Light Absorption Properties Of Phytoplankton Blooms In The Adjacent Waters Of The Changjiang Estuary

This study was conducted within the range of 122.2°-124.1°E and 28.9°-32.1°N. The environmental factors from the field data were analyzed in March and August 2013 as the representives of winter and summer. Phytoplankton biomass and size structure as well as their responses to environmental factors were explored. Afterwards, the spatial and temporal variations of phytoplankton absorption properties were analyzed with the absorption parameterization model developed. This study also compared the absorption properties of phytoplankton bloom waters and non-bloom waters in summer.5 species of diatoms-Skeletonema costatum, Thalassiosira weissflogii, Chaetoceros debilis, Chaetoceros curvisetus, Phaeodactylum tricornutum- and 2 species of dinoflagellates - Karenia mikimotoi, Prorocentrum donghaiense -were cultivated separately in laboratory, with absorption properties of the three water color components explored and paramterized.The spatial distribution of phytoplankton biomass was seasonally varied. Cholorophyll-a concentration (Chla 浓度) gradually decreased from the west to the east in March, and decrease after an increase from the north to the south, with a range of 0.46～2.07 mg m-3. In August, Chla 浓度 generally decreased from the.northwest to the southeast, spanning a range of 0.24～37.73 mg m-3 with higher values distribution in consistency with the extending directions of the Changjiang Diluted Waters. When transitioned from the coastal and shallow waters of the northwest to offshore waters, the proportion of phytoplankton with larger cell size lowered while that of smaller phytoplankton elevated. Variations of Chla 浓度 directly affected the values of phytoplankton absorption coefficient (αPh(γ)). αph(440) spanned a respective range of 0.008～0.073m-1 and 0.012-0.822 m-1 in March and August. The specific absorption coefficient of phytoplankton (a*ph(γ)) spacially and temporally varied in March and August, with a range of 0.016～0.123 m2 mg-1 and 0.012～0.064 m2 mg-1, respectively. In March, phytoplankton cell size was samller and Chla 浓度 lower while the ratio of non-potosynthetic caronoids higher than those of August, resulting in higer α ph(γ) than August. α Ph(γ) can be well fitted with the model built by Lee (1995) when the reference wavelength-440 nm and 675 nm were chosen. Results showed that the accuracy of the parameteriztion model with the reference wavelength being 440 nm was higher than that with the reference wavelength being 675 nm, and the model based on Case A waters showed better accuracy in retrieving aph(X) from Case A waters while model based on Case B waters showed better accuracy in retrieving αPh(γ) from Case B waters. aph(440)(αPh(675)) and Chla 浓度 were in power law function. When took the data from August alone or from both March and August into consideration, αph(440)(aph(675)) fitted better with Chla 浓度 , and the αph(γ) retrieval model had better accuracy.During August, phytoplankton blooms broke out in 10 out of 52 investigation stations, among which diatom blooms occurred in 6 stations while 3 stations were predominated by dinoflagellate. Phytoplankton absorption coefficients of both bloom and non-bloom waters exhibited large variations, while phytoplankton specific absorption coefficients spanned much narrower range, with the average α*ph(440) of bloom and non-bloom waters being 0.023 and 0.035 m2 mg-1, respectively. When transitioned from bloom to non-bloom waters, the proportion of phytoplankton with larger cell size lowered while that of smaller phytoplankton elevated, causing a less extent of package effect and thus higher specific absorption coefficients. Distinctive absorption spectra were observed between different types of bloom (such as diatom and dinoflagellate blooms) with similar phytoplankton cell size, mostly attributed to distinctive accessory pigment composition. The ratios of diadinoxanthin and chlorophyll-c2 concentration to chlorophyll-a concentration in dinoflagellate blooms were higher than those in diatom blooms, which mostly contributed to the shoulder peaks at 465 nm in dinoflagellate blooms. Colored dissolved organic matter (CDOM) is an important component that influences ocean color in the adjacent waters of the Changjiang estuary. The absorption coefficient of CDOM, αg(γ) at 440 nm sharply increased during phytoplankton blooms. A weak positive relationship was found between αg(440) and Chla 浓度, and the spectral slope of CDOM, Sg, tend to increase under bloom conditions, indicating that some of the colored dissolved organic matter (CDOM) from bloom waters may come from phytoplankton degradation. Compared with non-bloom conditons, the absortion coefficient of non-algal particles (NAP), αd(γ) also exhibited distict properties under bloom conditions. αd(440) showed good relationship with Chla 浓度 when phytoplankton blooms broke up, suggesting that organic detritus are main content of NAP.During the uni-algae culture experiements, absorption properties of 7 algae species, the detritus and CDOM from culture solution were explored. Results showed that all 7 species exhibited similar absorption spectra shape. αPh(γ) of all 7 species can be well fitted with the model built by Lee (1995) when the reference wavelength -440 nm and 675 nm were chosen and the parameteriztion model with the reference wavelength being 440 nm and 675 nm showed similar accuracy for the diatoms and Karenia mikimotoi while the model for Prorocentrum donghaiense was better with the reference wavelength being 675 nm. Prorocentrum donghaiense and Karenia mikimotoi had evident absorption peak while 5 diatom spcecies not. The absorption shoulder near 495 nm by diatoms and dinoflagellates resulted from the existence of fucoxanthin and peridinin, respectively. The diatoms had a distinct trough near 643 nm as the absorption of Chla has a depression near 650 nm. However, in Prorocentrum donghaiense, the absorption due to much higher content of accessory pigments cholorophyll c filled this trough and made the absorption curve almost flat between 630 and 650 nm, since the in-vivo absorption of cholorophyll c has a strong peak near 643 nm. The magnitude of αd(γ) in the visible range were far lower than their corresponding αph(γ), with the ratio of αd(440) to the absorption coefficients of the total particles at 440 nm no larger than 13% among diatoms and 27% among dinoflagellates. αd(γ) had residual absorption peak at 412 nm attributed to phaeopigments. The average spectral slope of NAP, Sd, among 5 diatom species and 2 dianoflagellate species were 0.014 nm-1 and 0.012 nm-1, respectively, while the averaged Sg were in the range of 0.013-0.019 nm-1 among diatoms and the exact value of 0.016 nm-1 among dianoflagellates.This study analyzed the differences between bloom and non-bloom waters and amony different algaes from the perspective of light absorption properties, which plays an important role in understanding the light absorption properties of the adjacent waters of the Changjiang estuary and building the phytoplankton bloom retrieval algorithms based on the Inherent Optical Properties.