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Solving the green tide puzzle
Decaying mats of sea lettuce were noticed at Lands End, Leisure Island and elsewhere along the Knysna Estuary during the peak summer months this year.
KNYSNA NEWS - "Until there is persistent and determined effort by society and its collective components to tighten up its lax environmental morals we can expect repetition of these [green algal] blooms and suffer their impacts on the biology of the estuary and its environmental quality. We have been warned!"
These are the word of Professor Brian Allanson of the Knysna Basin Project in response to the occurrence in recent months of a 'green tide', in the form of an over-abundance of a seaweed commonly known as sea lettuce.
Allanson explained that the extensive red tide of microplankton of last summer, was this summer replaced by a macroplankton plant, Ulva, made up of sheets of single cells floating indiscriminately in the water and distributed in the lower basin of the estuary.
While this plant has been present in the the lower estuary for many years, this summer has seen an explosion of the plant which, at low tide, covered the sand and muddy shores with slippery green sheets.
Especially affected by the overwhelming abundance of the plant, were Land's End, leisure Island, Costa Sarda and upper Ashmead where fragments of the plant sheets are picked up by the tide and spread over the adjacent salt marsh.
The dead fragments of the plant sheets take on the appearance of 'loo' paper, much to the consternation of residents and visitors who then point accusing fingers at the Knysna Wastewater Treatment Works (WWTW).
Allanson explained that he had to take into consideration all the available data to ascertain what precipitated the Knysna bloom, which did not occur in other estuaries, and also to identify the specific local conditions that triggered the rapid and sustained growth of the plant. This data, when reviewed alongside literature indicating that eutrophication (a process where water bodies receive excess nutrients, especially nitrate and phosphorus) is associated with blooms of the macro-algae Ulva, give a strong indication of the local conditions conducive to the current 'green tide'.
Allanson's research pointed him into the the likely sources of nutrients, nitrogen and phosphorus compounds in the catchment of the Knysna Estuary: the final effluent of the WWTW, storm water flow, agricultural seepage and an oceanic phenomenon called upwelling. Upwelling occurs when seasonal South-Easterly winds (usually associated with summer) contribute to conditions that result in deep, colder nutrient-dense water rising to replace warmer surface water near the shore. Much of the nutrients, especially nitrate, are retained in the lower estuary, despite the movement of the tides.
Anther contributor to high nutrient levels is the summer rain increasing the flow and nitrate loading in the Knysna River. Add these changes to the higher WWTW outflow during the summer, and it is easy to anticipate high quantities of nitrate in the estuary.
Allanson believes that these factors and the upwelling during the summer 2014 and 2015 provided the trigger that set in motion ideal conditions for algal bloom.
"It would appear that we have joined the international community from USA to Australia faced with the task of reducing the enriching nutrients of nitrogen and phosphorus hidden in our wastes and so freely thrown into the estuaries around which we live," says Allanson.
According to SANParks spokesperson for the Garden Route National Park Nandi Mgwadlamba, authorities in Knysna (SANParks, Knysna Municipality, Eden District Municipality and the Knysna Basin Project) are actively investigating causes of the recent algal bloom. There is also suspicion is that the same nutrients suspected of causing a red tide in January 2014, lingered long enough to trigger another bloom, of sea lettuce.
"SANParks already has a great working relationship with farmers through various projects including the removal of invasive alien species on private land around the park (buffer zones). Through such projects landowners have made a significant contribution to conserving fynbos and mountain catchment areas."
Johan de Klerk, area manager of Knysna, says SANParks scientists will also be brought in to assist. "It’s all hands on deck."
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