太湖梅梁湾沿岸带水体生物学与光学特性

基于 1998～ 1999年周年 4季原位水下光场观测资料及中国科学院太湖湖泊生态系统研究站 1992～ 2 0 0 1年悬浮物、叶绿素 a、透明度长期历史观测资料分析了太湖梅梁湾沿岸带第 2号站点水体的生物学与光学特性 ,探讨了水下光合有效辐射(PAR)总量的日变化、垂直分布 ;光衰减系数的季节变化及光谱分布 ;影响光衰减系数的主要水色因子。结果表明 ,无论是 PAR还是光谱衰减系数其值都很高 ,其中 PAR衰减系数在 1.4 0～ 5 .30 / m间变化 ,均值为 2 .4 3± 0 .5 5 / m,秋季最大、夏季最小 ,真光层深度在 0 .87～ 3.2 9m间变化 ,均值为 1.98± 0 .4 1m;水下光谱在蓝光波段衰减最强烈 ,其次是红光、绿光 ,随着深度增加光谱成分出现绿移和红移现象 ,绿红光占得比例越来越大 ;光谱衰减系数随着波长的增加大致呈下降趋势 ,但在 6 70 nm附近有个峰值 ;基于线性相关分析发现在混浊的沿岸带水体中影响光衰减主要因子为水体中的悬浮物和有色可溶性有机物 ,叶绿素 a对 PAR衰减系数的贡献率只占到 1.5 9%～ 14 .2 1%。     

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.     

Light acclimation of Chlamydomonas acidophila accumulating in the hypolimnion of an acidic lake (pH 2.6)

1. The unicellular green alga Chlamydomonas acidophila accumulates in a thin phytoplankton layer in the hypolimnion (deep chlorophyll maximum, DCM) of an extremely acidic lake (Lake 111, pH 2.6, Lusatia, Germany), in which the underwater light spectrum is distorted and red‐shifted. 2. Chlamydomonas acidophila exhibited a significantly higher absorption efficiency and a higher cellular chlorophyll b content when incubated in the red shifted underwater light of Lake 111 than in a typical, blue‐green dominated, light spectrum. 3. Chlamydomonas acidophila has excellent low light acclimation properties (increased chlorophyll b content, increased oxygen yield and a low light saturation point for photosynthesis) that support survival of the species in the low light climate of the DCM. 4. In situ acclimation to the DCM under low light and temperature decreased maximum photosynthetic rate in autotrophic C. acidophila cultures, whereas the presence of glucose under these conditions enhanced photosynthetic efficiency and capacity. 5. The adaptive abilities of C. acidophila to light and temperature shown in this study, in combination with the absence of potent competitors because of low lake pH, most probably enable the unusual dominance of the green alga in the DCM of Lake 111.     

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.     

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.     

Light availability as a possible regulator of cyanobacteria species composition in a shallow subtropical lake

1. Variations in the relative biovolumes of dominant cyanobacterial taxa were evaluated in the context of environmental conditions using canonical correlation analysis (CCorrA) and Redundancy Analysis (RDA). The objective was to test a conceptual model in which underwater irradiance determines dominance by bloom-forming (high light adapted) or non-blooming (low light adapted) taxa. 2. The data set consisted of 404 contiguous observations, collected over a 3-year period at eight pelagic sites, in shallow Lake Okeechobee, Florida, U.S.A. Data included species biovolumes, total phosphorus (TP), total nitrogen (TN), dissolved oxygen (DO) and chlorophyll a concentrations, as well as two indices: underwater irradiance (Secchi depth) and the ratio of Secchi:total depth. 3. The first environmental canonical variable was strongly correlated with the two light-related indices, and negatively correlated with TP. This reflects the predominant role of resuspended P-rich lake sediments in controlling underwater irradiance in the shallow lake. The first species canonical variable displayed a strong negative correlation with Lyngbya limnetica and L. contorta, and positive correlations with Anabaena circinalis, Aphanizomenon flos aquae and Microcystis spp. The results support the conceptual model; the first pair of canonical variables explained 55% of the variation in the species–environmental data set. RDA results provided further support for the hypothesis that irradiance was the major force controlling community structure. 4. One unexpected result was a positive association between Oscillatoria spp. dominance and indicators of high irradiance. This conflicts with past research indicating that Oscillatoria is a low light adapted taxon, and the finding that it is the most abundant taxon in Lake Okeechobee. This may reflect the fact that the two Lyngbya taxa were more strongly associated with low light conditions than Oscillatoria. CCorrA results indicated that Oscillatoria densities are strongly controlled by water temperature. There is a need for more detailed studies of cyanobacteria ecophysiology in order to explain fully the seasonality of phytoplankton in this and other shallow subtropical lakes.     

Water temperature and stratification depth independently shift cardinal events during plankton spring succession

In deep temperate lakes, the beginning of the growing season is triggered by thermal stratification, which alleviates light limitation of planktonic producers in the surface layer and prevents heat loss to deeper strata. The sequence of subsequent phenological events (phytoplankton spring bloom, grazer peak, clearwater phase) results in part from coupled phytoplankton–grazer interactions. Disentangling the separate, direct effects of correlated climatic drivers (stratification‐dependent underwater light climate vs. water temperature) from their indirect effects mediated through trophic feedbacks is impossible using observational field data, which challenges our understanding of global warming effects on seasonal plankton dynamics. We therefore manipulated water temperature and stratification depth independently in experimental field mesocosms containing ambient microplankton and inocula of the resident grazer Daphnia hyalina. Higher light availability in shallower surface layers accelerated primary production, warming accelerated consumption and growth of Daphnia, and both factors speeded up successional dynamics driven by trophic feedbacks. Specifically, phytoplankton peaked and decreased earlier and Daphnia populations increased and peaked earlier at both shallower stratification and higher temperature. The timing of ciliate dynamics was unrelated to both factors. Volumetric peak densities of phytoplankton, ciliates and Daphnia in the surface layer were also unaffected by temperature but declined with stratification depth in parallel with light availability. The latter relationship vanished, however, when population sizes were integrated over the entire water column. Overall our results suggest that, integrated over the entire water column of a deep lake, surface warming and shallower stratification independently speed up spring successional events, whereas the magnitudes of phytoplankton and zooplankton spring peaks are less sensitive to these factors. Therefore, accelerated dynamics under warming need not lead to a trophic mismatch (given similar grazer inocula at the time of stratification). We emphasize that entire water column dynamics must be studied to estimate global warming effects on lake ecosystems.     

Optical Properties and Light Climate in Lake Verevi

The optical properties and light climate during the ice-free period in the highly stratified Lake Verevi (Estonia) have been studied together with other lakes in same region since 1994. The upper water layer above the thermocline belongs to class “moderate” by optical classification of Estonian lakes but can turn “turbid” (concentration of chlorophyll a up to 73 mg m⁻³ and total suspended matter up to 13.2 g m⁻³) during late summer blooms. In the blue part of the spectrum, light is mainly attenuated by dissolved organic matter and in red part notably scattering but also absorption by phytoplanktonic pigments effect the spectral distribution of underwater light. Consequently, the underwater light is of greenish-yellow color (550–650 nm). Rapid change in optical properties occurs with an increase of all optically active substances close to thermocline (2.5–6 m). Optical measurements are often hampered beneath this layer so that modeling of the depth distribution of the diffuse attenuation coefficient is an useful compliment to field measurements. K_d,PAR ranges from 0.8 to 2.9 m⁻¹ in the surface layer, and model results suggest that it may be up to 5.8 m⁻¹ in the optically dense layer. This forms a barrier for light penetration into the hypolimnion.     

Unusual links between inherent and apparent optical properties in shallow lakes,the case of Taihu Lake

The spectral distribution of downwelling solar irradiance is an important factor in the radiative balance, primary productivity and biogeochemistry in most lakes. In the present study, we show the relative importance of different inherent and apparent optical properties in controlling the spectral attenuation of diffuse downwelling irradiance in a large shallow lake in eastern China. Most importantly, we show how elevated concentrations of suspended matter not only increase attenuation, but are linked to a “spectral shift” in major attenuation peaks, with important consequences on biogeochemical processes and remote sensing. The analysis of the lake optical properties in relation to the geographical distribution of submerged macrophytes indicates how heterogenic optical conditions play a role in controlling benthic primary production.     

Preservation of the light field in turbid lake and river water in laboratory-scale enclosure

The laboratory-scale enclosure (LSE), as a physical model ofthe water column, has been introduced for studies of sestondynamics in shallow, mixed lakes. Preserving the ‘natural’underwater light climate was among the design criteria. However,lateral exchange of light between the LSE cylinder and its waterbath caused deviations from conditions in the water column insitu. Two modifications of the original design are introducedby providing the LSE cylinder with (i) a masking plate and (ii)a reflective coating. The light fields in the original and modifieddesigns are compared for Osciiiatoria limnetica culture andWestern Scheldt Estuary water. These test suspensions had dryweight concentrations of 50 mg 1^–1 and differed widelyin scattering to absorption ratios, i.e. 4 and 12 for the cultureand natural water, respectively. Using the reflective coating,the LSE provided an underwater light field conforming to generaltheory for diffuse attenuation and reflectance. Using the maskingplate may suffice for cultures of cyanobacteria, but the reflectivecoating is needed for studies of phytoplankton growth in relationto the optical conditions in turbid lake and river water.     

Shading effects of free-floating plants on drainage-ditch invertebrates

In small, lentic ecosystems in agricultural areas, eutrophication often results in excessive growth of small, free-floating plants. A dense layer of plants on the water surface changes the underwater light climate drastically and in turn leads to hypoxic or even anoxic conditions. Knowledge of the effects on macroinvertebrates of reduced light conditions and oxygen stress as result of eutrophication is limited. We thus examined in a field situation the influence of an unpredictable, 10-day period of low oxygen availability as a result of poor underwater light conditions. In a before–after control–impact design, the underwater light climate and dissolved oxygen concentration of ditch sections were manipulated, and the macroinvertebrate assemblage composition was recorded during the 4 weeks before and after treatment. A poor underwater light climate in combination with normoxic conditions did not affect the invertebrate assemblage composition, but the combination of low-light intensity and anoxic conditions did alter it. Interestingly, these changes were not apparent directly after treatment but developed in the weeks following, indicating that although the invertebrates could cope with a shading-induced period of hypoxia, costs were associated with the event over a longer time period.     

The specific inherent optical properties of three sub-tropical and tropical water reservoirs in Queensland, Australia

The underwater light climate, which is a major influence on the ecology of aquatic systems, is affected by the absorption and scattering processes that take place within the water column. Knowledge of the specific inherent optical properties (SIOPs) of water quality parameters and their spatial variation is essential for the modelling of underwater light fields and remote sensing applications. We measured the SIOPs and water quality parameter concentrations of three large inland water impoundments in Queensland, Australia. The measurements ranged from 0.9 to 42.7 μg l⁻¹ for chlorophyll a concentration, 0.9–170.4 mg l⁻¹ for tripton concentration, 0.36–1.59 m⁻¹ for a _CDOM(440) and 0.15–2.5 m for Secchi depth. The SIOP measurements showed that there is sufficient intra-impoundment variation in the specific absorption and specific scattering of phytoplankton and tripton to require a well distributed network of measurement stations to fully characterise the SIOPs of the optical water quality parameters. While significantly different SIOP sets were measured for each of the study sites the measurements were consistent with published values in other inland waters. The multiple measurement stations were allocated into optical domains as a necessary step to parameterise a semi-analytical inversion remote sensing algorithm. This article also addresses the paucity of published global inland water SIOP sets by contributing Australian SIOP sets to allow international and national comparison.     

Basin-scale control on the phytoplankton biomass in Lake Victoria, Africa

The relative bio-optical variability within Lake Victoria was analyzed through the spatio-temporal decomposition of a 1997–2004 dataset of remotely-sensed reflectance ratios in the visible spectral range. Results show a regular seasonal pattern with a phase shift (around 2 months) between the south and north parts of the lake. Interannual trends suggested a teleconnection between the lake dynamics and El-Niño phenomena. Both seasonal and interannual patterns were associated to conditions of light limitation for phytoplankton growth and basin-scale hydrodynamics on phytoplankton access to light. Phytoplankton blooms developed during the periods of lake surface warming and water column stability. The temporal shift apparent in the bio-optical seasonal cycles was related to the differential cooling of the lake surface by southeastern monsoon winds. North-south differences in the exposure to trade winds are supported by the orography of the Eastern Great Rift Valley. The result is that surface layer warming begins in the northern part of the lake while the formation of cool and dense water continues in the southern part. The resulting buoyancy field is sufficient to induce a lake-wide convective circulation and the tilting of the isotherms along the north-south axis. Once surface warming spreads over the whole lake, the phytoplankton bloom dynamics are subjected to the internal seiche derived from the relaxation of thermocline tilting. In 1997–98, El-Niño phenomenon weakened the monsoon wind flow which led to an increase in water column stability and a higher phytoplankton optical signal throughout the lake. This suggests that phytoplankton response to expected climate scenarios will be opposite to that proposed for nutrient-limited great lakes. The present analysis of remotely-sensed bio-optical properties in combination with environmental data provides a novel basin-scale framework for research and management strategies in Lake Victoria.     

Climate drivers of diatom distribution in shallow subarctic lakes

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The spectral absorption and scattering properties of dissolved and particulate components in relation to the underwater light field of some tropical Australian fresh waters

SUMMARY. 1. The inherent optical properties (scattering coefficients and absorption coefficients across the photosynthetic waveband) are presented from diverse tropical water bodies (billabongs) in the Alligator Rivers Region of northern Australia.
2. The data are used to interpret observed characteristics of the underwater light field as exemplified by the spectral distribution, and overall rate of attenuation, of photosynthetically available radiation (PAR).
3. Attenuation of PAR, especially in the blue waveband, is caused primarily by intense light absorption by the yellow-brown humic pigments, both soluble and particulate, in the water.
4. It was estimated that in six moderately turbid billabongs, light scattering increased attenuation by an average of 58% above that attributable to absorption alone, whereas in a highly turbid billabong the increase was 111%.
5. A distinguishing feature of the optical character of these billabongs, compared with previously studied water bodies in southern Australia, is the great contribution to light absorption made by the particulate humic material.     

The influence of atmospheric wave dynamics on interannual variation in the surface temperature of lakes in the English Lake District

Surface water temperatures in four lakes of the English Lake District (T_L) are shown to be sensitive to climate change and a large‐scale atmospheric phenomenon known as tropospheric Rossby wave breaking (RWB). RWB occurs frequently near the English Lake District, bringing warm and moist air, or cool and dry air, from distant sources. RWB case examples and composites are used to show three dimensional circulations and anomalies of near‐surface temperature and humidity associated with the two types of RWB (anticyclonic and cyclonic). Statistical models of lake surface temperature are developed for each season using objectively identified variability patterns of anticyclonic and cyclonic RWB, along with an index of Northern Hemisphere annual mean surface temperature (T_NH) to account for climate change. The statistical models, depending on season, account for 54–69% of T_L variance. RWB alone contributes significantly during each season, accounting for 37–52% of T_L variance after the effect of T_NH is removed. RWB is a key physical mechanism underlying the North Atlantic Oscillation (NAO), a regional‐scale weather‐pattern that is frequently related to coherent lake properties. RWB may therefore be a more fundamental driver than the NAO in controlling interannual variation in the properties of lakes such as ice duration, metabolic rates, phenology, species composition and, via effects on stratification, underwater light‐climate, nutrient‐cycling and oxygen‐depletion. Variation in other meteorological features that are linked to RWB, such as precipitation, may have additional effects. RWB is also likely to influence terrestrial and marine environments.     

Phytoplankton biomass in Lake Xolotlán (Managua): Its seasonal and horizontal distribution

Some well-documented studies on restoring eutrophic lake systems in The Netherlands by fish stock management have been evaluated with the emphasis on the role of macrophytes. Furthermore, the factors determining the light climate for submerged macrophytes in a large shallow eutrophic lake (Lake Veluwe) have been assessed and the potential success of biomanipulation in large scale projects is discussed. Today relatively little attention has been paid to macrophyte management although the importance of macrophytes in lake restoration has been recognized regularly. The biomanipulation strategy was successful in small scale projects. In a large scale project, however, wind-induced resuspension may largely determine the underwater light climate through attenuation by the water column and periphytic layer. Therefore, restoration of relatively large waterbodies by fish stock management only is expected not to lead to any noteworthy improvement of the light climate for submerged macrophytes. Additional measures aimed at reducing wind-induced resuspension of sediment particles and reestablishing of the macrophyte stands are required for successful biomanipulation strategies. Water quality managers should pay more attention to macrophyte stands in biomanipulation projects because macrophytes enhance a more stable and diverse ecosystem. Restoration objectives and the methods of their achievement must be carefully planned since an abundant submerged macrophyte vegetation may have undesirable effects as well.     

Light acclimation, CO2 response and long-term capacity of underwater photosynthesis in three terrestrial plant species

To characterize underwater photosynthetic performance in some terrestrial plants, we determined (i) underwater light acclimation (ii) underwater photosynthetic response to dissolved CO₂, and (iii) underwater photosynthetic capacity during prolonged submergence in three species that differ in submergence tolerance: Phalaris arundinacea, Rumex crispus (both submergence-tolerant) and Arrhenatherum elatius (submergence-intolerant). None of the species had adjusted to low irradiance after 1 week of submergence. Under non-submerged (control) conditions, only R. crispus displayed shade acclimation. Submergence increased the apparent quantum yield in this species, presumably because of the enhanced CO₂ affinity of the elongated leaves. In control plants of the grass species P. arundinacea and A. elatius, CO₂ affinities were higher than for R. crispus. The underwater photosynthetic capacity of R. crispus increased during 1 month of submergence. In P. arundinacea photosynthesis remained constant during 1 month of submergence at normal irradiance; at low irradiance a reduction in photosynthetic capacity was observed after 2 weeks, although there was no tissue degeneration. In contrast, underwater photosynthesis of the submergence-intolerant species A. elatius collapsed rapidly under both irradiances, and this was accompanied by leaf decay. To describe photosynthesis versus irradiance curves, four models were evaluated. The hyperbolic tangent produced the best goodness-of-fit, whereas the rectangular hyperbola (Michaelis-Menten model) gave relatively poor results.     

Underwater light attenuation inhibits native submerged plants and facilitates the invasive co-occurring plant Cabomba caroliniana

Aim

Decreasing in the diversity and distribution of native submerged plants have been widely observed in recent decades. Global underwater darkening, which is mainly caused by radiation dimming and a decrease in transparency due to, e.g. eutrophication, has emerged as a general trend that strongly hampers the growth of submerged plants in lakes by decreasing the light available for photosynthesis. However, few studies have attempted to compare the responses of native and invasive submerged plants to underwater darkening. In this study, we aimed to compare the effects of light attenuation on the growth and photosynthesis traits of native and invasive submerged plants.

Location

East China.

Method

Through field investigations and a mesocosm experiment, the responses of functional traits of two representative native [water thyme (Hydrilla verticillata), Eurasian watermilfoil (Myriophyllum spicatum)] and one invasive [Carolina fanwort (Cabomba caroliniana)] plant species to various environmental factors, notably to underwater light attenuation, were studied.

Results

Underwater photosynthetically active radiation (PAR) exerted a substantial effect on the relative coverage and abundance of the three studied submerged plants in their natural freshwater habitats. Invasive C. caroliniana showed relatively superior growth (total biomass and relative growth rate) and photosynthesis traits (maximum quantum yield of photosystem II F_v/F_m, chlorophyll a content, chlorophyll b content and the ratio of Chl a and b contents) compared to the two native plants under low underwater PAR conditions. In contrast, under high underwater PAR conditions, C. caroliniana showed the opposite response.

Main Conclusions

Light attenuation inhibits the growth of native submerged plants but facilitates the growth of invasive plant species. Restoration of freshwater lakes by reducing deterioration from underwater darkening (for instance, by reducing of external nutrients loading) may therefore constrain the growth and spread of the invasive C. caroliniana.     

Ultraviolet radiation and bio-optics in Crater Lake,Oregon

Crater Lake, Oregon, is a mid-latitude caldera lake famous for its depth (594 m) and blue color. Recent underwater spectral measurements of solar radiation (300–800 nm) support earlier observations of unusual transparency and extend these to UV-B wavelengths. New data suggest that penetration of solar UVR into Crater Lake has a significant ecological impact. Evidence includes a correlation between water column chlorophyll-a and stratospheric ozone since 1984, the scarcity of organisms in the upper water column, and apparent UV screening pigments in phytoplankton that vary with depth. The lowest UV-B diffuse attenuation coefficients (K _d,320) were similar to those reported for the clearest natural waters elsewhere, and were lower than estimates for pure water published in 1981. Optical proxies for UVR attenuation were correlated with chlorophyll-a concentration (0–30 m) during typical dry summer months from 1984 to 2002. Using all proxies and measurements of UV transparency, decadal and longer cycles were apparent but no long-term trend since the first optical measurement in 1896.