Light, nutrients and primary productivity in Lake Biwa: An evaluation of the current ecosystem situation

Simple correlation and multiple regression analyses were performed to examine the relationship between primary productivity and environmental factors in the north basin of Lake Biwa. The primary production rates used in the analyses were estimated monthly or bimonthly during the growing season (April–November) in 1992, 1996 and 1997 with the ¹³C method. Elemental (C, N and P) contents of seston were used to assess nutrient conditions. Analyses revealed that 86% of variance in depth-integrated primary production rates (areal PP) can be explained by changes in light intensity, and sestonic C, N and P concentrations. Water temperature had no effect on areal PP. To assess relative effects of light and nutrients on PP, the P:B ratio was estimated by normalizing PP with sestonic C. The areal P:B ratio correlated most significantly with the sestonic N:P ratio, followed by light intensity. When regression analyses were made at each depth, however, the P:B ratio correlated significantly only with the sestonic N:P ratio at 0 and 1 m depths, while light intensity was also incorporated into the regressions at deeper than 2.5 m. In these regressions, the P:B ratio was negatively correlated with sestonic N:P ratio but positively with light intensity. The results suggest that the primary production rate in this lake was mainly limited by P relative to N supply rates, but was not free from light limitation in a large part of the epilimnion. In Lake Biwa, the vertical water mixing regime as well as the nutrient supply seem to be important in determining the growth and composition of primary producers, since the surface mixing layer extends into 10–15 m depths during most of the growing season.     

Relative importance of biotic and abiotic factors affecting bacterial abundance in Lake Biwa: an empirical analysis

To determine the relative importance of factors affecting bacterial abundance in Lake Biwa, correlation and multiple regression analyses were performed with relevant biotic and abiotic variables. Data used in the analyses were collected weekly from April 1997 to June 1998 at a pelagic site in the north basin. The bacterial abundance ranged from 1 to 7 × 10⁶ cells ml⁻¹, and its spatio-temporal pattern was virtually identical to that in previous studies conducted 12–15 years ago. In the surface layer (0–12.5 m), bacterial abundance was significantly correlated with water temperature and with protozoan and metazoan grazers, but not with chlorophyll a and nutrient concentrations. The results suggest that loss factors rather than growth factors are more important in determining bacterial abundance in this lake. However, grazing effects on bacterial abundance differed among zooplankton. Bacterial abundance correlated negatively with phagotrophic nanoflagellates (PNF) and Daphnia, but positively with Eodiaptomus. Thus, PNF and Daphnia act to reduce the bacterial abundance, while Eodiaptomus acts to stimulate. In contrast, these biotic factors did not explain changes in bacterial abundance in the middle (12.5–25 m) and deep (>25 m) layers. Instead, the bacterial abundance in the deep layer was highly correlated with vertical mixing regimes, suggesting that bacterial abundance was directly or indirectly affected by abiotic factors. These results indicate that bacterial abundance in Lake Biwa was regulated by different factors at different depths. Received: February 8, 2000 / Accepted: August 29, 2000     

Determination of dissolved and particulate thiols in Lake Biwa water and extracted fulvic acids by solid phase extraction followed by HPLC with fluorescence detection

Thiols are important antioxidants that can modulate the bioavailability and biogeochemistry of many soft metals, although their detection remains challenging in both their reduced (R–S) and oxidized (R–S–S–R) forms. Here, a modified biochemical method was applied to determine the levels of dissolved and particulate thiols in Lake Biwa water and extracted Lake Biwa fulvic acids obtained at various depths. This method involves the use of the reducing agent tris(2-carboxyethyl)phosphine and the fluorescent label 7-fluorobenzofurazan-4-sulfonic acid ammonium salt (SBD-F), followed by solid-phase extraction and HPLC with fluorescence detection. Dissolved cysteine (Cys) (2.0–6.0 nM), glutathione (GSH) (2.8–5.1 nM), and N-acetyl-l-cysteine (NAC) (1.6–4.2 nM) were detected throughout the water column but were broadly consistent at depths of 5–20 m. In contrast, abundant levels of particulate cysteine (1.3–3.5?×?10² nM) and glutathione (1.6–3.1?×?10² nM) were detected down to depths of 15 m. The particulate cysteine and glutathione were significantly covariant, and the ratios between them reflected the differences in the plankton community composition and availability of these compounds. This work also studied the concentrations of Cys, GSH and NAC in Lake Biwa fulvic acids (LBFAs) for the first time (at 0 m: cysteine, 0.8 nM; glutathione, 1.6 nM; NAC, 2.5 nM; at 10 m: cysteine, 1.4 nM; glutathione, 0.6 nM; NAC, 1.6 nM). The nanomolar to sub-nanomolar concentrations of the particulate and dissolved Cys, GSH and NAC in the lake indicates that these are an important class of ligands for chalcophile metals and may influence the distribution of plankton communities from the epilimnion to the hypolimnion of the lake.     

Morphological and phylogenetic characterizations of freshwater Thioploca species from Lake Biwa,Japan, and Lake Constance,Germany

Filamentous, gliding, sulfide-oxidizing bacteria of the genus Thioploca were found on sediments in profundal areas of Lake Biwa, a Japanese freshwater mesotrophic lake, and were characterized morphologically and phylogenetically. The Lake Biwa Thioploca resembled morphologically Thioploca ingrica, a brackish water species from a Danish fjord. The diameters of individual trichomes were 3 to 5.6 microm; the diameters of complete Thioploca filaments ranged from 18 to 75 micro m. The cell lengths ranged from 1.2 to 3.8 micro m. In transmission electron microscope specimens stained with uranyl acetate, dense intracellular particles were found, which did not show any positive signals for phosphorus and sulfur in an X-ray analysis. The 16S rRNA gene of the Thioploca from Lake Biwa was amplified by using newly designed Thioploca-specific primers (706-Thioploca, Biwa160F, and Biwa829R) in combination with general bacterial primers in order to avoid nonspecific amplification of contaminating bacterial DNA. Denaturing gradient gel electrophoresis (DGGE) analysis of the three overlapping PCR products resulted in single DGGE bands, indicating that a single 16S rRNA gene had been amplified. With the same method, the Thioploca from Lake Constance was examined. The 16S rRNA sequence was verified by performing fluorescence in situ hybridization targeted at specific motifs of the Lake Biwa THIOPLOCA: Positive signals were obtained with the bacterial probe EUB-338, the gamma-proteobacterial probe GAM42a, and probe Biwa829 targeting the Lake Biwa THIOPLOCA: Based on the nearly complete 16S rRNA sequence and on morphological similarities, the Thioploca from Lake Biwa and the Thioploca from Lake Constance are closely related to T. ingrica and to each other.     

The production-to-respiration ratio and its implication in Lake Biwa,Japan

Production-to-respiration (P:R) ratio was estimated at an offshore site of Lake Biwa in order to examine whether the plankton and benthic community is subsidized with allochthonous organic carbon, and to clarify the role of this lake as potential source or sink of carbon dioxide. The respiration rate of protozoan and metazoan plankton was calculated from their biomass and empirical equations of oxygen consumption rates, and that of bacterioplankton was derived from their production rate and growth efficiency. In addition, the carbon mineralization rate in the lake sediments was estimated from the accumulation rate of organic carbon, which was determined using a ²¹⁰Pb dating technique. On an annual basis, the sum of respiration rates of heterotrophic plankton was comparable to net primary production rate measured by the ¹³C method. However, when the mineralization rate in the lake sediments was included, the areal P:R ratio was 0.89, suggesting that Lake Biwa is net heterotrophic at the offshore site with the community being subsidized with allochthonous organic carbon. Such a view was supported by the surface water pCO₂ that was on average higher than that of the atmosphere. However, the estimate of net CO₂ release rate was close to that of carbon burial rate in the sediments. The result suggests that the role of Lake Biwa in relation to atmospheric carbon is almost null at the offshore site, although the community is supported partially by organic carbon released from the surrounding areas.     

Morphological and Phylogenetic Characterizations of Freshwater Thioploca Species from Lake Biwa, Japan, and Lake Constance, Germany

Filamentous, gliding, sulfide-oxidizing bacteria of the genus Thioploca were found on sediments in profundal areas of Lake Biwa, a Japanese freshwater mesotrophic lake, and were characterized morphologically and phylogenetically. The Lake Biwa Thioploca resembled morphologically Thioploca ingrica, a brackish water species from a Danish fjord. The diameters of individual trichomes were 3 to 5.6 μm; the diameters of complete Thioploca filaments ranged from 18 to 75 μm. The cell lengths ranged from 1.2 to 3.8 μm. In transmission electron microscope specimens stained with uranyl acetate, dense intracellular particles were found, which did not show any positive signals for phosphorus and sulfur in an X-ray analysis. The 16S rRNA gene of the Thioploca from Lake Biwa was amplified by using newly designed Thioploca-specific primers (706-Thioploca, Biwa160F, and Biwa829R) in combination with general bacterial primers in order to avoid nonspecific amplification of contaminating bacterial DNA. Denaturing gradient gel electrophoresis (DGGE) analysis of the three overlapping PCR products resulted in single DGGE bands, indicating that a single 16S rRNA gene had been amplified. With the same method, the Thioploca from Lake Constance was examined. The 16S rRNA sequence was verified by performing fluorescence in situ hybridization targeted at specific motifs of the Lake Biwa Thioploca. Positive signals were obtained with the bacterial probe EUB-338, the γ-proteobacterial probe GAM42a, and probe Biwa829 targeting the Lake Biwa Thioploca. Based on the nearly complete 16S rRNA sequence and on morphological similarities, the Thioploca from Lake Biwa and the Thioploca from Lake Constance are closely related to T. ingrica and to each other.     

Protozoan growth rates in Antarctic lakes

The growth rates of heterotrophic nanoflagellates (HNAN), mixotrophic cryptophytes, dinoflagellates and ciliates in field assemblages from Ace Lake in the Vestfold Hills (eastern Antarctica) and Lakes Fryxell and Hoare (McMurdo Dry Valleys, western Antarctica), were determined during the austral summers of 1996/1997 and 1997/1998. The response of the nanoflagellates to temperature differed between lakes in eastern and western Antarctica. In Ace Lake the available bacterial food resources had little impact on growth rate, while temperature imposed an impact, whereas in Lake Hoare increased bacterial food resources elicited an increase in growth rate. However, the incorporation of published data from across Antarctica showed that temperature had the greater effect, but that growth is probably controlled by a suite of factors not solely related to bacterial food resources and temperature. Dinoflagellates had relatively high specific growth rates (0.0057–0.384 h⁻¹), which were comparable to Antarctic lake ciliates and to dinoflagellates from warmer, lower latitude locations. Temperature did not appear to impose any significant impact on growth rates. Mixotrophic cryptophytes in Lake Hoare had lower specific growth rates than HNAN (0.0029–0.0059 h⁻¹ and 0.0056–0.0127 h⁻¹, respectively). They showed a marked seasonal variation in growth rate, which was probably related to photosynthetically active radiation under the ice at different depths in the water column. Ciliates' growth rates showed no relationship between food supply and mean cell volume, but did show a response to temperature. Specific growth rates ranged between 0.0033 and 0.150 h⁻¹ for heterotrophic ciliates, 0.0143 h⁻¹ for a mixotrophic Plagiocampa species and 0.0075 h⁻¹ for the entirely autotrophic ciliate, Mesodinium rubrum. The data indicated that the scope for growth among planktonic Protozoa living in oligotrophic, cold extreme lake ecosystems is limited. These organisms are likely to suffer prolonged physiological stress, which may account for the highly variable growth rates seen within and between Antarctic lakes. Accepted: 7 December 1999     

The effects of water depth and density on the growth of a unionid clam*

SUMMARY. 1. Unionid clams from Narrow Lake, Alberta, were collected to quantify the natural variation in growth, to assess the natural variation in abundance, age and size distribution, and growth with water depth in the lake, and to conduct in situ experiments to directly test the effects of water depth (temperature) and clam abundance on clam growth. 2. The unionid clam, Anodonta grandis simpsoniana, showed wide variation in length at a given age. There were no significant differences in growth between clams collected at 1,3, 5, and 7m depths in the lake despite marked differences in water temperature. The wide variation in clam biomass within each depth zone may have masked possible effects of water depth. 3. The effect of water depth and variation in clam density on clam growth was tested directly by stocking clams into small enclosures at densities equivalent to 50, 150, 250, 350 and 450g m-2 (live weight) at each of 1, 3, 5 and 7 m depths in Narrow Lake (each depth and abundance treatment in triplicate). A uniform sandy substrate was used in all enclosures to eliminate any possible effect of substrate type on growth. 4. Mortality was negligible (0.9%) during the experiment. Clam density had no significant effect on clam growth which suggests that clam growth was not food limited in the lake. 5. Clams reared at 7 m grew more slowly than clams reared at 1, 3 and 5 m. Clams reared at 5 m grew more slowly than clams reared at 1 and 3m. Growth of clams reared at 1 and 3m did not differ. These differences in growth were strongly correlated with the measured differences in water temperature between depths. 6. Migration between depths probably accounts for the lack of a depth effect on clams growing in the natural habitat.     

Protistan grazing and viral lysis losses of bacterial carbon production in a large mesotrophic lake (Lake Biwa)

The grazing and lysis mortalities of planktonic bacteria were estimated using the modified dilution method and respiratory quinone (RQ) analysis in mesotrophic Lake Biwa, Japan. The planktonic bacterial assemblages in the lake consisted of various RQ subgroups with different growth and mortality rates. The sum of total bacterial mortalities due to protistan grazing and viral lysis accounted for 96.6 % (range 89.0–107.2 %) of daily total bacterial production. This is the first report that successfully demonstrates a balanced relationship between bacterial production and losses using the modified dilution method in a lake. The growth rates of ubiquinone (UQ)-containing bacteria were faster than those of menaquinone-containing bacteria. Especially the dominant and fastest growing bacterial groups in the present study were the bacterial groups containing UQ-8 or UQ-10. The sum of their production and loss accounted for 60 % of carbon fluxes within the microbial loop. Thus, a large portion of the carbon cycling through the bacterial community in Lake Biwa can be explained by the carbon fluxes through dominant bacterial groups.     

Major contribution of sulfide-derived sulfur to the benthic food web in a large freshwater lake

In freshwater systems, contributions of chemosynthetic products by sulfur-oxidizing bacteria in sediments as nutritional resources in benthic food webs remain unclear, even though chemosynthetic products might be an important nutritional resource for benthic food webs in deep-sea hydrothermal vents and shallow marine systems. To study geochemical aspects of this trophic pathway, we sampled sediment cores and benthic animals at two sites (90 and 50 m water depths) in the largest freshwater (mesotrophic) lake in Japan: Lake Biwa. Stable carbon, nitrogen, and sulfur isotopes of the sediments and animals were measured to elucidate the sulfur nutritional resources for the benthic food web precisely by calculating the contributions of the incorporation of sulfide-derived sulfur to the biomass and of the biogeochemical sulfur cycle supporting the sulfur nutritional resource. The recovered sediment cores showed increases in ³⁴S-depleted sulfide at 5 cm sediment depth and showed low sulfide concentration with high δ³⁴S in deeper layers, suggesting an association of microbial activities with sulfate reduction and sulfide oxidation in the sediments. The sulfur-oxidizing bacteria may contribute to benthic animal biomass. Calculations based on the biomass, sulfur content, and contribution to sulfide-derived sulfur of each animal comprising the benthic food web revealed that 58%–67% of the total biomass sulfur in the benthic food web of Lake Biwa is occupied by sulfide-derived sulfur. Such a large contribution implies that the chemosynthetic products of sulfur-oxidizing bacteria are important nutritional resources supporting benthic food webs in the lake ecosystems, at least in terms of sulfur. The results present a new trophic pathway for sulfur that has been overlooked in lake ecosystems with low-sulfate concentrations.     

Vertical and seasonal variations of bacterioplankton subgroups with different nucleic Acid contents: possible regulation by phosphorus

We used flow cytometry to examine seasonal variations in basin-scale distributions of bacterioplankton in Lake Biwa, Japan, a large mesotrophic freshwater lake with an oxygenated hypolimnion. The bacterial communities were divided into three subgroups: bacteria with very high nucleic acid contents (VHNA bacteria), bacteria with high nucleic acid contents (HNA bacteria), and bacteria with low nucleic acid contents (LNA bacteria). During the thermal stratification period, the relative abundance of VHNA bacteria (%VHNA) increased with depth, while the reverse trend was evident for LNA bacteria. Seasonally, the %VHNA was strongly positively correlated (r = 0.87; P < 0.001) with the concentration of dissolved inorganic phosphorus, but not with the concentration of chlorophyll a. The growth of VHNA bacteria was significantly enhanced by addition of phosphate or phosphate plus glucose but not by addition of glucose alone. Although the growth of VHNA and HNA bacteria generally exceeded that of LNA bacteria, our data also revealed that LNA bacteria grew faster than and were grazed as fast as VHNA bacteria in late August, when nutrient limitation was presumably severe. Based on these results, we hypothesize that in severely P-limited environments such as Lake Biwa, P limitation exerts more severe constraints on the growth of bacterial groups with higher nucleic acid contents, which allows LNA bacteria to be competitive and become an important component of the microbial loop.     

Lake Biwa and the ocean: geochemical similarity and difference

The average composition of water, bottom sediments, manganese (Mn) crusts, and Mn concretions from Lake Biwa (the largest freshwater lake in Japan) are re-examined, in conjunction with those of seawater, oceanic pelagic clay, and deep-sea Mn nodules. The purpose is to gain additional insights into the geochemical behaviors of elements in Lake Biwa and the ocean, which are quite different in ionic strength (or salinity), pH, water residence times, sediment accumulation rates, carbon fluxes to sediments, and the redox potential in sediments. Excluding a few millimeters of oxic surface sediment, there is no appreciable accumulation of Mn in the Lake Biwa bottom sediments due to reducing condition there. Consequently, other B-type cations [such as iron (Fe), gallium (Ga), copper (Cu), lead (Pb), cobalt (Co), tin (Sn), and bismuth (Bi), with subshell valence electron configuration of d ¹⁻¹⁰] are also less concentrated in the lake sediments than in the oceanic pelagic clay. In turn, B-type cations have much higher dissolved concentrations in the lake water than in the ocean. The rare earth elements (REE) mainly form organic complexes in the lake water and carbonate complexes in the ocean. REE are mostly associated with detritus aluminosilicate phases in Lake Biwa sediments but with phosphate phases in deep-sea sediments. Fe and Mn oxide phases are clearly separated in marine Mn nodules and crusts but not in Mn crusts and concretions from Lake Biwa. Useful parameters such as the enrichment factor (E _Al) and logarithms of the distribution coefficient (log K _d) of elements between solid and liquid phases were estimated in both systems for further discussions.     

Methane production in meromictic Ace Lake,Antarctica

Methane occurred in the monimolimnion, at depths greater than 11 m, of an antarctic meromictic lake, Ace Lake (depth 24.7 m). Although the water of the lake was of approximate marine salinity, bottom waters were depleted in sulfate (less than 1 mmol 1^–1). The temperature of the bottom waters of the lake were constantly between 1 °C and 2 °C. Rates of methanogenesis from ¹⁴C-labelled precursors (bicarbonate, formate and acetate) were determined in time course experiments with the detection of ¹⁴CH₄ produced by a gas chromatography-gas proportional counting system. Rates of ¹⁴CH₄ production were difficult to determine as the reactions were always near our limit of detection.Reliable determinations of rates of methanogenesis at some depths using some precursors were obtained, the fastest rate being 2.5 µmol kg^–1 day^–1 at depth 20 m. Assuming constant rates of methanogenesis with time, this would equate to a turnover of methane in the lake every two years.The slow rate of methanogenesis suggests that the methanogens in Ace Lake may be working at well below their optimum temperature although definitive statements regarding the presence of psychrophilic methanogens in this antarctic lake must await isolation attempts or longer field studies using alternative methodologies.     

The effect of low water levels on the water quality of Lake Biwa

Because of a lack of precipitation, water levels in Lake Biwa, Japan, were extremely low between the beginning of September 1984 and the end of February 1985. Approximately 13 million people depend upon the lake as a source of drinking water and for industrial use, and the severe water shortage became a serious concern for downstream communities. Also, there was concern that deterioration of water quality caused by rotting macrophytes and the release of nutrients from vegetation and nearshore sediments might create additional problems.In this paper, the release of nutrients from vegetation and sediments is examined under conditions which simulate both calm and turbulent water motions in the nearshore, and the magnitude of nutrient loadings are estimated in relation to the specific effects of low lake level.Sample stations were established around the south shore of Lake Biwa. Sampling was undertaken at the time of low water and during the rising water levels. Sediment samples were particle sized into 7 groups (<2000 µm). Other measured values ranged as follows: BOD (0.5–1.3), COD (1.2–3.5), TP (0.019–0.037), SRP (0.013–0.030), SOP (0.005–0.007), TN (0.45–0.90), NO₂-N (0.004–0.007), NO₃-N (0.04–0.08) and NH₄-N (0.026–0.053), all as mgL^-1. The sample data suggest that, overall, there was little impact on lake water quality as a result of low water levels. However, remedial actions may have had an important and beneficial impact on nearshore water quality in the southern basin of Lake Biwa.     

Influence of the Hydrological Cycle on the Bacterioplankton of an Impacted Clear Water Amazonian Lake

Abstract Free-living and attached bacterial population sizes were determined fortnightly from December 1991 to December 1992 in natural and disturbed areas of an Amazonian clear water lake (Batata Lake, Pará, Brazil) impacted by bauxite tailings. The bacterioplankton showed distinct patterns during different phases of the hydrological cycle. Total bacterial population size and rates of thymidine incorporation (measured during high and low water phases) were high during low water, with values ranging from 3.3 × 10⁵ to 1.1 × 10⁶ cells ml⁻¹, and from 0.28 to 4.01 μg C l⁻¹ h⁻¹, respectively. The population size of free-living bacteria was larger at the natural station, while no differences were observed between attached bacterial populations at both stations. However, production and turnover rate of attached bacteria were high at the disturbed area. During low water, bacterial growth appeared to be driven mainly by the input of dissolved organic carbon (DOC) from phytoplankton origin. During high water, bacterial abundance was reduced, probably as the result of dilution and the input of less labile DOC from floodplains. The presence of bauxite tailings seems to influence bacterial dynamics in an indirect way, probably due to shading of phytoplankton cells and, hence, reducing the DOC supply for bacterial growth. This study, the first on the microbial ecology of an Amazonian clear water lake, demonstrated that water level variations exert a strong influence on the bacterioplankton dynamics. Received: 9 January 1996; Accepted 6 November 1996     

The physical factor inducing spawning of the Biwa catfish, Silurus biwaensis

The physical factor inducing spawning of the Biwa catfish, Silurus biwaensis (Siluriformes: Siluridae), an endemic species of Lake Biwa, was investigated on the basis of field observations on 150 nights at the spawning site, the rocky shore of the Seta River, the lake's outlet, and environmental data from April to July in 1989–1994. The catfish gathered at the spawning site when the water level of the river rapidly rose and the rocky shoal became submerged, within a wide range of water temperature (15.0°–30.0°C); their spawning was induced in relation to increases in the submerged portion of the rocky shoal. This observation suggests that water level control operations in Lake Biwa might critically affect the spawning of this and other fish species. Received: December 17, 1999 / Revised: September 13, 2000 / Accepted: January 16, 2001     

Trematode fauna of prosobranch snails of the genus Semisulcospira in Lake Biwa and the connected drainage system

The parasite fauna of prosobranch snails of the genus Semisulcospira was surveyed in Lake Biwa and the adjacent water system. One aspidogastrean and 28 digenetic trematode taxa were detected in 19209 snails consisting of 10 morphological species. There was no trematode species peculiar to members of the subgenus Biwamelania that is endemic to the Lake Biwa water system. However, one species, Notocotylus magniovatus, was found only in the non-endemic subgenus Semisulcospira. Of 23 digenean taxa detected in more than one host, 13 were distributed in both the lake and the tributaries. Seven of these had host taxa, more than 1% of which were infected with the parasite in both the lake and the tributaries, four had such hosts only in the tributaries, and two had no such hosts. Three species detected only in Lake Biwa were previously reported from other rivers in Japan. In the seven species detected only in the tributaries, two species had life cycles that could be maintained only in rivers. These results indicate that the core areas for the distribution of parasites of Semisulcospira are tributaries, and the lake is a sink for these species. These results contradict the expectation that the parasite fauna should be richer in the lake than in tributaries because the lake is a stable habitat over a geological time scale and has more divergent freshwater animals than the adjacent water system.     

Composition of photosynthetic products in Lake Biwa, Japan; vertical and seasonal changes and their relation to environmental factors

The seasonal and vertical changes in photosynthetic productswere determined in Lake Biwa, which is the largest lake in Japan,by combined ¹³C and gas chromatography-mass spectrometry (¹³C-GC-MS).The temperature dependence of the production rates differedamong organic compounds. The seasonal change in the proteinspecific production rate (SPR) showed a strong temperature dependence,indicating that photoplankton growth rate was primarily governedby water temperature. In August and September, low protein SPRwas, however, observed, probably due to low inorganic nitrogenconcentration. Nitrogen deficiency in the late stagnation periodwas also suggested by high C:N ratios both in paniculate matterand photosynthetic products. The glucose SPR did not show anysignificant correlation with water temperature, irradiance andinorganic nitrogen concentration. Highly variable glucose SPRmight be due to high turnover of glucose, and vertical mixingin the mixed surface layer may be one of the most importantfactors causing variability of glucose SPR.     

Nutrient enrichment and nutrient regeneration stimulate bacterioplankton growth

Bacterial abundance results from predatory losses of individuals and replacement of losses through growth. Growth depends on sustained input of organic substrates and mineral nutrients. In this work we tested the hypothesis that bacterial growth in two oligotrophic Canadian shield lakes was limited by nitrogen (N) or phosphorus (P). We also determined whether consumer-regenerated resources contributed substantially to net bacterial growth. Two types of dilution assays were conducted to determine the response of bacteria to nutrient enrichment: diluted whole water (DWW, 1:9 whole/filtered with 0.2 m of filtered lake water) and diluted fractionated water (DFW, 1.0 m prefiltered then diluted as above). Replicate bottles in each dilution assay received either N (50 m), P (10 m), or both N and P enrichments. Controls received no nutrients. Resource-saturated growth rates and grazing rates were estimated from a standard dilution-growth approach. Bacterial growth was stimulated by addition of P alone and in combination with N. Consumers regenerated sufficient resources to support up to half the bacterial growth rate, but the benefit derived from consumers was minor when compared to mortality.     

Paddy herbicide inputs in the entire river inflow reaching Lake Biwa, Japan

This study estimated the inputs of four paddy herbicides in the entire river inflow reaching Lake Biwa, the largest lake in Japan, which serves as a water resource for 14 million people. The Uso River and the Hino River, the main contaminated rivers among the inflow rivers, were selected as daily and hourly monitoring sites to provide data on the seasonal trends in the concentration and load of herbicides and to determine the effect of rainfall events on load. The monitoring was also performed four times in 15 inflow rivers. The total input to the lake was calculated from the loads during fine weather conditions and additional loads during rainfall events. The former based on the lumped load from the two rivers and by prorating for the 15 rivers, and the latter was estimated from the relation between precipitation and increased load rate. The annual losses of herbicide from the basin to Lake Biwa were estimated to be 14.5% for bromobutide, 3.0% for pretilachlor, 5.2% for molinate, and 8.8% for simetryn. The loads caused by rainfall events accounted for 9%–18% of the total annual loads.