Total zooplankton biomass decreased but body size of Daphnia hyalina, the largest zooplankton species in the lake, remained unchanged, suggesting that the fish population remained dominated by planktivorous species. 6. Macrophyte growth was still largely absent after 20 years, although during the spring water clarity may have become sufficient for macrophytes to re‐establish.     

Regulation and Seasonal Dynamics of Extracellular Enzyme Activities in the Sediments of a Large Lowland River

We tested whether seasonal changes in the sources oforganic substances for microbial metabolism were reflected changes in the activities of five extracellular enzymes in the eighth order lowland River Elbe, Germany. Leucine aminopeptidase showed the highest activities in the water column and the sediments, followed by phosphatase > β-glucosidase > α-glucosidase > exo-1,4-β-glucanase. Individual enzymes exhibited characteristic seasonal dynamics, as indicated by their relative contribution to cumulative enzyme activity. Leucine aminopeptidase was significantly more active in spring and summer. In contrast, the carbohydrate-degrading enzymes peaked in autumn, and β-glucosidase activity peaked once again in winter. Thus, in sediments, the ratio of leucine aminopeptidase/β-glucosidase reached significant higher medians in spring and summer (5-cm depth: ratio 7.7; 20-cm depth: ratio 10.1) than in autumn and winter (5-cm depth: ratio 3.7, 20-cm depth: ratio 6.3). Therelative activity of phosphatase in the sediments was seasonally related to both the biomass of planktonic algae as well as to the high content of total particulate phosphorus in autumn and winter. Due to temporal shifts in organic matter supply and changes in the storage capacity of sediments, the seasonal peaks of enzyme activities in sediments exhibited a time lag of 2–3 months compared to that in the water column, along with a significant extension of peak width. Hence, our data show that the seasonal pattern of extracellular enzyme activities provides a sensitive approach to infer seasonal or temporary availability of organic matter in rivers from autochthonous and allochthonous sources. From the dynamics of individual enzyme activities, a consistent synoptic pattern of heterotrophic functioning in the studied river ecosystem could be derived. Our data support the revised riverine productivity model predicting that the metabolism of organic matter in high-order rivers is mainly fuelled by autochthonous production occurring in these reaches and riparian inputs.     

Physicochemical limnology of the Tongue River Reservoir,Montana

A one year physicochemical survey was conducted on the Tongue River Reservoir, a run of the river impoundment in southeastern Montana. The Tongue River was the only significant inflow and supplied 93, 96 and 97% of the nutrient, major ion and water inputs to the impoundment. Heat advected from inflowing water accounted for 17% of the energy gained during the summer heating cycle. The annual nutrient load to the reservoir from the river was 20.2 g m^–2 total nitrogen (TN) and 3.8 g m^–2 total phosphorus (TP). Due to the absence of reducing conditions at depth and the complex seasonal pattern of water movement through the reservoir, 99% of the TN load was discharged but 49% of the TP load was retained in the reservoir.