The Tensile Root Strength of Emergent Coastal Macrophytes

Abstract

Spartina patens is a dominant emergent macrophyte in fresh, intermediate, and brackish marshes along the Atlantic and Gulf coasts of United States where its biomechanical properties are a key component of wetland health and resilience. Its root biomass and tensile root strength are essential for anchorage, erosion protection, and are important determinants of soil strength. Nutrients and the herbicide atrazine are suspected of negatively impacting this wetland plant and others. The objectives of this study were to: 1) ascertain the tensile root strength of five emergent coastal macrophytes in coastal estuaries, and 2) test the effects of nutrient addition, atrazine exposure, flood duration, and possible interactive effects of these natural and anthropogenic stressors on the tensile root strength of S. patens. The tensile root strength of five coastal macrophytes declined with depth in four estuaries in southeastern Louisiana. The tensile strength of Panicum hemitomon and Sagittaria lancifolia growing in a wastewater treatment wetland also declined relative to a reference wetland site. The results from multiple greenhouse experiments demonstrated that combinations of nitrogen, phosphorus, and nitrogen+phosphorus resulted in the loss of about 50% tensile root strength of S. patens after four months. Atrazine treatments resulted in similar tensile root strength losses. The belowground biomass declined with nutrient and atrazine additions and in combination. The tensile root strength of S. patens varied depending on soil texture and flood duration regimes. The formation of aerenchyma tissue in response to flooding and the cessation of nutrient foraging by roots were the main factors that contributed to lower tensile root strength and less belowground biomass production. The field survey and greenhouse experiments results indicated that prolonged exposure to ambient levels of nutrient loads and atrazine weakens the tensile strength and degrades the belowground biomass. Prolonged inundation may exacerbate the effects of xenobiotics. The long-term effects of these multiple stressors may facilitate coastal land loss. Management efforts can ameliorate the effects of poor water quality on wetlands by amending agricultural practices and land use zoning.