Nitrogen dynamics in a eutrophic lake sediment

Nitrogen flux from sediment of a shallow lake and subsequent utilization by water hyacinth (Eichhornia crassipes [Mart] Solms) present in the water column were evaluated using an outdoor microcosm sediment-water column. Sediment N was enriched with ¹⁵N to quantitatively determine the movement of NH₄-N from the sediment to the overlying water column. During the first 30 days. 48% of the total N uptake by water hyacinth was derived from sediment ¹⁵NH₄-N. This had decreased to 14% after 183 days. Mass balance of N indicates that about 25% sediment NH₄-N was released into the overlying water, but only 17% was assimilated by water hyacinth. NH₄-N levels in the water column were very low, with very little or no concentration gradients. NH₄-N levels in the interstitial water of the sediment were in the range of 30–35 mg L^–1 for the lower depths (> 35 cm), while in the surface 5 cm of depth NH₄-N levels decreased to 3.2 mg L^–1. Simulated results also showed similar trends for the interstitial NH₄-N concentration of the sediment. The overall estimated NH₄-N flux from the sediment to the overlying water was 4.8 µg cm^–2 day^–1, and the soluble organic N flux was 5.8 µg N cm^–2 day^–1. Total N flux was 10.6 µg N cm^–2 day^–1.     

Seasonal changes in sediment and water chemistry of a subtropical shallow eutrophic lake

A field study was conducted (May 1981 to June 1982) to develop a data-base on seasonal changes of water and sediment chemistry of Lake Monroe (4 000 ha surface and ca. 2 m deep) located in central Florida, USA. This shallow eutrophic lake is a part of the St. Johns River. Quantitative samples of lake water and sediments were collected on a monthly basis from 16 stations and analyzed for various physico-chemical parameters. Relatively high levels of dissolved solids (mean electrical conductivity (EC) = 1832 µS cm¹) prevailed in the lake water, and seasonal changes in EC were probably associated with hydrologic flushing from external sources, such as incoming water from upstream as well as precipitation. Average monthly levels of total N and P during the study period were 1.82 and 0.21 mg l^–1, respectively. Nutrient concentrations in the water did not show any strong seasonal trends. Organic matter content of lake sediments ranged from 1 to 182 g C kg^–1 of dry sediment, reflecting considerable spatial variability. All nutrient elements in the sediments showed highly significant (P < 0.01) correlations with sediment organic C, though little or no significant relationship appeared at any sampling period between water and sediment chemistry of the lake. Temporal trends in water and sediment chemical parameters may have been concealed by periodic hydrologic flushing of the St. Johns River into Lake Monroe.Florida Agricultural Experiment Stations Journal Series No. 7836.     

Oxygen inhibition of nitrate uptake is a general regulatory mechanism in nitrate respiration

It has recently been demonstrated that oxygen inhibits nitrate uptake by denitrifying Pseudomonas aeruginosa. The purpose of the present investigation was to determine if this novel mechanism of regulation is universal for the regulation of nitrate respiration in other widely divergent species of bacteria. Nitrate transport by whole cell suspensions was completely and reversibly inhibited in 11 out of 12 species tested, whereas nitrate reduction by cell-free extracts was not affected by oxygen or was only partially inhibited in some cases. These results indicate that oxygen inhibition of nitrate uptake is a general regulatory phenomenon.     

Recent patterns of sediment accumulation in a small closed eutrophic lake revealed by the sediment records

A short-core palaeolimnological investigation was undertaken with the aim ofacquiring knowledge of sediment deposition. Analyses of the lithological composition of sediments from the whole-lake basin were performed on the small eutrophic L. Linajärv (northern Estonia) and the concentrations of mineral and organic matter were measured on 647 sub-samples from 14 sediment cores. The accumulation rate of the sediment sequences was established and C/N ratios of organic matter in some cores were recorded. Results indicate that the water depth, basin slopes and distance to the shore have the most important impact on the physical sediment properties. It was shown that variations in the mineral matter concentrations were influenced by the changes in deposition conditions in the areas with steep slopes. The study indicated that more objective information about the sedimentation mechanisms is obtained using analysis of the concentration ratio of mineral and organic matter since it reduces the implied role of diagenetic compaction.     

In situ plankton community respiration measurements show low respiratory quotients in a eutrophic lake

Planktonic community respiration is an important carbon cycling process, typically quantified by converting measured values of dissolved O₂ consumption rates into CO₂ production rates assuming a respiratory quotient of 1 (RQ = CO₂ per O₂ by moles). However, the true variability in planktonic RQs between different aquatic ecosystems is poorly understood. We conducted in situ RQ measurements in a eutrophic lake dominated by algal-derived substances and found that RQs were significantly below 1. In fact, many RQ values were extremely low (0.2–0.6), below theoretical RQs for oxidation of algal organic matter substrates (0.7–0.8), suggesting that other factors than substrate control need to be considered to understand the RQ. This view was further supported by lack of correlations between RQ and microbial variables known to be strongly substrate dependent, including bacterial growth efficiency and the functional capacity of the bacterioplankton community to degrade different compounds. Based on the measured dynamics in methane and nutrient pools, we discuss that methane oxidation and nitrification likely occurred in the lake, contributing to the unusually low RQs. Our findings demonstrate that planktonic RQs in productive lakes can systematically be below 1, suggesting that CO₂ emissions from these lakes may currently be overestimated.     

Resuspension in a shallow eutrophic lake

The frequency and the importance of wind-induced resuspension were studied in the shallow, eutrophic Lake Arresø, Denmark (41 km², mean depth 3 m). During storm events in autumn 1988 lake water samples were collected every 2–8 hours by an automatic sampler at a mid-lake station. The concentration of suspended solids and Tot-P was found to increase markedly. During storms up to 2 cm of the superficial sediment was resuspended, and the concentration of resuspended solids in the water column rose to 140 mg l^–1. The resuspended particles had a relatively high settling velocity and on average, a relatively short residence time in the water column of 7 hours.A model which describes the concentration of resuspended solids as a function of wind velocity and of settling velocity of the resuspended particles is presented. Using additional wind velocity data from a nearby meteorological station, the model has been used to calculate the frequency of resuspension events and concentration of resuspended solids for the period from May to November 1988.These calculations show that resuspension occurred about 50% of the time. Average flux of suspended solids from the sediment to the water was 300 g m^–2 d^–1 and during 50% of the time lake water concentration of suspended solids was more than 32 mg l^–1. A relationship between concentration of suspended solids and Secchi-depth is presented. Because of resuspension, Secchi-depth in Lake Arresø is reduced to 0.5 m.Resuspension also had a marked effect on Tot-P concentration in the lake water, and P input to the lake water being totally dominated by resuspension events.     

Zooplankton-phytoplankton interactions in a eutrophic lake

Enclosure experiments were made in a cyanobacteria dominatedlake (Lake Rotongaio) to assess the impact of zooplankton (>150µm) grazing on algal growth rates and determine the effectof diel and vertical changes in zooplankton grazing intensityand nutrient (NH₄-N) regeneration upon abundance of phytoplankton.The filamentous cyanobacterium Anabaena minutissima var. attenuataand diatom Cyclotella meneghiniana showed a negative linearchange in abundance with a gradient in zooplankton grazing intensity.Phytoflagellates were not grazed and showed a positive linearchange in abundance with increasing zooplankton biomass. Theseeffects, as well as shortening of filament length of Anabaena,were caused by raptorial feeding by the alanoid copepod Boeckellapropinqua which dominated the zooplankton. Phytoplankton growthwas not stimulated by addition of nutrients, suggesting nutrientregeneration was not important. Diel and vertical changes infeeding and NH₄-N regeneration rates were measured in Marchand June 1988. Diel differences were more pronounced in Marchwhen the water column was stratified. Specific feeding rateswere more important than vertical changes in zooplankton biomassin determining community grazing rates in March, but in Junewhen the water column was mixed, vertical distribution of zooplanktonbiomass was important. Zooplankton grazing was an importantloss process for phytoplankton in the lower part of the epilimnionin Lake Rotongaio.     

Lake restoration using a reed swamp to remove nutrients from non-point sources

The reed swamp adjacent to Lake Glumsø was partly separated from the lake by a dam and the inflow to the reed swamp controlled by pumping from the tributary. An investigation of the nutrient balances for the reed swamp showed an average daily denitrification rate of 252 mg/m2 from August 1988 to March 1989. Mineralization of the peat took place simultaneously, in sufficient amounts to supply carbon for the denitrification process. Ammonia and phosphorus were released in the ratio 7:1, corresponding to 111 mg N and 16 mg P/m²/ day. Most of the ammonia was nitrified and denitrified (included in the above mentioned denitrification rate). The net release of phosphorus implies that the method only can be applied satisfactorily in situations where nitrogen is the limiting nutrient. This is the case for Lake Glumso during the summer period, but not during the spring.This paper was presented at the INTECOL IV International Wetlands Conference in Columbus, Ohio, 1992, as part of a session organized by Prof. S. E. Jørgensen and sponsored by the International Lake Environment Committee.Corresponding Editor: Prof K. R. Reddy     

Hydrogen metabolism and sulfate-dependent inhibition of methanogenesis in a eutrophic lake sediment (Lake Mendota)

Hydrogen metabolism was studied in anoxic sediments of the stratified Lake Mendota; using a method which allowed the measurement of in situ H₂ concentrations and the headspace-free analysis of turnover of dissolved H₂. Addition of sulfate resulted in partial but immediate inhibition of H₂-dependent methanogenesis. Sulfate addition did not result in an immediate decrease in the steady state concentration of dissolved H₂, nor did it significantly stimulate the rate constant of H₂ turnover. Sulfate-induced decrease in dissolved H₂ was only observed after prolonged incubation or when endogenous H₂ production was stimulated by added glucose. The turnover of the in situ H₂ accounted for only 14% of the H₂-dependent methanogenesis from bicarbonate. While rates of methanogenesis increased during the season, rates of H₂ turnover decreased, accounting for only 2% of the H₂-dependent methanogenesis at the end of summer stratification. These observations indicate that increasing proportions of CH₄ were formed from H₂ being directly transferred in syntrophic methanogenic associations. The rapid inhibition of H₂-dependent methanogenesis by exogenous sulfate may be explained at least partially by assuming methanogenic associations in which syntrophic sulfate reducers change their metabolism from fermentative H₂ production to sulfate reduction.     

Abundance and feeding of microheterotrophic flagellates from a eutrophic lake

The growth of planktonic bacteria from a eutrophic lake was evaluated with microflagellate predators present and absent. Differential filtration (50 and 8 µm filters) was used to exclude ciliates and larger zoo-and phytoplankton from replicate experimental cultures. Additional filtration (1 µm filter) excluded heterotrophic microflagellates from a second set of experimental cultures, producing cultures that contained either bacteria and microflagellates or only bacteria. Growth of bacteria and microflagellates was evaluated by epifluorescent microscopy from repeated sampling over approximately 200 h. Bacterial numbers were reduced in the presence of microflagellates, and microflagellates were observed to contain bacterial prey. However, microflagellate numbers were high (about 10⁶ cells ml^-1) and were less than an order of magnitude lower than bacterial numbers. Bacteria growing in the presence of microflagellates did not show predator-prey population oscillations but had in-phase oscillations in numbers, suggesting that microflagellate predation in freshwater may not control numbers of planktonic bacteria. Clearance rates of heterotrophic microflagellates, estimated to be only 30 body volumes hr^-1, were insufficient to maintain flagellate growth, suggesting that other energy sources may be needed to maintain microflagellates in eutrophic freshwater ecosystems.     

Soil and sediment bacteria capable of aerobic nitrate respiration.

Several laboratory strains of gram-negative bacteria are known to be able to respire nitrate in the presence of oxygen, although the physiological advantage gained from this process is not entirely clear. The contribution that aerobic nitrate respiration makes to the environmental nitrogen cycle has not been studied. As a first step in addressing this question, a strategy which allows for the isolation of organisms capable of reducing nitrate to nitrite following aerobic growth has been developed. Twenty-nine such strains have been isolated from three soils and a freshwater sediment and shown to comprise members of three genera (Pseudomonas, Aeromonas, and Moraxella). All of these strains expressed a nitrate reductase with an active site located in the periplasmic compartment. Twenty-two of the strains showed significant rates of nitrate respiration in the presence of oxygen when assayed with physiological electron donors. Also isolated was one member of the gram-positive genus Arthrobacter, which was likewise able to respire nitrate in the presence of oxygen but appeared to express a different type of nitrate reductase. In the four environments studied, culturable bacteria capable of aerobic nitrate respiration were isolated in significant numbers (10(4) to 10(7) per g of soil or sediment) and in three cases were as abundant as, or more abundant than, culturable bacteria capable of denitrification. Thus, it seems likely that the corespiration of nitrate and oxygen may indeed make a significant contribution to the flux of nitrate to nitrite in the environment.     

5'-nucleotidase activity in a eutrophic lake and an oligotrophic lake

Differences in enzymatic hydrolysis of dissolved organic phosphorus and subsequent phosphorus uptake were compared by using dual-labeled (gamma-P and 2-H) ATP in oligotrophic Lake Michigan and a moderately eutrophic lake in southeastern Michigan. More than 50% of the phosphate that was hydrolyzed was immediately taken up into bacterium-sized particles in the eutrophic lake and at a near-shore site in Lake Michigan. Less than 50% of the hydrolyzed phosphate was taken up into bacterium-sized particles at an offshore site in Lake Michigan. It is hypothesized that differences in size-fractionated uptake were the result of greater phosphorus utilization capacity in bacteria in habitats where loading of organic carbon is greater. Substantial isotope dilution of labeled phosphate uptake by unlabeled phosphate occurred, which implied that the phosphate was hydrolyzed extracellularly in both systems. Comparable nucleotidase activities were measured in the eutrophic lake and Lake Michigan, but the significance of the phosphate regenerated relative to particulate phosphorus pools was an order of magnitude greater in Lake Michigan. Seventy percent of the nucleotidase activity was inhibited by 100 muM phosphate in the eutrophic lake, which suggests that most hydrolysis was by phosphatase. Therefore, nucleotidase activity may be more important to phosphorus regeneration in oligotrophic habitats than phosphatase activity.     

Alcian Blue-stained particles in a eutrophic lake

We used a neutral solution of Alcian Blue to stain transparentparticles in eutrophic Lake Frederiksborg Slotss0, Denmark.Alcian Blue-stained particles (ABSP) appeared to be similarto the so-called transparent exopolymer particles (TEP) identifiedwith an acidic solution of Alcian Blue. Our results on the abundance,size distribution and bacterial colonization of ABSP thereforereflect general patterns of TEP. The abundance of ABSP in thesize range 3–162 µm and retained by 3mu;m pore sizefilters averaged 3.62.4910⁵ ml^–1 (SD), which is amongthe highest concentrations reported for comparable size spectraof TEP. On average, 35 % of ABSP (by number) were colonizedby bacteria and 8.610⁵ bacteria ml^–1 lake water wereattached to ABSP, which corresponds to 7% of the total bacterialabundance.     

Desmids of the Staurastrum tetracerum-group from a eutrophic lake in mid-wales

Seven desmids have been found in the phytoplankton of a markedly eutrophic lake in mid Wales. All have grown well in laboratory cultures and so an opportunity has been provided to explore the taxonomy and morphological variability of four species, Staurastrum tetracerum, S. irregulare, S. bibrachiatum and S. pseudotetracerum, previously described inadequately. Suggestions for nomenclatural changes are made for S. irregulare and the very variable S. bibrachiatum, which is recorded from the British Isles for the first time. Reasons for the occurrence of the seven desmid species in the eutrophic lake plankton are discussed.     

Mineralization of dissolved organic phosphorus from a shallow eutrophic lake

Release of soluble reactive phosphorus (SRP) from dissolved organic phosphorus (DOP), concentrated by reverse osmosis of water samples from Lough Neagh Northern Ireland, was measured in the presence of enzymes and cultures of lake water bacteria in a basal liquid medium adjusted to the pH of lake water (7.6). No hydrolysis of unfractionated DOP was observed in the presence of alkaline phosphatase but a combination of alkaline phosphatase and phosphodiesterase mineralized 14% of DOP in a 30 day incubation period at 15 °C. A similar amount of mineralization was attained by phytase. Phytase induced the same degree of mineralization in a range of DOP fractions varying from MW > 100 000 to c. 500. A mixed culture of lake water bacteria mineralized 12% of unfractionated DOP. Single cultures of lake water bacteria displayed low mineralizing activity (mean of 49 cultures = 5% DOP hydrolysed). Results indicate that DOP from Lough Neagh in the above molecular weight range is predominantly recalcitrant to bacterial mineralization under natural lake conditions.     

Effects of temperature and oxygenavailability on greenhouse gas and nutrient dynamics in sediment of a eutrophic mid-boreal lake

The effects of oxygen conditions and temperature on dynamics of greenhousegases (CH₄, CO₂, N₂O) and nutrients(NH₄ ⁺, NO₂ ^–+NO₃ ^–, tot-P) were studied in sediment of hyper-eutrophic LakeKevätön, Finland. Undisturbed sediment cores were incubated at 6, 11,16, and 23 °C in a laboratory microcosm using a continuouswater flowtechnique with an oxic or anoxic water flow. The production of CO₂increased with increasing temperature in both oxic (Q₁₀ 3.2 ±0.6) and anoxic (Q₁₀ 2.3 ± 0.4) flows. The release ofCH₄ increased with temperature in anoxic conditions (Q₁₀2.3 ± 0.2), but was negligible with the oxic flow at all temperatures.The release of NH₄ ⁺ increased with temperature with the oxic and anoxic flows(Q₁₀ 2.4 ± 0.1). There was a net production of NO₂ ^–, NO₃ ^– and N₂O with the oxic flow at temperatures below16 °C. The release of phosphorus was greater from the anoxicsediments and increased with temperature with both the anoxic (Q₁₀2.9 ± 0.5) and oxic (Q₁₀ 1.9 ± 0.1) flows. It isprobable that the temperature of boreal lakes and the associated oxygendeficiency will increase as the climate becomes warmer. Our experiments showedthat this change would increase the global warming potential of greenhousegasesreleased from sediments of eutrophic lakes predominately attributable to theincrease in the CH₄ production. Furthermore, warming would alsoaccelerate the eutrophication of lakes by increasing release of phosphorus andmineral nitrogen from sediments, which further enhance CH₄productionin sediments.     

Ammonium excretion by some freshwater zoobenthos from a eutrophic lake

Relationships between the ammonium excretion rate of zoobenthos and temperature were examined for two chironomids, Chironomus, plumosus and Tokunagayusurika akamusi, one chaoborid, Chaoborus flavicans, and two tubificids, Limnodrilus spp. and Branchiura sowerbyi, which often dominate in eutrophic lakes. The representative Q₁₀ value and excretion rate at 15° C for each species were as follows: C. plumosus, 1.67, 1.40, (µg N mg dry w.^–1 d^–1 ); T. akamusi, 1.02 (< 15° C), 0.33; C. flavicans, 1.49, 2.87; Limnodrilus spp., 2.98, 0.59; and B. sowerbyi, 3.15, 0.79.     

Matrix bound phosphine formation and depletion in eutrophic lake sediment fermentation-simulation of different environmental factors

Closed anaerobic batch-fermentation of eutrophic lake sediment samples was performed under variation of four environmental fermentation factors (pH, temperature, water/sediment ratio and disturbance) to learn how the quantity of phosphine will change and if the quantity of phosphine can increase. The fermentation conditions where matrix bound phosphine (MBP) increased (doubled from 3193+/-520 to about 7982+/-1003 ng/kg) were: a pH of 8 and of 10 (as compared to 1, 2, 4, 6, 12), a temperature of 20 and 30 degrees C (as compared to 4 and 40 degrees C), a water/sediment ratio of 3:1 (as compared to 1:1, 2:1, 5:1) and a disturbance of 100 r/min (as compared to 0 r/min), respectively. Although, over the full time course of fermentation, the balance of phosphine production became negative again or did remain almost unchanged under most conditions. A pH of 1 or disturbance of 150 r/min was significant factors to decrease phosphine over the long term. Free phosphine had been detected but was of minor importance (in the order of 60.9+/-10.1 ng/m(3)). Overall, the fermentation conditions which had been most favorable for microbial life (moderate temperature (20 and 30 degrees C) and pH 8) were also most favorable for a positive phosphine balance. This is an indication, but no biochemical proof that a natural (biogenic, microbial, biochemical) NET PRODUCTION of phosphine or DE NOVO PRODUCTION of phosphine has occurred. MBP concentrations in lake sediments were discussed as to be strongly dependent on a balance of natural generation and depletion processes, dependent of the simulated parameters.     

Endogenic development of sediments in a eutrophic lake

Sedimentation in the Gouet reservoir (France), measured for 2 years at 8 stations, was maximal during summer, when river inputs were minimal. Physical and chemical conditions in the deposits indicate that the endogenous part of sedimentation was about 70% and resulted from significant diatom production. The high sedimentation rate on the bottom was favoured by the funnel morphology of the reservoir, the chronic lack of oxygen in the water column, and the repeated copper sulfate treatment. The former river meanders of the reservoir were the preferential deposit sites.