Performance of absorption coefficient measurements for the in situ determination of chlorophyll-a and total suspended matter

Abstract

The concentrations of chlorophyll-a ([chl-a]) and total suspended matter ([TSM]) are important parameters in biological oceanography. [Chl-a] is a commonly used proxy for estimating phytoplankton biomass while [TSM] also includes detrital material and mineral particles and thus influences light attenuation and photosynthetic activity in the water column. For characterizing the distribution (patchiness) of both parameters adequately over a longer time period, fast and effective measurement methods are required that can also be applied in situ or continuously. Thus, alternatively to direct determination of [chl-a] and [TSM], optical proxy values are often measured. The PSICAM is an integrating cavity approach for measuring absorption coefficients of water constituents with high precision which can be used also continuously (flow-through-PSICAM). In this study, the performance of these absorption measurements for [chl-a] and [TSM] determination was evaluated and compared with the performance of traditional approaches using chl-a fluorescence and turbidity measurements. Data were collected in the German Bight (North Sea) in 2010 and 2011. For [chl-a], fluorescence measurements are compared with pigment absorption coefficient values at a wavelength of 676 nm (aΦ 676 nm), while the [TSM]-proxies were turbidity and particle absorption at 700 nm (ap 700 nm). As reference data, HPLC-determined [chl-a] and gravimetrically determined [TSM] were used. Our results showed linear relationships between [chl-a] and fluorescence or aΦ 676 nm, respectively. Coefficients of determination (R2) were in a range of 0.71 to 0.88, with the higher values related to the absorption measurements. Furthermore, it was demonstrated that fluorescence underestimates [chl-a] depending on ambient photosynthetically active radiation (PAR). Linear relationships were also observed between [TSM] and its optical proxies with R2 values between 0.93 and 0.98. Turbidity measurements appeared to be influenced to a certain extent by the physical properties of the suspended material, resulting in a slightly higher variability than the ap 700 nm measurements. Absorption measurements turned out to be promising optical proxies for determining [TSM] and [chl-a] due to their lower variability compared with the other proxies. This improved accuracy could be already partially achieved also for continuous measurements. Moreover, a combination of the different optical methods has the potential to provide additional information besides concentration, such as the source of TSM in the water or physiological condition of the phytoplankton.