Salinity limits of vegetation in Cienega de Santa Clara, an oligotrophic marsh in the delta of the Colorado River, Mexico: Implications for an increase in salinity

Abstract

Greenhouse and field trials were conducted to determine the salinity limits for main vegetation types in Cienega de Santa Clara, an oligohaline marsh in the delta of the Colorado River in Mexico. The Cienega is the largest brackish marsh in the Sonoran Desert and supports numerous bird, mammal and invertebrate species, including threatened or endangered marsh birds. It is supported by brackish agricultural return flows from the USA and Mexico, and operation of the Yuma Desalting Plant (YDP) could reduce the volume and increase the salinity of inflows. Current inflows average 4m3s−1 at 2.8gL−1 Total Dissolved Solids (TDS), and the dominant vegetation type is Typha domingensis, with subdominant stands of Schoenoplectus americanus and other emergent species distributed amidst the Typha stands. A greenhouse experiment showed that under flooded-soil conditions, T. domingensis had a linear reduction in relative growth rate based on biomass production (RGRBiomass) with salinity, with zero RGRBiomass at 8.3gL−1 TDS and a 50% reduction at 4.0gL−1 TDS. S. americanus was about twice as salt-tolerant, with zero RGRBiomass at 12.5gL−1 TDS and 50% RGRBiomass at 9gL−1 TDS. The results are consistent with other studies that show a mean reduction in RGRBiomass of 13.3% per g L−1 TDS for Typha spp. and 4.3% per g L−1 TDS for Schoenoplectus spp. Field surveys showed that T. domingensis stands were restricted to salinities of 6.5gL−1 TDS or less, and that annual biomass production was reduced by 85% in a stand at 5.9gL−1 TDS compared to a stand at 3.0gL−1 TDS. Normalized Difference Vegetation Index (NDVI) values of T. domingensis stands from satellite imagery showed a linear decline with increasing salinity in the marsh. It was concluded that 6gL−1 TDS is the approximate upper limit for vigorous stands of T. domingensis, and that replacement by S. americanus is a possibility if salinities increase. Implications for marsh vegetation structure and habitat value are discussed under different possible operating scenarios for the YDP.