Dynamics of estuarine drift macroalgae: growth cycles and contributions to sediments in shallow areas

Blooms of opportunistic macroalgae play a major role in nutrient cycling in shallow coastal areas. However, their short life cycles and rapid biomass remineralization can promote feedbacks on sediment nutrient recycling, leading to self-regenerated blooms and the consequent disruption of local biogeochemical cycles. We investigated the potential for algal biomass-sediment nutrient feedbacks in shallow estuarine areas under natural scenarios of biomass accumulation. To do so, the environmental factors driving the dynamics of drift macroalgal blooms and their biomass contributions to sedimentary organic matter were assessed in a 2 yr field survey in shallow bays of the Patos Lagoon estuary, southern Brazil. Interactive effects of hydrological and water physico-chemical parameters modulated the abundance and persistence of algal biomass. The magnitude of the bloom biomass among bays was positively correlated to wind exposure, reflecting the importance of drifting mat advection by onshore wind-driven waves and currents on biomass accumulation. Large biomass accumulations represented a substantial nutrient stock within the system. However, no significant increases in the sediment total organic carbon or total nitrogen were observed. Isotopic analysis revealed overall low contributions of macroalgal biomass to the sedimentary organic matter during senescence, highlighting possible dual effects of wind exposure on biomass accumulation and deposition. Our findings suggest that, in highly hydro dynamic systems, feedbacks between biomass accumulation and sediment nutrient regeneration are unlikely to occur. Given the unknown fate of algal-bound nutrients, more studies are necessary to determine the effects of drift macroalgae-dominated phases on the long-term balance of nutrients within the estuarine system.