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Calculating Optical Water Quality Targets to Restore and Protect

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Calculating Optical Water Quality Targets to Restore and Protect Estuaries Vol. 24, No. 3, p. 381–397 June 2001 Calculating Optical Water Quality Targets to Restore and Protect Submersed Aquatic Vegetation: Overcoming Problems in Partitioning the Diffuse Attenuation Coefficient for Photosynthetically Active Radiation CHARLES L. GALLEGOS* Smithsonian Environmental Research Center, P. O. Box 28, Edgewater, Maryland 20137 ABSTRACT: Submersed aquatic vegetation (SAV) is an important component of shallow water estuarine systems that has declined drastically in recent decades. SAV has particularly high light requirements, and losses of SAV have, in many cases, been attributed to increased light attenuation in the water column, frequently due to coastal eutrophication. The desire to restore these valuable habitats to their historical levels has created the need for a simple but accurate manage- ment tool for translating light requirements into water quality targets capable of supporting SAV communities. A pro- cedure for calculating water quality targets for concentrations of chlorophyll and total suspended solids (TSS) is derived, based on representing the diffuse attenuation coefficient for photosynthetically active radiation, Kd(PAR), as a linear function of contributions due to water plus colored dissolved organic matter (CDOM), chlorophyll, and TSS. It is assumed that Kd(PAR) conforms to the Lambert-Beer law. Target concentrations are determined as the intersection of a line representing intended reduction of TSS and chlorophyll by management actions, with another line describing the dependence of TSS on chlorophyll at a constant value of Kd(PAR). The validity of applying the Lambert-Beer law to Kd(PAR) in estuarine waters was tested by comparing the performance of a linear model of Kd(PAR) with data simulated using a more realistic model of light attenuation. The linear regression model tended to underestimate Kd(PAR) at high light attenuation, resulting in erroneous predictions of target concentrations at shallow restoration depths. The errors result more from the wide spectral bandwidth of PAR, than from irrecoverable nonlinearities in the diffuse attenuation coefficient per se. In spite of the failure of the Lambert-Beer law applied to Kd(PAR), the variation of TSS with chlo- rophyll at constant Kd(PAR) determined by the more mechanistic attenuation model was, nevertheless, highly linear. Use of the management tool based on intersecting lines is still possible, but coefficients in the line describing the dependence of TSS on chlorophyll at constant Kd(PAR) must be determined empirically by application of an optical model suitably calibrated for the region of interest. An example application of the procedure to data from the Rhode River, Maryland, indicates that approximately 15% reduction in both TSS and chlorophyll concentrations, or 50% reduction in chlorophyll alone, will be needed to restore conditions for growth of SAV to levels that existed in the late 1960s. Introduction in mesohaline and polyhaline estuaries appears to Submersed aquatic vegetation (SAV) is an im- require a long-term average of about 22% of sur- portant component of coastal ecosystems. It is be- face irradiance to survive, while that in oligohaline lieved that nearly 600,000 acres of SAV covered the and tidal freshwater regions appear to require c. bottom of Chesapeake Bay in historic times, but 13% of surface irradiance (Batiuk et al. 1992; Den- catastrophic declines occurred during the 1960s nison et al. 1993; Carter et al. 2000). A wide variety and 1970s (Orth and Moore 1983). Coverage in of methods has been used to estimate the light re- recent years has increased about 70% above the quirement of SAV, but the most reliable estimates lowest areal coverage of 40,000 acres recorded in generally are determined by comparing in situ 1984. Because of the ecological importance of SAV, depth distributions of SAV with the long-term me- the Chesapeake Bay Program has set a goal to re- dian value of diffuse attenuation coefficient, store 114,000 acres of SAV throughout historically Kd(PAR), for downwelling photosynthetically active vegetated areas of the Bay by the year 2005. radiation (PAR, 400 to 700 nm). The principal factor believed to limit the distri- The diffuse attenuation coefficient is the empir- bution of SAV is the availability of light at the plant ical descriptor of the penetration of cosine-weight- surface (Duarte 1991; Dennison et al. 1993). SAV ed downwelling irradiance underwater, i.e., ϭ Ϫ Ed(Z) Ed(0)exp[ Kd(PAR)Z] (1) * Tele: 443/482-2240; fax: 443/482-2380; e-mail: ϭ [email protected]. where Ed(Z) downwelling cosine-weighted irra- ᮊ 2001 Estuarine Research Federation 381 382 C. L. Gallegos ϭ diance (PAR) at depth Z, and Ed(0) downwelling For a diagnostic tool to be useful to managers, it cosine-weighted irradiance just beneath the air-wa- needs to be accessible to non-specialists, able to ter interface. Let Zmax denote the maximum depth batch-process extensive water quality data sets, and of colonization of SAV. The 22% requirement is be usable with very limited optical data. based on the observed tendency for the dimen- The simultaneous need for accuracy and ease of sionless product, ZmaxKd(PAR), to cluster around use presents potentially conflicting requirements. 1.5 (Duarte 1991; Dennison et al. 1993). Of course, Models believed to faithfully represent the pro- other factors influence the maximum depth of col- cesses of radiative transfer underwater are written onization by SAV (Koch 2001), especially light at- for monochromatic light [i.e., a complete spec- tenuation at the leaf surface by epiphytic algae and trum must be simulated to determine Kd(PAR)] attached matrix of organic detritus and inorganic and require specification of a, b, (functions of silts and clays (Twilley et al. 1985; Kemp et al. wavelength), and the scattering phase function 2000). Variability in the optical density of material (Mobley et al. 1993). Although extremely useful attached to the leaf surface may explain some of for research problems, such as generating empiri- the scatter in plots of Zmax against percentage of cal relationships between IOPs and AOPs (Kirk incident light availability (Dennison et al. 1993). 1984), such models are still too complex for use Adequate penetration of light through the water in the water quality management community. At column, quantified by Kd(PAR), is a necessary but the other extreme, some researchers have applied not sufficient condition for colonization by SAV. the Lambert-Beer law to the diffuse attenuation co- In order to take management action to restore efficient by modeling Kd(PAR) as a linear function conditions that permit growth of SAV, it is neces- of water quality concentrations (Lorenzen 1972; sary to know how the value of Kd(PAR) depends Smith 1982; Verduin 1982; Stefan et al. 1983). Lin- on the suspended and dissolved matter in the wa- ear representation of Kd(PAR) may meet the ease- ter. The diffuse attenuation coefficient is referred of-use criteria, but the inherent inaccuracies in do- to as an apparent optical property (AOP), because ing so are as yet unquantified (c.f., Kirk 1994). its value depends not only on the concentrations Gordon (1989) used a Monte Carlo model of an of light attenuating components in the water, but ocean-atmosphere system to examine the severity also on the angular distribution of the underwater of violation of the Lambert-Beer law by narrow light field (Morel and Smith 1982). It is affected waveband spectral irradiance in Case 1 waters, i.e., by factors in addition to water quality, e.g., surface waters in which the optical properties are gov- waves, solar incidence angle, depth, and cloud cov- erned by phytoplankton pigments and their detri- er. Optical properties that depend only on the con- tal degradation products. He demonstrated that tents of the water (i.e., that are independent of the the diffuse attenuation coefficient for narrow-band ambient light field) are referred to as inherent op- irradiance obeyed the Lambert-Beer law to a high tical properties (IOPs, Kirk 1994). degree of accuracy in Case 1 waters, provided the One goal in the field of hydrologic optics has measurements were divided by an easily calculated been to determine the relationship between AOPs distribution coefficient that corrects for the effects and IOPs (Mobley 1994). The IOPs most relevant of variation in sun angle, the relative proportion to the problem of determining water quality con- of diffuse sky irradiance, and sea state. Extension centrations suitable for restoring SAV are the ab- to Case 2 waters (in which the optical properties sorption coefficient, a, and the scattering coeffi- are substantially affected by colored dissolved or- cient, b. The absorption and scattering coefficients ganic matter (CDOM) and non-algal particulate can be expressed as the sum of contributions due matter) did not significantly increase errors as long to different optically active constituents, and the as waters with high concentrations of non-absorb- effect of each component is directly proportional ing particles were avoided (Gordon 1989). To ap- to its concentration: i.e., IOPs are additive and lin- ply the Lambert-Beer law to Kd(PAR) in situations ear and thus obey the Lambert-Beer law. This is useful for setting water quality management goals not true generally of AOPs, such as Kd(PAR) (Kirk requires extension of the law to broadband irra- 1994). Nevertheless, the need exists in the man- diance, and application in waters with high con- agement community for a diagnostic tool able to centrations of terrigenous suspended solids. The accurately and quantitatively attribute light atten- errors incurred by doing so require further ex- uation to the content of the water. Accuracy is amination. needed to assure that the actions taken to improve In this paper I use a series of models of decreas- water quality will result in sufficient reduction in ing complexity and increasing ease of use to ex- light attenuation to promote expansion of SAV, amine the severity of systematic errors incurred by while maintaining cost effectiveness by not dictat- applying the Lambert-Beer law to Kd(PAR), partic- ing larger reductions than are actually necessary.
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