Ecology of understory kelp environments. II. Effects of kelps on recruitment of benthic invertebrates

Abstract

Previous studies demonstrated that understory kelps altered rates of mass and particulate transport beneath kelp canopies. This study examined the implications of these physical effects, plus shading by canopies, on the recruitment of four taxa of benthic invertebrates. Using field manipulations of understory kelp density and substratum orientation, we examined the effect of kelps on invertebrate recruitment and assessed the mechanisms involved. These mechanisms included reduced flow, increased sedimentation, reduced light intensity and reduced microalgal cover. All effects were significant to recruitment, but their relative importance varied among taxa. Two species examined, the serpulid polychaete Pseudochitinopoma occidental (Bush) and the cheilostome bryozoan Membranipora membranacea (Linnaeus) have long-lived planktonic larvae. Kelp canopies significantly affected recruitment of these two species, but in different ways. Pseudochitinopoma recruitment was reduced by sedimentation and light but was unaffected by ambient flow directly. Recruitment of Membranipora exhibited patterns consistent with that of passive transport of larvae with higher recruitment in high flow, no-kelp environments. Its recruitment was reduced by higher sedimentation beneath canopies, but showed no response to changes in light intensity. Spirorbid polychaetes have short planktonic phases of unknown durations. Kelps significantly affected their recruitment but difference in recruitment rates between kelp-covered and exposed substrata varied in direction with season. Spirorbid recruitment was reduced by light and high levels of sedimentation, but their recruitment patterns were unaffected by ambient flow directly. The cyclostome bryozoan Tubulipora sp. has a very short planktonic period (seconds to minutes) and factors affecting its recruitment were consequently difficult to decipher. It apparently responded negatively to higher light intensity and increased sedimentation. The effects of kelp canopies on recruitment depend on the complex interaction of these variables.