Even a small change in topography can help tides drive an oxygen pump

Pioneer species moving into tidal flats are under attack twice a day. This might be bearable for grown plants, but seedlings are vulnerable. Ideally, a plant would grow as quickly as possible. A new study by Greg Fivash and colleagues has examined the effect of microtopography. Astonishingly, they have found that even a 2cm variation in height can make a difference to growth rates.

The study compared the growth of Salicornia procumbens, an annual herbaceous plant. Fivash and colleagues grew it in a number of pots, some with a small mound above the usual level, some hollowed slightly, and some just flat. Yet, as Fivash explained, Salicornia might not be the first plant you associate with mounds in marshland. “Salicornia is actually a species does not typically form sediment mounds on its own. This is something you might expect of the clonal grass species like those in the well-known Spartina group. Instead, Salicornia is a species that tends to invade a mudflat en-masse when conditions are right. Because of its small stature and annual nature, it takes a massive invasion of Salicornia for it to begin to alter its environment and to carry over between years. However, we know it can happen because of historical satellite data of Salicornia converting kilometers of intertidal mudflat to salt marsh in a very short time. One such area was Hoogeplaat in the Western Scheldt in the Netherlands.”

“Besides the pipe-dream that we could use an annual species to make restoration occur on a faster time scale, Salicornia is just a great species for doing experiments in the lab. Because of its succulent, leaf-less body plan, we were able to track its growth by taking photographs that were extremely accurate at predicting the mass of the plant. That way, we were able to increase the replicate number by an incredibly high amount. The first experiment in this paper features 17,393 individual plants. This would not have been possible with any other salt marsh pioneer group.”

Looking at how the plants grew, Fivash and colleagues found that plants on the slightly raised mounds around 25% faster. The results of the experiment came as a surprise to some people in the team. “I was not entirely surprised that the 2 cm sediment elevation would have some effect on the plants, Fivash said. “My supervisor, on the other hand, told me after everything was done that he was extremely surprised that the experiment worked. I, however, had already seen the phenomenon in another experiment in the field and the effect of the raised surface elevation was our best leading hypothesis for explaining our otherwise inexplicable field results. I have to admit though that we still have not managed to replicate the intensity of the growth effect that we see in the field in a lab setting.”

So why did the plants do so well? The tides in the experiment lifted seeds away from the mounds. In contrast, they stayed in the pots with hollows. “This is likely to be a realistic approximation of how seeds would collect in hollows over a mudflat, but it does not mean that you will actually find more seedlings in these places,” said Fivash. “In fact, it is quite the opposite, and that likely has to do with processes that happen later in life. The poor sediment consolidation inside of those pools tends to make them more erodible, and thus more dangerous for plants living in them. That is combined with the evidence we show that plants will grow somewhat slower in environments that don’t drain well and will, therefore, be even more vulnerable to uprooting by waves and tidal flows.”

However, it’s not a matter of less competition that aids the plant. Oddly, the plants on the higher surfaces benefit from receiving more oxygen, thanks to periodic flooding. Fivash explained how flooding helps the plants breathe. “The main idea is that when sediments drain, the spaces in the sediment empty of water and fill with air. This air is de-oxygenated by microbes living in the sediment, so it doesn’t appear to alleviate the anoxia in the sediment directly (at least not in sediments that are not completely dry). Then, when the tides return, the tidal water, which is well mixed and well oxygenated, pushes all the de-oxygenated gas out of the sediment, because it is obviously denser than the gas in the sediment spaces. This process provides a lot of oxygen to the sediment, even a few centimeters below the surface in some cases.”

“Furthermore, the whole process seems to be exaggerated in sediment mounds. In our experiments, they maintained high oxygen levels for much longer, and the effect also reached much deeper in the sediment. That makes it a strong candidate explanation for the growth benefits we see in our recruits.”

Fivash said he was working on another paper that he hopes to finish this year, where he explores how microtopgraphy can aid restoration projection. “From preliminary results, this certainly appears to be applicable in restoration, also for other salt marsh species,” he said. “Mossman et al. in Journal of Ecology have also recently demonstrated in their own field experiments that these mounds have a positive effect for other species in the pioneer zone. However, I think the real benefit would be if we could produce habitat like sediment mounds (or any other beneficial environment that we discover) that causes establishment naturally. That way we could potentially convert much larger areas with less effort than would be needed to do the same with planting.”

“My personal feeling is that the role of mudflat topography has been under-served in research about salt marsh establishment. If we can convince other scientists to consider the importance of this phenomenon, it could lead to a major breakthrough in how we go about attempting to restore and expand salt marshes. Mudflat topology is something that can be manipulated in many ways. Some already exist, and others are likely still to be discovered. If we can make the connection between the establishment of salt marsh vegetation and these topographic features, then it would open doors to some very new applications.”

Interestingly while there are practical applications, Fivash also notes that the research also has value for some basic evolutionary science. “On a completely different note, from an evolutionary standpoint, the realization that tidal movement can alleviate anoxia in the sediment could be an eye-opener for those studying how wetland species combat that particular environmental constraint.”