Naoshige Goto

First investigation of ultraoligotrophic alpine Lake Puma Yumco in the pre-Himalayas, China

Lake Puma Yumco is a typical alpine lake (altitude 5030 m) located in the pre-Himalayas of Tibet, China, and this study was the first limnological investigation ever conducted on it. Lake Puma Yumco (28°34″N, 90°24″E) has the following morphometric properties: maximum length 31 km, maximum width 14 km, mean width 9 km, shoreline 90 km, surface area 280 km2, and shoreline development 1.5. Transparency was approximately 10 m, even in the thawing season. The extinction coefficient of the lake water was calculated as 0.15 m−1. Annual maximum transparency was estimated from the depth of the Chara zone to be 30 m. Dissolved oxygen was 7 mg O2 l−1 and showed saturated values, and salinity was 360 mg l−1. The chemical type of the lake water was Mg-Ca-HCO3-SO4, and it was slightly alkaline in character. Total nitrogenous nutrients (sum of ammonia, nitrite, nitrate, and urea nitrogen), phosphate, and silicate were extremely low at 1, 0.02, and 9 µM, respectively. Dissolved organic carbon, nitrogen, and phosphorus concentrations were 160, 11, and 0.08 µM and the molar ratio was calculated as 2100 : 140 : 1. Chlorophyll a concentration was 0.2 mg m−3. Phytoplankton and zooplankton were dominated by Aphanocapsa sp. and Diaptomidae. Both nitrogen and phosphorus appear to be the limiting parameters for phytoplankton growth. Organic carbon and nitrogen contents in lake sediments were low and the sediments contained a large amount of CaCO3. The grain size of sediment was that of silt-sand in most cases. The present results indicate that the pre-Himalayan alpine freshwater Lake Puma Yumco is an ultraoligotrophic lake.

Development of filamentous green algae in the benthic algal community in a littoral sand-beach zone of Lake Biwa

Temporal changes of biomass and dominant species in benthic algal communities were investigated in a littoral sand-beach zone in the north basin of Lake Biwa from December 1999 to September 2000. Chlorophyll-a amounts of benthic algal communities per unit area of the sandy sediments rapidly increased from late April to June. Increases in biomass of the benthic algal communities are considered to result from the propagation of filamentous green algae Oedogonium sp. and Spirogyra sp. The cell numbers of filamentous green algae and chlorophyll-a amounts of benthic algal communities at depths of 30 and 50 cm at a station protected by a breakwater in May were significantly higher than those of a station exposed directly to wave activity. Thus, the biomass accumulation of the benthic algal communities seems to be regulated strongly by wave disturbance. The development of filamentous green algae may contribute to the increase in biomass of the benthic algal community and to the changes in seasonal patterns of biomass in the sand-beach zone of Lake Biwa. We consider that the development of the filamentous green algal community in the littoral zone of Lake Biwa is the result of eutrophication.

Relationships between electron transport rates determined by pulse amplitude modulated (PAM) chlorophyll fluorescence and photosynthetic rates by traditional and common methods in natural freshwater phytoplankton

The relationships between the electron transport rate (ETR) measured by pulse amplitude modulated (PAM) fluorometer and the rate of O 2 production and 14 C fixation by traditional and most common methods were estimated in natural phytoplankton communities to determine the molar ratio of oxygen produced and carbon fixation per electron transported. The observations were carried out in Lake Biwa, Japan approximately once a month for a year. A remarkable linear relationship was observed between the rate of O 2 production and C fixation to ETR at low-to-medium irradiances during the sampling period, but not at higher irradiances. The annual average O 2 production/ETR and C fixation/ETR ratios at low-to-medium irradiances were 0.117 and 0.095, respectively, and the photosynthetic quotient (O 2 production/C fixation) calculated from the two ratios was 1.23, which is the most common value as the photosynthetic quotient. The same was true for cultured algal species. These results indicate that the O 2 production/ETR and C fixation/ETR ratios are reliable values at low-to-medium irradiances, and that PAM measurements can be used to quantify primary production rates based on traditional methods using the ratios. However, it must be noted that the determination of the average O 2 production/ETR and C fixation/ETR ratios in a natural phytoplankton community requires long-term observation because those ratios change seasonally over a certain range.

Seasonal changes in carbon and nitrogen productivity in the north basin of Lake Biwa, Japan

One o f the most significant biogeochemical processes in the euphotic layer o f freshwater environments is carbon and nitrogen production by phytoplankton assemblages. lnformation regarding the uptake of nitrogenous nutrients in freshwater lakes has recently been accumulated to elucidate its importance as a nitrogen source for phytoplankton growth and to demonstrate its significance in the biogeochemica1 nitrogen cycle (MIYAZAKI et al. 1985, WHALEN & ALEXANDER 1986, MITAMURA & SAIJO 1986a, TAKAMURA et al. 1987, BINHE & ALEXANDER 1993, PRESING et al. 1998, 2001). MITAMURA & SAIJO (1986b) measured the in situ uptake rate o f nitrogenous nutrients in the euphotic layer ofLake Biwa and demonstrated that the principal nitrogenous compounds that sustain the standing crop ofphytoplankton were arnmonia, urea and nitrate, and that phytoplankton preferentially utilized arnmonia. MITAMURA & SAuo (1986b) and MITAMURA et al. (1995) observed that relative nitrogen uptake rates by phytoplankton were ranked as arnmonia > urea > nitrate in temperate and tropical natural lakes. Several studies have reported that the regenerated forms of nitrogenous nutrients such as arnmonia and urea play significant roles as nitrogen sources for phytoplankton. To provide further information on the biogeochemical carbon and nitrogen cycling in freshwater bodies, the in situ uptake rates of arnmonia, nitrate and urea nitrogen were seasonally measured in relation to the photosynthetic carbon uptake of phytoplankton populations in the euphotic layer of the north basin of LakeBiwa.

Denitrification activities in epilimnetic sediments in littoral wetland areas of Lake Biwa

The ecotone, which lies between land and water, shows characteristics ofboth freshwater and terrestrial ecosystems. The ecotone ecosystem comprises a wide diversity of environments due to various factors such as changes o f water level in the lake, wave erosion, chemical characteristics of inflow water and density of large emergent plants. In the wetlands in ecotone regions oflakes, the chemical state ofbioelements and organic matter in incoming water is significantly changed by biological functions such as the uptake of nutrients by large emergent plants, decomposition of organic matter by benthic microorganisms, and reduction in nitrate through denitrification under anaerobic conditions. Therefore, in lake ecosystems, the littoral wetlands play a significant ro le as a natural buffer, holding back incoming water. In this way, lakes maintain the stability of their biogeochemical cycle. The aim ofthe present study was to clarify the contribution of denitrification by littoral wetlands of Lake Biwa to the nitrogen cycle ofthe lake. The denitrification process involves a biochemical reduction that emits nitrogen as nitrogen gas. Studies of denitrification have previously been conducted in a large number ofwetlands, mainly focusing on the relationship between denitrification and differences in sediment environments or root zones o f emergent plants (REDDY et al. 1989, KERN et al. 1996). However, few investigations have focused on the contribution of denitrification by littoral wetlands to the nitrogen cycle o f a lake. Eutrophication in Lake Biwa has rapidly advanced since the 1960s due to increasing economic activities. Thus, denitrification is considered to play a major role in purification ofthe lake by reducing nitrate concentrations. The waterside ecotone of Lake Biwa has many wetlands separated from the main body of water by sandy beaches. Since these wetlands are dominated by large emergent plants, their sediments are covered with an abundance of detrital organic matter. An enormous biomass of microorganisms is supported by the rich organic matter, which is decomposed by numerous microorganisms, creating anoxic conditions. This feature o f wetland is thought to intensify the denitrification in littoral wetlands of the lake. This study reports on the relationships between the denitrification rates (in silu and potential) and the environmental parameters in littoral wetlands of Lake Biwa.

Seasonal changes of nutrient concentration and algal biomass in the stagnant water system of a Kizu River sand bar, Japan

River water flows downstream through many hydrological storage zones such as stream pools, backwaters and hyporheic zones. Such sites, termed transient storage zones (BENCALA & WALTERS 1983), play an important role in regulating nutrient retention, transformation and transports in streams (VALLET et al. 1996, MULHOLLAND et al. 1997, BUTTURINI & SABATER 1999). Nutrient concentrations in river water are possibly influenced by abiotic and biotic processes during their presence in those zones. Stagnant waters on a bar, isolated from channel flows, have longer hydrological retention times in surface transient storage zones. In stagnant waters on a bar, in contrast to freeflowing river water, planktonic algae can grow, so that algal primary producers in stagnant waters are composed of not only phytobenthos but also phytoplankton. Stagnant waters in floodplains, particularly in parafluvial zones, are considered transient water bodies frequently disturbed by fluctuations in a stream discharge, as VAN DER NAT et al. (2003) demonstrated in the Tagliamento River (northeast Italy). These characteristics suggest that stagnant waters isolated from channel flows constitute a unique component in river ecosystems. In this study, we investigated the seasonal and spatial pattems of nutrient concentrations in stagnant waters ona sand bar ofthe Kizu River. These pattems were also compared with the pattems of flooding and of phytobenthos and phytoplankton biomasses to detect the factors that influence nutrient dynamics in stagnant waters.

Production of extracellular organic carbon in the total primary production by freshwater benthic algae at the littoral zone and inflow river of Lake Biwa

In general, a high proportion of benthic algae flora consists of diatoms belonging to the pennates (Ro uN o et al. 1990). One o f the conspicuous characteristic of diatoms is the production of an adhesive extracellular secretion. This mucilaginous substance is secreted from the raphe and/or mucilage pores of diatoms to the outside ofthe cell, covering it in various forms such as stalks, tubes, apical pads, adhering films and fibrils (HOAGLA;-..;o et al. 1993). These secretions are highly hydrated, carbohydrate-rich substances containing extracellular polymeric substances (EPS) consisting mainly of acidic polysaccharides that may be carboxylated and/or su1fated to varying degrees. Various functions have been proposed for benthic algae secretions in aquatic ecosystems, including protection against rapid changes in enhabited environments, sessile adhesion, motility, stabilization of sediments, and adsorption of dissolved organic matter and metals (DECHO 1990, HOAGLAI\D et ai. 1993, MILLER et al. 1996 ). Moreover, from the viewpoint o f food webs in inshore ecosystems, the extracellular organic carbon produced by benthic algae p1ays an important role as an organic carbon source for heterotrophic organisms (DECHO & LOPEZ 1993, KAWAMURA et al. 1998, GOTO et al. 200 l). Recent studies have shown that the production 1evel of extracellular organic matter by marine microphytobenthos is much higher than that ofmarine phytoplankton, ranging from about 30-80% o f total primary production (GOTO et al. 1999, MIDDELBURG et al. 2000, SMITH & UNDERWOOD 2000, WOLFSTEIN et al. 2002). In addition, GoTo et al. (1999) found that mos t of the extracel1ular organic matter produced by benthic diatoms inhabiting an intertidal flat was made up of colloidal organic carbon (EDTA-extractable organic carbon) containing 41-53% EPS. Although extracellular organic matter containing EPS is considered to p1ay various important ro1es in inshore ecosystems, as mentioned above, the production level o f extracellu1ar organic matter an d EPS by benthic algae in freshwater ecosystems such as rivers and lakes has yet to be clarified. Therefore, in this study, we elucidated the ratio of each organic carbon fraction to the carbon photoassimilated by cu1tured freshwater benthic a1gae and benthic alga1 communities in comparison to phytoplankton in natural fresh waters.