Total Particulate Absorption Research Articles

A neural network approach for deriving absorption coefficients of ocean water constituents from total light absorption and particulate absorption coefficients

Inherent Optical Properties (IOPs) of oceanic and coastal waters provide useful information for studying light availability in various ocean layers, primary productivity, particulate matter, and aquatic photochemistry. The total spectral absorption coefficient, a(λ) and spectral particulate absorption coefficient, ap(λ) are the IOPs that are some of the extensively measured IOPs with the existing absorption instruments. The total spectral absorption coefficient is often expressed as the sum of subcomponent IOPs, phytoplankton (aph(λ)), combined absorption due gelbstoff and detritus (adg(λ)) and the water itself. Similarly, ap can be subdivided into aph(λ) and ad(λ), the detrital absorption coefficients. The existing models partition these IOPs into subcomponent IOPs by assuming spectral shapes for subcomponent IOPs and are computationally intensive. Hence, in this study, we implemented Neural Networks to derive the subcomponent IOPs due to their lower computational time requirement and their capability to model complex relationships. We trained and validated four Neural Networks (NN) for retrieval of IOPs from a(λ) and ap(λ). NNs - I &II are used to retrieve aph and adg at 443 nm from a(λ). Similarly, NNs - III &IV are used to derive aph(443) and ad(443) from ap(λ). The four NNs are trained and tested using synthetic data created encompassing a wide range of optical properties observed in natural waters. For validation of the four NNs, the International Ocean Color Coordinating Group (IOCCG) simulated dataset, NASA bio-Optical Marine Algorithm Dataset (NOMAD) and global bio-optical insitu dataset (GBI) are used. The R2 values obtained for GBI in the retrieval of aph(443) and adg(443) using NNs - I &II are 0.95 and 0.97, respectively. Similarly, aph and ad retrieved using NN–III & IV resulted in R2 values of 0.94 and 0.94 for the NOMAD dataset. Upon comparing the performance NNs - I &II with Quasi-Analytical Algorithm (QAA) in retrieving IOPs using the IOCCG dataset, the NNs exhibited lower errors. We also developed a hybrid algorithm, QAANN, to demonstrate the capability of NNs - I &II for remote sensing applications. QAANN resulted in 11–43% lower mean absolute percentage error (MAPE) compared to QAA, Generalized Inherent Optical Property (GIOP) and Garver-Siegel-Maritorena (GSM) models for aph(443) retrieval using remote sensing reflectance from IOCCG simulated dataset. In the case of adg(443), QAANN resulted in 5–10% less MAPE compared to other models for the same dataset. These results suggest that the developed NNs can be applied to satellite remote sensing data to retrieve subcomponent IOPs with improved accuracy.

Mass-specific light absorption coefficients of mineral particles in aqueous suspension for the ultraviolet to near-infrared radiation spectral region (200-2500 nm).

Light absorption by in-water suspended natural particles in the near-infrared radiation (NIR; 780-3000 nm) region has received little attention. Minerogenic matter is thought to be one source for NIR light absorption in aquatic environments. Here, mass-specific particulate light absorption coefficients of several particulate single minerals and mineral samples for the spectral range of 200-2500 nm are presented. The current methodology allows very sensitive measurements of particle suspension with a detection limit of about 2×10-6m2g-1 for the mass-specific absorption coefficient. Except for one, all mineral materials examined possessed significant light absorption throughout the full spectral range considered. The spectra revealed absorption features of specific elements (like iron) and from water structures (H2O, O-H bonds) in the mineral or crystal structure that have been known from reflectance measurements of minerals. The specific absorption in the NIR was as high as 0.02m2g-1 for laterite earths samples, but also below the detection limit for a steatite sample in a narrow spectral region (1600-1800 nm). The specific absorption by mineral particles in the NIR was, hence, highly variable from strong absorbing black minerals (magnetite) to low absorbing white clays. The information in the absorption coefficient spectrum can be used not only to identify specific mineral in natural particle assemblages but also to quantify their contribution to total particulate absorption in the NIR.

Particulate absorption properties in the Red Sea from hyperspectral particulate absorption spectra

This paper aims to describe the variability of particulate absorption properties using a unique hyperspectral dataset collected in the Red Sea as part of the TARA Oceans expedition. The absorption contributions by phytoplankton (aph) and non-algal particles (aNAP) to the total particulate absorption coefficients are determined using a numerical decomposition method (NDM). The NDM is validated by comparing the NDM derived values of aph and aNAP with simulated values of aph and aNAP are found to be in excellent agreement for the selected wavelengths (i.e., 443, 490, 555, and 676 nm) with high correlation coefficient (R2), low root mean square error (RMSE), mean relative error (MRE), and with a slope close to unity. Further analyses showed that the total particulate absorption coefficients (i.e., ap(443)average = 0.01995 m−1) were dominated by phytoplankton absorption (i.e., aph(443)average = 0.01743 m−1) with a smaller contribution by non-algal particles absorption (i.e., aNAP(443)average = 0.002524 m−1). The chlorophyll a is computed using the absorption based Line Height Method (LHM). The derived chlorophyll-specific absorption ((a*ph = aph(λ)/ChlLH)) showed more variability in the blue part of spectrum as compared to the red part of spectrum representative of the package effect and changes in pigment composition. A new parametrization proposed also enabled the reconstruction of a*ph(λ) for the Red Sea. Comparison of derived spectral constants with the spectral constants of existing models showed that our study A(λ) values are consistent with the existing values, despite there is a divergence with the B(λ) values. This study provides valuable information derived from the particulate absorption properties and its spectral variability and this would help us to determine the relationship between the phytoplankton absorption coefficients and chlorophyll a and its host of variables for the Red Sea.

Absorption characteristics of particulates and CDOM in spring in the Lake Xingkai

Field surveys were carried out in the Lake Xingkai( including both the Great Lake Xingkai and the Small Lake Xingkai)in May 2013,and water samples were collected for examination of the absorption features. In the laboratory,tripton and chromophoric dissolved organic matter( CDOM) absorption of 20 water samples were measured through quantitative filter technique. Absorption coefficients were used to analyse the absorption characteristics,source,spatial distribution and relative contribution of optical active constituents over the range of PAR( 400-700 nm). The results showed that the absorption spectra of tripton are similar to nonalgal particles,indicating less phytoplankton presence in the water column. The total particulate absorption is dominated by the nonalgal particles,of which the contribution rate is up to 50% of the total absorption,and Sgis higher than that in water bodies being investigated elsewhere. Positive correlation of the particles was observed within ap( 440),ad( 440) and total suspended matter,inorganic suspended matter and organic suspended matter,respectively,while weak correlation was recorded in terms of chlorophyll-a( Chl. a) compared with results reported in literatures. In the current study,low correlations of the particles were observed within ag( 440),Chl. a and dissolve dissolved organic carbon( DOC). This is probably due to the fact that more colorless DOCs are present in the Lake Xingkai. Absorption coefficient and water quality parameters showed insignificant difference between the Great Lake Xingkai and the Small Lake Xingkai,though the latter is greatly impacted by farmland irrigation discharge,fisheries and tourism sewage.

Specific inherent optical quantities of complex turbid inland waters, from the perspective of water classification

For optically complex turbid productive waters, the optical behavior of suspended particles is the keynote of characterizing the unordered variations of inherent optical properties (IOPs). Multiple bio-optical measurements and sampling of optically active substances were performed in Lake Taihu, Lake Chaohu, and Lake Dianchi, and Three Gorges reservoir of China, in 2008, 2009, and 2010. On the basis of obtaining adequate observation data, we developed an improved and robust water classification approach, by which complex water conditions were divided into three types, i.e., Type 1 (Normalized Trough Depth at 675 nm, hereafter NTD675, ≥0.092), Type 2 (0 < NTD675 < 0.092), and Type 3 (NTD675 ≤ 0). Furthermore, the specific inherent optical quantities for suspended particles, including the specific absorption coefficient of non-algal particles (a*(nap)), the specific absorption coefficient of phytoplankton (a*(ph)), and the specific scattering coefficient of the suspended particles (b*(p)), were determined for the three classified types of waters. The validation results showed that our proposed values for these specific inherent optical quantities presented relatively high predictive accuracies, with most mean absolute percentage errors (MAPE) near 30%, and more importantly, performed much better than that of non-classified waters. Additionally, relative contributions of phytoplankton and non-algal particles to the total particulate absorption and scattering, as well as the spectra, were also analyzed, and the differences among the three classified types of waters were clarified. Overall, the results obtained in this study provide us with new knowledge for understanding complex varied inherent optical properties of highly turbid productive waters.

Characterization of water constituents spectra absorption in Chagan Lake of Jilin Province, Northeast China

Research on the optical characteristics of water color constituents in Chagan Lake of Jilin Province, Northeast China was carried out in order to investigate the variability of the spectra absorption parameters as inputs to bio-optical models and remote sensing algorithms for converting observed spectral signals into water quality information. Samples of total particulates, non-algal particles and colored dissolved organic matter (CDOM) were first prepared by quantitative filter technique (QFT) and then absorption coefficients of these color producing agents were determined by spectrophotometry. Spectral characteristics of absorption coefficients by total particulate matter, spectral specific absorption dependency on chlorophyll concentration (Chl-a) of phytoplankton, spectral absorption slopes variation for CDOM and non-algal particles and their corresponding reasons were examined and clarified over five months of 2009 and 2010 in this study. Results suggest that total particulate spectral absorption in Chagan Lake is mainly dominated by non-algal particles in most cases, but phytoplankton could be the dominant contributor when chlorophyll concentration is high (up to 84.48 mg/m3 in autumn 2010). The specific absorption coefficients of phytoplankton particulate (a*ph(λ)) dependency on Chl-a is significantly variable due to relative contributions of package effect and accessory pigments, and the parameters of power function are clearly biased on a long time span. The sources of variability in spectral absorption slopes of CDOM and non-algal particles are mainly attributed to the changing proportions of high molecular weight humic acids and mineral suspended sediments in waters, respectively.

Seasonal Variability of Optical Properties in a Highly Turbid Lake (Laguna Chascomús, Argentina)

We study the underwater light field seasonality in a turbid lake, Laguna Chascomús (Buenos Aires, Argentina). We report (1) relationships between optical properties (OPs) and optically active substances (OASs); (2) relationships between inherent (IOPs) and apparent (AOPs) optical properties; and (3) the seasonal variability in OASs and OPs. Light absorption was dominated by the particulate fraction. The contributions of phytoplankton pigments and unpigmented components were similar. The best predictors of total particulate absorption, unpigmented particulate absorption, turbidity and vertical attenuation coefficient were total suspended solids or their ash content. Many OASs and OPs varied seasonally. The concentrations of OASs were higher during spring and summer, resulting in lower transparency and higher turbidity. However, mass-specific absorption coefficients displayed lower values during spring and summer. Thus, the higher light attenuation observed during spring and summer resulted from higher concentrations of relatively less absorptive OASs. Collectively, these results suggest that: (1) light extinction is enhanced during spring and summer; (2) the enhanced light extinction is due to changes in the particulate fraction; (3) the enhanced light extinction is mostly due to an increase in the amount of particulate material; and (4) the increase of particulate matter also enhanced light extinction through increased scattering.

Light absorption properties and absorption budget of Southeast Pacific waters

Absorption coefficients of phytoplankton, nonalgal particles (NAPs), and colored dissolved organic matter (CDOM), and their relative contributions to total light absorption, are essential variables for bio‐optical and biogeochemical models. However, their actual variations in the open ocean remain poorly documented, particularly for clear waters because of the difficulty in measuring very low absorption coefficients. The Biogeochemistry and Optics South Pacific Experiment (BIOSOPE) cruise investigated a large range of oceanic regimes, from mesotrophic waters around the Marquesas Islands to hyperoligotrophic waters in the subtropical gyre and eutrophic waters in the upwelling area off Chile. The spectral absorption coefficients of phytoplankton and NAPs were determined using the filter technique, while the CDOM absorption coefficients were measured using a 2 m capillary waveguide. Over the whole transect, the absorption coefficients of both dissolved and particulate components covered approximately two orders of magnitude; in the gyre, they were among the lowest ever reported for open ocean waters. In the oligotrophic and mesotrophic waters, absorption coefficients of phytoplankton and NAPs were notably lower than those measured in other oceanic areas with similar chlorophyll contents, indicating some deviation from the standard chlorophyll‐absorption relationships. The contribution of absorption by NAPs to total particulate absorption showed large vertical and horizontal variations. CDOM absorption coefficients covaried with algal biomass, albeit with a high scatter. The spectral slopes of both NAP and CDOM absorption revealed structured spatial variability in relation with the trophic conditions. The relative contributions of each component to total nonwater absorption were (at a given wavelength) weakly variable over the transect, at least within the euphotic layer.

Particulate absorption of solar radiation: anthropogenic aerosols vs. dust

Abstract. Particulate solar absorption is a critical factor in determining the value and even sign of the direct radiative forcing of aerosols. The heating to the atmosphere and cooling to the Earth's surface caused by this absorption are hypothesized to have significant climate impacts. We find that anthropogenic aerosols play an important role around the globe in total particulate absorption of solar radiation. The global-average anthropogenic fraction in total aerosol absorbing optical depth exceeds 65% in all seasons. Combining the potentially highest dust absorption with the lowest anthropogenic absorption within our model range, this fraction would still exceed 47% in most seasons except for boreal spring (36%) when dust abundance reaches its peak. Nevertheless, dust aerosol is still a critical absorbing constituent over places including North Africa, the entire tropical Atlantic, and during boreal spring in most part of Eurasian continent. The equality in absorbing solar radiation of dust and anthropogenic aerosols appears to be particularly important over Indian subcontinent and nearby regions as well as North Africa.

Parameterization of light absorption by components of seawater in optically complex coastal waters of the Crimea Peninsula (Black Sea)

The absorption of sunlight by oceanic constituents significantly contributes to the spectral distribution of the water-leaving radiance. Here it is shown that current parameterizations of absorption coefficients do not apply to the optically complex waters of the Crimea Peninsula. Based on in situ measurements, parameterizations of phytoplankton, nonalgal, and total particulate absorption coefficients are proposed. Their performance is evaluated using a log-log regression combined with a low-pass filter and the nonlinear least-square method. Statistical significance of the estimated parameters is verified using the bootstrap method. The parameterizations are relevant for chlorophyll a concentrations ranging from 0.45 up to 2 mg/m(3).

Partitioning particulate absorption coefficient into contributions of phytoplankton and nonalgal particles: A case study in the northern South China Sea

We measured the absorption coefficients of suspended particles ( a p( λ)) during three cruises from coastal waters to open ocean in the northern South China Sea (NSCS). The absorption contributions of phytoplankton ( a ph( λ)) and nonalgal particles ( a NAP( λ)) were determined using the methanol extraction method. Based on the dataset of about 360 samples, we examined the spectral relationships of the particle absorption coefficients. The results show that a p( λ) spectra are well linearly correlated with a p(443) over the wavebands between 420–650 nm; a ph( λ) could be well expressed as the second-order quadratic equations of a ph(443) among the blue-green wavebands, and a NAP( λ) follows the general exponential function. Based on these spectral relationships, a model was proposed for partitioning the total particulate absorption coefficients into the contributions of phytoplankton and nonalgal particles using the nonlinear optimization method. The model was validated by comparing the computed results with in situ absorption coefficients. In some wavebands, such as 412 nm, 443 nm, 490 nm and 683 nm, we obtained good correlations with the percentage root mean square error (RMSE) values being controlled within 25% and the slopes being closer to 1.0. For samples from coastal waters, the discrepancy was a little large, which might be due to the higher absorption contributions from certain pheopigments. Overall, this model provides us much insight into phytoplankton absorption retrieval from in situ measurements and remote sensing ocean color data.

Variations in the mass‐specific absorption coefficient of mineral particles suspended in water

We examined the light‐absorption properties of various samples of mineral particles suspended in water, which included pure mineral species (quartz, calcite, illite, kaolinite, and montmorillonite) and natural particulate assemblages such as desert dust originating from different locations in the Sahara. The absorption coefficient was measured in the spectral region from ultraviolet (UV) to near‐infrared on particle suspensions, using a special measurement geometry that reduced the scattering error to a very small level. The concentrations of the total mass of particles in suspension, as well as the mineralogical and elemental composition of particulate samples, were also determined. For the samples of pure mineral species with negligible contamination with iron, absorption was undetectable in the visible spectral region. All of the examined natural assemblages of mixed mineral species showed a significant content of iron (5–29% by weight), and all these samples exhibited significant absorption in the UV and blue‐green (400–550 nm) spectral regions. The spectral shape of absorption was similar to that determined on pure iron hydroxide suspension, with some spectral features (shoulders and changes in slope) superposed onto a general increase of absorption toward short wavelengths in the UV. The mass‐specific absorption coefficient of the mineral samples, a*m(λ), obtained by normalization of the absorption coefficient to the dry‐mass concentration of particles, *m showed a significant positive correlation with Fe content. No such relationship was found for other elements present in particles. The range of a*m(λ) values covered more than an order of magnitude (from &lt;0.1 to &lt;1 m2 g−1 near 400 nm), but the normalization of absorption coefficient to iron concentration led to a considerable reduction in variability among the samples. The iron‐specific absorption coefficient, a*Fe(λ), varied from &lt;1 to 4 m2 (g Fe)−1 near 400 nm for the natural mixtures of mineral species. Although iron was a major pigmenting agent, its concentration could explain only part of variability observed in the absorption properties of mineral particles.

Sources of spatial variability in light absorbing components along an equatorial transect from 165°E to 150°W

Spatial variability of major light‐absorbing components in seawater was analyzed at the equator from 165°E to 150°W during the Zonal Flux Cruise aboard the R/V Thompson from 20 April to 10 May 1996. Spectral absorption coefficients were separated into phytoplankton, nonphytoplankton chromophoric particulate material (CPM) and chromophoric dissolved organic material (CDOM). CPM and phytoplankton absorption account for about 20% and 80% of the total particulate absorption, respectively, above 100 m. The &lt;0.2 μm CDOM absorption accounts for nearly 80% of the total absorption below 100 m. A significant portion of spatial variability in particulate absorption was due to conservative processes in the upper 100 m. Non‐conservative spatial variability of phytoplankton absorption was zonally determined by the biomass changes of a spectrally invariant taxonomic community and vertically by photoacclimation. Distributions of size‐fractionated CDOM absorption are suggestive of the presence of new (0.2 to 0.7 μm) and old (&lt;0.2 μm) DOM pools. Photochemical reactions and microbial activity are nonconservative processes that act upon these pools, respectively. New DOM was found primarily in the upper water column, while an analysis of variance showed that &lt;0.2 μm CDOM originating from deep water composes the background CDOM in the Equatorial Pacific.

Regional patterns of particulate spectral absorption in the Pearl River estuary

Spectral absorption coefficients of the total particulate, a(p)(lambda), nonalgal particulate, a(d)(lambda), and phytoplankton pigment, a(ph)(lambda), in the Pearl River estuary and its vicinity waters were determined using the quantitative filter technique. The particulate absorption ap(443) ranged from 0.04 to 1.82 m(-1), with the corresponding a(ph)(443) ranging from 0.016 to 0.484 m(-1). Two typical spectral patterns are found for the total particulate absorption. For the first typical spectral pattern, the total particulate absorption spectra are similar to that of nonalgal particulate, with values of absorption coefficient decreasing with wavelength. In contrast, for the second spectral pattern the spectral absorptions by total particulate are very similar to that of phytoplankton pigment. The spectral dependency of absorption by nonalgal particulate follows an exponential increase toward short wavelengths, with an average slope of 0.012 +/- 0.002 nm(-1). The nonalgal absorption and the fraction of the nonalgal particulate absorption to the total particulate absorption exhibit a distinct trend of decreasing with salinity of the surface water. Phytoplankton pigment absorption exhibits a clear trend of increasing nonlinearly with chlorophyll a concentration. The relationships between the phytoplankton pigment absorption and chlorophyll a concentration can be described by power law, with the determination coefficient r(2) of 0.82. But only weak relationships between a(p)(lambda) and chlorophyll a concentration are observed, with the determination coefficient r(2) of 0.42. The relatively large scatter around ap(443) versus chl-a relationship would be attributed to the effects of loading of the nonalgal particulate absorption. Our analysis indicated that such relationships similar to that for Case I waters can be applicable to optically complex Case II waters if the effects caused by nonalgal are corrected. The chlorophyll-specific absorption coefficients of phytoplankton pigment are not constant, it increases with decreasing chlorophyll a level. To improve the accuracy of bio-optical algorithms for remote sensing in coastal waters, further investigations on the variations of specific absorption of chlorophyll pigment must be made.

The effects of variability in the inherent optical properties on estimations of chlorophyll a by remote sensing in Swedish freshwaters

During late summer of 1997 measurements were made of the inherent optical properties in Lake Mälaren, Sweden. These included measurements of the total particulate absorption, phytoplankton absorption, yellow substance absorption and total backscattering. Measurements were also made of concentrations known to affect the inherent optical properties; chlorophyll a and suspended particulate organic and inorganic matter. At the same time measurements of underwater radiance reflectance were made. The above measurements were used to better define the relationships between the inherent optical properties and the concentrations affecting them. Correlation analyses were also made in order to study covariation of the concentrations of suspended and dissolved substances in the data set. These relationships were then used to better parameterize a semi-analytical model predicting remote sensing reflectance. Multivariate sensitivity analyses showed that large variations on the Rr(700–710)/Rr(678–685) ratio vs. chlorophyll concentrations could be expected in a heterogeneous environment such as Lake Mälaren. This could lead to large errors in subsequent estimates of chlorophyll concentrations.

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.

Determining the contribution of pigments and the nonalgal fractions to total absorption: Toward an improved algorithm

Three methods that estimate the algal and nonalgal contributions to the total particulate matter light absorption were analyzed against the experimental Kishino method in a wide variety of environments. We found that one method gave better overall correlations and slopes closer to 1 than did the other methods, but the method overestimated the nonalgal fraction when the nonalgal absorption to total particulate matter absorption proportion in the sample was low. As a result, a correction is suggested and results are compared with those of the other methods, showing that the combined use of the original algorithm and the correction do configure a suitable basis for differentiating the algal and nonalgal fractions using as input only the total particulate light absorption data.

Partitioning particulate light absorption: A budget for a Mediterranean bay

The contribution of different components of the plankton (autotrophs and heterotrophic bacteria, heterotrophic flagellates, and mixo‐ and heterotrophic ciliates) and suspended inorganic particles to light absorbed by particles in a Mediterranean bay was examined based on a 2‐year time series of particulate light absorption (400–700, 400, and 675 nm), the biomass of planktonic microorganisms, and the mass of suspended inorganic particles. The average (±SE) particulate light absorption coefficient for the photosynthetically active radiation (PAR) range (0.035 ± 0.002 m−1) was characteristic of relatively clear coastal waters but showed great variability on occasions. A substantial fraction (53–73%, depending on the wavelength examined) of this variability could be accounted for by changes in the abundance of inorganic suspended matter, as well as planktonic organisms. The specific light absorption by autotrophs was less variable over the three spectral bands considered than those of microheterotrophs and inorganic particles, which dropped sharply with increasing wavelength. Inorganic particles contributed, on the average, 48 and 74% of the total particulate absorption for the PAR waveband and at 400 nm, respectively, with their contribution to light absorption at 675 nm being negligible. Autotrophs dominated light absorption at 675 nm (on average 45.8% of total particulate absorption), whereas mixo‐ and heterotrophic ciliates and bacteria together contributed, on average, 22.5% of the total light absorption at this wavelength. The combined light absorption coefficient of microheterotrophs at 400 nm (0.0126 m−1) was similar to that of autotrophs (0.013 m−1). These results documented the dominant role that inorganic particles play in the absorption of blue light in the Bay of Blanes and showed that the particulate light absorption by autotrophs was often comparable to that of heterotrophs.

Regional models for phytoplankton absorption as a function of chlorophyll a concentration

Empirical relationships for predicting phytoplanktonic absorption at 676 and 436 nm from water column chlorophyll a concentration are presented for distinct geographic regions defined by latitude. The forms of the predictive equations are controlled by underlying biological mechanisms and lend insight into these mechanisms. These region‐specific models allow prediction of phytoplanktonic absorption from more easily measured parameters such as chlorophyll a concentration or in situ fluorescence and increase accuracy in modeling optical properties or primary production rates from specific absorption coefficients. Phytoplanktonic absorption can be predicted from chlorophyll a concentrations estimated from satellite‐based ocean color measurements. Temperate and tropical regions exhibited statistically indistinguishable relationships at low chlorophyll so these regions were combined and treated as one. Nonlinear relationships between phytoplanktonic absorption at both 436 and 676 nm and chlorophyll a concentration for the combined temperate/tropical region suggested that pigment packaging effects were important and variable. Higher slopes between absorption and chlorophyll a at low chlorophyll supported the concept of low pigment packaging effects (thus higher specific absorption) in oligotrophic, low chlorophyll a waters. Subpolar waters displayed a distinct pattern and were defined as a separate region. Near‐linear and linear relationships between phytoplanktonic absorption at 436 and 676 nm and chlorophyll a concentration indicated that influences of pigment packaging on phytoplankton specific absorption coefficients were relatively constant and uncoupled from water column chlorophyll a concentration in the subpolar region. Optical depth correlated inversely with specific absorption at 436 nm in the subpolar region, illustrating the role of photoadaptation in determining specific absorption and predictive relationships. Differences between predictive quadratic equations for particulate and phytoplanktonic absorption indicated that detritus made only a small contribution to the nonlinear nature of the relationships. Absorption by detritus at 436 nm ranged from 25 to 90% of the total particulate absorption. The proportion of absorption by detritus did not exhibit any strong patterns as a function of chlorophyll a concentration, arguing against the previously invoked explanation that proportional increases in absorption by detritus as environments become more oligotrophic cause the observed nonlinearity between absorption and chlorophyll a concentration.

Particulate and dissolved spectral absorption on the continental shelf of the southeastern United States

Visible absorption spectra of particulate and dissolved materials were characterized on the continental shelf off the southeastern United States (the South Atlantic Bight), emphasizing cross‐shelf and seasonal variability. A coastal front separates turbid coastal waters from clearer midshelf waters. Spatial and seasonal patterns were evident in absorption coefficients for phytoplankton, detritus, and colored dissolved organic matter (CDOM); spectral shape parameters for CDOM and detritus; and phytoplankton chlorophyll‐specific absorption. The magnitude of CDOM absorption reflected seasonal differences in freshwater discharge and the salinity of the midshelf waters. In the spring of 1993 (high discharge), CDOM absorption at 443 nm was &gt;10 times that of total particulate absorption between 12 and 50 km offshore (0.28–0.69 m−1 versus 0.027–0.062 m−1) and up to 10 times the CDOM absorption measured in the previous summer (low discharge). Phytoplankton chlorophyll‐specific absorption in the blue increased with distance from shore (from &lt;0.03 m2 mg−1 in inner shelf waters to ∼0.1 m2 mg−1 at the most seaward stations in summer) and, for similar chlorophyll concentrations, was higher in summer than in the winter‐spring. These spatial and seasonal patterns in phytoplankton chlorophyll‐specific absorption can be attributed to a shift in phytoplankton species composition (from predominantly diatoms inshore to a cyanobacteria‐dominated assemblage midshelf in summer), pigment packaging, and higher carotenoid:chlorophyll with distance from shore.