Seasonal changes in the underwater light climate of two Canadian shield lakes

This study examines the seasonal variation in the underwater spectral distribution of light in a mesotrophic (Lake Cromwell) and an oligo-mesotrophic (Lake Croche) temperate lake. Gilvin is primarily responsible for the strong selective attenuation of blue light in both lakes. As a result of differing gilvin concentrations light transmission maxima of downwelling and upwelling spectra are near 615 nm in Lake Cromwell and 599 nm in Lake Croche. With increases in depth both upwelling and downwelling radiance fluxes decrease, are shifted to longer wavelengths and become more monochromatic. The greatest penetration of light occurs in the summer and spring after which a gradual decrease occurs through fall to a minimum value in winter. Under the winter cover the P ₅₀ of downwelling light shifts 10 to 20 nm towards shorter wavelengths. Seasonal changes in downwelling irradiance are related to solar altitude, concentration of suspended particles, phytoplankton populations, amount of gilvin, mixing and winter cover. The brownish colouration of these lakes is explained by reflectance of spectrally impure orangish-red light.     

Long and short term variations in suspended particulate material: the influence on light available to the phytoplankton community

The magnitude and frequency of events leading to changes in turbidity have been studied in a large (61 km²), shallow (mean depth 3.4 m) wind-exposed lake basin at the western end of Lake Mälaren, Sweden. In this paper we couple changes in suspended particulate inorganic material (SPIM) resulting from wind driven sediment resuspension, and variations in the discharge and sediment load, to spectral variations in subsurface light and estimates of photosynthetically active radiation (PAR). To accomplish this we use a semi-analytical model which predicts the spectral variations in downwelling irradiance (E _d()) and the attenuation coefficient of downwelling irradiance (K _d()), as a function of the concentrations of chlorophyll, dissolved yellow substances, and suspended inorganic and organic particulate material. Unusually high river discharge, led to large inputs of yellow substances and large in lake yellow substance concentrations (a _ys(420) 20 m^–1), causing variations in yellow substance concentration to have the greatest role in influencing temporal trends in the attenuation of PAR and variations in the depth of the euphotic zone (Z _eup). In spite of this, variations in SPIM could account for approximately 60% of the variation in Z _eup attributed to changes in yellow substances alone. Our results show that changes in suspended sediment concentration leads to both long-term and short-term changes in the attenuation of PAR, even in the presence of high concentrations of dissolved yellow substances.     

The role of phytoplankton in determining the underwater light climate in Lake Constance

At all seasons, the underwater light field of meso-eutrophic large (480 km²) deep (mean: 100 m) Lake Constance was studied in conjunction with the assessments of vertical distributions of phytoplankton chlorophyll concentrations. Vertical profiles of scalar, downwelling and upwelling fluxes of photosynthetically available radiation, as well as fluxes of spectral irradiance between 400 and 700 nm wavelength were measured.The overall transparency of the water for PAR is highly dependent on chlorophyll concentration. However, the spectral composition of underwater light is narrowing with water depth regardless of phytoplankton biomass.Green light is transmitted best, even at extremely low chlorophyll concentrations. This is explained by the selective absorption of blue light by dissolved organic substances and red light by the water molecules. Nevertheless, significant correlations were found between vertical attenuation coefficients of downwelling spectral irradiance and chlorophyll concentrations at all wavelengths. The slopes of the regression lines were used as estimates of chlorophyll-specific spectral vertical light attenuation coefficients (K _c()).The proportions of total upwelling relative to total downwelling irradiance (reflectance) increased with water depth, even when phytoplankton were homogeneously distributed over the water column. Under such conditions, reflectance of monochromatic light remained constant. Lower reflectance of PAR in shallow water is explained by smaller bandwidths of upwelling relative to downwelling light near the water surface. In deeper water, by contrast, the spectra of both upwelling and downwelling irradiance are narrowed to the most penetrating components in the green spectral range. Reflectance of PAR was significantly correlated with chlorophyll concentration and varied from 1% and 1-% at low and high phytoplankton biomass, respectively. Over the spectrum, reflectance exhibited a maximum in the green range. Moreover, in deeper layers, a red maximum was observed which is attributed to natural fluorescence by phytoplankton chlorophyll.     

Photic Volume in Photobioreactors Supporting Ultrahigh Population Densities of the Photoautotroph Spirulina platensis

Characterization of the photic zone and light penetration depth in cultures with ultrahigh cell densities represents a major issue in mass cultures of phytoautotrophic microorganisms grown in enclosed photobioreactors. In a study of the effect of underwater optical properties on the penetration depth of photosynthetically active radiation, the inherent optical properties of algal suspensions, i.e., absorption and scattering coefficients, as well as their apparent optical properties, i.e., the reflectance and the vertical attenuation coefficient of downwelling irradiance, were determined by using high-spectral-resolution radiometric measurements. The vertical attenuation coefficient was used to estimate quantitatively the depth of light penetration into a reactor containing an ultrahigh cell density (chlorophyll concentration, up to 300,000 mg m(sup-3)). For such a high cell density, the photic volume in the reactor was found to be extremely small; nevertheless, it differed between the blue and red light (less than 0.06 mm) and the green light (about 0.5 mm). This suggests a singular role for green light under the unique circumstances existing in ultrahigh-cell-density cultures of photoautotrophs.     

In situ spectroradiometric estimation of microalgal biomass in first-year sea ice

Summary First- and multi-year sea ice are colonized by microalgae, whose biomass modifies the spectral distribution of underice downwelling irradiance. It is proposed that an index of algal biomass in the first-year ice may be derived from the ratio of underice irradiance at a wavelength where absorption by chlorophyll a is high to a wavelength where absorption by the photosynthetic pigments is low and transmission through the ice is high. In southeastern Hudson Bay (Canadian Arctic), the irradiance ratio (671540 nm) accounts for 55% of the variance in chlorophyll a, indicating that the in situ biomass of algae in first-year ice can be estimated from spectral measurements of underice downwelling irradiance.Contribution to the programme of GIROQ (Groupe interuniversitaire de recherches océanographiques du Québec)     

Optical teledetection of the vertical attenuation coefficient for downward quantum irradiance of photosynthetically available radiation in turbid inland waters

Depth profiles of downward quantum irradiance of photosynthetically available radiation in situ and spectral subsurface irradiance reflectance, obtained from water-leaving radiance, were determined in different inland water types. These included the large, shallow and eutrophic IJssel lagoon in the Netherlands, eighteen Dutch lakes differing in depth and trophic state, and the large, shallow and eutrophic Lake Tai in China. The attenuation coefficient for downward irradiance ranged from 0.7 to 5.4 m^-1. An empirical relationship with the backscattering coefficient in the near-infrared waveband, derived from the reflectance, and three spectral reflectance band ratios explained 91% of the variation. The potential assessment of the spatial distribution of phytoplankton production through shipboard optical teledetection of the concentration of chlorophyll-a and the attenuation coefficient for downward irradiance is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Visual predation during springtime foraging of the North Atlantic right whale (Eubalaena glacialis)

To assess the role that vision plays in the ability of the North Atlantic right whale (Eubalaena glacialis) to detect its primary prey species, the calanoid copepod Calanus finmarchicus, we have compared the absorbance spectrum of the E. glacialis rod visual pigment, the transmittance spectra of C. finmarchicus carotenoid pigments, as well as the downwelling irradiance and horizontal radiance spectra collected during springtime at three locations in the western Gulf of Maine. The E. glacialis rod visual pigment absorbs light maximally at 493 nm, while microspectrophotometric measurements of the C. finmarchicus carotenoid pigments reveal transmission spectra with minima matching very well with the E. glacialis rod visual pigment absorbance spectra maximum. Springtime spectral downwelling irradiance and horizontal radiance values from the surface waters of Cape Cod Bay and at all depths in Great South Channel overlap the E. glacialis rod absorbance spectrum, allowing C. finmarchicus to appear as a high‐contrast dark silhouette against a bright background spacelight, thus facilitating visually guided contrast foraging. In contrast, spectral downwelling irradiance and horizontal radiance at depth in Cape Cod Bay, and all depths in Wilkinson Basin, do not overlap the E. glacialis rod absorbance spectrum, providing little if any useful light for contrast vision.     

A comparison of in situ and simulated in situ methods for estimating oceanic primary production

Primary production data measured by in situ (IS) and ‘simulated’in situ (SIS) incubations were compared. To minimize differencesbetween the two types of incubations, SIS experiments were conductedin temperature-controlled incubators in which the spectral distributionand irradiance were adjusted to approximate IS conditions. ISavailable irradiance (I_Is) was computed from vertical attenuationof integrated surface irradiance. Vertical attenuation was estimatedusing a spectral irradiance model, validated by measured profilesof the vertical attenuation coefficient. IS incubations werecarried out using two methods. The first involved deploymentof bottles on a drifting array for whole-day (dawn to dusk)incubations. The second method employed an autonomous submersibleincubation device that performed short term (<1 h) incubationsat multiple depths. Differences between whole-day IS and SISincubation estimates were attributed partially to differencesbetween I_IS and SIS-available irradiance (I_SIS). Photosynthesis-irradiance(P-I) properties of IS and SIS populations from the whole-dayincubations were not significantly different. P-I propertiesof the short-term IS and SIS populations were significantlydifferent, although estimates of P^B (mg C mg Chl^–1 h^–1)from contemporaneous IS and SIS incubations did not differ by>40%. Integrated water-column primary production (IPP) estimatedusing P-I models derived from SIS data were within 15% of ISestimates of IPP.     

The spectral distribution and attenuation of underwater irradiance in Tasmanian inland waters

SUMMARY. 1. Measurements were made of the attenuation and spectral distribution of downwelling and upwelling photosynthetically-available radiation (PAR) in all the principal types of natural waters found in Tasmania. 2. Most lakes in the State are clear and non-turbid, with water itself and the low concentrations of gilvin being the principal determinants of the green underwater light climate. Many others are deeply coloured by dissolved and colloidal organic material (gilvin, gelbstoff) which rapidly attenuates short wavelengths, specifying a shallow, predominantly red euphotic zone. 3. A spectrophotometric measure of colour, the absorption coefficient at 440 nm, is statistically related to measurements on the platinum scale with good precision. 4. Few Tasmanian lakes are turbid but in those that are the underwater light climate is almost identical to that of non-turbid, humic lakes. 5. Reflectance, R, varied with depth but not in the asymptotic way previously encountered. A linear relationship existed between the scattering coefficient, b, and nephelometric turbidity, but not at the approximate 1:1 ratio reported elsewhere. 6. Most Tasmanian lakes are oligotrophic or dystrophic and phytoplankton rarely influenced the underwater light field. 7. Seasonal variation in optical character is not great in natural lakes and their optical properties and light fields can be used typologically. 8. Simple and multiple regression analysis showed that Secchi depth was a poor predictor of euphotic depth but the optical properties and the underwater light field of inaccessible lakes could be reasonably predicted from laboratory measurements made on small water samples, using regressions developed for a wide range of lake types and by reference to the quantaradiometric scans of lakes with comparable optical properties. 9. An optical classification of Tasmanian lakes made by cluster analysis agreed reasonably well with one based on edaphic, vegetational and chemical criteria.     

Daily variations in the optical properties of a small lake

1. The major components of the underwater light field (ULF: vertical attenuation, absorption, scattering and suspensoids including plankton fractions) of Las Madres Lake, a small wind‐sheltered, oligohumic lake in Central Spain, were investigated daily over a period of 3 months at the onset of vernal circulation. 2. Gilvin, arising mostly from the decomposition of reeds in the littoral in autumn, was the main component of vertical attenuation, and its variability explained the highest fraction of absorption variability. Tripton appeared to be the main factor responsible for scattering, and might have resulted from dust deposition from the surrounding mining land. The plankton community played a minor role in attenuation, absorption or scattering throughout the investigation period. 3. Vertical and horizontal mixing dynamics may control the ULF to a certain extent, as most optical properties changed within different mixing periods and poor advective exchanges may have resulted in uneven distribution of water colour in this small lake. 4. Time series analysis showed that most autocorrelations were shorter than a week, inherent properties (absorption, scattering) being delayed longer than apparent properties (attenuation, transparency) as a result of their lower dependence on solar irradiance. A 2‐day lag was observed in cross‐correlations between either gilvin and absorption or tripton and scattering. When different mixing periods at early circulation were considered, however, ULF components changed their relationships and delays with suspensoids and dissolved substances over such periods, probably tracking the dynamics of their controlling factors. 5. Our study, and others at daily, weekly, seasonal, interannual and long‐term scales, demonstrates that ULF is a system upon which different processes are operating at different time scales. Contrary to expectations, however, the variability in the ULF does not increase with time scale and depends partly upon the trophic status of lakes.     

Attenuation of visible light by phytoplankton in a vertically structured ocean: solutions and applications

To find the solution to problems in applied marine optics, wedo not always need to compute the submarine irradiance at alldepths; in many cases, it is sufficient to know the averageattenuation of light for finite layers of arbitrary thickness.If multiple scattering can be neglected, the average attenuationof light in a finite layer of the water column depends onlyon the vertical distribution of the attenuating substances;in open-ocean waters, the most important of these is phytoplankton.It is shown how the average attenuation of monochromatic lightin an arbitrary layer can be determined when the vertical pigmentprofile takes one of two standardized forms: a shifted Gaussianor a triangle. The choice of efficient algorithms to computethe attenuation in a layer using these models is discussed.The extension to polychromatic light involves the selectionof a function to represent the spectral distribution of thespecific absorption coefficient for phytoplankton, as determinedby observation. Chebyshev polynomial representation is shownto be convenient for applications which require the calculationof a weighted wavelength integral. An efficient procedure forthe evaluation of these integrals, Gauss-Chebyshev quadrature,is presented and evaluated. The extension to the computationof bulk properties for discrete layers is straightforward.     

Measurements of spectral optical properties and their relation to biogeochemical variables and processes in Crater Lake,Crater Lake National Park,OR

Spectral inherent optical properties (IOPs) have been measured at Crater Lake, OR, an extremely clear sub-alpine lake. Indeed Pure water IOPs are major contributors to the total IOPs, and thus to the color of the lake. Variations in the spatial distribution of IOPs were observed in June and September 2001, and reflect biogeochemical processes in the lake. Absorption by colored dissolved organic material increases with depth and between June and September in the upper 300 m. This pattern is consistent with a net release of dissolved organic materials from primary and secondary production through the summer and its photo-oxidation near the surface. Waters fed by a tributary near the lake’s rim exhibited low levels of absorption by dissolved organic materials. Scattering is mostly dominated by organic particulate material, though inorganic material is found to enter the lake from the rim following a rain storm. Several similarities to oceanic oligotrophic regions are observed: (a) The Beam attenuation correlates well with particulate organic material (POM) and the relationship is similar to that observed in the open ocean. (b) The specific absorption of colored dissolved organic material has a value similar to that of open ocean humic material. (c) The distribution of chlorophyll with depth does not follow the distribution of particulate organic material due to photo-acclimation resulting in a subsurface pigment maximum located about 50 m below the POM maximum.     

Inorganic tripton in the Finger Lakes of New York: importance to optical characteristics

Inorganic particles in the upper waters of the 11 Finger Lakes of New York are morphometrically and elementally characterized by individual particle analysis conducted with scanning electron microscopy interfaced with automated image and X-ray analyses (IPA/SAX). Coupled measurements of Secchi disk transparency (SD), the attenuation coefficient for downwelling irradiance (K_d), the beam attenuation coefficient at 660 nm, and turbidity (T_n) were made to support evaluation of the importance of non-living, inorganic particles (inorganic tripton) in regulating these optical features of water quality. Wide differences in levels of inorganic tripton, represented in terms of particle projected area per unit volume (PAV_in), and the optical measures are reported for these lakes. However, generally similar size distributions are observed for the inorganic tripton for the lakes. Terrigenous suspensoids, in the form of clay minerals, dominated the inorganic tripton particle assemblage of nine lakes, while CaCO₃, formed autochthonously, dominated in the other two and was a noteworthy contributor in four others. PAV_in is demonstrated to be an important regulator of the optical properties of these lakes, performing substantially better than chlorophyll in predicting SD, and T_n, and interlake differences in these optical measures.Contribution No. 226 of the Upstate Freshwater Institute     

Bio-optical Characteristics and the Vertical Distribution of Photosynthetic Pigments and Photosynthesis in an Artificial Cyanobacterial Mat

Abstract Zonations of photosynthesis and photopigments in artificial cyanobacterial mats were studied with (i) oxygen and pH microsensors, (ii) fiber-optic microprobes for field radiance, scalar irradiance, and PSII fluorescence, and (iii) a light microscope equipped with a spectrometer for spectral absorbance and fluorescence measurements. Our analysis revealed the presence of several distinct 1–2 mm thick cyanobacterial layers mixed with patches of anoxygenic photosynthetic bacteria. Strong attenuation of visible light confined the euphotic zone to the uppermost 3 mm of the mat, where oxygen levels of 3–4 times air saturation and a pH peak of up to pH 8.8 were observed under saturating irradiance (413 μmol photon m⁻² s⁻¹). Oxygen penetration was 5 mm in light and decreased to 1 mm in darkness. Volumetric oxygen consumption in the photic and aphotic zones of illuminated mat was 5.5 and 2.9 times higher, respectively, than oxygen consumption in dark incubated mats. Scalar irradiance reached 100–150% of incident irradiance in the upper 0.5 mm of the mat due to intense scattering in the matrix of cells, exopolymers, and carbonate precipitates. In deeper mat layers scalar irradiance decreased nearly exponentially, and highest attenuation coefficients of 6–7 mm⁻¹ were found in cyanobacterial layers, where photosynthesis and photopigment fluorescence also peaked. Visible light was attenuated >100 times more strongly than near infrared light. Microscope spectrometry on thin sections of mats allowed detailed spectral absorbance and fluorescence measurements at defined positions relative to the mat surface. Besides strong spectral signals of cyanobacterial photopigments (Chl a and phycobiliproteins), the presence of both green and purple photosynthetic bacteria was evident from spectral signals of Bchl a and Bchl c. Microprofiles of photopigment absorbance correlated well with microdistributions of phototrophs determined in an accompanying study. Received: 20 December 1999; Accepted: 10 June 2000; Online Publication: 28 August 2000     

Fluorescence characteristics of a natural assemblage of freshwater picocyanobacteria

The fluorescence characteristics of a freshwater assemblageof picocyanobacteria weredetermined in a mesotrophic lake usingmicrospectrofluorometry. In Jack's Lake. Ontario. 72–98%of the assemblage was comprised of cells with a single excitationpeak for chlorophyll (Chl) a (emission at 680 nm) at 565 ±3)nm. theexcitation spectra for Chl a resembling the spectralcomposition of downwelling irradiance. The assemblage was, therefore,dominated by a single phycobiliproteinpigment type, similarto type 2 phycoerythrin (PE) marine Synechococcus strains. Theshape of excitation spectra did not change significantly withdepth down to 0.6% of incident irradiance or between samplingdates, although the relative intensity of the PE excitationpeak was generally greater for populations below the thermoclinecompared to surface populations during summer stratification.Two separate populations of PE-containing picocyanobacteria,distinguished based on their morphology and plane of division,could be further separated based on their emission spectra (usingblue excitation): a Synechocystis type cell (PE-Sys) consistentlyhad a more pronounced peak at 665 nm from allophycocyanin comparedto a Synechococcus (PE-Syn) type cell. In addition, the ratioof the PE to Chl a peak emissions was higher in PE-Sys and increasedsignificantly with depth below the thermocline. While nitrogenwas limiting in the lake in summer, experimental additions ofnitrogen did not significantly affect this ratio in surfacewater populations, but increased the ratio in PE-Syn populationsat the base of the photic zone. For surface assemblages of picocyanobacteria,high irradiance may be more-important in regulating fluorescencecharacteristics than nitrogen stress. ¹Deceased September 7, 1993     

ABSORPTION AND UTILIZATION OF IRRADIANCE BY CYANOBACTERIAL MATS IN TWO ICE-COVERED ANTARCTIC LAKES WITH CONTRASTING LIGHT CLIMATES

We investigated the under-ice light climate and the efficiency with which light was absorbed and utilized by benthic algal mats in Lakes Hoare and Vanda, two perennially ice-covered lakes in the McMurdo Dry Valleys area of Southern Victoria Land, Antarctica. The ice cover and water column of Lake Vanda were much more transparent than those of Lake Hoare (18% vs. 2% transmission though ice and attenuation coefficients for downwelling irradiance of 0.05 vs. 0.12 m ^{− 1}, respectively). In both lakes the under-ice spectra were dominated by blue-green wavelengths. The benthic flora under perennial ice covers of both lakes comprised thick mucilaginous mats, dominated by cyanobacteria. The mats were well suited to absorb the dominant blue-green wavelengths of the under-ice light, with phycoerythrin being present at high concentrations. The pigment systems of the benthic mats absorbed 30%–50% of the light that reached them, varying with depth and lake. There was a tendency for the percentage of absorption to increase as ambient irradiance decreased. The efficiency of utilization of absorbed irradiance was examined by constructing absorbed irradiance/oxygen evolution curves to estimate community quantum yield. Mats from 13 m in Lake Hoare showed the highest quantum yields, approaching 1 mol of carbon fixed for every 8 mol quanta absorbed under light-limiting conditions. Lake Vanda mats had lower quantum yields, but these increased with depth. Calculated in situ irradiance occasionally exceeded the measured saturating irradiance for oxygen evolution in both lakes, thus efficiency in situ was below the maximum at times. As in other environments, optimization strategies allowed efficient capture and utilization of the lower and middle ranges of experienced irradiance but led to a compromised capacity to use the highest irradiances encountered at each depth.     

The productive and optical status of the oligotrophic waters of the Southern Aegean Sea (Cretan Sea), Eastern Mediterranean

Primary production, pigment concentrations and spectral measurementsof downwelling irradiance were made at four stations in fourseasons (spring, summer, autumn, winter) during 1994 in thewaters of the South Aegean Sea (Cretan Sea), Eastern Mediterranean.Rates of production were determined using in Situ incubationtechniques and included measurements at the surface microlayer.Depth-integrated values averaged over season were 5.66 mg Cm^–2 h^–1 for primary production and the correspondingchlorophyll (Ch1) a and phaeophytin (Phaeo) a values had meansof 4.87 and 1.21 mg m^–3 respectively. The assimilationratio remained very low (mean over season: 1.19 mg C mg^–2Chl a h^–1 as did the Phaeo a/Chl a ratio (mean over season:0.24). The annual production for the area was estimated to yield24.79 g C m^–2 year^–1. Primary production and Chla estimates showed statistically significant seasonal, spatialand depth variations. The spectral values of the attenuationcoefficient Kd (     

Inherent and apparent optical measurements in the Hudson/Raritan Estuary

Duringan August, 1999 field campaign, measurements were made to establish hydrologicoptical properties of the Hudson/Raritan Estuary (New York-New Jersey): 1)concurrent above-and below-surface spectral irradiance; 2) sampling forlaboratory determination of inherent optical properties; and 3) concentrationsof optically-important water quality parameters. We used a bio optical modelbased on to predict thesubsurface irradiance reflectance from optically important water constituents.This model was then validated with the measured reflectance spectra from thefield spectroradiometers. Modeling of reflectance is a prerequisite forprocessing remote sensing data to desired thematic maps. These are key input tothe geographic information system (GIS) used to manage the water qualitycondition of the estuary.     

Light climate and plankton in the deep chlorophyll maxima in North Patagonian Andean lakes

The light climate at the deep chlorophyll maxima (DCM) was analysedin a set of lakes of the North Patagonian Andean region. Apparentand inherent optical properties in relation to the chlorophylla vertical distribution were investigated in seven lakes includingdeep (Z_max > 90 m) and shallow (Z_max < 12 m) ones. Samplingwas carried out during the thermal stratification period (summer)of the deep lakes since in the shallower lakes no stable thermalstratification was detected. The large deep lakes presentedvery low diffuse attenuation coefficients of photosyntheticallyactive radiation (K_d PAR), and a DCM situated at 0.98–2%of surface PAR irradiance, coinciding with the maximum abundanceof the mixotrophic ciliate Ophrydium naumanni and autotrophicpicoplankton. Both fractions seemed to be favoured by dim lightconditions of particular wavelengths, since at these DCM layersmainly green and blue wavelengths prevailed (<600 nm). Incontrast, shallow lakes showed higher K_d PAR values, with ahigher concentration of dissolved yellow substances, which causedsubstantial differences in the spectral quality that may havecontributed to explain the absence of this ciliate populationin these lakes.     

Transport and accumulation of bloom-forming cyanobacteria in a large, mid-latitude lake: the gyre-Microcystis hypothesis

 Toxic cyanobacterial blooms have occurred in the near-shore waters of the North Basin of Lake Biwa, Japan, since 1994, and have been attributed to deterioration of water quality in the enriched littoral zone of the lake. From 1997 onwards, the bloom-forming cyanobacteria have been observed with increasing frequency in the deep offshore waters of the North Basin. In the present study, we examined the mechanisms responsible for these bloom populations in the main body of the lake. Specifically, we addressed the hypothesis that buoyant, nutrient-replete colonies of cyanobacteria are generated inshore, are advected offshore by large-scale horizontal transport processes, and subsequently accumulate in the downwelling center of large surface gyres that characterize the overall circulation pattern in the epilimnion of the North Basin. Diel variations of Microcystis biomass at the center and the edge of the Lake Biwa gyre were monitored at 6-h intervals on August 23–24, 2000, and the horizontal distribution of buoyant Microcystis was determined on October 6. The hydrodynamic structure of the first gyre was determined over the preceding 2 days by an on-board Acoustic Doppler Current Profiler (ADCP). The gyre was characterized by a counterclockwise horizontal current that could potentially advect material large distances offshore, a downwelling current near the center of the gyre, and an upwelling current at the edge of the gyre, caused by the radial pressure gradients. The biomass of Microcystis near the water surface was greater at the center than at the edge of the gyre, and the biomass at 5 m depth at the edge of the gyre was greater than that at the water surface or at the thermocline near the edge of the gyre. The results are consistent with the gyre-Microcystis hypothesis, and show the potential for accumulation of large concentrations of cyanobacteria in deep offshore lake environments that are normally considered unsuitable for cyanobacterial blooms. Received: July 16, 2001 / Accepted: March 6, 2002