The vertical attenuation of irradiance as a function of turbidity: a case of the Huanghai (Yellow) Sea in spring

The planar photosynthetically available radiation （PAR）, turbidity and concentration of chlorophyll a （chl a）, were measured at 26 stations in the Huanghai （Yellow） Sea during a cruise of China SOLAS from 19 to 27 March 2005. Due to low chl a （〈0.35 mg · m^-3 ） in upper layers （above 5 m）, suspended particulate matter became the major factor that influenced the turbidity in early spring. The calculated vertical diffuse attenuation coefficient of PAR, K PAR , varied with water depths with a maximum value in the upper 5 m layer. The mean K PAR in survey area was 0.277 ± 0.07 m^-1 that is considerably higher than most of the other case 2 waters. Within the survey area, K PAR also showed distinct regional characteristics, corresponding to the distribution of turbidity. Based on measurements, the relationship between K PAR and turbidity as well as chl a was established. It was suggested that suspended particulate matter plays an important role in light attenuation in the Huanghai （Yellow） Sea in spring.

Variation of diffuse attenuation coefficient of downwelling irradiance in the Arctic Ocean

The diffuse attenuation coefficient （Kd） for downwelling irradiance is calculated from solar irradiance data measured in the Arctic Ocean during 3rd and 4th Chinese National Arctic Research Expedition （CHINARE）, including 18 stations and nine stations selected for irradiance profiles in seawater respectively. In this study, the variation of attenuation coefficient in the Arctic Ocean was studied, and the following results were obtained. First, the relationship between attenuation coefficient and chlorophyll concentration in the Arctic Ocean has the form of a power function. The best fit is at 443 nm, and its determination coefficient is more than 0.7. With increasing wavelength, the determination coefficient decreases abruptly. At 550 nm, it even reaches a value lower than 0.2. However, the exponent fitted is only half of that adapted in low-latitude ocean because of the lower chlorophyll-specific absorption in the Arctic Ocean. The upshot was that, in the case of the same chlorophyll concentration, the attenuation caused by phytoplankton chlorophyll in the Arctic Ocean is lower than in low-latitude ocean. Second, the spectral model, which exhibits the relationship of attenuation coefficients between 490 nm and other wavelength, was built and provided a new method to estimate the attenuation coefficient at other wavelength, if the attenuation coefficient at 490 nm was known. Third, the impact factors on attenuation coefficient, including sea ice and sea water mass, were discussed. The influence of sea ice on attenuation coefficient is indirect and is determined through the control of enter- ing solar radiation. The linear relationship between averaging sea ice concentration （ASIC, from 158 Julian day to observation day） and the depth of maximum chlorophyll is fitted by a simple linear equation. In addition, the sea water mass, such as the ACW （Alaskan Coastal Water）, directly affects the amount of chlo- rophyll through taking more nutrient, and results in the higher attenuation coefficient in the layer of 30-60 m. Consequently, the spectral model of diffuse attenuation coefficient, the relationship between attenuation coefficient and chlorophyll and the linear relationship between the ASIC and the depth of maximum chlorophyll, together provide probability for simulating the process of diffuse attenuation coefficient during summer in the Arctic Ocean.

Characteristics of water masses and bio-optical properties of the Bering Sea shelf during 2007–2009

The hydrographic and bio-optical properties of the Bering Sea shelf were analyzed based on in-situ measurements obtained during four cruises from 2007 to 2009.According to the temperature and salinity of the seawater,the spring water masses on the Bering Sea shelf were classified as the Alaskan Coast Water,Bering Sea Shelf Water,Anadyr Water,Spring Mixed Layer Water,Remnant Winter Water,and Winter Water,each of which had varying chlorophyll a concentrations.Among them,the highest chlorophyll a concentration occurred in the nutrient-rich Anadyr Water((7.57±6.16)mg/m^(3) in spring).The spectrum-dependent diffuse attenuation coefficient(Kd(λ))of the water column for downwelling irradiance was also calculated,exhibiting a decrease at 412-555 nm and then an increase within the range of 0.17-0.48 m-1in spring.Furthermore,a strong correlation between the chlorophyll a concentration and the attenuation coefficient was found at visible wavelengths on the Bering Sea shelf.Spatially,the chlorophyll a concentration was higher on the northern shelf((5.18±3.78)mg/m^(3))than on the southern shelf((3.64±2.51)mg/m^(3)),which was consistent with the distribution of the attenuation coefficient.Seasonally,the consumption of nutrients by blooms resulted in minimum chlorophyll a concentration((0.78±0.51)mg/m^(3))and attenuation coefficient values in summer.In terms of the vertical structure,both the attenuation coefficient and the chlorophyll a concentration tended to reach maximum values at the same depth,and the depth of the maximum values increased as the surface temperature increased in summer.Moreover,an empirical model was fitted with a power function based on the correlation between the chlorophyll a concentration and the attenuation coefficient at 412-555 nm.In addition,a spectral model was constructed according to the relationship between the attenuation coefficients at 490 nm and at other wavelengths,which provides a method for estimating the bio-optical properties of the Bering Sea shelf.

Observation and simulation of the diffuse attenuation coefficient of downwelling irradiance in the Polar Ocean

The three-stream radiation transfer model is used to investigate the fluctuation in the underwater diffuse attenuation coefficient of downwelling irradiance in the polar ocean with a high solar zenith angle and different direct radiation proportions.First,the applicability of the three-stream radiation model in the polar region is validated by using 18 in situ observation data from September to October 2009 in the Beaufort Sea.Statistics show that in the absence of sea ice,the average relative errors between the simulation and observation values for 490 nm downwelling irradiance (E_(d)(490)) and its diffuse attenuation coefficient (K_(d)(490)) are 7.04%and 9.88%,respectively.At the stations surrounded by sea ice,the radiation is relatively small due to ice blocking,and the average relative errors simulated by the model reach 15.89%and 15.55%,respectively.Second,simulations with different chlorophyll concentrations and different proportions of direct radiation reveal that a high solar zenith angle has a greater impact on K_(d)(490) in the surface water.K_(d)(490) is less affected by the light field (affected by the solar zenith angle and the proportion of direct radiation) at depths greater than 30 m,and meets the linear relationship with the inherent optical parameters(the sum of the absorption coefficient and backscattering coefficient).The surface K_(d)(490) is still consistent with that at a depth of more than 50 meters under a high solar zenith angle,implying that the surface K_(d)(490) can also be considered as an inherent optical parameter at a high solar zenith angle (greater than 60 degrees).The relative error of obtaining surface K_(d)(490) by using the linear relationship at the 50 m layer is found to be less than 8%in the seawater with chlorophyll concentration greater than0.05 mg m^(-3).The effect of the solar zenith angle and proportion of direct radiation can be ignored when measuring the diffuse attenuation coefficient in the polar region.Finally,the model can correct the ice-induced fluctuation in downward irradiance,allowing for optical research of seawater beneath the ice in the polar ocean.

Seasonal and spatial distributions of euphotic zone and longterm variations in water transparency in a clear oligotrophic Lake Fuxian, China

To assess the seasonal and spatial variations and long-term trends in water optical properties in Lake Fuxian, investigations based on field work in four seasons and a long-term analysis of data from 1980 to 2014 were conducted. The results show that there was no significant variation in the euphotic depth（Z（eu）） across the four seasons, and no significant correlations between Z（eu） and potential influencing factors in seasons other than summer, suggesting that the water itself may be a major factor regulating the Z（eu）in general. Nevertheless, significant differences in Z（eu） between the north region（NR） and the south region（SR） were observed in all seasonal tests except spring. This finding relates to a higher abundance of chromophoric dissolved organic matter（CDOM） in the NR due to runoff, especially in the rainy seasons（summer and autumn）.CDOM and its terrigenous component had an important impact on Z（eu）in summer, with the highest precipitation, and impacts from suspended solids and non-algal particles were also found in the NR in summer. The Secchi disk depth in the lake decreased clearly over the years,with significantly negative correlations with the increasing permanganate index and air temperature, implying that organic contaminants（CDOM and/or phytoplankton） are important regulators of water transparency. We estimate that the combined effects of climate warming and changes in land use and land cover are also indirect regulating factors. These findings should be considered in the protection of Lake Fuxian, owing to the importance of light penetration in aquatic ecosystems.