Seiches stimulate transient biogeochemical changes in a microtidal coastal ecosystem

Abstract

We examined the short-term hydrodynamic variability associated with atmospherically forced seiches at a microtidal Mediterranean embayment, its influence in nutrient pumping from the sediment, and the response of phytoplankton. During June 2007, 2 episodes of high-frequency atmospheric pressure fluctuations generated resonant sea-level oscillations. The observed sea-level oscillations varied slightly in both spectral composition and energy, exhibiting 3 predominant oscillation periods: 8.1, 15.2, and 32.9 min. Unusual pulsed increases in dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphate (PO4) concentrations in nearshore seawater were observed a few hours after the commencement of the first seiche. Peak nutrient concentrations in DIN (26 µmol l–1) and PO4 (0.35 µmol l–1) were comparable to sediment porewater values, suggesting that the source of enrichment was the hydraulically driven flux through the permeable sediment. Rapid responses in the size spectra of the phytoplankton community, mainly composed of flagellates, were observed in response to perturbations induced by the seiches. Numerical analysis of the submarine porewater exchange revealed that the 8.1 min sea-level oscillations efficiently forced benthic fluxes. This seiche period particularly affected the surveyed area. Our results show a mechanism by which large-scale forcing drives the local ecosystem dynamics, and emphasizes the importance of episodic events in the bottom-up control of microalgal communities in shallow coastal areas