Bottom-up control in the benthos of ocean-exposed sandy beaches?

Bottom-up control is a fundamental structuring force in food webs. Food webs of ocean-exposed sandy beaches are predicted to be bottom-up controlled systems, underpinned by imported organic matter rather than in situ primary production. This ecological model of resource-based regulation of biological assemblages is juxtaposed against a prevailing paradigm built around a dominance of physical drivers in sandy beach ecosystems. Surprisingly, given the apparently ubiquitous energetic subsidies of beach food webs, the central premise of bottom-up control has not been tested. Here we experimentally manipulated in situ nutrient levels on a sandy beach to test food web responses at the levels of primary producers (benthic microalgae) and their grazers (meiofauna). The meiofauna community as a whole appeared most strongly influenced by the local physical environment, particularly changes in sediment grain size this supports the traditional environmental control paradigm'. We also detected a significant, positive response of two consumer groups of the meiofauna (nematodes, ostracods) to nutrient enrichment that supports a model of biological, bottom-up control. Although the predicted response of elevated producer biomass following nutrient enrichment was not detected, intense grazing pressure on new, stimulated production may have masked positive responses by the primary producers. Multichannel regulation of food webs is likely for many exposed sandy beaches, albeit an often lower importance of in situ bottom-up forces compared with stronger environmental control.