Conceptual Approach to Light Limitation

It has been shown that the light environment during the growing season is the most important period determining survival of seagrasses (Moore et al. 1997; Dixon 2000, Batiuk et al. 2000). The amount of light required for growth and reproduction (flowering) is the cumulative light received during the growing period of the plant's life history, typically spring and summer months for temperate seagrasses. Light attenuation by the water column is the major variable related to seagrass decline. Low light levels, below some minimum physiological requirement (typically 15-25% of incident surface light = Io), usually results in a loss of seagrasses. Light is attenuated down the water column resulting in less light available at the bottom (Iz) than at the surface (Io). Factors that contribute to light attenuation can include (Fig. 1):

1. turbidity, expressed as total suspended particulate matter (SPM),

2. phytoplankton, which both absorb and scatter light, expressed in chlorophyll concentration (Chl a),

3. colored dissolved organic matter (CDOM) leaching from decaying vegetation and peat deposits,

4. macroalgae and epiphytic microalgae that grow on the seagrass. These are usually most problematic when eutrophication is taking place.

One of the goals of our preliminary research has been to determine the importance of each of these factors on light attenuation to seagrasses in North Carolina during different seasons. We have established a collaborative effort with Charles Gallegos (Smithsonian Environ. Research Ctr) to refine a bio-optical water quality model he has developed for North Carolina conditions.

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. Light absorption in water is the sum of contributions due to water itself, colored dissolved organic matter (CDOM), phytoplankton pigments, and non-algal particulate matter which consists of mineral and detrital particles, heterotrophic plankton, and the non-pigmented portion of algalcells.

Absorption spectra by different components exhibit characteristic shapes, which are determined by measuring the absorption by different components in isolation.  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 Chl a, absorption by non-algal particulates is normalized to the concentration of total suspended solids (TSS), and absorption by CDOM is normalized to its value at 440 nm.  This decomposition allows us to express the total absorption spectrum, at(l), as a sum of the 4 components,

(1) where l=wavelength, aw(l) is the absorption by pure water, aX*(l) are the specific-absorption spectra of CDOM (X=CDOM), phytoplankton (X=f), and non-algal particulates (X=p-f); the scale factors for the components are the absorption by CDOM at 440 nm, (aCDOM(440), the concentration of chlorophyll, [CHL], and the concentration of suspended particulates, [TSS].  A similar procedure is used to model scattering coefficient as a function of TSS.  The total absorption and scattering spectra may then be used with any of a number of radiative transfer programs available to predict the penetration of light underwater.

The bio-optical model is useful because it permits us to determine the relative contributions of the different water quality parameters to light attenuation at different positions in the estuary.  Comparing light attenuation calculated at the deep survival limit of seagrasses with water quality concentrations measured there allows the determination of ranges of water clarity that permit expansion or cause contraction of the seagrass bed.  Basing the model on inherent optical properties has the advantage that extrapolation beyond the range of water quality concentrations encountered during model development is possible, because the absorption and scattering coefficients are linearly related to the relevant water quality concentrations (Fig. 1).  Such an exercise can be used to determine the availability of light at the edge of the seagrass bed in response to hypothetical scenarios, such as accelerated eutrophication resulting from increased nutrient loading in the watershed.  This bio-optical model has already been calibrated to conditions typical for Chesapeake Bay, MD (Gallegos 2001) and Indian River Lagoon, Fla (Gallegos and Kenworthy 1996), estuaries with significant seagrass habitats.

Figure 1: A (left) Conceptual diagram of light attenuation down the water column (PLW), and the relative contributions of turbidity, chlorophyll, and color to attenuation, resulting in significantly reduced light at depth. Additional attenuation can occur at the leaf surface due to epiphyte fouling (PLL), which occurs primarily under heavily eutrophic conditions.

Figure1: B (below) Graphical representation of the bio-optical model. Components of light attenuation in the water column are presented on the axes as concentrations in which they are typically measured. Median concentrations for one sample are plotted on this graph and compared to a minimum-light water quality requirement for a given depth (red 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.

Created and maintained by: Alan Joyner