Growth and age structure of the swan mussel Anodonta cygnea (L.) at different depths in lake Mattsee (Salzburg, Austria)

Unionid clams were collected at 1–2 m, 3–4 m and 6–7 m depth in lake Mattsee, a moderately mesotrophic lake, to investigate the effect of depth on clam growth and age structure. No significant differences in age structure of Anodonta cygnea were found (p=0.65). Three and ten years old clams were present at all depths, but in different percentages. Whereas at 1–2 m 13.3% of the collected clams were <4 years old, this percentage was 4.4% at 6–7 m and 7.1% at 3–4 m. A greater percentage (6.7%) of older mussels (9, 10 years) were collected at 6–7 m than at 1–2 m (2.2%). Growth declined with depth. Total length at a given age of clams at 1–2 m and 3–4 m did not differ (p=0.54), whereas differences were significant between clams at 1–2 m and 6–7 m (p<0.05) as well as between 3–4 m and 6–7 m (p<0.05). The Growth constant k was highest at 1–2 m depth.     

Depth distribution of photosynthetic pigments and diatoms in the sediments of a microtidal fjord

The depth distribution of photosynthetic pigments and benthic marine diatoms was investigated in late spring at three different sites on the Swedish west coast. At each site, sediment cores were taken at six depths (7–35 m) by scuba divers. It was hypothesized that (1) living benthic diatoms constitute a substantial part of the benthic microflora even at depths where the light levels are <1% of the surface irradiance, and (2) the changing light environment along the depth gradient will be reflected in (a) the composition of diatom assemblages, and (b) different pigment ratios. Sediment microalgal communities were analysed using epifluorescence microscopy (to study live cells), light microscopy and scanning electron microscopy (diatom preparations), and HPLC (photosynthetic pigments). Pigments were calculated as concentrations (mg m^–2) and as ratios relative to chlorophyll a. Hypothesis (1) was accepted. At 20 m, the irradiance was 0.2% of surface irradiance and at 7 m, 1%. Living (epifluorescent) benthic diatoms were found down to 20 m at all sites. The cell counts corroborated the diatom pigment concentrations, decreasing with depth from 7 to 25 m, levelling out between 25 and 35 m. There were significant positive correlations between chlorophyll a and living (epifluorescent) benthic diatoms and between the diatom pigment fucoxanthin and chlorophyll a. Hypothesis (2) was only partly accepted because it could not be shown that light was the main environmental factor. A principal component analysis on diatom species showed that pelagic forms characterized the deeper locations (25–35 m), and epipelic–epipsammic taxa the shallower sites (7–20 m). Redundancy analyses showed a significant relationship between diatom taxa and environmental factors – temperature, salinity, and light intensities explained 57% of diatom taxa variations.     

Macrobenthos in Montedoglio Reservior, central Italy

An investigation was carried out on the macrobenthos of Montedoglio Reservoir, an artificial lake in central Italy. Seven qualitative and quantitative samplings, performed by drag netting and airlifting at 5 stations, revealed a total of 48 taxa during 1992–94. The highest density and most frequent terms were Tubificidae, Chironomidae of the genera Procladius, Tanypus, Chironomus and Polypedilium and Chaoboridae. Tubificidae were found mainly at a depth of 7–9 m, Chironomidae at 1–3 m. The modest density and low biological diversity values are probably attributable to the substrate characteristics and a drop in hypolymnic oxygen during thermic stratification. The oxygen stress at the lake bed, which was unexpected given the scarce planktonic productivity, may have been caused by recent impoundment.     

The impact of the River Ntahangwa, the most polluted Burundian affluent of Lake Tanganyika, on the water quality of the lake

Sources of pollution at the north end of Lake Tanganyika are concentrated around Bujumbura, Burundi. The impact of River Ntahangwa on the water quality of this part of Lake Tanganyika was compared with that of an unpolluted river, the Mugere. A shallow bay, not influenced by a river, was used as a reference station. Chemical analyses were carried out at four week intervals during nearly a year. Samples were taken at different distances from the shore and at different depths. River impact was detected up to 100 m from both river mouths. Even at 50 m from the mouth of the polluted River Ntahangwa, no important decrease of dissolved oxygen was detected. Chlorophyll-a maxima were found 100 m from the Ntahangwa mouth, but this was also the case 100 m from shore in the unpolluted bay. Lake water at 60 m depth was saturated with oxygen during the dry season, as it used to be in 1956. The rivers studied descent initially within the lake due to greater sediment load and lower water temperature. Nevertheless, it appears that these rivers mix completely in the lake within 100–300 m from the river mouths. The direct effects from organic pollution of the Ntahangwa on the lake seem to be rather negligible. Appropriate anti-erosion measures should reduce massive sediment discharge into the lake.     

Irradiance Regulation of Photosynthesis and Respiration in Modern Marine Microbialites Built by Benthic Cyanobacteria in a Tropical Lagoon (New Caledonia)

Microbialites are organosedimentary deposits that have built up as a result of the growth and binding of detrital sediment by a benthic microbial community. This study focuses on microbialites built by monospecific populations of cyanobacteria in the south-west lagoon of New Caledonia, where they have been observed down to 20–25 m depth. The aim was to study their photosynthetic and respiratory responses to various light intensities. The Phormidium sp. TK1 microbialite was collected at 19 m depth and the P. crosbyanum (Tilden) microbialite was collected at 0.5 and 13 m depth. Phormidium sp. TK1 showed all the characteristic features of a low-light adapted species. The initial slope of the Photosynthesis versus Irradiance curve for this microbialite was close to the maximum quantum yield indicating an efficient light absorption and utilization at low light. The photosynthesis maximum was located 0.2–0.4 mm below the surface and did not shift with changing light intensity. Respiration rates were low and not enhanced by light; photoinhibition was observed at higher light intensities. In Phormidium crosbyanum (Tilden) microbialites, the photosynthesis maximum shifted downward to lower depths with increasing light, probably as a result of phototactic migration of cyanobacterial filaments, and light-enhanced respiration was observed at light intensities above light saturation. The photosynthetic para- meters measured in P. crosbyanum indicate that P. crosbyanum is capable of photo-acclimation at high light intensities. The gross productivity of the different microbialites was comparable to values measured in cyanobacterial stromatolites observed in other shallow environments. However, the microbialites studied here were characterized by a lower respiration / production ratio which indicates a higher growth efficiency.     

Factors influencing depth distribution of soft bottom inhabiting Laminaria saccharina (L.) Lamour. in Kiel Bay,Western Baltic

The kelp Laminaria saccharina dominates soft bottoms in 4–10 m depth in Kiel Bay. Experimental sporophytes transplanted to 2 and 5 m depth showed the typical annual growth pattern of Laminaria species. Surprisingly, 2 m plants died after the first resting phase, whereas 5 m plants survived and showed outgrowth of a new blade generation. Thalli at both depths were infected with the brown algal endophyte Streblonema aecidioides, with host deformations being significantly stronger in 2 m plants. Growth rates of infected sporophytes were reduced. Exclusion of UV light in 2 m depth resulted in less infected thalli. Discs excised from L. saccharina and cultivated in different photon fluence rates from 10–600 µmol m^–2 s^–1 did not differ in growth rate, photosynthesis or dark respiration. Hence, an exclusion of L. saccharina from shallow depths caused by high light cannot be concluded. We suggest the biological interaction with the endophyte S. aecidioides, amplified by UV light, to be most important for the exclusion of L. saccharina from shallow depths in the western Baltic.     

Caesium-137 and lead-210 dating of recent sediments from Mondsee (Austria)

Mean annual sedimentation rates over the last 20–30 years were determined in the pre-alpine Mondsee (Upper Austria) using Cs-137 and Pb(Po)-210 profiles for sediment core dating and two natural sediment markers. Lower sedimentation rates of about 2–3 mm yr^–1 were observed in the central part of the lake near the shore at 18–20 m and in the southern part at 30 m depth. Higher sedimentation rates of 4–7 mm yr^–1 were found in the central part of the lake at 47 and 65 m and in the northern bay at 18 and 41 m depth. At both these sites the Pb-210 profiles were strongly disturbed in the upper zone of the sediment cores, whereas the Cs-137 pattern remained intact. The higher annual sediment accumulation rates can be explained only partly by deposition of allochthonous material discharged by the streams, enhanced eutrophication in these parts of the lake, erosion and sediment focusing by turbidity currents being also probable.     

Depth Distribution and Abundance of the Common Bully, Gobiomorphus cotidianus (Eleotridae), in Three Oligotrophic New Zealand Lakes, One of which is Turbid

The depth distribution of the common bully, Gobiomorphus cotidianus, a small benthic forage fish, was measured by trapping at set depths from 0–70m in three large oligotrophic lakes, including one where inorganic sediment from a glacially-fed river produces turbid conditions. Bullies occurred at all depths from 0.5–70m in the clear lakes, but none were present below 25m in the turbid lake. Two groups of bullies were present in the clear lakes; a high-density, littoral stock at depths of 0.5–25m, and a low-density, profundal stock at depths of 30–70m. These groups were further distinguished by differing buoyancy requirements and feeding habits. The swimbladders of littoral bullies contained gas, but those of the profundal bullies, which fed more than littoral bullies by both day and night, did not. The variation in mean CPUE with depth within the littoral zone was not related to water temperature, oxygen concentration, or conductivity. Nor was it related to a reduction in light levels or to reduced water transparency caused by increased turbidity. It may therefore be controlled by biotic factors. The absence of a profundal stock below the littoral zone in the turbid lake indicates that the settlement of fines from turbid inputs may affect bully abundance in deeper waters. As conventional measures of the abundance of benthic fish in lakes are often restricted to littoral habitats, and do not reflect changes in abundance with depth, an index of overall abundance based on depth distribution was developed to allow comparisons between lakes.     

Limnological studies in a large,deep, oligotrophic lake (Lake Ohrid,Yugoslavia). A summary of nutritional radiobioassay responses of the pelagial phytoplankton

In situ carbon-14 bioassay techniques were used during 1972–1974 to estimate nutritional preferences and requirements of the pelagial phytoplankton in Lake Ohrid, Yugoslavia.Bioassay measurements, conducted spatially and temporally, and corrected appropriately, showed a strong stimulation to phytoplankton photosynthesis rates during most seasons in epilimnetic waters following microadditions of inorganic silica and iron. Photosynthetic stimulation was additive for individual elements and synergistic when simultaneous additions were made. Marked stimulation occurred for both elements during the spring, especially in upper hypolimnetic waters (50–75 m) and correlated strongly with dominant diatom populations.The addition of an organic chelator, nitrilotriaceticacid (NTA), commonly stimulated photosynthetic rates and, frequently, as deep as 15o m, considerably below the depth of optimal light availability for photosynthesis. With minor exception, the simultaneous addition of this chelator and inorganic iron were always capable of producing stimulatory responses.Inorganic phosphorus was preferred over nitrogen and generally found to be more stimulatory. Phosphorus stimulation was restricted primarily to periods of spring and summer production. The addition of glucose, acetate and glycine produced positive responses, while the addition of several vitamins showed little effect except for a stimulation from microamounts of B₁₂. The addition of two organic growth substances, Gibberellic acid (GA) and Indoleacetic acid (IAA), both stimulated phytoplankton photosynthesis.The results of more than 140 bioassay measurements indicate the pelagial phytoplankton are severely restricted nutritionally due to specific physical-chemical interactions occurring in Lake Ohrid. Similar mechanisms have been postulated and evaluated in other hand-water lake systems. In spite of the great depth of the lake (300 m+) and probable age, these mechanisms are undoubtedly responsible for the low phytoplankton production occuring in the lake.     

Colonization and succession of submerged macrophytes in shallow Lake Væng during the first five years following fish manipulation

Colonization of submerged macrophytes and changes in species composition were studied in shallow Lake Væng during the first five years (1987–91) following fish manipulation in 1986–1988 and a resultant significant improvement in lake water transparency. No submerged macrophytes were present in the lake from 1981–1986, during which time the summer mean Secchi depth ranged from 0.6 and 0.8 m. From 1987 to 1990, Secchi depth increased from 0.9 m to 1.8 m and macrophyte coverage consequently increased (1 % of the lake area in 1987, 2% in 1988, 50% in 1989, 80% in 1990 and 90% in 1991). At the same time, the macrophytes became taller, and the weedbeds more dense. The macrophytes colonized from the exposed and deeper part of the lake towards the sheltered and more shallow part of the lake, a colonization pattern that was confirmed by transplantation experiments. The delay in colonization of the shallow parts may be caused by waterfowl grazing. The vegetation was initially dominated by Potamogeton crispus L., but there was a gradual change during 1988–1989 and Elodea canadensis Michx became exclusively dominant in 1990–1991.     

Submerged aquatic vegetation correlations with depth and light attenuating materials in a shallow subtropical lake

A 3-year study was done to quantify the biomass of submerged aquatic vegetation (SAV) and its relationship with environmental attributes in Lake Okeechobee, the largest lake in the southeastern United States. Plants were sampled on 21 occasions at sites located along 15 fixed transects around the shoreline, giving rise to 721 observations of SAV species (Chara spp., Vallisneria americana, Hydrilla verticillata, Potamogeton illinoinensis) dry weight biomass. Environmental sampling focused on factors that attenuate light, including phytoplankton chlorophyll a (chl a), total suspended solids (TSS), non-volatile suspended solids (NVSS) and color. Depth and Secchi transparency also were measured. Based on regression analysis, NVSS was considerably more important in attenuating light than chl a or color. Total biomass of SAV varied from 0 to 271 g dw m^–2, with a mean of 4.7 g dw m^–2, and strong dominance by Chara. The SAV biomass was lower than average for Florida lakes, and may reflect the influence of suspended solids on underwater irradiance, as well as high water level in the late 1990s. Dense SAV was found only where depth was < 2 m and TSS < 20–30 mg l^–1. At locations where high biomass of SAV occurred, the plants may have influenced water quality, because concentrations of TSS, NVSS, and chl a were 2–3 fold lower than at sites with no plants. The potential effects of SAV also were apparent at a regional scale. The shoreline region of the lake displayed a pattern of rising and falling chl a and NVSS with water depth. This occurred both at sites with and without plants, suggesting that it may be driven by physical processes, such as water circulation patterns, which are influenced by depth. However, the pattern was dampened at sites with SAV, indicating a potential to influence these attributes of water quality.     

Primary production of ice algae on a seasonally-ice-covered,continental shelf

Summary Ice algae samples were collected from the winter pack ice off Labrador during March 1984. The population was dominated by centric diatoms. Chlorophyll concentrations ranged from 40 to 190 mg m^–3, and particulate organic carbon from 2 to 10 g m^–3. Assimilation numbers for the ice algae ranged from 1.4 to 2.8 mg C (mg chl)^–1 h^–1 with a mean of 2.3, and were not significantly different from the assimilation numbers of the pelagic community beneath the ice. The ice algae were not photoinhibited at light intensities approaching surface light intensities. It is postulated that the dynamic nature of the ice field permits near-surface light intensities to reach the ice algae community at irregular intervals thereby suppressing photoinhibition.     

The diversity,biomass and production of zooplankton in Lake Inarijärvi

About 650 zooplankton samples were collected from Lake Inarijärvi in 1977–1979 from the littoral and pelagial zones of the lake. One hundred and twenty-three zooplankton taxa were found and most of them can be considered euplanktonic.The most important species were Holopedium gibberum, Daphnia cristata, Cyclops spp. and Eudiaptomus spp. Mean pelagial zooplankton biomass was 0.29 g m^–3 in the 0–5 m depth zone, 0.17 g m^–3 in 5–10 m and 0.11 g m^–3 in 10–20 m.The zooplankton biomass at a sandy shore was about 0.09 g m^–3, at a stony shore 0.05 g m^–3 and at a vegetated shore 0.76 g m^–3. About 70% of the whole zooplankton production consisted of crustaceans.The sum of herbivore and carnivore zooplankton production in the pelagial area during the summer was 210–330 kg ha^–1 × 3 months.     

Effects of thermal effluents from the Bergum power station on the phytoplankton in the Bergumermeer

Summary The Bergum power station (600 MW) of the Friesian Provincial Electricity Board is situated at the northern shore of the Lake Bergum. The lake has a mean depth of 1.3 m and a surface area of 4.4 km². Its northern half is separated by a break-water into an intake area in the north-west and a discharge area in the north-east.The Lake Bergum is connected with other water bodies in the northern provinces of the Netherlands by four canals. The whole yaer various amounts of water enter Lake Bergum mainly from the western canal (Prinses Margrietkanaal) and to a lesser extent from the southern canal (De Lits). In wet seasons lake water flows off, mainly after passing the power station, to the northern canal (De Zwemmer); then the heated water (22 m³.sec_–1) does not enter the discharge area of the lake. When evapo-transpiration exceeds precipitation lake water flows off mainly to the eastern canal (Kolonelsdiep). In these relatively dry periods most of the heated water returns to the lake in the discharge area.We found that the mean increase in water temperature effected by the condensors of the power station was ca. 5°C; the maximum increase was 7.5°C. On average about 25% of the whole lake had a noticable higher (1°C) temperature than the intake water, only 6.5% was about 2°C above ambient temperatures.For about 3.5 years (1974–Sept. 1978) water samples for analysis of the chlorophyll concentrations of the different areas within the lake and the surrounding canals were taken every week during the growing season, and fortnightly during the winter period. The chlorophyll concentrations of the intake water were about 5% higher than those of the discharge water leaving the power station. Near the mouth of the northern canal in the discharge area still small, but significant lower chlorophyll concentrations were found. The southern half of the lake, in which practically no elevated water temperatures were found, had significant higher chlorophyll concentrations (10–15%) than the intake area. Water entering the lake from the western canal had significant (10–15%) lower chlorophyll concentrations than the intake area of the lake. Probably, relatively chlorophyll-poor canal water and chlorophyll-rich water from the southern lake area mix in the intake area. While the water passes the power station the chlorophyll concentrations decrease. In the discharge area of the lake the chlorophyll concentrations of the discharge water gradually increase again to values equal to those of the intake area.During the last 2 years of the research period oxygen production and consumption experiments were conducted almost every month. In each experiment light and dark botties containing intake and discharge water were suspended in water with both water temperatures. The light intensities during the incubation periods (2–3 hours) were chosen according to maximum production values. The incubations were started within one hour and/or one day after sampling. Directly after sampling gross productivity of the intake water incubated at discharge temperatures was about 1.5 times as high as at intake temperatures. The gross productivity of the discharge water was always somewhat lower than the gross productivity of the intake water incubated at corresponding temperatures. After one day this inhibiting effect of passage through the power station had increased, even when the discharge water had been cooled down to intake temperatures immediately after sampling.The oxygen consumption of the discharge water incubated at discharge temperatures as well as at intake temperatures was about 1.3 times the oxygen consumption of the intake water at intake temperatures. After one day the discharge water, which had stayed at discharge temperatures, consumed 1.6–1.7 times as much as the intake water incubated at intake temperatures. The oxygen consumption of the discharge water which had been cooled down to intake temperatures directly after sampling, was after one day still 1.3 times the oxygen consumption of the intake water at intake temperatures.This research was financially supported by the Ministerie van Volksgezondheld en Millieuhygiëne (Ministry of Public Health and the Environment). An extensive report (in Dutch) will be published this year.     

Incomplete mixing of limed water and acidic runoff restricts recruitment of lake spawning brown trout in Hovvatn, southern Norway

Repeated liming of Hovvatn during the 1981–1995 period assured successful reintroduction of lake spawning brown trout, Salmo trutta. Poor natural recruitment to the population was associated with low survival during early life stages (before hatching) as shown by the 0.5, 3.5, 0.9 and 1.0% of live embryos found in natural redds during the 1992–1995 period, respectively. The low survival was most likely caused by the combination of shallow spawning areas (<2.0 m) and acidic runoff (pH 4.0–4.8) which overlayed the limed part of the water body during the ice covered period. It is therefore concluded that this type of episodic acidification poses a major threat to lake spawning salmonids, and that it can retard or inhibit biotic recovery towards preacidified conditions expected as a result of liming. Addition of limestone gravel (8–32 mm) onto spawning grounds was an efficient alternative liming strategy as 33–36% live embryos were found in this substrate. Conversely, the trout actively avoided additions of shellsand, a behaviour most likely caused by the small particle size of shellsand (3–7 mm) relative to natural spawning gravel.     

Integrated cultivation of salmonids and seaweeds in open systems

Bacterial abundance and production in a vertical profile in Lake Kariba (17dgS), Zimbabwe, were affected by solar irradiance. At the surface, 1.87 × 10⁹ bacteria 1^–1 were found and abundance peaked at 10 m (2.5 × 10⁹ bacteria l^-1), then decreasing with depth. Bacterial reproduction at the surface(0.145 µg C1^–1 h^–1) was nearly four times less than the production at 10 m although bacterial numbers were only 26% less. Thus, bacterial production per cell was lower at the surface than deeper down, suggesting that bacterial production is inhibited at the surface.Bacterial production in GF/F filtered lake water in Whirl Pack bags showed an exponential decrease down to 3 m depth. The inhibition was well in accordance with light extinction in the UV region. Phosphatase activity was low in light exposed bags compared to dark, indicating photolysis of extracellular enzymes, or phototransformation of recalcitrant DOM, which substitutes enzyme activity. Hypolimnetic enzyme activity was less affected by solar light than epilimnetic.     

Vertical Distribution of the Far East Trepang Apostichopus japonicusin Vostok Bay,Sea of Japan

The abundance, weight, and age structure of aggregations of the Far East trepang Apostichopus japonicuswere studied in Vostok Bay, Sea of Japan, during the first part of September 2000. The highest density of aggregations (8.3 ± 0.5 ind/m²) and biomass (131.88 ± 10.50 g/m²) of animals was characteristic for coastal sites at the depth 0.5–1.5 m. With anincrease in depth the biomass and density of A. japonicusreduced 1.5 and 3 (depth 5–6 m), 220 and 830 times (depth 8–15 m) respectively. The ratio of large animals concurrently increased in the aggregation structure. The specifics of the distribution and spatial variability of the aggregation structure are discussed in relation to uncontrolled fishing of holothurians of large sizes.     

Field biology of Halimeda tuna (Bryopsidales, Chlorophyta) across a depth gradient: comparative growth, survivorship, recruitment, and reproduction

Growth, survivorship, recruitment, and reproduction of Halimeda tuna, a dominant green alga in many reef systems of the Florida Keys, were monitored at a shallow back reef (4–7m) and deep reef slope (15–22 m) on Conch Reef. Despite lower light intensities and similar grazing pressures, amphipod infestations, and epiphyte loads at both sites, the deeper site exhibited significantly higher growth rates in summer months over a 4-year period than found for the shallow population, possibly because of higher nutrient levels at depth and photoinhibition of shallow plants. Sexual reproductive events occurred simultaneously across the entire reef, with up to 5% of the population at both sites developing gametangia. New upright axes formed from zygotes, asexual fragmentation, or vegetative runners. Plants appear to have persistent basal stumps that survive harsh environmental conditions, even if upright, photosynthetic axes are removed. Sexual reproduction and smothering by epiphyte overgrowth are hypothesized to be two causes of death for individuals.     

Species richness,composition, and abundance of fish larvae and juveniles inhabiting natural and developed shorelines of a glacial Iowa lake

Synopsis Young-of-the-year fish communities in naturally vegetated sites were compared with those inhabiting nearby sites where lakeshore development (i.e., construction of homes, boat docks, and beaches) reduced nearshore macrophyte species richness and abundance. The study was conducted in a 2266 hectare, glacially formed, eutrophic lake in northwestern Iowa during the summers of 1987 and 1988. Study sites were divided into 3 depth zones, and fishes were collected by seining (0–1 m), plexiglass traps (1–2 m), and a nonclosing Tucker trawl (2–3 m). Species richness and total fish abundance were consistently greater in natural than in developed sites in both nearshore (0–1 m) and intermediate (1–2 m) depth zones, but differed little between natural and developed sites in the offshore (2–3 m) depth zone. Nearly 50% of the species sampled, including yellow perch Perca flavescens and bluegill Lepomis macrochirus, inhabited limnetic areas as larvae before migrating inshore as juveniles. Eighteen of the 20 fish species collected as juveniles were in greater abundance in natural than in developed sites. Smallmouth bass Micropterus dolomieui was the only game species consistently found in equal or greater abundance in developed sites. Within all sites, juvenile fishes were generally most abundant where macrophyte abundance and species richness were greatest. Findings from this study demonstrate the importance of nearshore aquatic vegetation to fishes during their first summer of life. If nearshore vegetation beds of lakes continue to be regarded as a nuisance and indiscriminately removed, important fish nursery habitat will be lost. The short-term result will likely be reduced year-class strength of vegetation-dependent species. More importantly, the long-term effects will be changes in fish community richness and composition which will, in turn, alter the lake's fishery.     

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.