Effects of zooplankton on the sinking flux of organic carbon in Lake Biwa

To clarify the roles of zooplankton in the sedimentation of seston from the epilimnion, the sinking flux of particulate carbon was measured along with primary production rate and zooplankton biomass from July 1996 to October 1997 at a pelagic site in the north basin of Lake Biwa. During the study period, the flux varied seasonally from 66 to 510 mg C m⁻² day⁻¹ and was low in summer when zooplankton, composed mainly of Eodiaptomus japonicus and Daphnia galeata, were abundant. Simple correlation analysis revealed that the sinking flux correlated neither with the primary production rate nor with the amount of sestonic carbon above the sediment trap. However, the particle elimination rate, estimated as the difference between the primary production rate and the sinking flux, correlated positively with the zooplankton biomass. These results suggest that zooplankton play a substantial role in decreasing the sinking flux in Lake Biwa. Received: March 6, 2000 / Accepted: October 7, 2000     

Plankton and primary productivity of Lake Manzala,Egypt

Primary production and distribution and abundance of phyto- and zooplankton of lake Manzala were investigated from June 1985 to June 1986.Primary production varied from 4.1 to 28.7 g O₂ m^–2 d^–1 with the highest values 24.2 and 28.7 g O₂ m^–2 d^–1 recorded in the eastern and southern sectors and the lowest value 4.1 g O₂ m^–2 d^–1 recorded in the northern sector. The seasonal mean production for the lake was estimated at 13.3 g O₂ m^–2 d^–1.Mean phytoplankton abundance ranged from 32.7 10⁷ to 76.1 10⁷ cells m^–3 with a mean value of 48.10⁷ cells m^–3. Diatoms were the dominant phytoplankton group comprising 52 to 90 % by number. The greatest relative abundance (87 to 90%) was recorded in the southern sector.Mean zooplankton abundance ranged from 30.1 10³ to 44.4 10³ organisms m^–3 in the eastern sector to 5.5.10³ in the northern sector. In response of eutrophication, the species composition changed significantly over the last 20 years. Cladocerans represented less than 1% of zooplankton during 1959/60, but 75% in 1985/86. Rotifers constituted 40% in 1959/60, and only 1% in 1985/86. Cirriped larvae declined from 21% to 1%.     

Spatial distribution and seasonal changes of pesticides in Lake Biwa,Japan

The spatial distribution and seasonal variation in the concentrations in Lake Biwa of pesticides used in paddy fields were studied. Lake Biwa is the largest lake in Japan and is a recognized water resource for 14 million people in the Kinki district. Samples were collected nine times from April to December 2001 at ten sites within the lake and at the mouths of six influent rivers. Weekly sampling was also carried out at a single site on an effluent river. Among the 20 pesticides analyzed, the detection frequencies in surface water were almost 100% for simetryn, bromobutide, and isoprothiolane; around 75% for molinate and pyroquilon; around 30% for three herbicides and one fungicide; and almost zero for the remaining substances. The maximum concentrations of pesticides detected frequently in the lake were in the range 0.1–0.4µgl^–1. The occurrence of a few pesticides below the thermocline may be explained by thermal stratification and vertical circulation. Although the thermocline suppressed vertical diffusion in spring and summer during pesticide application periods, a few pesticides remaining at the surface of the lake in winter were transported to the hypolimnion by vertical circulation and remained there even after the reestablishment of the thermocline. The half-lives of pesticides in the lake were estimated to be more than a year for simetryn, half a year for bromobutide, 1.5 months for molinate, and 1 month for dimepiperate. The main cause of elimination for molinate and dimepiperate was estimated to be degradation, that for simetryn was outflow, and for bromobutide both degradation and outflow were significant.     

Annual cycle of circulations in Lake Biwa, part 1: model validation

We perform a series of model studies of the physical environment in Lake Biwa. In part 1, provided here, we formulate and validate a numerical model which consists of three-dimensional momentum and continuity equations for a Boussinesq fluid under the hydrostatic approximation in a rotating frame, the advective–diffusive equation for water temperature, and the full nonlinear equation of state for freshwater. Water motion is driven by heat flux and wind forcing which are evaluated from the time series of meteorological parameters observed at weather stations around and on the lake using bulk formulae. This model for the first time accurately simulates the annual and interannual variations found in observed water temperature and velocity fields in the 2004–2006 hindcast experiment. A current system of cyclonic (counterclockwise) and anticyclonic (clockwise) circular gyres appears above the seasonal thermocline during a stratified season, and a bottom flow appears along the eastern slope to sink cold dense water from the shallow eastern region to the deep lake bottom during an unstratified season. Full overturning of the water column during winter causes bottom water temperature to change interannually depending on the severity of winter. These successful results endorse the benefits of the model when investigating the physical environment of the lake over longer timescales.     

Annual cycle of circulations in Lake Biwa, part 2: mechanisms

We investigate the mechanisms of circulations in Lake Biwa using model results that are driven by the momentum and heat fluxes evaluated from meteorological parameters observed around and on the lake. The kinetic energy budget shows that the wind stress over the lake, which has positive (negative) curl over the northern (southern) region of the lake throughout the year, generates the current system of multiple gyres in the stratified season (May–January), while the thermal forcing makes a minor contribution. This mechanism is quite different from the previous picture, in which the thermal forcing was dominant in forming the gyre system during the heating season. In the cooling season, the overturning of the lake occurs not in a simple one-dimensional (vertical) process, but in a time-dependent three-dimensional one. Cold water that is formed in the shallow eastern region descends the bottom slope to the deep lake in December to March while such a descent hardly occurs in the western region. A large shallow area (small heat capacity) as well as intense cooling in the eastern region effectively produces cold water. Wind again plays an important role by intensifying the cooling through its higher speed and by driving slope current. Further, the fluctuating wind direction causes significantly time-dependent behaviors of the slope current or bottom water formation.     

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.     

Seasonal and vertical variation in the C : P ratio of suspended and settling seston of lakes

In the absence of field data it is often assumed that the C : P ratio of the seston in lakes corresponds to the ideal ratio of 106 : 1, reported by Redfield et al. (1963) for marine phytoplankton. However, we found out that in nearly all lakes studied C : P ratios deviate systematically from this value especially during periods, when phosphorus is growth limiting. Thus, during summer the sestonic C : P ratio generally exceeds 106 in the epilimnion, but decreases with depth in the hypolimnion. Possible mechanisms which may lead to these seasonal and spatial changes in the C : P ratio are discussed.     

Echosounding observations of coverage,height, PVI,and biomass of submerged macrophytes in the southern basin of Lake Biwa,Japan

We have developed a procedure to process echosounding data to map the distribution of submerged aquatic macrophytes in the southern basin of Lake Biwa, a water body that has a surface area of 52 km² and a mean depth of 4 m. Echosounding observations were made along 27 transect lines spaced at 500-m intervals on August 4 and September 2 and 30, 2003. Quantitative vegetation data including percent coverage, mean vegetation height, and percent vegetation infestation were directly determined using image data from the echosounder recorded digitally on videotape. Based on the image data from an echosounder, a regression model was developed for estimating biomass of submerged macrophytes. The regression model using the total echo strength as the explanatory variable could reliably estimate macrophyte biomass up to 300 g m⁻². Distribution maps of macrophyte height and biomass suggest that the recent summer decline of submerged macrophytes started earlier in shallow areas (<3 m of depth) than deep areas (>4 m) in the southern basin of Lake Biwa.     

Distribution of dissolved organic carbon and dissolved fulvic acid in mesotrophic Lake Biwa, Japan

The dissolved organic carbon (DOC) concentrations in mesotrophic Lake Biwa were determined by a total organic carbon (TOC) analyzer, and DOC molecular size distributions were determined by size exclusion chromatography (SEC) using a fluorescence detector at excitation/emission (Ex/Em) levels of 300/425 nm with the eluent at pH 9.7. The fluorescence wavelengths for detection were chosen from the result of excitation–emission matrix spectrometry (EEM) analysis for dissolved fulvic acid (DFA) extracted from Ado River (peak A, Ex/Em = 260–270/430–440 nm; peak B, Ex/Em = 300–310/420–430 nm). Ado River DFA was eluted with a retention time (RT) of 7.4–8.9 min and the apparent molecular weight was estimated at 22–87 kDa based on the elution curve for the spherical protein molecular weight standard. A DFA peak eluted at the same retention time as Ado River DFA also appeared in all the samples of Lake Biwa water. From the linear relationship between the peak areas with an RT of 7.4–8.9 min by SEC analysis and DOC values of DFA by TOC analysis of a series of DFA samples (r² = 0.9995), the concentrations of DFA in the lake water were roughly calculated. DFA was distributed within the range 0.25–0.43 mg C l⁻¹ and accounted for 15%–41% of DOC, with the highest ratios observed at a depth of 70 m in August and the lowest at 2.5 m in May.     

Horizontal distribution of carbon and nitrogen and their isotopic compositions in the surface sediment of Lake Biwa

The horizontal distribution of the abundance and isotopic composition of carbon and nitrogen was studied on surface sediment samples (0–15 cm) collected from the entire area of Lake Biwa, the largest freshwater lake in Japan. As water depth increased, a marked increase in organic matter content was observed at the sampling sites, especially in the western North Basin, characterized by a steep slope. In the northwestern North Basin, which has no major inflowing streams, the sediments contained large amounts of organic matter, suggesting the possibility of lateral transportation of sedimented matter from other places by lake currents. The total amounts of carbon and nitrogen in the top-2 cm of sediment of the entire area of Lake Biwa were estimated to be 9.2 × 10⁴ tC and 1.0 × 10⁴ tN. The δ¹⁵N values in the littoral sediment were low and close to those in the inflowing river sediment, suggesting selective sedimentation of allochthonous organic matter onto the littoral area. In the North Basin, vertical profiles of organic matter content and δ¹³C values of the sediments in the littoral area showed a smaller downward decrease than in the profundal area, whereas δ¹⁵N values decreased with sediment depth in both areas. It was suggested that the littoral sediments contained abundant amounts of allochthonous and relatively refractory organic matter. Further, it was suggested that the autochthonous organic matter originated from primary production deposited mainly on the profundal zone and was easily decomposed in early diagenesis after sedimentation. Received: July 30, 1999 / Accepted: December 10, 1999     

The C:N:P ratio of phytoplankton determines the relative amounts of dissolved inorganic nitrogen and phosphorus released during aerobic decomposition

Three phytoplankton assemblages, with different C:N:P ratios of 314:55:1, 103:17:1, and 57:5.5:1 (by weight), were prepared by growing lake phytoplankton in artificial media with different N:P supply ratios and then decomposed under aerobic conditions for three months.There was no net release of dissolved inorganic phosphorus (DIP) from the phytoplankton assemblage with the highest C:N:P ratio, in contrast with an abundant liberation of DIP from that with the lowest C:N:P ratio. On the other hand, there was an abundant release of dissolved inorganic nitrogen (DIN) from the phytoplankton assemblage with the highest C:N:P ratio, in contrast with little or virtually no release of DIN from those with lower C:N:P ratios. Thus, it was concluded that the C:N:P ratio of phytoplankton is an important parameter to determine the relative amounts of DIP and DIN released, when they are decomposed under aerobic conditions.Contribution from Otsu Hydrobiological Station, Kyoto University, No. 316 (Foreign Language Series).Contribution from Otsu Hydrobiological Station, Kyoto University, No. 316 (Foreign Language Series).     

Effects of zooplankton on nutrient availability and seston C : N : P stoichiometry in inshore waters of Lake Biwa, Japan

Forty-eight-hour experimental manipulations of zooplankton biomass were performed to examine the potential effects of zooplankton on nutrient availability and phytoplankton biomass (as measured by seston concentration) and C : N : P stoichiometry in eutrophic nearshore waters of Lake Biwa, Japan. Increasing zooplankton, both mixed-species communities and Daphnia alone, consistently reduced seston concentration, indicating that nearshore phytoplankton were generally edible. The zooplankton clearance rates of inshore phytoplankton were similar to rates measured previously for offshore phytoplankton. Increased zooplankton biomass led to increased concentrations of nutrients (NH₄-N, soluble reactive phosphorus [SRP]). Net release rates were higher than those found in previous measurements made offshore, reflecting the nutrient-rich nature of inshore seston. Zooplankton nutrient recycling consistently decreased TIN : SRP ratios (TIN = NH₄ + NO₃ + NO₂). This effect probably resulted from the low N : P ratios of nearshore seston, which were lower than those commonly found in crustacean zooplankton and thus resulted in low retention efficiency of P (relative to N) by the zooplankton. Thus, zooplankton grazing inshore may ameliorate algal blooms due to direct consumption but tends to create nutrient supply conditions with low N : P, potentially favoring cyanobacteria. In comparison with previous findings for offshore, it appears that potential zooplankton effects on phytoplankton and nutrient dynamics differ qualitatively in inshore and offshore regions of Lake Biwa. Received: September 4, 2000 / Accepted: January 23, 2001     

Effects of temperature, and availability of nitrogen and phosphorus on the abundance of Anabaena and Microcystis in Lake Biwa, Japan: an experimental approach

Under optimal nutrient conditions, both Microcystis sp. and Anabaena sp. isolated from Lake Biwa grew optimally at 28–32°C but differed in maximal growth rates, phosphate uptake kinetics, maximal phosphorus quotas, and growth responses to nitrogen and phosphorus limitation. The maximal growth rates of Microcystis and Anabaena were 1.6 and 1.25 divisions day⁻¹, respectively. With phosphate and nitrate in the growth-limiting range, the growth of Microcystis was optimal at an N : P ratio of 100 : 1 (by weight) and declined at lower (nitrogen limitation) and higher (phosphorus limitation) ratios. In contrast, Anabaena growth rates did not change at N : P ratios from 1000 : 1 to 10 : 1. Starting with cells containing the maximal phosphorus quota, Microcystis growth in minus-phosphorus medium ceased in 7–9 days, compared with 12–13 days for Anabaena. The phosphate turnover time in cultures starved to their minimum cell quotas was 7.9 min for Microcystis and 0.6 min for Anabaena. Microcystis had a higher K _s (0.12 μg P l⁻¹ 10⁻⁶ cells) and lower V _max (9.63 μg P l⁻¹ h⁻¹ 10⁻⁶ cells), than Anabaena (K _s 0.02 μg P l⁻¹ h⁻¹ 10⁻⁶ cells; V _max 46.25 63 μg P l⁻¹ h⁻¹ 10⁻⁶ cells), suggesting that Microcystis would not be able to grow well in phosphorus-limited waters. We conclude that in spite of the higher growth rate under ideal conditions, Microcystis does not usually bloom in the North Basin because of low availability of phosphorus and nitrogen. Although Anabaena has an efficient phosphorus-uptake system, its main strategy for growth in low-phosphorus environments may depend on storage of phosphorus during periods of abundant phosphorus supply, which are rare in the North Basin. Received: July 31, 2000 / Accepted: October 18, 2000     

Thermal stratification, nutrient dynamics, and phytoplankton productivity during the onset of spring phytoplankton growth in Lake Baikal, Russia

Lake Baikal, Russian Siberia, was sampled in July 1990 during the period of spring mixing and initiation of thermal stratification. Vertical profiles of temperature, dissolved nutrients (nitrate and soluble reactive phosphorus), phytoplankton biomass, and primary productivity were determined in an eleven-station transect encompassing the entire 636 km length of the lake. Pronounced horizontal variability in hydrodynamic conditions was observed, with the southern region of the lake being strongly thermally stratified while the middle and north basins were largely isothermal through July. The extent of depletion of surface water nutrients, and the magnitude of phytoplankton biomass and productivity, were found to be strongly correlated with the degree of thermal stratification. Horizontal differences likely reflected the contribution of two important factors: variation in the timing of ice-out in different parts of the lake (driving large-scale patterns of thermal stratification and other limnological properties) and localized effects of river inflows that may contribute to the preliminary stabilization of the water column in the face of intense turbulent spring mixing (driving meso-scale patterns). Examination of the relationships between surface water inorganic N and P depletion suggested that during the spring and early summer, phytoplankton growth in unstratified portions of the lake was largely unconstrained by nutrient supplies. As summer progressed, the importance of co-limitation by both N and P became more apparent. Uptake and regeneration rates, measured directly using the stable isotope ¹⁵N, revealed that phytoplankton in stratified portions of the lake relied primarily on NH₄ as their N source. Rates of NH₄ regeneration were in approximate equilibrium with uptake; both processes were dominated by organisms <2 µm. This pattern is similar to that observed for oligotrophic marine systems. Our study underscores the importance of hydrodynamic conditions in influencing patterns of biological productivity and nutrient dynamics that occur in Lake Baikal during its brief growing season.     

Phytoplankton production and growth rate in Lake Tanganyika: evidence of a decline in primary productivity in recent decades

1. This study focused on phytoplankton production in Lake Tanganyika. We provide new estimates of daily and annual primary production, as well as growth rates of phytoplankton, and we compare them with values published in former studies.
2. Chlorophyll- a (chl- a ) in the mixed layer ranged from 5 to 120 mg chl- a  m⁻² and varied significantly between rainy and dry seasons. Particulate organic carbon concentrations were significantly higher in the south basin (with 196 and 166 mg C m⁻³ in the dry and the rainy season, respectively) than in the north basin (112 and 109 mg C m⁻³, respectively).
3. Carbon : phosphorus (C : P) ratios varied according to season. Phosphorus limitation seemed to occur more frequently than nitrogen limitation, especially during the rainy season. Severe P deficiencies were rare.
4. Measured particulate daily primary production ranged from 110 to 1410 mg C m⁻² day⁻¹; seasonal contrasts were well marked in the north basin, but less in the south basin, where primary production peaks occurred also in the rainy season. Estimates of annual primary production, based on daily primary production calculated from chl- a and water transparency, gave values lower than those reported in previous studies. Picophytoplankton accounted on average for 56% of total particulate production in the south basin during the wet season of 2003.
5. Phytoplankton growth rates, calculated from primary production, ranged from 0.055 to 0.282 day⁻¹; these are lower than previously published values for Lake Tanganyika.     

Primary productivity and nutrients in the sediment retention basin of Lake Monroe

Lake Monroe is the largest body of water in Indiana with a daily mean productivity of 220 mg · C · m⁻² · day⁻¹ in an observed range from 26 to 714 m · C · M⁻² day⁻¹. It is a medium soft reservoir; the acid combining capacity varies from 0.28 to 0.71 meq · l⁻¹ with a mean slightly above 0.5 meq · l⁻¹. The results of diurnal changes in major nutrients, the C, N, and P ratios, and bioassay experiments indicated that phosphorus is the major limiting nutrient on algal photosynthesis in this lake. Surface photo-inhibition may be used to indicate the sufficiency of light for the species of algae in the water. The low productivity in December, January, February, and early March can be attributed to light limitation due to low water transparency. Contribution no. 312 of the Great Lakes Research Division, University of Michigan. Contribution no. 312 of the Great Lakes Research Division, University of Michigan.     

Seasonal variation in biomass and photosynthetic activity of epilithic algae on a rock at the upper littoral area in the north basin of Lake Biwa, Japan

Vertical distributions of biomass and the photosynthetic activity of epilithic algae were observed seasonally with regard to their physical and chemical parameters along a rocky shoreline of the upper littoral area in the north basin of Lake Biwa. Chlorophyll a amounts in epilithic algae showed a marked change with depth from spring to early summer. The algal biomass showed higher values on the lower part of a large rock in early spring, and gradually increased on the upper part as a result of the growth of Spirogyra sp., a filamentous Chlorophyceae. This growth seems to be associated with a rise in water temperature, with the biomass at the upper depths seeming to be affected by a hydraulic disturbance. The standing crop of epilithic algae showed a maximum value in June and declined to a minimum in August because of the corresponding proliferation or disappearance of Spirogyra as well as changes in the water level. The maximum value of the assimilation number calculated from the potential photosynthetic rate of epilithic algae was obtained at the lower depths in June when Spirogyra grew vigorously. Appreciably high assimilation numbers were also observed in summer and fall, although chlorophyll a amounts were low. The present study is the first in a series to clarify the contribution of epilithic algae on rocky areas to productivity throughout the entire littoral ecosystem.     

Trend,seasonality, cycle,and irregular fluctuations in primary productivity at Lake Tahoe,California-Nevada,USA

Primary productivity has been measured routinely at Lake Tahoe since 1967, and a number of mechanisms underlying variability in the productivity record have now been identified. A long-term trend due to nutrient loading dominates the series. Seasonality also is prominent, apparently controlled by direct physical factors unrelated to the trophic cascade. A 3-yr cycle has been detected and several possible mechanisms are considered. Irregular fluctuations also are present, caused in part by isolated events (a forest fire) and recurring but variable phenomena (spring mixing). Except possibly for the 3-yr cycle, the known sources of variability appear to operate bottom-up through direct physical and chemical effects on the phytoplankton.     

Flow cytometric assessment of cytotype distributions within local populations of Phragmites australis (Poaceae) around Lake Biwa,the largest lake in Japan

Phragmites australis (Poaceae) is a clonal perennial that is an important component of wetland ecosystems worldwide. Using flow cytometry, we examined the cytotype distributions within five populations of P. australis located in the vicinity of Lake Biwa. As in previous reports, two ploidy levels, octoploid (2n = 8x = 96) and decaploid (2n = 10x = 120), were identified, which are assumed to be the main cytotypes around the lake. The coexistence of two cytotypes was detected in four of the five populations, suggesting the relatively common occurrence of mixed ploidy levels in the populations around Lake Biwa. Although intermingled cytotype distributions were observed in some populations, the 9x cytotype, that is, the expected outcome of inter‐cytotype crosses, was not observed, indicating limited gene flow between the two cytotypes.     

Characterization of suspended solids in Lake Biwa by measuring their elemental composition of Al,Si, P,S, K,Ca, Ti,Mn, and Fe

We investigated the concentrations of particulate Al, Si, P, S, K, Ca, Ti, Mn, and Fe at various areas of Lake Biwa, such as the limnetic zone in the northern basin, the offshore zone, the dredged area and Akanoi Bay in the southern basin, and reed community areas in the shore. In the wide and deep northern basin, the density of suspended solids was low, and substantial fractions of particulate Si, P, S, and Ca were biogenic matter produced by phytoplankton. In the offshore zone in the narrow and shallow southern basin, the concentration ratios of particulate Si, K, Ca, Ti, and Fe against particulate Al were almost constant throughout the year and were similar to those found in crusts. Therefore, these elements are most likely derived from terrigenous clay matter. In the dredged area where the hypolimnion became anoxic during the stratification period, the concentrations of particulate Fe, Mn, P, S, and Ca increased because of the formation of hydrous Fe and Mn oxides and Fe sulfide. In Akanoi Bay, the ratios of Mn/Al and Fe/Al were high because of the stirring of sediments enriched with Mn and Fe oxides. In the reed community areas, the concentration of suspended solids correlated well with particulate Fe and P, but not with Al or chlorophyll a; therefore these areas seem to be rich in humic organic matter enriched with P and Fe due to microbial activity. Thus, the elemental composition of suspended solids reflects the chemical, biological, and physical processes in the lake and gives us useful indices of the characteristics of the water. Received: February 5, 2001 / Accepted: August 28, 2001