The role of submerged macrophytes and macroalgae in nutrient cycling: A budget approach

Abstract This study used a budget approach to determine the effect of submerged macrophytes and macroalgae on the storage of N and P in an estuary. Above and below ground tissue content of nitrogen and phosphorus were determined for the various macrophyte species. The mouth of the estuary was artificially breached in February 2011 with a volume of 0.3 × 10 6  m 3 and closed a week later. A 1:100 year flood with volume close to 3 × 10 6  m 3 breached the mouth naturally in June 2011 flushing water and sediment out of the estuary. In order to track the change in the nutrient acquisition of the submerged macrophytes and macroalgae over a closed-mouth state, the nutrient budget was constructed for the period February 2011 to July 2011, from the time the mouth closed until it opened again. Relative to other inputs the sediment contributed 30% of the TN and 40% TP toward the nutrient budget, while the submerged macrophytes and macroalgae stored 20–30 % TN and 30–38 % TP. The river and precipitation contributed less than 3% of the TN and TP input. It was previously thought that the sediments of South African temporarily open/closed estuaries did not have the necessary organic stock to fuel primary production. However this research showed this to be incorrect. Submerged macrophytes and macroalgae significantly influenced nutrient cycling and this is the first detailed account of incorporating vegetation into a nutrient budget without relying solely on C:N:P ratios.