Predicting Productivity Change Resulting from Potential Freshwater Diversions


Jan. 24, 2007


Montagna, P

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Galveston Bay Estuary Program


There is a need to predict environmental consequences of when planning water resource development and for adaptive management of operational water systems. Prediction is most powerful when based on mechanistic understanding of how the environment works and how physical drivers effect biological resources. Typically, prediction is created through a modeling exercise, but there are many different modeling approaches and goals. The product of such exercises is just as diverse. One approach that appears to be successful is to model productivity of the communities most affected by freshwater inflow. One example is the lower Colorado River basin, which supports a diverse ecological community that relies heavily on the quality and quantity of water moving through the system. A planned water project has the potential to alter the flow regime for the lower Colorado River and consequently Matagorda Bay; hence, the need to assess the potential impact of these flow regime modifications. In the bay health assessment, the study of benthic indicators is an integral part of determining the potential impacts or benefits on the aquatic resources of Matagorda Bay with and without the project. A bio-energetic model to relate macrobenthic productivity to salinity within and among four Texas estuaries was developed in 1996. A 5-year data set (1990-1995) of macrobenthic biomass, was used to calibrate the model. The benthos were divided into two trophic groups: deposit feeders (that consume detritus or sediment organic matter) and suspension feeders (that filter phytoplankton or graze on benthic diatoms). Simulations using this calibration show that the period between 1996 and 2000 validated the model. After 2001, blue crab populations crashed releasing benthos from predation pressure and the model predicts higher benthic productivity than found. The results of the simulations demonstrate that suspension feeders are good indicators of the importance of freshwater inflow on maintaining secondary production and they decline with increased salinity. However, deposit feeders increase productivity with increasing salinity. Although predicted total productivity does not change with freshwater diversions and increasing salinity, community composition and feeding guild structure does change.




flow modification, macrobenthic biomass, macrobenthic productivity, models, prediction, salinity, simulations, water resource development, water systems