The role of mechanical and chemical processes in Rhodalgal sediment production and implications for ocean acidification (Baja California, Mexico)

Rhodoliths are free-living coralline algae that produce carbonate sediments in shallow water marine systems worldwide. Rhodalgal sediments, which result from rhodolith breakage and chemical weathering, mix together with living rhodoliths and form shallow water habitats known as rhodolith beds, support a diverse assemblage of organisms. Rhodoliths and sediment cores collected from the El Requeso?n rhodolith bed in Bahi?a Concepcio?n, Me?xico were used to study the basic mechanical and chemical processes involved in rhodalgal sediment production and basic framework of a rhodolith bed. Results showed four major groups of rhodalgal sediments produced from rhodolith breakdown: "cores," "branches," "crumbs," and �dust� that ranges in size from pebbles, sands, to silts. Dissolution of rhodoliths was evident at seawater below pH 7.5 and at 30% dissolution, core breakdown was accelerated and smaller branches were produced. The general vertical trend of coarse-fine-coarse sedimentary texture indicates the temporal dynamics of a rhodolith bed, suggesting movement in south-north-south direction or expansion-shrinkage-expansion of the active part of the bed from past to present. The effect of 30% dissolution seen in the experiment suggests that dissolution occurring in future high CO2 ocean conditions would cause structural changes to shift towards more compacted framework with smaller interstitial spaces, hence changing the habitat quality of the bed.