Detection and Quantification of Snow Algae with an Airborne Imaging Spectrometer

We describe spectral reflectance measurements of snow containing the snow alga Chlamydomonas nivalis and a model to retrieve snow algal concentrations from airborne imaging spectrometer data. Because cells of C. nivalis absorb at specific wavelengths in regions indicative of carotenoids (astaxanthin esters, lutein, β-carotene) and chlorophylls a and b, the spectral signature of snow containing C. nivalis is distinct from that of snow without algae. The spectral reflectance of snow containing C. nivalis is separable from that of snow without algae due to carotenoid absorption in the wavelength range from 0.4 to 0.58 μm and chlorophyll a and b absorption in the wavelength range from 0.6 to 0.7 μm. The integral of the scaled chlorophyll a and b absorption feature (I_0.68) varies with algal concentration (C_a). Using the relationship C_a = 81019.2 I_0.68 + 845.2, we inverted Airborne Visible Infrared Imaging Spectrometer reflectance data collected in the Tioga Pass region of the Sierra Nevada in California to determine algal concentration. For the 5.5-km² region imaged, the mean algal concentration was 1,306 cells ml⁻¹, the standard deviation was 1,740 cells ml⁻¹, and the coefficient of variation was 1.33. The retrieved spatial distribution was consistent with observations made in the field. From the spatial estimates of algal concentration, we calculated a total imaged algal biomass of 16.55 kg for the 0.495-km² snow-covered area, which gave an areal biomass concentration of 0.033 g/m².     

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

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

三峡库区古夫河着生藻类叶绿素a的时空分布特征及其影响因素

古夫河系长江三峡水库湖北省境内香溪河的一条支流,发源于神农架林区并最终汇入三峡水库香溪河库湾,是流域生态学研究的热点水体。本文对古夫河2010年9月至2011年8月干流及主要支流21个样点的着生藻类叶绿素a和14项环境因子进行了调查,采用多元统计方法对调查数据进行了方差分析、聚类分析、偏相关分析和逐步回归分析,研究了着生藻类叶绿素a的时空分布特征及其主要环境影响因子。结果表明：叶绿素a含量范围为0.07—145.96 mg/m2,平均值为11.63 mg/m2。不同样点的叶绿素a含量差异显著,其中古夫河干流上游低于下游,支流竹园河上游高于下游,表现为人为影响大的区域高于人为影响小的区域;不同季节着生藻类叶绿素a含量差异显著,表现出冬春季高、夏秋季低的趋势。古夫河着生藻类叶绿素a与总磷和硬度呈极显著正相关,与电导率、氨氮和总氮呈显著正相关,而与流速呈极显著负相关;水体总磷是古夫河流域着生藻类生长的第一限制因子,流速对着生藻类的生长具有显著抑制作用;古夫河着生藻类生物量空间格局可能由其生境尺度（营养盐）和流域尺度（硬度和电导率）的特征决定,而古夫河着生藻类生物量时间变化主要受水动力（流速）的影响。     

ESTIMATION OF CHLOROPHYLL a FROM TIME SERIES MEASUREMENTS OF HIGH SPECTRAL RESOLUTION REFLECTANCE IN AN EUTROPHIC LAKE

We acquired high spectral resolution reflectance data in Carter Lake, a eutrophic oxbow on the Iowa–Nebraska border, from April 1995 to April 1996. Chlorophyll a, total seston, sestonic organic matter, Secchi depth, and nephelometric turbidity were determined for each respective spectral measurement. Changes in algal taxonomic structure and abundance coincided with the development and senescence of a midsummer through autumn bloom of Anabaena. Taxonomic structure was more diverse in late winter and spring when Synedra sp. (diatom) and several chlorophytes and dinoflagellates were present. Overall, chlorophyll a varied from about 20 to 280 μg·L^?1, Secchi transparency from 18 to 74 cm, and seston dry weight from 11 to 48 mg·L^?1 in February and September, respectively. Particulate matter completely dominated lake water light attenuation. Dissolved organic matter had low optical activity. The most sensitive spectral feature to variation in chlorophyll a concentration was the magnitude of the scattering peak near 700 nm. The 700-nm peak correlated to chlorophyll concentration through the relationships between algal pigment absorption near 670 nm and the cell biomass and surface-related scattering signal in the near infrared. An algorithm relating the height of the 700-nm reflectance peak above a reference baseline between 670 and 850 nm to chlorophyll a was accurate and robust despite large variations in optical constituents caused by both strong seasonality in the algal system and short-term variations in seston from wind-induced sediment resuspension. The present algorithms were successfully used in other systems with different seasonality and productivity patterns. The coefficients of the models relating chlorophyll a and spectral reflectance variables appeared to be ecosystem specific: both the intercept and slope for the models in this study were moderately lower than for several other recently published results. We validated our algorithm coefficients with a second, independent dataset. The standard error for chlorophyll a prediction was ±28 μg·L^?1.     

Analysis of absorption spectra changes induced by temperature lowering on phycobilisomes, thylakoids and chlorophyll-protein complexes.

Using fourth derivative analysis, differences between room and low temperature absorption spectra were studied. The positions of most absorption bands of the water-soluble, accessory pigment complex, the phycobilisome, remained unchanged after cooling. The stability of the wavelength positions of chlorophyll a forms in vivo as a function of temperature (Gulyaev, B.A. and Litvin, F.F. (1967) Biofizika 12, 845--854) was generally confirmed. The wavelength positions of all chlorophyll a forms in the P-700 chlorophyll a protein complex were unchanged when the preparations were cooled to -196 degrees C. Likewise, with other chlorophyll-containing materials: the light-harvesting chlorophyll a/b protein complex and the thylakoids of higher plants, algae, and cyanobacteria, the wavelengths positions of most chlorophyll a forms were stable upon cooling. An exception was a 680 nm chlorophyll a band which was generally split at low temperature into two bands with the materials investigated. An interpretation of the multiplicity of chlorophyll spectral forms and the spectral changes induced by cooling for these forms is given using exciton theory and the energy-coupling variation of chlorophyll a molecules.     

Comparative reflectance properties of algal cultures with manipulated densities

A comparative study was conducted with representatives of four ecologically important freshwater algal phyla (Chlorophyta, Cyanophyta, Bacillariophyta and Pyrrophyta) in order to (1) analyze the relationship between reflectance spectral features and algal density, and (2) to elucidate and characterize possible diagnostic spectral reflectance features for identification of dominant groups in algal bloom states. Algae (two chlorophytes, a cyanophyte and a bacillariophyte) were cultured in mesocosm tanks and then their densities were reduced through dilution with clear water in four experiments. In a fifth experiment, use was made of the pyrrophyte Peridinium gatunense, which bloomed in Lake Kinneret, Israel, practically forming a unialgal ‘culture’. The basic quantitative procedures for spectroradiometeric measurements and support data collection (pigment concentrations) were consistent among the experiments. Several spectral reflectance characteristics were found to be common to all the species examined in the study: a depression between 440 and 500 nm, a salient trough around 670 nm, and prominent peaks centered around 570 nm and 700 nm. The increase in magnitude of a near infra-red peak and shift of its position towards longer wavelengths with increased chlorophyll- a concentration was found to be a common trait for all the species studied. Other specific spectral reflectance features were also found and are discussed here. Quantitatively different relationships of magnitude and position of spectral reflectance features with chlorophyll- a concentration reflect different optical properties of algae (their scattering and absorption). This may be useful for taxonomic characterization by remote sensing. This revised version was published online in June 2006 with corrections to the Cover Date.     

Experimental Evidence on Nutrient and Substrate Limitation of Baltic Sea sea-ice Algae and Bacteria

The effect of nutrient limitation on Baltic Sea ice algae, and substrate and nutrient limitation on ice bacteria, was studied in a series of in situ -experiments conducted during the winter of 2002 in northern Baltic Sea. Community level changes in algal biomass (chlorophyll a) and productivity, and bacterial thymidine and leucine incorporation were followed for one week after the addition of nutrient and/or organic carbon rich filtered seawater to the experimental units. The results showed the major contribution of snow cover to the algal responses during the beginning of the ice-covered season. Algal communities were able to grow even in January if no snow was present. Nutrient addition did occasionally have an effect on algal biomass and productivity in the ice. Surprisingly, seeding effect from the ice to the underlying water was negatively affected by the nutrient availability in March. Bacterial limitation varied between nutrient (phosphorus) and substrate limitations. The results showed, that limitation in both algal and bacterial communities changed periodically in the northern Baltic Sea ice.     

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

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

Inherent optical properties of suspended particulates in four temperate lakes: application of in situ spectroscopy

Instrumentation measuring hyperspectral particle attenuation and absorption was used to assess particle concentration and size, chlorophyll, and spectral characteristics as a function of depth in four temperate lakes of different trophy. Partitioning the absorption coefficient permitted us to analyze properties of phytoplankton absorption as a function of ambient illumination and hydrographic conditions. Stratification was found to be a controlling factor in the size distribution and concentration of particles. Bloom cycles (chlorophyll > 10 mg m^?3) were observed to evolve over several weeks but on occasion did change rapidly. Total chlorophyll concentration revealed the majority of the lakes did not follow the typical seasonal succession of biomass associated with temperate waters. Particle and chlorophyll concentration maxima did not always coincide, cautioning the use of chlorophyll a as a surrogate for algal biomass. Phytoplankton near the base of the euphotic zone, including a deep chlorophyll maximum in an oligotrophic system, were found to exhibit significant chromatic adaptation. Unique absorption peaks identified the ubiquitous presence of cyanobacteria in all four lakes. Finally, particle resuspension and possible nepheloid layers were observed in the two smallest lakes.     

Comparing Springtime Ice-Algal Chlorophyll a and Physical Properties of Multi-Year and First-Year Sea Ice from the Lincoln Sea

With near-complete replacement of Arctic multi-year ice (MYI) by first-year ice (FYI) predicted to occur within this century, it remains uncertain how the loss of MYI will impact the abundance and distribution of sea ice associated algae. In this study we compare the chlorophyll a (chl a) concentrations and physical properties of MYI and FYI from the Lincoln Sea during 3 spring seasons (2010-2012). Cores were analysed for texture, salinity, and chl a. We identified annual growth layers for 7 of 11 MYI cores and found no significant differences in chl a concentration between the bottom first-year-ice portions of MYI, upper old-ice portions of MYI, and FYI cores. Overall, the maximum chl a concentrations were observed at the bottom of young FYI. However, there were no significant differences in chl a concentrations between MYI and FYI. This suggests little or no change in algal biomass with a shift from MYI to FYI and that the spatial extent and regional variability of refrozen leads and younger FYI will likely be key factors governing future changes in Arctic sea ice algal biomass. Bottom-integrated chl a concentrations showed negative logistic relationships with snow depth and bulk (snow plus ice) integrated extinction coefficients; indicating a strong influence of snow cover in controlling bottom ice algal biomass. The maximum bottom MYI chl a concentration was observed in a hummock, representing the thickest ice with lowest snow depth of this study. Hence, in this and other studies MYI chl a biomass may be under-estimated due to an under-representation of thick MYI (e.g., hummocks), which typically have a relatively thin snowpack allowing for increased light transmission. Therefore, we suggest the on-going loss of MYI in the Arctic Ocean may have a larger impact on ice–associated production than generally assumed.     

Changes in photosynthetic rate and pigment content of blue-green algae in Lake Mendota.

Blue-green algal blooms were present in Lake Mendota (Dane County, Wis.) from June to November 1976. Concentrations of total algal biomass and of particular algal species were monitored and compared with the pigment contents (chlorophyll a and phycocyanin) and photosynthetic rate of the algal populations. The specific photosynthetic rate (micrograms of C fixed per microgram of chlorophyll a per hour) was a good measure of the physiological state of the algae because this quantity increased just before each population increase and decreased before algal densities diminished. Since the quantity of light in the epilimnion which was available for photosynthesis by algal cells decreased in summer when the high algal densities attenuated incoming radiation, we investigated the possibility that the organisms would utilize lower light intensities more efficiently by increasing their pigment contents. Although some evidence of enhanced utilization of low light levels was found in the period from July to October, this result was not due to increasing chlorophyll and phycocyanin contents. There was a decrease in the phycocyanin content of the algae during this period, perhaps related to the availability of inorganic nitrogen.     

Changes in photosynthetic rate and pigment content of blue-green algae in Lake Mendota.

Blue-green algal blooms were present in Lake Mendota (Dane County, Wis.) from June to November 1976. Concentrations of total algal biomass and of particular algal species were monitored and compared with the pigment contents (chlorophyll a and phycocyanin) and photosynthetic rate of the algal populations. The specific photosynthetic rate (micrograms of C fixed per microgram of chlorophyll a per hour) was a good measure of the physiological state of the algae because this quantity increased just before each population increase and decreased before algal densities diminished. Since the quantity of light in the epilimnion which was available for photosynthesis by algal cells decreased in summer when the high algal densities attenuated incoming radiation, we investigated the possibility that the organisms would utilize lower light intensities more efficiently by increasing their pigment contents. Although some evidence of enhanced utilization of low light levels was found in the period from July to October, this result was not due to increasing chlorophyll and phycocyanin contents. There was a decrease in the phycocyanin content of the algae during this period, perhaps related to the availability of inorganic nitrogen.     

A fluorometric method for the differentiation of algal populations in vivo and in situ

Fingerprints of excitation spectra of chlorophyll (Chl) fluorescence can be used to differentiate `spectral groups' of microalgae in vivo and in situ in, for example, vertical profiles within a few seconds. The investigated spectral groups of algae (green group, Chlorophyta; blue, Cyanobacteria; brown, Heterokontophyta, Haptophyta, Dinophyta; mixed, Cryptophyta) are each characterised by a specific composition of photosynthetic antenna pigments and, consequently, by a specific excitation spectrum of the Chl fluorescence. Particularly relevant are Chl a, Chl c, phycocyanobilin, phycoerythrobilin, fucoxanthin and peridinin. A laboratory-based instrument and a submersible instrument were constructed containing light-emitting diodes to excite Chl fluorescence in five distinct wavelength ranges. Norm spectra were determined for the four spectral algal groups (several species per group). Using these norm spectra and the actual five-point excitation spectrum of a water sample, a separate estimate of the respective Chl concentration is rapidly obtained for each algal group. The results of dilution experiments are presented. In vivo and in situ measurements are compared with results obtained by HPLC analysis. Depth profiles of the distribution of spectral algal groups taken over a time period of few seconds are shown. The method for algae differentiation described here opens up new research areas, monitoring and supervision tasks related to photosynthetic primary production in aquatic environments. This revised version was published online in June 2006 with corrections to the Cover Date.     

Photosynthetic action spectra of marine algae

A polarographic oxygen determination, with tissue in direct contact with a stationary platinum electrode, has been used to measure the photosynthetic response of marine algae. These were exposed to monochromatic light, of equal energy, at some 35 points through the visible spectrum (derived from a monochromator). Ulva and Monostroma (green algae) show action spectra which correspond very closely to their absorption spectra. Coilodesme (a brown alga) shows almost as good correspondence, including the spectral region absorbed by the carotenoid, fucoxanthin. In green and brown algae, light absorbed by both chlorophyll and carotenoids seems photosynthetically effective, although some inactive absorption by carotenoids is indicated. Action spectra for a wide variety of red algae, however, show marked deviations from their corresponding absorption spectra. The photosynthetic rates are high in the spectral regions absorbed by the water-soluble "phycobilin" pigments (phycoerythrin and phycocyanin), while the light absorbed by chlorophyll and carotenoids is poorly utilized for oxygen production. In red algae containing chiefly phycoerythrin, the action spectrum closely resembles that of the water-extracted pigment, with peaks corresponding to its absorption maxima (495, 540, and 565 mµ). Such algae include Delesseria, Schizymenia, and Porphyrella. In the genus Porphyra, there is a series P. nereocystis, P. naiadum, and P. perforata, with increasingly more phycocyanin and less phycoerythrin: the action spectra reflect this, with increasing activity in the orange-red region (600 to 640 mµ) where phycocyanin absorbs. In all these red algae, photosynthesis is almost minimal at 435 mµ and 675 mµ, where chlorophyll shows maximum absorption. Although the chlorophylls (and carotenoids) are present in quantities comparable to the green algae, their function is apparently not that of a primary light absorber; this role is taken over by the phycobilins. In this respect the red algae (Rhodophyta) appear unique among photosynthetic plants.     

Biological,chemical and physical factors influencing the seasonal change of polysaccharide concentration in irrigation-channel sediment

Variation of polysaccharide concentration in irrigation-channel sediment was determined concurrently with biological, chemical and physical factors influencing the benthic algal community. Phenol-sulphuric acid method was used to measure polysaccharide concentration. Polysaccharide concentration, biomass of benthic algae, and species composition changed spatially and temporally. Fluctuations of total suspended solid (TSS) concentration and exposure of channel bed to direct sunlight had major effects on algal growth and polysaccharide production. Polysaccharide concentration was correlated to chlorophyll a concentration (r=0.73, P<0.001) and algal biomass (r=0.57, P<0.001). Fragilaria construens and Aulacoseira (Melosira) italica were the most common diatoms in the benthic flora. Chlorophyll a concentration in the sediment showed a strong negative correlation (r=-0.99, P<0.001) with the seasonal variation of TSS concentration in channel water. The polysaccharides produced by benthic microorganisms play a major role in clogging channel bed and thereby reducing seepage from earthen irrigation channels. Correlations between polysaccharide concentration and chlorophyll a (and algal biomass) further indicate the importance of benthic algae for polysaccharide production. Since availability of light to the algal flora is critical for the production of polysaccharides, the effect of clogging can be maximized by exposing the channel bed to direct sunlight during non-irrigation period (winter).     

Shifting nutrient-mediated interactions between algae and bacteria in a microcosm: evidence from alkaline phosphatase assay

The impacts of different nutrient additions (N + P, N + P + C, 4N + P, 4N + P + C, N + 2P) on the growth of algae and bacteria were studied in a microcosm experiment. Since alkaline phosphatase activity (APA) provides an indication of phosphorus deficiency, the higher value for algal APA in the treatments with excess nitrogen and for bacterial APA in the treatments with excess carbon suggested that, algal and bacterial phosphorus-limited status were induced by abundant nitrogen and carbon input, respectively. Bacterial phosphorus-limited status was weakened due to higher bacterial competition for phosphorus, compared to algae. In comparison with the bacterial and specific bacterial APA, higher values of algal and specific algal APA were found, which showed a gradual increase that coincided with the increase of chlorophyll a concentration. This fact indicated not only a stronger phosphorus demand by algae than by bacteria, but also a complementary relationship for phosphorus demand between algae and bacteria. However, this commensalism could be interfered by glucose input resulting in the decline of chlorophyll a concentration. Furthermore, the correlation between bacterial numbers and chlorophyll a concentration was positive in treatments without carbon and blurry in treatments with carbon. These observations validate a hypothesis that carbon addition can stimulate bacterial growth justifying bacterial nutrient demand, which decreases the availability of nutrients to algae and affects nutrient relationship between algae and bacteria. However, this interference would terminate after algal and bacterial adaption to carbon input.     

Effects of snow removal and algal photoacclimation on growth and export of ice algae

Net growth of ice algae in response to changes in overlying snow cover was studied after manipulating snow thickness on land-fast, Arctic sea ice. Parallel laboratory experiments measured the effect of changing irradiance on growth rate of the ice diatom, Nitzschia frigida. After complete removal of thick snow (≥9 cm), in situ ice algae biomass declined (over 7–12 days), while removal of thin snow layers (4–5 cm), or partial snow removal, increased net algal growth. Ice bottom ablation sometimes followed snow removal, but did not always result in net loss of algae. Similarly, in laboratory experiments, small increases in irradiance increased algal growth rate, while greater light shifts suppressed growth for 3–6 days. However, N. frigida could acclimate to relatively high irradiance (110 μmol photons m² s⁻¹). The results suggest that algal loss following removal of a thick snow layer was due to the combination of photoinhibition and bottom ablation. The smaller relative increase in irradiance after removal of thin or partial snow layers allowed algae to maintain high specific-growth rates that compensated for loss from physical mechanisms. Thus, the response of ice algae to snow loss depends both on the amount of change in snow depth and algal photophysiology. The complex response of ice algae growth and export loss to frequently changing snow fields may contribute to horizontal and temporal patchiness of ecologically and biogeochemically important variables in sea ice and should be considered in predictions of how climate change will affect Arctic marine ecosystems.     

Compaitive Study on Liposoluble Compoands in Autotrophic and Heterotrophic Chlorella protothecoides

Both autotrophically and heterotrophically grown Chlorella protothecoides cells have been obtained in cell cultures. The content of liposoluble compounds in the cells of heterotrophic algae occupied 72% of the total cells in dry weight, which was more than 4 times as high as that in the autotrophic algal cells. There existed remarkbly different distribution patterns of the hydrocarbons in thesetwo kinds of cells. The hydrocarbons in autotrophic cells were characterised by the predominance of C17 normal alkanes, wheraes the heterotrophic cells were rich in normal alkanes of higher molecular weight or longer carbon chain with C25 as the dominant carbon. The structure of the compounds in benzene fraction is not quite clear, but the compounds in autotrophis sample may be related to the degeneration of the pigments. The compounds in heterotrophic sample probably come from lipid acids. The visible--ultraviolet absorption spectrum of the pigment compounds demonstrated the absorption peaks of the acetone extract from the autotrophic cells at 432.5, 451.5, 472.5 and 661.5 nm, reflecting the existence of chlorophyll and carotenoid, both with a rather high concentration. However, the acetone extract from the hetertrophic algal cells only showed absorption peaks at 427.4, 450.8 and 477.5 nm. The absorption peaks of the original green cells completely disappeared at 432.5 and 661.5 nm, reflecting the disappearance of chlorophyll in cells on the whole; the remaining absorption peaks only reflected the existence of carotenoid, but its concentration had already been greatly reduced. The resuls from comparative experiments were of essential significance on the study of physiological metabolism in heterotrophically grown C. protothecoides and on the exploration and application of the lipid compounds in this kind of algae.     

The use of spectral fluorescence methods to detect changes in the phytoplankton community

In vivo fluorescence methods are efficient toolsfor studying the seasonal and spatial dynamics ofphytoplankton. Traditionally the measurements are madeusing single excitation-emission wavelengthcombination. During a cruise in the Gulf of Riga(Baltic Sea) we supplemented this technique bymeasuring the spectral fluorescence signal (SFS) andfixed wavelength fluorescence intensities at theexcitation maxima of main accessory pigments. Thesemethods allowed the rapid collection of quantitativefluorescence data and chemotaxonomic diagnostics ofthe phytoplankton community. The chlorophylla-specific fluorescence intensities (R) and thespectral fluorescence fingerprints were analysedtogether with concentrations of chlorophyll a indifferent algal size-groups, phytoplankton biomass andtaxonomic position. The lower level of R in thesouthern gulf was related to the higher proportion ofcyanobacteria relative to total biomass and the lowerabundance of small algae. The phycoerythrinfluorescence signal was obviously due to the largecyanobacteria. The basin-wide shift in the shape ofchlorophyll a excitation spectra was caused bythe variable proportions of differently pigmentedcyanobacteria, diatoms and cryptomonads. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of phytoplankton species composition on absorption spectra and modeled hyperspectral reflectance

Understanding the spectral characteristics of remotely-sensed reflectance by different phytoplankton species can assist in the development of algorithms to identify various algal groups using satellite ocean color remote sensing. One of the main challenges is to separate the effect of species composition on the reflectance spectrum from other factors such as pigment concentration and particle size structure. Measuring the absorption spectra of nine different cultured algae, and estimating the reflectance of the different species, provides a useful approach to study the effects of species composition on the bio-optical properties. The results show that the absorption spectra of different species exhibit different spectral characteristics and that species composition can significantly change the absorption characteristics at four main peaks (438, 536, 600 and 650 nm). A ‘distance angle index’ was used to compare different phytoplankton species. Results indicate that this index can be used to identify species from the absorption spectra, using a database of standard absorption spectra of known species as reference. By taking into account the role of species composition in the phytoplankton absorption model, the performance of the model can be improved by up to 5%. A reflectance-species model is developed to estimate the remotely-sensed reflectance from the absorption spectra, and the reflectance of different phytoplankton species at the same chlorophyll-a concentration is compared, to understand effects of species composition on the reflectance spectra. Different phytoplankton species can cause up to 33% difference in the modeled reflectance at short wavelengths under the condition of the same chlorophyll-a concentration, and variations in the reflectance spectrum correspond to the colors of the algae. The standard deviation of the reflectance among different species shows that the variations from 400 to 450 nm are sensitive to species composition at low chlorophyll-a concentrations, whereas variations in the 510 to 550 nm range are more sensitive under high chlorophyll-a concentrations. For this reason, the green bands may be more suitable for estimating species composition from hyperspectral satellite data during bloom conditions, whereas the blue bands may be more helpful in detection of species under low chlorophyll-a concentrations. In this theoretical approach, variations in reflectance at the same chlorophyll-a concentration can be used to identify phytoplankton species. Another approach to identify phytoplankton species from remotely-sensed hyperspectral reflectance measurements would be to derive the absorption spectra of phytoplankton from the reflectance measurements, and compare these with a standard database of absorption spectra.