I’ve been out several times this week and we’ve found clean, green and brown water in False Bay during this week’s dives. We’ll do double tank dives on both days of the weekend, starting early. I haven’t picked sites for the weekend as we will go looking for clean water when we launch. If you want to be on board, you know what to do.
Things to do
A Pint of Science is a series of evening science talks from Monday to Wednesday next week, combined with the exciting prospect, for enthusiasts, of beer. The talks will be held at the Empire Cafe in Muizenberg and Sgt Pepper in Long Street. Tickets are R35. Visit the website and see if anything takes your fancy.
We had a small gremlin interfere with our newsletter timing yesterday and for this we apologise.
We had decent conditions on Wednesday with dives in the vicinity of Roman Rock. There was a dirty layer on the surface, but underneath there was clear water with visibility of about 12 metres. Today we are taking visitors from Port Elizabeth to explore some local dive sites.
We ventured out to Long Beach last weekend and found the conditions to be less than ideal: surgy, especially for a 10 year old student, and with 3-4 m visibility. Winter diving is around the corner and we look forward to improved conditions.
The forecast for the weekend is not that great. The visibility is reasonable, but there is currently a 17 second period swell in False Bay and that won’t do much to maintain the viz. Saturday looks to be the best option for boating, but an early start is needed as the wind picks up around midday. I will make the call late tomorrow afternoon as to whether we have launch weather or not. Text, email, call, or Whatsapp me if you want to dive.
This is also a good opportunity to remember what a privilege it is to dive with the cowsharks by viewing a video Jerrel recently compiled from footage taken at the site just over a year ago. We have a cowshark diving protocol as a reminder of how we approach this amazing dive.
Dive gear sale
Monty of Scuba Culture is having a stock clearance sale, so if you’re in need of a hose, a cutting tool, or something else shiny or cool, contact him to find out what he’s got available.
Please remember to bring your permit to dive in a marine protected area with you when you come for a dive. Ideally they should be on the boat with you when you come diving (as that’s where they’ll get checked). If you don’t have a permit, the post office can help. We also have temporary permits available, valid for a month, but not very cost effective.
Saturday & Sunday: Boat or shore dives on both days
We spent Saturday morning shadowing a swimmer doing the Freedom Swim from Robben Island to Big Bay… Around 7 kilometres in 14 degree water that at times looked like brown onion soup filled with jellyfish. The swimmers are a brave and dedicated bunch and I admire them. Our swimmer did the crossing in just over two hours, but some of the swimsuit folks spent 5 hours in the water, bravely swimming into a humping current.
The forecast looks like a piece of cake for the weekend… Light winds, not much swell and warm sunny skies. False Bay, however, looks a little off, colour-wise, and there are large colour fronts scattered across much of the bay. There is no certainty on where they will go with the light winds over the next couple of days so it is going to be a matter of making the “call to action” early each morning.
I have both shore dive and boat dive students ready to go, so will do either shore or boat based on what it looks like when we wake up, early, on both Saturday and Sunday.
Text, Whatsapp, email or carrier pigeon you desires for the weekend and I will add you to the early morning wake up call list…
If ever you had the urge to show a friend the beauty of the underwater world that you enjoy, now is your chance. Haul them off to the Two Oceans Aquarium and show them some of the stunning creatures captured on camera at a wide range of dive sites scattered along Cape Town’s shores. The photo exhibition runs for two months and is included in your entry fee to the aquarium. Read more about it here.
Summer weather patterns and wind direction are sure taking their time heading off. This far into March and we have not yet had any spectacular diving. Well it has to change sometime, but I don’t think the time is this weekend.
Saturday looks good from a wind point of view, however the swell is supposed to peak at 4 metres on Friday afternoon with a 14 second period. That will make for very surgy diving, and even deep dives will feel the 14 seconds. The swells don’t always arrive as predicted but this evening just before sunset it was already at 3 metres and starting to ruffle a few feathers.
Sadly, once again, I think we are set for a dry weekend. Sunday has more wind and less swell, and visibility will most likely be good out of Hout Bay but 15 knots of wind, 2-3 metre swells will not make for great diving. I have a sand dune at home so if you are at a loose end you are welcome to pop in and build a castle.
The water is really doing its best to limit diving right now. It is either murky green or way too windy for decent diving on a regular basis. At some point it will change, but I doubt it will change in time for the weekend. There are areas in False Bay that are clean and free of red tide, but you need to go and look for them.
Saturday has pretty strong south easterly wind and a fair amount of swell, Sunday is not looking much better. I have students who require shore and boat dives so I will launch at 6.00am onSaturday for a quick look and double tank dive, and plan the weekend around what we find.
If you’d like to be notified of diving plans that may materialise if we find suitable conditions, let me know!
It would seem from the forecast that it is a open and shut case of where to go and what to do this weekend. To be honest I am not too sure of the right thing to do! Both the Atlantic and False Bay are a colour that does not exactly inspire one to throw on a wetsuit.
The wind has blown more easterly and north easterly today than was expected, so it will not have done much for the visibility on either side. Sunday is out of the question as the forecast is for humping south easter, so that leaves Saturday.
I am launching from Hout Bay tomorrow afternoon and will have a better idea of whether it is clean enough for Saturday. The other option is shore diving at Long Beach. I reckon that there is about a strong chance that the water won’t be clean enough for any diving at all, though.
Today we took a load of surfers out to Dungeons. Amongst them were a few first timers at Dungeons and the happy cheers and bear hugs when they caught their first serious Dungeons roller was a sight worth seeing. Dungeons is a spectacular sight so if you haven’t been there do so at least once in your life.
Back to diving… We dived the Atlantic last weekend. Maori Bay was cold and clean-ish. Visibility was around 10 metres but then the temperature was also in the single digits. Die Josie was a lot cleaner and just as cold.
On Sunday we dived in False Bay – doing Search and Recovery for an Advanced course in 2 m visibility makes it a little more realistic!
Well… There is swell and wind in the forecast. The swell was not all that noticeable in False Bay today but was very surf-worthy at Dungeons and Muizenberg today. The wind is forecast at around 30 km/h for both days and for students doing their first boat dives I think it’s not that good an idea. So I have no launches planned for this weekend.
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.