Dry sea lettuce as it appeared earlier this year, behind Costa Sarda and Leisure Isle.
KNYSNA NEWS - In the past few weeks several people have approached SANParks to report 'pollution' in the Knysna Estuary, but instead they point out sea lettuce, an algae.
Nandi Mgwadlamba, spokesperson for SANParks, explained that sea lettuce (Ulva lactuca) is a form of green algae that comes from the sea (often seen free-floating) and can be consumed by herbivorous fish, sea animals, slugs and others. "It is bright green in colour but can be white or black when dry."
Professor Brian Allanson of the Knysna Basin Project released a report earlier in the year explaining how dead and decaying algae push up the greyish, oxidized algae to the surface. According to the report he also suspected that an increased level of hydrogen and phosphate levels in the water which had led to last year's red tide, had led to this year’s visible sea lettuce.
Mgwadlamba said: "The surfacing of sea lettuce to its current level is a natural phenomenon and not related to the water treatment works at all. In fact as the tide rises, fragments of these sheets are lifted and float to the surface from where they are transported onto the salt marsh under the stress of winds.
With ebb tide the fragments are caught on top of the salt marsh where they dry out and create the appearance of toilet paper. Sea Lettuce is not pollution, it is a form of green algae, which comes from the sea."
Green seaweed, sheet-like in appearance. It ranges in size from 6 inches to 2 feet.
It can be found in estuarine and coastal areas where there is salty water.
Scientists say it is most common in waters that are rich in nutrients.
Can be free-floating in the water or attached to rocks, pilings and other hard surfaces.
Often found washed up on the shore.
Potential causes of an algal bloom
Sea lettuce is a naturally occurring algae that i strongly influenced by uncontrollable factors such as wind, tides and coastal currents which affect water temperature and nutrient levels. In most instances, Westerly winds drive coastal water offshore. When this occurs, deeper water from the ocean wells up to replace coastal water, bringing along cooler nutrient-rich water which enters the estuary.
Human sources of nutrients are cited as other contributors in global salty water and estuarine systems.
Would such sea lettuce be edible?
Sea lettuce can be used to make compost when mixed with other compostable material.
Anyone needing more information can contact either SANParks at 044 302 5600 or Professor Allanson of the Knysna Basin Project at firstname.lastname@example.org.
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