Abstract
As an essential parameter for all kinds of aquatic life, light influences life cycles and the behavior of various marine organisms. However, its primary role is that of a driver for photosynthesis and thus primary production, forming the basis of the marine food web. As a simplification when dealing with light, a common measure (e.g. used in biogeochemical models) is the photosynthetically active radiation (PAR), which integrates the spectral distribution of photon flux between 400 – 700 nm into a single value. While passing through the water column, light is attenuated by the water itself and its optically active substances (OAS, e.g. phytoplankton, chromophoric dissolved organic matter (CDOM), and non-algal particles), summarized in the diffuse attenuation coefficient of downwelling radiation (Kd). Existing parameterizations for light attenuation in models often consider only phytoplankton as parameter, which is not sufficient for coastal areas where the contributions of CDOM and suspended mineral particles can be substantial. Furthermore, they mostly ignore the spectral variability of Kd by attenuating PAR with only a single coefficient. For this reason, this study proposes a parameterization of Kd that involves all relevant OAS and that attenuates PAR in three bands (trimodal approach). For this, the hyperspectral underwater light field was examined on three expeditions in different areas of the North Sea and along the British and Irish coasts. The derived Kd spectra were stepwise decomposed in the contributions of the different OAS and used in combination with direct OAS measurements to derive substance specific attenuation coefficients for the three bands. For comparison, also a monomodal and a spectral parameterization were developed. Evaluation showed that the trimodal approach was almost as accurate as the full spectral approach, while requiring only marginally more computational performance as the classical monomodal approach. Being therefore an excellent compromise between these factors, it can act as a valuable, yet computational affordable addition to biogeochemical models in order to improve their performance in coastal waters.
Highlights
Light is a parameter essential for aquatic life
Since for photosynthetic electron transport each photon, when absorbed, is of equal efficiency, the biological relevant light is commonly summarized as photosynthetically active radiation (PAR), which is the integrated number of photons from 400 to 700 nm
The derived spectra of Kd cdom (λ), Kd inorganic suspended particulate matter (iSPM) (λ), and Kd phyt (λ) are subsequently related to the respective optically active substances (OAS) by linear regression, in order to obtain the Kd∗ values required for the model formulation
Summary
Light is a parameter essential for aquatic life. It transfers heat to the upper water column, eventually leading to stratification, by that shaping the abiotic conditions for a considerable period of the year. It has been shown that the use of spectral light data significantly alters the result of ecosystem models (Mobley et al, 2015), there are a couple of reasons for using spectrally integrated, PAR-based approaches This includes on the one hand the sparsity of available spectral data and the greater effort in evaluating a spectral model (Thewes et al, 2020), and the increase in computational effort. In attempting to overcome this issue while still considering computational efficiency, bi- and multimodal parameterizations have been developed (Paulson and Simpson, 1977; Morel, 1988; Zielinski et al, 2002; Dutkiewicz et al, 2015), which have shown to be advantageous In such approaches, the PAR spectrum is divided into two or more spectral bands, either spaced or driven by considerations related to the attenuation properties of the OAS. Strengths and weaknesses of the chosen approach are discussed
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