Patterns of sediment reworking and transport over small spatial scales on an intertidal sandflat, Manukau Harbour, New Zealand

Measurements of physical sediment reworking and transport were conducted at 22 experimental sites within a 250×500 m study site on a sandflat at Wiroa Island (Manukau Harbour, New Zealand), in order to examine spatial patterns of sediment transport, and its relationship to passive advection of benthic fauna ( Turner et al., 1997). Sediment reworking and transport were measured four times during February 1994 as replacement of dyed sand in pans of sediment buried in the intertidal zone, change in total height of the sediment column in the pans, and as deposition in tube traps with openings flush with the bed (bedload traps) and at 15 cm above the bed (water-column traps). Sediment reworking replaced about 2–3 mm of sand per day, with increasing cumulative transport to a depth of 20 mm during the study period. In addition, there were site-specific differences among sampling dates. Spatial structure in sediment reworking was analyzed by trend surface analysis. Depending on date, variance in reworking was influenced by location within the study site, tidal shear stress (model generated), and elevation on the sandflat. Analysis of residuals demonstrated that sediment reworking at times contained inherent spatial structure after accounting for the effects of other explanatory variables. Bedload trap rates in the final sampling period accounted for most of the variance in deposition indicated by sediment height. Sediment reworking and transport are variable over scales of 10 1–10 2 m, as well as over a period of days, such that measurements determined in single point studies cannot necessarily be extrapolated over larger spatial scales. Patterns of sediment reworking and transport patterns provide a template against which to compare patterns of faunal transport. However, the linkage will be most apparent when 1) sediment reworking and transport are substantial in magnitude, 2) there is significant XY spatial structure to the pattern of sediment transport at the scale of the study, and 3) the fauna of interest are at least potentially transported as bedload (e.g. shelled forms).