Suspended Particulate Loading on the Macrobenthos in a Highly Turbid Fjord: Knight Inlet, British Columbia

Abstract

Knight Inlet is a deep, low-energy but well-oxygenated fjord. The effects of turbidity are principally those of seasonally very high vertical flux from the sediment-laden surface layer. Submersible observations of the benthos are related to suspended load and sedimentation rate data. The bayhead delta slope receives a continual rain of glacial silt (30 cm/yr) and has summer sedimentation rates of 4 kg∙m−2∙d−1.The epifauna of the steep fjord walls shows a marked down-inlet increase in abundance, and the number of species increases from 9 to 45. Brachiopods (Laqueus californianus), solitary corals (Caryophyllia/Cyathoceros), and encrusting sponges are remarkably tolerant of high turbidity. The high sedimentation rates affect the epifauna in three ways: (1) subjection to the ubiquitous "gentle rain" of floes and agglomerates; (2) more locally, microturbidity flows set off by biological resuspension of unstable slope sediment; and (3) major slides of rock and/or sediment. Submarine soil-creep limits the slope infauna, by burrows requiring repeated reexcavation.Down-inlet decrease in sedimentation rate is accompanied by a change in feeding type. In the upper inlet, suspension feeders taking particles < 8 μm (e.g. sabellids, brachiopods) probably utilize bacteria associated with the high vertical flux of SPM. In the middle inlet, selective suspension feeders (e.g. crinoids, gorgonians) sort the SPM for larger particles. In the lower inlet, filter feeders (e.g. barnacles, glass sponges) increase, especially where currents are stronger. The soft-bottom detritus feeders also show a change: from epibenthic forms (e.g. pandalid shrimp, spider crabs) in the upper inlet, through burrowing shrimp in the middle inlet, to ophiuroids in the lower inlet. These changes are correlated with the greater amounts of organic carbon preserved in bottom sediments as they are less diluted by glacially derived inorganic material.The modern-day "turbid association" of brachiopods, solitary corals, and sponges recalls typical outer shelf-basinal shale facies of the Palaeozoic. Turbid, low-energy environments were thus colonized early but faunas have since become less diverse.Key words: fjord, benthos, sedimentation, submersible