Sedimentology, ichnology, ecology and anthropogenic modification of muddy tidal flats in a cold-temperate environment: Chignecto Bay, Canada

In Chignecto Bay, upper Bay of Fundy, Canada, the muddy tidal flats exhibit distinctive sedimentological and ichnological characteristics indicative of winter conditions and the development of ice. From late spring to autumn (May–October: mean temperature +13.6 °C), the mud flats sustain a high infaunal biomass and sediment deposition is dominated by tidal processes. Neap–spring tidal rhythmites, fluid-mud deposition and high levels of bioturbation are all characteristic of summer deposits. During the winter, temperatures remain below zero (December–March: mean temperature −6.3 °C), and ice forms in the bay and periodically on the mud flats. Ice rafts and blocks are common on the tidal-flat surface, and these blocks deform the muddy sediment, cut deep scours and deposit allochthonous sediment (including gravel) across the flats. Intermittent storms in the autumn and winter also contribute to sediment scouring and erosion. Annual die-offs of infauna are reflected by reduced bioturbation in winter deposits. Each spring, renewed larval recruitment and opportunistic colonization results in increasing levels of bioturbation, and the trace suite is dominated by a low-diversity assemblage of diminutive, vertical burrows. The summer–winter cyclicity in infaunal colonization manifests, ichnologically, as a distinctive bioturbated–non-bioturbated bedset character.

Infaunal populations in Chignecto Bay peak in the late spring and in the late summer–early autumn. Both population peaks are exploited by vertebrates, including migrating Atlantic sturgeon (late spring) and migratory shorebirds (late summer–early autumn). In particular, the upper Bay of Fundy, including Chignecto Bay, is a staging ground for approximately 42–74% of the world’s Semipalmated sandpipers (Calidris pusilla) on their annual migration from the Arctic to South America. Fortunately, anthropogenic modification of Chignecto Bay, and its associated bays and rivers (e.g. construction and dismantling of a causeway over the Petitcodiac River), has not had a significant effect, to date, on the mud-flat habitat or the infaunal biomass.