Exchange of nutrients across the sediment–water interface in intertidal ria systems (SW Europe)

Concentrations of nitrate, nitrite, ammonium, phosphate and silicate were determined in river water, tidal water that floods the intertidal sediment (flooding water) and pore water of those sediments in the Northern Galician Rias of Ortigueira and Viveiro (NW Iberian Peninsula). The field surveys were done in the productive seasons of spring and summer 2008. Short-sediment cores and tidal flooding water were sampled at the intertidal area during the first 20 min that the tide inundates the sampling site. Nutrient fluxes of rivers (Lourido and Landro) flowing into the rias were in the order of H4SiO4 > NO3− > NH4+ > HPO 4 − 2 Nutrients input from those rivers were low relative to the nutrient discharge of the entire coastal area. Striking changes of nutrient concentrations in flooding and pore waters of intertidal sediments were observed in the short periods of tidal inundation. Nutrient fluxes driven by molecular diffusion and tide-induced transport across the sediment–water interface were quantified and compared to the nutrient river contribution. Diffusive fluxes ranged from 9.3 to 13.7 nmol·cm− 2·d− 1 for nitrate and nitrite, − 1.32 to 30.1 nmol·cm− 2·d− 1 for ammonium, − 0.01 to 0.49 nmol·cm− 2·d− 1 for phosphate, and − 13.2 to 0.2 nmol·cm− 2·d− 1 for silicate. Tide-induced transport always exceeded diffusive fluxes, with differences reaching up to four orders of magnitude for silicate. The overall results of this study emphasize the relevance of tidal water movement in promoting the sediment–water exchange of nutrients in intertidal sub-ecosystems.