CONTROLS OF TIDAL CHANNEL CONNECTIVITY ON SEDIMENT FLUX INTO AMAZONIAN MANGROVE FORESTS

To determine how the connectivity of tidal channels regulates sediment accumulation on mangrove flats, hydrodynamic and suspended-sediment measurements as well as sediment cores were collected within Amazonian mangrove forests during the dry and wet seasons. Two tide-dominated channels were studied: a channel with limited connectivity that terminates inland and opens to a minor estuary and a channel with high connectivity that opens at each end to an estuary, one minor and one major. Results indicate that channels with greater connectivity exhibit higher magnitude flux imports and exports and serve as conduits for sediment from source to sink zones, while channels with less connectivity can import sediment to the mangrove flats throughout the year.

In the terminating channel, tidal asymmetry is controlled by prolonged water storage in the highly frictional mangrove forest, which interacts with offshore tidal forcing. Flood tide velocities remained consistent [generally ~0.25-0.50 m s^-1] and suspended-sediment concentrations were relatively high as water flowed over the banks and into the mangrove forests. Velocities peaked during the tidal pulse of ebb tide [>1 m s^-1], but sediment resuspension was less substantial than that of flood tide in both time duration and magnitude. Thus, sediment was imported into the mangrove forest in both the dry [~2.6 kg m² day^-1] and wet [~12.2 kg m² day^-1] seasons.

Tidal asymmetry and sediment flux in the highly-connected channel is controlled by out-of-phase water levels in the two estuaries as well as pooling associated with sandy shoals. During both rising and falling tides, comparable concentrations of sediment were in suspension and overbank water velocity was almost exclusively towards the minor estuary [often >1 m s^-1]. Thus, net water and sediment fluxes were towards the minor estuary during both seasons, and sediment was exported out of the mangrove forest to the channel during the dry season [~2.5 kg m² day^-1] and imported during the wet season [~18.5 kg m² day^-1].

These results demonstrate that channels with more connectivity are less efficient at importing sediment into adjacent mangrove forests during the dry season but more efficient during the wet season because they are more directly impacted by fluctuations in fluvial sediment source availability.