4.8.8 CONTROL OF PRODUCTION 



Nutrients appear to be the major abiotic variable 

 controlling the primary production rate, although 

 salinity has an important effect on the kinds of 

 phytoplankton present. 



Saline sediments are typically rich in phosphorus 

 (Pomeroy et al. 1965) and have a strong buffering 

 ability for phosphorus, so nitrogen is more likely to 

 be limiting in brackish and sahne waters. Ryther and 

 Dunstan (1971) documented this for Long Island 

 Sound, New York. They found that about twice the 

 amount of phosphate as can be used by the phyto- 

 plankton is normally present, whereas nitrogen 

 is avaUable in limiting amounts. 



Addition of excessive nutrients, usually nitrogen 

 and/or phosphorus, leads to an excessively eutrophic 

 state. The high nutrient levels stimulate growth of a 

 few algal species, which rapidly reach high population 

 densities. Thus eutrophication is accompanied by 

 dramatic changes in the composition of the commu- 

 nity with a progressive deterioration of water quality, 

 often anoxic conditions of sediments, advent of algal 

 blooms, and the elimination of desirable (commer- 

 cially important) fishes and sheUfishes. Normally, algal 

 blooms and oxygen problems associated with eutro- 

 phication occur in the warmer months of the year. 

 This condition can occur in fresh, brackish, or saline 

 waters. In fresh waters, blue-green algae such as 

 Microcystis, Anabaena, Anabaenopsis, and Spirulina 

 dominate. In brackish and sahne waters the common 

 blooming genera include such small coccoid algae as 

 Monodus, Nanochloris, and Stichococcus. 



The severity of eutrophication in a water body is 

 strongly controlled by the flushing rate. Rapidly 

 flushed areas can tolerate higher levels of nutrient 

 inflow than can stagnant areas. Since coastal bays and 

 lakes are usually inundated daily by tidal waters, they 

 tend to be better flushed than freshwater areas and 

 less subject to excessive states of eutrophication. 

 Since inland Chenier Plain water bodies are usually 

 very shallow (2 m or 6.6 ft), they are particularly 

 sensitive to high nutrient loading levels. Craig and 

 Day (1977) suggest a critical phosphorus loading level 

 of 0.4 g/m /yr for Louisiana freshwater areas. They 

 also cited permissible and dangerous loading levels of 

 phosphorus and nitrogen from studies by VoUen- 

 weider ( 1 968) and Brezonik and Shannon (1971). 



4.9 INLAND OPEN WATER HABITAT 



For the most part, the inland open water habitat 

 is maintained by rainfall, the inflow of freshwater and 

 sediments from rivers and runoff, and from tidal 

 action and seawater inflow from the Gulf. The water 

 bodies composing this habitat are shallow, seldom 

 exceeding 3 m (10 ft), except for deep channels such 

 as tidal passes and navigation channels. The area 

 covered by this habitat type (fig. 4-34) is 2,008 km^ 

 (775 mi^), 35% of the Chenier Plain aquatic system. 



Shape and size of inland water bodies vary 

 widely, since linear canals and rivers, as well as lakes 

 and ponds, are included. The boundary between these 

 water bodies and the surrounding wetlands is gently 

 sloping, except where the water body is dredged and 

 a spoil bank is formed. SaUnities within the inland 

 open water habitat vary from fresh to nearly full 

 ocean strength, reflecting both proximity to the Gulf 

 and the local hydrologic regime. 



Generally inland open waters are somewhat 

 turbid, emphasizing the importance of the interactions 

 of fine bottom sediments, shallow depth, and turbu- 

 lence. This turbulence is caused by wind-driven water 

 currents, and sometimes by boat traffic. The water 

 column is relatively homogeneous and well-mixed. 

 During the summer, water temperature is usually 

 high, often above 30°C (86T), and the amount of 

 dissolved oxygen sometimes can become dangerously 

 low for aquatic animals. 



4.9.1 PRODUCERS 



The inland open water habitat provides a gradient 

 of subhabitats that range from saline (up to 25o/oo) 

 through brackish to freshwater. Plant communities 

 associated with each subhabitat vary with salinity 

 ranges. 



In the sahne areas of the inland open water habi- 

 tat, the large proportion of water to wetland and the 

 high frequency of marsh flooding by estuarine waters 

 leads to a pronounced interaction between the aquatic 

 and wetland habitats. The inland open water habitat 

 is shallow and turbid with a muddy substrate. These 

 conditions are unfavorable for the growth of large 

 rooted aquatic plants and most of the primary pro- 

 duction in these areas results from phytoplankton. 

 Most numerous of the phytoplankton are the diatoms, 

 coccoid blue-green algae, and coccoid green algae. 

 Only one study (Denoux 1976) was found concerning 

 phytoplankton in the Chenier Plain area, and it lists 

 numbers of phytoplankton and not the types found 

 there. Appendix 6.3 hsts those phytoplankton found 

 in inland open water habitat in southeastern 

 Louisiana. 



A few species of phytoplankton, such as Nitzchia 

 closterium, are found across the whole salinity range 

 into freshwater, but most freshwater species are 

 seldom present where salinity is significant. The most 

 numerous forms are diatoms and blue-green algae; 

 the presence of the latter often reflects excessive 

 eutrophic states. 



Plant growth in brackish marsh areas shows a 

 marked difference between summer and winter con- 

 ditions. During the winter, tides and tidal currents are 

 generally low in amphtude, and water bodies clear up 

 allowing the growth of several macrophytes adapted 

 to reduced temperatures. Large mats of filamentous 

 green algae sometimes clog the less sahne waterways. 

 During the summer, higher turbidity levels restrict 

 prima ly production to phytoplankton, except for the 

 shallowest areas which are colonized by benthic dia- 

 toms (Bahr and Hebrard 1976). 



203 



