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. Comparativeexperimental 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 westcoast 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 westcoast 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 ...

Context-Dependency in the Effects of Nutrient Loading and Consumers on the Availability of Space in Marine Rocky Environments

“…8B). These findings, while adding to the growing body of evidence that background productivity regulates interactive effects of consumers and nutrients on primary producer assemblages [14], [24], [25], [47], show that, within a regional context, the relative strength of these forces can vary across habitats.…”

“…Baltic [13], [27], New Zealand [24], NE Pacific [52], NW Atlantic [27]), our patterns provide an alternative perspective from that which suggests stronger consumer control of primary producers in relatively nutrient-poor environments [14], [23]. The larger negative effect of grazers on algal turfs at elevated than ambient nutrient levels on SA intertidal rocky shores clearly shows that inputs of nutrients can directly increase rates of algal consumption by intertidal grazers under oligotrophic conditions.…”

“…This synthesis suggests that the relative importance of top-down versus bottom-up forces varies among different functional groups of algae and according to background productivity levels. Further evidence of this context-dependency in the role of consumers and nutrients in structuring benthic communities has been provided by experiments investigating algae-herbivore interactions among regions characterized by different oceanographic conditions [9], [24], [25]. No empirical study has, however, assessed how the response of primary producers to alterations in nutrient inputs and grazing pressure can vary between contiguous habitats in high- versus low-productivity systems.…”

Estimates of Particulate Organic Carbon Flowing from the Pelagic Environment to the Benthos through Sponge Assemblages

“…Furthermore, this study shows that sponge assemblages are important components in temperate rocky habitats and that they play a key role in the transfer of POC from the water column to the benthos. This is particularly relevant since sponge feeding within the microbial loop could represent a significant biomass link with sponges being a sink for picoplankton (Bac, Prochlo and Synecho), and the linkages between sponges and the water column may have important implications for determining overall community structure [71].…”

Global Diversity and Phylogeny of the Asteroidea (Echinodermata)

“…Asteroids such as the North Pacific Pisaster have been important in ecological studies addressing the role of competition, reproduction [27], [28], [29], [30], [31] and community structure [32], [33], [34]. Paine [32] idealized Pisaster as the textbook example of a keystone species.…”