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 Christopher David McQuaid ⁎, Bruce Petrus Mostert Department of Zoology and Entomology, Rhodes University, Grahamstown 6140, South Africa abstract article info Article history: Received 18 November 2009 Received in revised form 7 January 2010 Accepted 8 January 2010 Keywords: Bottom-up regulation Intertidal Mussels Perna perna Rocky shores 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. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Ecological communities are influenced by both top-down effects that flow down through the food chain (e.g. predation and competition), and by the bottom-up supply of resources such as food, nutrients or propagules. Generally, top-down effects are understood to involve relatively small-scale species interactions happening at scales of 10 s m, while bottom-up factors are perceived as acting over much larger spatial and temporal scales (Menge, 2000), though the two can be closely interlinked (Menge et al., 1997b). In the case of intertidal communities, the small-scale of top-down effects comes from the fact that most benthic organisms are physically small and will perceive their environment and interact with one another on very small physical scales. On the other hand, these communities are interlinked by the transport by water of materials that originate elsewhere. For example, larval transport ensures that populations with planktonic propagules are demographically open at some scale that is large relative to body size. Similarly, re-mineralization of nutrients occurs primarily below the euphotic zone so that nutrient generation and use are physically uncoupled though small-scale fertilisation of primary producers through guano input (Bosman and Hockey, 1986; Methratta, 2004) or the excreta of epifauna can occur (Probyn and Chapman, 1983; Williams and Carpenter, 1988). Consequently the bottom-up supply of propagules and nutrients is usually mediated by large-scale water flow. For filter-feeders, food particles originate elsewhere because phytoplankton require time in the water column to grow and divide while detritus requires time to degrade. However, the local supply of food may be regulated within shores through small-scale effects on hydrodynamics and this in turn has the potential to shape the community by affecting interactions among competitors for primary space (Erlandsson et al., 2005). Filter-feeding benthic macro-inverte- brates are ecologically important in coastal waters because they provide habitats for other species, and extract large amounts of phytoplankton from the water column during feeding (Ward and Shumway, 2004; Lassen et al., 2006) so that they act as a trophic link between the pelagic and benthic subsystems. Rates of filter-feeding depend heavily on hydrodynamic factors such as water column mixing and currents, with feeding generally being enhanced by processes that create turbulence (Lassen et al., 2006). Consequently there are clear site-scale effects of water flux on community structure. There are more filter-feeders and they show higher growth rates on Journal of Experimental Marine Biology and Ecology 384 (2010) 119–123 ⁎ Corresponding author. Tel.: + 27 46 603 8535. E-mail address: c.mcquaid@ru.ac.za (C.D. McQuaid). 0022-0981/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2010.01.005 Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe