Chromophoric dissolved organic matter (CDOM) absorption characteristics in relation to fluorescence in Lake Taihu,China, a large shallow subtropical lake

Absorption measurements from chromophoric dissolved organic matter (CDOM) and their relationships with dissolved organic carbon (DOC) and fluorescence were studied in Lake Taihu, a large, shallow, subtropical lake in China. Absorption spectra of lake water samples were measured from 240 nm to 800 nm. Highest values of a(λ), DOC and F _n (355) occurred near the river inflow to Meiliang Bay and decreased towards the central lake basin. A significant spatial difference was found between Meiliang Bay and the central lake basin in absorption coefficient, DOC-specific absorption coefficient, exponential slope coefficient, DOC concentration and fluorescence value. The spatial distribution of CDOM suggested that a major part of CDOM in the lake was from river input. CDOM absorption coefficients were correlated with DOC over the wavelength range 280–500 nm, and a(355) was also correlated with F _n (355), which showed that CDOM absorption could be inferred from DOC and fluorescence measurement. The coefficient of variation between a(λ) and DOC concentration decreased with increase in wavelength from 240 nm to 800 nm. Furthermore, a significant negative linear relationship was recorded between S value and CDOM absorption coefficient, as well as DOC-specific absorption coefficient. S value and DOC-specific absorption coefficient were used as a proxy for CDOM composition and source. Accurate CDOM absorption measurements are very useful in explaining UV attenuation and in developing, validating remote sensing model of water quality in Lake Taihu.     

A study of absorption characteristics of chromophoric dissolved organic matter and particles in Lake Taihu,China

Absorptions by non-phytoplankton particles and phytoplankton, and chromophoric dissolved organic matter (CDOM) were measured at 50 sites in large, shallow, Lake Taihu in winter and summer 2006 to study their seasonal and spatial variations, and their relative contributions to total absorption. The CDOM absorption was significantly higher in winter than in summer, due to degradation and release of fixed carbon in phytoplankton and submerged aquatic vegetation (SAV). The hyperbolic model was used to model the spectral absorption of CDOM, and the mean spectral slope of 6.38 nm⁻¹ was obtained. At most sites, the spectral absorption of non-phytoplankton particles was similar to that of the total particles, demonstrating that the absorption of the total particles is dominated by the absorption of non-phytoplankton particles. In summer, phytoplankton absorption increased markedly, due to frequent algal blooms especially in Meiliang Bay. In winter, the significant increase in non-phytoplankton particle absorption resulted from the increase of inorganic particulate matter caused by sediment resuspension. Strong linear relationships were found between a _d(440) and total suspended matter (TSM), organic suspended matter (OSM), and inorganic suspended matter (ISM). Strong linear relationships were also found between a _ph(440), a _ph(675) and chlorophyll a (Chl-a) concentration. The total relative contributions of non-phytoplankton particles over the range of photosynthetically active radiation (PAR) (400–700 nm) were 48.4 and 79.9% in summer and winter respectively. Non-phytoplankton particle absorption dominated the total absorption, especially in winter, in Lake Taihu, due to frequent sediment resuspension in the large shallow lake as a result of strong windy conditions. The results indicate that strong absorption by CDOM and non-phytoplankton particles at the blue wavelength has an impact on the spectral availability, and acts as a selection factor for the composition of the phytoplankton community, with cyanobacteria being the dominate species in Lake Taihu. Handling editor: L. Naselli-Flores     

Detecting the bloom-forming dinoflagellate Alexandrium tamarense using the absorption signature

Pulsed N inputs were simulated by exposing Alexandrium tamarense to sudden increase in concentration of three different N sources to evaluate an approach of using absorption coefficient (a _ph) ratio at three wavelengths to detect toxic dinoflagellates such as A. tamarense. The present study examined the absorption properties of A. tamarense for a range of concentrations under three different nitrogen sources and concluded that using absorption such as chlorophyll-specific absorption coefficient (a _ph ^* ) at a single wavelength is not appropriate for detecting bloom forming species due to variability of a _ph ^* caused by varying physiological states. With appropriate wavelengths established, the a _ph ratio approach with three wavelengths has the capability of detecting A. tamarense. Variable environmental conditions such as nutrient species, light quantity and light quality did not affect the a _ph ratios significantly, particularly the ratio at 510 and 675 relative to 555 nm. This ratio allows differentiation of A. tamarense from other phytoplankton species including other bloom-forming dinoflagellates. In addition, this ratio could also detect the presence of A. tamarense in mixed cultures from diluted samples of as low as 5% in cell abundance (c. 30% in Chl a concentration). The present study suggests that this ratio could be an adequate biomarker for dinoflagellates such as A. tamarense. This absorption ratio technique could provide means of interpreting field populations and enhancing the capability for detecting harmful species such as A. tamarense.     

Partitioning particulate scattering and absorption into contributions of phytoplankton and non-algal particles in winter in Lake Taihu (China)

Light scattering, backscattering, and absorption coefficients of particles were observed at 62 locations in Lake Taihu (China) in November 2008. A method using a priori knowledge and the measured data was proposed to partition particulate scattering and absorption into contributions of phytoplankton and non-algal particles. The results showed that phytoplankton weakly contributed to the particulate scattering and backscattering with the mean b _ph/b _p values usually below 10% and b _bph/b _bt values of 0.3–3.9% in the whole visible light spectrum, and an approximate relationship of b _bt ≈ b _bp ≈ b _bnap was regarded as reasonable in Lake Taihu. In contrast with scattering and backscattering, phytoplankton made more contributions to the particulate absorption with the mean a _ph/a _p values varying in a wide range of about 20–70%. Both the scattering and absorption spectra of non-algal particles can be modeled well by corresponding methods. A power function model was used to simulate the scattering spectra, which presented high predictive accuracies with MAPE values usually below 5% and RMSE values below 1.5 m⁻¹, while the spectral absorption model also performed well with mean S _nap being 0.0052 nm⁻¹ (standard deviation, SD = 0.0010 nm⁻¹). As to the phytoplankton absorption, a quadratic function model used was considered to have a good performance with corresponding parameters being supported at each wavelength in the spectral range of 400–700 nm. Additionally, two basic bio-optical parameters were determined, that is, b _nap^*(550) = 0.604 m² g⁻¹ and a _ph^*(675) = 0.0288 m² mg⁻¹. Overall, these results obtained in the present study supply us with new knowledge about optical properties of suspended particulates in an inland and highly turbid lake (Lake Taihu), which are beneficial to the development of analytical models of water color remote sensing.     

Optical characteristics of the harmful dinoflagellate Alexandrium tamarense in response to different nitrogen sources

The chlorophyll a specific absorption coefficient a_ph^* and absorption ratio (a_ph ratio) of Alexandrium tamarense at five concentrations of nitrate, ammonium and urea over a range from 6 to 100 μM were examined. The experimental results were compared to two coastal diatom species and a prymnesiophyte to identify differences in absorption ratios. Cells exposed to increasing nitrate concentrations were characterized by an increase in a_ph^* at 443, 490, 510, 555 and 675 nm. However, ammonium and urea induced low a_ph^* values at their lowest and highest concentrations. The a_ph relative to 510 or 555 nm was constant regardless of the concentration of the N source, but dependent on the N source. Oxidized N induced a lower a_ph ratio than the reduced form. Comparisons of the a_ph ratio among taxonomic groups revealed significant differences. The a_ph ratio of A. tamarense was 20–30 and >50% lower than those of two diatoms and a prymnesiophyte, respectively. The a_ph ratio of the present study could assist in increasing the capability for detecting harmful species such as A. tamarense.     

Parameterization of chlorophyll a-specific absorption coefficients and effects of their variations in a highly eutrophic lake: a case study at Lake Kasumigaura, Japan

The chlorophyll a-specific absorption coefficient ( a_\textph^* ( l) a_{\text{ph}}^{*} \left( \lambda \right) ) in a highly eutrophic lake can show characteristics distinct from that in the ocean due to the differences in the structure and composition of phytoplankton. In this study, investigated the variation of a_\textph^* ( l) a_{\text{ph}}^{*} \left( \lambda \right) in Lake Kasumigaura, a highly eutrophic lake in Japan, in association with the package effect and the effect of accessory pigments, and carried out the parameterization of a_\textph^* ( l) a_{\text{ph}}^{*} \left( \lambda \right) . Although a_\textph^* ( l) a_{\text{ph}}^{*} \left( \lambda \right) did not vary spatially, it did show significant temporal variation, with a particularly high value after spring-bloom. This high a_\textph^* ( l) a_{\text{ph}}^{*} \left( \lambda \right) in spring was attributed to a lower package effect and a higher proportion of carotenoid than the other samples. Although the value of a_\textph^* ( l) a_{\text{ph}}^{*} \left( \lambda \right) was correlated with the concentration of chlorophyll-a (Chl-a), the correlation coefficient was lower than those reported in the ocean. Some lake-water samples showed variations of the package effect and the effect of accessory pigments that were independent of the concentration of Chl-a, and these independent variations resulted in the weak correlation between a_\textph^* ( l) a_{\text{ph}}^{*} \left( \lambda \right) and the concentration of Chl-a. Together, these results suggest that the factors controlling a_\textph^* ( l) a_{\text{ph}}^{*} \left( \lambda \right) in highly eutrophic lakes are distinct from that in ocean samples.     

Retrieval of phycocyanin concentration from remote-sensing reflectance using a semi-analytic model in eutrophic lakes

With the rapid development of the economy in recent years, massive algal (blue-green algae in particular) blooms have often observed in Chinese eutrophic lakes. The concentration of the cyanobacterial pigment phycocyanin (PC), an accessory pigment unique to freshwater blue-green algae, is often used as a quantitative indicator of blue-green algae in eutrophic inland waters. The purpose of this study was to evaluate the semi-analytic PC retrieval algorithm proposed by Simis et al. and to explore the potential to improve this PC algorithm so that it is more suitable for eutrophic lakes, such as Taihu Lake. In this paper, we recalculated the correction coefficients γ and δ to calculate the absorptions of chlorophyll-a at 665 nm and the absorptions of phycocyanin at 620 nm in terms of in situ measurements and observed that the values of these coefficients differed from the values used by Simis et al. and Randolph et al. The two coefficients are site dependent due to the different bio-optical properties of lakes. We also observed that the specific PC absorption at 620 nm a_pc*(620) decreases exponentially with an increase in PC concentrations. Therefore, a non-linear power–function of a_pc*(620), instead of a constant value of a_pc*(620) as used by Simis et al., was proposed for our improved PC retrieval algorithm in Taihu Lake, yielding a squared correlation coefficient (R²) of 0.55 and a root mean square error (RMSE) of 58.89 μg/L. Compared with the original PC retrieval algorithm by Simis et al., the improved retrieval algorithm has generally superior performance. In evaluating the limitation of the PC retrieval algorithms, we observed that the ratio of the total suspended solids to phycocyanin can be used as a primary measure for retrieval performance. Validation in Dianchi Lake and an error analysis proved that the improved PC algorithm has a better universality and is more suitable for eutrophic lakes with higher PC concentrations.     

Scattering spectrum properties and their relationship to biogeochemical parameters: a case study in Taihu Lake

The scattering spectrum properties of highly turbid and eutrophic inland case 2 water from Taihu Lake were studied during three cruises from 2006 to 2007. The scattering [b _p(λ)] and backscattering [b _bp(λ)] coefficients and the backscattering probability (B) for Taihu Lake were found to show a clear spectral dependence, and this dependence was well simulated by a power-law function. This dependence, however, became weak when algae dominated the sample points. The mean values of the power-law index for b _p(λ), v, in Oct 2006, Mar 2007 and Nov 2007 were −0.6712, −0.8129 and −0.7600, respectively. To interpret the spectral characteristics and mechanisms of b _p(λ) and b _bp(λ), water samples were collected simultaneously for the biogeochemical characterization of suspended particles. The average values of the specific scattering coefficients for total suspended matter, inorganic suspended matter (ISPM) and organic suspended matter (OSPM) were 0.634 (550 nm), 1.057 (532 nm), and 0.396 g m⁻² (532 nm), respectively. The power-law index of b _bp(λ) (Y) was significantly related to ISPM/OSPM and b _bp(532 nm), but only weakly related to the particle size distribution index. The mean (spatial and wavelength) values of B in Oct 2006, Mar 2007, and Nov 2007 were 0.0108, 0.0138, and 0.0125, respectively. B decreases with increasing ISPM concentration because of the large contribution of ISPM to b _b(λ) and the strong restraint on b _bp(λ) caused by the multi-scattering effect under high-turbidity conditions.     

Photoadaptation of an ice algal community in thin sea ice,Saroma-Ko Lagoon,Hokkaido, Japan

We investigated the shade adaptation of a seasonally well-developed ice algal community in thin sea ice at Saroma-Ko Lagoon, Hokkaido, Japan on 3–4 March 2006 and 4–5 March 2007, by examining photosynthetic pigment concentrations, the chlorophyll a-specific light-absorption coefficient (a _ph *), and the light-saturation index (E _k ). The high proportions of photosynthetic pigments, including chlorophyll a, fucoxanthin, and chlorophyll c, and the low values of a _ph *₍₄₄₀₎ and a _ph *₍₆₇₅₎ suggested that the lagoon’s ice algal community was shade-adapted. The high ratio of E _k to total photosynthetically active radiation (PAR) in the ice algal habitat suggested that the degree of shade adaptation is weak. Scaling of E _k to total PAR could be extended to studies of the degree of photoadaptive succession of ice algal communities in the Northern Hemisphere. The degree of shade adaptation of ice algal communities in the Northern Hemisphere might be related to ice thickness, regardless of latitude.     

Development of optical criteria to discriminate various types of highly turbid lake waters

Our aim was to refine the optical classification of turbid waters in order to improve the performance of water color algorithms. Bio-optical measurements and sampling of optically active substances were performed in highly turbid lakes Taihu, Chaohu, and Dianchi, and in Three Gorges reservoir in China. Based on strong correlations between trough depths of remote sensing reflectance (R _rs(λ)) near 680 nm (denoted as TD680) and the ratios of inorganic suspended matter (ISM) to total suspended matter (TSM) concentrations, an empirical model was developed for water classification. In the 400–900 nm spectral range, different correlations between R _rs(λ), TSM and chlorophyll a (Chla) concentrations indicate discrepancies among the following ISM/TSM ranges: ISM/TSM ≤ 0.5, 0.5 < ISM/TSM < 0.8, and ISM/TSM ≥ 0.8. Corresponding findings support an important conclusion that only high ISM/TSM ratios, usually above 0.5, and using the more sensitive and stable near infrared spectral range (730–820 nm), can assure good performances of the TSM remote sensing algorithms. Meanwhile, the particulate absorption a _p(λ) and scattering b _p(λ) were strongly influenced by different ranges of ISM/TSM ratios. Typically the a _p(675) peaks became more and more vague as ISM/TSM increased, and the b _p(λ) values first decreased and then increased with a marked inflexion at ISM/TSM ≈ 0.5. The TD680 threshold values were derived to discriminate three types of turbid waters, i.e., Type 1 (TD680 ≥ 0.0082 sr⁻¹), Type 2 (0.0082 sr⁻¹ > TD680 > 0 sr⁻¹), and Type 3 (TD680 ≤ 0 sr⁻¹). This study provides a promising tool for identifying various types of highly turbid waters, and is significant for the development of class-based algorithms of water color remote sensing.     

太湖西北湖区2003-2012年间氮磷浓度及浮游植物主要类群变化趋势分析

基于2003-2012年太湖竺山湖和西部沿岸区水体理化指标与浮游植物丰度的逐月监测数据,分析了两个湖区氮磷营养盐状态和浮游植物丰度以及浮游植物主要类群的年际变化趋势及季节变化特征,探讨了浮游植物群落变化与水温及营养盐指标间的关系。结果表明：10年间两个湖区氮磷营养盐浓度总体呈下降趋势,以竺山湖TN、NH₃-N浓度和西部沿岸区NO₃-N浓度下降最为显著;浮游植物丰度总体呈上升趋势,蓝藻在群落结构中日益占据绝对优势;季节变化上,氮营养盐浓度表现为春冬季节高于夏秋季节,TP浓度和浮游植物丰度呈相反的变化趋势。Pearson相关分析显示,水温、NH₃-N浓度和TN/TP是影响蓝藻丰度及其在浮游植物群落中所占比例的主要因素。在温度和营养盐结构的共同作用下,10年间两个湖区蓝藻水华暴发时间逐渐提前,而消退时间逐渐滞后,水华持续时间逐年上升。在全球变暖背景下,太湖水华治理需执行更加严格的氮磷限制阈值,且在重污染的西北湖区控磷依然是关键。     

COMBINATION OF FLOW CYTOMETRY AND SINGLE CELL ABSORPTION SPECTROSCOPY TO STUDY THE PHYTOPLANKTON STRUCTURE AND TO CALCULATE THE CHL A SPECIFIC ABSORPTION COEFFICIENTS AT THE TAXON LEVEL1

The phytoplankton community structure of a hypertrophic lake was quantitatively determined with the aid of flow cytometry. The flow cytometry signals were calibrated to obtain cell‐specific information, such as the chl a content and the biovolume per cell. The reliability of this method was tested with laboratory cultures. The results of the phytoplankton structure in a hypertrophic lake with respect to chl distribution in the different algal groups obtained by flow cytometry were compared with the results from HPLC pigment fingerprinting. Both methods yield the percentage contribution of the different algal groups to total chl a. The chl a specific absorption coefficient of the phytoplankton (a^*_Phy) was determined via visible (VIS) spectroscopy of samples taken from a hypertrophic lake (Auensee) in 2003. The results indicated that a^*_Phy of the total cell suspension is dependent on the phytoplankton structure as well as on environmental factors. The linear relationship between a^*_Phy at 675 nm and the product of the chl a content per cell and the biovolume offered the possibility to normalize phytoplankton absorption spectra to acquire the taxon‐specific a^*_Phy. The estimated a^*_Phy (675 nm) values were used to normalize single cell absorption spectra at this wavelength to obtain the a^*_Phy between 400 and 750 nm for representatives of the major algal groups. Our measurements show that the absorption coefficient for the whole phytoplankton community varies within the season. Finally, we used the a^*_Phy and the chl a distribution to calculate the light absorption of each algal group in the hypertrophic lake.     

Bio-optical characterization of selected cyanobacteria strains present in marine and freshwater ecosystems

The optical properties, i.e., absorption and scattering spectra of ten strains of cyanobacteria from the Baltic Sea and Pomeranian lakes (Aphanizomenon flos-aquae KAC 15, Microcystis aeruginosa CCNP 1101, Anabaena sp. CCNP 1406, Synechocystis salina CCNP 1104, Phormidium sp. CCNP 1317, Nodularia spumigena CCNP 1401, Synechococcus sp. CCNP 1108, Nostoc sp. CCNP 1411, Cyanobacterium sp. CCNP 1105, Pseudanabaena cf. galeata CCNP 1312) grown under low light conditions were investigated. Moreover, the chlorophylls, carotenoids, and phycobilin composition as well as the size structure of chosen cyanobacteria were measured. Studied species revealed high diversity both in optical properties with the absorption spectra similarity index ranging from 0.67 to 0.94 and the pigment composition. The chlorophyll-specific absorption coefficient at 440 nm a _ph *(440) varied between 0.017 and 0.065 m² mg^?1. The influence of the package effect was only observed in the case of large filamentous cyanobacteria like N. spumigena or Nostoc sp. Interestingly, the package effect factor Q _a *(675) for large-celled Anabaena sp. was 0.92. Besides chlorophyll a, only echinenone, β-carotene, and phycocyanin were present in all analyzed cyanobacteria strains. Zeaxanthin, which is widely used as a marker pigment for cyanobacteria, was absent in the toxic N. spumigena and Anabaena sp., which are the species that occur in the Baltic Sea most frequently causing summer cyanobacterial blooms. The investigation also showed that the sample preservation technique can introduce some major errors within the absorption band affected by the phycocyanin absorption.     

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.     

Simultaneous Monitoring of Phosphine and of Phosphorus Species in Taihu Lake Sediments and Phosphine Emission from Lake Sediments

Phosphine (PH₃) was monitored in the Taihu Lake in China by a GC/NPD method, coupled with cryo-trapping enrichment technology. Results showed that PH₃ was universally detected in sediments, lake water and atmosphere of the Taihu Lake area. Total phosphorus (TP_s) and fractions of different phosphorus species in lake sediments were separately measured as dissolved phosphate (DP), phosphorus bound to aluminum (Al-P), iron (Fe-P) and calcium (Ca-P), occluded phosphorus (OP), and organic phosphorus (Org-P) by sequential chemical extraction. High PH₃ levels were correlated with high TP_s values in sediments and with eutrophication at different sites. In addition, a positive linear correlation equation was obtained between the concentrations of PH₃ in lake sediments and of the phosphorus fractions. The resulting multiple linear regression equation is PH₃ = −165 + 63.3 DP + 0.736 Al-P + 2.33 Ca-P + 2.29 Org-P. The flux of PH₃ across the sediment–water interface was estimated from sediment core incubation in May and October 2002. The annual average sediment–water flux of PH₃ was estimated at ca. 0.0138±0.005 pg dm⁻² h⁻¹, the average yearly emission value of PH₃ from Taihu Lake sediments to water was calculated to be 28.3±10.2 g year⁻¹, which causes a water PH₃ concentration of up to 0.178±0.064 pmol dm⁻³. The real importance of PH₃ could be higher, because PH₃ could be consumed in the oxic sediment–water boundary layer and in the water column. Spatial and temporal distributions of total phosphorus (TP_w) and chlorophyll a (Chl-a) in the water column of Taihu Lake were measured over the study period. Higher water PH₃ has also been found where the TP_w content was high. Similarly, high Chl-a was consistent with higher water PH₃. Positive relationships between PH₃ and TP_w (average R² = 0.47±0.26) and Chl-a (average R² = 0.23±0.31) were observed in Taihu Lake water.     

EFFECTS OF TEMPERATURE ON GROWTH,LIGHT ABSORPTION,AND QUANTUM YIELD IN DUNALIELLA TERTIOLECTA (CHLOROPHYCEAE)1

The effects of growth temperature on the marine chlorophyte Dunaliella tertiolecta Butcher were studied to provide a more mechanistic understanding of the role of environmental factors in regulating bio-optical properties of phytoplankton. Specific attention was focused on quantities that are relevant for modeling of growth and photosynthesis. Characteristics including chlorophyll a (chl z)-specific light absorption (a*_ph(λ)), C:chl a ratio, and quantum yield for growth (φ_μ) varied as functions of temperature under conditions of excess light and nutrients. As temperature increased over the range examined (12°-28°C), intracellular concentrations of chl a increased by a factor of 2 and a*_ph(λ) values decreased by more than 50% at blue to green wavelengths. The lower values of a*_ph(λ) were due to both a decrease in the abundance of accessory pigments relative to chl a and an increase in pigment package effects arising from higher intracellular pigment concentrations. Intracellular pigment concentration increased as a consequence of higher cellular pigment quotas combined with lower cell volume. At high growth temperatures, slightly more light was absorbed on a per-cell-C basis, but the dramatic increases in growth rate from μ= 0.5 d^?1 at 12° C to μ= 2.2 d^?1 at 28°C were primarily due to an increase in φ_μ (0.015–0.041 mol C (mol quanta)^?1). By comparison with previous work on this species, we conclude the effects of temperature on a*_ph(λ) and φ_μ are comparable to those observed for light and nutrient limitation. Patterns of variability in a*_ph(λ)and φ_μ as a function of growth rate at different temperatures are similar to those previously documented for this species grown at the same irradiance but under a range of nitrogen-limited conditions. These results are discussed in the context of implications for bio-optical modeling of aquatic primary production by phytoplankton.     

Photochemical degradation of chromophoric-dissolved organic matter exposed to simulated UV-B and natural solar radiation

Photochemical degradation of chromophoric-dissolved organic matter (CDOM) by UV-B radiation decreases CDOM absorption in the UV region and fluorescence intensity, and alters CDOM composition. CDOM absorption, fluorescence, and the spectral slope indicating the CDOM composition were studied using 0.22-μm-filtered samples of Meiliang Bay water from Lake Taihu that were exposed to short-term (0–12 h) simulated UV-B radiation and long-term (0–12 days) natural solar radiation in summer. CDOM absorption coefficient and fluorescence decreased with increasing exposure time, which relates to the amounts of absorbed light energy. The decreases of CDOM absorption and normalized fluorescence corresponded to first order kinetics reactions. Different decreases of CDOM absorption and fluorescence at different wavelengths suggested that the composition of CDOM changed when it absorbed ultraviolet radiation. Photochemical degradation increased the spectral slope during 275–295 nm region (S _275–295) but decreased the spectral slope during 275–295 nm region (S _350–400). The slope ratio S _R (S _275–295:S _350–400) increased in the photochemical process, which could be used as an indicator of photobleaching and composition change of CDOM. Our results show that photochemical degradation is important in the cycling of CDOM, which indicated change in the composition of CDOM. Handling editor: Luigi Naselli-Flores     

Aromatic amino acid oxidation products as antioxidants

The accumulation of UV photolysis products of amino acids tyrosine and tryptophan, which possess antioxidant activity, has been studied by the method of luminol-dependent chemiluminescence. The amount of antioxidant products was judged by the value of the total antioxidant potential of a UV-irradiated solution, the measure of which was the distance between the peaks of the chemiluminescence curve in the system 2,2′-azo-bis(2-amidinopropane) hydrochloride + luminol with a UV-irradiated and an unirradiated sample (induction period, τ _i ). Simultaneously, the absorption and fluorescence spectra of unirradiared and UV-irradiated amino acid solutions were recorded. It was shown that exposure of a tryptophan solution to radiation led to accumulation of a fluorescent product N-formyl kynurenine (λ_em = 325 nm, λ_max = 440 nm), and the curve of its accumulation was similar to the growth of antioxidant potential. When a tyrosine solution was irradiated, the main fluorescent product was dityrosine (λ_em = 310 nm, λ_max = 415 nm). Nevertheless, the dose dependences of the formation of dityrosine and the total antioxidant potential were completely different. It was found that another product of tyrosine UV photolysis, dihydroxyphenylalanine, possessed pronounced antioxidant activity. It was concluded that the main antioxidant produced under UV irradiation of tryptophan is formyl kynurenine, and under irradiation of tyrosine it is dihydroxyphenylalanine.     

Photoadaptation in marine phytoplankton : changes in spectral absorption and excitation of chlorophyll a fluorescence

The optical properties of marine phytoplankton were examined by measuring the absorption spectra and fluorescence excitation spectra of chlorophyll a for natural marine particles collected on glass fiber filters. Samples were collected at different depths from stations in temperate waters of the Southern California Bight and in polar waters of the Scotia and Ross Seas. At all stations, phytoplankton fluorescence excitation and absorption spectra changed systematically with depth and vertical stability of the water columns. In samples from deeper waters, both absorption and chlorophyll a fluorescence excitation spectra showed enhancement in the blue-to-green portion of the spectrum (470-560 nm) relative to that at 440 nm. Since similar changes in absorption and excitation were induced by incubating sea water samples at different light intensities, the changes in optical properties can be attributed to photoadaptation of the phytoplankton. The data indicate that in the natural populations studied, shade adaptation caused increases in the concentration of photosynthetic accessory pigments relative to chlorophyll a. These changes in cellular pigment composition were detectable within less than 1 day. Comparisons of absorption spectra with fluorescence excitation spectra indicate an apparent increase in the efficiency of sensitization of chlorophyll a fluorescence in the blue and green spectral regions for low light populations.     

Study of optical properties of electropolymerized melanin films by photopyroelectric spectroscopy

Photopyroelectric (PPE) spectroscopy, in the 350–1,075 nm wavelength range, was used to study the optical properties of electropolymerized melanin films on indium tin oxide (ITO) coated glass. The PPE intensity signal as a function of the wavelength λ, V _n(λ) and its phase F _n(λ) were independently measured. Using the PPE signal intensity and the thermal and optical properties of the pyroelectric detector, we were able to calculate the optical absorption coefficient β of melanin in the solid-state. We believe this to be the first such measurement of its kind on this material. Additionally, we found an optical gap in these melanin films at 1.70 eV.