Sulfate reduction processes in sediments at different sites in Lake Kinneret,Israel

Lake Kinneret, Israel, is a warm (13–30°C) monomictic lake that stratifies in April and turns over in December. Between January and June each year, a heavy bloom (up to 250 g wet weight n^–2 2) of the dinoflagellate Peridinium gatunense dominates the phytoplankton biomass. In early summer, the bloom collapses, and the sinking Peridinium biomass serves as a trigger for intense sulfate-reduction activity throughout the hypolimnion and within the sediments. The availability of organic matter and sulfate was high shortly after the bloom crash and the beginning of stratification and was lowest in December before overturn. Sulfate-reduction rates at three different sites in the lake were studied. In the sediments, the rates varied seasonally and among stations from 5 to 1600 nmol SO₄ ^–2 reduced cm^–3 day^–1, with respect to the distance from the Jordan River, depth, organic content, and stratification period. During years of low lake water levels, intense sulfate reduction occurred in the hypolimnion, resulting in anoxia and high concentrations of H₂S (>400 m). In years with high water levels, early bloom, and delayed stratification, higher rates of sulfate reduction were recorded in the sediments, probably as a result of a greater fraction of the primary production (organic matter) reaching the bottom. Correspondence to: O. Hadas.     

Vertical diffusion and nutrient transport in a tropical lake (Lake McIlwaine,Rhodesia)

Measurements of the vertical temperature in tropical Lake McIlwaine were used to calculate the time-averaged ( 6 months) vertical diffusivity coefficient (K_z) in the metalimnion and hypolimnion. The mean value of K_z (0.21 cm² s^–1) was correlated with the lake surface area. The mass transport rates of PO₄-P and NH₄-N, upward from the hypolimnion to the metalimnion, were calculated using K_z and measured values of the nutrient concentration gradients. During a period of 4.5 months when the water was stably stratified, PO₄-P was transported upward at a mean rate of 42 kg day^–1 and NH₄-N at a mean rate of 162 kg day^–1 over the entire lake.     

Sulphate reduction in the hypolimnion and sediments of Lake Kinneret, Israel

1. Lake Kinneret is a warm (13–30°C) monomictic lake. Between January and June a heavy annual bloom of the dinoflagellate Peridinium gatunense dominates phytoplankton biomass (250 g m^?2). At the beginning of the summer, degradation and decomposition of the Peridinium biomass occurs, serving as a trigger for intense sulphate reduction in the hypolimnion and sediments. 2. The rates of sulphate reduction in the sediments varied seasonally from 12 to 1700 nmol SO₄.^?2 reduced cm^?3 day^?1 in December and July, respectively. The availability of organic matter and sulphate is high in June after the crash of the Peridinium bloom and the beginning of stratification and is lowest in December before overturn. 3. Sulphate concentrations in the hypolimnion range between 0.52 mM and 0.20 mM during mixing (January-April) and before overturn (December), respectively. The depletion in sulphate in the hypolimnion is stoichiometrically correlated to the increase in sulphide. The lake is not depleted of sulphate at any time, so the sulphate reduction process in Lake Kinneret is not limited by sulphate concentrations except in the sediments just before overturn.     

Interactions between sediment and water in a shallow and hypertrophic lake: a study on phytoplankton collapses in Lake Søbygård,Denmark

Short-term changes in phytoplankton and zooplankton biomass have occurred 1–3 times every summer for the past 5 years in the shallow and hypertrophic Lake Søbygård, Denmark. These changes markedly affected lake water characteristics as well as the sediment/water interaction. Thus during a collapse of the phytoplankton biomass in 1985, lasting for about 2 weeks, the lake water became almost anoxic, followed by rapid increase in nitrogen and phosphorus at rates of 100–400 mg N M^–2 day^–1 and 100–200 mg P m^–1 day^–1. Average external loading during this period was about 350 mg N m^–2 day^–1 and 5 mg P m^–2 day^–1, respectively.Due to high phytoplankton biomass and subsequently a high sedimentation and recycling of nutrients, gross release rates of phosphorus and nitrogen were several times higher than net release rates. The net summer sediment release of phosphorus was usually about 40 mg P m^–2 day^–1, corresponding to a 2–3 fold increase in the net phosphorus release during the collapse. The nitrogen and phosphorus increase during the collapse is considered to be due primarily to a decreased sedimentation because of low algal biomass. The nutrient interactions between sediment and lake water during phytoplankton collapse, therefore, were changed from being dominated by both a large input and a large sedimentation of nutrients to a dominance of only a large input. Nitrogen was derived from both the inlet and sediment, whereas phosphorus was preferentially derived from the sediment. Different temperature levels may be a main reason for the different release rates from year to year.     

Sulfur cycling in the water column of Little Rock Lake, Wisconsin

The S cycle in the water column of a small, soft-water lake was studied for 9 years as part of an experimental study of the effects of acid rain on lakes. The two basins of the lake were artificially separated, and one basin was experimentally acidified with sulfuric acid while the other served as a reference or control. Spatial and seasonal patterns of sulfate uptake by plankton (53–70 mmol m^–2 yr^–1), deposition of sulfur to sediments in settling seston (53 mmol m^–2 yr^–1), and sulfate diffusion (0–39 mmol m^–2 yr^–1) into sediments were examined. Measurements of inputs (12–108 mmol m^–2 yr^–1) and outputs (5.5–25 mmol m^–2 yr^–1) allowed construction of a mass balance that was then compared with rates of S accumulation in sediments cores (10–28 mmol m^–2 yr^–1) and measured fluxes of S into the sediments. Because of the low SO₄ ^2– concentrations (µmole L^–1) in the lake, annual uptake by plankton (53–70 mmol m^–2 yr^–1) represented a large fraction (>50%) of the SO₄ ^2– inventory in the lake. Despite this large flux through the plankton, only small seasonal fluctuations in SO₄ ^2– concentrations (µmole L^–1) were observed; rapid mineralization of organic matter (half-life <3 months) prevented sulfate depletion in the water column. The turnover time for sulfate in the water column is only 1.4 yr; much less than the 11-yr turnover time of a conservative ion in this seepage lake. Sulfate diffusion into and reduction in the sediments (0–160 µmole m^–2 d^–1) caused SO₄ ^2– depletion in the hypolimnion. Modeling of seasonal changes in lake-water SO₄ ^2– concentrations indicated that only 30–50% of the diffusive flux of sulfate to the sediments was permanently incorporated in solid phases, and about 15% of sulfur in settling seston was buried in the sediments. The utility of sulfur mass balances for seepage lakes would be enhanced if uncertainty about the deposition velocity for both sulfate aerosols and SO₂, uncertainty in calculation of a lake-wide rate of S accumulation in sediments, and uncertainty in the measured diffusive fluxes could be further constrained.     

Oxygen consumption and ammonia accumulation in the hypolimnion of Walker Lake, Nevada

Walker Lake (area = 140 km², Z _mean = 19.3 m) is a large, terminal lake in western Nevada. As a result of anthropogenic desiccation, the lake has decreased in volume by 75% since the 1880s. The hypolimnion of the lake, now too small to meet the oxygen demand exerted by decaying matter, rapidly goes anoxic after thermal stratification. Field and laboratory studies were conducted to examine the feasibility of using oxygenation to avoid hypolimnetic anoxia and subsequent accumulation of ammonia in the hypolimnion, and to estimate the required DO capacity of an oxygenation system for the lake. The accumulation of inorganic nitrogen in water overlaying sediment was measured in laboratory chambers under various DO levels. Rates of ammonia accumulation ranged from 16.8 to 23.5 mg-N m^–2 d^–1 in chambers with 0, 2.5 and 4.8 mg L^–1 DO, and ammonia release was not significantly different between treatments. Beggiatoa sp. on the sediment surface of the moderately aerated chambers (2.5 and 4.8 mg L^–1 DO) indicated that oxygen penetration into sediment was minimal. In contrast, ammonia accumulation was reversed in chambers with 10 mg L^–1 DO, where oxygen penetration into sediment stimulated nitrification and denitrification. Ammonia accumulation in anoxic chambers (18.1 and 20.6 mg-N m^–2 d^–1) was similar to ammonia accumulation in the hypolimnion from July through September of 1998 (16.5 mg-N m^–2 d^–1). Areal hypolimnetic oxygen demand averaged 1.2 g O₂ m^–2 d^–1 for 1994–1996 and 1998. Sediment oxygen demand (SOD) determined in experimental chambers averaged approximately 0.14 g O₂ m^–2 d^–1. Continuous water currents at the sediment-water interface of 5–6 cm s^–1 resulted in a substantial increase in SOD (0.38 g O₂ m^–2 d^–1). The recommended oxygen delivery capacity of an oxygenation system, taking into account increased SOD due to mixing in the hypolimnion after system start-up, is 215 Mg d^–1. Experimental results suggest that the system should maintain high levels of DO at the sediment-water interface (10 mg L^–1) to insure adequate oxygen penetration into the sediments, and a subsequent inhibition of ammonia accumulation in the hypolimnion of the lake.     

The response of the sedimentological regime in Lake Kinneret to lower lake levels

The surface level of Lake Kinneret is regulated to remain between –209 m and –213 m msl. During the stratified period, soluble reactive phosphorus (SRP), ammonium (NH _inf4 ^sup+ ) and dissolved sulphide (H₂S) accumulate in the hypolimnion. The concentration of these solutes, which are direct and indirect products of the decomposition of organic matter, increase considerably in summers with lower lake levels. A numerical model describing depth-averaged hypolimnion and epilimnion current velocities for high and low lake levels was adapted for Lake Kinneret. Simulated hypolimnetic currents were shown to be stronger for low lake levels as a result of the fact that low lake levels are characterized by a thinner hypolimnion while the thickness of the epilimnion remains unchanged. We suggest that the stronger hypolimnetic currents have the following consequences: 1. turbulence is induced, 2. the enhanced turbulence results in higher resuspension, 3. because SO⁴= is available to bacteria on resuspended particles, mineralization rates are enhanced, and 4. focusing of fine sediments and associated organic matter to the pelagic zone is enhanced.     

Initial Effects of the Mount St. Helens Eruption on Nitrogen Cycle and Related Chemical Processes in Ryan Lake

Ryan Lake, a 1.6-hectare basin lake near the periphery of the tree blowdown area in the blast zone 19 km north of Mount St. Helens, was studied from August to October 1980 to determine the microbial and chemical response of the lake to the eruption. Nutrient enrichment through the addition of fresh volcanic material and the organic debris from the surrounding conifer forest stimulated intense microbial activity. Concentrations of such nutrients as phosphorus, sulfur, manganese, iron, and dissolved organic carbon were markedly elevated. Nitrogen cycle activity was especially important to the lake ecosystem in regulating biogeochemical cycling owing to the limiting abundance of nitrogen compounds. Nitrogen fixation, both aerobic and anaerobic, was active from aerobic benthic and planktonic cyanobacteria with rates up to 210 nmol of N₂ cm⁻¹ h⁻¹ and 667 nmol of N₂ liter⁻¹ h⁻¹, respectively, and from anaerobic bacteria with rates reaching 220 nmol of N₂ liter⁻¹ h⁻¹. Nitrification was limited to the aerobic epilimnion and littoral zones where rates were 43 and 261 nmol of NO₂ liter⁻¹ day⁻¹, respectively. Potential denitrification rates were as high as 30 μmol of N₂O liter⁻¹ day⁻¹ in the anaerobic hypolimnion. Total bacterial numbers ranged from 1 × 10⁶ to 3 × 10⁸ ml⁻¹ with the number of viable sulfur-metal-oxidizing bacteria reaching 2 × 10⁶ ml⁻¹ in the hypolimnion. A general scenario for the microbial cycling of nitrogen, carbon, sulfur, and metals is presented for volcanically impacted lakes. The important role of nitrogen as these lakes recover from the cataclysmic eruption and proceed back towards their prior status as oligotrophic alpine lakes is emphasized.     

Nitrogen Dynamics in the Steeply Stratified,Temperate Lake Verevi,Estonia

The dynamics of different nitrogen compounds and nitrification in diverse habitats of a stratified Lake Verevi (Estonia) was investigated in 2000–2001. Also planktonic N₂-fixation (N₂fix) was measured in August of the observed years. The nitrogen that accumulated in the hypolimnion was trapped in the non-mixed layer during most of the vegetation period causing a concentration of an order of magnitude higher than in the epilimnion. The ammonium level remained low in the epilimnion (maximum 577 mgN m⁻³, average 115 mgN m⁻³) in spite of high concentrations in the hypolimnion (maximum 12223 mgN m⁻³, average 4807 mgN m⁻³). The concentrations of NO₂⁻ and NO₃⁻ remained on a low level both in the epilimnion (average 0.94 and 9.09 mgN m⁻³, respectively) and hypolimnion (average 0.47 and 5.05 mgN m⁻³, respectively). N₂fix and nitrification ranged from 0.30 to 2.80 mgN m⁻³ day⁻¹ and 6.0 to 107 mgN m⁻³ day⁻¹, respectively; the most intensive processes occurred in 07.08.00 at depths of 2 and 5 m, accordingly. The role of N₂fix in the total nitrogen budget of Lake Verevi (in August 2000 and 2001) was negligible while episodically in the nitrogen-depleted epilimnion the N₂fix could substantially contribute to the pool of mineral nitrogen. Nitrification was unable to influence nitrogen dynamics in the epilimnion while some temporary coupling with ammonium dynamics in the hypolimnion was documented.     

Bacterioplankton in the acidified Lake Gårdsjön

Total bacterial numbers in different strata of lake water and in inlet and outlet streams have been recorded during a yearly cycle. A calculated mean cell volume of 0.342 µm² has then been used to estimate bacterial biomass in the lake. Change of biomass during the year was substantial and the range was from about 0.1 g · m^–3 to about 1.0–1.2 g · m^–3. The seasonal development included a spring-early summer increase followed by a decrease to the minimum in July–August. Correlation between epi- and hypolimnion was high and in both strata two dominant autumn peaks in biomass appeared. With the exception of the last autumn peak the development of bacterial biomass was closely related to development of phytoplankton biomass and production.     

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.     

Sequential kinetic parameter estimation of anaerobic fluidized bed bioreactor using an optimization technique

Mathematical model parameters for the methanogenic degradation of propylene glycol were estimated in a sequential manner by means of an optimization technique. Model parameters determined from an initial experimental data set using one bioreactor were then verified with the results from a second bioreactor. The proposed methodology is a useful tool to obtain model parameters for continuous flow reactors with completely mixed regime. Abbrevations: S – substrate concentration (mg COD l^–1); S _in – influent substrate concentration (mg COD l^–1); D _L – dilution rate (day^–1); – stoichiometric coefficients (ND); nx – number of microbial species (ND); X _S – fixed biomass concentration (mg biomass l^–1); X _L – suspended biomass concentration of (mg biomass l^–1); k _d – decay rate of biomass (day^–1); b _S – specific detachment rate of biofilm (day^–1); – specific growth rate of biomass (day^–1); _m – maximum specific growth rate of biomass (day^–1); K _S – half saturation constant (mg COD l^–1); K _I – inhibition constant (mg COD l^–1).     

Sediment Nitrification, Denitrification, and Nitrous Oxide Production in a Deep Arctic Lake

We used a combination of ¹⁵N tracer methods and a C₂H₂ blockage technique to determine the role of sediment nitrification and denitrification in a deep oligotrophic arctic lake. Inorganic nitrogen concentrations ranged between 40 and 600 nmol · cm⁻³, increasing with depth below the sediment-water interface. Nitrate concentrations were at least 10 times lower, and nitrate was only detectable within the top 0 to 6 cm of sediment. Eh and pH profiles showed an oxidized surface zone underlain by more reduced conditions. The lake water never became anoxic. Sediment Eh values ranged from −7 to 484 mV, decreasing with depth, whereas pH ranged from 6.0 to 7.3, usually increasing with depth. The average nitrification rate (49 ng of N · cm⁻³ · day⁻¹) was similar to the average denitrification rate (44 ng of N · cm⁻³ · day⁻¹). In situ N₂O production from nitrification and denitrification ranged from 0 to 25 ng of N · cm⁻³ · day⁻¹. Denitrification appears to depend on the supply of nitrate by nitrification, such that the two processes are coupled functionally in this sediment system. However, the low rates result in only a small nitrogen loss.     

Sulfate reduction in fine-grained sediments in the Eastern Scheldt,southwest Netherlands

Sulfate reduction and sulfide accumulation were examined in fine-grained sediments from rapidly accreting abandoned channels and mussel culture areas in the Eastern Scheldt, which covered 4 and 5% of the total surface area, respectively.Reduction rates were measured in batch experiments in which the SO₄ ^2– depletion was measured during anoxic incubation. The reduction rates in summer varied between 14–68 mmol SO₄ ^2– m^–2 day^–1 and were related to the sedimentation rate. In the most rapidly accreting channels, SO₄ ^2– was exhausted below 15–50 cm and methanogenesis became the terminal process of organic carbon oxidationOne-dimensional modelling of sulfate profiles in mussel banks indicated that the subsurface influx of SO₄ ^2– was almost of the same order as the diffusive flux at the sediment-seawater interface, during the initial stages of the mussel bank accretion. The energy dissipation of waves and tidal currents on the mussel bank surface increased the apparent sediment diffusivity up to 3-fold, especially in the winterThe results indicate that acid volatile sulfide (AVS) was the major, in-situ reduced, sulfur compound in the sediment. The sulfidation of easily extractable iron was nearly complete. Pyrite concentrations (40–80 M S cm^–3) were as high as the AVS concentrations, but there was apparently no in-situ transformation of AVS into pyrite. The detrital pyrite originated from eroding marine sediments elsewhere     

Disruption of sulfur cycling and acid neutralization in lakes at low pH

Experimental acidification of a softwater lake to below pH 5 fundamentally changed the sulfur cycle and lowered internal alkalinity generation (IAG). Prior to reaching pH 4.5, the balance of sulfur reduction and oxidation reactions within the lake was in favour of reduction, and the lake was a net sink for sulfate. In the four years at pH 4.5 the balance of reduction and oxidation reactions was in favour of oxidation, and there was a net production of sulfate (SO₄ ^2–) within the lake. Evidence indicating a decrease in net SO₄ ^2– reduction at pH 4.5 was also obtained in an anthropogenically acidified lake that had been acidified for many decades. In both lakes, the decrease in net SO₄ ^2– reduction appeared to be linked not to a simple inhibition of SO₄ ^2– reduction but rather to changes in benthic ecosystem structure, especially the development of metaphytic filamentous green algae, which altered the balance between SO₄ ^2– reduction and sulfur oxidation.At pH's above 4.5, net SO₄ ^2– reduction was the major contributor to IAG in the experimental lake, as it is in many previously studied lakes at pH 5 and above. At pH 4.5, the change in net annual SO₄ ^2– reduction (a decrease of 110%) resulted in a 38% decrease in total IAG. Because of the important role of net SO₄ ^2– reduction in acid neutralization in softwater lakes, models for predicting acidification and recovery of lakes may need to be modified for lakes acidified to pH <5.     

The annual cycle of heat content and mechanical stability of hypersaline Deep Lake,Vestfold Hills,Antarctica

Deep Lake, a hypersaline lake of about ten times seawater concentration, rarely freezes and is characterized by a monomictic thermal cycle, Winter circulation, at c. –17 °C, lasts for two to three months. In summer, epilimnetic temperatures from 7–11 °C result in large vertical thermal gradients (21–26 °C) which combine with the enhanced rate of density change per degree Celsius, accompanying such high salt concentration, to produce a particularly stable density configuration in Deep Lake (Schmidt stability c. 8000 g-cm cm^–2; 0.785 J cm^–2). The Birgean annual heat budget (c. 24500 cal cm^–2; 102.7 10³ J cm^–2) is comparable to that of a temperate lake with a similar mean depth, despite the comparatively high ratio of Birgean wind work to annual heat budget (0.37 g-cm cal^–1). Deep lake retains c. 50% of the incident solar radiation during the short summer heating period; within the range estimated for first class lakes in North America. Extended daylight hours certainly contribute to the high maximum rate of heating in the lake (444 cal cm^–2 day^–1; 1.86 10³ J cm^–2 day^–1). Deep Lake cools at a rate less than half its average heating rate. Partitioning the total stability into thermal and saline components shows that salinity can contribute up to c. 20% of the maximum summer Schmidt stability. In early summer, the effect of small melt-streams is to increase stability by diluting the epilimnion. In autumn, evaporative water loss can overtake this effect, creating small de-stabilizing salinity gradients. The usually short-term stabilizing influence of snowfall and drift is less predictable, but is probably more common in winter when strong winds are most frequent.Hypersalinity has a profound effect on the physical cycle of Deep Lake, through freezing point depression and the increased rate of density change with temperature. These changes affect the lake's biota, both in relation to osmotic stress, and by effectively exposing them to a more thermally extreme environment. A comparison between Deep Lake and a smaller lake of similar salinity (Lake Hunazoko, Skarvs Nes), demonstrates that it is inappropriate to consider the biological effects of salinity in isolation. The smaller lake offers warmer epilimnetic conditions for at least part of the summer, which may explain the much greater limnetic algal production in Lake Hunazoko.     

Biomass and icosapentaenoic acid productivities from an outdoor batch culture of Phaeodactylum tricornutum UTEX 640 in an airlift tubular photobioreactor

An outdoor tubular photobioreactor, based on an external-loop airlift, has been used to cultivate the diatom Phaeodactylum tricornutum UTEX 640, rich in icosapentaenoic acid (EPA). The system was operated in batch mode. The specific growth rate in the exponential phase, averaged over the whole day, was 0.254 day^–1 · A 25 gl^–1 maximum biomass concentration was obtained by the end of the culture period. The dissolved oxygen generated never rose over 210% of air saturation nor was there impediment to biomass generation or inhibition of photosynthesis. Furthermore, around 2 × 10¹⁷ quanta cm^–2 s^–1 light intensity seemed to saturate the photosynthetic activity. Biomass losses during the night could be reduced to less than 5% by maintaining the culture at 16°C at night. The maximum concentration and productivity of EPA were 423 mgl^–1 and 13 mgl^–1 day^–1, respectively, at the end of the culture, although a 21-mgl^–1 day^–1 net EPA generation rate was obtained at the linear growth phase.     

Winter microbial activity in Lake Tuusulanjärvi

Microbial heterotrophic activity, dark CO₂ assimilation, primary productivity and microbial ATP were measured monthly in the extremely eutrophic Lake Tuusulanjärvi during the winter of 1979–1980. Because of continuous water circulation caused by low temperature and artificial aeration of the lake, no winter stratification developed. Very low summertime ³H-glucose turnover times of 5 h increased to a level of 10–20 h from August to January. Winter maximum of 110 h was measured in March, and turnover times returned to 10–20 h in April, before the vernal bloom of algae occured. Oxygen saturation remained over 46% during the winter.High primary productivity was observed in November (400–500 mg C m^–3 day^–1), and measurable productivity was detected under ice in January (80 mg C m^–3 day^–1). Dark CO₂ assimilation increased to 14% of primary productivity in March. No correlation was found between ³H-glucose turnover rate and dark CO₂ assimilation. ATP correlated slightly better with primary productivity than with turnover rate. The single concentration method proved to be sensitive for winter heterotrophic activity measurement.     

Vertical Distribution of Zooplankton in a Strongly Stratified Hypertrophic Lake

The vertical and temporal distribution of metazooplankton in the small hypertrophic, strongly stratified, temperate Lake Verevi (Estonia) was studied during 1998–2001. The zooplankton of Lake Verevi is characteristic of hypertrophic lakes, with a small number of dominant species, rotifers being the main ones, and juveniles prevailing among copepods. In 1999–2001, the average abundance of metazooplankton in the lake was 1570 × 10³ ind m⁻³; in the epilimnion 2320 × 10³ ind m⁻³, in the metalimnion 2178 × 10³ ind m⁻³, and in the hypolimnion 237 × 10³ ind m⁻³. The average biomass of metazooplankton was 1.75 g m⁻³; in the epi-, meta- and hypolimnion, accordingly, 2.16, 2.85 and 0.26 g m⁻³. The highest abundances – 19,136 × 10³ ind m⁻³ and 12,008 × 10³ ind m⁻³ – were registered in the lower half of the metalimnion in 24 May and 5 June 2001, respectively. Rotifer Keratella cochlearis f. typica (Gosse, 1851) was the dominating species in abundance. In biomass, Asplanchna priodonta Gosse, 1850, among the rotifers, and Eudiaptomus graciloides (Lilljeborg, 1888), among the copepods, dominated. According to the data from 2000–2001, the abundance and biomass of both copepods and rotifers were highest in spring. Zooplankton was scarce in the hypolimnion, and no peaks were observed there. During the summers of 1998 and 1999, when thermal stratification was particularly strong, zooplankton was the most abundant in the upper half of the metalimnion, and a distinct peak of biomass occurred in the second fourth of the metalimnion. Probably, the main factors affecting the vertical distribution of zooplankton in L. Verevi are fish, Chaoborus larvae, and chemocline, while food, like phytoplankton, composition and abundance may affect more the seasonal development of zooplankton.     

A comparison of oxygen,nitrate, and sulfate respiration in coastal marine sediments

Aerobic respiration with oxygen and anaerobic respiration with nitrate (denitrification) and sulfate (sulfate reduction) were measured during winter and summer in two coastal marine sediments (Denmark). Both aerobic respiration and denitrification took place in the oxidized surface layer, whereas sulfate reduction was most significant in the deeper, reduced sediment. The low availability of nitrate apparently limited the activity of denitrification during summer to less than 0.2 mmoles NO ₃ ^– m^–2 day^–1, whereas activities of 1.0–3.0 mmoles NO ₃ ^– m^–2 day^–1 were measured during winter. Sulfate reduction, on the contrary, increased from 2.6–7.6 mmoles SO ₄ ^2– m^–2 day^–1 during winter to 9.8–15.1 mmoles SO ₄ ^2– m^–2 day^–1 during summer. The aerobic respiration was high during summer, 135–140 mmoles O₂ m^–2 day^–1, as compared to estimated winter activities of about 30 mmoles O₂ m^–2 day^–1. The little importance of denitrification relative to aerobic respiration and sulfate reduction is discussed in relation to the availability and distribution of oxygen, nitrate, and sulfate in the sediments and to the detritus mineralization.