A comparison of physicochemical variables across plant zones in a mangal/salt marsh community in Louisiana
Three vegetation zones were delineated in a mangal / salt marsh community at Bay Champagne, Louisiana — a zone dominated byAvicennia germinans adjacent to the bay, an inland zone dominated bySpartina alterniflora, and a transition zone between the two containing both species. Parallel transects that intersected each zone were established perpendicular to the shore of the bay and sampled to determine if these zones differed on the basis of soil texture, elevation, redox potential, and selected interstitial water variables. Redox potential and interstitial water sulfide, ammonium, P, Ca, Mg, K, Fe, Mn, Cu, Zn, pH, and salinity were measured 5 times during the year to account for seasonal variation.
Factor analysis identified five factors, accounting for 80.6% of the variation in the data. These factors were interpreted as representing seawater, sulfide, nitrogen/phosphorus, Eh, and copper variables. Factor scores in theAvicennia zone were significantly different than in the other zones for the salinity and sulfide factors, with higher salinity scores and lower sulfide scores.
Analysis of variance revealed highly significant zone differences for most individual variables. TheAvicennia zone was characterized by the highest relative elevation and soil bulk density. Higher concentrations of ions associated with sea water, such as potassium, calcium, and magnesium, occurred in theAvicennia zone, which also caused interstitial water salinity to be higher in that zone.
The transition andSpartina zones were more biochemically reduced than theAvicennia zone, as shown by lower redox potential measurements, higher sulfide concentrations, and higher sulfide factor scores in the former. Iron and manganese were lower in the more reduced zones, probably due to precipitation with sulfide. The transition andSpartina zones only differed in interstitial water potassium and sulfide and relative elevation, with some additional seasonal differences in other variables. TheSpartina zone had a lower elevation and usually higher sulfide concentrations than the transition zone. We hypothesize that the relatively high sulfide and lower elevation of theSpartina zone may limit the establishment ofAvicennia propagules there.
Key WordsAvicennia germinans Spartina alterniflora distribution sulfide elevation redox potential Louisiana species zonation
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