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Abstract
Light availability in lakes or reservoirs is affected by optically active components (OACs) in the water. Light plays a key role in the distribution of phytoplankton and hydrophytes, thus, is a good indicator of the trophic state of an aquatic system. Diffuse attenuation of photosynthetic active radiation (PAR) (Kd(PAR)) is commonly used to quantitatively assess the light availability. The PAR and the concentration of OACs were measured at 206 sites, which covered 26 lakes and reservoirs in Northeast China. The spatial distribution of Kd(PAR) was depicted and its association with the OACs was assessed by grey incidences(GIs) and linear regression analysis. Kd(PAR) varied from 0.45 to 15.04 m−1. This investigation revealed that reservoirs in the east part of Northeast China were clear with small Kd(PAR) values, while lakes located in plain areas, where the source of total suspended matter (TSM) varied, displayed high Kd(PAR) values. The GIs and linear regression analysis indicated that the TSM was the dominant factor in determining Kd(PAR) values and best correlated with Kd(PAR) (R2 = 0.906, RMSE = 0.709). Most importantly, we have demonstrated that the TSM concentration is a reliable measurement for the estimation of the Kd(PAR) as 74% of the data produced a relative error (RE) of less than 0.4 in a leave-one-out cross validation (LOO-CV) analysis. Spatial transferability assessment of the model also revealed that TSM performed well as a determining factor of the Kd(PAR) for the majority of the lakes. However, a few exceptions were identified where the optically regulating dominant factors were chlorophyll-a (Chl-a) and/or the chromophroic dissolved organic matter (CDOM). These extreme cases represent lakes with exceptionally clear waters.
Highlights
Diffuse attenuation of photosynthetic active radiation (PAR), expressed as Kd (PAR), indicates the ability of solar radiation to penetrate a water column
The results indicated that Kd (PAR) was strongly correlated to total suspended matter (TSM), while there was no obvious relationship between Kd (PAR) and Chl-a or chromophroic dissolved organic matter (CDOM) (Figure 2)
Based on the data collected in Northeast China, the relationships between Kd (PAR) and optically active components (OACs) were studied
Summary
Diffuse attenuation of photosynthetic active radiation (PAR), expressed as Kd (PAR), indicates the ability of solar radiation to penetrate a water column. Optical remote sensing imagery is a cost-efficient method to obtain Kd (PAR) values at large regional scale due to the correlation between Kd (PAR) and water leaving radiance, and due to its spatial and temporal resolution. Though the applications may be confined to specified regions or seasons, empirical models have been widely used to derive Kd (PAR) from remote sensing data for both case-I and case-II waters [12,23,24,25]. The Kd (PAR) can be expressed as a function of the dominant one or some optically active components (OACs) whose concentrations can be estimated from remote sensing data empirically or semi-analytically. OACs with in situ data was necessary before indirectly deriving Kd (PAR) from remote sensing data for inland waters.
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