Abstract
Organic matter (OM) other than living phytoplankton is known to affect fluorometric in situ assessments of chlorophyll in lakes. For this reason, calibrating fluorometric measurements for OM error is important. In this study, chlorophyll (Chl) fluorescence was measured in situ in multiple Finnish lakes using two sondes equipped with Chl fluorometers (ex.470/em.650–700 nm). OM absorbance (A420) was measured from water samples, and one of the two sondes was also equipped with in situ fluorometer for OM (ex.350/em.430 nm). The sonde with Chl and OM fluorometers was also deployed continuously on an automated water quality monitoring station on Lake Konnevesi. For data from multiple lakes, inclusion of water colour estimates into the calibration model improved the predictability of Chl assessments markedly. When OM absorbance or in situ OM fluorescence was used in the calibration model, predictability between the in situ Chl and laboratory Chl a assessments was also enhanced. However, correction was not superior to the one done with the water colour estimate. Our results demonstrated that correction with water colour assessments or in situ measurements of OM fluorescence offers practical means to overcome the variation due to OM when assessing Chl in humic lakes in situ.
Highlights
Phytoplankton biomass is widely used as an indicator of eutrophication in the status assessment of surface waters
Our results demonstrated that correction with water colour assessments or in situ measurements of organic matter (OM) fluorescence offers practical means to overcome the variation due to OM when assessing Chl in humic lakes in situ
Our study demonstrated the use of water colour as an estimate of OM in controlling the OM fluorescence in Chl in situ fluorometer data
Summary
Phytoplankton biomass is widely used as an indicator of eutrophication in the status assessment of surface waters. Chlorophyll a concentration is used as a proxy for phytoplankton biomass, traditionally quantified in laboratory from water samples by ethanol extraction followed by spectrophotometric measurement at wavelengths 665 and 750 nm (Lorenzen, 1967); ISO 10260, 1992). The shortcomings of this protocol are well known: it requires large sample volumes, sample transportation, storage and handling. We use ‘‘Chl a’’ to refer to chlorophyll a concentration measured in laboratory, in contrast to ‘‘Chl’’ that refers to in situ fluorescence of chlorophyll
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