Rhodoliths (free-living, non-geniculate red coralline algae growing as balls, branched twigs, or rosettes) often form dense aggregations, termed rhodolith beds, at depths of up to 150 m in tropical to polar seas. The important contribution of rhodolith beds to marine biodiversity and global calcium carbonate production has, in part, triggered the recent increase in number of studies of factors and processes regulating their structure and function with, however, little attention to feeding

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... onships. The present thesis investigated trophodynamics of a rhodolith (Lithothamnion glaciale) bed in St. Philip's, Newfoundland (Canada) with a combination of lipid, fatty acid, and stable isotope analyses. I quantified lipid composition, energy transfer, essential nutrients, and seasonal and spatial variability among six dominant echinoderm, bivalve, gastropod, and polychaete species, two algal species, seawater, and sediment beneath individual rhodoliths. Results suggest strong benthic-pelagic coupling in that rhodolith bed organisms utilize phytoplankton, especially diatoms, as a main food source during blooms. I identified three distinct trophic levels among associated macrofauna and flora (producers, suspension/filter feeders and grazers, and predators), while discovering a potential resource partitioning relationship among organisms in which first- and second-order consumers share a common resource (diatoms or kelp). Diets shifted in response to seasonal food availability and life history requirements, but were unaffected by riverine input proximity. Based on my findings, I conceptualized a partial food web detailing the interpreted linkages among food sources and organisms in the studied rhodolith bed. This proposed food web delineates relationships among organisms and essential nutrients in cold water systems, while demonstrating the interconnectedness of various rhodolith bed components.