Insights into the cause of an Ulva lactuca Linnaeus bloom in the Knysna Estuary

Recently a large bloom of opportunistic macroalgae, with Ulva lactuca as the dominant species, covered the lower reach of the permanently open Knysna Estuary. The occurrence of an opportunistic species is globally considered a sign of eutrophication. The aims of this study were to determine the impact of the bloom on the Zostera capensis beds and investigate why the bloom persisted in the lower reach of the estuary. Changes in vegetation cover were measured along five permanent line transects in the intertidal zone. Water column samples were collected for nutrient analysis and benthic chambers were deployed in Ashmead Channel to investigate the flux of nutrients from the sediment. Algal tissue was collected and analysed for N, P and four metals (Cd, Cu, Pb and Zn) to evaluate if the algae could potentially be harvested and used as fertilizer. The results showed that there was a decrease in Z. capensis abundance, which had been displaced by the algal bloom. Recycled nutrients specifically, ammonium and soluble reactive phosphorus, from the sediment and input from a nearby wastewater treatment works acted as a consistent source of nutrients that enabled the bloom to persist in Ashmead Channel. The low concentration of metals and high concentration of N and P in the tissue of U. lactuca mean this algae has the potential to be used as a fertilizer or composted if harvested. Harvesting of the macroalgae will enable the recovery of the Z. capensis beds and remediation of the water column via the elimination of the excess N and P stored in the macroalgae.