TOP-DOWN AND BOTTOM-UP REGULATION OF NEW ZEALAND ROCKY INTERTIDAL COMMUNITIES

Abstract

Studies on the west coast of North America suggest that nearshore oceanographic conditions can have important effects on rocky intertidal community structure and dynamics. Specifically, upwelling-dependent processes in coastal waters can affect both top-down and bottom-up processes on adjacent rocky shores. As a first step in testing the prediction that similar linkages occur elsewhere, we investigated the effects and rates of predation, grazing, and recruitment on rocky intertidal community dynamics at upwelling and non-upwelling sites on the South Island of New Zealand. Comparative-experimental studies were done at each of two sites on both the east and west coasts of the South Island. We quantified benthic community structure, maximal wave force, nearshore sea-surface temperature, air temperature at low tide, nutrient concentrations, survival of mussels, rates and effects of predation, rates and effects of limpet grazing, recruitment of mussels and barnacles, and RNA:DNA ratios (a growth index) of mussels. Overall, zonation patterns were similar on the upper shore on both coasts: barnacles (Chamaesipho columna) dominated the high zone, and mussels (Mytilus galloprovincialis) the middle zone. In the low zone, however, community structure differed markedly between coasts. East-coast low-zone communities were dominated by mussels with a moderate canopy of kelp, primarily Durvillea spp., while mussels were largely absent from west-coast low zones. Food webs were similar on the different coasts and included predaceous whelks, sea stars, oystercatchers, and herbivorous limpets. Field experiments showed that only sea stars and limpets had strong effects at west-coast sites, and only limpets had strong effects at east-coast sites. The sea star Stichaster australis, previously identified as a keystone species on the west coast of the North Island, was common and important on the west coast of the South Island but was absent from the east coast. Physical conditions (wave forces, low-tide air temperature) were comparable on the two coasts, suggesting that other factors caused the differences in low-zone community structure. Experiments and observations indicated that predation, grazing, prey recruitment, and mussel growth were greater on the west than on the east coast. While some between-coast contrasts in community dynamics could emanate from differences in species composition (e.g., the absence of S. australis from the east coast), the higher west-coast rates of most of the ecological processes studied suggest that between-coast differences may also depend on other factors. Among the alternatives, a difference in nearshore oceanographic conditions on the opposite coasts of the South Island seems most likely. Prior oceanographic research, and our onshore measurements of sea-surface temperature and nutrients indicate that summer upwelling may be relatively frequent on the west coast and rare on the east coast. While detailed oceanographic studies synchronized with benthic studies in nearshore coastal environments are needed to evaluate this hypothesis, present evidence is consistent with the view that rocky intertidal community structure and dynamics vary with large-scale oceanographic conditions in nearshore coastal environments around New Zealand.