Growth and age structure of the swan mussel Anodonta cygnea (L.) at different depths in lake Mattsee (Salzburg, Austria)

Unionid clams were collected at 1–2 m, 3–4 m and 6–7 m depth in lake Mattsee, a moderately mesotrophic lake, to investigate the effect of depth on clam growth and age structure. No significant differences in age structure of Anodonta cygnea were found (p=0.65). Three and ten years old clams were present at all depths, but in different percentages. Whereas at 1–2 m 13.3% of the collected clams were <4 years old, this percentage was 4.4% at 6–7 m and 7.1% at 3–4 m. A greater percentage (6.7%) of older mussels (9, 10 years) were collected at 6–7 m than at 1–2 m (2.2%). Growth declined with depth. Total length at a given age of clams at 1–2 m and 3–4 m did not differ (p=0.54), whereas differences were significant between clams at 1–2 m and 6–7 m (p<0.05) as well as between 3–4 m and 6–7 m (p<0.05). The Growth constant k was highest at 1–2 m depth.     

Continuous underwater light measurement near Helgoland (North Sea) and its significance for characteristic light limits in the sublittoral region

Underwater irradiance was measured at intervals of 20 min for one year at 2 water depths (2.5 and 3.5 m below M.L.W.S.) and in 3 spectral regions in the sublittoral region of the rocky island of Helgoland. Data are presented for spectral and total irradiance at water depths ranging from 2 to 15 m (below M.L.W.S.). 90% of the total annual light reaching sublittoral habitats is received during the period from April to September, when Jerlov water type 7 (occasionally water type 5) dominates. During the other half of the year, the water is very turbid, and transparency is so low that long dark periods occur even at moderate water depths. The total annual light received at the lower kelp limit (Laminaria hyperborea), at 8 m water depth, is 15 MJ m^–2 year^–1 or 70 E m^–2 year^–1, which corresponds to 0.7% of surface irradiance (visible). At the lower algal limit (15 m water depth) these values are 1 MJ m^–2 year^–1 or 6 E m^–2 year^–1, corresponding to 0.05% of surface irradiance. These data are similar to measurements at the same limits in several different geographical areas, and may determine the depth at which these limits occur.     

Vertical distribution in and isolation of bacteria from Lake Vanda: an Antarctic lake

Vertical distribution of bacteria in Lake Vanda, an Antarctic meromictic lake, was examined by the acridine orange epifluorescence direct count method. Total bacteria were 10⁴–10⁵ cells · ml^–1 in the water at 55 m depth and above, and increased drastically to 10⁷ cells · ml^–1 in the bottom water. Filamentous or long rodshaped bacteria occurred at a high frequency in the upper layers, but in the bottom layers most bacteria were coccoidal or short rods. Mean bacterial cell volume in water of between 10 m and 60 m deep was fairly large compared with common bacterial populations in seawater and lake water. Aerobic heterotrophic bacteria were recovered from the water of a depth of 30 m and above, and were assumed to belong to Caulobacter. Viable heterotrophic bacteria were not recovered from the high salinity deep water by media prepared with the same deep water. Phototrophic purple non-sulphur bacteria were isolated by enrichment cultures from water at 55 m depth.     

Denitrification in the water column of an intermittently anoxic fjord

Denitrification was studied in the water column in the Bunnefjord, inner part of the Oslofjord in southern Norway, using a ¹⁵N-technique (the isotope pairing method). The fjord is 150 m deep and during our surveys in September–December 1998 hydrogen sulphide was present in the deep water below 80 m. No significant denitrification was found in water samples from the surface layer (4 m depth), but high rates were observed within a deep density gradient between 62 and 78 m depth. Oxygen concentration within this layer was low (<21 mmol m^–3), and the concentration of NO₃ decreased from ca. 15 mmolm^–3 at 62 m depth to not detectable below 78 m. Pronounced peaks of NO₂ up to 4.4 mmol m^–3 were observed at 70–78 m depth. The maximum denitrification rate of 1.5 mmol N m^–3 d^–1 was observed at 70 m depth. Integrated for the whole layer, the denitrification rate was 13 mmol N m^–2 d^–1. A significant linear correlation was found between the denitrification rate and the ambient nitrate concentration which indicated that the rate was primarily controlled by the availability of nitrate in the O₂-poor water. Compared to rates reported for coastal water, denitrification in the water column in the Bunnefjord was high and the process appears to be a major sink of bioavailable nitrogen in the fjord.     

Respiratory electron transport system (ETS) — activity of the plankton and sediment in Lake Balaton (Hungary)

The ETS-activity of microplankton (1–60 µm), net zooplankton (60–250 µm and > 250 µm) and sediment was measured in the hypertrophic Keszthely-basin and in the meso-eutrophic Siófok-basin of Lake Balaton in 1988–1989.Six times higher microplanktonic and 1.35–1.75 times higher zooplanktonic ETS-activities l^–1 were found in the Keszthely-basin than in the Siófok-basin. The equivalent relationship for the microplanktonic ETS-activity m^–2 was 3.83–3.93 to 1 in 1988 and 1989. This ratio in the case of the zooplankton was 1.07–1.09 to 1. The zooplanktonic ETS-activity represented 6.0–10.8% of the total measured sestonic ETS-activity in the Keszthely-basin and 19.0–30.5% of that in the Siófok-basin in 1988 and 1989. The results suggest a decreasing trend in the contribution of net zooplankton metabolic activity in the whole metabolism of the water column with increasing primary production.The sediment was biologically active to 30–35 cm depth in the Keszthely-basin and to 15–20 cm depth in the Siófok-basin. In the Keszthely-basin and in the Siófok-basin, relative decrease in the sediment ETS-activity cm^–3 with depth were 10% cm^–1 and 25% cm^–1 respectively. The sediment ETS-activity m^–2 was much higher than that in the plankton: 7.8 times in the Keszthely-basin and 9.3 times in the Siófok-basin. The Keszthely-Siófok basins ratio for the sediment ETS-activity m^–2 was 3 to 1.According to the ETS-activity data, Lake Balaton plankton itself can oxidize all the organic matter produced in the water column. The sestonic and sediment respiratory potentials together are much higher than is necessary for complete oxidation of the plankton-assimilated carbon.     

Annual productivity of Spirulina (Arthrospira) and nutrient removal in a pig wastewater recycling process under tropical conditions

An evaluation was made of the annual productivity of Spirulina (Arthrospira) and its ability to remove nutrients in outdoor raceways treating anaerobic effluents from pig wastewater under tropical conditions. The study was based at a pilot plant at La Mancha beach, State of Veracruz, Mexico. Batch or semi-continuous cultures were established at different seasons during four consecutive years. The protein content of the harvested biomass and the N and P removal from the ponds were also evaluated. Anaerobic effluents from digested pig waste were added in a proportion of 2% (v/v) to untreated sea-water diluted 1:4 with fresh water supplemented with 2 g L^–1 sodium bicarbonate, at days 0, 3 and 5. A straight filament strain of Spirulina adapted to grow in this complex medium was utilized. A pH value 9.5 ± 0.2 was maintained. The productivity of batch cultures during summer 1998 was significantly more with a pond depth of 0.10 m than with a depth 0.065 m. The average productivity of semi-continuous cultures during summer 1999 was 14.4 g m^–2 d^–1 with a pond depth of 0.15 m and 15.1 g m^–2 d^–1 with a depth of 0.20 m. The average annual productivity for semi-continuous cultures operating with depths of 0.10 m for winter and 0.15 and 0.25 m for the rest of the year, was 11.8 g m^–2 d^–1. This is the highest value reported for a Spirulina cultivation system utilising sea-water. The average protein content of the semi-continuous cultures was 48.9% ash-free dry weight. NH₄-N removal was in the range 84–96% and P removal in the range of 72–87%, depending on the depth of the culture and the season.     

Production and decomposition processes in a saline meromictic lake

Bacterial and phytoplankton cell number and productivity were measured in the mixolimnion and chemocline of saline meromictic Mahoney Lake during the spring (Apr.–May) and fall (Oct.) between 1982 and 1987. High levels of bacterial productivity (methyl ³H-thymidine incorporation), cell numbers, and heterotrophic assimilation of ¹⁴C-glucose and ¹⁴C-acetate in the mixolimnion shifted from near surface (1.5 m), at a secondary chemocline, to deeper water (4–7 m) as this zone of microstratification gradually weakened during a several year drying trend in the watershed. In the mixolimnion, bacterial carbon (13–261 µgC 1^–1) was often similar to phytoplankton carbon (44–300 µgC 1^–1) and represented between 14–57% of the total microbial (phytoplankton + bacteria) carbon depending on the depth interval. Phototrophic purple sulphur bacteria were stratified at the permanent primary chemocline (7.5–8.3 m) in a dense layer (POC 250 mg 1^–1, bacteriochlorophyll a 1500–70001µ 1^–1), where H₂S changed from 0.1 to 2.5 mM over a 0.2 m depth interval. This phototrophic bacterial layer contributed between 17–66% of the total primary production (115–476 mgC m^–2 d^–1) in the vertical water column. Microorganisms in the phototrophic bacterial layer showed a higher uptake rate for acetate (0.5–3.7 µC 1^–1 h^–1) than for glucose (0.3–1.4 µgC 1^–1 h^–1) and this heterotrophic activity as well as bacterial productivity were 1 to 2 orders of magnitude higher in the dense plate than in the mixolimnetic waters above. Primary phytoplanktonic production in the mixolimnion was limited by phosphorus while light penetration appeared to regulate phototrophic productivity of the purple sulphur bacteria.     

Chemical composition of interstitial water and diffusive fluxes within the diatomaceous sediment in Lake Myvatn, Iceland

The sub-arctic Lake Myvatn is one of the most productive lakes in the Northern Hemisphere, despite an ice-cover of 190 days per year. This is due to relatively high solar radiation, nutrient rich inflow waters, N₂ fixation and internal nutrient loading. In order to define direction and magnitude of diffusive fluxes, soil water samplers were used to collect interstitial water from 25–150 cm depth, from within the diatomaceous sediment at the bottom of Lake Myvatn. Water depth at the sampling site was 225 cm. The pH of the interstitial water ranged from 7.16 to 7.30, while the pH of the lake water was 9.80–10.00. The concentrations of most solutes were similar 16 cm above the bottom of the lake at the sampling site and at the lake outlet. The concentrations of NO₃, S, F, O₂, Al, Cr, Mo, V, U, Sn and Sb were higher in the lake water than in the interstitial water. They will therefore diffuse from the lake water into the interstitial water. The concentrations of orthophosphates, PO₄, and total dissolved P were highest at 25 cm depth, but Co and NH₄ concentrations were highest at 50 to 100 cm depth. Thus they diffuse both up towards the lake bottom and down deeper into the sediments. The concentrations of Na, K, Ca, Mg, Sr, Mn, Li and alkalinity were greater within the sediments than in the lake water and increased continuously with depth. The Si concentration of the interstitial water was higher than in the lake water, it was highest at 25 cm depth and decreased slightly down into the sediments. The concentration gradient was greatest for bicarbonate, HCO₃ ^–, 1.5×10^–7 mol cm^–3 cm^–1, and then in declining order for the solutes with the highest gradient; NH₄, Si, Na, Ca, Mg, -S (diffusion into the sediments), K, PO₄, Cl, Fe and Mn. The estimated annual diffusive flux of PO₄ for Lake Myvatn was 0.1 g P m^–2 yr^–1, about 10% of the total PO₄ input to Lake Myvatn. The H₄SiO₄° flux was 1.3 g Si m^–2 yr^–1, <1% of both the input and the annual net Si fixation by diatoms within the lake and the diffusive flux of dissolved inorganic carbon was 1% of the annual net C fixation by diatoms. Annual diffusive flux of NH₄ ⁺ was 1.9 g N m^–2 yr^–1 similar to the input of fixed N to the lake and 24% of the net N fixation within Lake Myvatn. Thus it is important for the nitrogen budget of Lake Myvatn and the primary production in the lake since fixed nitrogen is the rate determining nutrient for primary production.     

Sources of water used by riparian Eucalyptus camaldulensis overlying highly saline groundwater

Water sources of Eucalyptus camaldulensis Dehn. trees were investigated on a semiarid floodplain in south-eastern Australia. The trees investigated ranged in distance from 0.5 to 40 m from a stream, with electrical conductivity 0.8 dSm^–1, and grew over groundwater with electrical conductivity ranging from 30 to 50 dSm^–1. The sources of water being used by the trees were investigated using the naturally occurring stable isotopes of water and measurements of soil water potential. Xylem water potential and leaf conductance were also examined to identify the trees' response to using these sources of water. Trees at distances greater than about 15 m from the stream used no stream water. The trees used groundwater in summer and a combination of groundwater and rain-derived surface-soil water (0.05–0.15 m depth) in winter. In doing so they suffered water stress at electrical conductivities higher than approximately 40 dSm^–1 (equivalent to approximately –1.4 MPa). Trees adjacent to the stream used stream water directly in summer, but may have used stream water from the soil profile in winter, after the stream had risen and recharged the soil water. E. camaldulensis appeared to be partially opportunistic in the sources of water they used.     

Vegetative growth response of young olive trees (Olea europaea L., cv. Arbequina) to soil salinity and waterlogging

High-density olive orchards are increasing around the world, many of which may be potentially affected by salinity and waterlogging (hypoxia), two important stresses common in irrigated fields in arid and semi-arid climates. However, the response of olive to these stresses under field conditions is not well established. Therefore, our objective was to evaluate the vegetative growth response of young olive trees (Olea europaea L., cv. Arbequina) grown in a spatially variable waterlogged, saline-sodic field. We monitored the growth in trunk diameter of 341, 3-year-old olives between September 1999 and September 2000. Field contour maps were developed delineating soil salinity (ECa), relative ground elevation (RGE) and water table depth (WTD). Soil samples were also collected and analyzed for ECe and SARe in order to characterize the salinity and sodicity profiles and develop the ECa-ECe calibration equation. The infiltration rate (IR) of the crusted and uncrusted soil and the penetration resistance (PR) were also measured. The field was characterized by spatially variable ECe (2–15 dS m^–1), SARe (3–40), RGE (–4 to +4 cm) and WTD (0.5–1.9 m, with corresponding ground water EC values between 12 and 6 dS m^–1). Steady-state IR of crusted soil was only 7% of the uncrusted soil. Since the field was heavily irrigated by flooding, waterlogging conditions were related to low RGE values. Soil salinity was negatively correlated (R ² = 0.83, P<0.001) with RGE (ponded water) and WTD (upward flux), due to the evapo-concentration of water and salts at the soil surface. Thus, inverted salinity profiles developed in high salinity areas. Fifty-five percent of the olives were dead 3.5 years after planted, and most of them were located in areas of high ECe (> 10 dS m^–1), low RGE (< – 1.5 cm) and low WTD (< 1.2 m). The surviving trees had vegetative salinity tolerance values of ECe threshold = 4 dS m^–1 and slope = –12% (i.e., percent decline per unit increase in ECe above the treshold), indicating that the Arbequina olive is moderately tolerant to salinity. The RGE and WTD thresholds for olive's survival were > 0.1 cm and > 1.6 m, respectively. Thus, very small changes in ground elevation had a significant effect on olive's survival or death. The coupled effects of salinity and waterlogging (hypoxia) stresses were most detrimental for olive's growth.     

Use of artificial eggs in studies of washout depth and drift distance for salmonid eggs

Colour-coded artificial trout eggs were used in investigations of washout depth in a natural stream and of drift distance relative to water velocity in an experimental channel and in a section of natural stream.Washout depth was studied in a spawning riffle of a stream whose bankful discharge is 5.6 m³ s^–1. During an experiment when spates never exceeded 6.5 m³ s^–1 egg washout was severe at 5 cm depth within the gravel, variable at 10 cm and negligible at 15 cm. During another experiment when a spate of 9.0 m³ s^–1 (return period 10–20 years) occurred, washout was severe at 5 and 10 cm depth and variable at 15 cm. There was also evidence that some eggs were moved short distances downstream within the gravel without being washed out.Within experimental channels, drift distance could be predicted from multiple regressions relating logarithms of water velocity, percentage of eggs settled and distance from point of release. At a water velocity of 100 cm s^–1 at 0.6 depth, 50% of eggs would settle within 8 m of the point of release. At water velocities of 75 to 100 cm s^–1 drifting eggs would, on average, travel at c. 60% of water velocity and make 1 to 2 bed contacts m^–1 of travel.A similar multiple regression can be applied to data from a natural stream channel. It predicts much larger drift distances (50% settled in 42 m at 100 cm s ^–1 ). However, in the natural channel, settlement appears aggregated and the validity of the concept of permanent settlement is in doubt.     

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.     

Relationship between current velocity,depth and the invertebrate community in a stable river system

The relationship between invertebrate densities, current velocity and water depth was studied in the Dan River, northern Israel. Maximum current preferences ranged from 5–120 cm sec^–1, and depth preferences ranged from 5–60 cm. Thirty-five taxa of invertebrates were collected by means of colonization cages. Larval and adult stages of 3 Elmidae (Coleoptera) species were treated separately: Limnius letourneuxi, Grouvellinus caucasicus and Elmis rioloides. Differences in current preference were observed between larval and adult stages of the same species of Elmidae. Taxa were also grouped according to preference for turbulence. Wide ranges of depth and current velocity preferences were observed. Most of the taxa were found at between 80–100 cm sec^–1 and at depths of less than 30 cm. A correlation between species diversity and current velocity was established. Velocities of 60–80 cm sec^–1 contained the greatest overlap of faunal preference. The sensitivity of selected species to stream flow reduction is discussed.IES Laboratory, Department of Zoology, Hebrew University; and the Nature Reserves Authority     

Effects of water movement on prey capture and distribution of reef corals

Coral reefs comprise a variety of microhabitats, each with a characteristic pattern of water movement. Variation in flow microhabitat is likely to influence the distribution and abundance of suspension feeders, including the corals. Water flow was measured concurrently with wave heights at 8 depths along the forereef slope in Salt River Canyon, St Croix, U.S.V.I. The greatest flow speeds occurred on the shallow forereef at 7 m depth, where oscillatory wave-induced flow reached speeds over 50 cm s^–1. From 7 m to at least 15 m depth, flow decreased and was primarily bidirectional. Below 15 m depth, flow decreased even further, to less than one fifth of that experienced by shallow corals, and was unidirectional. The relationship between particle capture by the corals Meandrina meandrites and Madracis decactis and water flow was studied in the field. Colony morphology and the resulting modification of flow influenced the relationship of flow to feeding success; prey capture by the branching Madracis colonies increased with flow, while that of the flat Meandrina colonies did not. Such relationships may contribute to differences in distribution of corals of divergent morphologies. In transect surveys from 7 to 45 m depth,; branching and mounding corals with tentacular feeding modes were most common in the shallow forereef habitats, and plating corals with small polyps (ciliary mucus feeders) were ubiquitous in the deeper zones.This paper was presented at the Fifth International Conference on Coelenterate Biology at Southampton, UK in July 1989. A synopsis appears in the Proceedings (Hydrobiologia 216/217: 247–248, 1991).This paper was presented at the Fifth International Conference on Coelenterate Biology at Southampton, UK in July 1989. A synopsis appears in the Proceedings (Hydrobiologia 216/217: 247–248, 1991).     

Water chemistry and nutrient budgets in an undisturbed evergreen rainforest of southern Chile

In a pristine evergreen rainforest of Nothofagus betuloides, located at the Cordillera de los Andes in southern Chile (41 °S), concentrations and fluxes of nutrients in bulk precipitation, cloud water, throughfall water, stemflow water, soil infiltration and percolation water and runoff water were measured. The main objectives of this study were to investigate canopy-soil-atmosphere interactions and to calculate input-output budgets. From May 1999 till April 2000, the experimental watershed received 8121 mm water (86% incident precipitation, 14% cloud water), of which the canopy intercepted 16%. Runoff water volume amounted 9527 mm. Bulk deposition of inorganic (DIN) and organic (DON) nitrogen amounted 3.6 kg ha^–1 year^–1 and 8.2 kg ha^–1 year^–1 respectively. Occult deposition (clouds + fog) contributes for 40% to the atmospheric nitrogen input (bulk + occult deposition) of the forest. An important part of the atmospheric ammonium deposition is retained within the canopy or converted to nitrate or organic nitrogen by epiphytic bacteria or lichens. Also the export of inorganic (0.9 kg ha^–1 year^–1) and organic (5.2 kg ha^–1 year^–1) nitrogen via runoff is lower than the input to the forest floor via throughfall and stemflow water (3.2 kg DIN ha–1 year^–1 and 5.6 kg DON ha^–1 year^–1). The low concentrations of NO ₃ ^– and NH ₄ ⁺ under the rooting depth suggest an effective biological immobilization by vegetation and soil microflora. Dry deposition and foliar leaching of base cations (K⁺, Ca²⁺, Mg²⁺) was estimated using a canopy budget model. Bulk deposition accounted for about 50% of the total atmospheric input. Calculated dry and occult deposition are both of equal value (about 25%). Foliar leaching of K⁺, Ca²⁺, and Mg²⁺ accounted for 45%, 38% and 6% of throughfall deposition respectively. On an annual basis, the experimental watershed was a net source for Na⁺, Ca²⁺ and Mg²⁺.     

A comparison of the abilities of three sympatric species of Petrotilapia (Cichlidae,Lake Malawi) to penetrate deep water

Synopsis The ability of three sympatric species ofPetrotolapia, from Monkey Bay, Lake Malawi, to penetrate deep water was tested using pressure chambers. The three species were found to differ in terms of the maximum pressure (= depth) to which each was able to equilibrate. Mean maximum equilibration pressures were for the form designated MOL 39.0 m, for MBB 35.5 m and for the form MOC 30.4 m. The rate of compensation to pressure changes was slower in MBB (2.59 m day^–1) than in other two forms (MOL = 3.97 m day^–1, MOC = 3.98 day^–1). Natural depth distribution of all three forms, in particular MBB and MOC, is not determined by limits imposed by swin bladder physiology.     

An ecological study of the macrophytic vegetation of the Doodhadhari Lake,Raipur, M. P., India

Summary Physical factors operating in Doodhadhari Lake were studied periodically during 1967–68. Thermal stratification in the lake was found to be induced by solar radiation during daytime and destroyed by nocturnal cooling and mixing of the lake waters as evidenced by temperature records. Light intensities at different strata were found to be reduced by plant growth. At 0.5 m depth and 1 m depth considerable reduction in light intensities was found. The floating-leaved rooted plants and free-floating sudd communities curtailed the light transmission to 0.0 – 0.1% at 0.5 m depth. Visibility also decreases considerably owing to shading by the plants at different stations.A part of the thesis submitted for the Degree of Doctor of Philosophy.     

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.     

A bloom of Dunaliella parva in the Dead Sea in 1992: biological and biogeochemical aspects

A bloom of the unicellular green alga Dunaliella parva (up to 15 000 cells m1^–1) developed in the upper 5 m of the water column of the Dead Sea in May-June 1992. This was the first mass development of Dunaliella observed in the lake since 1980, when another bloom was reported (up to 8800 cells m1^–1). For a bloom of Dunaliella to develop in the Dead Sea, two conditions must be fulfilled: the salinity of the upper water layers must become sufficiently low as a result of dilution with rain floods, and phosphate must be available. During the period 1983–1991 the lake was holomictic, hardly any dilution with rainwater occurred, and no Dunaliella cells were observed. Heavy rain floods in the winter of 1991–1992 caused a new stratification, in which the upper 5 m of the water column became diluted to about 70% of their former salinity. Measurements of the isotopic composition of inorganic carbon in the upper water layer during the bloom (¹³C = 5.1) indicate a strong fractionation when compared with the estimated –3.4 prior to the bloom. The particulate organic carbon formed was highly enriched in light carbon isotopes ( ¹³ C = – 13.5). The algal bloom rapidly declined during the months June–July, probably as a result of the formation of resting stages, which sank to the bloom. A smaller secondary bloom (up to 1850 cells m1–1) developed between 6 and 10 m depth at the end of the summer. Salinity values at this deep chlorophyll maximum were much beyond those conductive for the growth of Dunaliella, and the factors responsible for the development of this bloom are still unclear.     

Water chemistry of Lake Ladoga and Russian-Finnish intercalibration of analyses

As part of the Russian-Finnish research studies on Lake Ladoga, joint expeditions were organized in 1992 and 1993. Water samples were collected for intercalibration of chemical analysis methods and to monitor the chemical quality of the lake water.In August of 1992 water samples were taken from northern Lake Ladoga for intercalibration of Russian and Finnish analysis methods. In August 1993 water samples were collected from 23 sampling stations in all parts of the lake; some of these were also used for intercalibration purposes.The oxygen, colour and COD_Mn results were at the same level in the intercalibration. In 1993, the P_tot results obtained were acceptable. In N_tot, Fe and Mn analysis there seemed to be systematic and random errors between some results.The Secchi depth ranged from 1.5 m to 3.3 m. The average concentrations for the total phosphorus ranged from 15 µg 1^–1 to 29 µg 1^–1. The total nitrogen values were from 620 µg 1^–1 to 690 µg 1^–1. The N:P ratio varied from 24 to 40. The concentration of phosphorus indicated mesotrophic or even eutrophic conditions in the lake. Phosphorus seemed to be the limiting nutrient to bacteria and algae.