Nutrient production in rhodolith beds: Impact of a foundation species and its associates

Foundation species harboring associated fauna and micro-organisms provide an op - portunity to explore nutrient contributions from excretion and decomposition (recycled nutrients) relative to new inputs. Rhodoliths, free-living non-geniculate coralline algae, support di verse cryptofaunal and microbial assemblages which could supply regenerated nutrients to their hosts, but studies of such nutrient dynamics are limited. We assessed temporal variability in concentrations of dissolved inorganic nutrients (ammonium: NH₄ ⁺, nitrate + nitrite: NO_x ^-, and ortho phos - phate: PO₄ ^3-) associated with rhodolith beds on the Gulf of California and Pacific sides of Baja Cali fornia Sur, Mexico, and considered potential sources of nutrient enrichment from cryptofauna, microbes and allochthonous inputs. Data suggest rhodoliths and/or sediments provide NH₄ ⁺, NO_x ^- and PO₄ ^3-in excess of water column availability, regardless of season or year. Cryptofaunal biomass was positively correlated with rhodolith diameter at all sites, and in some cases so was nutrient concentration. Ammonium and PO₄ ^3-enrichment in rhodoliths may be attributed to cryptofaunal excretion or decomposition by micro-organisms. While seasonal delivery of new nutrients (NO_x ^-) via wind-induced water motion occurred in the Gulf, NO_x ^-in the Pacific was likely im - pacted by microbial-mediated nitrification. Light and dark incubations showed that rhodolith assemblages may initially take up NH₄ ⁺ over NO_x ^-, but all nutrients were consumed in the presence and absence of light. This study highlights the importance of rhodoliths in coastal and estuarine environments not only as foundation species, but also for their role in nutrient cycling, especially in areas of seasonal nutrient limitation.