Distribution and onshore migration behaviour of burbot larvae in Lake Constance, Germany

Larval abundance of burbot Lota lota in the pelagic zone of Lake Constance, Germany, peaked during April and remained at a constant but lower level until mid‐June. From the end of May onwards a significant diel vertical migration pattern was observed: while burbot larvae were distributed above as well as below the thermocline in the daytime with the highest percentages of abundance at water depths of 30 and 50 m (below the thermocline) on 5 and 25 June, they were concentrated from 2 to 15 m (above the thermocline) during the night. This migration pattern coincided with a significant decline in the food items preferred by burbot at this life stage, cyclopoid copepods. This phase of diel vertical migration lasted for c . 4 weeks until the burbot larvae suddenly disappeared from the pelagic zone. This indicated a short phase, lasting only a few days, of larval settlement into profundal benthic habitats. This was supported by a significant, abrupt decrease in the absolute growth rates of the larvae at day 56 after hatching. The day–night pattern of vertical migration observed could be assumed to be a transition phase from a pelagic to a benthic lifestyle, as reported for juveniles of several other, marine gadoid species.     

Absence of typical diel vertical migration in Daphnia: varying role of water clarity, food, and dissolved oxygen in Lake Eymir, Turkey

Diel vertical migration (DVM) is a complex and dynamic behaviour against predation because the reaction of migrating organisms to light intensity plays a primary role, but is modified by other factors. In the relatively shallow but thermally stratified Lake Eymir, Daphnia pulex de Geers utilized vertical refugia afforded by the hypolimnion during both day and night. Differences in general vulnerability to fish predation determined the differences in their mean residence depths (MRDs) of different population categories such as most conspicuous and vulnerable individuals of adult with eggs inhabited the deepest depth, whereas juveniles stayed close the thermocline. In late spring, profoundly high amplitude of displacement within the hypolimnion, probably due to the hypolimnion being well-lit and relatively well-oxygenated for the fish and rather unsafe for the large-sized daphnids, was recorded. Therefore, the large-sized daphnids daytime refuge was close to the bottom whereas at night they moved upward to benefit from warmer water temperature along with food availability in the presence of fish predation but still remained below the thermocline. In summer, the insignificant amplitude of the hypolimnetic, which later became epilimnetic, displacements were probably due to the near-anoxic condition found below the thermocline. This might have deterred the fish, thus providing a safer refuge for daphnids in the below thermocline, which afterwards became the above thermocline. Low oxygen availability was regarded as the summer proximate factor. The abundant food and warmer water conditions found in the below/above thermocline also accounted for absence of DVM in summer. Consequently, this study suggests that DVM by Daphnia is an adaptation that is plastic to changing environmental conditions.     

Vertical distribution of epipelagic siphonophores at the confluence between Benguela waters and the Angola Current over 48 hours

The diel distribution of epipelagic siphonophores at a station off northern Namibia (18°00 S 10°30 E) was studied. This area is characterized by the mixing of surface waters of the Angola Current with the waters of the northernmost Benguela region. During the sampling period, the continuous flow of the Angola Current gave rise to a marked thermocline at ca 30 m depth. In order to study the diel vertical distribution patterns of epipelagic species under these hydrographic conditions, narrow depth horizons in the top 200 m of the water column were sampled repeatedly over a 48 h period. Thirty-four species were collected. Sphaeronectes gracilis and Chelophyes appendiculata predominated. Increasing numbers of species and individuals were observed on the second day of sampling, particularly in the layers above the thermocline. Only a few species showed an upward nocturnal migration that crossed the boundary between 2 water masses. The differences in the depth distributions of the various species were related to 3 distinct phenomena: the existence of 2 separate siphonophore populations associated with each of the 2 water masses; vertical migration by the most abundant species, which were able to cross the thermocline; and differences in patch size for the most epipelagic species.     

Environmental factors influencing diurnal distribution of zooplankton and ichthyoplankton

The diurnal vertical distribution of a large number of speciesof zooplankton, icbthyoplankton and micronekton were determinedin the top 150 m in three locations in the Shelf Water, on theNova Scotia Shelf, and Slope and on Georges Bank during springand fall periods. Species were categorized as to their trophiclevel and their type of diurnal migration behaviour. The influenceof temperature, salinity, and water density on the diurnal verticaldistribution of the species was examined. Temperature was foundto have the greatest influence on the distribution of the largestnumber of species. Diurnal migration behavior of the same speciesin Shelf and Slope water and at different times of the yearwas examined. Results showed that species changed their behaviorin the two water masses, while some species changed their migrationbehavior at different times of the year. During the night inApril the most abundant copepod species, Calanus finmarchicus,making up about 80% of the biomass, was found concentrated abovethe thermocline and the main chlorophyll layer. The majorityof the less abundant species of copepods were found below thethermocline and the chlorophyll layer. At night in August thetwo most abundant copepod species, Centropoger typicus and Paracalanusparvus, making up at least 80% of the zooplankton biomass, werealso concentrated above the thermocline and the main chlorophyllLayer. Three species of copepods were concentrated at the depthof the main chlorophyll layer and two species were concentratedbelow the chlorophyll layer and thermocline. The vertical distributionof other zooplankton and ichthyoplankton species was examinedin relation to the thermocline and chlorophyll layer. Relationshipsbetween concentrations of six species of fish larvae and allspecies of copepods in the same samples showed a general increasein the numbers of larvae m^–3 as the numbers of copepodsm^–3 increased in a range of 500–4000 m^–3.However, the concentration of Merluccius bilinearis decreasedas the concentration of copepods exceeded 4000 m^–3 suggestingthat high concentrations of copepods may not be a favourableenvironment for the larvae.     

Vertical distribution and abundance of mesoplanktonic medusae and siphonophores from the Weddell Sea,Antarctica

The composition, abundance and vertical distribution of mesoplanktonic cnidarians collected along a transect across the Weddell Sea have been analysed. The transect was characterized by a thermocline, approximately between 200 and 100 m, which deepened significantly towards the shelf edges. In total, 10 species of medusae and 18 species of siphonophores were identified. The most abundant medusae were Pantachogon scotti (up to 11,671 specimens/1,000 m³) and Arctapodema ampla (up to 960 specimens/1000 m³). The most abundant siphonophores were Muggiaea bargmannae (up to 1,172 nectophores/1,000 m³) and Dimophyes arctica (up to 230 nectophores/1,000 m³). Five assemblages of planktonic cnidarians were distinguished: (a) epipelagic species located in and above the thermocline; (b) epi- and upper mesopelagic species located in, above and just below the thermocline; (c) epi- and mesopelagic species located in and below the thermocline; (d) mesopelagic species; (e) lower mesopelagic species. Differences in the depth distribution of the various species gave rise to a clear partitioning of the mesoplanktonic cnidarian population throughout the water column. This vertical partitioning was related to the existence of a thermocline, the structure of the water column and the vertical distribution of prey.     

基于水声学方法的天目湖鱼类季节和昼夜空间分布研究

采用水声学方法系统地对天目湖春夏秋冬四季鱼类水平和垂直空间分布进行了调查研究,同时对昼夜探测的差异性进行了比较,并对不同季节间鱼类聚群形态进行了探讨。研究结果表明,天目湖鱼类在不同季节间存在规律性水平迁移,从全湖角度分析,天目湖鱼类资源昼夜间的水平空间分布无明显差异特征,但在春末和夏季的局部区域里,昼夜间鱼类存在一定近岸—远岸的水平迁移;不同季节和昼夜间,鱼类垂直分布差异明显,且在存在温跃层的夏季7月昼间,鱼类密度垂直分布与水温的垂直分布关系密切,温跃层以下的鱼类密度基本为0;天目湖鱼类在四个季节都属于成群分布类型,但季节间鱼类聚群形态不同,在冬季的1月呈现出典型的聚集,相对于昼间,春夏季鱼类在夜间分布更为均匀、分散,且水平探测表明在夜间水体表层存在大量鱼类分布。     

The biology of pygmy whitefish, Prosopium coulterii, in a closed sub-boreal lake: spatial distribution and diel movements

Using gillnets and trap nets, we examined the spatial distribution, diel movements, and environmental tolerances of pygmy whitefish, Prosopium coulterii, in a small boreal lake in north-central British Columbia. Most gillnets were set below the thermocline but we also fished a shore net in the littoral zone. During the ice-free season (May to November) there was a strong diel onshore–offshore movement: during the day pygmy whitefish were offshore and below the thermocline (water temperatures of 4–6°C) but at night they were inshore and above the thermocline (water temperatures of 12–18°C). This onshore–offshore movement occurred close to the bottom and, regardless of where they were caught, most fish were <4 m off the bottom. Oxygen concentrations in most of the hypolimnion dropped to <5.0 mg l⁻¹ in June and by late August to <1.0 mg l⁻¹; indicating pygmy whitefish can tolerate low oxygen conditions. The catch of pygmy whitefish in gillnets set below the thermocline was highly skewed: 53% of the nets were empty, 37% caught 18 or less fish, and 10% caught 70% of the total catch (742 fish). Trap nets produced similarly skewed results: most trap net sets caught no pygmy whitefish but one set caught over 2,000 individuals. Our catch data suggest that in Dina Lake #1 some pygmy whitefish aggregate.     

山东半岛南部产卵场温跃层对鳀鱼鱼卵垂直分布的作用

2000～2004年5～7月"北斗"号生物资源调查船对山东半岛南部产卵场进行多学科综合调查期间,于2003年6月17～18日和6月27～30日分别在35°01.9′N、120°32.9′E海区和35°03.3′N、120°26.4′E海区进行了25 h和64 h的水团追踪观测调查,鳀鱼鱼卵的数量、垂直分布与水温的分布为重点调查内容.调查结果表明:水温垂直分布的变化、海水混合的强弱、水体的辐聚/辐散等物理过程对水体中鳀鱼鱼卵数量和鳀鱼鱼卵在不同水层的分布有很大影响.温跃层分布深度的波动引起水体的垂直交换(混合)、水平对流(辐聚/辐散)和跨跃层的物质交换.温跃层深度变浅时,上混合层水体的厚度变薄,跨跃层的向上混合,将下混合层水体中和温跃层中的鳀鱼鱼卵输运到温跃层和上混合层中,同时表层水体发生辐散现象,使得下混合层、温跃层和上混合层水体中鳀鱼鱼卵数量减少、密度减小,导致整个水柱内鳀鱼鱼卵的数量减少;温跃层深度加深时,上混合层水体的厚度增加,跨跃层的向下混合,将上混合层水体中和温跃层中的鳀鱼鱼卵输运到温跃层和下混合层水体中,同时表层水体发生辐聚现象,使得上混合层、温跃层和下混合层水体中的鳀鱼鱼卵数量增多、密度增大,导致整个水柱内鳀鱼鱼卵的数量增多;温跃层水体中鱼卵数量和密度的变化较上混合层和下混合层水体更为显著.水体中温跃层的存在、温跃层分布深度的波动及其波动的程度对上混合层和温跃层中鳀鱼鱼卵的数量和密度的变化起着主导作用,导致鳀鱼鱼卵数量垂直分布的剧烈变化.     

A simulation study of the feedback of phytoplankton on thermal structure via light extinction

1. A simulation study of the feedback of phytoplankton biomass on temperature stratification in the large, monomictic Lake Constance was undertaken. Phytoplankton biomass affects the light extinction coefficient (LEC) of the water and, in turn, the vertical distribution of short‐wave radiation, which shapes the temperature stratification in the lake. 2. A sensitivity study of the variation in LEC using the hydrodynamic model DYRESM showed that a high LEC is associated with stronger stratification, shallower thermoclines, higher surface temperatures and reduced heat content during the heating phase of the lake. During the cooling phase, a shallower thermocline at high LEC leads to a faster decrease in water temperature so that during autumn, a high LEC is associated with lower surface temperatures. Thermal structure was particularly sensitive to changes in LEC when its value was below 0.5 m^?1. 3. When LEC is simulated dynamically with the coupled hydrodynamic–ecological model DYRESM‐CAEDYM, its value is a function of phytoplankton dynamics that change vertically and temporally. Comparing simulations with and without dynamic LEC (i.e. with and without phytoplankton dynamics) produced a complex picture: during the vegetation period, we often found a warmer surface layer and colder water beneath in the simulations with dynamic LEC, as expected from the higher LEC when phytoplankton is abundant. However, since phytoplankton biomass (as LEC) fluctuates and because of occasional cooling phases, the patterns were comparatively weak and not consistent over the whole growing season. 4. The most obvious patterns emerged by comparing simulations of oligotrophic and eutrophic conditions. In the eutrophic state, with its higher LEC, stratification was stronger and characterized by higher surface water temperatures, a shallower thermocline and colder water temperatures between 5 and 10 m depth. 5. Statistical analysis of long‐term data of water temperatures in Lake Constance, corrected for external forcing by air temperature, revealed a significant tendency towards warmer temperatures at 7.5 and 10 m depths with decreasing LECs over the course of reoligotrophication. This finding is consistent with our model results.     

Seasonal Vertical Migration and Aestivation of <Emphasis Type="Italic">Rhyacodrilus hiemalis</Emphasis> (Tubificidae,Clitellata) in the Sediment of Lake Biwa,Japan

The vertical distribution of the tubificid worm Rhyacodrilus hiemalis Ohtaka, the numerically dominant species of oligochaete in the littoral of Lake Biwa, was studied with special reference to seasonal vertical migration in the lake sediment. Monthly collections of lake sediment cores were made using PVC tubes. Core sections of sample sediments ranged from 76 to 117 cm. The vertical distribution of the worms showed no diurnal variation; therefore diel vertical migration was not evident. Seasonal downward migration started in April, and upward migration started in October. From December to March, almost all worms remained in the near-surface sediment layer (surface to 30 cm deep), while from July to September almost all worms remained deeper than 30 cm. However, few individuals migrated deeper than 90 cm. No discontinuous layers were found in grain size composition, water content, loss on ignition, particulate carbon, nitrogen or phosphorus. In deep sediment there was no free oxygen, as evidenced by negative ORP values. For 4 months in summer, R. hiemalis aestivated, probably utilizing anaerobic respiration. It appeared that R. hiemalis moved deeper in the sediment in response to sediment temperature, because sediment temperatures in the deep layers seemed to converge at around 20–21 °C in the summer months. The life history traits of seasonal vertical migration and summer aestivation perhaps arose as an adaptation to the climatic conditions accompanying the geographical origin of R. hiemalis, and they also serve to minimize predation risk during summer when most invertebrate predatory fishes are active.     

Observations on the vertical distribution of zooplankton, including post-larval teleosts, off Plymouth in the presence of a thermocline and a chlorophyll-dense layer

During 1973 – 1977, when Sagitta elegans was returningto dominance over S. setosa in Plymouth waters, there was often,in summer in stratified conditions, a community characterizedby S. setosa in the warm water above the thermocline, and atypical S. elegans community in the colder layer below the thermocline.This vertical separation persisted at night even though somespecies from the S. elegans community migrated up into the warmlayer. Such vertical migration was not stopped by a temperaturediscontinuity of from 3° to 6°C, but fewer individuals,or a lesser proportion of species, took part at the higher temperaturedifference. Only a small part of the zooplankton, comprisingsmall individuals of Limacina retroversa, the appendicularians,and, possibly, post-larvae of Arnoglossus laterna, showed signsof aggregation at the thermocline or the chlorophyll maximumclose to it.     

Do light quality and low nutrient concentration favour picocyanobacteria below the thermocline of the oligotrophic Lake Stechlin?

In May 1995, a short-term study on the vertical distributionof phytoplankton in the holomictic, oligotrophic Lake Stechlinrevealed a deep chlorophyll maximum (DCM) due to an increasein phytoplankton biomass below the thermocline. The DCM wasdominated by centric diatoms, which probably just passed thosewater layers while sinking to the sediment, and by picocyanobacteria.The DCM was situated well above the 1% level of photosyntheticallyactive radiation, but received atmost exclusively light of wavelengthsbetween 500 and 600 nm. The dominant picoyanobacteria were pre-adaptedto this environment by the possession of phycoerythrin. Thecompetitive advantage of picocyanobacteria compared with otherphycoerythrin-containing phototrophs was probably the most efficientnutrient uptake under low nutrient conditions as a consequenceof the small size of picoplankton.     

Vertical distribution of planktonic rotifers in a karstic meromictic lake

Vertical distribution of planktonic rotifers is described in relation to temperature and oxygen in Lake La Cruz, a single-doline, closed karstic lake (121 m diameter and 25 m maximum depth) which shows iron meromixis. Samples were taken by peristaltic pumping at 10 cm depth intervals in the oxycline zone from June 1987 to September 1988. A model of rotifer vertical structure in stratified lakes is proposed. Rotifers concentrate their populations at the depths with intense gradients. As stratification develops some rotifer populations show a downward migration following the thermocline and some others show an upward migration following the oxycline. The production-respiration balance in the lake, and so the position of the oxycline with respect to the thermocline and the layer of maximum production, depends on meteorological conditions. A shift in the dominance of congeneric or related species can occur in consecutive years. In Lake La Cruz, mixing conditions and subterranean inflow in spring were much more intense in 1988 than 1987, and the distance between production and decomposition depths was smaller in 1988. Anuraeopsis miraclei, an oxycline-bound species with high abundance in 1987, was displaced by A. fissa in 1988. A. fissa, which was a metalimnetic species during early summer, reached peak densities (3 × 10⁴ ind l^–1) at the oxycline, equaling the abundance of A. miraclei the preceeding year.     

Catastrophic transition in vertical distributions of phytoplankton: alternative equilibria in a water column

We showed a catastrophic transition between a surface maximum pattern and a subsurface maximum pattern of phytoplankton in a water column by a mathematical model considering the vertical distribution of phytoplankton and two resources, light and a limiting nutrient. In our model, we assumed that a water column consists of two layers: a complete mixing layer above a seasonal thermocline and an incomplete mixing layer below it. From numerical calculation of the model, we obtained that there are two stable vertical patterns of phytoplankton over a certain range of parameters of the model: a pattern having its maximum below the thermocline and another having its maximum above the thermocline. As other models having multiple stable equilibria, our model also exhibits a hysteresis effect and catastrophic transition when one of the parameters of the model changes continuously. These results indicate the possibility of the existence of alternative equilibria of vertical patterns of phytoplankton even if the trophic status and physical condition of the water column are similar. Moreover, the catastrophic transition between the steady states suggests one of the possible mechanisms of autumn algal blooms.     

Tactic responses of the parasitic mite, Psoroptes ovis, to light and temperature

The astigmatid mite, Psoroptes ovis (Hering) (Acari: Psoroptidae), is an obligate, non-burrowing ectoparasite of vertebrates, of particular economic importance in domestic sheep flocks where it causes clinical psoroptic mange. To help understand the behaviour which facilitates transmission via the environment, the responses of P. ovis derived from rabbits (syn. Psoroptes cuniculi) to temperature and light were examined in the laboratory. On a vertical surface of uniform temperature, the presence and direction of illumination had a significant effect on the distance and direction moved by the mites. In darkness or with illumination from both above and below, the mites moved relatively little, but this movement was upwards. In contrast, with illumination from above only, mites moved downwards. When the direction of the illumination was reversed so that it came from below only, the mites moved upwards. On a vertical surface with a temperature gradient, in darkness or with illumination from both above and below, the mites moved up or down towards the area of highest temperature, depending on whether this was above or below, respectively. However, the movement of the mites in response to the temperature gradient was strongly displaced up or down by the presence of unidirectional illumination from above or below, respectively. The results indicate that the movement of these mites is strongly directed towards areas of high temperature but away from higher light intensity. These behaviours might be expected to maintain the position of the mites on a host animal and help them locate the skin surface of a new host when displaced into the environment.     

A metalimnetic oxygen minimum indirectly contributing to the low biomass of cladocerans in Lake Hiidenvesi – a diurnal study on the refuge effect

The diurnal vertical migrations of smelt (Osmerus eperlanus), larvae of phantom midge (Chaoborus flavicans) and cladoceran zooplankton in eutrophic Lake Hiidenvesi were studied in order to clarify the factors behind the low zooplankton biomass. In the study area, an oxygen minimum occurred in the metalimnion in the 10–15 m depth. No diurnal fluctuations in the position of the minimum were observed. Cladocerans inhabited the epilimnion throughout the study period and their vertical movements were restricted to above the thermocline and above the oxygen minimum. C. flavicansconducted a diurnal migration. During the day, the majority of the population inhabited the 12 – 15 m depth just in the oxygen minimum, while during darkness they were found in the uppermost 8 m. Smelts started ascending towards the water surface before sunset and reached the uppermost 3 m around 23:00. During daytime, the majority of smelts inhabited the depth of 7–9 m, where the water temperature was unfavourably high for them (18 °C). Smelts thus probably avoided the steep oxygen gradient in the metalimnion, whereas Chaoborusused the oxygen minimum as a refuge against predation. Those smelts that were found in the same water layers as Chaoborusused the larvae as their main prey. The metalimnetic oxygen minimum thus seemed to favour the coexistence of vertebrate and invertebrate predators, leading to a depression of cladoceran zooplankton.     

Diel variation in the vertical distribution of fish and plankton in Lake Kinneret: a 24-h study of ecological overlap

Diel vertical migration (DVM) behaviour is a predator avoidance mechanism observed within many zooplankton species in the presence of zooplanktivorous fish. A 24-h survey was carried out in June 1998 to investigate diel variation in the vertical distribution of fish, zooplankton and phytoplankton (chlorophyll) in Lake Kinneret, Israel. Fish revealed diel variation in vertical distribution but had no spatial overlap with zooplankton, and consequently no apparent influence on zooplankton dispersal. Zooplankton revealed some diel variation in distribution being affected by thermocline and oxycline position and movement of the internal the internal seiche wave. Cyclopoid species closely follow the movement of the seiche wave implying that, due to their greater motility, they are following conditions that are suitable to them. The Cladocera species and small rotifers only partly, which may be part of their phototaxic behaviour. Physical forces like convection, horizontal and vertical forcing probably have a role in contributing to a homogeneous distribution of the plankton by preventing stratification or interfering with the more motile zooplankton which may be attempting to migrate.     

Specific gravity and vertical distribution of sprat eggs in the Baltic Sea

During peak spawning of sprat Sprattus sprattus in the Baltic Sea in May–June egg specific gravity averaged ± s . d . 1·00858 ± 0·00116 g cm⁻³ but was significantly higher in the beginning and significantly lower towards the end of the spawning season. A close relationship was found between egg diameter and egg specific gravity ( r ² = 0·71). This relationship, however, changed during the spawning season indicating that some other factor was involved causing the decrease in specific gravity during the spawning period. The vertical egg distribution changed during the spawning season: eggs were distributed mainly in the deep layers early in the season, occurred in and above the permanent halocline during peak spawning, and above the halocline towards the end of the spawning season. Consequently, poor oxygen conditions in the deep layers and low temperatures in layers between the halocline and the developing thermocline may affect egg development. Thus, opportunities for egg development vary over the spawning season and among spawning areas, and depending on frequency of saline water inflows into the Baltic Sea and severity of winters, between years.     

水体热分层对万峰湖水环境的影响

2009年9月(秋季)和2010年1月(冬季)对南盘江流域峡谷型水库--万峰湖的水温和水化学(DO、pH、总磷)进行监测,结果表明:万峰湖水体在秋、冬季节均存在明显分层,秋季.水体分为3层,0～10 m为混匀层,10～50 m为斜温层,50 m以下为滞水层,这种温度分层阻止了水的对流混合,引起显著的水化学分层,形成底部厌氧层.冬季,水温下降,在水下50 m左右分层,表层随深度增加水温下降,下层为滞水层,水温较为均匀.相关性分析表明,万峰湖在2009年9月(秋季)和2010年1月(冬季)水温和DO、pH、总磷之间均为极显著相关,水化学分层与温度分层同步.     

Distribution of CH4 in the north basin of Lake Biwa and tributary rivers

To determine the origin of CH₄, the vertical distribution of CH₄ around the thermocline in the north basin of Lake Biwa and the horizontal distribution in the Yasu and Ado rivers were measured. In 1995, CH₄ concentrations (ranging from 200 to 1000 nM) exceeding the saturation level were observed just above or in the thermocline in the pelagic region off the Yasu River. These values were higher than those just below the thermocline and in the hypolimnion. CH₄ concentrations in the mouths of the Yasu and Ado rivers were much higher than at other stations, around 2500 and 2000 nM, respectively. Due to the drought in 1994 when there was no water from the Yasu River flowing into the north basin, CH₄ concentration just above or in the thermocline in the pelagic region off the Yasu River ranged from 49 to 74 nM. It is thus concluded that the high level of CH₄ observed in the mouth of the Yasu River is one of the sources of the high CH₄ concentrations in the pelagic region off the Yasu River. On the other hand, the CH₄ concentration in the pelagic region off the Ado River was about 50 nM. It is thought that water flowing in from the Ado River diffused readily into the lake water because the depth of the lake in the region off the Ado River declines steeply. The maximum concentration of CH₄ in the river mouths indicates that these areas are important sites for clarifying the mechanism of the decrease in dissolved oxygen in Lake Biwa and the Yodo River watershed. Received: October 25, 2000 / Accepted: May 8, 2001