Date: 23 April 2013
Summer diving in the Atlantic can be incredible, but only after a couple of days of convincing southeasterly wind have pushed the surface water offshore and allowed it to be replaced by crystal clear water from the deep ocean. This process is called upwelling, and on South Africa’s west coast it occurs at a rate of up to 30 metres per day (compared to an average of less than 5-10 metres per day elsewhere in the world).
The powerful upwelling system supports very abundant fisheries, because in the water from the deep sea – which is so clear when it arrives at our coast – are many nutrients (decaying marine debris, for example) that provides abundant food for planktonic micro organisms. These micro organisms are the bottom of a food chain that extends up to the fish that are caught in the west coast fisheries.
As soon as the nutrients from the deep are brought up into shallow water, plankton starts to grow and blossom in the light of the sun, feeding on these nutrients. Phytoplankton (plant plankton) or algae blooms can turn the water in the Atlantic into pea soup in a matter of hours. The water feels thick, like cough syrup, and contains strings of algae that can cause irritation to one’s skin. This process of algae blooming starts as soon as the wind stops and the sun shines, which is why there’s such a small window of time during which Atlantic diving is good after a south easterly wind in Cape Town. After a day of hot sun, the algae is having a party.
On this particular occasion there had been a brief south easter – fairly strong but not for more than a day and a half – followed by a hot, sunny day. We decided to take our chances on the SS Maori. When we approached Maori Bay we could see a lot of foam on the water, which is a bad sign. The foam is created from the proteins and carbohydrates released by algal blooms as they break down. It is also these proteins that make the water feel viscous at times.
It turned out that the visibility was not, as the foam warned us, that great – perhaps four metres. The water was very green, as you can see in the photo above. While the divers were in the water, I watched the amount of foam in Maori Bay grow and grow. It happens really fast.
We decided to move away from Maori Bay for our second dive, and went across to the base of Chapmans Peak to dive at Die Josie. Clare took this photo from Chapmans Peak (she had to leave early) of Seahorse at the dive site; you can just see the boat as a speck opposite the rocks sticking out in the middle of the photo.
We had better visibility at Die Josie, but by the time we left the algal bloom was closing in and the Atlantic diving was finished for the moment.
For a nifty picture of a growing algal bloom taken underwater, check out the photo of me and Christo at the safety stop at Monty’s Pinnacles. The algal bloom starts on the surface and works its way downwards; in this example it had spread through the first five metres. We were half in and half out of the green water. There are some photos of an algal bloom that had spread through the first 20 metres of water in Clare’s post on Sea Point Ridge Pinnacles.
