Modeling submersed macrophyte growth in relation to underwater light climate: modeling approaches and application potential

The underwater light climate is one of the most important determinants of submersed aquatic vegetation. Because of the recent, large-scale, declines in aquatic vegetation, largely attributed to deterioration of the underwater light climate, interest in tools to predict the wax and wane of aquatic macrophyte populations has greatly increased. This paper summarizes two modeling approaches that can be applied to assess impacts of changes in underwater light climate on submersed vegetation. The first, stand-alone, model type focuses on metabolism and biomass formation of submersed freshwater macrophytes with difference in phenologies. This type is illustrated by examples from various sites using models developed for the freshwater macrophytes Hydrilla verticillata (L.f.) Royle (HYDRIL) and Myriophyllum spicatum L. (MILFO), and also by an example ecological risk assessment. The models (HYDRIL and MILFO) track carbon flow through the vegetation in meter-squared (m(2)) water columns. The models include descriptions of various factors that affect biomass dynamics, such as site-characteristic changes in climate, latitude, light attenuation within the water column, carbon assimilation rate at light saturation, temperature, wintering strategies, grazing and mechanical control (removal of shoot biomass). Simulated biomass, net assimilation and maintenance respiration over a relatively short (1-5 year) period agree well with measured values. The models are, therefore, believed to be suitable for predicting plant community production, growth and survival characteristics over relatively short periods over a large range of sites. The feasibility of using a macrophyte growth model of the HYDRIL type for ecological risk assessment is demonstrated. It is used to evaluate the consequences of management changes in large rivers for the survival of submersed vegetation. The current assessment evaluates the potential impact of increased commercial navigation traffic on the growth of Potamogeton pectinatus L. in Pool 4 of the Upper Mississippi River, U.S.A. In this case, navigational traffic scenarios were translated into suspended solids concentrations and underwater light climate, with the latter being used as inputs into the aquatic plant growth model. Model results demonstrate that the scenario increases in commercial traffic cause minimal decreases in growth and vegetative reproduction. Results indicate that this growth model can be a useful tool in ecological risk assessment, since the required stress-response relationships could be established. The second, integrated, model type focuses on the role of seagrass and other primary producers in estuarine littoral zone material cycling (carbon and nitrogen) at the Goodwin Islands, Virginia, U.S.A. The latter model was used to explore the effects of changes in underwater light climate and inorganic nitrogen on the biomass formation of Zostera marina in one of four interacting habitats at different elevations, coupled by a hydrodynamic model. Simulation results indicate that the water quality recently met the light extinction coefficient standard viewed as critical for the presence of healthy seagrass meadows, but that increased material loading into the littoral zone is expected to increase turbidity in the water column, causing a decreased production of seagrass and increased production of phytoplankton, lower overall system production, more rapid turnover of nitrogen, and a less vegetated habitat for fauna. Submersed macrophyte growth models can be useful tools to enhance our understanding of macrophyte responses, that is, their interactions with environmental factors and anthropogenic stressors. They can be used for predictions of future states based on assumptions of known interactions, but they can not be expected to provide precise predictions of future states.