Our recent Friday talk in the Fernkloof Hall took us off the hills and into our coastal waters.
Dr Nuette Gordon, a marine biologist with Abagold, set the scene by emphasizing the influence the two major currents, the warm Agulhas and the cold Benguela have on our coastal ecosystem. We are familiar with the large brown kelps and the smaller red and green seaweeds but less aware of the other primary producers, the microalgae or phytoplankton. Their basic requirements, as with all plant life, are temperature, light and nutrients. Their density and diversity within the water column are sensitive indicators of varying ecological conditions.
A “red tide” is named for the discolouration of the water due to particular microalgae which have distinctive photosynthetic pigments. Some of these produce toxins. As the water is not always red and has nothing to do with tides, the preferred term to use is Harmful Algal Bloom (HAB). Algal blooms are seasonal. If various factors such as warm currents and increased nutrients combine a HAB can occur within days. Walker Bay is particularly vulnerable in this regard, being in the mixing area of the Agulhas and Benguela currents.
A variety of planktonic diatoms and dinoflagellates can be involved and the toxins themselves vary in their effect on organisms and people. The impact of HABs on an ecosystem is however more far reaching. An extended foodchain may feel the effect of the toxins and many other organisms will suffer from the depletion of oxygen in the water due to bacterial decomposition.
The economic impacts on such industries as fishing, tourism and aquaculture are well known. The 2017 HAB had a very serious impact on the abalone industry with a stock loss of 30% and a reduction in growth of 35% from June to December 2017. This has led to intensive examination of all husbandry and monitoring systems at Abagold. Within the research and development of new technologies are programmes leading to early warning of sea conditions and algal blooms. The National Oceans and Coastal Information Management System (OCIMS) and the CSIR are working closely with Abagold Hermanus.
Red Tides are a natural occurrence. However, the maintenance of natural filters (wetlands and estuaries), the reduction of fertiliser run-off, and even coastal planning (waste-water discharge, etc) are all to some extent, within our control. With climate change, incidences of HABs are likely to increase and consequently global effort is required to mitigate against the effects of increased carbon dioxide and greenhouse emissions.