Reeds as indicators of nutrient enrichment in a small temporarily open/ closed South African estuary

Abstract

This study investigated the use of δ¹⁵N levels in Phragmites australis reeds as an indicator of nutrient input into the small temporarily open/closed East Kleinemonde Estuary, Eastern Cape, South Africa. Nutrient (NH₄ and SRP) concentrations decreased from the bank towards the main estuary channel, suggesting that nutrients introduced into the estuary in groundwater and surface runoff were taken up by the fringe of reeds. The roots, rhizomes, stems and leaves of Phragmites at the site with the greatest Phragmites biomass and density had significantly higher δ¹⁵N (∼20‰) than the leaves at a site with no groundwater input, consistent with nutrient enrichment by septic tank wastewater and stormwater runoff. This study suggests that the δ¹⁵N level in reeds can be used to detect enrichment in small estuaries threatened by changing land use and deteriorating water quality.