During March and April, water levels within semi-impoundments were reduced approximately 5.1- 

 10.2 cm below marsh level for three to four weeks. Light penetration to the bottom was increased, 

 encouraging the growth of aquatic vegetation. Similar water level regimes were found to enhance 

 production of widgeon grass (Joanen and Glasgow 1965). Water levels were maintained 

 approximately at marsh level for the remainder of the growing season. During droughts, however, 

 water levels within semi-impoundments occasionally fell to levels equal to those of a partial 

 drawdown (Figure 4). 



Water Hyacinth Control 



Controlled introductions of brackish water were used to eradicate water hyacinths within infested 

 semi-impoundments at LPWMA Prior to the introduction of brackish water, semi-impoundments 

 were partially dewatered. This procedure reduced the dilution of brackish water within semi- 

 impoundments and increased treatment effectiveness. When continually exposed to salinities of 4 

 ppt, water hyacinths died within 3 weeks. High salinity waters (8-16 ppt) were introduced to affect 

 a quicker kill and to overcome any immediate or future salinity dilution (due to precipitation). 

 During the summer of 1985, salinities of 10 ppt or higher were held for approximately 2 weeks 

 throughout semi-impoundment 3 (Figure 5). This brackish water introduction was one of the most 

 successful ever conducted at LPWMA Although water hyacinths were totally eradicated from 

 several semi-impoundments, freshwater aquatic plants were also killed by this introduction of 

 brackish water. Within a month, however, heavy rains associated with frequent summer 

 thundershowers diluted salinities to less than 2 ppt. Desirable aquatic vegetation soon reappeared 

 when the water freshened but were not as abundant as in normal years. Considering the 

 effectiveness, the expense, and the personnel needed to control water hyacinths through the use 

 of herbicides, controlled introduction of brackish water was a far superior method despite its 

 temporary adverse effect on freshwater aquatic vegetation. 



Introduction of Estuarine Organisms 



Brackish water was frequently introduced into semi-impoundments at LPWMA to allow ingress 

 of estuarine organisms, especially white shrimp (Penaeus setiferus), brown shrimp (Penaeus aztecus), 

 and blue crab (Callinectes sapidus). Brackish water and juvenile estuarine organisms occurred at 

 LPWMA primarily in association with late spring and early summer droughts. Recruitment of post- 

 larval white shrimp into the estuary (during June and July) often coincided with the occurrence of 

 brackish water at LPWMA Therefore, introduction of estuarine organisms at LPWMA focused 

 on the available and marketable white shrimp. 



When juvenile shrimp were thought to be present or were collected in Little Pecan Bayou, water 

 control structures were operated to allow ingress of estuarine organisms. In semi-impoundments 

 where widgeon grass was the dominant aquatic plant, free water exchange was allowed if turbid 

 waters or low tides did not threaten aquatic vegetation. In most semi-impoundments, freshwater 

 aquatics were dominant. Water exchange in these units was allowed if salinities remained at or 

 below 4-5 ppt. When salinities of outside waters were low, flap gates were raised and stop-logs 

 removed to maximize water exchange. Tidal flushing also appeared to reduce the severity of 

 filamentous algae blooms which often formed mats at the water's surface and smothered out desired 

 aquatic vegetation. When the salinities of outside waters were high, water exchange was restricted 

 by varying the weir crest elevations. 



Water Level Management During Waterfowl Season 



A month prior to waterfowl season, water control structures were set to draw water levels down 

 to marsh level or slightly below. This procedure helped to make submerged aquatic plants more 



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