Kentaro Nozaki

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.

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.

Seasonal dynamics of primary production in the pelagic zone of southern Lake Baikal

Abstract We measured primary production by phytoplankton in the south basin of Lake Baikal, Russia, by in situ 13C-bicarbonate incubations within the period March–October in two consecutive years (1999 and 2000). Primary production was highest in the subsurface layer, possibly due to near-surface photoinhibition of photosynthesis, even under 0.8 m of ice cover in March. Areal primary production varied from 79 mg C m−2 day−1 (March) to 424 mg C m−2 day−1 (August), and annual primary production was roughly estimated as 75 g C m−2 year−1, both of which are within the lower range of previous estimates. Size fractionation measurements revealed that phytoplankton in the <20 μm fraction accounted for 72%, 96%, and 85% of total primary production in March, August, and October, respectively. The contribution of picophytoplankton (<2 μm) to total primary production ranged from 41% to 62%. A large fraction (82%–98%) of particulate organic carbon was associated with particles in the <20 μm fraction. These results suggest that nano- and picophytoplankton play an important role as primary producers in the pelagic ecosystem of Lake Baikal.

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.

Push-up response of stonefly larvae in low-oxygen conditions

Under conditions of low oxygen availability, the larvae of the stonefly Oyamia lugubris McLachlan demonstrate a ‘push-up’ behavior that is thought to enhance respiratory efficiency. We conducted an experiment to investigate the effect of the oxygen supply on this behavior in winter and summer by using a lotic chamber and natural water. From the experiment, we determined the critical oxygen supply level below which the stonefly larvae are compelled to do push-ups. There was a small difference in the critical oxygen supply level between the seasons. This result emphasizes that a novel measurement of the oxygen availability, that is, the oxygen supply, could be an important determinant of the distribution of aquatic insects.

A seasonal change in the distribution of a stream-dwelling stonefly nymph reflects oxygen supply and water flow

We examined the effects of oxygen availability, which has been viewed as a minor factor in streams, on the seasonal and spatial microhabitat distribution of a stonefly. Surveys were conducted in winter and summer in a mountain stream by collecting stones from the streambed and determining the presence or absence of the insect. At each stone sampling, we also measured physical conditions. The probability of the stonefly presence increased significantly with current velocity in summer, but not in winter. Because current influences oxygen renewal rates, our results suggest that the distribution of the insect could be restricted by oxygen.

Photosynthetic potential of phytoplankton in the deep water of Lake Baikal, Russia

We examined the photosynthetic activity of the phytoplankton community collected from the surface to a depth of 1000 m in the south basin of Lake Baikal. Experiments were conducted in June (mixing period) and August (stratified period). The carbon fixation rate was measured by the use of a 13C tracer after the incubation of samples under light conditions in the upper water column. Photosynthetic fixation of 13C was detected for samples collected from a depth of 500 m, indicating the viability of phytoplankton in deep water. The community composition was dominated by Bacillariophyceae in deep water. The finding of lower activity at a depth of 200 m than that at a depth of 500 m in August suggests that the spring diatom bloom could be a significant source of viable populations at a depth of 500 m. Photosynthetic activity was not detected in samples collected at a depth of 1000 m.

Composition, biomass, and photosynthetic activity of the benthic algal communities in a littoral zone of Lake Baikal in summer

Abstract The composition, biomass, and photosynthetic activity of benthic algal communities were investigated in a littoral zone of the south basin of Lake Baikal in July 1998. A transect survey revealed a vertical transition of the dominant taxonomic groups: Ulothrix and diatoms dominated in the shallowest station (depth, 0.5 m), whereas only diatoms were found in a station with a depth of 1 m. In deeper stations (depth, 2–3 m), an endemic filamentous green alga, Draparnaldioides baicalensis, was the predominant form, but in addition to this species, Tetraspora and diatoms were also abundant in algal communities collected in the deepest stations (depth, 5–6 m). Biomass expressed as chlorophyll a (Chl a) varied in the range of 8 mg Chl a · m−2 (at a depth of 1 m) and 300 mg Chl a · m−2 (at a depth of 3 m). The maximum photosynthetic rate (of carbon, C) of the community dominated by Ulothrix was 0.24 mg C · mg Chl a−1 · h−1, whereas the corresponding rate of the community dominated by Draparnaldioides was 1.51 mg C · mg Chl a−1 · h−1.

Nutrient accumulation by a bloom of the filamentous green alga Spirogyra sp. in the littoral zone of the north basin of Lake Biwa

In reeent years, littoral blooms of a large filamentous green alga (FGA) Spirogyra sp. have been observed in the north basin ofLake Biwa (MATSUOKA et al. 1995, NoZAKI et al. 1998). FGA blooms dominated by Cladophora glomerata, Spirogyra sp. and/or Ulothrix zonata have been reported in Lake Biwa, as well as other lakes loeated in the north temperate region, sinee the 1980s (see NoZAKI et al. 1998). FGA propagation has often been a biologieal indieator of water quality disturbanee in lake environments (e.g. jACKSON 1988, PIECZYNSKA et al. 1988, GRAHAM et al. 1995, TURNER et al. 1995a). However, FGA bloom effeets on the littoral zone eeosystem (e.g. algal eommunity strueture, primary produetion, nutrient cydes, and the habitats of other aquatie organisms) have seldom been studied (TURNER et al. 1995b, NoZAKI et al. 1998, NoZAKI 1999). One of the important roles of algal eommunities is as sinks for earbon (C), nitrogen (N) and phosphorus (P). In the present study, we deseribed how partieulate earbon (Pe), nitrogen (PN) and phosphorus (PC) are aeeumulated by a bloom of Spirogyra sp. in the littoral zone in the north basin ofLake Biwa.

Oxygen production and bubble formation by algal mat on the bed in a slow sand filter pond

A slow sand filter pond for drinking water, similar in shape to a swimming pool with a water depth of about l m, was used in this study. The bottom was composed of a sand layer for filtration. There was a constant downward current, the speed of which was normally about 4-5 m/day (10-20 em/h), and a constant nutrient supply to the filter basin from an intake of water. There was sufficient radiation for photosynthesis on the shallow bottom and photosynthetic organisms grew in this environment. However, phytoplankton did not grow due to the presence of a water current and only some filamentous algae grew on the bottom under these conditions (NAKAMOTO & EzuRE 1989). A heavy algal bloom of filamentous diatoms was observed in the pond in which the intake water contained enough nutrients to allow algal growth. Bubbles were formed by photosynthesis in the algal mat on the sand surface. The algal mat was easily lifted up to the surface due to the buoyancy of the bubbles, and then disappeared through the scum outlet. The slow sand filter acted as a continuous culture system for filamentous algae. This paper describes the formation of oxygen bubbles and its role in this system.