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), an d compared with the change in benthic nitrogen pools. Data from April 1994 (night and day flood periods), July and November 1994 were analysed in orde r to study possible convective transport induced by the changing thermal re gime on a tidal time scale. An ecological consequence of the flooding of wa rm, permeable sediment beds by cool water was the reversal of porewater den sity gradients, leading to a quick exchange of porewater for flood water, i n 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 dep th scale of 6-10 cm. The Peclet number ranged from 1.4 to 28 in heavily bio turbated environments (D-Bs, for N solutes = 5 x 10(-9) m(2) s(-1)), and fr om 70 to 1400 in non-bioturbated environments (D-Bs for N solutes approxima te to 10(-10) m(2) s(-1)). The results indicate that convective turnover of porewaters in permeable tidal flats is abrupt, may occur with daily freque ncy, and may have three orders of magnitude more impact on sediment-water f luxes than diffusion alone. Convective flow is a major component of sedimen t-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 discuss ed, showing ii to be a very powerful potential accelerator of organic matte r diagenesis in tidal systems due to its frequency and range of action. (C) 2000 Elsevier Science B.V. All rights reserved.