Outer reef

Whenever I travel, I am always observing. As we’ve been diving the wrecks this week, I’ve made mental notes of what lived on each one and tried to detect patterns. A couple things have stuck out to me.

First, the wrecks actually have lower biodiversity than I expected. My only other time diving in the tropical Pacific was in Palau, and to some extent, I was expecting to see the same level of species richness that I had observed there. Not so – if I had to guess, I would say the largest, most species-rich wreck in Truk Lagoon was home to 30 benthic invertebrates, whereas a natural reef the same size in Palau hosted > 100.

Shipwrecks very often have lower biodiversity than nearby natural hard-bottom habitats, and in fact, figuring out why this happens is one of the major questions in my scientific career right now. In most cases, I think it has to do with dispersal – the types of organisms that live on hard-bottom reefs simply can’t disperse out to an isolated, island-like shipwreck. This idea was part of my PhD thesis, and I spent most of last summer figuring out what species can and cannot disperse to shipwrecks in New England.

I very much wanted to see a natural coral reef in Truk Lagoon to compare its species richness to the shipwrecks. We approached the dive guide, and he agreed we could have a dive on a natural reef. We loaded our gear into the boat, motored out to the edge of the lagoon, and jumped into the sea.

It’s not very often that I get answer to my scientific questions right away. Detecting patterns in nature most often requires extensive observation, careful data collection, and deep analysis. Today was one of the rare times that a simple, instantaneous observation answered a scientific question.

As I descended through the water column, I surveyed the corals below me. Clumps of yellow covered the seafloor. Yellow, yellow, yellow, everywhere I looked. The entire reef was dominated by a single species: Porites lobata.

Some of you may remember Porites lobata. It’s the species that Hanny and I were collecting in Palau last September-October. It’s a very common stony coral in the tropical Pacific, and it is uniquely resistant to bleaching.

We drifted along the reef with the current, and I continued to observe the animals beneath me. Small colonies of other stony corals were present, but the size of the patches suggested they were very young, probably new recruits from the last 1 – 2 years. Between the living Porites were calcium carbonate skeletons – spiky shapes and dome shapes and upright branching shapes of corals long dead. Ideas started coalescing in my mind.

Diverse dead corals + dominance of the living corals by a species that’s resilient to bleaching + only young colonists of other species = there must have been a massive bleaching event on the reef 2 – 3 years ago, and Porites lobata was the only species to survive. I looked it up, and there was actually a massive El Niño in 2014 – 2016 that decimated corals across the Pacific. I realized I was looking at the evidence right beneath me.

The wheels in my head kept turning, and I realized that the low biodiversity I was seeing on the shipwrecks in Truk Lagoon was not the result of dispersal. It wasn’t that only a few species of coral larvae could travel all the way from the natural reef and reach the wrecks. It was that only a few species could survive bleaching events, like Porites lobata. The patterns I was observing in the shipwreck communities were driven more by resilience to environmental disturbance than by dispersal.

In one way, coming to this conclusion was satisfying because it explained the patterns I was seeing, but in another way, it was disappointing because it meant Truk Lagoon shipwrecks are not a good system in which to study dispersal, so there’s little chance I could ever return here for work. Either way, I greatly enjoyed our dive on the outer reef. It’s amazing what you can learn just by observing the world.