Abiotic control of underwater light in a drinking water reservoir: Photon budget analysis and implications for water quality monitoring

Abstract

In optically complex inland waters, the underwater attenuation of photosynthetically active radiation (PAR) is controlled by a variable combination of absorption and scattering components of the lake or river water. Here we applied a photon budget approach to identify the main optical components affecting PAR attenuation in Lake St. Charles, a drinking water reservoir for Quebec City, Canada. This analysis showed the dominant role of colored dissolved organic matter (CDOM) absorption (average of 44% of total absorption during the sampling period), but with large changes over depth in the absolute and relative contribution of the individual absorption components (water, nonalgal particulates, phytoplankton and CDOM) to PAR attenuation. This pronounced vertical variation occurred because of the large spectral changes in the light field with depth, and it strongly affected the average in situ diffuse absorption coefficients in the water column. For example, the diffuse absorption coefficient for pure-water in the ambient light field was 10-fold higher than the value previously measured in the blue open ocean and erroneously applied to lakes and coastal waters. Photon absorption budget calculations for a range of limnological conditions confirmed that phytoplankton had little direct influence on underwater light, even at chlorophyll a values above those observed during harmful algal blooms in the lake. These results imply that traditional measures of water quality such as Secchi depth and radiometric transparency do not provide a meaningful estimate of the biological state of the water column in CDOM-colored lakes and reservoirs.

The Effect of Optical Properties on Secchi Depth and Implications for Eutrophication Management

“…There have been several local or regional investigations on how much the different optical components contribute to K d in coastal waters (Lund-Hansen, 2004;Kratzer and Tett, 2009;Aas et al, 2014;Murray et al, 2015) or lakes (Thrane et al, 2014;Watanabe et al, 2015). In Himmerfjärden bay in the Baltic Sea, included in this study, inorganic SPM had the strongest effect on the coastal spatial gradient in K d (Kratzer and Tett, 2009) although the K d (and thus also the Z SD ) in the Baltic Sea is generally governed by CDOM absorption due to its dark, humicrich waters (Kowalczuk et al, 2006;Skoog et al, 2011;Gustafsson et al, 2014;Harvey et al, 2015a).…”

The influence of glacial melt water on bio-optical properties in two contrasting Greenlandic fjords

“…The photon budget summarizes the fate of photons removed from the water column (Markager et al, 2004;Watanabe et al, submitted for publication) by absorption. Converting radiometric units (W m À2 ) to the more ecologically relevant photon flux density (mmol m À2 s À1 ), we calculate over discrete vertical intervals of the water column the number of photons absorbed by each of the four absorbing components: CDOM, phytoplankton pigment, non-algal particulate matter (NAP) and the water itself.…”

A quantitative blueness index for oligotrophic waters: Application to Lake Tahoe, California–Nevada

“…The application of in situ instrumentation for continuous radiometric measurements enables the researcher to observe changes in water color at various time scales, from short term responses to a transient change in limnological conditions (e.g., an algal bloom caused by upwelling events, Schladow et al ) to long‐term changes associated with eutrophication. However, the B w index would not be an appropriate index of water quality changes in lakes and reservoirs that are strongly influenced by CDOM, for example, in boreal forest regions where even large changes in phytoplankton biomass are masked by high concentrations of light‐absorbing humic materials (Watanabe et al ).…”