Paper: Gollner, S., Govenar, B., Martinez Arbizu, P., Mills, S., Le Bris, N., Weinbauer, M., Shank, T. M. and Bright, M. (2015). Differences in recovery between deep-sea hydrothermal vent and vent-proximate communities after a volcanic eruption. Deep Sea Res. Part I Oceanogr. Res. Pap. 106, 167–182.
Hydrothermal vents are highly productive areas at the bottom of the ocean that are formed over cracks in the seafloor. At these special places, metal-rich, superheated water spills out and is able to support large communities of life thousands of meters away from the surface of the ocean (Figure 1).
If you were able to journey down to the vent and smell it, it would smell like rotten eggs due to the high concentrations of hydrogen sulfide. The tubeworms pictured above – as well as other organisms – are able to use that smelly sulfide to make energy, much like plants use sunlight. Sulfides are useful to more than just these assemblages of strange and specialized animals, though. Humans can use these sulfide deposits as well, and these deep-sea hotspots are likely to be the target of deep-sea mining in the near future. Because not much is known about these communities and the foreign animals that live there, we’re not sure how mining efforts would affect these ecosystems. The researchers in this study set out to figure out what happens to vent communities when they’re disturbed by a natural force: volcanic eruptions.
Hydrothermal vent systems are located along major tectonic boundaries, like the mid-Atlantic Ridge and the East Pacific Rise, because these areas have a lot of cracks in the crust that leak superheated water (Figure 2).
Major tectonic boundaries are also associated with volcanoes because at these places, new seafloor is constantly being created by lava flows from eruptions. The authors of this paper looked at vent sites before a major eruption, then went back to the same site one, two, and four years after to see how the community of animals had changed on and around the vent. Understanding the natural recovery of the ecosystem is a step to understanding how the ecosystems both on and around the vent will recover if disturbed by human mining efforts.
To accomplish their goal, the researchers had to evaluate the community of animals before and after the volcanic eruption. For most of their “before” data, the researchers relied on other studies that they had performed in 2001-2002 and 2003-2004. Determining abundances and community diversity after the eruption in 2006 took a little more work. To capture the animals for analysis, the researchers used a submersible (Figure 3) with a robotic arm to lay down kitchen sponges on three different vent communities in the East Pacific Rise. The porous sponges gave the animals a place to settle and were therefore a good, removable estimator of the animals that settle directly on the vent or on the surrounding rock.
They deployed the sponges for varying lengths of time after the eruption: one set of sponges was deployed for one month right after the eruption, a second set of sponges was deployed for a year after that, and then a third set of sponges was deployed for another two years after that. Staggering the sponges allowed the researchers to see a snapshot of the animal community. The animals which were quick to return showed up on the first set of sponges, and the last animals to return (if they returned at all) showed up on the third set. They put sponges on both the vent and on the surrounding rock.
The sponges were picked up by Alvin after they had been in the vent community for the required length of time. From there, the researchers had to shake out the sponge and count every single animal that came out of it to determine the abundance (number of animals per sponge) and determine the species to calculate community diversity (number of different species per sponge). They did that with all the sponges (vent and surrounding rock) to compare which habitat had higher abundance or a higher diversity.
Results and Significance
In community ecology, diversity is a better indicator of ecosystem recovery than abundance because abundance is largely determined by the mode of dispersal of the animal (swimming animals are more likely to recolonize an environment more quickly than animals anchored to a substrate, for example). As a result, the researchers focused more on the diversity of the two different habitats before and after the volcanic eruption. They divided the animals into two major categories: meiofauna (small animals less than 1 mm) and macrofauna (larger animals over 1 mm).
At vent systems, it took four years for 42% of the meiofauna and 39% of the macrofauna species to come back to the site. Most of the species that did come back to the habitat had a large range and were abundant before the volcano erupted. The group of animals with the highest return rate were copepods, a highly mobile type of organism that disperses widely in the water column (Figure 4).
In the basalt rock adjacent to the vent, it took four years for 28% of the meiofauna and 67% of the macrofauna species to come back to the site. The meiofauna that didn’t come back were animals that had existed in the community before, but not at a high abundance; they were rare species before the eruption and non-existent at the habitat after the eruption.
Some species found after the eruption were not there before the eruption, suggesting that there could be a niche for new colonizers in a post-disturbance habitat. These “pioneer species” may be good at dispersing themselves through water, but are outcompeted once the community establishes itself.
More information is needed to make any conclusions, but the relatively quick recolonization of abundances and a middling level of diversity in both these habitats suggests that communities can establish themselves again even after a large disturbance like an eruption. While there are still many questions to consider before we start deep-sea mining in earnest, like if there are chemicals released during the mining process or if there are other side effects, it’s heartening to think that we may not be destroying these communities we still don’t know that much about.
Do we need to have all the facts before we make a decision about something like deep-sea mining? How do we determine if these habitats are worth conserving?