Growth controls of rhodoliths (Lithothamnion glaciale) and relationships between structural complexity and macrofaunal diversity in subarctic rhodolith beds

Bélanger, David (2020) Growth controls of rhodoliths (Lithothamnion glaciale) and relationships between structural complexity and macrofaunal diversity in subarctic rhodolith beds. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Coastal benthic ecosystems are major contributors to oceans global productivity and biodiversity. Research has historically focused on charismatic ecosystems such as coral reefs, kelp beds and seagrass meadows. This paradigm may overshadow the biogeochemical functions and ecological importance to coastal oceans of other less studied communities. Rhodoliths are benthic, unattached, slow-growing coralline red algae. Rhodoliths may aggregate into extensive and diverse communities called rhodolith beds, which occur within the photic zone in all oceans, from the tropics to the poles. This thesis used long-term laboratory and field experiments as well as seasonal surveys to characterize controls of growth in Lithothamnion glaciale rhodoliths and to investigate the relationship between structural complexity and the diversity of rhodolith-associated macrofauna in a subarctic rhodolith bed from southeastern Newfoundland. Results showed that rhodolith growth is negatively affected by elevated nutrient (N and P) concentrations and biofouling. Rhodolith growth was mainly controlled by irradiance and was unaffected by temperatures between ~1 and 17°C, but appeared to be inhibited by temperatures ≤ 0.5°C. Rhodolith bed structure showed little annual spatial and temporal variations. Macrofaunal density scaled positively with total rhodolith volume per surface area, whereas biomass did not. Results also suggest that rhodolith morphology exerts a control on the diversity of macrofauna associated to rhodoliths. Macrofaunal assemblages varied spatially and temporally in most taxonomic groups with few, uncommon taxa being generally responsible for dissimilarity between sites and among seasons. These findings provide novel insights into the ecology and vulnerability of rhodoliths to anthropogenic threats and climate change while elevating the importance of subarctic L. glaciale beds as a key ecosystem in the Northwest Atlantic.

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