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An Optical Water Quality Model for Conservation and Restoration of Seagrasses
ISSUE: Land and resource use is impacting water quality in estuaries by increasing nutrient flux, enhancing suspended sediment loads and modifying freshwater inputs.
All of these factors have an effect on optical water quality and primary production. We are developing an optical modeling tool which will enable resource mangers to identify the specific factors causing the degradation of water quality and the decline of seagrasses.
The modeling approach focuses on decomposing the components of light attenuation through the water column into its regularly measure constituents, namely total suspended solids (TSS), phytoplankton chlorophyll, and colored dissolved organic matter (CDOM). By determining the relative contributions of these constituents to light attenuation, the importance of each constituent can be evaluated. Using this model in a management-oriented approach allows a direct link to be made between loading and attenuation, enabling managers to implement plans and regulations to conserve and restore seagrasses.
APPROACH: The first objective in bio-optical modeling is to determine the contribution of different substances in the water to the spectral absorption and scattering coefficients by field sampling. Absorption spectra by different components exhibit characteristic shapes, which are determined by measuring the absorption by different components in isolation in the laboratory The absorption by the different components is normalized by its relevant water quality measure, to determine the specific-absorption spectrum of each component. Specific-absorption spectra are a measure of the incremental effect of a unit change in concentration of a parameter on the total absorption spectrum. Absorption by phytoplankton is normalized to Chlorophyll a (Chl a), absorption by non-algal particulates is normalized to the concentration of suspended particulate matter (SPM), and absorption by CDOM is normalized to its value at 440 nm. This decomposition allows us to express the total absorption spectrum as a sum of the 4 components. This absorption spectra been calibrated in the Chesapeake Bay, the Indian River Lagoon in Florida and in Back Sound North Carolina. We are now beginning calibration in the coastal embayments of Massachusetts.
OUTCOME: Once the optical model is calibrated samples are collected at water quality monitoring stations and the results are plotted graphically where two components of light attenuation (Chl a, TSS) are presented on the axes. The third constituent, CDOM, can be easily plotted on the Z-axis on a 3D plot to show all three components of attenuation. Median concentrations for one water-quality sample are plotted on this graph and compared to a minimum-light water quality requirement for a given depth (line of constant attenuation), which is calculated using a radiative-transfer model and knowledge of seagrass species light requirements. Target minimum water-clarity requirements for seagrass survival are found at the intersection of vectors perpendicular to the axes or the origin from the median sample concentration. The target concentrations in this figure suggest that both TSS and Chl a need to be reduced to meet the minimum light requirements of this seagrass species. Managers can use this graph to identify which management scenario will have the most effect on protecting seagrasses. Read More from the NCCOS Projects Explorer ›