Document: ROB-3-80-9

Controls on macrophyte production and abundance in estuaries: Interaction between physical stress and resource competition.

DAOUST, R.J.*, J.T.MORRIS and C.T.NIETCH

University of South Carolina Columbia SC 29208 USA ¹

Abstract:
Contemporary ecological theories often present physical stress and resource competition as opposing, disparate mechanisms for controlling macrophyte primary production in ecosystems. In most cases these theories have been tested in ecosystems in which one factor is clearly of greater import than the other. Estuaries, in contrast, represent inherently dynamic and complex systems in which both physical stress and resource competition vary in accordance with changing salinity, and relative phosphorus (P) and nitrogen (N) availability. Research has shown that correlations between leaf tissue N:P ratios and aboveground abundance/productivity can be used to assess the role of nutrient availability in controlling macrophyte community structure. We used this approach to investigate the importance of nutrient availability in the Edisto River, SC estuary. Our 7 sites spanned the estuarine salinity gradient (average salinities ranged from 0 to 20 ppt). In addition, we quantified porewater sulfide (S^-2) and chloride concentrations (Cl^-) as a measure of physical stress. Peak biomass occurred in October at all sites. Among sites, the oligohaline site (salinity = 7 ppt) had the greatest biomass and highest productivity rates (1989 g DW m^-2 and 11.06 g DW m^-2 day^-1). Significant relationships between N:P ratios and abundance were found for most saltwater sites and at the freshwater site. At the oligohaline sites, however, N:P ratios were uncorrelated with both productivity and abundance. This was also true for saltwater sites where S^-2 concentrations were high (>300 uM). These data suggest that saltwater sites are N-limited and that N-availability is important in determining production patterns at this end of estuaries. In addition, freshwater sites appear to P-limited and, consequently, P-availability plays a major role in controlling plant community structure there. In contrast, however, nutrient availability seems to be unimportant in the oligohaline zone. This is also true when physical factors such as S^-2 stress becomes overwhelming. Our results, therefore, illustrate that both physical stress and resource competition play important roles in determining patterns of macrophyte production in estuaries.

Keywords: Nitrogen-limitation, Phosphorus-limitation, Macrophyte primary production, saltmarsh

This abstract is being presented at: 4:30 PM in session:
Oral Session #65: Wetlands, Estuaries and Salt Marshes.