Sulfide eruption!

The past three weeks have been devastating for the oyster growers on Namibia’s coast at Walvis Bay. A large phytoplankton bloom, which turned the water into a thick chocolate of spiky dinoflagellates, was followed by a sulfide eruption, when the toxic gas that accumulates in the sediment as a by-product of anaerobic decomposition bubbled up. Hydrogen sulfide is itself toxic, and it reacts with oxygen in seawater, causing hypoxia. The color of the water turns an almost tropical aquamarine, but accompanied by the stench of rotting eggs. Close on the heels of this sulfide eruption came another phytoplankton bloom. Growers are reporting losses of literally millions of oysters, from 25% to more than 70% of their crops. They have little buffer for these losses, since many of the companies are just 2 or 3 years old, and people were counting on the next harvest to pay off debts from start-up costs. Some companies will probably fold altogether.

Oysters are not the only organisms that suffered from the toxic environmental conditions of the past few weeks. From 3/9 to 3/11 we visited rocky intertidal sites around Swakopmund during reasonably good low tides. Initially, we were so excited by what we saw: spiny lobsters, hundreds of them, wiggling their antennae from under any available intertidal ledge, or just hiding in the algae. “Wow”, I thought, “I’ve never seen so many intertidal predators in one place. This must be what it’s like to visit a “pristine” beach!” To the contrary: The lobsters had gathered there in hordes to escape the low-oxygen conditions of deeper water. And the beaches were hardly pristine, as people descended from above to strip as many of the easily-accessible lobsters as they were able to fit into mesh bags, lunch boxes, or plastic shopping bags. We saw some seasoned lobster-hunters in wet suits and gloves, but also some unusual suspects – businessmen in collar shirts and loafers, squabbling couples – all taking advantage of the coincidence of low tides and low oxygen. All sorts of regulations were violated – including catch limits (people would collect some, then stand in a parking lot to sell what they’d collected, then collect more) and size limits (lobsters are not supposed to be kept below 65 mm carapace length, but few people were bothering to measure, justifying that these lobsters were going to die anyway). Any rock small enough to be lifted had been overturned and not turned back. Solitary antennae washed back and forth in the waves and accumulated in the wrack on the beach, as people latched on to the most obvious, but frail, parts of the lobsters to catch them. Watching the interaction between people and these beautiful, vulnerable, probably keystone creatures sickened me.

Meanwhile, many other organisms were also suffering, but primarily from the environmental conditions, not from humans. Mobile species tended to move into the intertidal zone, notably small sharks and large octopus. Sessile species often succumbed: the first to go were the soft-bodied “red bait” (large Pyura ascidians), which I initially thought were horse dung on the beach; many of the lower mussels also died, and then dragged their epiphytes on shore, including kelp and bryozoans. On the oyster longlines in Walvis Bay, epiphytic hydroids had died and discolored, but interestingly the newly-settled native clams were still alive amongst the hydroid strands, even if just a few mm long. On the beach south of Swakopmund, it was clear that many fish had also been unable to escape the low-oxygen conditions: species included the serpent eel (pencil thin and more than a foot long with a beaky snout), clingfish known as rocksuckers, and many more that we didn’t take the time to identify. Death and devastation, the beach littered with bodies – we had never seen such a tangle of the ocean’s denizens cast up on shore like this.

This year’s environmental conditions have been strange. Of course, we have no grounds for personal comparison, but we read the newspapers and we ask around. Even though the inland rains started late (late January rather than November), they have been torrential: reservoirs are overflowing, rivers are flooding, and agricultural land and human infrastructure are under water. The ocean has been warmer than normal for 4 months, with temperatures well above 20C, compared to rare excursions above 18C in most years. Based on reading ahead of time, we expected strong and persistent southwest winds on the coast, but we have only experienced a scattered few days of these winds, with the more common pattern to be W or NW or simply weak winds. We do not know – in fact, no one knows, although there are many hypotheses – any clear relationship between this year’s weather and the devastation for coastal organisms. However, 2006 and 2008 were high-rain years for Namibia and also brought large sulfide eruptions. The weather patterns that bring rain could also trigger sulfide (directly from freshwater inflow to the coast, or from some joint meteorological conditions). We do know that the unusually warm ocean temperatures stem from the lack of upwelling, because little deep cold water comes to the surface, and the warm Angola current pushes south. Earlier this year, these warm water conditions were also associated with oyster mortalities, even before the blooms and eruptions. We also know that the winds determine the severity of sulfide eruptions in Walvis Bay, and the absence of SW winds to push the water offshore allowed phytoplankton or sulfide to accumulate within the bay.

Today, we participated in an emergency meeting of the oyster growers to discuss combined responses to their losses. This was an opportunity to collate information about the spatial and temporal patterns of mortality. Did oysters die in response to toxic phytoplankton, Vibrio bacteria after the phytoplankton crashed, or due to sulfide and low oxygen? (Unfortunately, this seems impossible to resolve because so many environmental changes happened simultaneously, and oysters can buffer themselves somewhat by closing up for a while.) Were oysters closer to the ocean/ mouth of Walvis Bay buffered from mortality because they received some clean water advected with tides? (From what we have heard, this appears to be the case.) Did oysters deeper in the water suffer higher mortality? (This also seems to be the case, consistent with a decline in oxygen concentration from the surface to depth.) We heard that oysters suffered higher mortality if recently cleaned of fouling organisms, and we suspect that this is because cleaning these fast-growing subtidal oysters chips their shells, so they cannot protect themselves by closing up against poor environmental conditions. So what could be done to toughen them up? Did the native clams survive as epibionts on the baskets throughout the bay, and if so, why not cultivate them? (They taste fine steamed in butter and garlic; personal observation).

The oysters growing onshore in the salt ponds at Walvis Bay and Swakopmund have weathered the recent ocean conditions with little trouble, except perhaps mortality of oyster larvae in the hatchery associated with the initial phytoplankton bloom. Because ocean water is pumped vigorously up into the salt ponds, it is well aerated by the time it reaches the oysters.

This has been an awful, but awesome event to witness as an ecologist – as a farmer, it would just be awful. I wonder if living in Africa for a year is a bit like having a terminal illness. Each seasonal event, each visited scene will never be experienced again.