This will be a quick newsletter as we are downwind of a raging vegetation fire and the smoke is unbelievably thick. Even our tortoises are going to spend the night indoors. Firefighters, many of whom are volunteers, have been have a rough time of late and I admire their endurance, skills and management of these situations.
We had really good conditions in Hout Bay last weekend with clean and cold being the main factor. 8 degrees celcius is chilly but the viz was easy 15 metres.
The south easter has humped all week long and False Bay is a little messy. The visibility in the yacht basin today was actually very good but I think the rest of False Bay would have been messy. The yacht basin is protected from the south easter by the naval harbour.
The forecast leads me to believe that Hout Bay will be good on Saturday and False Bay on Sunday. I would also like to slip in a night dive on Saturday if the conditions permit.
Perhaps you have wondered what causes the patterns of strange coloured water in False Bay during the summer months. Perhaps you have dived in it, and wondered why sometimes you can’t see your hand in front of your face! Wonder no more – I am here to help.
Frequent visitors to and residents of the shores of False Bay will observe that at certain times of the year, the ocean is marked by bands and arcs of sharply contrasting coloured water. This phenomenon is known as a colour front. In oceanography, a front is the interface or boundary between two separate masses of water. In this case, the water masses are easy to discern, because they are of different colours. There are usually other characteristics of the water on each side of the front that differ, too. Fronts are either convergent (the water masses are moving towards each other) or divergent. The presence of marine debris (like pieces of kelp) at the front boundary suggests that it is convergent.
Causes of colour fronts in False Bay
Prior to 2005, there was much conjecture about the causes of these fronts (including the usual pollution bugbear), but little evidence to support any of the theories. By sampling, the fronts were found not to be caused by pollution, or by plankton blooms in the surf zone. It was known that a colour front was most likely to occur in False Bay after a period of southerly or south easterly wind lasting a few days. October and November seem to be prime months for the phenomenon.
When a large, obvious colour front arose near Simon’s Town in November 2005 with milky green water on one side, and darker blue-green water on the other, researchers from UCT and IMT sprang into action, sampling the water on each side of the boundary so that they could measure its characteristics. Speed is of the essence in these situations; colour fronts can disappear quickly. The one in the picture below is busy decaying – notice the smudged boundary.
Measurements revealed that the milky green water overlaid the clearer, bluer water, down to a depth of 11-12 metres (this will vary from front to front). The milky water did not extend to the ocean floor. Scuba divers around the Cape Peninsula will be familiar with the experience of diving through two or more layers of water, with varying turbidity (clarity) and temperature! (Here is picture of Tony and Christo diving near Oudekraal in the Atlantic that shows what the boundary between two layers of water can look like.)
The researchers found that the milky coloured greenish water was full of fine, almost neutrally buoyant particles of calcium-rich sediment. The green-blue water contained much less calcium, but relatively more silicon, which would suggest the presence of diatoms (a kind of phytoplankton – you can think of them as teeny tiny plant-like organisms) or sand in the water. The origins of the calcium-enriched sediment in the milky water are interesting: one source is from the shallows (less than 30 metres deep) of north western corner of False Bay, where the ocean floor is made up of rocks that are rich in calcium carbonate (such calcrete and limestone), some areas covered by a thin layer of sand.
The second probable origin for the particles of calcium-rich material is the interface between the sea and the land at the northern end of False Bay. The cliffs at Wolfgat/Swartklip at the head of the bay are made of calcrete, and at Swartklip the beach narrows to the extent that the cliffs erode directly into the water when the sea is high. Strong southerly winds create a wide (of the order of one kilometre) surf zone at Muizenberg and Strandfontein; a spring tide also adds to ideal conditions for the generation of a colour front.
The temperature of the milky water was found to be slightly (0.4 degrees Celcius) higher than the green-blue water. This measurement will also vary from front to front. The researchers speculate that the temperature difference could be because the milky water originated in the surf zone, which is shallower and therefore warmer, or because the high concentration of suspended particles in the milky water caused greater absorption of heat from the sun.
Here’s the tl;dr: strong southerly and/or south easterly winds, perhaps coupled with spring tide conditions, set up a very wide surf zone along the northern end of False Bay, which disturbs the sediment on the ocean bottom and drives the waves further up the beach than usual. Particles of buoyant calcium carbonate from the sea floor and eroded from the cliffs at Swartklip are lifted up into the water column, changing its colour to a milky-green shade. Wind-driven circulation patterns in the bay push the front from its original location in a southerly direction, towards Simon’s Town.
What to do?
Contrary to what your friends on social media may claim, not all colour changes in the ocean around Cape Town can be attributed to a giant sewerage plume. Hardly any of them can, in fact. In summer, the reason for the ocean looking green, red or even brown is likely to do with a plankton bloom of some description, or related to suspended sediments (as in this case) or other naturally arising material in the water. Instead of using this as an opportunity to become hysterical on the internet, how about celebrating the incredibly dynamic system that we can observe, living near the ocean? Drive up a mountain next to the ocean and take in the spectacle from on high. Dip your face in the water and see what it does to the viz. Take some pictures for posterity. And – if you don’t know what’s causing it – try to find and question someone who does know, like a scientist, or consult a good non-fiction book, to find out some facts.
The south easter has been blowing for a while now… Almost non stop. Windy, our wind generator at home who generates about a third of our monthly electricity requirements, has been having such a great time that she almost fell off the roof this week. She brakes herself when the wind reaches 78 km/h, and on several occasions the wind gusts exceeded that.
The Atlantic went from clean and cold, to cleaner and colder. The water colour in False Bay is not that bad but the surface conditions are pretty hectic. Fortunately I have had a lot of students requiring pool time so we have at least been getting wet.
There is wind and some swell for the weekend but I do believe that there is a possibility that Hout Bay will be good on Sunday. I have a bunch of students so will plan for both morning and afternoon dives, wind and swell dependent.
If the wind is driving you crazy too, check out the suggestions for land-based marine-related entertainment in last week’s newsletter.
We are back from a trip up north… Really far north – so far north that the temperatures, so I was told, were 0 – 2 degrees Celcius but to me felt like minus 50 degrees. It was great diving in False Bay this week in 22 degree water.
The weekend is a little on the questionable side as the wind and swell forecasts are not promising. I think the best option is going to be Table Bay, and If we go it will be to North and South Paw. I will take a look there tomorrow at the conditions and confirm late afternoon to those on the list if we are diving. Let me know if you’re keen to dive by email, text, or message in a bottle.
The last two months have been what I would call an almost ideal summer.The weather has been terrific and the hectic south easters we had a year ago have not been much of an issue. Whilst we still await the icy crystal clean Atlantic to materialise, the viz has been good more often than not on both sides of the mountain. Roll on January.
The coming weekend’s forecast is almost a carbon copy of last weekend, so we will shore dive tomorrow and have early start on Saturday before the wind arrives. Sunday we will follow a “let’s wait and see” plan as it’s not too clear whether the wind will be as strong as the forecast says it will. That leaves Monday,Tuesday, and Wednesday for us to wrap up diving for 2015.
Taking a break
We will be closed from 24 December until 3 January 2016. If we don’t see you before then, have a merry Christmas and a jolly new year. Be safe and get some rest. We would would like to thank everyone that has dived with us throughout the year. Your support is appreciated and I know I have had a lot of fun, laughs good and bad viz, and an abundance of cake.
Same again is not a phrase we can use for December this year. Last December was appalling by comparison and although we are only 10 days into the month, we have already done twice the number of dives we did in the whole of December last year.
Conditions this past weekend were great and they have been the same all week. The westerly wind today has made things even better and tomorrow and Saturday should be pretty good… In False Bay. The Atlantic looks a little dark and the water temperature is 16 degrees Celcius, which doesn’t herald good viz that side.
We are launching tomorrow and Saturday, but I think Sunday will be a little too windy for diving. The forecast is for it to blow hard from midday on Saturday, so we will launch for a double tanker at 6.00 am on Saturday from Simon’s Town jetty.
Remember our free try dives in the pool until Christmas eve. We have had some bookings and the pool is warm! It’s a great opportunity to introduce your friends and family to scuba diving. Get in touch if you want to bring someone over – booking is essential.
Please have your MPA permits up to date – you can get one for R94.00 at the post office. Take along your ID document.
On the subject of Marine Protected Areas, please read this and send in your comments about the imminent opening of the Tsitsikamma MPA to fishing. Allowing fishing in a marine protected area is a bit of a contradiction in terms, so I encourage you to read about the proposal and let your voice be heard on the subject.
This past week has not really been a diving friendly week. There has been a fair amount of swell on and off and also a decent amount of humping south easterly wind. The weekend shows promise for the Atlantic, which is currently still dirty but is showing signs of improvement. There is a good chance it will be clean by Saturday or Sunday. The swell goes south east, so I think Granger Bay may be a possibility, and Hout Bay will almost certainly work.
Sunday will probably have less viz than Saturday but also a bit less swell, which is important for Hout Bay launches. If by Saturday afternoon the Atlantic is not clean we will dive False Bay instead (warm water is better than cold water in poor visibility!).
Weekend diving has become a scarce commodity of late and the best diving days have fallen in the week. This weekend looks a lot better than last weekend: there is very little swell and no howling winds to deal with. False Bay needs some westerly wind to clean it up, so I am going to choose Sunday as the better option for this weekend’s diving. It’s windier than Saturday but less likely to rain, plus False Bay will have had a bit more time to improve. The Atlantic was crystal clear yesterday but I have my doubts it is going to stay that way for the weekend with the hot day we have just had.
We are night diving on Saturday evening as part of the international Diversnight event, and will meet at Long Beach at around 7.15 pm. The idea is to be in the water at 15 minutes past 8.00 pm (2015). Text me if you are joining, make sure you bring your permit to dive in an MPA, and if you need to rent kit or a cylinder, I need to know by tomorrow (Friday) evening please.
Be sure your permit to dive in an MPA is up to date. It’s particularly important that you bring it on Saturday evening if you plan to dive, so don’t wait until lunchtime on Saturday to look for it!
We had an early start last weekend and launched from Simon’s Town at 7.30 am to get in and out before the wind arrived. Conditions were good and we dived the Atlantis area.
On Tuesday we launched from Hout Bay and despite a few days of south easterly winds the visibility was around 6 metres, but the swell created quite considerable surge. Both dives were around deep Tafelberg Reef and the surge could be felt below 30 metres.
The forecast for the weekend looks a little bleak. Much wind and much swell means slim odds of finding nice diving conditions anywhere. However should conditions change we will keep you posted by text and Whatsapp, and go diving.
Saturday and/or Sunday: Early launches from OPBC (Saturday) / Simon’s Town jetty (Sunday) depending on weather and demand
Last Saturday we launched the boat in Hout Bay for a team of researchers to deploy and retrieve three of the acoustic tracking receivers that listen for tagged sharks and other animals along South Africa’s coastline, providing information about their movements to scientists. The receivers are part the Acoustic Tracking Array Program (ATAP), and you can read more about how it works here.
We launched from Hout Bay again on Tuesday, and the 3 metre swell could be felt at 30 metres on Tafelberg Reef. The visibility was OK, but the water was really cold.
Weekend dive plans
False Bay looks a little messy right now and tomorrow’s wind is not going to help. Today and tomorrow the swell is around 3 metres and goes southerly. Both days of the weekend have strange weather patterns in the forecast.
I reckon Saturday would be best at OPBC and Sunday would be best for False Bay. Wind speed builds mid-morning so either way you are looking at a really early start – and by that I mean launched by 7.00 am. Both days lend themselves to double tanking to avoid the wind.
A 7.00 am launch is not everyone’s cup of tea so I suggest we decide where to go based on who is diving. You can text, Whatsapp, facebook message, email or carrier pigeon your desires to my door if diving is on your mind.