Integrated indicators of estuarine watershed pollution: Seagrass, epiphytes, sediment and benthic macrofauna

Pollution causes degradation and loss of ecological and economic value in seagrass habitats globally. These impacts can scale across ecosystem components. Here we assessed multiple seagrass associated metrics in Zostera capensis across a gradient of watershed pollution at the Knysna Estuary, South Africa to determine which indices were most indicative of pollution. The most nutrient-enriched site (N and P) was linked to a wastewater treatment works. Above- and belowground seagrass biomass decreased in response to high nutrient concentrations; cover and shoot length did not change. This is indicative of long-term exposure to nutrient enrichment. In contrast, short-term responses to nutrient enrichment were observed in epiphyte biomass which increased with high nutrient concentrations. The δ¹⁵N isotopic signatures in Z. capensis leaves were not consistently indicative of nutrient enrichment, which could be due to poorly treated wastewater that depleted δ¹⁵N signals. There was a close coupling between community composition of macrofauna associated with the rhizosphere sediment and site-specific nutrient enrichment profiles. High abundance of annelids (pollution-tolerant group) and low abundance of amphipods (pollution-sensitive group) was observed at the most polluted site. Macrofauna trends closely matched sediment characteristics such as redox potential, with a higher abundance and richness observed in more oxygenated sediments. This study showed that complementary indicators of nutrient enrichment such as seagrass, epiphytes, sediment and benthic macrofauna can be effectively integrated to reveal pollution impacts on coastal environments. Future management to protect the endangered seagrass Z. capensis can reduce watershed pollution and maintain ecosystem services through seagrass buffer zones.