IN-SITU CHEMICAL INHIBITION OF BENTHIC DIATOM GROWTH AFFECTS RECRUITMENT OF COMPETING, PERMANENT AND TEMPORARY MEIOFAUNA

To reduce the supply of a potentially limiting food for meiobenthos, w e developed a new method to inhibit benthic diatom growth by perfusing pore waters with herbicide diffusing from a buried gel. Placing the g el in a bottom-sealed chamber constrained the chemical gradient to the vertical dimension so that concentrations in overlying pore waters co uld be predicted from a one-dimensional diffusion model. Our goal was to determine whether the meiofaunal oligochaete Amphichaeta leydigii a nd juveniles of the polychaete Hobsonia florida, whose adults deposit feed, share benthic diatoms as a limiting resource during recruitment. We verified that gels containing the herbicide DCMU inhibited microal gal growth in the laboratory. The field DCMU manipulation also signifi cantly reduced diatom abundance relative to control gels. The recruiti ng H. florida population was divided at median size into two classes. Development of feeding tentacles begins at that size-suggesting that s ize classes differ in feeding mode. Smaller juveniles showed no signif icant response to the treatment, while larger juveniles and oligochaet es both had significantly reduced abundances in DCMU patches. No other dominant meiofauna showed significant responses to the treatment. Cal culated concentrations of DCMU in surficial sediments were approximate ly 0.01-0.1 x those that cause lethal or sublethal effects on animals, and nontarget invertebrates showed no significant effects-implying th at A. leydigii and H. florida abundances varied because of the effect of DCMU on diatoms rather than as a direct response to the chemical it self This evidence of a shared resource suggests food limitation as a competitive bottleneck between the meiofaunal oligochaete and recruiti ng H. florida juveniles and a critical stage in the life history of th e deposit-feeding polychaete.