Is the Iron Gate I reservoir on the Danube River a sink for dissolved silica?

Damming rivers changes sediment and nutrient cycles downstream of a dam in many direct and indirect ways. The Iron Gates I reservoir on the Yugoslavian-Romanian border is the largest impoundment by volume on the Danube River holding 3.2billionm³ of water. Silica retention within the reservoir in the form of diatom frustules was postulated to be as high as 600ktyear^–1 in previous studies using indirect methods. This amount of dissolved silicate was not delivered to the coastal Black Sea, and presumably caused a shift in the phytoplankton community there, and subsequent drastic decline in fishery. We directly quantified the amount of dissolved silicate (DSi) entering and leaving the reservoir for 11 continuous months. The budget based on these data reveals two important facts: (1) only about 4% of incoming DSi was retained in the reservoir; (2) the DSi concentrations were relatively low in the rivers upstream of the reservoir compared to regional and global averages. Thus damming the Danube at the Iron Gates could not have caused the decline in DSi concentrations documented downstream of the impoundment. Rather, this change in DSi must have occurred in the headwaters of the Danube River. Potential reasons include the construction of many dams upstream of the Iron Gates, hydrologic changes resulting in lower groundwater levels, and clogging of the riverbed limiting groundwater–river exchange.     

Whole-Catchment Manipulations of Internal and External Loading Reveal the Sensitivity of a Century-Old Reservoir to Hypoxia

Climate change is predicted to have widespread impacts on freshwater lake and reservoir nutrient budgets by altering both hypolimnetic hypoxia and runoff, which will in turn alter the magnitude of internal and external nutrient loads. To examine the effects of these potential climate scenarios on nitrogen (N) and phosphorus (P) budgets, we conducted a whole-catchment manipulation of hypolimnetic oxygen conditions and external loads to Falling Creek Reservoir (FCR), an old, eutrophic reservoir in a reforested catchment with a history of agricultural land use. Throughout 2 years of monitoring, internal N and P loading during hypoxic conditions dominated the hypolimnetic mass of nutrients in FCR, regardless of changes in external loading. FCR commonly functioned as a net sink of N and P, except during hypoxic conditions, when the reservoir was a net source of ammonium (\( {\text{NH}}_{4}^{ + } \)) to downstream. We observed extremely high nitrate–nitrite (\( {\text{NO}}_{3}^{ - } {-}{\text{NO}}_{2}^{ - } \)), soluble reactive P (SRP), total nitrogen (TN), and total phosphorus (TP) retention rates, indicating that the reservoir served as a sink for greater than 70% of \( {\text{NO}}_{3}^{ - } {-}{\text{NO}}_{2}^{ - } \) inputs and greater than 30% of SRP, TN, and TP inputs, on average. Our study is notable in the length of time since reforestation (>80 years) that a reservoir is still exhibiting high N and P internal loading during hypoxia, potentially as a result of the considerable store of accumulated nutrients in its sediment from historical agricultural runoff. Our whole-catchment manipulations highlight the importance of understanding how multiple aspects of global change, waterbody and catchment characteristics, and land use history will interact to alter nutrient budgets in the future.     

The role of sediments for phosphorus retention in the Kirinya wetland (Uganda)

Phosphorus dynamics were examined and modelled in a Cyperus papyrus and Phragmites mauritanus wetland on the Ugandan coast of Lake Victoria receiving secondary treated wastewater. Using a series of transversal transects, concentrations of nitrogen (N) and phosphorus (P) were found to decrease gradually as water moved downstream, giving nutrient retention capacities which ranged between 40% and 60%. Near-zero oxygen and nitrate concentrations were observed as well. To investigate the phosphorus retention characteristics in more detail, laboratory experiments were carried out on sediment samples and sediment cores retrieved from points along the wetland. Following a P shock load to cores of the wetland sediment, it was possible to determine a sediment P uptake rate of 0.016 day⁻¹. Sediment P adsorption studies were also performed, showing significant Freundlich and Langmuir isotherm behaviour. With these data a maximum P adsorption capacity of 4 mg P/g for the wetland sediment could be estimated. A plug-flow model was used to evaluate the phosphorus retention dynamics of the Kirinya wetland. A good correspondence between the actual and simulated P retention was observed. Comparing the daily P uptake (g/m³day) in the Kirinya wetland with the maximum sediment P uptake capacity, it can be concluded that the total P retention capacity of the wetland will only be sufficient for 30 more years under the present P loading and wetland management.     

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.     

Soil system budgets of N,Si and P in an agricultural irrigated watershed: surplus,differential export and underlying mechanisms

Ongoing socio-economic and climatic changes can differentially affect the biogeochemistry of the key nutrients nitrogen (N), silica (Si), and phosphorus (P) by altering their soil budgets, their transfer to aquatic environments and their ecological stoichiometry. This may lead to cascade consequences for aquatic communities and biogeochemical processes. Soil budgets, river export, and N, Si, and P ecological stoichiometry were assessed in a heavy impacted basin (Mincio River, Italy) in two decades (1991–2000; 2001–2010). The main aim was to analyse element-specific mechanisms of terrestrial-aquatic transport and retention within aquatic habitats. Budget results suggest a net accumulation (inputs exceeding outputs) of all nutrients in agricultural lands, mainly due to livestock manure, with a reduction for N (196 kg N ha^?1 year^?1 in 2000, and 132 kg N ha^?1 year^?1 in 2010), and constant values for Si (up to 3 kg Si ha^?1 year^?1) and P (43 kg P ha^?1 year^?1) along the study period. River export of N and P accounted for 3–27% and?~?2% of N and P soil net accumulation, respectively, while Si export was significantly greater (25 kg Si ha^?1 year^?1) than Si net accumulation on farmlands. The stoichiometry of net nutrient accumulation in soils was not reflected by the stoichiometry of nutrient riverine export, due to element-specific mechanisms. We speculate that N and Si vertical and horizontal mobilization is increased by the irrigation loop, while P retention is favored by limited erosion due to limited slopes in the Mincio River basin. The simultaneous analysis of N, Si and P allows us to better understand the different paths, transformation and retention mechanisms at the watershed scale.     

Effects of phosphorus and nitrogen amendments on the growth of Egeria najas

The effect of nutrient addition on the growth of E. najas was evaluated in a dose response experiment using sand amended with phosphorus (P) and nitrogen (N), and in enrichment trials with N and P amendments to natural sediments. Plants, water and sediment came from lagoons of the Upper Paraná River Floodplain and from Itaipu Reservoir (Brazil). Relative growth rates (RGRs) of E. najas shoots, based on dry mass (DM), varied from 0.03 to 0.060 d⁻¹ for both nutrients. Root:shoot biomass ratios were related to sediment exchangeable P (r = −0.419; P = 0.03) and N (r = −0.54; P = 0.006), however root RGR was not related to sediment nutrient concentrations. When natural sediments were amended with N and P, neither shoot nor root RGRs differed among treatments for substrata from either the reservoir or the floodplain lagoons (P > 0.05). Comparison of nutrient concentrations measured in natural sediments collected from several sites in both the Upper Paraná River Floodplain (range 49–213 μg P g⁻¹ DM; 36–373 μg N g⁻¹ DM) and Itaipu Reservoir (range 43–402 μg P g⁻¹ DM; 7.9–238 μg N g⁻¹ DM) showed that sediment N and P from these systems usually exceeded minimum requirements necessary for E. najas growth, as measured in the dose response experiment. Together, these results indicate that E. najas, at least in early stages of development, responds to sediment nutrient amendments and relies upon bottom sediments to meet its N and P requirements and that for at least two Brazilian ecosystems, growth of this species is not limited by insufficient sediment N or P. Thus, reducing N and P in water is not enough to control E. najas growth in short time periods in these ecosystems.     

Sediment nutrient accumulation and nutrient availability in two tidal freshwater marshes along the Mattaponi River, Virginia, USA

Sediment deposition is the main mechanism of nutrient delivery to tidal freshwater marshes (TFMs). We quantified sediment nutrient accumulation in TFMs upstream and downstream of a proposed water withdrawal project on the Mattaponi River, Virginia. Our goal was to assess nutrient availability by comparing relative rates of carbon (C), nitrogen (N), and phosphorus (P) accumulated in sediments with the C, N, and P stoichiometries of surface soils and above ground plant tissues. Surface soil nutrient contents (0.60–0.92% N and 0.09–0.13% P) were low but within reported ranges for TFMs in the eastern US. In both marshes, soil nutrient pools and C, N, and P stoichiometries were closely associated with sedimentation patterns. Differences between marshes were more striking than spatial variations within marshes: both C, N, and P accumulation during summer, and annual P accumulation rates (0.16 and 0.04 g P m^–2 year^–1, respectively) in sediments were significantly higher at the downstream than at the upstream marsh. Nitrogen:P ratios <14 in above ground biomass, surface soils, and sediments suggest that N limits primary production in these marshes, but experimental additions of N and/or P did not significantly increase above ground productivity in either marsh. Lower soil N:P ratios are consistent with higher rates of sediment P accumulation at the downstream site, perhaps due to its greater proximity to the estuarine turbidity maximum.     

Importance of flood zones for nitrogen and phosphorus dynamics in the Danube Delta

The change of concentration of total reactive phosphorus (TRP) and dissolved inorganic nitrogen (DIN) was studied in the lower Danube river and in selected lakes situated in the wetland area of the Danube Delta. The differences Danube Delta in nutrient concentration in the river waters entering the delta and the delta in different sites (especially lakes) of the wetland area are considered to reflect retention in the system. The highest retention was found in periods of moderate and low water level when the surface-to-volume ratio of the lakes was high. In these periods the in-lake concentration of TRP and DIN could be as low as 11 and 23% of the values found in the inflowing river.     

Nutrient budgets and biogeochemistry in an experimental agricultural watershed in Southeastern China

During a two-year field study, an annual nutrient budget and cycles were developed for a small agricultural watershed. The study emphasized the integrated unit of the watershed in understanding the biogeochemistry. It was found that the total nutrient input was 39.1× 10⁴ kg nitrogen and 3.91×10⁴ kg phosphorus in the year 1995, of which the greatest input of nutrients to the watershed was chemical fertilizer application, reaching 34.7×10⁴ kg (676 kg/ha) nitrogen and 3.88×10⁴ kg (76 kg/ha) phosphorus. The total nutrient output from the watershed was 13.55×10⁴ kg nitrogen and 0.40×10⁴ kg phosphorus, while the largest output of nitrogen was denitrification, accounting for 44.1% of N output; the largest output of phosphorus was sale of crops, accounting for 99.4% of P output. The results show that the nutrient input is larger than output, demonstrating that there is nutrient surplus within the watershed, a surplus which may become a potential source of nonpoint pollution to area waters. The research showed that both denitrification and volatilization of nitrogen are key ways of nitrogen loss from the watershed. This suggests that careful management of fertilizer application will be important for the sustainable development of agriculture.The research demonstrated that a multipond system within the watershed had high retention rate for both water and nutrients, benefiting the water, nutrient and sediment recycling in the terrestrial ecosystem and helping to reduce agricultural nonpoint pollution at its source. Therefore, this unique watershed system should be recommended due to its great potential relevance for sustainable agricultural development.     

The value of wetlands for water quality improvement: an example from the St. Johns River watershed,Florida

Wetlands provide many valuable ecosystem functions such as sediment and nutrient retention, high biological productivity and biodiversity, flood control, and opportunities to recreate. Despite their importance, estimating the value of wetlands is difficult as the worth of these functions and services is not easily quantified. The overall objective of this study was to estimate the value of freshwater wetlands in the Saint Johns River (SJR) watershed, Florida based on their ability to remove nutrients, namely nitrogen (N) and phosphorus (P). We used a combination of literature review, geospatial analysis of land cover, and regression analysis to determine the total wetland area in the SJR watershed and the rates of nitrogen and phosphorus burial in the wetlands. We then estimated the economic value of these wetlands based on the replacement cost of nutrient removal by wastewater treatment plants. Nitrogen burial rates ranged from 27 g/m²/year to a background rate of 6.56 g/m²/year, and phosphorus burial rates range from 1.31 g/m²/year to a background of 0.11 g/m²/year. Using these rates, we calculate wetlands of the SJR catchment remove 79,873 MT of nitrogen annually just from burial in the soil, with a replacement cost of between $240 million to $150 billion per year. The amount of phosphorus buried yearly is more than 2400 MT with an annual replacement cost of $17 to $497 million. Though they are based on limited data and include a variety of watershed-scale research limitations, these findings highlight the significant potential value of conserving functional wetlands based solely on their nutrient retention functions. If we were to consider the benefits associated with other wetland functions such as flood control, biological productivity, and biodiversity in addition to their ability to retain nutrients, the value of the SJR wetlands would be even greater.     

Nutrient uptake and benthic regeneration in Danube Delta Lakes

We investigated the nutrient uptake capacity of three lakes (Uzlina, Matita and Rosu) within the Danube Delta during high water level in June and low water level in September 1999. Special emphasis was placed on nutrient cycling at the sediment-water interface and on the self-purification capacity of the lakes in the Danube Delta. In order to estimate the nutrient uptake of selected lakes we present in this paper the results of water analyses, benthic flux chamber experiments and deck incubation experiments of ¹⁵N-labeled sediment cores at the inflow and the outlet of the lakes. The external input of dissolved inorganic nitrogen and silica into the lakes decreases with increasing distance to the main Danube branches whereas the total dissolved phosphorus input is independent of the hydrological distance to the main branches. The nutrient loading is highest in the inflow channels, and decreases towards the outflow of the lakes. In June, the uptake of NO₃ ^–, TDP and Si(OH)₄ in the lakes was higher than in September. In contrast, NH₄ ⁺ uptake was more intense in September, when benthic release was more intense as well. On average, about 76% of the external plus internal nitrogen and phosphorus input into the lakes was taken up by macrophytes and phytoplankton during the growing season, whereas the uptake of external nutrient input amounted to about 43%. The benthic release of ammonia and silica increases from June to September and indicates, that part of the nutrients taken up during the growing season might be released during winter. We estimate the net impact of the Delta on the nutrient reduction of the Danube during the growing season is about 4.3%, assuming 10% of the Danube water is flowing through the Delta.     

Nutrient limitation of Microcystis aeruginosa in northern California Klamath River reservoirs

Nutrient limitations were investigated in Copco and Iron Gate Reservoirs, on the Klamath River in California, where blooms of the toxin-producing cyanobacterium Microcystis aeruginosa were first reported in 2005. Nutrient enrichment experiments conducted in situ in June and August, 2007 and 2008, determined responses in phytoplankton biomass, Microcystis abundance and microcystin concentration to additions of phosphorus and different forms of nitrogen (NH₄⁺, NO₃, and urea). Microcystis abundance was determined using quantitative PCR targeting the phycocyanin intergenic spacer cpcBA.Total phytoplankton biomass increased with additions of N both before and during Microcystis blooms, with no primary effects from P, suggesting overall N limitation for phytoplankton growth during the summer season. NH₄⁺ generally produced the greatest response in phytoplankton growth, while Microcystis abundance increased in response to all forms of N. Microcystis doubling time in the in situ experiments was 1.24–1.39 days when N was not limiting growth. The results from this study suggest availability of N during the summer is a key growth-limiting factor for the initiation and maintenance of toxic Microcystis blooms in Copco and Iron Gate Reservoirs in the Klamath River.     

Nutrient limitation of Microcystis aeruginosa in northern California Klamath River reservoirs

Nutrient limitations were investigated in Copco and Iron Gate Reservoirs, on the Klamath River in California, where blooms of the toxin-producing cyanobacterium Microcystis aeruginosa were first reported in 2005. Nutrient enrichment experiments conducted in situ in June and August, 2007 and 2008, determined responses in phytoplankton biomass, Microcystis abundance and microcystin concentration to additions of phosphorus and different forms of nitrogen (NH₄⁺, NO₃, and urea). Microcystis abundance was determined using quantitative PCR targeting the phycocyanin intergenic spacer cpcBA.Total phytoplankton biomass increased with additions of N both before and during Microcystis blooms, with no primary effects from P, suggesting overall N limitation for phytoplankton growth during the summer season. NH₄⁺ generally produced the greatest response in phytoplankton growth, while Microcystis abundance increased in response to all forms of N. Microcystis doubling time in the in situ experiments was 1.24–1.39 days when N was not limiting growth. The results from this study suggest availability of N during the summer is a key growth-limiting factor for the initiation and maintenance of toxic Microcystis blooms in Copco and Iron Gate Reservoirs in the Klamath River.     

Biogeochemical mass-balances (C, N, P, Si) in three large reservoirs of the Seine Basin (France)

Three major reservoirs (Marne, Seine and Aube), situated in the upstream basin of the river Seine represent a storage capacity of 800 10⁶ m³. In order to quantify the possible role of these reservoirs as a sink or source of nutrients and organic matter for the river system, an input/output mass-balance of suspended matter, organic carbon, inorganic nitrogen forms, phosphorus and reactive silica was established, providing reliable estimates of their retention/elimination and export. The study was carried out over 3 years (1993, 1994 and 1995) in differing hydrological conditions. The retention times varied from 0.3 to 0.8 year, depending on the reservoir and the year, but was longer in 1993 that was a drier year than 1994 and 1995, hydrologically quite similar.Regarding retention (or elimination) and export, the behaviour of the three studied reservoirs was similar. A clear loss or retention of nitrogen, phosphorus and silica was observed in the reservoirs and represented about 40% of the incoming flux of nitrate, 50% of silica, and 60% of phosphate. The retention was lower for total phosphorus than for phosphate. The reservoirs are also sites of suspended matter deposition except during the decennial drawdown, when suspended matter is exported. For inorganic nitrogen, the average amount of nitrate retained in the Seine basin reservoirs upstream from Paris is 5000 tonnes y^–1 that is almost equal to the estimated retention by deposition or denitrification in river channel sediments for the whole drainage network. The retention in the reservoirs represents about 12% of the total flux of nitrate at the outlet of the basin upstream from Paris, and 5% at the mouth of the Seine River.We also calculated inlake C, N, P, Si budgets on the basis of direct process measurements. Measurements of planktonic primary and bacterial activity production led to annual net production of 4200 and 580 tonnes of carbon, respectively. A reasonable value (450 tonnes of carbon) of grazing was calculated. Corresponding N, P, Si fluxes were drawn from appropriate C:N:P:Si ratios. Benthic fluxes were measured with bell jars. The retention of P and Si represents a small fraction of important internal fluxes of phytoplanktonic uptake and recycling, while inorganic nitrogen retention depends mostly on benthic denitrification. The behaviour of P and Si differs in that P is mainly recycled in the water column, while Si dissolution occurs at the sediment interface. Nitrogen is recycled in both the planktonic and the benthic phase.     

Integrated stream and wetland restoration: A watershed approach to improved water quality on the landscape

Water quality in Upper Sandy Creek, a headwater stream for the Cape Fear River in the North Carolina Piedmont, is impaired due to high N and P concentrations, sediment load, and coliform bacteria. The creek and floodplain ecosystem had become dysfunctional due to the effects of altered storm water delivery following urban watershed development where the impervious surface reached nearly 30% in some sub-watersheds. At Duke University, an 8-ha Stream and Wetland Assessment Management Park (SWAMP) was created in the lower portion of the watershed to assess the cumulative effect of restoring multiple portions of stream and former adjacent wetlands, with specific goals of quantifying water quality improvements. To accomplish these goals, a three-phase stream/riparian floodplain restoration (600 m), storm water reservoir/wetland complex (1.6 ha) along with a surface flow treatment wetland (0.5 ha) was ecologically designed to increase the stream wetland connection, and restore groundwater wetland hydrology. The multi-phased restoration of Sandy Creek and adjacent wetlands resulted in functioning riparian hydrology, which reduced downstream water pulses, nutrients, coliform bacteria, sediment, and stream erosion. Storm water event nutrient budgets indicated a substantial attenuation of N and P within the SWAMP project. Most notably, (NO₂⁻ + NO₃⁻)-N loads were reduced by 64% and P loads were reduced by 28%. Sediment retention in the stormwater reservoir and riparian wetlands showed accretion rates of 1.8 cm year⁻¹ and 1.1 cm year⁻¹, respectively. Sediment retention totaled nearly 500 MT year⁻¹.     

Retention of nitrogen, phosphorus and silicon in a large semi-arid riverine lake system

Lakes and reservoirs (impoundments) are often viewed as a sink for nutrients within the river continuum. To date, most studies on nutrient retention within impoundments are derived from the temperate climate zones of Europe and North America, only consider one nutrient, and are often short-term (1–2 years). Here, we present a long-term (17 year) data set and nutrient (nitrogen, phosphorus and silica) budget for two connected semi-arid lakes (the Lower Lakes) at the terminus of the River Murray, Australia. Most of the filterable reactive phosphorus and nitrate entering the lakes were retained (77 and 92%, respectively). Total phosphorus (TP) was also strongly retained (55% of the annual TP load on average) and the annual TP retention rates could be predicted as a function of the areal hydraulic loading rate (annual lake outflow/lake surface area). On average, there was a slight net retention (7%) of the annual total nitrogen (TN) load but a slight net export (6% of the load) of organic N. TN retention as function of the areal hydraulic loading rate was lower than expected from existing models, possibly because of high nitrogen fixation rates in the Lower Lakes. Silica was retained (39%) at similar rates to those observed in previous studies. There was also a marked increase in the TN:TP and TN:Si ratios within the lake (TN:TP~30 and TN:Si~0.67) compared to those entering (TN:TP~15, TN:Si~0.45), as a consequence of the relatively low net retention of nitrogen.     

Stream section types of the Danube River in Serbia according to the distribution of macroinvertebrates

The aim of this study was to use the data on the distribution of aquatic macroinvertebrates obtained from 14 sites within a 413 km long stretch of the Danube River in Serbia to show the relevance of the bordering zone between the Middle and Lower Danube. A total of 68 macroinvertebrate taxa were observed. Molluscs were the major component with regard to species richness and relative abundance. Lithoglyphus naticoides (C. Pfeiffer, 1828) was the most abundant species and Unio tumidus (Retzius, 1788) was the most frequent species. Product-moment correlation coefficients or Pearson r coefficient was used to analyse the relation between the sites based on macroinvertebrate distribution. The data obtained by product-moment correlation served as input for cluster analyses. According to a cluster analyses Danube River in Serbia could be separated in the free-flowing sector, the stretch with a backwater effect and the area of the Iron Gate.     

Phosphorus Fractions and Fluxes in the Water Column and Sediments of a Tropical Reservoir (Lobo‐Broa – SP)

Sedimentation rates of organic and inorganic matter, chlorophyll a, P fractions, Ca, Mn, Fe and Al, were determined by sediment traps in a tropical oligo‐mesotrophic reservoir of São Paulo (Brazil). Vertical profiles of the sediments were analyzed for organic content, metals, P and surface P fraction composition. Estimated mean sedimentation rates, corrected for resuspension were: total solids, 1068 g m^—2 y^—1 (OM = 44.7%); chlorophyll a, 2.1 g m^—2 y^—1 and total phosphorus, 2.9 g m^—2 y^—1. The predominant P fraction in the settling flux was associated with aluminum minerals while surface sediments were dominated by organic P. The reservoir exhibited low sediment retention of P (13.0%), Al (9.9%), Fe (9.9%), Mn (1.4%) and Ca (traces), compared to trap sedimentation. This feature was related with a high vertical dynamics (resuspension and bottom release) and with the low retention time of the system.     

不同泥沙埋深对几种一年生草本枯落物分解及养分动态特征的影响

为探究三峡消落带泥沙淤积条件下一年生草本枯落物的分解及养分变化特征,通过模拟实验,采用分解袋法,研究不同泥沙埋深对4种一年生草本分解及养分动态的影响。所选择的三峡消落带4种一年生优势草本植物包括马唐（Digitaria sanguinalis）、稗（Echinochloa crusgali）、狼杷草（Bidens tripartita）、苍耳（Xanthium sibiricum）,设置了无掩埋（0 cm）、中度掩埋（5 cm）和深度掩埋（10 cm）3个泥沙掩埋深度,试验周期为180 d。结果表明：（1）在整个实验期间,4种一年生草本枯落物的分解速率均呈现先快后慢的特点,与0 cm泥沙埋深处理相比,5、10 cm埋深下4种一年生草本枯落物的分解速率均显著降低;（2）在分解结束时,4种一年生草本枯落物C含量变化不大,P元素处于释放状态,而在泥沙掩埋下N元素处于富集状态;（3）除马唐和稗枯落物的P含量变化外,4种一年生草本枯落物C、N、P含量均表现为0 cm掩埋处理显著低于5、10 cm处理组;相反,4种一年生草本枯落物K含量则表现为0 cm掩埋处理显著高于5、10 cm处理组。研究表明消落带大量泥沙淤积抑制枯落物的分解和C、N、P元素释放,有利于降低对三峡库区水体富营养化及营养物质悬浮的贡献率,一定程度上,对库区水质保护有正面效应。     

Migratory Atlantic salmon as vectors for the transfer of energy and nutrients between freshwater and marine environments

SUMMARY 1. Annual energy, carbon, nitrogen and phosphorus fluxes across the river mouth by Atlantic salmon were estimated for 18 years (1976–94) in the Norwegian River Imsa. The total energy content of the emigrating smolts in each year varied considerably with a mean value of 237 × 10³ kJ. That of returning adults also varied between years with a mean value of 141 × 10⁴ kJ. One‐sea‐winter salmon (grilse) made up 65% of the total energy content of the spawners in the river. Dead carcasses remaining in the river after spawning were estimated to have a mean annual energy content of 175 × 10³ kJ. 2. The net annual energy flux from the sea to the river varied between 48 × 10³ kJ (1987) and 152 × 10⁴ kJ (1989) with a mean of 616 × 10³ kJ, and a coefficient of variation of 67%. Average net marine import of the returning adults was 83 × 10⁴ kJ year^?1 with a coefficient of variation of 52%. Mean annual export of C, N and P to sea by the smolts was 595, 131 and 22 kg, and by kelts 1535, 352 and 70 kg, respectively, whereas gross import via the adults was 3176 kg C, 735 kg N and 132 kg P. The annual flux across the river mouth was 1046 kg C, 253 kg N and 39 kg P. The net marine import were 1585 kg C, 371 kg N and 60 kg P. The net flux was estimated at 0.2% for nitrogen and 5% for phosphorus of the total river load. 3. The energy flux caused by Atlantic salmon spawning in the River Imsa was relatively high because the general nutrient load in the river is low. Thus, even though most Atlantic salmon survive spawning, their contribution to the nutrient flux in the river is significant.