Environmental controls, climate change and fish: talking about coral reefs at XI SOUFBA

Written by Carla Elliff

The Covid-19 pandemic has in many ways changed how scientists think of conferences. Over the course of a few months, the pre-defined schedules of researchers all over the world have been turned upside-down, leaving us in an uncertainty of “what to do?”. A recent survey from the Young Ecosystem Services Specialists (YESS) network has shown that over 90% of their respondents have been affected by conference postponements and cancellations , with 79% not being able to present their research as planned. These numbers are very concerning especially to early-career researchers.

An option that is being increasingly explored is  transitioning events to an online format. However, there are some very important aspects to consider with this option. An online conference cannot simply be the exact replica of an in-person event. Programs must be thought to accommodate the reality of participants, such as a different time zone or that the conference will likely be an extra activity on top of a full day of work. Making an online event accessible also requires thinking about alternatives for those with poor internet connections, thus requiring logistics such as pre-recorded talks that can be accessed when most convenient. However, this bold move has allowed many events to still be held under these challenging circumstances.

Last week I was fortunate to see first-hand one such bold initiative: the XI Semana de Oceanografia da UFBA (XI Oceanography Week of the Federal University of Bahia – Brazil), or simply XI SOUFBA.

This annual event is entirely organized by undergraduates in oceanography and for the first time, it was held online. The Federal University of Bahia (UFBA) holds a special place in my heart, since it was there that I did my master’s and doctorate studies on coral reef ecosystem services. So, when I was invited to be a part of a round table about corals of course I said yes!

I was joined by Prof. Carlos Werner from the Federal University of Southern Bahia (UFSB) and by Prof. Ruy Kikuchi from UFBA. Each one of us would tackle a different aspect about coral reefs.

Kikuchi was the first to speak and focused on what environmental controls shape coral reef evolution from a geological perspective. He began with an important aspect: corals or coral reefs? Corals are basically colonies made up of polyps, while coral reefs are whole ecosystems composed of corals (and many more organisms). It’s like comparing a building full of apartments to a whole city. There are buildings of all shapes and sizes in a city that can steadily grow into a metropolis under certain circumstances. These circumstances are the environmental controls. For a coral reef there are many controlling factors that influence how much and in what ways it can grow, but I’ll quickly focus on two very important ones presented at XI SOUFBA: light and temperature.

Corals live in symbiosis with zooxanthellae, microalgae that provide energy to their host through photosynthesis. This means that if corals are not within reach of natural light, they lose this precious relationship and cannot survive (keeping in mind that we are not talking about deep-water corals, that is a whole different story!). This loss of light can occur because we simply went deeper into the ocean or because water turbidity increased, meaning there are more particles such as sediments in the water, making it harder for the light to shine through. Temperature is also tricky for the zooxanthellae. If sea surface temperatures rise too much, the microalgae are expelled from the coral tissue because they start to produce toxic substances and, again, the coral is left without its symbiont. All organisms have an interval of temperature in which they thrive, and this is a very sensitive matter with corals.

While temperature is a naturally oscillating parameter, climate change is making our oceans warmer over longer periods of time. And this is where I came in: my talk was all about the implications of climate change to coral reefs. Global warming is perhaps the most recognized impact of climate change by the general public, but we must understand that this is only one part of the problem. Ocean acidification and rising sea levels are two other important components to this equation. Coral reefs are feeling the blow from lots of different angles and have been compared to “canaries in the coal mine”, meaning their decline is a sign that things are not well for our oceans as a whole. These three components (warming oceans, acidification and sea-level rise) act synergistically and can lead to several concerning outcomes. These include coral bleaching, increased occurrence of diseases, more fragile reef structures and “drowned” reefs.

Coral bleaching happens when the zooxanthellae are expelled. The microalgae gives the coral their color and, when they are gone, we are left with a transparent tissue that allows us to see all the way to the carbonate skeleton of the colony. Bleaching is reversible, but if it goes on for long enough it can lead to mass mortality – and this is exactly what has been happening more and more over the past few years with warmer oceans. This increase in temperature has also benefited pathogens that cause diseases in corals, making them more frequent and virulent. Changing our focus now from corals to reefs, the whole structure of a reef can be damaged beyond repair in face of climate change. Acidification makes it more difficult for corals and other reef-builders to secrete calcium carbonate for their skeletons. This can lead to a fragile structure that is vulnerable to erosion and can break more easily with the stronger and more frequent storm surges observed. Finally, rapid sea-level rises will most likely take us to that same scenario in which the reef is too far away from natural light to go on.

And what about the inhabitants of these coral reefs? Werner brought several reflections on the relationship between coral reefs and fish. Biodiversity is closely linked to habitats, so changes to a coral reef influences the fish communities that live there, without a doubt. For example, studies have shown that some reef fish prefer to live in the nooks and crannies of branching corals, which provide greater protection than massive corals. So, if something happens to these branching corals (bleaching, diseases, breaks…) the fish will change too. Greater fish abundance has also been associated with greater rugosity on a coral reef. Rugosity is an extremely important parameter for coral reef ecosystem services, showing that a more complex structure tends to be a richer environment.

Werner also touched on the topic of artificial reefs and what characteristics make them more attractive for fish. He showed that while many aspects can be simulated on an artificial structure, real reefs are always more complex, diverse and richer than their human-made counterparts. Which goes to show that we shouldn’t expect to simply replace all reefs with artificial versions if we lose them in the future.

This is only a summary of what was discussed, and I invite all readers to check out the full presentations on the YouTube:

Thanks again to the XI Semana de Oceanografia da UFBA for including coral reefs in their program and for inviting me to share my work! Check out more about other topics covered, from Antarctica to the Ocean Decade to Representation in Marine Sciences, at @semanadeoceano

And, if you are interested in knowing more about the work carried out by Kikuchi and Werner, you can follow their labs at @recor.igeo.ufba and @lecomarufsb

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