CDOM retrieval using measurements of downwelling irradiance

Abstract

As it can strongly influence the availability of light and thus primary production, coloured dissolved organic matter (CDOM) affects the function of lake ecosystems. Therefore reliable methods are required for the monitoring of CDOM concentration. A new method using downwelling irradiance was tested for applicability in four selected lakes of the Bavarian Osterseen Lake District, which consists of 19 naturally connected freshwater lakes of different trophic level. The method separates between the direct and diffuse part of the incident light in order to handle the strong variability of the underwater light field. It is implemented in the software WASI, which is capable to retrieve water constituents by inverse modeling. During field campaigns downwelling irradiance measurements using RAMSES sensors were made in different depths. Simultaneously, water samples were taken in three depths (0.5 m, 2 m and Secchi disk depth), from which the absorption coefficient of CDOM, a Y , was derived in the range from 190 to 900 nm using photometric absorption measurements. Concentration (defined as a Y at 440 nm) ranged from 0.33 to 1.55 m -1 with a mean of 0.71 m -1 ± 0.04 m -1 , the spectral slope at 440 nm from 0.0120 to 0.0184 nm -1 with a mean of 0.0145 ± 0.0008 nm -1 . These laboratory measurements from water samples were compared to CDOM concentration obtained by inverse modeling of downwelling irradiance measurements using WASI. For sensor depths lower than 1 to 1.5 m large uncertainties were observed. The measurements in 2 m depth and at Secchi disk depth yielded good correlation between water sample and WASI derived data (R 2 = 0.87) with a mean standard deviation of 0.06 m -1 for the determined CDOM concentrations. This new method is an alternative to laboratory analysis of water samples from in situ measurements of CDOM concentration.

Retrieval of Water Constituents from Hyperspectral In-Situ Measurements under Variable Cloud Cover—A Case Study at Lake Stechlin (Germany)

“…This is in accordance with previous studies which showed that, for accurate Chl-a concentration retrieval, a layer of water above the sensor of at least 1 m (depending on overall water constituent concentration ranges) was required [44,51]. a CDOM (440), compared to results from water analytics, showed a deviation of about 57% from PSICAM and 32% from LWCC measurements.…”

“…To the authors' best knowledge, this is the only model that can handle the high variability of E − d (λ, z) measurements induced by wave focusing. Inverse modeling allows the derivation of the sensor depth z, the concentration of phytoplankton and the absorption coefficient of CDOM if f dd and f ds are treated as fit parameters together with z, C Chla and a CDOM (440) [44,51].…”

Water Constituents and Water Depth Retrieval from Sentinel-2A—A First Evaluation in an Oligotrophic Lake

“…For lakes with chl-a and CDOM concentrations similar to Lake Starnberg, this critical depth is in the order of 1.0-1.5 m [71,72]. Following this principle, we estimated the CDOM parameters a CDOM (440) and S from RAMSES E d (z < 0, λ) measurements in water.…”

“…The downwelling irradiance in water, ( < 0, ) , can be used for determining the concentration of phytoplankton [71] and CDOM [72] if sensor depth z is large enough to capture the impact of absorption by water constituents on the spectral signature of ( < 0, ). For lakes with chl-a and CDOM concentrations similar to Lake Starnberg, this critical depth is in the order of 1.0-1.5 m [71,72].…”