Seasonal and spatial variations in the source and transport of sinking particles in the Strait of Georgia, British Columbia, Canada

Abstract

Three sequential sediment traps were deployed at 150 m in the Strait of Georgia from March 1996 to February 1998, one at the southern end of the Strait, one located centrally, opposite the main channel of the Fraser River, and one to the north of the Fraser River channels. Subsequently, all three traps were moored at the central site on a single mooring (150, 225 and 300 m) from February 1998 to January 1999. The highest total flux in the <500 μm fraction (2.2 g cm −2 a −1) was recorded at the southern station and the lowest at the northern station (0.044 g cm −2 a −1). An annual flux peak at all three stations coincided approximately with the Fraser River freshet (May/June), and the southern station exhibited a second flux peak in September, which seemed not to be directly associated with the river's discharge. Lithogenic particles dominated the flux at all three stations, representing about 85% of the total flux at the southern and central stations and 60% at the northern station, although that proportion varied widely throughout the year at all three stations. According to stable isotope composition (δ 13C and δ 15N), the majority of the biogenic material was marine-derived, particularly at the northern station (approximately 80% of organic matter marine-derived), with much of the variation in the trap samples explained by variation in the proportions of marine and terrigenous organic matter. However, a second source of variation was evident in the isotopic composition of the marine-derived organic matter, probably because of seasonal changes in nutrient availability, productivity and the length of the local food chain. As a result, the sinking organic matter in the Strait of Georgia can be described as a mixture of three end-members—terrigenous, marine (bloom) and marine (non-bloom). The flux and compositional data, together with data from sediment cores collected at the same locations, imply that most of the particles settle in the southern Strait, where they may then be transported northward along the bottom toward the central and northern stations.