Ok, so there had to be a video. Here are some of the goose barnacles (Lepas testudinata) that I found on Noordhoek beach last month. Bear in mind that they are out of their natural habitat (which is sea water) and were struggling in the sun. (I did try to drag some of the kelp back to the sea, but the pieces were huge and high up on the sand.) This group of barnacles was quite frisky, and shows you what a raft of kelp colonised by goose barnacles might look like if you swam underneath it.
A (lovely, rain-bringing) onshore wind left great rafts of kelp all over Noordhoek beach one weekend in mid May. Finding anything of substance on this beach is unusual; it’s on an exposed piece of coastline and all but the most robust objects are dashed to pieces before they arrive on the sand. Seeing all the washed up kelp also reminded me that frequenting the beaches inside False Bay, that are daily cleaned of washed up kelp by the City of Cape Town, is liable to give one a skewed idea of just how much kelp naturally washes up on the sand.
Kelp stipe covered in goose barnacles
This time, there was kelp, and lots of it. Several of the pieces of kelp had been colonised by goose barnacles. There are several species of goose barnacle that occur off South Africa’s coast, but these ones are Lepas testudinata. They are incredibly strange looking animals, and some of them were still alive and writhing slowly in the drying sun.
In parts of the world (I’m looking at you, Iberian peninsula), goose barnacles are an expensive delicacy. I have nothing to say about that.
Goose barnacles, with my paw for scale
Lepas testudinata larvae most often attach to free-floating pieces of kelp (Ecklonia maxima) and plastic debris, which is why you have probably never seen these mesmerisingly gross-looking creatures while on a dive. In the picture below, you can see that they’re attached to the bottom of a kelp holdfast, where it would ordinarily attach to the rock. This shows that they attached after the kelp broke off.
A kelp holdfast encrusted with goose barnacles
Each barnacle is possessed of a long fleshy peduncle, or stalk, which attaches to the kelp holdfast, stipe or fronds. On the end of the peduncle is a carapace (shell) made up of five separate pieces. The large part of the barnacle on the end of the peduncle (what you’d think of as its body), covered by the carapace, is called the capitulum. The apparatus that the barnacle uses for feeding – essentially six pairs of hairy legs – reside inside the carapace, along with the mouth. There’s some more detail and a nice diagram at this link. If you are familiar with other kinds of barnacles – the volcano-shaped ones that live on rocks, ships, whales and piers for example, then most of this (except the peduncle) should sound familiar to you.
Lepas testudinata goose barnacles
Research done around South Africa’s coast (published here) by Otto Whitehead, Aiden Biccard and Charles Griffiths, identified the marked preference of Lepas testudinata for attaching to kelp. The researchers surveyed a selection of beaches around South Africa’s coast, from the west coast of the Cape Peninsula up to northern KwaZulu Natal, between June and October 2009. When they found goose barnacles washed up, they recorded the species of barnacle, the type of material they were attached to, the dimensions of the object, and its location. They also estimated the number of barnacles in each colony they found.
Lepas testudinata was the species they found most commonly, of the six species in total that they identified along the area of coast that was surveyed. (There’s a nice picture of the six species in their paper, which I used to identify the ones I found.) This species of goose barnacle was found to prefer kelp, as mentioned, and also tended to colonise large objects compared to the other species (this could, of course, be because pieces of kelp are usually larger than items such as bits of plastic, glass, feathers, and shells that some other species prefer).
Kelp fronds with goose barnacles
Lepas testudinata was the only species of goose barnacle that the researchers regularly found to form colonies comprising more than 1,000 individuals. It is also the only species of goose barnacle recorded by the survey that is only found in temperate (cooler) waters, which happens to be where kelp is found, too.
The researchers note that the goose barnacles of the Lepas testudinata species that they found on kelp seemed to have exceptionally long peduncles, some more than 25 centimetres long, and that this seems to differ from what has been previously known about them (which is that they have “short, spiny” peduncles). They suggest that perhaps the variety of Lepas testudinata that colonises kelp may even be a separate species from the one previously described (more research obviously required to ascertain this). You can see from my photographs that the peduncles of the washed up Noordhoek beach goose barnacle colonies are also quite long, some easily 20 centimetres in length.
Clusters of goose barnacles on a kelp stipe
They also found that the increasing prevalence of long-lasting and buoyant plastic marine debris and other anthropogenic objects around our coastline, which some species of goose barnacles preferentially attach to, gives these weird little creatures increased opportunities to form colonies, and to spread to new places. This is one of those interesting phenomena to keep in mind, as humans inexorably alter the environment. Some creatures will benefit in strange ways from warming oceans, and others will find new homes in the garbage we leave lying around.
Boat dives on Sunday or Monday / Shore dives on Monday (conditions dependent)
Traditionally Easter is a difficult time for diving. Many people are away and the weather does not always play ball. Add to this the traffic congestion from the Two Oceans marathonon Saturday… This weekend we may dive from Hout Bay on Sunday or Monday, or perhaps shore dives from Long Beach, wind dependent.
We are out tomorrow on a full day private charter but I do think Long Beach will be a good option if you feel like shore diving.
We send well wishes to everyone celebrating a religious observance this weekend. For those of you who celebrate Easter, here’s an egg for you:
Catshark egg on a sea fan
Plastic and Water
On the marathon topic, watch this video and see how the Two Oceans Aquarium and Old Mutual are teaming up to reduce the use and impact of single use plastic, and learn about the aquarium’s turtle rehabilitation program.
This article on how to responsibly stockpile (or just purchase) bottled water, is very helpful if you’re working on water security at home, but don’t want to contribute to an environmental apocalypse.
Capetonians are familiar with the tea-coloured water that runs in our mountain streams. Most people know that the brown colour comes from tannins, leached naturally from the indigenous fynbos vegetation. Perhaps less well known is the reason for the brown water that is sometimes seen in the surf zone along Muizenberg beach, stretching all the way to Strandfontein, Monwabisi and beyond.
Tea-coloured water at Muizenberg
The most frequent explanations that are offered on social media are, of course, pollution, “raw sewage”, and the like. This is not the reason for the brown water, and it does not necessarily impact the water’s safety or healthfulness for humans to swim in.
Like False Bay’s famous colour fronts, the reason for the brown waves at Muizenberg beach turns out to have much to do with the topography of False Bay, particularly of the kilometres-long beach at its head (Muizenberg-Strandfontein-Macassar-Monwabisi), and something called a diatom.
View of Muizenberg showing patches of brown water
Diatoms
Diatoms are a type of phytoplankton (plant plankton or microalgae). They are single celled, usually symmetrically shaped organisms that multiply by dividing in half at a constant rate. Their cell walls are made of silica, SiO2. Chicken keepers and gardeners may be familiar with diatomaceous earth – this is made up of the fossilised shells of ancient diatoms.
Diatoms are what are called primary producers or autotrophs, meaning that they generate organic material from carbon dioxide and other inorganic nutrients (for example nitrates and phosphates), through the process of photosynthesis, which uses light as an energy source. Primary producers sit at the base of the food chain and all life relies on them, directly or indirectly. Everything else produces organic material from other organic material (such as diatoms).
I am telling you all about diatoms because the brown water at Muizenberg contains an accumulation of a diatom that you can call Anaulus australis Drebes et Schultz the first time you mention it, but usually just Anaulus australis, or Anaulus for short. There are several members of the genus Anaulus, but usually just one tends to be dominant at each beach where these accumulations occur, and Anaulus australis is the main species found along the South African coast.
Analaus are pillow-shaped diatoms. If you wanted to see what an individual Anaulus diatom looked like, you’d use a microscope, but when enough of them are in one place, they can be seen to change the colour of the water. There’s a picture of them under a microscope at the bottom of this webpage (they also occur in Brazil). They occur at beaches with particular topograhical characteristics, which explains why you haven’t seen them at Camps Bay, Kogel Bay, or Scarborough.
At hospitable beaches, the diatoms are always there, spending much of the time lying dormant in the sand behind the surf zone. A proportion of the diatom population is able to survive for relatively long periods (estimated to be more than two months) like this, in the dark on the seabed, not photosynthesising or dividing, until the correct meteorological conditions arise for an accumulation. But first – what sorts of beaches are hospitable to Anaulus?
a nutrient source close to the surf zone (often an unconfined aquifer overlaid by a dune field)
Muizenberg and Strandfontein beach tick all these boxes. The beach stretches from Surfers Corner all the way across the top of False Bay to Monwabisi, a distance of over 20 kilometres. It is a high energy beach, meaning that it is exposed to large waves and strong winds, and is not protected by any offshore features such as sandbars or headlands that might reduce the force of the waves. Rip currents do occur at the beach, and both these and the exceptionally wide surf zone – wider during south easterly winds in summer – can be observed from the mountainside on Boyes Drive. (A rip current is like a hidden river flowing out to sea from the beach. The Sydney Morning Herald has an excellent visual explainer of rip currents here.)
The head of False Bay where Muizenberg is situated is incredibly nutrient-rich, much of it thanks to urbanisation. The canalised Zandvlei estuary – the only vaguely functional one on False Bay’s coast – is situated a short distance down the beach, and supplies nitrates, phosphates and other nutrients to the surf zone. Many of these nutrients are technically pollutants, added to the river further upstream. The Cape Flats Waste Water Treatment plant at Strandfontein also discharges 200 million litres of treated water per day (under normal, non-drought circumstances) via a canal onto Strandfontein beach. This is essentially an artificial estuary for Zeekoevlei. This waste water has spent some time working its way through the settlement ponds at Strandfontein, but is nevertheless rich in ammonia and other nutrients, and Anaulus accumulations are a very common sight in the surf around this discharge point. The dunes that run along Baden Powell drive overlay a high water table, and groundwater seepage – specially during times of heavy rainfall – may also leach nutrients out of the ground and into the surf zone.
Meteorological conditions
The meteorological conditions required for an Anaulus accumulation involve strong wind and a large swell. These act together to create rough sea conditions, which stir up the dormant diatoms from the ocean floor. The diatoms adhere to air bubbles in the surf zone, staying suspended in the water column, which is when you would notice the water turning brown. Exposed to light, they awaken from their dormant state and start to photosynthesise, take up nutrients, divide and multiply. The presence of rip currents creates an onshore-offshore flow all along the beach. This forms a semi-closed ecosystem, and the diatoms are essentially trapped in gyres in the waves. Longshore currents that run parallel to the beach transport Anaulus cells out of the surf zone at one end, and bring fresh (sea)water in at the other end of the beach.
It may seem surprising that anything manages to accumulate in the waves of a beach, but the surf zone is actually quite retentive, meaning that things that end up there often tend to stay there. (Incidentally, this is why it’s a terrible idea to discharge the byproduct of reverse osmosis seawater desalination – a super-salty brine – into the surf zone. It must be discharged offshore so that it can disperse and mix with the surrounding water.)
Diatoms in the surf zone at Muizenberg
You’ll notice that, contrary to what you may have seen when large amounts of plankton are under discussion, I’ve been using the word “accumulation” instead of “bloom” to talk about Anaulus. This is deliberate, because of the constant presence and constant rate of division of the diatoms. When the water goes brown, it doesn’t mean that Anaulus is suddenly multiplying faster than usual. It means that it’s all been gathered together in patches, is exposed to light and therefore photosynthesising (at its usual steady rate), and is thus more visible than it was when it was lying on the ocean floor.
Anaulus at Muizenberg in November 2017
The human factor
You may also be thinking that everything I’ve said about the nutrients that Anaulus requires to survive and thrive points to the fact that humans – and pollution – are ultimately responsible for these brown-water plankton accumulations at Muizenberg. Well yes, in a way. But accumulations of Anaulus australis and related species have been observed and documented for well over 100 years at suitable beaches around the world, and are a natural phenomenon.
Yes, we are providing more nutrients to the False Bay diatom population than they would otherwise have received without human settlement in the greater Cape Town area, but these accumulations would likely occur regardless. They are certainly more intense now than they would have been in the past, but estuaries are nutrient-rich locations even when not surrounded by a large city. Furthermore, the water table is high on the Cape Flats, which would supply nutrients to the surf zone regardless of whether humans lived nearby.
Anaulus is in fact performing a vital and useful function by mopping up the excess nutrients that the city discharges in the ocean. The mass of diatoms – primary producers – also provides a food source to bivalves such as mussels, and other invertebrates. We can be grateful that the excess nutrients that urbanisation directs towards the ocean at the head of False Bay leads only to accumulations of harmless diatoms, rather than to frequent occurrences of harmful algal blooms that can kill marine life and exacerbate respiratory problems in humans.
Muizenberg during a diatom aggregation
Sources
Most of the original scientific study on surf zone diatoms in South Africa was done by a group of researchers (primarily M Talbot, Eileen Campbell and Guy Bate) from the University of Port Elizabeth, working at the Sundays River Beach in the Eastern Cape. I did quite a bit of reading to research this post, but you can start with this paper for a description of the topographical characteristics of beaches where surf zone diatoms accumulate. The first few chapters of this Masters thesis also provide a good overall survey of what is known about surf zone diatoms.
Putting knowledge into practice
Not every instance of brown, foamy water at the beach will be an Anaulus accumulation. On the west coast of South Africa, for example, there are no beaches where Anaulus occurs, but you may see brownish foam that is the result of heavy wave action frothing up organic matter in the surf (nothing sinister – there is a lot of organic material in the ocean). A clue to help you distinguish diatom accumulations from other brown-water phenomena – apart from running through the checklist of required beach characteristics above – is that an Anaulus accumulation doesn’t stretch much beyond the back of the surf zone. If the brown water stretches beyond the furthest row of waves, it’s probably something else. (And this seems like an apposite time to remind you that sewage looks whitish-grey, not brown, when it’s pumped out into the ocean.)
The number of beaches worldwide where surf zone diatom accumulations occur is so small – less than 100 – that Odebrecht et al could enumerate them in a 2013 paper. I hope this helps to convince you that the brown water at Muizenberg beach (and beyond) is something special and interesting, not to be feared. Go surfing!
Early in November I attended an information session at the Kalk Bay Community Centre, where the City of Cape Town announced that they will be trialling an environmentally friendly cleaning process on five of the 19 tidal pools on the 260 kilometres of Cape Town’s coastline managed by the City. This coast stretches from Silwerstroom on the West Coast to Kogel Bay on the eastern shores of False Bay.
St James tidal pool
The presentation was made by team members from the City’s Recreation and Parks department, which – among other things – is responsible for beaches, outdoor signage, ablutions, lifesaving, environmental education, and administration of Blue Flag status for the beaches and marinas that earn it. This department is also responsible for the tidal pools. (Incidentally the City’s assortment of safe seawater bathing facilities includes two of the largest tidal pools in the southern hemisphere, at Monwabisi and Strand.)
Until now, the City would use chlorine to clean the walls (top and sides) and steps in the tidal pools. The cleaning would be done after draining the pool completely. This year, a supply chain management issue meant that there was no cleaning of the tidal pools between July and November. During this time, regular swimmers (some of them members of the Sea-Change project) noticed that marine life flourished in the pools, and engaged with the City to try to find a way to keep the tidal pools safe but also to preserve the diversity of marine species that had been thriving in the pools during the cleaning hiatus. Safety, of course, is why they are cleaned: slippery, algae-covered steps are dangerous.
The tidal pool at Millers Point
It was agreed that five of the pools – St James, Dalebrook, Wooley’s Pool, and the two pools at Kalk Bay station – would be subject to a trial of a new, environmentally friendly cleaning regimen. These pools are relatively close together in the north western corner of False Bay. The aim is still to ensure that the tops of the pool’s walls and steps are not slippery, and thus safe for bathers. But a second aim has been added by the City, which is to ensure the environmental integrity of the pools.
Under the new cleaning protocol, the following will be done:
the pools will be drained only when necessary, and only as far as is required to reach areas that are covered by water and in need of cleaning (for example, the steps at Dalebrook)
animals in harm’s way will be relocated
excess kelp and sea urchins will be removed from the pools
the tops of the walls and steps will be scraped to remove algae (the sides of the walls used to be scraped too, but this will no longer take place)
environmentally friendly chemicals will be used to remove the algal residue after scraping – no more chlorine and no more whitewashing!
All of the above means that the pools will be ready for use by the public immediately after cleaning, in contrast to the old protocol, which renders the pool unusable for a period after the cleaning crew has chlorined it.
I’ve asked the City for more information about the drainage procedure, and for more information about the earth-friendly chemicals that the cleaning contractor will use, but with no response so far. (If I get one I’ll obviously update this post.)
Buffels Bay tidal pool inside the Cape Point section of Table Mountain National Park
Many of the City of Cape Town’s tidal pools fall within the Table Mountain National Park Marine Protected Area, and it therefore makes perfect sense to aim to protect the animals living in them while maintaining public safety. Dr Maya Pfaff, another speaker at the information session, even suggested that some of the animals that may now thrive in the pools may actually help to keep the water clean. Mussels and feather duster worms filter the water and improve the clarity, algae take up nutrients, and limpets clean algae off the rocks.
Particularly over the festive season, the beaches and tidal pools around Cape Town are extremely busy. This is a wonderful opportunity for thousands of beach-goers to experience both safe swimming and a little bit more of what the ocean has to offer, instead of a sterile, salt-water pool devoid of healthy marine life. Bringing a snorkel and mask with you when next you go swimming will be well rewarded. To see some pictures of the amazing animals – from nudibranchs to a cuttlefish with eggs – in the St James tidal pool, check out Lisa’s instagram profile.
Do you swim regularly in any of the five pools in which the new cleaning regimen is being tested? What do you think about it? If you think that environmentally gentle cleaning of tidal pools is a good idea, what about letting the city know that you appreciate having tidal pools that are both safe and biodiverse. A short message on the City of Cape Town facebook page to say thank you and keep up the good work (and a request to extend it to the other tidal pools) is a good place to start!
You can read a news article about the new cleaning protocol here.
There is little doubt in my mind that diving this weekend will be for the hardcore only. Both Saturday and Sunday will feature howling south easterly winds which will make for rough surface conditions. The forecast seems to imply wind strength capable of affecting both sides of the mountain so there will be very few places to hide.
But wait, there’s more
There are some windless days coming up next week. If you’re a lucky one available for weekday diving next week, let me know your availability and I’ll keep you in the loop regarding plans.
We came across this small biscuit skate (Raja straeleni) while doing a night dive at the jetty in Simon’s Town, on the occasion of Diversnight 2017. They are found in the eastern Atlantic ocean all the way down to 700 metres depth, grow really slowly, and are frequent bycatch from hake trawling operations off the South African coastline. SASSI says don’t buy it. The species is data deficient on the IUCN Red List.
These skates have thorn-like stings along part of their tails, and this one seems to have a whole lot else going on in the tail region which looks as though it would help him camouflage among seaweed. (None of the biscuit skates pictured in our fish identification books have quite such fancy tail-gear.) Also watch how he flicks sand over himself for additional disguise when he stops moving.
Also, they can jump – perhaps a little known talent… Once, while Clare was on duty at the aquarium, a small one leaped right out of the shallow ray pool that used to be next to the touch pool, and landed on the floor. It was a quiet Sunday afternoon. A quick manhandle and he was replaced in his pool (and that exhibit was moved soon after)!
It feels very much as though winter has arrived (minus the rain), mostly in the early mornings. When daytime temperatures rise into the twenties, it’s not totally unpleasant! Sunday is one of those days, and we will launch from Simons Town jetty at 9.30 and 12.30. Both sites will be a maximum depth of 18 metres as I have a bunch of Open Water students to qualify.
Last weekend’s dives were pretty good; the visibility was better further south on Sunday. On Saturday we met an octopus at Long Beach, who tried very hard to persuade us he was a piece of kelp.
Octopus imitating kelp
This week has had a fair whack of wind on and off, and False Bay currently looks pretty grumpy with very little sign of visibility. Hout Bay also looked a little off this morning, but the wind should have improved that.
I will stick my neck out and say we will launch on Saturday, and it needs to be really early as the wind picks up around midday.
Where we will need to wait for a visual check late tomorrow to decide which side of the mountain we will dive. I don’t think the conditions will be great for training dives and we may end up in the cold Atlantic. Either way we meet at 7.30 am at either Hout Bay slipway or False Bay Yacht Club parking in Simons Town. Text me if you want to join in.
Saturday: Launching from Simons Town yacht club at 8.30 am, to be confirmed on Friday afternoon
Atlantis sea fans
The wind has been punishing, but there’s very little blowing tomorrow, which means the visibility could go either way. We will plan launches for Saturday morning, from the yacht club in Simons Town.
I’ll look at the conditions tomorrow afternoon. If you’ve let me know you’re keen to dive, I’ll let YOU know whether we can. So if you want to get wet on Saturday, please be in touch.
