Environmental parameters related to growth of submersed aquatic vegetation in the lower Mobile Delta, Alabama

Abstract

Light availability and soils influence growth of submersed aquatic vegetation (SAV). My objectives in this study were to determine relationships between these environmental parameters and distribution and abundance of species of SAV in the lower Mobile River Delta, Alabama. I established sampling sites (n = 22) in Eurasian watermilfoil (Myriophyllum spicatum; n = 4), wild celery (Vallisineria americana; n = 5), Southern naid (Najas guadalupensis; n = 4), mixed species (n = 4; milfoil and native species were co-dominants), and sparsely vegetated areas (n = 5). I measured water depth, Secchi depth, surface temperature, velocity, salinity, turbidity, and total suspended solids (TSS)) twice monthly in June – August 2003 and in March – August 2004 at each site, and I estimated species composition and biomass of SAV at sites annually. I combined measurements of turbidity and TSS using principal component analysis and used principal component (PC1) scores to estimate water clarity at sites. In April 2004, I placed experimental shade plots (1.8m2) that produced three levels of shade (30%, 60%, and 90%) at mixed species sites to test relative effects of reduced light on biomass of milfoil and native SAV species. I also collected soil cores (5cm x 20cm) at sites in March 2004 and measured soil parameters (soil texture, total carbon (C), total nitrogen (N), extractable phosphorus (P), and pH). PC1 scores of turbidity and TSS were lower at milfoil sites than at native SAV or sparsely vegetated sites on most sampling dates during the study period. PC1 scores also were greater during the early growing season (March-May) than during summer (June-August) in 2004. Total abundance of SAV was reduced by >69% in shade plots, but 30%, 60%, and 90% shade did not have different effects on growth of SAV species. Milfoil sites had greater amounts of C and N in soil cores than native SAV and sparsely vegetated sites. Milfoil sites also had greater amounts of clay and less P in soil cores (10-20 cm) than sparsely vegetated sites. My results identify differences between microhabitat conditions in milfoil, native SAV, and sparsely vegetated areas in the lower Mobile Delta, and these results provide evidence that milfoil and native SAV species may partition habitats in estuarine environments based on light availability and soil conditions.