Limnological effects of anthropogenic desiccation of a large, saline lake, Walker Lake, Nevada

Walker Lake is a monomictic, nitrogen-limited, terminal lake located in western Nevada. It is one of only eight large (Area>100 km², Z _{{ mean}}>15 m) saline lakes of moderate salinity (3–20 g l^–1) worldwide, and one of the few to support an endemic trout fishery (Oncorhynchus clarki henshawi). As a result of anthropogenic desiccation, between 1882 and 1996 the lake's volume has dropped from 11.1 to 2.7 km³ and salinity has increased from 2.6 to 12–13 g l^–1. This study, conducted between 1992 and 1998, examined the effects of desiccation on the limnology of the lake. Increases in salinity over the past two decades caused the extinction of two zooplankton species, Ceriodaphnia quadrangula and Acanthocyclops vernalis. Recent increases in salinity have not negatively affected the lake's dominant phytoplankton species, the filamentous blue-green algae Nodularia spumigena. In 1994 high salinity levels (14–15 g l^–1) caused a decrease in tui chub minnow populations, the main source of food for Lahontan cutthroat trout, and a subsequent decrease in the health of stocked trout. Lake shrinkage has resulted in hypolimnetic anoxia and hypolimnetic accumulation of ammonia (800–2000 g-N l^–1) and sulfide (15 mg l^–1) to levels toxic to trout. Internal loading of ammonia via hypolimnetic entrainment during summer wind mixing (170 Mg-N during a single event), vertical diffusion (225–500 Mg-N year^–1), and fall destratification (540–740 Mg-N year^–1) exceeds external nitrogen loading (<25 Mg-N year^–1). Increasing salinity in combination with factors related to hypolimnetic anoxia have stressed trout populations and caused a decline in trout size and longevity. If desiccation continues unabated, the lake will be too saline (>15–16 g l^–1) to support trout and chub fisheries in 20 years, and in 50–60 years the lake will reach hydrologic equilibrium at a volume of 1.0 km³ and a salinity of 34 g l^–1.     

The influence of road salts on the salinity and the meromictic stability of Lake Svinsjøen,southeastern Norway

Primarily as a result of road salting, the water masses ofLake Svinsjøen, a small meromictic lake in southeasternNorway, have been subject to great changes in salinity duringthe period 1947–1995. The greatest change in saltconcentration has occurred in the upper part of themonimolimnion (depth 10–15 m) where mean conductivityincreased 104.2 per cent, from 143 to 292 S cm^–1. Inthe upper mixolimnion (depth 0-5 m), mean conductivity rosefrom 130 to 238 S cm^–1 during the same period. Theions responsible for the salinity changes were Na⁺ andCl^– from de-icing salts, and Ca²⁺ and Cl^– fromsalts used to keep down dust from roads. Further sources ofCa²⁺ are the road asphalt and increased weathering andleaching of the lime-rich rocks caused by acid precipitation,the main source of the additional inputs of SO tothe lake. The salinity changes caused major changes inmeromictic stability, S . In the period1947-1966, S increased by 24 g cm cm^–2,and the maximum level of meromictic stability, 125 gcm^–2, was found in 1966. As a result of higher rate ofsalt accumulation in the upper part of the monimolimnion andin the mixolimnion, S decreased by 30 g cmcm^–2 during the period 1966-1991, and a simultaneousrise in the chemocline took place. In the period 1991-1995 anadditional decrease of 26 g cm cm^–2 occurred. Continuedectogenic inputs of salts through processes typical of thetime period investigated will in future further weaken thelake's meromictic stability, and may cause the demise ofmeromixis in Lake Svinsjøen, a development which may haveimportant implications for primary productivity of thelake.     

Predicting concentration of total phosphorus and chlorophyll a in a lake with short hydraulic residence time

The relationship between total phosphorus and chlorophyll a concentration was determined for Skinner Lake, Indiana over an annual cycle in 1978–79. Total nitrogen:total phosphorus ratios in the epilimnion ranged from 19 to 220 suggesting a phosphorus-dependent algal yield in the epilimnion. Approximately 90% of annual TP loading reached the lake via streamflow, and 93% of this entered during snowmelt and spring-overturn periods. At that time incoming water flushed the lake 2.4 times. Atmospheric loading accounted for 1.4% of annual TP load. Internal hypolimnetic TP loading occurred during summer stratification. Mean [chl a] for the ice-free period was 15.15 mg m^–3, within the range expected for eutrophic lakes.The 1978–79 data were used in conjuction with the Vollenweider & Kerekes (1980) model to produce a model specific for the Skinner Lake system. The model predicted mean epilimnetic total phosphorus and chlorophyll a concentrations from mean total phosphorus concentration in inlet streams and from lake water residence time during the period of spring overturn and summer stratification. The Skinner-specific model was tested in 1982 and it closely predicted observed mean epilimnetic [TP] and [chl a] during the ice-free period. This study shows that variability in lake models which average data over an annual period can be reduced by considering lake-specific seasonal variation in hydrology and external TP loading.     

Determining trophic state in Lake Whatcom,Washington (USA), a soft water lake exhibiting seasonal nitrogen limitation

We evaluated an eleven year data set to assess trophic state and nutrient limitation in Lake Whatcom, an oligotrophic, soft water, chain lake located in the Puget Sound lowlands of Washington (U.S.A.). Although total phosphorus (TP) and soluble reactive phosphate (SRP) concentrations were relatively low throughout the lake, there were significant differences between the northern basin (Site 1) and the other sampling sites (Sites 2–4). Nonparametric correlation coefficients (Kendall's ) were highest between chlorophyll (CHL), Secchi depth (SD), total nitrogen (TN), and dissolved inorganic nitrogen (DIN). Late summer algal biomass correlated best with DIN and TP. Trophic State Indices based on TP, TN, CHL and SD revealed that although algal growth was most likely phosphorus limited throughout the year, the northern basin of the lake may have developed nitrogen co-limitation during late summer and fall. During this period, N/P ratios were often less than 20, and in 1998 the epilimnetic DIN concentrations dropped below 20 g l^–1 while DIN/TP ratios fell below 4. Reviews of the literature suggest that while co-limitation by phosphorus and nitrogen is fairly common in unproductive lakes, the patterns seen in Lake Whatcom were more similar to those reported for eutrophic lakes experiencing secondary nitrogen limitation resulting from excess phosphorus loading.     

Phytoplankton dynamics in a deep, tropical, hyposaline lake

The annual variation of the phytoplankton assemblage of deep (64.6 m), hyposaline (8.5 g l^–1) Lake Alchichica, central Mexico (19 ° N, 97° W), was analyzed in relation to thermal regime, and nutrients concentrations. Lake Alchichica is warm monomictic with a 3-month circulation period during the dry, cold season. During the stratified period in the warm, wet season, the hypolimnion became anoxic. N–NH₃ ranged between non detectable (n.d.) and 0.98 mg l^–1, N–NO₂ between n.d. and 0.007 mg l^–1, N–NO₃ from 0.1 to 1.0 mg l^–1 and P–PO₄ from n.d. to 0.54 mg l^–1. Highest nutrient concentrations were found in the circulation period. Chlorophyll a varied from <1 to 19.8 g l^–1 but most values were <5 g l^–1. The euphotic zone (>1% PAR) usually comprised the top 15–20 m. Nineteen algae species were identified, most of them are typical inhabitants of salt lakes. Diatoms showed the highest species number (10) but the small chlorophyte Monoraphidium minutum, the single-cell cyanobacteria, Synechocystis aquatilis, and the colonial chlorophyte, Oocystis parva, were the numerical dominant species over the annual cycle. Chlorophytes, small cyanobacteria and diatoms dominated in the circulation period producing a bloom comparable to the spring bloom in temperate lakes. At the end of the circulation and at the beginning of stratification periods, the presence of a bloom of the nitrogen-fixing cyanobacteria, N. spumigena, indicated nitrogen-deficit conditions. The well-stratified season was characterized by low epilimnetic nutrients levels and the dominance of small single-cell cyanobacteria and colonial chlorophytes. Phytoplankton dynamics in tropical Lake Alchichica is similar to the pattern observed in some deep, hyposaline, North American temperate lakes.     

Annual cycle of phytoplankton in Ace Lake,an ice covered,saline meromictic lake

The annual cycle of phytoplankton in saline, meromictic Ace Lake (68°2S.4S, 78°11.1E) in the Vestfold Hills, Antarctica, was studied from January, 1979 to January 1980. Ace Lake has permanent gradients of temperature, salinity, dissolved oxygen, and hydrogen sulphide, and is ice covered with up to 2 m of ice for 10–12 months each year. The phytoplankton community had low diversity, consisting of only four species, all flagellates — a prasinophyte Pyramimonas gelidicola McFadden et al., a cryptophyte of the genus Cryptomonas; an unidentified colourless microflagellate, and an unarmoured dinoflagellate. These were restricted to the oxic zone of the lake from the surface to 10 m.The phytoplankton had a cycle of seven months of active growth over spring and summer. Low numbers of cells survived in the water column over winter. Spring growth was initiated below the ice by increased light penetration through the ice into the lake, enhanced at the time by the removal of surface snow which accumulated on the ice over winter. Peak phytoplankton biomass production was by the shade adapted P. gelidicola and occurred at the interface of the oxic and anoxic zones where substantial available nitrogen as ammonia is found.The three dominant phytoplankton species displayed distinct vertical stratification over the oxic zone. This stratification was not static and developed over spring as the flagellates migrated to preferred light climate zones. Mean cell volume of two of the flagellates varied significantly over the year. Minimum volumes were recorded in winter and volume increased progressively over spring to reach maximum mean cell volume in summer. Mean cell volume was positively correlated with light intensity (maximum ambient PAR at the respective depth for date of sample). Low cell volume in winter may be related to winter utilization of carbohydrate reserves by slow respiration, and may represent a survival mechanism.     

Seasonal variation in the microbial contamination of game carcasses in an Austrian hunting area

In 2003, 50 game carcasses (ungulates) originating from one Austrian hunting ground were subject to visual examination for (fecal) contamination of the body cavities and microbiological testing of the body cavities in order to assess variations in microbial surface contamination in the season June–August compared to October–December. No carcass tested positive for the bacterial pathogens Salmonella or Listeria. Bacterial surface counts in October–December (median values: total aerobic count: 4.12 log₁₀ colony-forming-units (cfu)/cm²; Enterobacteriaceae: 2.48 log₁₀ cfu/cm²) were significantly lower than those in June–August (median values: total aerobic count: 5.65 log₁₀ cfu/cm²; Enterobacteriaceae: 3.45 log₁₀ cfu/cm²). The cooling regime (0.4 °C, 62% relative humidity) allowed no microbial growth for 96 h but was associated with weight loss of the carcasses. All carcasses had undergone a precooling phase of 8–12 h, with temperatures of 17.8±1.2 °C in the season June–August and 9.8±1.2 °C in October–December. This temperature difference was identified as the most probable effector for the observed seasonal variation. The results demonstrate the need for a continuous cool chain after evisceration of game carcasses.     

Distribution and abundance of littoral benthic fauna in Canadian prairie saline lakes

The littoral benthos of 18 lakes in Alberta and Saskatchewan ranging in salinity from 3 to 126 (g1^–1 TDS) were investigated twice, in the spring and in the summer of 1986. Multiple Ekman dredge samples were taken at water depths of about 0.5, 1.0 and 2 metres in each transect. Two to three transects were used in each lake according to its estimated limnological diversity for a total of 114 stations. A total of 76 species was present varying from 29–31 species in the three lakes of lowest salinity (means of 3.1–5.55) to only 2 species in lakes exceeding 100. Species richness decreased rapidly in salinities greater than 15.Biomass maximum mean of 10.91 g m^–2 dry weight (maximum 63.0 g m^–2) occurred in culturally eutrophic Humboldt Lake (3.1) but one third as great in other low salinity lakes. However, biomass again increased to about 4.5 gm^–2 in two lakes of 15 As the salinity increased still further biomass declined steadily until a minimum of 0.0212 g m^–2 was recorded in most saline Aroma Lake (mean 119). Summer biomass (11 lakes) was greater than spring biomass (4 lakes) because some groups such as amphipods, corixids and ostracods became more abundant in summer. Wet weight biomass averaged 15.8 of dry weight biomass.Seasonality (spring or summer), sediment texture and organic matter content, water depth, pH, salinity (TDS) and the presence of aquatic plants ( plant cover) were considered in the matrix involving species dry weight biomass at each of 117 stations. TWINSPAN classification of the samples yielded a dendrogram with 18 indicator species. Successive dichotomies divided these indicator species into four main lake groups based on salinity, i.e., Group I: 3–10 (Gammarus, Glyptotendipes I, Chironomus cf. plumosus), Group II: 10–38%. (Hyalella, Enallagma,Bezzia), Group III: 38–63 (Hygrotus salinarius, Cricotopus ornatus), Group IV: >63 (Dolichopodidae, Ephydra hians). Each of these main groups was subdivided into smaller groups of lakes based on factors such as pH, seasonality (spring or summer species dominance), organic matter and plant cover. Depth of samples played no apparent role.     

Submerged Macrophyte-bed Effects on Water-Column Phosphorus,Chlorophyll <Emphasis Type="Italic">a</Emphasis>, and Bacterial Production

Submerged macrophytes are a major component of freshwater ecosystems, yet their net effect on water column phosphorus (P), algae, and bacterioplankton is not well understood. A 4-month mass-balance study during the summer quantified the net effect of a large (5.5 ha) undisturbed macrophyte bed on these water-column properties. The bed is located in a slow-flowing (0.05–0.1 cm s^–1) channel between two lakes, allowing for the quantification of inputs and outputs. The P budget for the study period showed that, despite considerable short-term variation, the macrophyte bed was a negligible net sink for P (0.06 mg m^–2 day^–1, range from –0.76 to +0.79 mg m^–2 day^–1), demonstrating that loading and uptake processes in the weedbed roughly balance over the summer. Chlorophyll a was disproportionately retained relative to particulate organic carbon (POC), indicating that the algal component of the POC was preferentially trapped. However, the principal contribution of the weedbed to the open water was a consistent positive influence on bacterioplankton production over the summer. Conservative extrapolations based on measured August specific exports (m^–2 day^–1) of P and bacterial production exiting the weedbed applied to five regional lakes varying in lake morphometry and macrophyte cover suggest that even in the most macrophyte dominated of lakes (66% cover), P loading from submerged weedbeds never exceeds 1% day^–1 of standing epilimnetic P levels, whereas subsidization of bacterioplankton production can reach upward of 20% day^–1. The presence of submerged macrophytes therefore differentially modifies algae and bacteria in the water column, while modestly altering P dynamics over the summer.     

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.     

The impact of zooplankton status on the management of Lake Kinneret (Israel)

Monthly averages of standing stock wet biomass of zooplankton in Lake Kinneret (Israel) varied between 11 and 76 g m^–2 during 1969–1981, with the exception of two months. Averaged contributions of different groups were: Cladocera 58%, Copepoda 35% and Rotifera 7%. Total standing crop wet biomass is highest during January–June, averages varied between 35 and 50 g m^–2, and decreases during summer–fall (23–36 g m^–2). The winter biomass of Cladocera fluctuated between 22 and 35 g m^–2 and dropped to a range of 9–23 g m^–2 in summer, whereas copepod biomass varied very little around an average of 18 g (ww) m^–2 with the exception of low values from April to June. The stock biomass of Rotifera is relatively high during winter floods season (December-March) whilst in summer it is very low.Young stages of fish in Lake Kinneret feed mostly on zooplankton and zoobenthic forms. The most abundant fish in the Kinneret ecosystem, Mirogrex terraesanctae terraesanctae, also feed on zooplankton at the adult stage throughout the year, and herbivorous fish consume zooplankton during the summer when lake plankton resources are limited.The summer ecosystem of Lake Kinneret is characterised as a steady state type, in which the impact of the zooplankton-chain is of great importance. Increase of predation pressure on zooplankton by fish can disequilibrate the balanced trophic relations existing between nannoplankton production and zooplankton grazing capacity. Such a situation can lead to organics accumulation as nannoplankton blooms, resulting in water quality deterioration. Management options aimed at preventing collapse of zooplankton populations are discussed.     

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.     

The phytoplankton of Lake Liddell,New South Wales: chlorophyll a concentrations,species seasonal succession,and covariation with nutrients

Samples of the phytoplankton in a freshwater lake, Lake Liddell, New South Wales (Lat: 32° 22 S, Long. 150° 1 E) were collected every 4 weeks between October 1987 and November 1988. Chlorophyll a concentrations ranged from 1.8 g 1^–1 to 9.1 g 1^–1 and were positively correlated with the following nutrient parameters: total and nett mass additions of nitrate/nitrite-N and total-N, total additions of Kjeldahl-N, and nett mass addition N-P ratios. There was no correlation between lake nutrient concentrations and chlorophyll a. Factors other than nutrient concentrations appeared to be effecting chlorophyll a concentrations as summer levels were low despite nutrient concentrations being at a maximum. In spring and summer the phytoplankton was dominated by chlorophytes, with dinoflagellates and diatoms most abundant in autumn. During winter cyanobacteria were the most abundant. The relative abundance of chlorophytes was positively correlated with in lake nitrate/nitrite-N concentrations whereas the relative abundance of cyanobacteria was negatively correlated with this parameter. Based on chlorophyll a concentrations and the phytoplankton flora Lake Liddell can be classified as mesotrophic.     

Limnological control of brine shrimp population dynamics and cyst production in the Great Salt Lake, Utah

In the Great Salt Lake of Utah, the brine shrimp Artemia franciscanaKellogg is an important food resource for birds and they produce dormant cysts that are harvested and used extensively in the aquaculture industry. We analyzed the limnological factors controlling Artemia growth and cyst production over 12 months in 1994 and 1995. Laboratory experiments showed that inter-brood intervals were highly dependent on temperature and slightly on food level. At optimal temperatures and nutritious food, juveniles reached reproductive size within 7 d in the laboratory. In winter when temperatures were less than 3 °C, Artemia were absent from the lake, phytoplankton abundance was high (13 Chl a g l^–1), and the dominant grazers were ciliated protozoans. In the spring, cysts hatched when phytoplankton was abundant (15–30 g Chl a l^–1), and the Artemia grew and produced large clutches of ovoviviparous eggs. Estimated naupliar production from these eggs was 80 l^–1 from April to May. Despite the high production of nauplii, Artemia densities declined to 8 l^–1by June and the growing shrimp population grazed down the phytoplankton resource to <1 g Chl a l^–1. With the depleted phytoplankton food resource during the summer, Artemia growth slowed, lipid indices decreased, clutch sizes declined, and females switched primarily to oviparous cyst production. During the summer, there was limited production of ovoviviparous eggs, and limited recruitment of juveniles, probably due to low food. Although oviparous reproduction began in June, more than 90% of the cysts were produced after July when female densities had declined to 1.5 l^–1, but nearly all of them were producing cysts. Estimated cyst production was 650000 m^–2, or 4.54 × 10⁶ kg dry weight for the entire lake. The reported commercial harvest took 21% of the 1994 cyst production. That harvest had little impact on the subsequent year's population, as Artemia densities were ultimately controlled by algal production in the lake.     

Cultivation of the agarophyte Gelidium pristoides in Algoa Bay,South Africa

Biofilms were allowed to develop on wooden slides of the River Red Gum (Eucalyptus camaldulensis Dehnh., Myrtaceae) submerged in two billabongs of south-eastern Australia. The slides were placed in the photic zone and the aphotic zone, and the biofilms sampled after eight week's growth over the summer of 1989–1990 and winter of 1990. Bacterial numbers, estimated with epifluorescence microscopy, ranged from 4–78 × 10⁶ cells cm^–2. Bacteria were more abundant in the photic zone than the aphotic zone, and more abundant in summer than winter. Fewer than 0.5% of the bacteria could be cultivated on nutrient agar plates. Concentrations of phospholipids ranged from 8–79 ng cm^–2, which corresponded to bacterial abundances of 2–17 × 10⁶ cells cm^–2. Fifty five phospholipid fatty acids (PLFA) were identified, of which 16:0 (13–29% of total PFLA) was the most common. Other abundant PFLA included 16:17c (6–28%), 18:26 (3–16%), 18:33 (4–12%), 18:19c (3–5%), 18:l7c (5–11%) and 18:0 (2–8%). Minor PLFA included 14:0, i and a 15:0, 15:0, 16:l5c, 16:113c, 18:36, 18:43, 20:46 and 20:53. The PLFA profiles of the biofilms were quite different from those of the sediments and plankton. There was a clear distinction between the PLFA profiles of summer and winter biofilms, but less evidence for unequivocal site or light-regime effects.     

Sulfte-reduction process in sediments of Lake Kinneret,Israel

Monomictic Lake Kinneret is stratified during summer and autumn, resulting in a hypolimnion rich in H₂S (3–7 mg 1^–1). In winter and spring every year a bloom of dinoflagallate Peridinium gatunense produces an average biomass of 150000 ton wet weight. Part of this biomass sinks to the hypolimnion and sediments where it is decomposed and mineralized, with some of the mineralization due to the activity of sulfate-reducing bacteria (SRB). The sulfate-reduction potential of the upper sediment layer at the deepest part of the lake (42 m) was measured. The activity of the enzyme arylsulfatase was also monitored. Rates of sulfate-reduction ranged from a minimum of 12 nmoles SO_f4 ^p2–-reduced cm^–3 day^–1 in December before lake overturn to a maximum of 1673 nmoles SO_f4 ^p2– reduced cm^–3 day^–1 in July during stratification. These rates are considerably higher than those recorded from other freshwater lakes in the world and are probably limited more by the availability of organic matter than by sulfate concentrations.     

Factors influencing the phytoplankton steady state assemblages in a drinking-water reservoir (Ömerli reservoir, Istanbul)

In situ growth of heterotrophic nanoflagellates (HNF) in Lake Donghu, a eutrophic shallow lake in mainland China, was studied from January 1999 to March 2000 using a modified Weisse protocol. The study results indicated that the growth rates of HNF showed pronounced seasonal variation (–0.37–1.25 d^–1), reaching the maximum during spring to early summer. When the water temperature was higher than 25.5°C, HNF growth was inversely proportional to water temperature. There was an effect by bacterial abundance and autotrophic picoplankton on HNF growth that depended on location. HNF biomass was the highest in late spring, and the HNF production ranged from –2.25 to 35.45 mg l^–1 d^–1 with mean of 3.17 mg l^–1d^–1. When considered in the context of biomass and production data for zooplankton in Lake Donghu, it was evident that HNF contributed significantly to the total zooplankton production in Lake Donghu. These in situ studies indicate that temperature and food supply are the major determinants of HNF abundance and productivity.     

Internal phosphorus loading to shallow Edinboro Lake in northwestern Pennsylvania

The summer stratification phosphorus budget for eutrophic Edinboro Lake in northwestern Pennsylvania was determined. Phosphorus loading from internal sources contributed 141 kg, (79%) and 55 kg, (68%) of the mass phosphorus increase in the lake in 1981 and 1982, respectively. Calculated anaerobic sediment release rates of total phosphorus were 9.9 and 3.7 mg m^–2 day^–1 for these two years. The observed summer maximum chlorophyll a concentration was 1.5–3 times greater than that predicted by existing models. Year-to-year variability in the internal phosphorus load for this lake and others is discussed. Without a data base that will permit the comparison of lakes and with and without a significant supply of internal phosphorus, prediction of the relative importance of internal loading in a particular lake will be difficult.     

Photolithotrophy,photoheterotrophy, and chemoheterotrophy: Patterns of resource utilization on an annual and a diurnal basis within a pelagic microbial community

An annual investigation of rates of photolithotrophy, photoheterotrophy, and chemoheterotrophy utilizing glucose and bicarbonate was made within the pelagic zone of a small, hardwater, southwestern Michigan lake. Sampling proceeded on a monthly, diurnal, and depth-wise basis. Annual mean photoheterotrophic uptake was estimated at 2.6g C m^–3h^–1. Two periods of relatively high activity were observed: one during spring overturn and the second during the late summer period. In general, greatest contributions to overall carbon cycling occurred during morning to midday incubation periods and at intermediate depths within the water column. Rates of chemoheterotrophy averaged 6.9g C m^–3h^–1 and were relatively uniform throughout the annual period. Greatest overall chemoheterotrophic activity was associated with periods of overturn. In general, this activity increased throughout the day and with increasing depth within the water column. The annual mean for photolithotrophic fixation was 1.33 mg C m^–3h^–1. Greatest contributions to rates of photosynthesis were associated with epilimnetic waters during early morning and midday incubations. Relatively minor contributions to inorganic fixation were made by waters below the 6-meter contour. Spring overturn and late summer represented periods of particularly great photolithotrophic activity. Quantitative comparisons among carbon pathways indicate that rates of pelagic heterotrophy, both photo- and chemoheterotrophy combined, contribute small quantities of carbon to overall carbon metabolism in this oligotrophic system. Qualitative comparisons among pathways indicate strong spatial and temporal separation. The late summer period showed greatest seasonal separation of the three pathways. Spring values represented a period of relatively high activity for all three pathways. On a depth-wise basis, photolithotrophic activity was greatest near the surface and chemolithotrophic activity greatest near the bottom. Photoheterotrophy took an intermediate position between the two. Diurnally, photoheterotrophy and photolithotrophy showed greatest activity during midday and early morning periods, whereas chemoheterotrophy increased throughout the daylight period and reached maximal values in sunset incubations.     

Some ecological aspects of Lake Qarun,Fayoum, Egypt. Part II. Production of plankton and benthic organisms

Distribution and abundance of phyto-, zooplankton and benthic organisms in Lake Qarun were investigated during the period from January 1974 to December 1977.Average number of phytoplankton cells was 152,300 cells/L and its biomass was 0.365 g/C/m³; average number of zooplankton was 31.44 × 10³/m³ and its biomass was 194.19 mg/m³. The average number of benthic fauna was 19889/m² and its biomass was 400.22 g/m² (dry wt.). Therefore, Lake Qarun may be considered as a highly eutrophic body of water.Freshwater planktonic species, that used to inhabit the lake, such as Diaptomus salinus and the cladoceran Moina salinarum, disappeared completely when the salinity of the lake water reached 30–34 However, some Rotatoria were able to withstand the high salinity. The new composition of the zooplankton community shows that the marine zooplankton species include not only Acartia latisetosa and Cirripedia nauplii, but also other species such as Polychaeta, Obelia medusae, etc.The benthos of Lake Qarun is characterised by an intensive growth of few species. The major part (i.e. 93.54% by weight) of bottom fauna in the lake is Mollusca, mainly Cerastoderma glaucum (69·84% by weight).