Clarifying water clarity: A call to use metrics best suited to corresponding research and management goals in aquatic ecosystems

Abstract

Water clarity is a subjective term and can be measured multiple ways. Different metrics such as light attenuation and Secchi depth vary in effectiveness depending on the research or management application. In this essay, we argue that different questions merit different water clarity metrics. In coastal and inland waters, empirical relationships to estimate light attenuation can yield clarity estimates that either under-or overestimate the underwater light climate for restoration goals, such as potential habitat available for submerged aquatic vegetation. Best practices in reporting water clarity measurements include regionally specific, temporally representative calibrations and communicating the metric that was actually measured. An intentional choice of the water clarity metric best suited to the research or management question yields the most useful results.The term "water clarity" is inherently ambiguous. Water clarity generally refers to the distance that light penetrates through water, as well as the visibility of objects through water. In lakes, rivers, estuaries, coastal zones, and the open ocean, water clarity is an essential measurement for monitoring programs and a wide variety of research applications. For example, water clarity is used to assess habitat quality for submerged aquatic vegetation (SAV), to study visual predation, and to model primary production. Water clarity is measured using multiple methods, some focused on the depth of light penetration, some based on the depth of object visibility, and others based on the amounts of components present (Fig. 1).General metrics presented here include: Secchi disk depth (Z SD ) (Secchi and Cialdi 1866; Tyler 1968; Preisendorfer 1986), the downwelling and scalar light attenuation coefficients of photosynthetically active radiation (K d (PAR) and K o (PAR)) (Kirk 1994), turbidity (Zaneveld et al. 1980Davies-Colley and Smith 2001;Sampedro and Salgueiro 2015;Eidam et al. 2022), and beam attenuation (Bishop 1999). In this paper, the terms K d and K o will refer to K d (PAR) and K o (PAR). Z SD , K o , and K d are apparent optical properties, descriptors of water bodies that depend on both the substances present and the light field (Mobley 2022). Component-based metrics include colored dissolved organic matter (CDOM) commonly measured by its light absorption (aCDOM; m À1 ) (Green and Blough 1994) or

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“…Both approaches rely on the mechanistically derived relationship between Secchi disk depth (S) and the attenuation coefficient of downwelling irradiance, K S = 1.48/S (Lee et al 2018). For a general discussion, see Turner et al (2022).…”

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