Figure 2-8: View of water column habitat in central 

 Rincon Bayou. 



Photo courtesy of the Bureau of Reclamation. 



The water column food-web consists of phytoplankton 

 (single-celled plants) being eaten by zooplankton 

 (single-celled animals), which in turn are eaten by fish. 

 Primary production (the conversion of solar energy to 

 chemical energy by photosynthesis) by phytoplankton 

 in estuanne water can be relatively high but is typically 

 much less than in salt marshes. There are two cycles of 

 energy in the water column food- web. The first may 

 be referred to as the "microbial loop," which includes 

 only small flagellated phytoplankton and zooplankton 

 and bacteria. Flagellates are a very diverse group of 

 plankton that can travel short distances by beating their 

 whip-Hke flagella. The small phytoplankton are preyed 

 upon by small zooplankton, and, when the small 

 phytoplankton and small zooplankton die, they may be 

 decomposed by bacteria in the water. This food-web 

 c\'cle is small, transfers energy rapidly, and its 

 components are tighdy coupled. However, energy 

 cycled in this microbial food-web are not usually 

 transferred to laigher trophic levels (e.^., fish). 



The second food-web cycle of the estuarine water 

 column consists of the larger phytoplankton, such as 

 diatoms, which are eaten by zooplankton and some 

 fish. In addition, some zooplankton are eaten by larger 

 zooplankton and the lar\'al and adult forms of some 

 fish. Even larger predatory fish (e.g., red drum, 

 Sdaenops ocellatus) then eat these plantkivourous fish. 



This food-web cycle is larger, and it transfers energy 

 slower than the microbial loop cycle, but both cycles 

 are important in nutrient processing within the estuary. 



Open Water, Muddy Bottom 



The most common benthic habitat in the Nueces 

 Estuar)- is the unvegetated muddy bottom (Figure 2-9). 

 Movement of water over the surface of the mud keeps 

 the sediment oxygenated to about one centimeter in 

 depth. Below this region is a strongly reduced 

 en\aronment due to the presence of ox)'gen-consuming 

 microbial animals. Mud is easily re-suspended, and 

 muddy bottoms may therefore experience frequent 

 erosion or deposition of sediment. Turbidit)' tends to 

 be high at the surface, which restricts the presence of 

 light-dependant primarj' producers and filter feeders. 

 Deposit feeders, however, can be present in high 

 abundance, diversity and biomass. 



Detritus is the most important source of carbon for 

 muddy bottom habitats. This material may originate 

 from terrestrial sources transported by freshwater 

 inflow, marine sources derived from marshes, 

 seagrasses or sedimented phytoplankton. In the 

 benthic muds, there are three types of animals which 

 utilize detritus, including non-selective deposit feeders, 



Figure 2-9: View of muddy bottom habitat in upper 

 Rincon Bayou during a period of low water. 



Photo courtesy of the Bureau of Reclamation. 



2-10 ♦ Study Area 



