Activities by Hediste diversicolor under different light regimes: Experimental quantification of particle reworking using time-resolved imaging

Abstract

Particle reworking by the ragworm Hediste diversicolor was assessed by quantifying the transport of fluorescent particles (luminophores) added to the surface sediment during a 10day experiment. Plexiglass cores and thin glass aquaria with fauna and controls were exposed to either 12hour light/dark cycles or constant darkness. Luminophore distributions were evaluated by side-view imaging of the aquaria together with destructive sectioning and quantification of tracer distributions in both types of microcosms at the end of experiments. Tracer distributions were evaluated by the gallery-diffusor model, from which the biodiffusion (Db) and the non-local transport (r) coefficients were determined. In addition, the maximum penetration depth (MPD) of luminophores, the 2-D redistribution coefficient, and the transport rate were used as a suite of proxies to quantify particle reworking by fauna.All measured reworking proxies demonstrated a general decrease in transport of luminophores in darkness compared to light/dark cycles. The difference was significant for proxies determined from sectioning. Imaging of particle transport demonstrated that rates were ~30% higher during light/dark cycles, with ~constant amount of particles transported on a daily basis. The effect of light was consistent in the two microcosm types. However, there was a significant difference in Db, while r and MPD were not significantly different between the cores and aquaria. Overall, these results suggest a light-triggered surface feeding by H. diversicolor. Our study highlights the importance of experimental settings for quantification of particle transport by fauna, and that light conditions and types of experimental microcosms need to be carefully considered during investigations of bioturbation in illuminated environments.