Light Absorption Coefficient Of Phytoplankton Research Articles

Light Absorption by Optically Active Components in the Arctic Region (August 2020) and the Possibility of Application to Satellite Products for Water Quality Assessment

In August 2020, during the 80th cruise of the R/V “Akademik Mstislav Keldysh”, the chlorophyll a concentration (Chl-a) and spectral coefficients of light absorption by phytoplankton pigments, non-algal particles (NAP) and colored dissolved organic matter (CDOM) were measured in the Norwegian Sea, the Barents Sea and the adjacent area of the Arctic Ocean. It was shown that the spatial distribution of the three light-absorbing components in the explored Arctic region was non-homogenous. It was revealed that CDOM contributed largely to the total non-water light absorption (atot(λ) = aph(λ) + aNAP(λ) + aCDOM(λ)) in the blue spectral range in the Arctic Ocean and the Barents Sea. The fraction of NAP in the total non-water absorption was low (less than 20%). The depth of the euphotic zone depended on atot(λ) in the surface water layer, which was described by a power equation. The Arctic Ocean, the Norwegian Sea and the Barents Sea did not differ in the Chl-a-specific light absorption coefficients of phytoplankton. In the blue maximum of phytoplankton absorption spectra, Chl-a-specific light absorption coefficients of phytoplankton in the upper mixed layer (UML) were higher than those below the UML. Relationships between phytoplankton absorption coefficients and Chl-a were derived by least squares fitting to power functions for the whole visible domain with a 1 nm interval. The OCI, OC3 and GIOP algorithms were validated using a database of co-located results (day-to-day) of in situ measurements (n = 63) and the ocean color scanner data: the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Terra (EOS AM) and Aqua (EOS PM) satellites, the Visible and Infrared Imager/Radiometer Suite (VIIRS) onboard the Suomi National Polar-orbiting Partnership (S-NPP) and JPSS-1 satellites (also known as NOAA-20), and the Ocean and the Land Color Imager (OLCI) onboard the Sentinel-3A and Sentinel-3B satellites. The comparison showed that despite the technological progress in optical scanners and the algorithms refinement, the considered standard products (chlor_a, chl_ocx, aph_443, adg_443) carried little information about inherent optical properties in Arctic waters. Based on the statistic metrics (Bias, MdAD, MAE and RMSE), it was concluded that refinement of the algorithm for retrieval of water bio-optical properties based on remote sensing data was required for the Arctic region.

Interannual variability of surface bio-optical characteristics in the frontal zones of the Indian sector of the Southern Ocean during austral summer

Surface bio-optical constituents (Chl-a, light-absorption coefficient of phytoplankton (aph (443)), detritus (ad(443)), coloured dissolved organic matter (aCDOM(443))), and hydrographic variables were studied across the fronts along 57.5°E during 2011, 2013 and 2015 in the Indian sector of Southern Ocean to understand their interannual variability and phytoplankton pigment package effect. Latitudinal increases of Chl-a, aph(443), and aCDOM(443) were observed in 2011 and 2013. Mean Chl-a was higher in 2013 than in 2015 and 2011. The Chl-a specific absorption (a*ph(443)) latitudinally decreased implying ‘pigment package effect’ with increasing latitude. Interannually increase of aph blue (443)/red (675) ratio suggested higher non-photosynthetic absorption during 2013 and 2015 than in 2011. The relative contributions of bio-optical constituents to total absorption (at) showed interannual decrease in aph(443) and increase in aCDOM(443) and ad(443) inferring phytoplankton growth and decay phases. The robust aph(443)-Chl-a relationship during 2011 was used to derive aph(443) for other years, which holds good for in situ and satellite-derived counterparts. aCDOM(443) positively correlated with Chl-a and negatively with CDOM-spectral slope (S300-500), suggesting autochthonous sources. Satellite-retrieved Chl-a, aph(443) showed significant correlation (p < 0.001) with in situ data. This study provides baseline bio-optical data for understanding phytoplankton size-class, and improvement of bio-optical algorithms in this relatively inaccessible polar region.

Bio-optical depiction of a polar ocean under global change: Exploring the regional absorption traits

The Southern Ocean (SO) is a vital ecoregion that plays a crucial role in modulation of the global carbon cycle and changing climate. Light absorption properties of clear waters are challenging to study due to very low absorption coefficients typical for these waters, and the SO is no exception to this. The present study examines the bio-optical variability of light absorption coefficients of phytoplankton and non-phytoplankton constituents during the Indian Scientific Expedition to the Southern Ocean (ISESO) in the austral summer 2017–18. We characterize the light absorption coefficients of bio-optical constituents, quantify their relative contributions to total absorption, derive phytoplankton size classes from two absorption-based approaches and compare the results with hyperspectral radiometric observations. Our observations suggest a prevalence of the pigment package effect (phenomenon interfering with photosynthetic efficiency of phytoplankton) at both, the frontal (40°S–60°S) and coastal (60°S–69°S) domains, inducing deviations from the expected bio-optical classification of the study region. The total light absorption budget indicated a clear dominance of phytoplankton absorption at the frontal (41.93–85.27%) as well as coastal (36.77–93.71%) region. Further, the dominance of microphytoplankton indicated by the aph (443)/aph (675) Blue/Red spectral ratios supported the impact of pigment package on phytoplankton absorption at both, the frontal (0.5–3.2) and coastal (0.9–3.7) regions. The global absorption-based (that links the phytoplankton absorption to their size classes) model captured smaller sized (pico and nano) phytoplankton better than the larger (micro). A satisfactory agreement was observed between the Quasi-Analytical Algorithm (radiometer based) and Quantitative Filter Technique (in-situ measurements) for aph retrieval. The OC5 algorithm of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) was well-correlated with in situ Chlorophyll-a in the region. Our findings improve the existing understanding of bio-optical variability in the SO and highlight the essential considerations that should be taken while assessing the study region through global ocean colour platforms. They also highlight the necessity to assess the impact of pigment package on optical variability while designing satellite data tools at regional a level.

Variations in the light absorption coefficients of phytoplankton, non-algal particles and dissolved organic matter in reservoirs across China

Reservoirs were critical sources of drinking water for many large cities around the world, but progress in the development of large-scale monitoring protocols to obtain timely information about water quality had been hampered by the complex nature of inland waters and the various optical conditions exhibited by these aquatic ecosystems. In this study, we systematically investigated the absorption coefficient of different optically-active constituents (OACs) in 120 reservoirs of different trophic states across five eco-regions in China. The relationships were found between phytoplankton absorption coefficient at 675 nm (aph (675)) and Chlorophyll a (Chla) concentration in different regions (R2:0.60–0.82). The non-algal particle (NAP) absorption coefficient (aNAP) showed an increasing trend for reservoirs with trophic states. Significant correlation (p < 0.05) was observed between chromophoric dissolved organic matter (CDOM) absorption and water chemical parameters. The influencing factors for contributing the relative proportion of OACs absorption including the hydrological factors and water quality factors were analyzed. The non-water absorption budget from our data showed the variations of the dominant absorption types which underscored the need to develop and parameterize region-specific bio-optical models for large-scale assessment in water reservoirs.

Study of absorption characteristics of phytoplankton, particles and colored dissolved organic matter in Lake Baikal (July 2018 and September 2019)

Results of two scientific cruises carried out in July 2018 and September 2019 were used for an analysis of variability in spectral bio-optical properties of Lake Baikal. Chlorophyll a concentration, spectral light absorption coefficient of phytoplankton, non-algal particles and colored dissolved organic matter varied in ~order of magnitude between stations. Vertical distribution of these bio-optical parameters depended on water column stability in euphotic layer. The chlorophyll a profiles were characterized by maximum near the bottom of euphotic zone. In the layer of deep chlorophyll a maximum the phytoplankton absorption spectra had a specific feature – a local maximum at 550-570 nm. It was associated with relative increasing of phycoerythrin containing species in phytoplankton community. The absorption budget at 440 nm showed that the CDOM was the main optically active component, which provided ~ 50 % contribution to total absorption on averaged over Lake Baikal surface.

Estimation of Kd(PAR) in inland waters across China in relation to the light absorption of optically active components.

The comprehensive analysis of the relationships between the attenuation of photosynthetic active radiation (Kd(PAR)) and light absorption is an imperative requirement to retrieve Kd(PAR) from remote sensing data for aquatic environments. The spatial distributions of the Kd(PAR) and light absorption of optically active components (aOACs) were routinely estimated in China lakes and reservoirs. Spatial Kd(PAR) was relatively dependent on the inorganic particles (average relative contribution of 57.95%). The aOACs could explain 70-87% of Kd(PAR) variations. A linear model is used to predict Kd(PAR), as a function of light absorption coefficient of phytoplankton (aphy), colored dissolved organic matter (aCDOM), and inorganic particles (aNAP): Kd(PAR) = 0.41 + 0.57 × aCDOM + 0.96 × aNAP + 0.57 × aphy (R2 = 0.87, n = 741, p < 0.001). In the lakes with low TSM concentration and non-eutrophic lakes with high TSM, aCDOM was the most powerful predicting factor on Kd(PAR). In eutrophic lakes with high TSM, aNAP had the most significant impact on Kd(PAR). This study allowed Kd(PAR) to be predicted from aOACs values in the inland waters.

Semianalytical Derivation of Phytoplankton, CDOM, and Detritus Absorption Coefficients From the Landsat 8/OLI Reflectance in Coastal Waters

AbstractThe Operational Land Imager (OLI) onboard Landsat 8 has potential for mapping the water bio‐optical properties with high spatial resolution. Landsat 8/OLI generates the remote sensing reflectance (Rrs) at four visible bands (λ1‐4 = 443, 482, 561, and 655 nm) and is lack of a 412‐nm band commonly included for ocean color sensors. This spectral configuration has limited the use of Landsat 8/OLI reflectance product for analytical derivation of light absorption coefficients of phytoplankton, colored dissolved organic matter (CDOM), and detritus. In this study, we proposed a hybrid approach to fill this gap. First, we developed an algorithm to estimate the reflectance in a virtual band centered at λ0 = 412 nm from the OLI reflectance spectra Rrs(λ1‐4). Both the estimated Rrs(λ0) and measured Rrs(λ1‐4) were then used together to retrieve the water component absorption coefficients with existing algorithms including the quasi‐analytical algorithm. We assessed the model performance using in situ measurements from the global waters. It was found that the proposed approach could estimate Rrs(412) with a median absolute percentage difference of ~9%. The subsequent retrievals of the component absorption coefficients were satisfactorily accurate, with median absolute percentage difference roughly equal to 35%, 40%, and 60% for phytoplankton, CDOM, and detritus, respectively. The results suggest the feasibility to generate analytically the component absorption coefficients from the Landsat 8/OLI reflectance.

Remote sensing of chlorophyll-a in coastal waters based on the light absorption coefficient of phytoplankton

Remote sensing of chlorophyll-a concentration, [Chl-a], has been difficult in coastal waters like the Chesapeake Bay owing largely to terrestrial substances (such as minerals and humus) that are optically significant but do not covary with phytoplankton. Here we revisit the semi-analytical pathway of deriving [Chl-a] based on the light absorption coefficient of phytoplankton by introducing the generalized stacked-constraints model (GSCM) to partition satellite-derived total light absorption coefficient of water (with pure-water contribution subtracted), anw(λ), into phytoplankton, aph(λ), and non-phytoplankton components, where anw(λ) is derived from satellite remote-sensing reflectance, Rrs(λ), using the Quasi-Analytical Algorithm. The GSCM-derived aph(λ) was compared with field matchups of [Chl-a]. We show that semi-analytical approaches can provide superior [Chl-a] product compared with reflectance-band-ratio algorithms when the accuracy of satellite-derived aph(λ) is sufficiently improved, in this case with the GSCM. However, the improvement is at the cost of significantly reduced data availability because the GSCM may provide no feasible solutions when input anw(λ) data are subject to large errors. This in turn highlights the needs for improved atmospheric correction and upstream models capable of preserving actual spectral shapes of Rrs(λ) and anw(λ), respectively.

Light Absorption by Phytoplankton in the Upper Mixed Layer of the Black Sea: Seasonality and Parametrization

Standard NASA ocean color algorithm OC4 was developed on the basis of ocean optical data and while appropriate for Case 1 oceanic waters could not be adequately applied for the Black Sea waters due to its different bio-optical properties. OC4 algorithm is shown to overestimate chlorophyll concentration (Chl-a) in summer and underestimate Chl-a during early spring phytoplankton blooms in the Black Sea. For correct conversion of satellite data to Chl-a, primary production and other indicators regional algorithms should be developed taking into account bio-optical properties of the Black Sea waters. Light absorption by phytoplankton pigments – a_ph (λ) have been measured in open sea and shelf Black Sea waters in different seasons since 1998. It was shown that the first optical depth was located within the upper mixed layer (UML) for most of the year with the exception of the spring when seasonal stratification was developing. As a result spectral features of water leaving radiance were determined by optical properties of the UML. Significant seasonal differences in Chl-a specific light absorption coefficients of phytoplankton within UML have been revealed. These differences were caused by adaptive changes of composition and intracellular pigment concentration due to variable environment conditions – mainly light intensity. Empirical relationships between a_ph (λ) and Chl-a were derived by least squares fitting to power functions for different seasons. Incorporation of these results will refine the regional ocean color models and provide improved and seasonally adjusted estimates of chlorophyll a concentration, downwelling radiance and primary production in the Black Sea based on satellite data.

Bio-optical properties during the summer season in the Sea of Okhotsk

The Sea of Okhotsk is one of the most productive ocean regions in the world. However, the in situ bio-optical properties, which are crucial for satellite ocean-color of the productivity, remain uncertain in this region because little data have been available. We conducted an in situ observation and evaluated the bio-optical properties in terms of chlorophyll a (Chl a) concentration, spectral remote sensing reflectance (Rrs), and the light absorption coefficients of phytoplankton (aφ), non-algal particles (aNAP), and chromophoric dissolved organic matter (aCDOM) in the summer of 2006. The data covered a wide range of Chl a levels in surface waters from 0.3 to 8.5mgm−3. At 443nm, aCDOM dominated (64% on average) the total non-water absorption (at-w) in this study area. Based on the in situ Rrs data, surface Chl a concentrations that were estimated using the sea-viewing wide field-of-view sensor (SeaWiFS) OC4v6 and the moderate-resolution imaging spectroradiometer (MODIS) OC3M algorithms were significantly higher than the in situ data by more than 160% and 260%, respectively. In particular, the largest overestimation occurred in the region where aCDOM at 443nm accounted for more than 80% of at-w near the mouth of the Amur River. However, except the CDOM-rich stations, the performance of the OC4v6 and OC3M algorithms became better (i.e., their mean normalized biases were reduced to 50% and 66%, respectively). We conclude that the operational global algorithms were applicable to the summer season in the Sea of Okhotsk except the CDOM-rich region, in which new approaches for ocean-color algorithms (i.e., local algorithms) would be required.

Variability in the light absorption coefficients of phytoplankton, non-algal particles, and colored dissolved organic matter in a subtropical bay (Brazil)

This study characterized the variability in magnitudes and spectral shapes of the absorption coefficients of phytoplankton, detritus, and colored dissolved organic matter (CDOM) in a dynamic bay (Santos Bay) in southeastern Brazil in response to the contributions of the main estuarine channel and large tide variations, therefore in different time scales. Two strategies were adopted: (1) monthly year-round sampling in the estuarine channel and Santos Bay and (2) sampling in Santos Bay during spring/neap tides and cold/warm months. Chlorophyll-a concentration and CDOM absorption were higher during warm (wet) months, while the relative contribution of organic and inorganic particles was driven by neap/spring tide cycles. Salinity partially accounted for changes in optical variables, especially for CDOM absorption and total suspended matter (TSM) during cold months and neap tides, respectively. The spectral shapes of detritus and CDOM absorption showed relatively little variability for the entire dataset and were not considered feasible for monitoring purposes. The spectral shape of phytoplankton absorption (index of cell size) varied broadly, with no remarkable dependence on the sampling conditions. Comparison of absorption coefficients measured by the Quantitative Filter Technique (QFT) and Transmittance Reflectance (TR) method showed higher phytoplankton coefficients toward longer visible wavelengths (flatter spectra) and shallower slopes of detritus absorption yielded by the TR method. Our results also suggest that measurements at the near red spectral region result from not only scattering signals but also non-algal particle absorption.

Remote sensing of size structure of phytoplankton communities using optical properties of the Chukchi and Bering Sea shelf region

Abstract. Recent ocean warming and subsequent sea ice decline resulting from climate change could affect the northward shift of the ecosystem structure in the Chukchi Sea and Bering Sea shelf region (Grebmeier et al., 2006b). The size structure of phytoplankton communities provides an index of trophic levels that is crucial to understanding the mechanisms underlying such ecosystem changes and their implications for the future. This study proposes a new ocean color algorithm for deriving this characteristic by using the region's optical properties. The size derivation model (SDM) estimates the phytoplankton size index FL on the basis of size-fractionated chlorophyll-a (chl-a) using the light absorption coefficient of phytoplankton, aph(λ), and the backscattering coefficient of suspended particles including algae, bbp(λ). FL was defined as the ratio of algal biomass attributed to cells larger than 5 μm to the total. It was expressed by a multiple regression model using the aph(λ) ratio, aph(488)/aph(555), which varies with phytoplankton pigment composition, and the spectral slope of bbp(λ), γ, which is an index of the mean suspended particle size. A validation study demonstrated that 69% of unknown data are correctly derived within FL range of ±20%. The spatial distributions of FL for the cold August of 2006 and the warm August of 2007 were compared to examine application of the SDM to satellite remote sensing. The results suggested that phytoplankton size was responsive to changes in sea surface temperature. Further analysis of satellite-derived FL values and other environmental factors can advance our understanding of ecosystem structure changes in the shelf region of the Chukchi and Bering Seas.

Seasonal variability in the light absorption properties of western Arctic waters: Parameterization of the individual components of absorption for ocean color applications

[1] The light absorption properties of particulate and dissolved materials strongly influence the propagation of visible light in oceanic waters and therefore the accuracy of ocean color algorithms. While the general absorption properties of these materials have been reported for Arctic waters, their seasonal variability remains unknown. We investigated the light absorption coefficients of phytoplankton [aϕ(λ)], nonalgal particles [aNAP(λ)], and colored dissolved organic matter [aCDOM(λ)] in both coastal and oceanic waters of the western Arctic Ocean from spring to autumn. Values for the chlorophyll a–specific absorption coefficient of phytoplankton [a*ϕ(440)] declined significantly from the ice melt period in the early spring to the summer. Using high-performance liquid chromatography, we show that the decrease in a*ϕ(440) was due to a strong package effect that overwhelmed the influence of the pigment composition. A decrease in the aNAP(λ) values from spring and summer to autumn likely originated from a decrease in the concentration of phytoplanktonic detritus. The aCDOM(λ) near the surface decreased by 34% from spring to summer as a result of photobleaching by solar radiation. The colored dissolved organic matter (CDOM) absorption values then increased significantly during autumn, resulting from the cumulative injection of Alaskan Coastal Waters into the Arctic as well as CDOM generated in situ. Our results suggest that all of the absorption components are tightly linked to biogeochemical processes, and thus the seasonal variability in aϕ(λ), aNAP(λ), and aCDOM(λ) should be taken into account in bio-optical models.

A phytoplankton absorption-based primary productivity model for remote sensing in the Southern Ocean

Recent global environmental changes such as an increase in sea surface temperature (SST) are likely to impact primary productivity of phytoplankton in the Southern Ocean. However, models to estimate net primary production using satellite data use SST and uncertain estimation of chlorophyll a (chl-a) concentration. A primary productivity model for satellite ocean color data from the Southern Ocean, which is based on the light absorption coefficient of phytoplankton to reduce uncertainties of sea surface chl-a estimations and bias in optimal values of chl-a normalized productivity derived from SST, has been developed. The new model was able to estimate net primary productivity in the water column (PPeu) without dependency on temperature when in the range of −2 to 25°C, and it explained 51% of the observed variability in PPeu with a root mean square error (RMSE) of 0.15. Application of the model revealed that the SST dependent model has overestimated PPeu in warmer waters around the Subtropical Front, and underestimated PPeu in colder waters poleward of the Sub-Antarctic Front. This absorption-based primary productivity model contributes to a study of the relationship among spatio-temporal variations in the physical environment, and biogeochemical cycles in the Southern Ocean.

Bio-optical characteristics of the western Arctic Ocean: implications for ocean color algorithms

Global ocean color algorithms designed to estimate chlorophyll a concentration (chla) are not accurate at high latitudes. Although a regional Arctic OC4L algorithm was designed to be applicable to high northern latitudes, its applicability remains uncertain. To examine these issues, we investigated the light absorption coefficients of phytoplankton, non-algal particles (NAP), and colored dissolved organic matter (CDOM) and remote sensing reflectance, Rrs(λ), covering most of the western Arctic Ocean. A higher pigment packaging effect was identified relative to that at lower latitudes. The CDOM absorption dominated and accounted for 76% of the total non-water absorption at 443 nm and did not covary with variations in chla. This absorption is significantly higher than those in other marine environments. We also examined the backscattering coefficient of particles obtained from the inversion of Rrs(λ) and found that it covaried well with variation in NAP absorption. Our evaluation shows that when turbid waters (Rrs(670) > 0.00042 sr–1) are excluded, the performance of the OC4L is good and much better than that of the sea-viewing wide field-of-view sensor (SeaWiFS) OC4V4 and the moderate-resolution imaging spectroradiometer (MODIS) OC3M (root mean square error (RMSE) of 0.13, 0.21, and 0.22, respectively). The reason why the OC4L performs well despite strong CDOM absorption is discussed.

Long-term monitoring of primary production in coastal waters by an improved natural fluorescence method

In order to apply the natural fluorescence method for the long-term monitoring of chl a and primary production, the seasonal variation in the light absorption coefficient of phytoplankton (a* ph ), maximum quantum yield ratio of photosynthesis to fluorescence [(Φ c /Φ f ) max ], and its light saturation index (k cf ) were examined. The mooring of a natural fluorescence sensor was conducted during winter to early spring (January to April 1998), late spring (May 2000) and summer (July to August 2000) at a station in Otsuchi Bay on the north east coast of Japan. The bio-optical parameters, a* ph , (Φ c /Φ f ) max and k cf , were measured by water sampling and incubation experiments twice a week. The temporal variation in a* ph was expressed as an exponential of chl a. This relationship varied seasonally. The variations in (Φ c /Φ f ) max were related to chl a during winter to early spring, and to chl a and temperature during late spring and summer. The value of k cf was relatively low and stable during the winter and early spring; it reached higher values during late spring to summer, when temperature and light intensities were also higher. Based on these relationships, the long-term monitoring of chl a and primary production from natural fluorescence was realized in coastal water where environmental conditions and phytoplankton abundance fluctuated markedly. By developing algorithms in various areas and seasons, as the present study has shown, the natural fluorescence method is expected to be useful for continuous monitoring of primary production, with a high spatial resolution by a mooring web of sensors, even in coastal waters.

Variations in the light absorption coefficients of phytoplankton, nonalgal particles, and dissolved organic matter in coastal waters around Europe

We measured the absorption properties of phytoplankton, nonalgal particles (NAP), and colored dissolved organic matter (CDOM) at about 350 stations in various coastal waters around Europe including the English Channel, Adriatic Sea, Baltic Sea, Mediterranean Sea, and North Sea. For comparison, we also collected data in the open ocean waters of North Atlantic. The exponential slope of the CDOM absorption spectrum varied within a narrow range around 0.0176 nm−1 (SD = 0.0020 nm−1). When data from all the regions were considered altogether, the relationship between phytoplankton absorption and chlorophyll concentration was generally similar to the one previously established for open oceanic waters. Our coastal data, however, show that significant departures from the general trend may occur due to peculiar pigment composition and cell size. In some coastal areas, high phaeopigment concentrations gave rise to especially high blue‐to‐red ratio of phytoplankton absorption. The NAP absorption covaried with the particle dry weight. Most absorption spectra of these particles were well described by an exponential function with a slope averaging 0.0123 nm−1 (SD = 0.0013 nm−1). In some highly turbid waters, the spectra exhibited a signature possibly associated with iron oxides. In the Baltic Sea, NAP absorption systematically showed lower values at wavelengths shorter than 440 nm than predicted from the fitted exponential function. Overall, the variability in the absorption properties of European coastal waters showed some consistent patterns despite the high diversity of the examined waters. Distinct features were identified in the phytoplankton and NAP components. An absorption budget is presented and parameterizations are proposed.

Particulate light absorption and the prediction of phytoplankton biomass and planktonic metabolism in northeastern Spanish aquatic ecosystems

Examination of particulate light absorption and microplankton metabolism in 36 northeastern Spanish aquatic ecosystems, ranging from alpine rivers to inland saline lakes and the open Mediterranean Sea, revealed the existence of general relationships between particulate light absorption and the biomass of phytoplankton and microplankton metabolism. The particulate absorption spectra reflected a dominance of nonphotosynthetic, likely detrital, particles in rivers and a dominance of phytoplankton in coastal lagoons. There was a strong relationship between the light absorbed by phytoplankton and the chlorophyll a (Chl a) concentration of the systems, which indicated an average (±SE) Chl a specific absorption coefficient of 0.0233 ± 0.0020 m2·mg Chl a-1 for these widely diverse systems. Chl a concentration was a weaker predictor of the total particulate light absorption coefficient, pointing to an important role of nonphytoplanktonic particles in light absorption. Gross production was very closely related to the light absorption coefficient of phytoplankton, whereas community respiration was strongly correlated with the total particulate light absorption coefficient, indicating the optical signatures of sestonic particles to be reliable predictors of planktonic biomass and metabolism in aquatic ecosystems.