Density-driven convection during flooding of warm, permeable intertidal sediments: the ecological importance of the convective turnover pump

Vertical temperature profiles during exposure and flooding of a sandy tidal flat were investigated at a single site in the Sado estuary (Portugal), and compared with the change in benthic nitrogen pools. Data from April 1994 (night and day flood periods), July and November 1994 were analysed in order to study possible convective transport induced by the changing thermal regime on a tidal time scale. An ecological consequence of the flooding of warm, permeable sediment beds by cool water was the reversal of porewater density gradients, leading to a quick exchange of porewater for flood water, in what constitutes a rapid, powerful pathway for benthic solute removal in tidal ecosystems. The matrix-averaged interstitial velocity of up- and down-draught plumes of water ranged between 10−7 and 10−6 m s−1 over a depth scale of 6–10 cm. The Peclet number ranged from 1.4 to 28 in heavily bioturbated environments (DBs for N solutes=5×10−9 m2 s−1), and from 70 to 1400 in non-bioturbated environments (DBs for N solutes ≈10−10 m2 s−1). The results indicate that convective turnover of porewaters in permeable tidal flats is abrupt, may occur with daily frequency, and may have three orders of magnitude more impact on sediment-water fluxes than diffusion alone. Convective flow is a major component of sediment-water fluxes in tidal areas and crucial to accurate budget studies on the sandy intertidal. On the basis of these results and recent literature, the potential ecological importance of the Convective Turnover Pump is discussed, showing it to be a very powerful potential accelerator of organic matter diagenesis in tidal systems due to its frequency and range of action.