Phytoplankton ecology of Loch Achray (Scotland)

Loch Achray is a warm monomictic lake in the Scottish highlands with water temperature ranging from 4 °C to 18.7 °C. The oxygen distribution is mainly clinograde with saturation values ranging from 76 to 112%. The general pattern of nutrient variation is irregular particularly for phosphate. Dissolved silica (measured as silicate) showed a range of 24 μg at. Si l⁻¹; phosphate varied from 0.24 to 0.03 μg at. P l⁻¹ while nitrate ranged from 3.5 to 15.2 μg at. N l⁻¹. The pH remained below 7 for the entire year and the alkalinity was very low, ranging from 2.5 to 6 mg CaCO₃ 1⁻¹. The examination of net samples showed Loch Achray to possess a poor phytoplankton population, consisting mainly of desmids and a few diatoms. The quantitative investigation showed a different picture of variation. There was no clear phytoplankton increase till May when a simultaneous increase occurred involving Melosira, Asterionella, Tabellaria fenestrata and Cyclotella. In July the population was dominated by Anabaena, while Cyclotella and Staurastrum became dominant in August.     

Primary production and phytoplankton in two small,polyhumic forest lakes in southern Finland

Primary production and phytoplankton in polyhumic lakes showed a very distinct seasonal succession. A vigorous spring maximum produced by Chlamydomonas green algae at the beginning of the growing season and two summer maxima composed mainly of Mallomonas caudata Iwanoff were typical. The annual primary production was ca. 6 g org. C · m^–2 in both lakes. The mean epilimnetic biomass was 1.1 in the first lake and 2.2 g · m^–2 (ww) in the second one. The maximum phytoplankton biomass, 14 g · m^–2, was observed during the vernal peak in May.     

Grazing and assimilation rates of natural populations of planktonic rotifersKeratella cochlearis,Keratella quadrata andKellicottia longispina in a eutrophic lake (Aydat,France)

The filtering rates of¹⁴C labelledChlamydomonas sp. by 3 dominant species of rotifers were studied in eutrophic Lake Aydat. They varied from 4 to 53 µl ind^–1 h^–1 forKeratella cochlearis, from 2 to 56 µl.^–1h^–1 forKeratella quadrata and from 3 to 52 µl ind^–1 h^–1 forKellicottia longispina. Their maximum assimilation efficiency was 32%. At the measured grazing rates, these populations could clear the water in less than two days during July. In Lake Aydat, the rotifers community could play an important role in the regulation of seasonal succession of phytoplankton and bacteria.     

Primary production of phytoplankton and standing crops of zooplankton and zoobenthos in hypertrophic Lake Teganuma

Primary production of phytoplankton and standing crops of zooplankton and zoobenthos were intensively surveyed in Lake Teganuma during May 1983–April 1984. The annual mean chlorophyll a concentrations were as high as 304 µg · l^–1–383 µg · l^–1. The daily gross primary production of phytoplankton was high throughout the year. The peak production rate was recorded in August and September, when blue-green algae bloomed. The annual gross primary production was estimated as 1450 g C · m^–2 · y^–1, extremely high as compared with other temperate eutrophic lakes. Zooplankton was predominantly composed of rotifers. The annual mean standing crop of zooplankton was 0.182 g C · m^–2 around the middle between the inlets and the outlet and was lower than in most other temperate eutrophic lakes. Zoobenthos was mostly composed of Oligochaeta and chironomids. The annual mean standing crop of zoobenthos ranged from 0.052 g C · m^–2 to 0.265 g C · m^–2, the lowest values among temperate eutrophic lakes, which is in contrast to the high primary production.     

Phytoplankton and phosphorus in southern Georgian Bay, 1973–1982, and implications for phosphorus loading controls

The phytoplankton and nutrient status of the embayments between Penetanguishene and Waubaushene in southern Georgian Bay (Severn Sound) were examined during the ice-free periods of 1973–1982 because the area showed symptoms of excessive nutrient enrichment. Four wastewater treatment plants currently discharge to the area, another is under construction and a sixth plant has been proposed. Except for Penetang Bay, the area is well-mixed by prevailing winds but is somewhat isolated from the main part of Georgian Bay. Average phytoplankton biomasses throughout the area were 10–20× higher than values from adjacent Nottawasaga Bay where, during 1980, total phytoplankton biomass ranged between 0.15 and 0.25 mm³ · l^–1. Total phosphorus concentrations were highest in Penetang Bay, ranging between 30 and 49 µg P · l^–1 (May–September means) over the 10 year period. Phosphorus concentrations in Nottawasaga Bay averaged 4–6 µg P · l^–1 and no significant differences were detected among the sampling stations; however, phytoplankton densities at stations near urban centres and river inflows were significantly higher than at more remote offshore sites and attests to the use of phytoplankton as a sensitive measure of trophic status in Georgian Bay.Although improvement of Severn Sound water quality to a level comparable to that presently existing in Nottawasaga Bay may never be practical, steps are being taken to control high industrial phosphorus loading and to lessen bypassing of sewage treatment facilities previously hydraulically overloaded during periods of heavy runoff. These measures, along with an evaluation of other major sources of nutrients to Severn Sound, should enable a refinement of the nutrient management programme for Severn Sound and some improvement in trophic status.     

High biomass and production of picoplankton in a Chinese integrated fish culture pond

The phytoplankton dynamics of a Chinese integrated fish culture pond in the suburbs of Shanghai were studied in September and October 1989. The chlorophyll a concentration was high with a range of 62.5–127.3 µg l^–1; however, daily net production of phytoplankton was relatively low, with a range of 0.53–1.94 gC m ^–2 d^–1. Of the total phytoplankton biomass, 70–87% was composed of nanoplankton (<10 µm) and picoplankton, probably because of the selective feeding by phytoplanktivorous carp. In particular, the chlorophyll a concentration of picoplankton was 2.1 – 14.1 mg m ^–3, and its contribution to total phytoplankton production rate was high (18–68%).     

Build-up and decline of summer phytoplankton biomass in the eastern Weddell Sea,Antarctica

The seasonal development and decline of phytoplankton was investigated in the eastern Weddell Sea during summer and fall 1991. During the first half of the study (15 Jan–13 Feb) in an area off Vestkapp, favourable irradiance/mixing regimes initiated net phytoplankton growth in ice-free waters on the shelf and in stretches of open water over the partially ice-covered deep ocean. Chi a concentrations in the upper water column were moderate (0.2–0.8 g l^–1), but significantly above winter values. Later in the season (16 Feb–11 March), a phytoplankton bloom with surface Chl a concentrations ranging from 1.6–2.3 g l^–1 was encountered in an area further to the east. We suggest that the upper water column must have been stratified in this region for time scales of weeks to faciliate bloom development. Bacterial biomass and productivity generally paralleled the seasonal development of the phytoplankton. Nitrate concentrations in the upper mixed layer were substantially lower than would be expected from the existing phytoplankton standing stock, suggesting that heterotrophic consumption of organic matter by bacteria and zooplankton removed a large fraction of the primary production. The shallow seasonal pycnocline was eventually eroded by the passage of a storm, resulting in a homogeneous distribution of phytoplankton biomass over the entire water column, followed by sedimentation and deposition of phytodetritus on the sea floor. After the storm induced destratification, bacterial productivity was particularly high, amounting to more than half of the primary production (range: 10%–120%) in the upper water column. Subsequently, phytoplankton biomass in the upper water column decreased to values <1 g Chl a l^–1. The combination of low incident irradiances and incessant deep mixing prevented the phytoplankton biomass to increase again. During the last week of the investigation, extensive new-ice formation was observed. A major fraction of the residual surface plankton was incorporated into new sea ice, thus terminating the pelagic growth season of the phytoplankton in the eastern Weddell Sea.     

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.     

Seasonal change in the proportion of bacterial and phytoplankton production along a salinity gradient in a shallow estuary

We intended to evaluate the relative contribution of primary production versus allochthonous carbon in the production of bacterial biomass in a mesotrophic estuary. Different spatial and temporal ranges were observed in the values of bacterioplankton biomass (31–273 g C l^–1) and production (0.1–16.0 g C l^–1 h^–1, 1.5–36.8 mg C m^–2 h^–1) as well as in phytoplankton abundance (50–1700 g C l^–1) and primary production (0.1–512.9 g C l^–1 h^–1, 1.5–512.9 mg C m^–2 h^–1). Bacterial specific growth rate (0.10–1.68 d^–1) during the year did not fluctuate as much as phytoplankton specific growth rate (0.02–0.74 d^–1). Along the salinity gradient and towards the inner estuary, bacterio- and phytoplankton biomass and production increased steadily both in the warm and cold seasons. The maximum geographical increase observed in these variables was 12 times more for the bacterial community and 8 times more for the phytoplankton community. The warm to cold season ratios of the biological variables varied geographically and according to these variables. The increase at the warm season achieved its maximum in the biomass production, particularly in the marine zone and at high tide (20 and 112 times higher in bacterial and phytoplankton production, respectively). The seasonal variation in specific growth rate was most noticeable in phytoplankton, with seasonal ratios of 3–26. The bacterial community of the marine zone responded positively – generating seasonal ratios of 1–13 in bacterial specific growth rate – to the strong warm season increment in phytoplankton growth rate in this zone. In the brackish water zone where even during the warm season allochthonous carbon accounted for 41% (on average) of the bacterial carbon demand, the seasonal ratio of bacterial specific growth rate varied from about 1 to 2. During the warm season, an average of 21% of the primary production was potentially sufficient to support the whole bacterial production. During the cold months, however, the total primary production would be either required or even insufficient to support bacterial production. The estuary turned then into a mostly heterotrophic system. However, the calculated annual production of biomass by bacterio- and phytoplankton in the whole ecosystem showed that auto- and heterotrophic production was balanced in this estuary.     

Can suspension feeding by bivalves regulate phytoplankton biomass in Lake Waccamaw, North Carolina?

Suspension feeding by bivalves has been hypothesized to control phytoplankton biomass in shallow aquatic ecosystems. Lake Waccamaw, North Carolina, USA is a shallow lake with a diverse bivalve assemblage and low to moderate phytoplankton biomass levels. Filtration and ingestion rates of two relatively abundant species in the lake, the endemic unionid, Elliptio waccamawensis, and an introduced species, Corbicula fluminea, were measured in experiments using natural phytoplankton for durations of 1 to 6 days. Measured filtration and ingestion rates averaged 1.78 and 1.121 ind.^–1 d^–1, much too low to control phytoplankton at the observed phytoplankton biomass levels and growth rates. Measured ingestion rates averaged 4.80 and 1.50 µg chlorophyll a ind.^–1 d^–1, too low to support individuals of either species. The abundance of benthic microalgae in Lake Waccamaw reaches 200 mg chlorophyll a m^–2 in the littoral zone and averages almost an order of magnitude higher than depth-integrated phytoplankton chlorophyll a. Total microalgal biomass in the lake is therefore not controlled by suspension feeding by bivalves.     

Effects of phosphorus and nitrogen enrichment on the phytoplankton in a tropical reservoir (Lobo Reservoir,Brazil)

The effects of enrichment with phosphate (0–500 µg. 1^–1) and forms of nitrogen (nitrate, nitrite, ammonia an and urea) (0–3500 µgg. ^–1) on the phytoplankton growth of Lobo Reservoir (Brazil) were studied in July, 1979. Suspended matter, chlorophyll a, cell concentrations and the carotenoid:cchlorophyll ratio were estimated following 14 days of in situ incubation. Phosphate alone caused no significant effects, but enrichment with nitrogen caused a substantial increase on the growth of phytoplankton. Comparison between the different forms of nitrogen showed insignificant effects after their additions with 350 µg. ^–1 and in combination with phosphate. However, when nitrogen was added in large quantities (3 500 µg. ^–1), significant differences between the nitrogeneous forms were found, with urea causing the strongest effect. In July, nitrogen is mhe main limiting nutrient to phytoplankton growth of Lobo Reservoir.Supported by CNPq and FAPESP.     

Characteristics of plankton communities in Chinese integrated fish ponds: effects of excessive grazing by planktivorous carps on plankton communities

Biomass and production of plankton communities were investigated in two Chinese integrated fish culture ponds in August, Dianshanhu Pond (with high density of planktivorous carp) and Pingwang Pond (with low density of planktivorous carp). The plankton communities were composed of rotifers, protozoans, phytoplankton (<40 µm) and bacteria. The large phytoplankton (>40 µm), cladocerans and copepods were rare because of grazing pressure by the carp. The density or biomass of bacteria (1.93 × 10⁷ and 2.20 × 10⁷ cells ml^–1 on average in Dianshanhu and Pingwang Ponds, respectively), picophytoplankton (24.6 and 18.5 mg m^–3 Chla on average) and rotifers (5372 and 20733 ind. 1^–1 on average) exceeded the maximum values reported for natural waters.The average [³H]thymidine uptake rates were 694 and 904 pmoles 1^–1 h^–1 (13.4 and 20.6 µgC 1^–1) and the bacterial production by the >2 µm fraction amounted 21–28% of total [³H] thymidine uptake rate in both ponds. The mean chlorophylla concentrations were 59.1 and 183 mg m^–3 in Dianshanhu and Pingwang Ponds, respectively. 82.4% and 65.3% of the total Chla was contributed by the <10 µm nano- and picophytoplankton in each pond, respectively. In particular, the picophytoplankton contribution amounted 41.2% of thtal Chla in Dianshanhu Pond. Primary production was 2.5 and 3.4 gC m^–2 d^–1 in each pond, respectively, and >50% of production was contributed by picophytoplankton. The mean biomasses of protozoa were 168 µg 1^–1 and 445 µg 1^–1 and those of rotifers were 763 µg 1^–1 and 1186 µg 1^–1 in Dianshanhu and Pingwang Ponds, respectively. The ecological efficiencies expressed in terms of the ratios of primary production to zooplankton production were 0.22 and 0.31, for the two ponds.     

Excretion of photosynthetically fixed organic carbon by metalimnetic phytoplankton

The effects of light intensity, oxygen concentration, and pH on the rates of photosynthesis and net excretion by metalimnetic phytoplankton populations of Little Crooked Lake, Indiana, were studied. Photosynthetic rates increased from 1.42 to 3.14 mg C·mg^–1 chlorophylla·hour^–1 within a range of light intensities from 65 to 150E·m^–2·sec^–1, whereas net excretion remained constant at 0.05 mg C·mg^–1 chlorophylla·hour^–1. Bacteria assimilated approximately 50% of the carbon released by the phytoplankton under these conditions. Excreted carbon (organic compounds either assimilated by bacteria or dissolved in the lake water) was produced by phytoplankton at rates of 0.02–0.15 mg C·mg^–1 chlorophylla·hour^–1. These rates were 6%–13% of the photosynthetic rates of the phytoplankton. Both total excretion of carbon and bacterial assimilation of excreted carbon increased at high light intensities whereas net excretion remained fairly constant. Elevated oxygen concentrations in samples incubated at 150E· m^–2·sec^–1 decreased rates of both photosynthesis and net excretion. The photosynthetic rate increased from 3.0 to 5.0 mg C·mg^–1 chlorophylla· hour^–1 as the pH was raised from 7.5 to 8.8. Net excretion within this range decreased slightly. Calculation of total primary production using a numerical model showed that whereas 225.8 g C·m^–2 was photosynthetically fixed between 12 May and 24 August 1982, a maximum of about 9.3 g C·m^–2 was released extracellularly.     

Seasonal dynamics of rotifer and crustacean zooplankton populations in a eutrophic,monomictic lake with a note on rotifer sampling techniques

The abundances, biomass, and seasonal succession of rotifer and crustacean zooplankton were examined in a man-made, eutrophic lake, Lake Oglethorpe, over a 13 month period. There was an inverse correlation between the abundance of rotifers and crustaceans. Rotifers were most abundant and dominated (>69%) the rotifer-crustacean biomass during summer months (June–September) while crustacean zooplankton dominated during the remainder of the year (>89%). Peak biomasses of crustaceans were observed in the fall (151 µg dry wt l^–1 in October) and spring (89.66 µg dry wt l^–1 in May). Mean annual biomass levels were 46.99 µg dry wt l^–1 for crustaceans and 19.26 µg dry wt l^–1 for rotifers. Trichocerca rousseleti, Polyarthra sp., Keratella cochlearis and Kellicottia bostoniensis were the most abundant rotifers in the lake. Diaptomus siciloides and Daphnia parvula were the most abundant crustaceans. Lake Oglethorpe is distinct in having an unusually high abundance of rotifers (range 217–7980 l^–1). These high densities can be attributed not only to the eutrophic conditions of the lake but also to the detailed sampling methods employed in this study.The research was supported by National Science Foundation grants DEB 7725354 and DEB 8005582 to Dr. K. G. Porter. It is lake Oglethorpe Limnological Association Contribution No. 25 and Contribution No. 371 of the Harbor Branch Foundation, Inc.     

Seasonal variation in phytoplankton primary production in relation to light and nutrients in Lake Awasa,Ethiopia

The biomass and primary production of phytoplankton in Lake Awasa, Ethiopia was measured over a 14 month period, November 1983 to March 1985. The lake had a mean phytoplankton biomass of 34 mg chl a m^–3 (n = 14). The seasonal variation in phytoplankton biomass of the euphotic zone (mg chl a m^–2 h^–1) was muted with a CV (standard deviation/mean) of 31%. The vertical distribution of photosynthetic activity was of a typical pattern for phytoplankton with light inhibition on all but overcast days. The maximum specific rates of photosynthesis or photosynthetic capacity (Ø_max) for the lake approached 19 mg O₂ (mg chl a)^–1 h^–1, with high values during periods of low phytoplankton biomass. Areal rates of photosynthesis ranged between 0.30 to 0.73 g O₂ m^–2 h^–1 and 3.3 to 7.8 g O₂ m^–2 d^–1. The efficiency of utilisation of PhAR incident on the lake surface varied from 2.4 to 4.1 mmol E^–1 with the highest efficiency observed corresponding to the lowest surface radiation. Calculated on a caloric basis, the efficiency ranged between 1.7 and 2.9%. The temporal pattern of primary production by phytoplankton showed limited variability (CV = 21 %).     

Phytoplankton and zooplankton (Cladocera,Copepoda) relationship in the eutrophicated River Danube (Danubialia Hungarica,CXI)

The seasonal variation in primary production, individual numbers, and biomass of phyto- and zooplankton was studied in the River Danube in 1981. The secondary production of two dominant zooplankton species (Bosmina longirostris and Acanthocyclops robustus) was also estimated. In the growing season (April–Sept.) individual numbers dry weights and chlorophyll a contents of phytoplankton ranged between 30–90 × 10⁶ individuals, l^–1, 3–12 mg l^–1, and 50–170 µg l^–1, respectively. Species of Thalassiosiraceae (Bacillariophyta) dominated in the phytoplankton with a subdominance of Chlorococcales in summer. Individual numbers and dry weights of crustacean zooplankton ranged between 1400–6500 individuals m^–3, and 1.2–12 mg m^–3, respectively. The daily mean gross primary production was 970 mg C m^–3 d^–1, and the net production was 660 mg C m^–3 d^–1. Acanthocyclops robustus populations produced 0.2 mg C m^–3 d^–1 as an average, and Bosmina longirostris populations 0.07 mg C m^–3 d^–1. The ecological efficiency between phytoplankton and crustacean zooplankton was 0.03%.     

Species composition and phytoplankton biomass in a tropical African lake (Lake Awassa,Ethiopia)

The species composition and phytoplankton biomass of Lake Awassa, Ethiopia were studied from September 1985 to July 1986 in relation to some limnological features of the lake. During the study period, three phases of thermal stratification were recognized: a period of unstable stratification and near-complete mixing was followed by a stable stratification period and another period of complete mixing. Complete mixing was associated with cooling of air temperature with an influx of cool rain and high rainfall. The underwater light penetration showed a similar pattern over the whole period with the highest in the red, and the lowest in the blue spectral region. Euphotic depth varied between 1.6 and 3.0 meters with the highest measurements corresponding to the stable stratification period. PO₄-P concentrations ranged between 23 and 45 µg l^–1 and NO₃-N concentrations varied between 7 and 14 µg l^–1 during the study period. Both nutrients showed increasing values associated with mixing periods and/or the rainy season.A total of 100 phytoplankton species were identified with 48% of the taxa represented by green algae, 30% by blue-green algae, 11% by diatoms, and the rest by chrysophytes, dinoflagellates, cryptomonads and euglenoids. The dominant phytoplankton species were Lyngbya nyassae, Botryococcus braunii and Microcystis species. Seasonal biomass variation was pronounced in the first two species but not in Mycrocystis. Phytoplankton biomass increased following the mixing period in December, and thermal destratification during May to July which was also a period with high rainfall and relatively high nutrient concentration. While the seasonal variation of the total phytoplankton community in Lake Awassa was relatively low (coefficient of variation < 20%), it was higher in some of the individual component species.     

Phytoplankton ecology of the Lake of Menteith,Scotland

The results discussed in this paper represent the first seasonal ecological study carried out on the phytoplankton of the Lake of Menteith. All measured nutrients reached maximum levels during the winter, with silicate showing particularly high concentrations (up to 85 µg at Si l^–1). During the summer period phosphate, nitrate and silicate showed almost complete exhaustion in surface waters. The lake water was consistently alkaline, never falling below pH 7, while the alkalinity ranged from 20 to 24 mg CaCO₃ l^–1. Generally, the nutrient status of the main inflow had a rapid effect on the water quality of the lake.The region of the lake under investigation showed no thermal stratification at any period of the year, although continuous thermal gradients were recorded in the winter. The continual circulation of the water mass probably prevented oxygen saturation from falling below 77% even following a large phytoplankton bloom and subsequent decomposition.From an examination of net phytoplankton samples the Lake of Menteith could be described as blue-green or blue-green/diatom in nature. From the quantitative study, large pulses of Melosira, Asterionella and Fragilaria were recorded in the spring. The disappearance of the species appears to be related to silicate limitation. The summer growth of Asterionella may have been promoted by a nitrogen source other than nitrate and nitrite, both of which were reduced to critical levels. This alternative source of combined nitrogen may have been contributed by nitrogen-fixing algae in the lake. Three species of Anabaena were recorded, all of which produced large populations during the year.Department of Botany, The University of GlasgowPresent Address: Department of Biology, College of Science, University of Sulaimaniyah, Sulaimaniyah, Iraq     

Planktonic bacterial biomass and seasonal pattern of the heterotrophic uptake and respiration of glucose and amino acids in the shallow sandpit lake of Créteil (Paris Suburb,France)

Biomass and activity of planktonic bacteria were investigated during a one year study in a shallow sandpit lake. The shallowness of the lake helped keep the water column homogeneous regarding bacterioplankton. Small free-living bacteria (0.03 µm³ cell^–1) dominated the populations throughout the period studied. Bacterial abundances varied from 1 to 11 × 10⁶ cells ml^–1. Kinetic parameters (V _max, K + S and T) were determined with ¹⁴C labelled compounds (glucose and amino acids mixture). V _max values were high and averaged 0.056 and 0.050 µgCl^–1 h^–1 for glucose and amino acids respectively. Maximal V _max values were observed in summer at the highest temperatures, but also in early spring. T values were much greater in winter. K + S values were significantly higher for amino acids (3 µg Cl^–1) than for glucose (1 µg Cl^–1). A low percentage of mineralization (about 25% for both tracers) could be the expression of the high growth efficiency expected when bacteria are growing at the expense of low molecular weight compounds as phytoplankton exudates.     

Heterotrophic glucose uptake and respiration in Lake Kinneret

The turnover times of glucose, averaged for 0–10 m in the upper waters of Lake Kinneret and measured by the addition of single or multiple concentrations of substrate, ranged from 23 to 188 hours and 1 to 87 hours respectively. Potential uptake rates (estimated as V_max) ranged from 0.095 to 1.94 µg glucose l^–1h^–1, while measured uptake rates varied from 0.09 to 1.1 µg glucose l^–1h^–1. Concentrations of dissolved carbohydrates and glucose averaged 0.71 mg glucose equivalents l^–1 and 39 µg glucose l^–1 respectively. No evident relationships between glucose cycling and any fractions of dissolved organic matter, phytoplankton biomass or primary productivity were found. Turnover times were generally most rapid immediately after the decline of the spring Peridinium bloom. The respiration percentage of incorporated glucose ranged from 25% to 61% with highest values during the summer months. Respiration may be influenced by the nature of the indigenous bacterial population as well as by temperature. Daily heterotrophic glucose carbon uptake was about 9% of the photosynthetic incorporation and could provide a bacterial yield of about 7 × 10⁴ ml^–1d^–1.