The effects of within-shore water movement on growth of the intertidal mussel Perna perna: An experimental field test of bottom-up control at centimetre scales

Top-down effects on intertidal communities concern species interactions such as competition and predation, and occur on small-scales. In contrast, bottom-up factors concern the supply of resources such as food and recruits. These are generally understood to be large-scale effects that are oceanographically controlled.Here we examined the effects of small-scale bottom-up factors on the growth of intertidal mussels to test the hypothesis that such factors can also operate on small (cm m) spatial scales. L-shaped aluminium baffles were used to reduce flow around mussels on very small (cm) scales in before/after experiments run at different sites in 2004 and 2006. Each experiment had two phases, each phase lasting approximately 100 days. At the start of phase one, mussels were individually marked at the growing edge of the shell in 10 control and 10 treatment patches before applying the treatment. At the start of phase 2, the same individuals were marked again at the new growing edge and baffles were placed around treatment patches. After phase one, there was no significant difference in growth rates between treatments in either year. In both years, applying the treatment reduced growth rates by approximately 30% compared to control patches. Condition index at the end of phase two showed the reverse and was lower for control mussels. Untreated control patches also showed a significant temporal effect (i.e. differences in growth between the two phases) during 2004 but not 2006. The results clearly show that bottom-up factors associated with food-supply operate at very small, within-shore scales, strongly influencing growth rates and possibly competitive abilities. They also indicate that bottom-up and top-down effects can interact across multiple spatial scales.