Physical disturbances in an Australian kelp community. II. Effects on under-storey species due to differences in kelp cover

Physical disturbances in a sublittoral kelp community were investigated for their roles in structuring benthc assemblages. Effects of storms that lead to partial and/or complete denudation of kelp plants were investigated in a series of manipulative field experiments. Effects on understorey specles due to differential damage to kelp plants were examined by sampling replicate plots of treatments that imitated the various kinds of damage to kelps. Partial damage to kelp canopies led to similar effects on species to complete removal of kelps, because damaged plants invariably died. Encrusting algae and sponges decreased in cover in manipulated areas while microalgae and then brown turfing species increased in cover. Consequences of living on the borders of clearings in the kelp forest were investigated. Understorey assemblages here contained abundances of both those species found under the kelp canopy and those in the centres of clearings. Effects on understorey species due to thinning the kelp canopy to various densities was investigated. Thnning kelp canopies had similar effects for most species as complete removal of canopies. Fluctuations in abundances of certain species required the removal of at least 50 % of the kelp canopy. Effects on colonizing species of providing clean primary substrata under kelp canopies (as when kelp holdfasts are detached) were investigated. The identity of the various micro-algae that colonized areas of bare substrata under the canopy showed marked variabhty among replicate sites. Results illustrated several complex influences that physical disturbances have on the structure of a kelp community, and ~ndicate the need for comprehensive experimental studies of the effects of physical, disturbances.