The two most important and fundamental processes describing coral reef ecosystem functioning are photosynthesis (primary production) and calcification. They determine energy flow, carbon cycling, and habitat provision (calcium carbonate structures) in reefs. In corals, as well as in other photosynthesizing calcifiers (e.g., crustose coralline red algae), photosynthesis and calcification are related to each other (e.g., higher calcification during the day then at night). However, considerable differences in their responsiveness to changing environmental conditions are evident as well. Numerous manipulation experiments have shown how the metabolism of corals is affected and/or able to adjust to changes of single and multiple environmental conditions, including stressful conditions that exceed naturally experienced conditions. However, much less is known about temporal and spatial dynamics of coral (and coral reef community) metabolism in-situ where multiple abiotic and biotic environmental factors, as well as species interactions, affect metabolic rates. The MABEE lab addresses research questions that relate to the temporal and spatial dynamics of reef organism and reef community metabolism under in-situ or near-natural conditions. The strong seasonal environmental fluctuations in Bermuda and the long-standing ocean time series, the Bermuda Atlantic Time-Series Study (BATS) provide an ideal framework to study the capacity for metabolic adjustments in corals and other photosynthesizing reef organisms. This knowledge forms an important basis for further studies that aim to understand how reefs and their functional processes may change under progressing global change.
NASA COral Reef Airborne Laboratory (CORAL) & light-use-efficiencies (LUEs)
When Sawall assumed a research position at BIOS in June 2016, she participated in the NASA Coral Reef Airborne Laboratory (CORAL) mission (2015-2019) led by BIOS senior scientist Eric Hochberg. The mission of CORAL was to investigate coral reef conditions, namely benthic community structure, primary production, and calcification via remote sensing, which would ultimately allow for reef monitoring on a global scale. Here, Sawall assessed community-scale light use efficiencies (LUEs) of different reef photosynthesizers using outdoor fumes (picture 1.4 in slideshow at top of page). These LUEs are defined as the daily gross photosynthetic rates (or calcification rate) per daily absorbed light, and are required to derive primary production of reefs from airborne hyperspectral imagery (Sawall et al. 2018). The ultimate goal of her work is to be able to model LUE based on community type and environmental condition, similar to what is already done for remote sensing of productivity in terrestrial ecosystems (e.g., satellite-based MODIS-GP).
Students involved in the CORAL LUE-flume experiments:
- Ashley Miller, University of Bremen, Germany: 6-months internship funded by CABIOS and ERASMUS (2017-2018). Data used for Master Thesis
- Kelly Chimpen MacLeod, Towson University, MD, USA: 3-months NSF-REU internship (2017).
- David Flesher, Arizona State University, AZ, USA: 3-months NSF-REU internship (2018). Data used for Honors Thesis.
- Allison Doolittle, Los Angeles Harbor College, CA, USA: 3-months NSF-REU internship (2019).
Students involved in LUE-related side projects:
- Zoe Pearson, University of Southampton, UK: 10-weeks fieldwork (self-funded; 2017). Data used for Master Thesis and presented at the European Coral Reef Symposium, Oxford, UK (2017).
- Anna Nicosia, Lehigh University, PA, USA: 10-weeks Iacocca International Internship (2019, picture 1.8).
- Katie Maguire, high school graduate: 8-weeks Bermuda Program internship (2019).
In-situ community metabolism
A common limitation of in-situ community measurements is that they are logistically challenging. Sawall, Eric Hochberg, and Nick Bates were awarded the BIOS Cawthorn Innovation Fund (2018 – 2019; $150k) to advance the rather new “gradient flux” (GF) approach for reef metabolism measurements. This approach makes use of the benthic boundary layer overlying the benthic community, and of the gradient of molecules that are taken up or released by the community’s metabolic activity (e.g., O2, carbonates, and bicarbonates – total alkalinity; picture 1.1). After considerable troubleshooting and with further input from Matthew Long (Woods Hole Oceanographic Institute, USA), we were able to significantly advance the GF approach for measurement of photosynthesis, respiration, and calcification rates. The major advancements of this approach compared to common respirometry approaches are (i) independence of local topography and water depth, (ii) high temporal resolution of metabolic rates (<1h), and (iii) autonomous data collection for >1 week.
As part of this award two remote access water samplers (RAS-500, McLane, USA; picture 1.2 & 1.3) were purchased that are also available for other (non-related) projects.
In-situ organism metabolism and nutrient fluxes
All living organisms and their environment are in constant exchange of components, including O2, CO2, inorganic nutrients, and a range of inorganic and organic carbon compounds. To understand (i) the flow of energy, carbon, and nutrients through organisms in their natural habitat, (ii) how the environment affects organism energy and carbon budgets, and (iii) the relative importance of different taxa contributing to reef community processes, in-situ incubations are necessary. In collaboration with engineers at GEOMAR Helmholtz Center for Ocean Research Kiel, Germany (Ralf Schwarz), Sawall built an innovative fully-automated in-situ incubation chamber setup (BIO-RESORT; 3 years developing time, estimated value $120k, expected finishing date mid 2020) that allows for continuous measurements of metabolic rates in six units simultaneously for up to one week at a time. Next to photosynthesis and respiration rate measurements (with O2 sensors), custom-built automated water samplers connected to each incubation chamber allow her to determine rates of nutrient uptake and release, calcification, and feeding. The BIO-RESORT is an advanced version of the “Coral Hotel” constructed by GEOMAR and previously deployed in the Red Sea. As part of the recently awarded Cawthorn Innovation Fund to Sawall (PI) and BIOS researcher Tim Noyes (co-PI; 2020-2021, $150k), the BIO-RESORT will be thoroughly tested and used initially to study the energy budgets of key species in Bermuda's coral reefs and seagrass meadows.
A second objective of this Cawthorn Innovation project is to improve the determination of gross photosynthesis rates using a stable oxygen isotope tracing method. The determination of real gross photosynthesis (GP = net photosynthesis + respiration) is a persisting challenge for many researchers, and impossible to quantify accurately using common approaches based on concentration changes in O2 or CO2 in the surrounding water (respirometry, picture 1.7). Respirometry-derived GP is based on the assumption that daytime respiration is equal to nighttime respiration, an assumption that has been proven to be outdated, leading to an underestimation of energy acquisition. To improve in-situ GP estimates, we will test and apply an oxygen stable isotopes approach (tracing 18O of 18O-labeled H2O). Accurate GP determination will significantly improve our understanding of energy and carbon flow through organisms and ecosystems.
Students involved in the project:
- Charlie Schneider, Colorado College, CO, USA: 3-months NSF-REU internship (2019).