DISCUSSION 



Inorganic Nitrogen 



Salinity 



In general, open water salinity values appeared to be 

 influenced by precipitation and flow-mediated 

 freshwater inputs. Typically, salinity values were 

 decreased by freshwater inputs and increased by 

 drought periods. The greatest peaks in salinity 

 occurred during dry periods. The extent and duration 

 of the freshwater event determined the degree to which 

 the salinity fell, and the time span over which the 

 salinity remained low. For example, a summer event in 

 1997 reduced open water salinity levels by 40 ppt at the 

 Reference Station. However, the salinity had increased 

 by 30 ppt only two months following the event. The 

 magnitude of the increase was most likely due to the 

 timing of the event. The event occurred during the 

 summer, and, after the precipitation ceased, air 

 temperatures were high and resulted in rapid 

 evaporation. In contrast, the October 1998 event 

 reduced open water salinity values at the Reference 

 Station from 33 ppt in June 1998 to 11 ppt ia 

 December 1998. Salinity levels remained low 

 throughout the winter and spring 1999, most likely 

 because the event occurred during the cooler part of 

 the year, and because there was steady rainfall during 

 the beginning of the year. 



Pore water salinity concentrations were almost always 

 higher than open water values. The periods when 

 values were similar were because the transects or part 

 of the transects were flooded with open water. This 

 trend was similar to that described by Hackney and 

 De la Cruz (1978) for a Mississippi tidal marsh. The 

 increased pore water values appear to be due 

 evaporation of water from the soil, as well as lack of 

 dilution from fresh water, as the highest pore water 

 values were seen during drought periods (e.^., 85 to 

 92 ppt in December 1999, which had 0.25 cm of rain). 

 Changes in pore water salinity concentrations were 

 usually reflected by changes in open water salimty 

 values. 



Open water NH4'^ and NO^" + NO,' values showed 

 only small responses to freshwater inundation. A 

 problem with evaluating the impact of the project on 

 these values was that it must be assumed that 

 instantaneous nitrogen levels measured on different 

 sampling dates were representative of the same body of 

 water. However, in reality, the channel water was not 

 stagnant, and moved continually based on tidal 

 oscillations. 



These problems were probably minor in the analysis of 

 vegetation responses as the nitrogen available for plant 

 uptake was found in the soils, which, unlike channel 

 water, did not move. The role of pore water nitrogen 

 in determining the biomass and productivity of salt 

 marsh vegetation has been investigated repeatedly. In 

 general, it has been demonstrated that nitrogen 

 fertilization of marsh plants has significandy increased 

 plant biomass, indicating nitrogen as a limiting nutrient 

 in estuaries. Several investigators have concluded that 

 nitrogen scarcity is responsible for stunting the growth 

 oi Spartina altemiflora (Broome et al. 1975; Gallagher 

 1975; VaHela and Teal 1974; VaUela et al. 1978). 

 Mendelssohn (1979) demonstrated that nitrogen 

 fertilization increased the aerial living standing crop of 

 the short form oi S. altemiflora from 49% to 172%. 

 De La Cruz et al. (1979) found significant increases in 

 the annual aboveground net primary productivity of 

 four tidal marsh communities seven months after 

 ammonium nitrate fertilization. 



In this study, pore water NH4* levels increased 

 dramatically from the beginning to the end of the study 

 period. Most increases were seen after the October 

 1998 composite hydrographic event. The change does 

 not appear to be flow related, as similar increases were 

 measured at all three stations. However, the 

 concentrations were most likely too high to be purely 

 rainfall meditated. The change may have been a result 

 of cyanobacterial mats that formed on the soil surface 

 following the event. Many cyanobacteria species are 

 capable of taking atmospheric nitrogen and fixing it 

 into a biologically available form NH4*. These mats 

 remained on the soil surface for over a year (they were 

 at the stations through the December 1 999 sampling 



ChapUrSix ♦ 6-45 



