restricted tolerance ranges for salin- 

 ity, because of the difficulty of main- 

 taining an optimal osmotic level within 

 their tissues. Many strategies have 

 evolved to allow different organisms to 

 cope with the hyposalinity of freshwater 

 existence, and the hypersalinity of 

 marine existence. Estuaries pose a 

 special problem however, because they 

 sometimes undergo drastic changes in 

 salinity over short time periods (days 

 or hours). Relatively few organisms have 

 evolved to handle this variability. 

 Estuaries have fewer species (but 

 greater organismal abundance) than 

 either freshwater or marine ecosystems 

 (Remane and Schlieper 1971). 



Five major groups of organisms are 

 found in the estuarine open water habi- 

 tat: (1) autotrophs (phytoplankton and 

 benthic algae); (2) zooplankton; (3) 

 nekton; (4) macro and meiobenthic organ- 

 isms; and (5) microbial forms. 



Autotrophs 



Primary production in the estuarine 

 open water habitat is often limited by 

 light rather than by nutrient concen- 

 tration. Light striking the water sur- 

 face is often completely absorbed in the 

 surface meter. Occasionally during low 

 surface runoff and calm periods, water 

 clarity may increase enough to allow 

 significant production by benthic algae. 



Light limitation thus precludes the 

 development in most areas of submergent 

 macrophyte communities (see ESTUARINE 

 AQUATIC BED (15)). Benthic algae (es- 

 pecially diatoms) are also less impor- 

 tant in this habitat than, for example, 

 in Georgia coastal estuaries, where 

 large tidal variations allow light 

 penetration to the sediment surface. 

 Most primary production in the open 

 water habitat is the result of unicel- 

 lular plants, primarily phytoplankton. 



Zooplankton 



Zooplankton in the estuarine open 

 water habitat may be divided into two 

 major classes, holoplankton (permanent 

 members of the zooplankton) and raero- 

 plankton (larval animals that become 



benthic or nektonic at maturity) . Holo- 

 plankton is often dominated by a single 

 copepod (Acartia tonsa ) that is ubiqui- 

 tous and characteristic of estuaries 

 over a wide range of latitudes. Acartia 

 is well adapted to estuarine existence, 

 in that it tolerates wide ranges in 

 salinity and temperature. This copepod 

 is also a cosmopolitan feeder that can 

 filter phytoplankton while swimming, and 

 can also feed in surface sediments on 

 the bottom. In a benthic study of Lake 

 Pontchartrain 8,030 individuals/m 2 of A. 

 tonsa were collected in bottom samples 

 in some areas (Sikora and Sikora 1982). 

 Other major holoplankters are cteno- 

 phores, arrow worms, and several other 

 species of copepods. 



During the summer, total zooplank- 

 ton biomass may be locally dominated at 

 times by meroplankton. For example, the 

 larvae of blue crabs, barnacles, and 

 bivalve molluscs may temporarily make up 

 most of the zooplankton community in 

 estuaries, especially in the higher 

 salinity areas. 



Nekton 



The estuarine habitat is perhaps 

 most valuable as a nursery ground area. 

 Immature nekton are carried inshore by 

 favorable water currents during their 

 peak growth period when their food re- 

 quirements are highest. It has been 

 noted that the annual movement of many 

 of these animals occurs during spring, 

 coinciding with the time of high organic 

 export from marshes (Happ et al. 1977). 



A study was recently made of the 

 Barataria basin as a nursery area for a 

 variety of nektonic species (Daud 1979; 

 Rogers 1979; Simoneaux 1979; Smith 1979; 

 and Chambers 1980). Chambers demon- 

 strated that some commercially important 

 marine-spawned nekton migrate into 

 freshwater areas during late winter and 

 spring as larvae, and emigrate offshore 

 as subadults during summer and fall. 

 These include blue crabs ( Callinectes 

 sapidus ) , croaker ( Micropogonias undula- 

 tus ) , mullet ( Mugil cephalus ), menhaden 

 ( Brevoortia patronus ) , flounder (Para- 

 licthys lethostigma ) , and the brown 

 shrimp ( Peneaus aztecus ) . 



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