Week 4 trip to the coast

1/23-27/08: We stayed Wednesday and Thursday nights at the Seagull B&B in Swakopmund, Friday and Saturday nights at Kleines Nest in Walvis Bay. Trip objective – learn as much as we could about the coastal organisms by exploring multiple sites at low tide. To that end, we also purchased 8 field guides (actually, only one for marine organisms, called Two Oceans reflecting its focus on the eastern and western sides of South Africa). Altogether, we looked at about 3 places around Swakopmund, all rocky intertidal, 2 places in the Walvis Bay lagoon, and two sandy beaches on either side of the “mouth” (it’s a wide one) of Walvis Bay. On our final day, we joined a lot of tourists on a 4-hour speedboat tour of Walvis Bay by water, including the Cape Fur Seal colony, derelict/confiscated Russian fishing boat, lighthouse, close pass by two cargo ships waiting to go into port, hanging oyster culture, and “largest table in the world” built almost 100 years ago to collect guano. Along the way, we saw Benguela endemic dolphins, bottlenose dolphins, and a mola mola. Three rather tame (but still very large with sharp teeth!) seals leaped onto the boat to get a treat and a scratch, pelicans flew alongside to catch fish that were tossed to them, and gulls and cormorants took more fish out of hand. A lot of close encounters with charismatic megafauna, but not much environmental education or interpretation. The skipper served sparkling wine, juice, quiche, brotchen, and oysters on the half shell. By the way, Walvis is pronounced Valfish.

As temporary residents in Namibia, we are thankful for the low biodiversity of the coast. Two Oceans is a thick book of vertebrates, invertebrates, and algae, but most of the species are restricted to the eastern side of the Cape, reflecting oceanic connections to biodiverse regions of the south Pacific. Fewer species occupy the western side, and fewer still extend their range past the Orange River into Namibia. Why would this be? Possibly, the alternation of warm Angola and cold Benguela currents make conditions too variable, or sulfur “blooms” kill off many species, but likely there is a huge effect of substrate: the Namibian coastline is 60-80% sand (we’ve seen variable reports), with small pockets of sand-scoured rock interspersed. Sandy beaches are notoriously species-poor, so perhaps there is just not enough rock around to support many species. But, again, this low diversity is a happy occurrence for newly-arrived marine ecologists. It’s also interesting to hear that essentially no Tatoosh-type experimental research has been carried out on the rocky shores – the scientists here are too busy working on economic problems of fisheries and aquaculture. The only rocky intertidal “fisheries” (artisenal or otherwise) are for rock lobsters, which we saw being collected by guys wading out in wetsuits into the surf and reaching into crevices in the rock.
Standard rocky intertidal zonation in Namibia:
Highest = 1. Afrolittorina – definitely hides during hot days and low tides, but similar in size and shape to Washington’s littorine snails. Small grazers of epilithic scum.
2. Chthamalus – just one barnacle species that’s obvious. All the individuals we observed were similarly-sized, <5 mm, which suggests pulsed recruitment.
2. Siphonaria – a limpet-like grazer that can reach rather large size (3 cm). Small ones seemed to be spatially separate from the larger ones. Possibly they have cleared out some bare space among the barnacles by bull-dozing, and we wondered what would happen to the “bare” rock if Siphonaria were removed. This species apparently does well even in sand-scoured areas: we saw rock-tips poking up out of the sand that were covered with Siphonaria.
3. Semimytilus – mussel that does not often exceed 2 cm in length. In sandy areas, it grew into hummocks, catching sand between the byssal threads and the rock. This seems not to be a very stable existence, as large areas of the bed were sometimes obviously washed away, leaving only the border where mussels were actually connected directly to the rock.
3. Limpets – intermixed with Semimytilus. Some of the classic species of ecological fame in South Africa are missing here (arganvillea, cochlear), and as yet we have a hard time distinguishing the two large species of the Namibian coast (Scutellastra granularis, Cymbula granatina). Both have scalloped edges to the shell and occur at high density. A third species (Cymbula miniata) we have found as shells in beach drift but have not noticed live – probably a bit too low to be safely accessible given the week’s tides and waves.
4. Perna perna – a much larger mussel, probably the competitive dominant. Hey! There’s a competitively dominant mussel here, too! (But what is it outcompeting? There’s not much else.) (And is there a keystone predator that keeps mussels in check? Seastars are decidedly absent; the two common crabs (Pilumnoides rubus, Plagusia chabrus) are herbivorous; perhaps spiny lobsters could set the lower limit.)
In the mussel zone, we saw anemones with their columns pimpled with shell hash (Aulactinia reynaudi), a fuzzy red alga reminiscent of Endocladia on Washington’s shores, with the same ability to grow on rock or epiphytically on mussel shells, and occasional clumps of Gunnarea capensis, a tubeworm.
Interspersed among Semimytilus and Perna, which tend to be brown, are two species of black-shelled mussel, particularly interesting because one is native and one not. From Two Oceans, we understand that it might be possible to tell them apart based on whether there are pits (Mytilus galloprovincialis) or not (Choromytilus meridionalis) in the ligament near the hinge on the inside of the shell. Obviously, this characteristic does not make it possible to identify live individuals. We are still looking for reliable shape/ color trends. We have now picked up probably 3 dozen shells from beach drift, and they all seem to have pits. So, either M. galloprovincialis is more common than we’ve been led to believe, or we don’t really have a “pits” clue.
In our quest to find predators on this shore, we kept a close look out for predatory snails such as whelks. We found a few individuals at just one of the rocky sites we visited, and these appear to be Burnupena – they look a lot like Nucella on Washington’s coast but their diet does not involve barnacles or mussels; they’re scavengers. Washed up in the drift, we’ve seen shells of Nucella dubia, which is indeed a predator and supposedly broadly distributed in the intertidal zone. We’ll have to continue to keep an eye out for these in real life! The beach drift has been full of clumps of egg capsules that clearly derive from a neogastropod such as these snails. The capsules must pop off the rock after the young snails emerge, as they generally have 1-mm holes at the top that have served as an escape hatch. Given the number of egg capsules in the drift, whoever laid them must be very abundant. It’s odd and fun and challenging to be in a place where a question as simple as “who laid those?” is entirely unknown to us.
5. Below the mussels, red algae go crazy! A quick look at 2 Oceans shows why: whereas in most other taxa, there are just a handful of species that occur on the west coast, among reds, 43 species are likely to occur in Namibia, and just 23 are not. (I based my count on distribution maps included with each species in the book, and I assumed that anything shown to reach the Orange River, which serves as the South Africa-Nambia border, could also extend north. We’ve already found many such species on intertidal rocks in Namibia, and we’re not sure if the distribution maps are curtailed because sampling has been lower in Namibia, or because habitat is relatively rare.) We haven’t really even attempted to begin identifying them yet. Most sites were a rather jumbled mess of rocks and boulders, so zonation was a bit obscure. But one site had a shallow bench that showed a dramatic line between higher fleshy reds and lower erect pink coralline algae.
6. And, of course, Namibia’s only kelp species – Laminaria pallida – is anchored in the low intertidal with blades that float to the water surface so are obvious from a distance. It is legal to collect beach-drift Laminaria to use as a feed for abalone, but harvest of living kelp is unlikely ever to be allowed.
One more comment about the rocky intertidal zone: there is not very much bare space. Species interactions, therefore, look like they could be very strong. We were surprised at how much life covered the rocks, since a coastal hydrodynamics hypothesis might suggest that larvae are entrained in upwelled water that moves away from the coast, thus leading to recruitment limitation and very few new arrivals on the shore. What might account for the discrepancy? It is possible that many of the species do not have planktonic larvae but instead produce local offspring (but we know this is not true for barnacles and mussels). It appears that, over 100s of km, this stretch of coastline is a retention zone even under strong, persistent upwelling, perhaps because the upwelling occurs slightly farther offshore (is the continental shelf wider here?) This retention zone explains the high productivity that drops to the bottom, accumulating and decomposing until unknown forces lead to catastrophic release of hydrogen sulfide. It may also explain the high cover we observed on rocks.

Wave-exposed sandy beaches in Namibia:
On the outside of the sandy peninsula that constitutes the western side of Walvis Bay, we encountered surf clams (Donax serra) at incredible abundance. Anywhere we dug over a wide range from about lower high water to the waves, we found several clams in each 10x10 cm area. An incredibly detailed study of these clams was done as a PhD thesis, including mark-recapture for density, growth and recruitment at multiple sites, chemical composition, and condition index, so we can take this species off the To Do list! It has been fun to compare the western peninsula of Walvis Bay with the western peninsula of Willapa Bay – Willapa’s is longer (but not by much), taller and wider. Walvis’ has no trees (but of course not, it’s the desert) and only extends several meters above the water. The lighthouse appears to be sort of in the middle of the peninsula, which is extending north at a rate of about 20 m per year. (Willapa’s is not growing quite so fast.)

Walvis Bay lagoon:
Imagine a north-facing bay, red sand dunes on the east side, low-lying sand peninsula on the west, divided into three parts. The northern third holds the port, with a channel dredged for 7 km through a shallow bay (20-30 m depth). The middle third is a functional lagoon that dries almost entirely at low tide. Water has to move in and out on the western edge, near the port, because a sand spit extends from the peninsula on the west. The southern third is cut off by an extensive network of salt ponds. The Kuiseb River historically entered here, in the one-in-10 years that enough water fell in Namibia to reach the ocean. This delta is now cut off from the lagoon by the salt ponds and road. We spent most of our time walking out in the middle section, and the boat ride occurred in the northern section. The southern section looked and smelled terrible – foam and feathers and salt and sulfurous stench and black goo under the feathery crust wherever a disturbance went through. The most amazing part of the middle section is… FLAMINGOS. Thousands of them, both the pale Greater Flamingos, which show bright pink and black on their wings when they fly, and the more dramatically pink all over Lesser Flamingos. The Walvis Bay population is supposed to represent 60-80% (respectively?) of the whole number in southern Africa. I think we were lucky that the rains were late, because they should all fly inland to breed as soon as sufficient freshwater is available in pans. The greater flamingos do an amazing dance to feed: they alternately pick up their feet, stomping in a circle with their head submerged upside down. The sediments that they suspend are filtered for small invertebrates and algae. They leave donut-shaped depressions about a half-meter wide, and the tideflats are packed with these donuts. Of course, we don’t know how long an individual donut lasts once it’s been stomped out, but the overall sense is one of extraordinary disturbance by birds. Indeed, 2 Oceans says that the presence of flamingos dramatically reduces the abundance of infaunal organisms. (But it would still be fun to do a flamingo exclosure: the swimming beach, perhaps an unreplicated flamingo “exclosure” where we saw no donuts, had a high abundance of tube-building worms and heart clams.) The lesser flamingos filter feed cyanobacteria (giving them their pink color), so are not responsible for sediment disturbance. Flamingos seem to feed in water that is about mid-calf deep. Consequently, as the tide ebbs and flows, the flamingos also seem to flow like water back and forth across the lagoon. At low tide, we were able to see many birds quite close on the eastern side of the lagoon, but at high tide, most had congregated in a vast pink horizon on the other side of the water.

At one point, after I’d just seen a pigeon, I exclaimed, “Wow, we haven’t seen very many birds in Walvis Bay!” And then I laughed. Land-birds are indeed uncommon, particularly compared to the busy, melodic tufts of fluff we’ve seen inland. (Many of the birds in Windhoek look strangely familiar – because they include the masked weaverbirds and longtailed grey capped mousebirds housed in the African Savannah aviary at the Woodland Park Zoo in Seattle. I guess they do just fine in close proximity to humans! English sparrows are also in Namibia.) But Walvis Bay is also full of birds: cormorants, pelicans, grey herons, small waders such as stilts, avocets, curlews, large and small gulls, large and small terns, even some ducks. But the flamingos are definitely the highlight. They would still be pink and beautiful in a zoo, but not nearly so ecologically interesting!