Rhodolith beds are recognized internationally as a unique ecosystem - built by free-living coralline algae - providing substrate and habitat for numerous algae and sessile invertebrates. In addition, their ability to calcify and their high abundance and biomass make rhodoliths major carbonate producers. Hence, giving the increasing role of marine ecosystems in the removal and storage of carbon (blue carbon), rhodolith beds may represent a not yet considered significant carbon store.
The EU-funded RHODOCAR project addresses this question by determining the carbon fluxes associated to rhodolith and rhodolith-bed community metabolism, carbonate production and storage, as well as their responses to global and local stressors. This information allows assessing the importance of rhodolith beds as natural carbon sinks, thus, help ascertain whether these ecosystems meet the requirements to be integrated into climate mitigation policy, and will further allow quantifying the effects of global climate change on their carbon sequestration and storage ability. In addition, it will help recognizing potential interactions between global and local stressors and hence, aid in the development of effective local conservation and management strategies. RHODOCAR uses an integrative physiological approach to determine and scale up individual and community productivity and the responses to global and local stressors to define the implications for ecosystem functioning and services. This approach includes a combination of laboratory, multi-factorial mesocosm and in situ experimentation, thus covering a wide range of complexity (cellular-organism-community). In addition, the project is designed to operate at a wide geographical scale, including rhodolith beds from different latitudes.
The project is divided into the following objectives that also represent the different work packages:
Work Package 1: Mechanistic understanding of rhodolith calcification
Short-term laboratory experiments were used to (a) assess the link between photosynthesis and calcification in different rhodolith species, (b) determine carbon-use strategies and physiological pathways to identify potential species-specific differences, and (c) assess the importance of rhodolith morphology on calcification and ocean acidification responses.
Results: Rhodolith species vary in their biological control of the calcification mechanism due to species-specific and morphological differences, which indicate the importance of rhodolith-community composition regarding the impacts of environmental stressors (e.g., climate change)
Work Package 2: Impacts of multiple global and local stressors
The effects of the increasingly more frequent and intense heatwaves on a rhodolith bed in Southern Portugal were investigated through a mid-term mesocosm experiment (weeks), in which different temperature regimes were simulated. The location of the studied rhodolith bed is influenced by frequent inversions between upwelling and downwelling conditions and the associated strong and fast temperature decreases and increases, respectively, which also allowed determining the effects of recurrent thermal stress on rhodolith primary and carbonate production.
Results: Impacts of environmental changes, such as marine heatwaves, are mitigated in rhodolith beds in highly fluctuating environments, where the recurrent stress increases their resilience.
Work Package 3: Carbon sequestration and storage potential of rhodolith communities: Implications for Blue Carbon
Field campaigns at different locations were performed, combined with on-site laboratory measurements of collected rhodolith samples. The biomass of rhodoliths (living and dead) per rhodolith bed area was determined through field collections and these data sets were combined with laboratory measurements of primary and carbonate productivity of the living rhodoliths, carbonate production/dissolution measured in dead rhodoliths and environmental in-situ light data for the respective rhodolith bed. This budgeting approach allowed determining and comparing the daily productivity and associated carbon fluxes of rhodoliths per m2 of rhodolith bed among the different locations.
Results: Most studied rhodolith beds can act as carbon sinks, though the magnitude depends on rhodolith biomass, predominant rhodolith species and environmental conditions, such as light availability.