Florian Roth, postdoc researcher talks about his paper High rates of carbon and dinitrogen fixation suggest a critical role of benthic pioneer communities in the energy and nutrient dynamics of coral reefs, what makes this work different and what got him into ecology.

What’s your paper about?

This paper is about the role of benthic pioneer communities in the carbon and nitrogen dynamics of coral reefs. Specifically, we evaluated how heterogeneous assemblages of algae and sessile invertebrates – organisms that quickly settle and cover all available space after coral mortality – contribute to the initial energy and nutrient demands of reef communities following coral loss.

What is the background behind your paper?

Coral reefs are some of the most biodiverse ecosystems worldwide. Reef-building corals build the foundation of these ecosystems and provide a structural “home” for fish and countless benthic and pelagic organisms. Corals also mediate the transfer of carbon and nitrogen within, across, and away from the reef, thereby enabling other organisms to thrive.

However, coral reef communities are highly dynamic and unstable. The death of corals (from the die-off of individual colonies to even entire populations due to environmental stress) exposes their bare coral skeletons made of calcium carbonate. This phenomenon is particularly common after mass coral mortality following prolonged bleaching events associated with marine heatwaves; something that is, unfortunately, observed more and more frequent globally.

While the bare coral skeletons can still provide the structural framework for other organisms temporally, the original ability to mediate carbon and nitrogen cycles is lost. Many studies have shown that heterogenous assemblages of algae and sessile invertebrates play an essential role in the biological succession and reorganization of the reef benthos after disturbance. However, their significance for critical ecosystem functions previously mediated by corals, such as the efficient cycling of carbon and nitrogen within the reef, remained utterly uncertain.

What are the key messages of your article?

Supporting previous studies, we too observed that many types of organisms settle quickly and cover all bare space in a reef within a matter of weeks. Importantly, we could show that these so-called “pioneer communities” can capture just as much carbon and nitrogen as fully-grown, mature coral communities. Elemental and stable isotope analysis further revealed that most of the organic material generated by pioneer communities can be exported to the wider reef, and may, thus constitute an important source of energy for the coral reef food web.

How is your paper new or different from other work in this area?

The existing literature mainly focuses on recruitment and succession patterns of pioneer communities to predict trajectories of reef recovery (or shifts to alternate states) after coral mortality. Here, we expanded on this knowledge by having a look at the community-wide metabolism of these species’ assemblages. Especially in nutrient-poor reef environments, where the cycling of carbon and nitrogen is fundamental to the overall ecosystem functioning, the contribution of carbon and nitrogen inputs by pioneer communities may play a crucial, yet overlooked role. However, it is important to consider that those pioneer communities constitute only the first stage of reef development. They can only help to maintain the reef trophodynamics if a healthy fish community, clean water, and the structural integrity of a reef are preserved.

Does this article raise any new research questions?

As usual, answering one research question gives way for many new. Our study provides first insights into the carbon and nitrogen dynamics of benthic pioneer communities in tropical coral reefs. However, as sensitive ecosystems, coral reefs are constantly transformed in their structural composition and ecosystem functioning due to anthropogenic disturbances and the impacts of climate change. In the future, we thus need to find out how the cycling of carbon and nitrogen is affected by changing environmental conditions (e.g., warming or ocean acidification) but also by altering community structures (e.g., phase-shifts to algal-dominance after coral loss). Likely, the role of benthic pioneer communities will be affected by these changes too – something that needs yet to be investigated.

About the research

Did you have any problems setting up the experiment/gathering your data?

As for any field study, we too had our challenges. While almost everywhere around the world researchers are looking forward to performing fieldwork during the summer months, we struggled quite a bit with the climatic conditions in Saudi Arabia. Winter is pleasant with both water and air temperatures around 25 °C. However, in summer, water temperature in the shallow reefs can rise to >32 °C, while air temperature often reaches 45 °C and more. During this period, it was a challenge to perform the quite physically-demanding work in the field. On the small and open boats, there was no way to escape the heat; not even jumping into the water helped to cool off.

Yet, the reward: access to some of the most pristine and mesmerizing coral reefs of the Red Sea.

Were you surprised by anything when working on it?

To be honest, we did not expect that the rates of carbon and dinitrogen fixation by benthic pioneer communities would be that high. In the end, these communities are so inconspicuous and often grow only a few millimetres tall; yet, they do a remarkable job of capturing carbon and nitrogen. In fact, despite staying small, these pioneer communities capture just as much carbon and nitrogen as fully-grown and mature coral communities of the region.

About the Author

How did you get involved in ecology?

I started as a biology bachelor student in landlocked Düsseldorf, Germany. However, soon I found my passion for marine ecology during an internship at a field station on the island of Giglio in Italy. In 2012, I began my master degree in International Studies in Aquatic Tropical Ecology (ISATEC) in Bremen, Germany. There, I had the chance to conduct an extensive field study for my master thesis assessing benthic recruitment and succession patterns in upwelling-influenced coral reefs of Costa Rica. I developed a broader research interest in integrating ecological, metabolic, and biogeochemical data to understand how benthic coral reef organisms interact with their environment during my PhD at the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia, from 2015 until 2019. In particular, my dissertation focused on quantitative data on critical functions (e.g., recruitment, primary production, calcification, and nutrient recycling) of classical and phase-shifted novel reef communities, on tipping points for the collapse of community functions, and potential future winners and losers.

What’s your current position?

After finishing my PhD, I started a post-doctoral position within the “Baltic Bridge” program between the University of Helsinki, Finland, and the Baltic Sea Centre in Stockholm, Sweden. While I still focus on carbon and nitrogen dynamics in shallow coastal habitats, I took the challenge to work with an entirely new ecosystem for me – the Baltic Sea.

What project/article are you most proud of?

Most work during my PhD was part of a project called “NICE” (NItrogen cycling in Coral reef Environments) that involves collaboration partners from Germany and Saudi Arabia. The projects’ overall aim is to understand the processing of nitrogen in coral reefs in response to seasonality and a range of key environmental change factors. While the project is still ongoing, I was extremely happy to be part of this project; for its fascinating topic, but also to be able to work in multidisciplinary team with motivated, creative early-stage researchers. Working across different disciplines (e.g., ecology, microbiology, and biogeochemistry) and scales (from intracellular processes over single organisms towards whole communities), I was delighted to see one key paper being published last year in Methods in Ecology and Evolution (Roth et al. 2019, An in situ approach for measuring biogeochemical fluxes in structurally complex benthic communities. Methods in Ecology and Evolution, 10(5), 712-725, doi.org/10.1111/2041-210X.13151) that may help other researchers to quantify community-wide fluxes of carbon and nitrogen in aquatic habitats.

What is the best thing about being an ecologist?

I love many things about being an ecologist. It is an extremely varied job and surely not for those who prefer routines. For someone like me, who thrives on a challenge, it’s ideal. I enjoy analysing data and using that information to creatively find out how nature works – it often requires you to think outside the box, and to collaborate within and outside your discipline. However, foremost, I enjoy being in the field and experiencing nature in its purest form. Not many people get to go diving in a coral reef at work…

What do you do in your spare time?

I am a travel and outdoor enthusiast. Luckily, all of my research expeditions brought me to amazing places, where I could explore the lush jungle of Costa Rica, go surfing in Brazil, climb the desert mountains of Saudi Arabia or, in my new position, go kayaking in the Stockholm archipelago.

Read High rates of carbon and dinitrogen fixation suggest a critical role of benthic pioneer communities in the energy and nutrient dynamics of coral reefs here!