Sunday: Boat dives from Simon’s Town jetty at 10.00 and 12.00, likely to Roman Rock (suitable for Open Water divers)
The weekend looks good for diving. We’ll be boat diving on Sunday. If you want to join us, you know what to do!
Green turtle on the move
Turtles
It’s that time of year when turtle hatchlings get caught up in currents that bring them to Western Cape shores, where it’s too cold for them to survive. If you find one on the beach, please keep it dry and warm, and get it to the Two Oceans Aquarium as soon as possible. Read more about what to do on the aquarium blog here, and here.
Sunday and Monday: Boat dives (location to be decided)
Last weekend we did some boating in Table Bay as a support boat for the Robben Island to Big Bay Freedom Swim. The remnants of the swell on Thursday and Friday was enough to give the swimmers a sizeable challenge with strong currents, choppy surface conditions and very cold patches of water.
Colin swimming across Table Bay
The swell climbed from under 2 metres to a little over 5 metres this morning. This means diving tomorrow is pretty much out, as is diving on Saturday (thanks also to the Two Oceans Marathon). The swell drops off during the day on Saturday so both Sunday and Monday should deliver some reasonable diving conditions.
It is difficult to say whether Hout Bay or False Bay would be better on Sunday and Monday, but I will make that decision late on Saturday afternoon. I have Open Water and Advanced Open Water students so one day is likely to include a deep dive to more than 18 metres.
If you are keen to dive on Sunday or Monday, let me know and I’ll schedule you in!
Colour front seen from Sunny Cove on 27 November 2015
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.
Colour fronts
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.
Colour front in north western False Bay on 13 November 2014
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.
Milky-white water near Swartklip on 29 November 2014
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.
Colour front at Smitswinkel Bay on 24 October 2014
Summary
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.
Scientists have discovered giant eddies, 100 kilometres across, making their leisurely way across the world’s oceans. The rotational effect extends up to a kilometre below the ocean’s surface. They are quite beautiful when visualised (if you were in one you probably wouldn’t notice – they move very slowly). Wired.com covered this discovery in an article last year.
Read the Wired article here. Bear in mind that the colours in the maps represent height of the ocean surface, not temperature!
Flotsametrics and the Floating World- Curtis Ebbesmeyer & Eric Scigliano
Flotsametrics
I found Flotsametrics to be a profound and moving memoir slash ocean science book. Curtis Ebbesmeyer is an oceanographer who cut his teeth in the oil industry, and later moved to private consulting. His late-life interest in beachcombing, and reading the debris he finds in order to chart the paths of ocean currents, was sparked by a question his mother asked him upon reading about a shipment of Nike shoes that had burst out of its container and floated all over the world.
The chapters of Flotsametrics are organised chronologically and by theme, and Ebbesmeyer mingles his life story with revelations about the oceanographic discoveries and projects he was part of at each stage. Cadavers, bath toys, messages in bottles (lots of these), drifting Japanese junks (seriously fascinating!), hockey gloves, sneakers, sea beans, and plain old garbage give up secrets of ocean circulation.
I stopped and re-read several parts of the book, particularly in the chapters that dealt with the death of Ebbesmeyer’s friends and family members. His meditations on releasing his loved ones’ ashes into the ocean gyres are quite beautiful and profound, notwithstanding an alcohol-fuelled incident in which he and a group of friends flush another friend’s ashes down a toilet in Seattle!
Thanks to Rochelle, I came across this article about a shipment of nautical-themed Lego that spilled out of a container 17 years ago, and is washing up (still) in Cornwall. Ebbesmeyer is lending his expertise here, as well. There is a New York Times review of Flotsametricshere, and one from The Guardian here. If you have to choose between Flotsametrics and Moby Duck, which deals with some overlapping themes, I would recommend Flotsametrics a thousand times over. The author actually has something (many things) to say!
Conditions that make for great surfing generally mean poor diving, and vice versa.
The second point above is exactly why reading a book on what makes waves turned out to be an extremely enlightening experience.
Dr Tony Butt lives in Spain on the exposed north Atlantic coast (good waves), but apparently spends the southern winter in Cape Town, surfing Dungeons where possible I imagine! He is a physical oceanographer. In Surf Science he explains where waves come from, and what leads to different kinds of surfing conditions.
His explanation of the origin of the waves that reach the coast starts at the very beginning, with an explanation of the drivers of weather systems. This section caused me to jump up and down with excitement, and to speak many long sentences to long suffering husband Tony. His expository method is easy to follow at each step, and for the first time I feel that I have a grip on . First we build up a mental model of the earth as a stationary globe with no land, and then gradually add rotation about its own axis, rotation around the sun (seasons), and the presence of continents to the model. At each step further understanding of the average world weather patterns (north Atlantic high, roaring forties, and so on) is improved.
The book goes on to cover the propagation of swell, and what happens when waves arrive in shallow water, break, and possibly allow surfing. Butt also deals with water temperature, tides, and rip currents, and concludes with a few chapters on surf forecasting models and how to read them. Throughout the concepts are illustrated with straightforward diagrams, and end-of-chapter boxes contain formulae for the mathematically inclined.
This is a shorter and more richly illustrated book than The Wavewatcher’s Companion, serving a different audience. Both surfers and divers can benefit from Surf Science, as well as the yachting fraternity, NSRI crew members, and anyone else who has an interest in the mechanics of the ocean. This volume has more the flavour of a textbook, with no unnecessary information, anecdote or humourous asides. There is a place for both books!
You can get a copy here or here, or here (if you’re in South Africa).
The Fluid Envelope of Our Planet: How the Study of Ocean Currents Became a Science – Eric L. Mills
The Fluid Envelope of Our Planet
This beautifully-titled book is a slow moving history of physical oceanography, tracing the contributions and occasional theoretical dead ends that, over the decades, have led us to our current understanding of ocean circulation. Eric Mills has a special interest in the history of the marine sciences, and – rather than get bogged down in too much explanation of the underlying processes – focuses here on the history and development of the science. His account is not normative in the sense that I sometimes didn’t know why a particular scientist’s ideas were wrong (not being a particularly hardcore student of physical oceanography), and he concentrates on providing a historical account.
The arrangement of the book traces the history of oceanography around the globe, from Canada to Berlin to Monaco to Scandinavia, and finally to the United States. I didn’t realise just how mathematical the field is. Mills makes clear the immense challenges in doing empirical oceanographic studies of the deep sea, which may be a contributing factor to the rate of development of the theoretical side of the science.
Mills provides interesting detail of the protagonists in his story, lifting them out of history as the often quirky individuals that they were (much as Trevor Norton does in Stars Beneath the Sea – although this is a more serious historical work). I enjoyed the fact that Columbus Iselin, Director of the Woods Hole Oceanographic Institute, complained in 1939 that:
The main difficulty with oceanography is that the Lord made the ocean too big and this is the chief obstacle, which we must use our collective ingenuity to overcome.
Mills’s writing is detailed and a pleasure to read. There are comprehensive references as well as a list of the canonical textbooks in the field, at the end of the book. I’d recommend this book to you if you have a special interest in the history of science.
You can get a copy of this book here or here. South Africans can find it here.
Mapping the Deep: The Extraordinary Story of Ocean Science – Robert Kunzig
Mapping the Deep
Robert Kunzig won the Aventis Science Book of the Year award in 2001 for this book (the 2011 winner was The Wavewatcher’s Companion!). It is an absolutely fantastic piece of science writing, charting the state of the art in ocean science as well as the historical processes that led us to where we are today (or were in 2000).
Kunzig mostly used scientific papers and interviews with the scientists themselves as his primary sources, making complex mechanisms understandable without loss of information in the transmission.
The first few chapters of the book deal with the challenges of finding out what the ocean floor looks like, and of representing it in a useful way. Those beautiful maps of the ocean floor that I pored over in the atlas as a child, or in Sylvia Earle and Linda Glover’s wonderful Ocean: An Illustrated Atlasaren’t as precise as you may think they are – we often repeat the aphorism about how little of the ocean we have explored – but conveniently forget that this also precludes us from making detailed maps of it. The United States Navy probably has the most comprehensive sea floor maps, but they aren’t sharing.
Kunzig devotes several chapters to the type of life found in the ocean – he is not so much concerned with coastal as pelagic life forms, and devotes many fascinating pages to hydrothermal vent communities without once mentioning Robert Ballard. I experienced serious job envy reading about the blue water diving that Bill Hamner did to collect and study pelagic marine organisms such as jellies and plankton.
The influence of humans on the ocean’s environment is the subject of the next few chapters. There is a terrible, compulsively readable chapter about the New England cod fishery that was so convincingly destroyed, even with the assistance of government scientists. The final sections of the book deal with climate change and the ocean.
If you’re in South Africa, get a copy of the book here, otherwise try here or here. This is an updated edition of The Restless Sea, so don’t buy that one!
Journalist Trevor Corson spent two years lobstering off the coast of Maine in the United States, giving him an excellent understanding of the rigours of that (or any) fishery, and its seasonal dynamics. (It also invested him, perhaps too deeply to allow him sufficient objectivity to tell this story, in relationships with the fishermen working there.)
He centres his book on the community of Little Cranberry island off Maine, a ridiculously beautiful island with a small permanent population, sustained by fishing for American lobster. These crustaceans can weigh up to 20 kilograms and are the heaviest in the world. He interweaves stories of the lobstermen at work with stories of the scientists who study the animals, and with details of the life of lobsters. (The title of the book is apt.) His characterisations of all of the individuals who appear in the book are fully formed, and by the end one feels a warm familiarity for many of them.
At the heart of the book is a conflict between scientists employed by the United States government to make recommendations regarding the management of its fisheries, who insisted for years that the lobsters of Maine are overfished, and the lobster fishing community, which has seen catches rising, has implemented their own measures to protect very small and very large lobsters and females (one of which is notching the tails of actively breeding females to indicate that they shouldn’t be kept), and is convinced that the stock is healthy.
Corson makes no secret of his antipathy for the “government scientists” who predicted the fishery’s disappearance and his disdain for their fears, but fails to explain why they are so convinced the stock is in danger of collapse. New England, the primary range of the American lobster, is the site of the devastating failure of a massive and lucrative cod fishery (in short, they were all eaten), and perhaps this failure informs the scientists concerned with excessive caution.
The Maine lobster fishery has, since 1995, evolved a system of co-management between regulations set by the government to manage the fishery, and management systems imposed and maintained by the fishermen themselves. This is an innovative and effective way of avoiding a “tragedy of the commons” (here’s Garrett Hardin’s original paper) that can arise when a common resource is free to be exploited by everyone. The Maine lobster fishery is presented at the end of Elinor Ostrom’s 1998 paper on coping with tragedies of the commons, as an example of how decentralised (or polycentric) management systems can work for common resources such as fisheries, clean water, or clean air. With very few government patrols, the fishery is kept regulated largely by its participants – the fishermen themselves. This is a fascinating success story. James Acheson, an economic anthropologist, has studied this fishery extensively. Some of his papers can be found here and here. Attempts to manage marine resources in a centralised, top down manner don’t work (c.f. abalone in South Africa – or all our fisheries, for that matter), and co-management solutions in which industry participates with government seem to be a very viable and effective solution.
The scientists Corson profiles are renegades (at least in comparison to the government’s people), who favour experiments and field work (involving scuba dives, ROVs and many laboratory tanks of lobster) over calculations and theory, and who back up the claims of the fishermen that the lobster population is healthy. I’m afraid the demonising of the government scientists didn’t sit well with me, and perhaps with a little more explanation or attempt to understand their perspective, Corson could have presented a more nuanced take on the matter. Lobster catches continue to boom in Maine, but because of larval lobsters’ dependence on ocean currents and water temperatures to usher in each phase of their development, global warming and the resulting changes in ocean currents and ocean temperatures could impact this fishery adversely in the future.
The details about lobster biology and behaviour that Corson presents are fascinating, and, I suspect, an excellent layperson’s substitute for wading through all the scientific literature on the life and times of the American lobster. He explains how the scientists reached their conclusions, too, giving detailed descriptions of experiments involving superglue, plastic tube, and long term observation of both wild and captive lobsters. I think (hope) a lot of what I learned here is generalisable to our own West coast rock lobsters: as I pointed out to Tony while I was explaining to him how male American lobsters urinate on one another from their heads while they fight vigorously over territory and mates, we don’t often observe lobsters behaving when we encounter them on dives. We sometimes see them eating, but most often they scoot backwards into a hiding place, or just gently wave their antennae at us. They have intricate social and reproductive lives, not to mention a dizzyingly complex life cycle (the difficulty of artificially replicating the conditions required for multiple larval transformations are part of the reason why they aren’t farmed like abalone). I will certainly be watching more closely next time I bump into one of these remarkable crustaceans.
Having turned our attention to the mighty Gulf Stream current yesterday, let’s think about the kinds of phenomena that the warm current can give rise to. As weather systems such as thunderstorms move over warm water, they draw water into the lower atmosphere by causing the seawater to evaporate. This water vapour is pushed up, leaving space for more, and the storm grows in power as long as it stays over the warm sea. The Gulf Stream is a warm current in a cold ocean, and is thus a major generator of hurricanes. NOAA explains it properly.
Outside Magazine published a detailed analysis of the sinking of the Bounty during Hurricane Sandy in late 2012. The Bountywas a replica of HMS Bounty, an eighteenth century three masted sailing ship. She was built in 1960 for a Marlon Brando movie about the mutiny that led to the establishment of the pit of despair that is the settlement on Pitcairn Island. (Why pit of despair? Read this.) After the movie was completed the vessel passed through several owners, all of whom struggled to finance the constant repairs and maintenance required on a wooden ship of that scale. Her story concluded in the glare of the world’s media attention, as the ship struggled to stay upright while sailing into the eye of Hurricane Sandy (for reasons unclear).
As the drama of the Bounty’s final hours unfolded on CNN and the Weather Channel, seamen and landlubbers alike were asking the same question: what was a square-rigged ship doing in the middle of a hurricane—a storm that had been forecast for days?
…
But the full answer to why the Bounty sank was much more complex than a captain’s rash decision. It was a story decades in the making, a veritable opera of near misses and fantastic schemes involving a dogged captain, a fiercely loyal crew, and an owner who was looking to sell.
This is a story of a nautical disaster told with a strong focus on the personalities involved. It’s quite a terrifying read.
