RELATIVE EFFECTS OF ELEVATED GRAZING PRESSURE AND COMPETITION FROM A RED ALGAL TURF ON 2 POSTSETTLEMENT STAGES OF FUCUS-EVANESCENS C. AG

In Nova Scotia, Canada, the lower shore of wave-exposed and semi-shelt ered portions of the coast is dominated by a dense turf of the red alg a Chondrus crispus C. Ag. The mid shore is dominated by three species of rockweed belonging to the genus Fucus, which are also physiological ly competent in the low zone, but may be inhibited by pre-emptive comp etitive exclusion by red algal turf and/or increased grazing pressure from abundant mesoherbivores. A grazer assay showed that herbivore pre ssure is > 50% higher in the Chondrus zone than in the rockweed zone. Fucus can colonize the low shore only through microrecruits which deve lop from planktonic zygotes. A main objective of this study was to tes t the hypothesis that early post-settlement stages represent a critica l phase for successful recruitment. We, therefore, tested the relative effects of competitive pressure from Chondrus and grazing on small (2 mm) and larger (6 cm) Fucus evanescens plants in a three-way factoria l experiment. Red algal turf and grazers independently depressed F. ev anescens growth, but only a combination of factors resulted in a signi ficant weight loss in the rockweeds. The effect size of grazing was mo re than double that of competition from the turf. Small and larger ind ividuals of F. evanescens were equally affected by grazing and competi tion. The results showed that colonization by rockweeds may only occur where there are gaps in the Chondrus canopy. We hypothesized that gra zer effects may decrease with increasing gap size. However, a controll ed experiment showed no differences in Fucus recruitment among the ran ge of naturally occurring gap sizes. That is, larger gaps do not promo te rockweed recruitment. In concordance with this observation, we foun d that grazer abundances were similar among gap sizes. Chondrus was sh own to be highly resistant to intense physical and herbivore-induced d isturbance. This combination ensures the persistence of a dense canopy which largely excludes rockweed recruitment from zygotes. Competitive dominance of Chondrus on the low shore is not obviously related to in dividual growth rates or canopy height. A positive correlation between growth rate and competitive ability may be expressed only in morpholo gically similar species which are likely to compete as germlings. (C) 1998 Elsevier Science B.V.