Can Ocean Conditions Explain the Lack of Biofouling on MH370 Debris?

RG Gyre

Almost immediately upon Blaine Alan Gibson’s discovery of the “No Step” debris fragment in Mozambique, questions were raised about the relative scarcity of marine life growing on it. These questions were redoubled after two more finds came to light, one from South Africa and the other from Mozambique, which both looked surprisingly pristine for objects that had been in the water for two years. I explored the issue in a post on this site entitled “Bioforensic Analysis of Suspected MH370 Debris.”

This weekend IG member Richard Godfrey addressed the question in a post on Duncan Steel’s website. “One possible explanation for this obvious difference between the flaperon and the other items,” he wrote, “might be linked to the differing routes taken by the floating debris.”

As a point of reference, I’ve reproduced the current chart from that post (above). Though in reality the currents are not nearly as deterministic as depicted–there is a randomness to the motion of floating objects that causes them to spread out, like a drop of ink in a bucket of water–it does accurately portray the overall movement of things. The black bar represents the area where Godfrey thinks the plane most likely impacted the water, northeast of the current seabed search zone. He points out that to get to the locations where they were found on the coast of Africa, the pieces would have to have either passed around the northern end or the southern end of  Madagascar.

In the image below I’ve sketched out what these paths might look like, more or less. The pink oval represents the central gyre seen in the current map above. The yellow line is a hypothetical path proposed by Godfrey that the flaperon might have taken on route to Réunion. The orange line is a hypothetical path that the capsized boat which washed up on Mayotte may have taken during its eight-month drift from northwestern Australia in 2013-2014. I suggest this is a plausible example of a “north route.” The purple line is an even more hypothetical proposal for a “south route” that I just sketched out freehand after watching some drift simulations.

North & South Routes

In the first part of his post, Godfrey tackles the question of whether the African debris might have traveled through water too cold to allow the growth of Lepas anatifera, the species of goose barnacle found on the Réunion flaperon:

If floating debris took a path passing slightly further south of Madagascar then it could remain in colder waters (especially between July and October) below 30S, under which circumstance barnacle attachment and growth is contra-indicated. Thus it might be that the three items found on the coast of Africa reached their destinations via such more-southerly routes… The Paindane item (‘676EB’) discovered at around 24S may well show some evidence of marine life, even though it most probably arrived via the southern route past Madagascar, mainly occupying cooler waters… The Mossel Bay find (‘Rolls Royce’) might not be expected to show evidence of marine life because it was discovered at around 34S and may well have spent most of its ocean transport time in the cooler waters below 30S.

To evaluate this idea, I consulted the newly published paper “Endorsing Darwin – Global biogeography of the epipelagic goose barnacles Lepas spp. (Cirripedia, Lepadomorpha) proves cryptic speciation” by Philipp H. Schiffer and Hans-Georg Herbig of Cologne University in Germany (preprint available here). According to this source, Lepas anatifera can be found in waters where the temperature is greater than 15 degrees Celsius. South of this line a sister species, Lepas australis, is found:

anatifera v australis distribution

To get a sense of where this transition zone occurs, I traced it out on Google Earth and superimposed a surface-temperature chart lifted from Godfrey’s post along with the previously described drift routes.

The southern boundary of anatifera’s range is the red line that passes through the seabed search rectangle:

SIO temp w routes

As is quite readily apparent, all the routes lie entirely within anatifera’s range. Note also that the southern boundary lies well south of the gyre, meaning that anything that drifts beyond it is going to be swept eastward. It’s entirely possible that a piece of debris might have neared Africa and then been swept south into cold water that killed the anatifera, but after that the piece would have been carried back towards Australia. In order to move back west it would have to have first drifted north back into anatifera habitat, where it would have had approximately a year to get re-colonized. Remember, Lepas reach sexual maturity in 60 days and achieve full size in six months to one year. So these pieces should have been carrying a load of biofouling similar to the Réunion flaperons even if their initial population was killed off by the cold.

Godfrey also raises another possibility: that the African pieces are clean because they passed through ocean regions too low in nutrients to permit the growth of marine organisms. To check this idea, I consulted with a NASA website that archives world-wide chlorophyll concentrations, which can be read as a proxy for ecosystem nutrient level. Here I’ve overlayed the same set of drift routes over a nutrient map for March 2014, when the water is near its warmest:

Mar nutrient map

And here are the nutrient levels in September, when the water is near its coldest:

Sept nutrient map

Broadly speaking, there is an area of relatively low nutrient levels in the middle of the SIO that grows and shrinks with the seasons, being biggest when the water is warmer. In the warmer latitudes transient high-nutrient patches can be  found, but they are transient in time and space. The southern end of anatifera’s range experiences consistently higher levels of nutrients, as does the ocean between Madagascar and the African mainland.

Godfrey writes:

Although it appears likely that the floating debris from MH370 was carried westwards towards Africa by the Indian Ocean South Equatorial Current through warm waters (i.e. where barnacle attachment and growth is feasible), these waters have relatively low concentrations of chlorophyll in the maps above, and therefore limited amounts of phytoplankton, and this militates against substantial barnacle growth.

The problem with this analysis is that the piece of debris which spent the greatest amount of time in the center of the Indian Ocean, with its low nutrient levels, is the flaperon, which has the greatest accumulation of Lepas, including some which have reached full size. The clean pieces, by contrast, have spent considerable time in the nutrient-rich waters near Madagascar.

Finally, I’d like to address an addendum to Godfrey’s piece by Don Thompson, who writes:

An alternative reason for the Réunion and Rodrigues items being barnacle-encrusted but not the other three might be as follows. The lepas (goose barnacle) colonisation may be a feature of proximity to coastlines inhabited by lepas colonies. Therefore, debris ‘dropped’ into a mid-ocean region (i.e. the crash site) might be expected to be ‘clean’ of lepas barnacles until free-swimming barnacle nauplii, released from reproducing coastal colonies, are encountered.

Again, Thompson has the situation reversed. Lepas are pelagic creatures which are adapted to rafting on the open ocean. Buoys placed far out to sea become heavily settled by them.

UPDATE 4-7-16: There seems to be some confusion about the lifestyle of the Lepas. Unlike some other genera of goose barnacle which can be found living in intertidal zones of the seashore (such as Pollicipes, a delicacy in Spain), those of the genus Lepas are obligate rafters, highly adapted to life floating free in the open ocean. Here’s an excerpt from Barnacles: Structure, function, development and evolution:

Lepas citation