Observations are presented of interactions between hydrodynamics and suspended particulate matter (SPM) on tidal, spring-neap and seasonal timescalesfrom datacollected in the Hilbre Channel at the mouth of the Dee Estuary (N.W. United Kingdom). Emphasis is placed upon the relationships between SPM andhydrodynamic controls, and the impact of specific biological components. Moored ADCP (Acoustic Doppler Current Profiler), temperature, salinityand LISST (Laser In-Situ Scattering Transmissometer)data were collected during February-March and May-June 2009, supplemented by ship-deployed 25h vertical profiles of transmissometer, LISST, CTD, and fluorescence data taken at the beginning and end of each mooring deployment.The Dee Estuary is a hypertidal estuary with a mean spring tidal range of 7.7 m and strong, near-rectilinear currents dominated by the M2and S2tidal components. During May-June, flood-dominant current velocities resulted in flood-dominant SPM flux (63% by volume, 39% by mass). During February-March, a strong horizontal SPM concentration gradient reduced the flood dominant SPM flux to 16 % by volume and 39 % by mass. It is proposed that low biological activity(measured using low fluorescence as a proxy) enabled erosion of fine sediments from mud flats and saltmarsh within the estuary, thus causinga large horizontal concentration gradient of the fine particle component of SPM. By contrast, during May-June, it is proposed that biological activityand atmospheric forcingincreased the bed sediment erosion threshold, thus suppressing the fine particle concentration gradient. Furthermore, it is proposed that this biological activity mediated the observed increases in the strength, size, density and settling velocity of the flocs that made up SPM. In May-June, SPM composition changeddue to a contribution of particles of uniform density but variable size, suspected to be either diatomsor aggregates ripped from the bed sediment of the intertidal areas.During February-March, flocs ruptured under the high turbulence conditions of floodand ebb tides, and reformed at high and low slack waters. By contrast, during May-June under comparable turbulence conditions, flocsresuspended on flood and ebb tides without significant breakup due to their increased strength.
Seasonal changes to SPM properties potentially influence SPM flux via two differentmechanisms:1.Biological and atmospheric suppression of resuspension in May-June reduces the horizontal concentration gradient that is observed in February-March. Consequently, flood dominance of SPM flux is greater in May-June. This must increase net up-estuary transport in May-June.2.SPM undergoes rupture and flocculation due to reduced floc strength in February-March. Flocculation and enhanced settling are greatest over an extended low water, so that flocs are deposited further up the estuary than their original starting point when they were resuspended by the flood. This should increase net up-estuary transport in February-March. These observations suggest that seasonal variations of SPM properties are important for net transport within estuariescharacterized by large intertidal flats.