The Green Tide in Knysna Estuary

The Green Tide in Knysna Estuary

The Green Tide in Knysna Estuary – Last summer we experienced a Red Tide made up of millions of single-celled plants which float suspended in the water column enjoying the warm and nutrient rich surface waters of the coastal seas and the Knysna Estuary. Fortunately this specific Tide was not overtly toxic so animal life of the estuary and particularly those that feed on the microplankton were not seriously affected.

This summer the microplankton was replaced by a macroplankton plant, Ulva, a sea weed, commonly known as ‘sea lettuce’. The plant is made up of sheets single cells which float indiscriminately in the water and are distributed with the tide in the lower basin of the estuary. They, in common with the microplankton, require light and large quantities of nutrients to grow, eventually bloom.

While this plant has been present throughout the lower estuary including the Ashmead Channel for many years it had not previously overwhelmed the intertidal shores or adjacent water. This summer has seen an explosion of the plant which at low tide covered the sand and muddy shores with slippery green sheets of sea lettuce.

During high summer the intertidal shore at Lands End, Leisure Isle, was covered by rows and rows of decaying plants thrown up by tides and winds – an unpleasant sight which prevented serious use of the beach. A similar accumulation of plant biomass occurred in upper Ashmead and fragments of the plant sheets are picked up by the tide and spread over the adjacent saltmarsh. The dead fragments of the plant take on the appearance of ‘loo’ paper much to the consternation of the public, blaming the sewage works for discharging raw sewage over the marshes. Understanding the biology of this plant provides an alternative and correct explanation of this feature of summer marshes.

Decaying mats of sea lettuce at Costa Sarda (left) and Lands End (right) during summer 2015.

I asked the question “What was so particular about this summer in the estuary that precipitated this bloom?” It did not bloom in other estuaries. This suggested that some feature of the summer estuary triggered the rapid and sustained growth of the plant.

A review of the ecological literature pointed to the world-wide and frequent occurrence of a green macro-algal floating plant that belongs to the genus Ulva in the family Ulvales, commonly known as ‘sea lettuce’. In all cases where blooms of the macro-algae occurred nutrient enrichment of the estuary waters is involved.

Careful review of the likely sources of nutrients, nitrogen and phosphorus compounds in the catchment of the Knysna Estuary pointed to the final effluent of the WWTW (sewage works), storm water flow, agricultural seepage and an oceanic phenomenon, ‘upwelling’ whereby seasonal south easterly winds induce an offshore movement of warm near shore surface water. This results in deep colder and nutrient rich water rising from deeper levels to take its place. In the Knysna Estuary this is identified by quite sudden drops in water temperature in the marine embayment – the strongly tidal area of the marine embayment downstream of the rail bridge.

This summer temperature drops of up to 12ª C were recorded that restricted swimmers from enjoying a summer activity. While this condition is relieved as the tide changes from flood to ebb, the overall effect is to sustain the nutrients, specially nitrate, in the lower estuary at levels that were higher than during periods when upwelling is less likely, e.g.winter.

It is likely that early summer rains increased the flow in the Knysna River and were accompanied by increases in nitrate loading. When these changes are added to the increases in WWTW outflow during summer, it is not unreasonable to expect nitrate quantities to exceed those of past summers. These changes linked to coastal upwelling during the summer of 2014/15, provided a trigger that set conditions for green algal growth to reach bloom proportions and accumulate in deep piles of decaying plants on at Costa Sarda, Ashmead and Lands End, Leisure Isle.

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.

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 blooms and suffer their impacts on the biology of the Estuary and its environmental quality. We have been warned !!

B R Allanson – The Knysna Basin Project