A biooptical model of irradiance distribution and photosynthesis in seagrass canopies

Abstract

Although extremely vulnerable to coastal eutrophication, seagrasses represent important structuring elements and sources of primary production in shallow waters. They also generate an optical signature that can be tracked remotely. Accurate knowledge of light absorption and scattering by submerged plant canopies permits the calculation of important plant-and ecosystem-level properties, including rates of photosynthesis, vegetation abundance, and distribution. The objectives of this study were to develop a realistic, yet simply parameterized two-flow model of plane irradiance distribution through a seagrass canopy submerged in an optically active water column, to evaluate its performance against in situ measurements, and to explore the impacts of variations in canopy architecture on irradiance distribution and photosynthesis within the canopy. Allometric functions derived from leaf length-frequency data enabled simple parameterization of canopy architecture. Model predictions of downwelling spectral irradiance distributions in seagrass canopies growing in both oligotrophic and eutrophic waters were within 15% of field measurements. Thus, the model provides a robust tool for investigating photosynthetic performance of seagrass canopies as functions of water quality, depth distribution, canopy architecture, and leaf orientation. Model predictions of upwelling irradiance were less reliable, particularly in the upper half of the canopies. The model was more sensitive to leaf orientation than leaf optical properties, seabed reflectance, or the average cosine of downwelling irradiance. Better knowledge of leaf orientation appears to be a fruitful avenue for improving our understanding of the interaction between seagrasses and the submarine light environment.

Satellite-based monitoring of tropical seagrass vegetation: current techniques and future developments

“…Some account for direct environmental effects on seagrass production, while others also include indirect effects such as shading by increased growth of algae following increased nutrient loads. A two-flow bio-optical model (Zimmerman, 2003) allowed the simulation of light climate over and in the seagrass canopy. Modelling predictions of downwelling spectral irradiance distributions were within 15% of field measurements.…”

Canopy Reflectance Modeling of Aquatic Vegetation for Algorithm Development: Global Sensitivity Analysis

“…Exploration of the radiative transfer process and analyses of the reflectance spectra of aquatic vegetation have increased in recent years. Currently, there are several models available that are able to simulate the spectral response of aquatic vegetation [18][19][20][21][22][23][24][25]. We have proposed the Aquatic Vegetation Radiative Transfer model (AVRT) based on the SAIL model framework.…”

Light in shallow waters: A brief research review

“…Furthermore, the details of how light is reflected appear to be controlled, at least in part, by factors such as sediment characteristics, the presence of polymers and other biogenous matter, and surface morphology, such as the presence of sand ripples and larger sand waves, although the details of these relationships are presently unknown. Radiance-based radiative transfer models are under development that include the effects of seafloor geometry (Mobley and Sundman 2003;Zaneveld and Boss 2003) and three-dimensional sessile structures extending into the overlying water column, as in the case of a seagrass canopy (Zimmerman 2003). It is also worth pointing out that significant advances are being made in simulating the structural details of complex benthic ecosystem components (Kaandrop and Kubler 2001) that could, in the future, be used to model the three-dimensional light field in complex benthic environments.…”