Figure 7-4: Conceptual model of the Nueces Delta ecosystem. Parts of the Nueces Estuary are bounded by shaded 

 rectangles. Circles are forcing functions from outside the system. Rounded rectangles represent standing stocks or 

 energy storage. Arrows represent flow rates of energy and materials. The diamond represents on/off switch. Pentagons 

 represent multipliers. 



food chain. Some benthic invertebrates then utilize 

 this marsh and water column detritus, contributing 

 themselves to the detrital food chain. Marsh 

 production is a function of sunlight, nutrients and soil 

 salinity. The soil salinity is affected by water column 

 salinity and elevation. All benthos are strongly affected 

 by salinity because of physiological tolerance to a 

 specific sahnity range, or by using salinity gradients as 

 cues for migration and reproduction. Of these 

 elements, fresh water is the most critical for 

 maintaining ecological functions in the upper Nueces 

 Delta. In natural systems, inflow acts as a continuous 

 forcing function. In this conceptual model, a diversion 



project acts as a "switch" that, when activated, restores 

 some freshwater inflow to the system. 



This conceptual model demonstrates an improved 

 understanding of the mechanisms controlling 

 production in the upper Nueces Delta. These 

 mechanisms indicate the importance of the 

 demonstration project in restoring fiinctionality to the 

 marsh-dominated ecosystem. Without fresh water 

 from river diversions, salinity concentrations increase, 

 average water levels decrease, and production of 

 primary and secondary producers declines. A loss of 

 fiinctionality in the Nueces Delta affects Nueces Bay 



Chapter Seven ^ 7-9 



