Comparison of denitrification rate estimation techniques in a large, shallow lake

A comparison was made between various incubation techniques and a mass balance model for estimating annual denitrification rates in Lake Okeechobee, Florida, a large, shallow lake. Incubation techniques involving isolation of the sediment from nitrate and labile organic carbon sources tended to produce low rate estimates, while diagenetic models based on nitrate profiles tended to produce unrealistically high rate estimates. Each technique has inherent errors when used with frequently suspended sediments, but most incubation techniques were in general agreement with mass balance calculations. Denitrification rate estimates range from 0.5 to 1.3 g N m⁻² yr⁻¹, which represents 9–23% of the average annual nitrogen inputs to the lake.     

Internal phosphorus loading in a shallow eutrophic lake

Internal loading of phosphorus has been implicated as a major eutrophicating factor in Long Lake, Washington (Kitsap County). As a result of such loading, summer total phosphorus concentrations approach or exceed 100 μgP l⁻¹. Most of the summer loading of phosphorus is thought to have been released directly from the rich, flocculent sediment in the mid and northern part of the lake as a result of high pH (up to 10) related to phytoplankton photosynthesis. The lake also supports a dense submersed macrophyte crop (areal weighted mean dry weight of about 220 g m⁻²) composed primarily of Elodea densa. Although excretion of phosphorus from healthy E. densa was found to be minimal, the potential contribution of P indirectly from sediment via macrophyte uptake and subsequent decomposition in the winter was on the order of 200–400 kg yr⁻¹ or about 25–50% of the external loading. Nevertheless, loading of phosphorus during predrawdown summers is thought to have originated largely as a direct release from sediment due to high pH. Estimates of sediment phosphorus release determined from laboratory experiments, mass balance and core analyses ranged from 2.2 to 5.6 mg m⁻² day⁻¹.As a component of the restoration program, the lake (mean depth of 2 m) was drawn down nearly 2 m for 4 months during the summer and fall of 1979. The lake's trophic status was expected to improve due to sediment consolidation and/or macrophyte reduction. The drawdown resulted in an 84% reduction in macrophyte biomass in 1980 but minimal sediment consolidation (0.1 m). Winter decomposition of the much smaller macrophyte crop, apparently provided insufficient phosphorus in the spring to stimulate phytoplankton and to enrich midlake sediments, which has probably occurred in previous years. Low water column pH during the postdrawndown summer of 1980 resulted from relatively low rates of plankton photosynthesis. During the summer of 1980 internal loading of phosphorus was reduced and total phosphorus remained below 50 μg l⁻¹.     

Enriching effects of Urban runoff on the productivity of a mesotrophic lake

Sewage was diverted from Lake Sammamish in September, 1968, and since then there have been no significant responses by trophic indicators which indicate the lake is beginning to recover. To explain this delay in response, extensive urban development in the lake's watershed was considered as a factor which might be acting to inhibit the lake's recovery. In vitro uni-algal experiments with water from 13 streams which drain urban and undeveloped areas showed that about half of the streams stimulated algal growth significantly, but these streams constitute less than 14 per cent of the lake's total water income. Of these six streams only two drain urban areas and account for less than 2 per cent of the water income. In situ experiments showed that no streams were significantly stimulatory to natural populations of phytoplankton. Other in situ studies showed that one urban stream and two from undeveloped areas caused substantial increases in periphyton growth after a 12-day period, although these increases were not statistically significant. Additional in situ nutrient limitation experiments in August of 1970 and 1971 revealed that P and N were limiting only in combination. Results from these studies do not support the contention that urban runoff is seriously enriching the limnetic region of Lake Sammamish.     

Impacts of agricultural phosphorus use in catchments on shallow lake water quality: About buffers, time delays and equilibria

Phosphorus (P) losses caused by intensive agriculture are known to have potentially large negative effects on the water quality of lakes. However, due to the buffering capacity of soils and lake ecosystems, such effects may appear long after intensive agriculture started. Here we present the study of a coupled shallow lake catchment model, which allows a glimpse of the magnitude of these buffer-related time delays. Results show that the buffering capacity of the lake water was negligible whereas buffering in the lake sediment postponed the final lake equilibrium for several decades. The surface soil layer in contact with runoff water was accountable for a delay of 5-50 years. The most important buffer, however, was the percolation soil layer that may cause a delay of 150-1700 years depending on agricultural P surplus levels. Although the buffers could postpone final lake equilibria for a considerable time, current and target agricultural surplus levels eventually led to very turbid conditions with total P concentrations of 2.0 and 0.6 mg L(-1) respectively. To secure permanent clear water states the current agricultural P surplus of 15 kg P ha(-1) yr(-1) should drop to 0.7 kg P ha(-1) yr(-1). We present several simple equations that can be used to estimate the sustainable P surplus levels, buffer related time delays and equilibrium P concentrations in other catchment-lake systems.     

Spatial and historical variation in sediment phosphorus fractions and mobility in a shallow lake

Temporal and spatial variation in sediment P composition and mobility were investigated in Loch Leven. Little change was observed in total sediment P (surface sediment at 4m depth), in comparison to a previous study (1990), despite significant reduction of external point sources of P. Labile P and residual P have both increased (0.007-0.039 mg PO(4)-P and 0.121-0.420 mg PO(4)-P per gram dry weight of sediment, respectively) since 1990. An analysis of P fractions, along a depth transect, indicated elevated labile P concentrations in shallow water sediment (<12 m overlying water depth). Regression analysis showed that spatial variability in reductant-adsorbed P was significantly related to sediment chlorophyll a concentration (R(2)=0.733, p<0.05). This may be linked to the production of oxygen, by benthic algae, resulting in the maintenance of an oxygenated layer at the sediment surface. Variation in labile P was best explained by overlying water temperature and equilibrium phosphate concentration (EPC0).     

Trihalomethane formation potential of dissolved organic matter in a shallow eutrophic lake

Dissolved organic matter (DOM) in water samples from the shallow eutrophic Lake Kasumigaura, the second largest lake in Japan, was fractionated and characterized by using resin adsorbents into 5 classes: aquatic humic substances (AHS), hydrophobic neutrals, hydrophilic acids (HiA), bases (BaS) and hydrophilic neutrals (HiN). Subsequently, the trihalomethane formation potential (THMFP), ultraviolet absorbance to dissolved organic carbon (UV:DOC) ratio, and molecular size distribution of the DOM, AHS and hydrophilic fractions (HiF) (HiF=HiA+BaS+HiN) were examined. The THMFP of HiF, normalized on a DOC basis, was found to be comparable to that of AHS (0.176 microM THM mg C(-1) vs. 0.195 microM THM mg C(-1), respectively). The importance of HiF over AHS as a THM precursor became more pronounced when THMFP was evaluated in terms of concentration. In this case, the THMFP of HiF was much greater than that of AHS (0.374 microM THM l(-1) vs. 0.229 microM THM l(-1), respectively). Molecular size distributions all exhibited a narrow size range and relatively low molecular weights. The weight-averaged molecular weights of DOM, AHS and HiF were 780, 957 and 606 g M(-1), respectively.     

Dissolved iron and its speciation in a shallow eutrophic lake and its inflowing rivers

It has been suggested that iron is a limiting factor on bloom-forming cyanobacteria in lake water. Although the availability of iron for phytoplankton depends significantly on its speciation, little is known about iron speciation in natural lake water. We investigated the horizontal distribution and temporal variation of dissolved iron and its chemical speciation in Lake Kasumigaura and its two inflowing rivers. Concentrations of dissolved iron and its organic ligands, determined by cathodic stripping voltammetry, clearly decreased as lake water flowed from the river entry points toward the center of the lake, indicating their riverine origin. The fraction of iron occurring in organic complexes tended to increase with the water flow. These results suggest that most of the dissolved iron in river water forms unstable soluble species, such as inorganic iron; thus, these unstable iron species may be scavenged in the mouths of rivers, and stable organic complexes of iron may flow to the center of the lake. Furthermore, most of the dissolved iron (88.2-99.9%) was present as organic complexes, and the inorganic iron level in Lake Kasumigaura (pFe' value=7.8-13.6) was similar to that observed in the open ocean. This result suggests that iron is an important factor determining the structure of the phytoplankton community in Lake Kasumigaura.     

Biological and chemical factors influencing shallow lake eutrophication: a long-term study

The focus of eutrophication research has tended to be upon short-term and experimental studies. However, given the range of factors that can influence eutrophication dynamics, and that these matter over a range of time scales, some discrete, some continuous, eutrophication dynamics may only be fully investigated when long-term, time-series data are available. The present study aims to evaluate the interacting effects of abiotic processes and biotic dynamics in explaining variations of phytoplankton biomass in a eutrophic shallow lake, Barton Broad (Norfolk, UK) using a long-term data set. Multivariate statistical analysis shows that the inter-relationships between phytoplankton variability, nutrient and grazing factors were highly sensitive to seasonal periodicity. In spring phytoplankton biomass was related to phosphorus, nitrogen and silicon. In summer phytoplankton biomass was associated with phosphorus, nitrogen and zooplankton. In autumn phytoplankton was related to phosphorus, nitrogen, silicon and zooplankton. In winter, no significant relationship could be established between phytoplankton and environmental variables. This paper improves our understanding of the governing role of nitrogen, phosphorus, silicon and zooplankton upon phytoplankton variability, and hence, improves management methods for eutrophic lakes.     

Phosphorus partitioning in a shallow lake: implications for water quality management

This paper describes the seasonal partitioning of phosphorus (P) across the sediment–water interface in Loch Leven, Scotland, and discusses the implications for future lake management strategy with respect to recovery from eutrophication. In a 10‐month survey, surface water total phosphorus (TP) concentrations were highest in late summer and lowest in early spring. In contrast, sediment TP concentrations were highest in mid‐winter and lowest in late summer. Water discharge at the main outflow of the loch was highest when water‐column TP was low and sediment TP high, and vice versa. Monthly sediment P uptake/release values showed significant cycling between the water‐column and the sediment and showed seasonal variation in four release‐sensitive P pools. Regulating the water level to increase flushing during sediment release periods and decrease flushing during uptake periods has the potential to significantly enhance the recovery of shallow lakes and reservoirs following historic nutrient loading.     

Transfer of mercury and methylmercury along macroinvertebrate food chains in a floodplain lake of the Beni River, Bolivian Amazonia

We have evaluated the mercury and methylmercury transfers to and within the macroinvertebrate communities of a floodplain lake of the Beni River basin, Bolivia, during three hydrological seasons and in two habitats (open water and vegetation belt). Using the stable isotopes δ¹³C and δ¹⁵N, six trophic chains were identified during a previous study. Four are based on only one source: seston, organic matter from the bottom sediment, periphyton and macrophytes. Two are based on mixed sources (seston and periphyton in one case, periphyton and macrophytes in the other). During sampling, we found only one taxon that had surface sediment organic matter as food source and very few taxa whose trophic source was constituted by macrophytes. The periphyton was the most important source during all seasons; it produced the longest chain, with three trophic positions. Whatever the season and trophic source, all collected macroinvertebrates contained methyl mercury and the latter was biomagnified in all trophic chains that we identified. The biomagnification of methylmercury through invertebrate trophic chains accurately reflected the existence and length of these chains. Biomagnification was virtually non-existent in the sediment-based chain, low and restricted to the dry season in the macrophyte-based chain. It was significant in the seston-based chain, but limited by the existence of only two trophic levels and restricted to the wet season. Finally, it was very effective in the periphyton-based chain, which offers the highest rate of contamination of the source but, above all, the largest number of trophic levels.     

Characterization of phosphorus-releasing bacteria in a small eutrophic shallow lake, Eastern China

Phosphorus contents and phosphorus-releasing bacteria were characterized in a small eutrophic lake, Eastern China. Total phosphorus and water soluble inorganic phosphate (WSIP) in water bodies were as high as the levels of hyper-eutrophic lakes. Calcium bound phosphate and organic phosphorus were 2 major forms of phosphorus existing in the sediments. Enumeration of inorganic phosphate-solubilizing bacteria (IPB) and organic phosphorus-mineralizing bacteria (OPB) with culture-dependent method showed that these bacterial groups were not very rich in the ecosystem. Molecular identification and phylogenetic analysis of the predominant IPB and OPB strains indicated that there existed various kinds of bacteria participating in the phosphorus release. Laboratory tests on phosphorus release abilities showed IPB strains could liberate more than 50 microgml(-1), while OPB strains produced less than 2 microgml(-1) WSIP every day.     

小岛连大岛大岛变半岛

南连宁波和周边城市 ,通过 5座跨海大桥把舟山本岛和它的 4个所属岛屿变成半岛的“大陆连岛工程”正在加速进行 ,目前 3座跨海大桥已相继建成 ;北接舟山洋山港和上海芦潮港 ,全长 31km的东海大桥经过一年的紧张施工 ,如今一座座桥墩已矗立海面 ,计划在年内完成 4 0 0 0根海上打桩任务。舟山有 1390个岛屿。由于岛屿分散 ,交通不便 ,海岛资源难以得到充分利用。通过一座座大桥 ,使岛城“登陆” ,成了舟山人的一个世纪之梦。总投资 80多亿元的舟山大陆连岛工程起于舟山本岛的 32 9国道 ,与规划中的宁波镇海中国沿海北线高速公路相连 ,共建跨海…     

Effects of light on sediment nutrient flux and water column nutrient stoichiometry in a shallow lake

The effects of light and temperature on nutrient cycling (silica (Si), nitrogen (N) and phosphorus (P)) between sediments and water in a shallow eutrophic lake (Loch Leven, Scotland), and consequent effects on water column nutrient stoichiometry, were assessed using a series of intact sediment core incubation experiments. Estimates of actual seasonal dark and light P-fluxes were assessed using 24-h incubations. Sediment-P uptake was observed in spring (7 degrees C) and release in autumn (12 degrees C) and summer (17 degrees C), with the highest release rates ( approximately 17 mgPO4-Pm(-2) sediment surface area d(-1)) occurring in summer. In a longer (21-day) experiment in which the effects of light (light (n=6) and dark (n=6)) and temperature (five 4-day cycles to represent: 7 degrees C-->13 degrees C-->23 degrees C-->13 degrees C-->7 degrees C) on water column nutrient concentrations were assessed, PO(4-)-P, total P (TP), SiO2 and total silica (TSi) concentrations in the water column were all significantly higher under dark conditions (ANOVA, alpha=0.05). NH4-N (ammonium N) water column concentrations were observed to be higher under dark conditions at low temperatures and higher under light conditions following a high-temperature (23 degrees C) treatment. No significant light effects were observed for water column total N (TN) concentration. Flux estimates for all nutrients measured are given. In terms of water column nutrient stoichiometry, TN:TP ratio was significantly higher under light conditions, TSi:TN was significantly lower under light conditions, and TSi:TP did not vary significantly between the dark and light treatments. The main processes acting to regulate diffusive nutrient release appeared to be photosynthetic elevation of bottom water pH and dissolved oxygen concentration (both significantly higher under light conditions) and direct microalgal sequestration. Thus, a feedback mechanism exists in recovering shallow lakes where benthic microalgae can affect the stoichiometry (to favour P/Si limitation) of the plankton, and also of the main source of nutrients back to the sediments via the disproportionate regulation of sediment P, Si and N release.     

杭州某大型内湖回填工程软基处理

杭州钱塘江南岸某大型内湖采用江底抽砂回填措施时,填入了大量江底淤泥,形成了深厚超软弱的填土地基,使后续的填土及建筑桩基工程施工无法进行。在多方案分析比较的基础上,通过现场试验,提出了分块、分级堆载固结的地基处理措施,并介绍了方案的实施要点。结合现场监测数据,分析了软基处理过程中地基沉降、孔隙水压力以及抗剪强度等的发展规律,结果表明排水系统的畅通与否直接关系到地基处理的效率,堆载在时间及空间上的合理安排是地基处理成败的关键,该项目的地基处理达到了预定的目标。     

Runoff-related agricultural impact in relation to macroinvertebrate communities of the Lourens River, South Africa

A field study at the Lourens River, South Africa, was undertaken during the pesticide application period between November 2001 and January 2002 in order to investigate the potential relation of agricultural pollution to the aquatic macroinvertebrate fauna. The upper regions of the Lourens River were free of contamination (LR1), whereas subsequent stretches flowing through a 400-ha orchard area (LR2) received transient insecticide peaks. Continuously operating suspended-particle samplers as well as flood samplers operating during runoff events were used to measure pesticide contamination. In addition, various physicochemical and morphological parameters were examined. A survey of the macroinvertebrate communities associated with the rocky substrates was carried out every three weeks. Community indices were calculated using the South African Scoring System (SASS 5) for bioassessment of water quality in rivers. The two sites differed in pesticide pollution as well as in average turbidity levels (LR1 5.5 mg/L; LR2 64.3 mg/L), but were similar in bottom substrate composition and most other abiotic factors. At the downstream site (LR2), pesticide values of 0.05 microg/L azinphos-methyl in water as well as 49 microg/kg azinphos-methyl, 94 microg/kg chlorpyrifos and 122 microg/kg total endosulfan in suspended particles were found during runoff conditions. The macroinvertebrate communities of the two sampling sites were similar in terms of number of total individuals, but differed significantly (ANOVA) in average number of taxa (LR1 11.7, LR2 8.9). Seven out of 17 investigated taxa occurred in significantly reduced numbers or were even absent at the downstream site LR2. The community characteristics determined by SASS 5 showed a significantly less sensitive community structure at the downstream site (TS 41; ASPT 4.6), indicating continuously lower water quality compared to site LR1 (TS 80; ASPT 6.9). It is concluded that the Lourens River macroinvertebrate communities are affected by agricultural pollution, with pesticides and increased turbidity as the most important stressors.     

南四湖流域平原区浅层地下水氮污染特征

本文在对浅层地下水"三氮"迁移转化原理和"三氮"污染控制因素进行分析的基础上,阐明了南四湖流域平原区浅层地下水"三氮"含量分布规律,并分析了形成原因。从区域分布看,NO3-N含量呈现湖东山前冲洪积平原区高于湖西冲积湖积平原区的特点;NO2-N含量虽然较低,但含量大于0.01mg/L的地段主要分布于南四湖沿岸和山前冲洪积平原部分地区;NH4-N绝大部分地区未检出。浅层地下水"三氮"含量的上述分布规律与非饱和带的岩性组成、地下水位埋深、农田灌溉方式、土壤含水量等密切相关。最后针对研究区浅层地下水氮污染的特点提出了"三氮"污染防治对策。     

Impacts of runoff from sulfuric soils on sediment chemistry in an estuarine lake

The impact of runoff from sulfuric soils in the heavily drained Cudgen Lake floodplain, eastern Australia on water quality and downstream coastal lake sediments has been examined. The oxidation of sulfidic soils and the transformation into sulfuric soils leads to changes not only in the upper soil profile but also affects drainage water quality and the chemistry of bottom sediments in receiving waters. Oxidation transforms the soil from a sink for sulfur and metals to a significant source for downstream environments. Sulfuric soils within the Cudgen Lake catchment contain 9.18 x 10(5) mol H+ per hectare as well as elevated concentration of metals (e.g. Al, Fe, Mn) and sulfate. These products of sulfidic soil oxidation are transported efficiently from the soil profile by the constructed drainage network and into the downstream lake system. The acid volatile sulfur (AVS), chromium reducible sulfur (CRS), total sulfur, organic carbon, and reactive iron contents present in the solid phase of the lake sediments are reported. The AVS/CRS, DOP and DOS values observed in the lake sediments show that natural monosulfide formation in the near surface sediments has been enhanced due to increased inputs of organic matter, sulfate, ferrous iron and other metals following development of the catchment. There are elevated concentrations of metals (e.g. As, Al, Cd, Cr, Hg, Zn and Pb) in the upper layer of monosulfidic lake sediments compared with the underlying pyritic sediments some of which exceed sediment quality guidelines. These metals could be released by dredging or through re-suspension during high flow conditions or enter the food chain.     

Nutrient and grazing factors in relation to phytoplankton level in a eutrophic shallow lake: the effect of low macrophyte abundance

The ability of submerged macrophytes to moderate the structure of food webs with respect to lake eutrophication management has been intensively studied in recent years. Many lake managers have adopted the option of increasing macrophyte abundance in order to restore eutrophic waters, with a view to improve water quality, increase water transparency and reduce phytoplankton biomass. These studies are mostly based upon the comparison of macrophyte-rich and macrophyte-free waters. There is less quantitative information on how different levels of macrophyte abundance, in particular very low levels, influence ecosystem structure, or vice versa, within the same ecosystem. This paper takes a multivariate statistical approach to examine ecosystem function with low macrophyte abundance (ca. 5%) in the context of nutrient-phytoplankton-zooplankton interaction in a eutrophic shallow lake. It shows that with low levels of macrophyte coverage, phytoplankton biomass was only limited by nutrient (TP and Si) availability, whilst nutrient (Si) as well as grazing (zooplankton and Daphnia) factors were both limiting phytoplankton levels in macrophyte-free waters. With respect to interactions between Daphnia and chlorophyll-a, the results suggest that low macrophyte cover does not result in a reduction of phytoplankton biomass due to the presence of Daphnia. Rather, low macrophyte cover provides a refuge that maintains constant Daphnia predation, so reducing chlorophyll-a fluctuation. These results add credence to the causal interpretation of different interactions amongst nutrients, phytoplankton and zooplankton at very low macrophyte densities.     

Progressive changes in water and sediment quality in a wetland system for control of highway runoff

Innovative wetland based systems were designed and installed on the Newbury Bypass, Berkshire, England to provide flow balancing and pollution control for road runoff. The systems were monitored over 18 months to evaluate performance, pollutant removal processes and offer improved design and operation codes for this new application of wetlands. Water quality, sediment accumulation rates, and metal concentrations in size-fractionated, settling solids and deposited sediments were determined in parts of the system to provide information on spatial and temporal variability. The results presented here show that over the long term, there were progressive changes in parts of the system for BOD and COD and for metal concentrations in the sediment fractions, which occurred with linear (or semi log-linear) rates, despite variability in flow rates, retention times and in pollutant loading to the system. Future work will continue monitoring to increase the data set, examine possible processes contributing to the regression constants, and test the potential use of the regressions in system modelling. Attempts at modelling road runoff treatment using wetlands must allow for progressions, since the systems can only be effective if they retain removed metals in the sediment sink.