Spiralling of particles by suspension feeders in a small lake-outlet stream
Suspension feeders in streams trap particles from the water column and their egesta can be captured by other individuals downstream. The resultant retention and cycling of organic matter has been termed spiralling. We investigated its significance in a lake-outlet stream that had dense aggregations of suspension-feeding black fly and midge larvae in a thin film of water. We examined the abundance of dye particles, added as a pulse, in larval guts at six sites along the outlet stream (0.15–6.94 m from the top of the dam). Samples were taken 15 min, 60 min, and 24 h after the dye had been added. From samples taken after 15 min we estimated the average spiralling length to be 5.6 m. After 24 h very few particles remained, but there were significantly more in both black fly and midge larvae at sites downstream than in those upstream.