Nutrient fluxes from sediments pose management challenges for the Knysna Estuary, South Africa

• Published in: African journal of aquatic science Vol. 45; no. 1-2; pp. 1 - 9
• Main Authors: Human, LRD, Weitz, R, Allanson, BR, Adams, JB
• Format: Journal Article
• Language: English
• Published: Grahamstown Taylor & Francis 02-04-2020; Taylor & Francis Ltd
• Subjects: Anoxia; anoxic sediment; Anoxic sediments; Benthos; Biodiversity; Bottom trawling; Dredging; Dredging wastes; Estuaries; Estuarine dynamics; eutrophication; fine sands; Fluxes; Lagoons; Mineral nutrients; Nutrient content; Nutrient dynamics; Nutrients; organic content; Organic matter; Phosphorus; pollutant retention; Pollutants; Regions; Seasonal variations; Sediment; Sediment-water interface; Sediments; Soil management; Wastewater; Wastewater treatment; water quality; South Africa
• ISSN: 1608-5914; 1727-9364
• DOI: 10.2989/16085914.2019.1671787

This study investigated seasonal dynamics of benthic nutrient fluxes using in situ benthic chambers deployed over a 24-hour period at the sediment water interface for four distinct regions of the Knysna Estuary. The sediment in the Upper Estuary, Lagoon and Bay acted as a source of NH
4
with a rate of 100-300 µmol m
−2
day
−1
N, and soluble reactive phosphorus (SRP) with a rate of 15 µmol m
−2
day
−1
P, whereas that of Ashmead channel exceeded these rates by far. The sediment acted as a sink for NO
3
−
with fluxes of between −200 and 100 µmol m
−2
h
−1
. Benthic nutrient fluxes in the estuary were positively related with sediment organic content and silt. Nutrient fluxes in the Ashmead channel were significantly greater than the other regions because of the anoxic sediment, reduced flushing and a legacy of pollutant and organic matter retention. This condition can only be improved by removing wastewater input and dredging. Such drastic interventions are necessary to improve the health of South Africa's top ranked estuary in terms of biodiversity importance.