| Summary: | The distribution of various burrowing and tube-dwelling organisms, their biogenic sedimentary structures, and the rates at which they turn over sediment are Investigated on three different tidal flat environments of the Fraser Delta. The organisms studied include Callianassa californiensis and Upogebia pugettensls, both thalassinldean burrowing shrimps, the burrowing
polychaete Abarenicola sp., the tube-dwelling polychaetes, Praxillela sp. and Spio sp., the bivalve Mya arenaria and the gastropods Batlllaria attramentaria and Nassarius mendicus. Thalassinidean shrimps are of most interest because they are widespread over the Delta, in particular Callianassa and because their distinctive burrows are well known in the geological record.
The 'marine' tidal flats of Boundary Bay on the inactive southern flank of the Fraser Delta are mantled with fine to very fine,well to very well sorted sands. The intertidal region has five floral/sedimentological zones delimited primarily by elevation and exposure and characterized by distinctive
macrofaunal assemblages. These are from the shoreline seawards, the saltmarsh, algal mat, upper sand wave, eelgrass and lower sand wave zones. Topography of both small and large scale of biogenic or physical origin
creates lateral heterogeneity within the biofacies of each zone.
An estimated 4.25 x 10⁸ Abarenicola on Boundary Bay tidal flats annually
rework about 10⁶m³ of sand. The bioturbation of this worm may be a factor limiting the extent of the algal mat zone. By irrigating its burrow, Abarenicola can separate a sand/clay mixture by floating the clay out in the head shaft irrigation current.
Thalassinidean burrowing shrimps are most abundant on the 'marine' tidal flats of southeastern Roberts Bank on the active Delta-front. These tidal flats can divided into four floral/ sedimentological zones: the saltmarsh, algal mat, sandflat and eelgrass zones. Thalassinidean burrowing shrimps dominate the sandflat zone. Upogebia densities are positively correlated to mud content
of the sediment. Callianassa show no clear grain size preference and are abundant in sediments ranging from 5 to 50% in mud content and from
2.6 to 4.0 Ø in median grain size. At their peak density (446 burrow openings
m⁻²) Callianassa rework the substrate they live in to a depth of 50 cm in about five months.
On central Roberts Bank a major transition from a 'marine' to a brackish environment occurs. A brackish marsh zone extending to much lower intertidal levels than the saltmarsh laterally replaces the algal mat zone and the upper half of the sandflat zone. A sandflat/mudflat zone cross-cut by channels displaces the eelgrass zone and lower half of the sandflat zone. The peak in Callianassa distribution moves to lower intertidal levels because of the presence of low salinity water at higher tidal levels and because of the absence of eelgrass in lower intertidal regions. Upogebia although physiologically
better adapted to cope with reduced salinity demonstrates lower tolerance of brackish water in its distribution than Callianassa, probably because the function of its mud-lined burrow as a conduit for suspension feeding and respiration exposes Upogebia to low salinity surface waters, while Callianassa, in its unlined burrow used for deposit feeding, is protected
from surface waters by high salinity interstitial waters. The distinction between these two types of burrow is considered to be very significant for paleoenvironmental reconstructions.
A new system of subdividing the intertidal region into exposure zones (the atmozone, amphizone and aquazone), based on critical tidal levels at which the maximum duration of continuous exposure or submergence 'jumps,' is advocated. It allows cross correlation between different tidal regions experiencing different types of astronomically controlled tides and much of the intertidal zonation of Fraser Delta tidal flats may be causally related to these exposure zones. === Science, Faculty of === Earth, Ocean and Atmospheric Sciences, Department of === Graduate
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