Sediment resuspension and boundary layer flow dramatically increase the growth rates of interface-feeding spionid polychaetes

Abstract

Most spionid polychaetes switch from surface deposit feeding to suspension feeding as current velocity and the flux of suspended food particles increase in the benthic boundary layer. This hydrodynamically mediated feeding behavior has been well studied, but the impact of flow on the growth rates of juvenile spionids has not been tested extensively. I performed two experiments in counter-rotating annular flumes to measure the growth of Polydora cornuta and Streblospio benedicti in a range of flow conditions. Experiment 1 tested the effects of sediment resuspension. The body volumes of individual worms were measured before and after a 7-day exposure to steady, unidirectional flow velocities ranging from 0.5 to 18 cm s −1 ( U 5mm=velocity measured 5mm above bottom). Sediment collected from the worms' field site was added to each flume as the only food resource. The sediment's critical erosion velocity was ∼13 cm s −1 ( U *=0.9 cm s −1). When flumes were set to U 5mm≤12 cm s −1, the relative growth rate (RGR) of small Polydora (initial body length=1–4 mm) equalled 0.19 day −1 on average, and the RGR of a larger class of juvenile Polydora (6–9 mm) equalled 0.06 day −1. When U 5mm=15 or 18 cm s −1 and some sediment became resuspended, the RGR of small and large Polydora equalled 0.37 and 0.22 day −1, respectively. Streblospio grew more slowly than Polydora, but showed a similar relative increase in RGR when flow exceeded the erosion velocity. To better test the effects of non-erosive flows, a second experiment was performed with phytoplankton added to the flumes. Experiment 2 tested four velocities ( U 5mm=3, 6, 9, or 12 cm s −1), and each flow treatment was replicated four times. Increases in velocity and the flux of microalgae significantly increased the RGR of Streblospio and Polydora, with an average RGR of 0.53 day −1 for small Polydora exposed to U 5 mm=12 cm s −1. The growth rates of Polydora and Streblospio in the flume experiments are much greater than those reported for similar polychaetes in still water, demonstrating that studies of spionid nutrition and population dynamics must consider realistic flow regimes.