Recent water samples in the Knysna estuary suggest a bloom in nitrate and phosphate levels were higher than expected for the estuary during summer. Additional samples collected at the Thesen Island Jetty indicate a decrease in the concentration of nutrients during the first collapse of the bloom. Local expert, Professor Brian Allanson of the Knysna Basin Project says the subsequent blooms had similar properties in the dispersion of the organism through the water column of the estuary. ‘Microscopic tests show the plant cells were infrequent and many were represented by the empty case. This is an indication that the active bloom is regressing and that the colouration of the water was due to decaying products linked with an increase in peaty water from the Knysna River due to the recent increases in river flows.
‘With the advance of Autumn daylight shortening and lower sea temperatures, the bloom is expected to become less and less’ according to Professor Allanson.
Dr Pitcher of the Department of Agriculture, Forestry and Fisheries (DAFF) has pointed out that species identified in Knysna earlier this year, like many other dinoflagellates, produces spores which sink to the bottom where they remain until conditions are once again suitable for rapid growth.
Professor Allanson’s report to the Knysna Pollution Committee made up of the Knysna Muncipality, SANParks, Eden District Municipality and the Knysna Basin Project, has suggested a need to set up a monitoring unit to be responsible for recording sea temperature and dinoflagellate abundance during the year. He sees a need for a nutrient analysis on water samples every month. Such action would provide data that could alert the team to changes in cell density and contribute to a coastwise survey which would provide early warnings of bloom events and assist the industry dependent upon live oysters.
Boilerplate: Nitrogen and phosphate levels are key water quality indicators in estuaries. They have a significant direct or indirect impact on plant growth, oxygen concentrations, water clarity and others.
Nitrogen serves as a primary limiting nutrient. Nitrogen levels control primary production. If the system is supplied with high levels of nitrogen, algal blooms will occur. Higher concentrations of nitrogen support less biodiversity.
***A peat is an accumulation of partially decayed vegetation or organic matter that is unique to natural areas called peatlands or mires.