Abstract
We studied the effects of particulate and dissolved optically active components on the attenuation of photosynthetic active radiation (PAR) in a tropical lake. The temporal and spatial distribution of tripton, Chl-a and aCDOM(440) and their relative contribution to the diffuse PAR attenuation coefficient (Kd) was investigated at 21 sites (dry and wet seasons and two intermediate periods) and at monthly interval at 1 pelagic site. Higher values of Kd were observed during the mixing period, characterized by a higher concentration of tripton and Chl-a compared to the stratified rainy season. In the spatial sampling PAR attenuation was dominated by tripton absorption/scattering (average relative contribution of 79%), followed by Chl-a (average 11.6%). In the monthly sampling tripton and Chl-a accounted for most of the Kd with relative contributions of 47.8% and 35.6%, respectively. Multiple linear regression analysis showed that Chl-a and tripton in combination explained 97% of the monthly variation in Kd (p<0.001), but Chl-a had more influence (higher regression coefficient). Thus, although most of light attenuation was due to tripton, seasonal variations in phytoplankton abundance were responsible for most of the temporal fluctuations in Kd.
Highlights
Availability of photosynthetically active radiation (PAR; 400 to 700 nm) in the water column provides the energy for primary producers and is a key determinant of the overall productivity of aquatic ecosystems (Parsons et al 1984, Kirk 1994).While it is well known that PAR is attenuated exponentially with depth, little is known of the causes for temporal and spatial variability in the vertical diffuse attenuation coefficient for downwelling PAR (Kd; m-1) in tropical lakes.Light attenuation in aquatic ecosystems is basically the sum of light absorbing and scattering optically active components present in water
Studies aiming to determine the relative contribution of optically active components to Kd have previously concentrated on shallow lakes (Somlyody and Koncsos 1991, Blom et al 1994, James et al 1997, Van Duin et al 2001, Pierson et al.2003, Zhang et al 2007a, b, Balogh et al 2009), estuaries and coastal lagoons (Phlips et al 1995, Christian and Sheng 2003, Lund-Hansen 2004, Kelble et al 2005, Yamaguchi et al 2013), while little information about partition of Kd is available on deep (Effler et al 2010) and tropical lakes (Loiselle et al 2008)
It has previously been demonstrated that mixing of the water column causes resuspension of particulate matter increasing turbidity and tripton concentrations (Blom et al 1994, James et al 1997, 2004, Phlips et al 1995, Zhang et al 2007b), which would explain the higher values of Kd and the highest average contribution of tripton (83.9%) during the mixed dry season
Summary
Availability of photosynthetically active radiation (PAR; 400 to 700 nm) in the water column provides the energy for primary producers and is a key determinant of the overall productivity of aquatic ecosystems (Parsons et al 1984, Kirk 1994).While it is well known that PAR is attenuated exponentially with depth, little is known of the causes for temporal and spatial variability in the vertical diffuse attenuation coefficient for downwelling PAR (Kd; m-1) in tropical lakes.Light attenuation in aquatic ecosystems is basically the sum of light absorbing and scattering optically active components present in water. Studies aiming to determine the relative contribution of optically active components to Kd have previously concentrated on shallow lakes (Somlyody and Koncsos 1991, Blom et al 1994, James et al 1997, Van Duin et al 2001, Pierson et al.2003, Zhang et al 2007a, b, Balogh et al 2009), estuaries and coastal lagoons (Phlips et al 1995, Christian and Sheng 2003, Lund-Hansen 2004, Kelble et al 2005, Yamaguchi et al 2013), while little information about partition of Kd is available on deep (Effler et al 2010) and tropical lakes (Loiselle et al 2008).
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