Studies on the west coast of North America suggest that nearshore oceanogra
phic conditions can have important effects on rocky intertidal community st
ructure and dynamics. Specifically, upwelling-dependent processes in coasta
l 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 oc
cur elsewhere, we investigated the effects and rates of predation, grazing,
and recruitment on rocky intertidal community dynamics at upwelling and no
n-upwelling sites on the South Island of New Zealand. Comparative-experimen
tal 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 wa
ve force, nearshore sea-surface temperature, air temperature at low tide, n
utrient 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 (Mytil
us 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 Durvil
lea 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 showe
d that only sea stars and limpets had strong effects at west-coast sites, a
nd only limpets had strong effects at east-coast sites. The sea star Sticha
ster australis, previously identified as a keystone species on the west coa
st of the North Island, was common and important on the west coast of the S
outh 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 communi
ty dynamics could emanate from differences in species composition (e.g., th
e absence of S. australis from the east coast), the higher west coast rates
of most of the ecological processes studied suggest that between-coast dif
ferences may also depend on other factors. Among the alternatives, a differ
ence in nearshore oceanographic conditions on the opposite coasts of the So
uth 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 eas
t coast. While detailed oceanographic studies synchronized with benthic stu
dies in nearshore coastal environments are needed to evaluate this hypothes
is, present evidence is consistent with the view that rocky intertidal comm
unity structure and dynamics vary with large-scale oceanographic conditions
in nearshore coastal environments around New Zealand.