in this marsh system, these effects are not expected to play as significant a 

 role as they do in brackish and salt marshes. 



Nutrients . Sources of nutrients in the fresh marsh include dissolved 

 organic and inorganic materials (from rain and upland drainage), peat decompo- 

 sition, and sediment deposition. The latter source is expected to be the 

 least significant if the system is removed from m.arine influences. 



The amount of extractable nutrients in a fresh marsh system is generally 

 lower than in brackish or salt marshes (Palmisano, 1970; Brupbacher et. al., 

 1973). However, within a given marsh type there are strong positive correla- 

 tions between water-soluble nutrients (sodium, magnesium, potassium, and 

 phosphorus) and organic matter content of soil materials. Phosphorus is not- 

 ably low in samples from fresh marsh systems. Palmisano (1970) stated that 

 phosphorus apparently accumulates in plant tissues in a form which is unavail- 

 able to succeeding generations of vegetation; this and other micronutrients, 

 through complexation with jrganic molecules, may be limiting to species in 

 organic soils. Bayley ana Odum (1976) also indicated that phosphorus is the 

 limiting nutrient in freshwater sawgrass marshes. 



The interconversion of unavailable and available nutrients is controlled 

 by several processes. Sodium concentration may influence the displacement of 

 other cations from exchange sites on clay micelles and organic molecules 

 (Palmisano, 1970). The magnitude of this effect is ultimately dependent upon 

 the amount of saltwater inundation. Fires convert some of the nutrients incor- 

 porated in peat and living biomass back into an available form. This may account 

 for the more vigorous growth frequently noted after marsh fires (Vogl, 1973). 

 Fire may also release some nutrients to the air, but the significance of such 

 losses in marsh systems remains unknown (Bayley and Odum, 1976). Submergence 

 and its effects on oxidation-reduction systems and microbial populations have 

 been discussed previously. In general, the reduced forms of nutrients that 

 result from flooding are more available to plants. However, the actual changes 

 in nutrient titers depend on the complex chemical characteristics of the soil 

 materials (Redman and Patrick, 1965; Brupbacher et al., 1973). 



Primary producers . Fresh marsh is characterized by a large number of 

 species"! Chabreck (1972) listed 93 species found in the fresh marshes of 

 Louisiana. In comparison, 40 and 17 species were listed for brackish and salt 

 marshes, respectively. This difference is presumably due to (1) wider ranges 

 in some ecologically important variables (topography, soil type, hydroperiod) 

 and (2) the decrease in extremes of salinity and inundation regimes found in 

 the fresh marsh ecosystem. Competition thus becomes more subtle a process 

 rather than an "all or none" situation, and the result is a large number of 

 coexisting species. Our miodel allows for categories of vegetation (submergent, 

 floating, emergent, meadows, phytoplankton) , but it is not concerned with 

 species. These plant categories are regulated primarily by water depth. Salt, 

 oxygen, soil type, and nutrient availability also have different effects and 

 play roles in determining the vegetative composition of an area. 



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