Production of Aquatic and Terrestrial Invertebrate Drift across Habitat Types in Small Streams

Invertebrate drift- the downstream transport of aquatic and terrestrial invertebrates – provides the primary prey base for many juvenile salmonids and is a key factor limiting productive capacity in small streams. Despite the importance of drift to salmonid production and habitat use in streams, spatial variation in drift production remains poorly understood. This is partly because biologists typically measure drift concentration (a measure of standing stock), rather than production, i.e., the net entry rate of invertebrates over a spatially explicit area of streambed. Currently, field estimates of drift production are virtually non-existent. We attempted to address this gap using a novel approach to measure drift production across different habitats types in small, coastal forest streams in Southwest British Columbia. We specifically tested several predictions of spatial drift dynamics: 1) that aquatic drift production would be greater in riffles than pools; 2) that drift production would increase with riffle length  and 3) that terrestrial drift production will be unrelated to pool-riffle habitat type. These predictions were broadly supported, and indicate that variation in habitat structure will affect prey production rates in addition to available habitat for salmonids.