Refining Habitat Requirements of Submersed Aquatic Vegetation: Role of Optical Models

Abstract

A model of the spectral diffuse attenuation coefficient of downwelling irradiance was constructed for Chincoteague Bay, Maryland, and the Rhode River, Maryland. The model is written in terms of absorption spectra of dissolved yellow substance, the chlorophyll-specific absorption of phytoplankton, and absorption and scattering by particulate matter (expressed as turbidity). Based on published light requirements for submersed aquatic vegetation (SAV) in Chesapeake Bay, the model is used to calculate the range of water-quality conditions that permit survival of SAV at various depths. Because the model is spectrally based, it can be used to calculate the attenuation of either photosynthetically active radiation (PAR, equally weighted quanta from 400 nm to 700 nm) or photosynthetically a.mble radiation (PUR, the integral of the quantum spectrum weighted by the pigment absorption spectrum of SAV). PUR is a more accurate measurement of light that can be absorbed by SAV and it is more strongly affected by phytoplankton chlorophyll in the water column than is PAR. For estuaries in which light attenuation is dominated by turbidity and chlorophyll, the model delimits regions in which turbidity alone (chlorophyll < 10 pg I-'), chlorophyll alone (turbidity < 1 NTU) or both factors (chlorophyll > 10 ug ll', turbidity > 1 NTU) must be reduced to improve survival depths for SAV.

Habitat requirements for submerged aquatic vegetation in Chesapeake Bay: Water quality, light regime, and physical-chemical factors

“…As a first approximation, the diffuse attenuation of light measured by K d can be partitioned into contributions from water and dissolved organic matter (K (WϩDOC) ), from phytoplankton chl a (K chl ), and from total suspended solids (K TSS ). Although wavelength-specificity of absorption by any substance and interactions with absorption by other substances would theoretically preclude linear partitioning of diffuse light attenuation (Gallegos 1994;Kirk 1994), we have assumed these non-linearities to be unimportant for most management applications (Gallegos 2001). The basic relationships can be described by the following simple equations: …”

Impact of the Spring 2000 phytoplankton bloom in Chesapeake Bay on optical properties and light penetration in the Rhode River, Maryland

“…This enabled us to calculate K d ( ) even during nighttime hours when K d technically is undefined, and thereby obtain a more complete record of bloom development than had we considered daylight hours only. We then calculated the diffuse attenuation coefficient for PAR, K d (PAR), from the calculated K d ( ) by interpolation and integration of the diffuse attenuation spectrum (see Gallegos 1994).…”

Proceedings of the Smithsonian Marine Science Symposium

“…Using a bio-optical model of light penetration in the mesohaline Chesapeake Bay, Gallegos (1994) determined that the 22% surface PAR requirement for seagrasses occurred at the same depth as the penetration of 16% of surface PUR. The distinction is potentially important because the penetration of PUR is more sensitive to the concentration of phytoplankton chlorophyll (i.e., eutrophication) than is the penetration of PAR, for the reason that phytoplankton chlorophyll absorption selectively removes those same wavelengths most effi ciently used in photosynthesis by seagrass.…”

“…By contrast, measurements of photosynthetically usable radiation (i.e., PUR; see Morel, 1978) weight quanta in proportion to the effi ciency with which they are absorbed. There are no sensors for direct measurement of PUR; it must be calculated from the underwater spectrum (measured or modeled) weighted by the relative absorption spectrum of the plant of interest.Using a bio-optical model of light penetration in the mesohaline Chesapeake Bay, Gallegos (1994) determined that the 22% surface PAR requirement for seagrasses occurred at the same depth as the penetration of 16% of surface PUR. The distinction is potentially important because the penetration of PUR is more sensitive to the concentration of phytoplankton chlorophyll (i.e., eutrophication) than is the penetration of PAR, for the reason that phytoplankton chlorophyll absorption selectively removes those same wavelengths most effi ciently used in photosynthesis by seagrass.…”

“…The distinction is potentially important because the penetration of PUR is more sensitive to the concentration of phytoplankton chlorophyll (i.e., eutrophication) than is the penetration of PAR, for the reason that phytoplankton chlorophyll absorption selectively removes those same wavelengths most effi ciently used in photosynthesis by seagrass. Thus, by basing light requirements on PUR rather than on PAR, we would predict greater restoration benefi t from chlorophyll reduction, and greater seagrass losses from chlorophyll increases, than by light requirements based on PAR (Gallegos, 1994).The objective of this work was to determine whether the distinction between PAR and PUR requirements could be determined from in situ depth distributions of seagrass communities. The distinction cannot be drawn from depth distributions at a single site such as Chesapeake Bay or the IRL, because within these systems the underwater spectrum is peaked in the green, and thus there is insuffi cient spectral variability in available light to differentiate between depth limits based on PAR compared with PUR.…”