We continue to have a superb time living on the beach and feeling a part of each day’s natural history. In the morning, we can tell which way the wind is blowing based on the location of the steady stream of cormorants traveling from guano platform to ocean. A million of them in a long line, they tend to fly into the wind. We have not yet determined the cues that cause them, on some days, to gather as a huge black blanket on the beach before heading out to sea. One day last week, we witnessed semelparous reproduction: A particular type of flying insect has been accumulating in ever greater numbers – Mr Klein calls them midges, and they are mosquito-sized, with no bite or sting, but a very dark lipid stain when smushed, highly attracted to light. One morning we found thousands of them stuck to the windows of Hotel California, interspersed with spiral egg cases about a cm long, surrounded in gelatinous mucus. By the end of the day, it was clear the insects had made the wrong choice for egg-laying: midges and mucus had all dried up. And we haven’t had to be nearly as vigilant about closing doors before turning on lights since then. Seasonal changes are also evident in the colors on the beach at low tide. When we arrived, the intertidal zone was red with Gracilariopsis (this identification tentative, but coincides with the monograph on Namibian algae published in the 90s). Now much of this has been reduced to perennial holdfasts, with a few female plants bumpy with carposporangia, and the intertidal zone is green with a flush of ulvoids. It’s somewhat embarrassing to admit that we have not yet determined the identity of the less than half dozen species of terrestrial plants living alongside us in the coastal desert. But, we know a bit about their natural history: Many of them accumulate mounds of wind-blown sand, where gerbils and ants then tunnel for security (and presumably food). Since our arrival here, many of these plants have been subtly flowering, more so on the down-wind (N) side. We can also see the dried remnants of annual plants that apparently completed their whole life cycle in the spring before we arrived.
We know that the tide has dropped, leaving dislodged mussels stranded on the beach, when we see the gulls flying up, dropping a large Perna, then following it to the ground to see if it has broken. The introduced clam in the salt pond, Ruditapes decussata (from the Channel Islands), was recently decimated by birds – hundreds of large shells lay broken around along the road. Alan has not been particularly impressed by gulls with such dysfunctional bills that they can’t even get into a dislodged bivalve, but Mr Klein says they’re actually quite smart: along areas of the coast with no hard substrate for dropping and cracking bivalves, they simply place Donax clams on the sand dunes until they gape from the heat and desiccation. While the gulls go for the large Perna mussels, turnstones seem to love the small Semimytilus. These have been washing up in clumps on the beach recently, probably dislodged by waves as the mussel beds have become thick, no longer attached to rock but to an accumulated layer of sand of several cm. It’s easy to see the pattern of patches within the mussel beds at low tide, and we suspect that, given the fast growth and small size of this mussel species, it would be possible to watch patch dynamics over months, rather than the years required by Paine and Levin on the Washington coast!
On the subject of mussels, I spent one illuminating afternoon looking for boring organisms in Perna. The idea came from our desire to begin testing top-down control of Polydora in the salt pond: what really caused the polychaete to essentially disappear in 2004? Was it isopods? Nemerteans? Since we have found so few Polydora in oyster shells – an infection rate of about 2% - we wondered if we could use spionids in mussel shells as a surrogate, then test to see if either isopods or nemerteans caused mortality. We know from walking along the beach that the wrack is full of bored mussels (you know, riddled with holes. You can’t be the other kind of bored when you’re dead!).
So, on a fair to middling low tide, JR walked down to the rocks by the Salt works, dressed in my normal Namibian field gear: bathing suit, wrap-around skirt, Crocs, wide-brimmed hat, long sleeves (it’s either that or a lot of sunscreen). The first thing I noticed was that the upper limit of Perna was just barely above the waves, which is not unusual given the rather small tidal amplitude, just a bit over 1 m. The second thing I noticed was that Perna at its upper limit is not bored, but instead seems to be sand-scoured except at its growing edge, sometimes to the point of having a concave outer shell surface. Finally, I found a few large mussels rolling around in a tidepool that had apparently been dislodged from lower down: they were covered with erect bryozoan epiphytes, red tufts of algae, and obviously bored. I put these in a bit of water in my bucket and carried them off hopefully to the ‘scopes at the hatchery, then spent the next 3 hours noticing a third thing: Most of the eroded burrows in Perna are full of phoronids!
For those of you not completely versed in marine biodiversity, I’ll simply state that Polydora is a polychaete annelid, a segmented worm, in a family characterized by the presence of two long palps on the head. I think I saw 2 long palps once in 15 shells, but was unable to extract any more – and in any case it may not even have been a boring spionid, but rather one that builds its tube in sediment. In contrast, phoronids are in an entirely separate invertebrate phylum. They are soft-bodied, unsegmented worms, with a horseshoe-shaped ring of tentacles on their head – this headdress made them quite unmistakable as soon as I found a shell that still contained live individuals. But then the next question: Did the phoronids make the tubes, or just occupy someone else’s burrow? Our satellite internet access at the beach came in handy once again, as I was able to search on “shell-boring phoronid” and learn that one of the 17 species of phoronids IN THE WORLD – and the smallest one, at that – makes burrows in mollusc shells. All the evidence points towards Namibian subtidal Perna perna full of Phoronis ovalis. It has been reported from a different Perna species in New Zealand, as well as from abalone in Chile. As far as we can tell from beach wrack, only one of the 4 mussel species on this coast hosts Phoronis ovalis, and we gather they are not a problem in aquaculture here. I guess that is good news for the oyster growers, but it puts another hold on our quest to discover the mystery of the missing Polydora.
Another quest we have set ourselves is a better understanding of Venerupis corrugatus, the native littleneck or steamer clam on this coast. We heard early on that this clam was ubiquitous, and indeed we’ve found it in mussel beds, intermixed with intertidal polychaetes, and washed up next to the Walvis Bay yacht club. Most impressively, we saw tiny (1-2 mm) individuals at incredible densities fouling the oyster culture gear in Walvis Bay, apparently a recent recruitment event at exactly the same time that so many oysters were dying in March! Two weeks later, the oyster gear coming out of Walvis Bay had clams around a cm long. This suggested to us that the native clam might be particularly well-adapted to survive low oxygen conditions and grow rapidly, perhaps an untapped aquaculture option! So, we tasted some “big” clams (they seem to get not much larger than 3 cm) with one of our oyster-growing friends and can now pronounce them delicious. How about a new market for Benguela clams?
Well, even though Venerupis corrugatus seems to weather much of what nature dishes out to it, it’s not very resistant to science… or perhaps to the blunders of curious scientists. We collected around 3000 of them in late March from oyster gear coming out of Walvis Bay. They probably got a little bit of initial mistreatment that was not our fault: a freshwater rinse, and a 1-hour car ride in a small tub of water. Then, we distributed the clams into 3 sand or gravel-filled trays and watched them burrow in – at least most of them. We had to leave for Windhoek soon after that, so anchored the trays in a salt pond canal… that reached nearly 30C due to a series of bright, hot days. Half of the clams died. When we returned from Windhoek, we placed the trays back in the hatchery tanks, where another 50% died over several days. Then, we anchored the trays in a different, cooler part of the salt pond canal, just where the water is pumped in from the ocean. Over the next few days, the trays silted up and sank, with another half of the clams dying. However, by this time the surviving clams averaged 15 mm, and we had found live clams throughout the canal that had recruited and grown on their own. This gave us the perfect opportunity to set up our first experiment (as opposed to simply measuring conditions in different locations): we planted out the surviving clams into PVC rings embedded in the sediment, then put bird/fish exclosures around half and cage “controls” (just 2 sides) around the others. Because greater flamingos feed on invertebrates in the sediment, and a variety of waders (curlews, stilts, sandpipers, lapwings) probe for food, we think that these cages may allow us to document their ecological impact on infaunal communities. It’s so exciting to do a flamingo exclosure! And so nice not to have to accommodate 3-meter tides (as in any tideflat exclosure experiment in Willapa Bay) – in fact, the water barely goes up and down depending on how vigorously the pumps are working at the inlet.This experiment went up about 3 weeks ago as a “pilot” to see what would happen to structures – 2 exclosures were apparently trampled by birds, perhaps invisible to them on a dark night; and 1 exclosure (so far) has been gnawed by a hyena! Perfect evidence of the importance of replication.
Showing posts with label Natural history. Show all posts
Showing posts with label Natural history. Show all posts
Friday, May 2, 2008
Monday, February 4, 2008
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!
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!
Monday, January 28, 2008
Rainy season
On the coast of Washington, a popular topic of conversation is the weather – when will the rain stop? Here in Namibia, the question is when will they start? The common weather pattern is for intermittent rain showers to begin in November, with fairly steady rains between January and March. In 2006, the summer rains were apparently quite heavy: There is an amazing set of pictures displayed in the Post Office, showing flooded roads, parks, houses, vehicles. “Rivers” that most years are literally streams of sand were cascading water. And of course lovely shots of clouds and rainbows. In contrast, 2007 was a drought year (the first drought of the millennium), and 2008 is also unfolding rather drily. We experienced our first rainstorm Jan. 5 – an easily-visible discrete patch of torrential downpour, straight grey lines between clouds and brown hills. We watched it come miles away, as we splashed in the Olympia swimming pool. First, the clouds gather together. Then, the wind picks up to a howl, as the air is forced away from distant rain. Then, large drops begin to fall, and finally the curtain arrives. Or not. The rain is remarkably patchy, pouring in one neighborhood while leaving another dry. The rains tend to fall in the evening, and the lightning after dark can be spectacular from our high perch, for instance crackling down in a jagged bolt or illuminating the grey rain against the black sky. Katie has declared that she doesn’t like lightning, But she knows that her parents find it fascinating!
In this dry country, the rains serve as the limiting resource for production, so their arrival is very important. In the past several weeks, we have witnessed the power of moisture. Recall, in week 2, that our drive to the coast was gauntleted by warthogs and baboons. Less than half of the trees had any leaves on them. Last week, on the same drive at the same time of day, we observed zero warthogs and baboons (no statistics needed). On our return trip (1/27), almost all trees had leaves, and the highway was gauntleted by flowers – yellow ones like buttercups, purple ones like lupine, and white ones like lilies. Pools of standing water have accumulated. Presumably, the animals have plenty of food and forage deep into the bush and no longer need to take advantage of roadside vegetation. Even the rocky, sandy piles around our apartment building have sprouted small acacias, blooming and fruiting so fast that we already know this particular species produces very prickly fruits!
The country looks green. Okay, you say, you’ve been away from rainy western Washington so long that your “green meter” has re-set. Anything that’s not simply red sand looks positively verdant. This is probably somewhat true, but we also have enough semi-quantitative evidence to know that we’re witnessing a change in season. The dripping 34C heat of our arrival has moderated to the mid-20s (but wet clothes still dry on the line in a matter of hours!). Similarly, our first days here were entirely cloud-free, just a brilliant blue day-sky and incredible stars at night. We’ve hardly seen any stars recently. Bare trees are leafing out and flowering; seeds are germinating in previously bare patches of ground. Where we were accustomed to patches of tall brown clumps of grass, we now see green new growth (and so do the cows, who will undoubtedly fill out a bit). To top it all off, we awoke Friday morning on the coast to the sound of water dripping through the roof! The coast typically gets 2 mm of rain annually, and this one rain shower must have exceeded the average – not to mention the capacity of the roof. (We were at a B&B, just waking up to go explore some rocky intertidal sites.)
In this dry country, the rains serve as the limiting resource for production, so their arrival is very important. In the past several weeks, we have witnessed the power of moisture. Recall, in week 2, that our drive to the coast was gauntleted by warthogs and baboons. Less than half of the trees had any leaves on them. Last week, on the same drive at the same time of day, we observed zero warthogs and baboons (no statistics needed). On our return trip (1/27), almost all trees had leaves, and the highway was gauntleted by flowers – yellow ones like buttercups, purple ones like lupine, and white ones like lilies. Pools of standing water have accumulated. Presumably, the animals have plenty of food and forage deep into the bush and no longer need to take advantage of roadside vegetation. Even the rocky, sandy piles around our apartment building have sprouted small acacias, blooming and fruiting so fast that we already know this particular species produces very prickly fruits!
The country looks green. Okay, you say, you’ve been away from rainy western Washington so long that your “green meter” has re-set. Anything that’s not simply red sand looks positively verdant. This is probably somewhat true, but we also have enough semi-quantitative evidence to know that we’re witnessing a change in season. The dripping 34C heat of our arrival has moderated to the mid-20s (but wet clothes still dry on the line in a matter of hours!). Similarly, our first days here were entirely cloud-free, just a brilliant blue day-sky and incredible stars at night. We’ve hardly seen any stars recently. Bare trees are leafing out and flowering; seeds are germinating in previously bare patches of ground. Where we were accustomed to patches of tall brown clumps of grass, we now see green new growth (and so do the cows, who will undoubtedly fill out a bit). To top it all off, we awoke Friday morning on the coast to the sound of water dripping through the roof! The coast typically gets 2 mm of rain annually, and this one rain shower must have exceeded the average – not to mention the capacity of the roof. (We were at a B&B, just waking up to go explore some rocky intertidal sites.)
Daan Viljoen nature reserve
1/19/08: The area around Windhoek is dotted with game reserves demarcated by fences – double-high if there is actually interest in keeping many of the antelope inside (or predators such as leopards and cheetahs outside). Most of the game reserves are private, although perfectly happy to have tourists visit, but our first excursion was to a reserve run by NWR (Namibian Wildlife Refuges?). On a bumpy, rocky, steep road precariously navigated by Corolla, we saw zebra ( a whole herd, plus one grazing right by the road), kudu (twisted horns), oryx (straight horns), hartebeest (ringed horns), wildebeest (and calves), warthog (and piglets), 7 giraffes so close that each one exceeded the field of view of our binoculars, banded mongooses (mongeese?), and several birds. Alan said: “Pinch me. I guess I’m in Africa.” But the reserve is proof that even native animals can overgraze an area (despite its size): the fence-line was clear because of the hazy green ground cover of new grass outside.
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