Aquatic Environment and Biodiversity Annual Review 2012

AEBAR 2012: Marine Biodiversity
and the highest δ 13 C values at the coastal sites (e.g. Leigh, Tawharanui and Kawau). Without
direct modelling of end point source signatures we cannot definitively determine the percentage
contribution of each carbon source. However, we suggest that the depleted δ 13 C of taxa from
offshore sites is the result of a pelagic source of C and the enriched δ 13 C at coastal sites is the
result of a more benthic input of C than at offshore sites, with sources including kelp detritus.
Taxa at the inner gulf sites are also likely to be subjected to a proportion of benthicaly-derived
enriched δ 13 C. There were no obvious effects of marine reserve status on the isotopic signatures
of study taxa with the exception of slightly enriched δ 13 C of kina and snapper at Leigh, and of
kina at Tawharanui.
Otolith microchemistry results for parore and snapper indicate strong connectivity between reef
and non-reef systems within the wider Hauraki Gulf ecosystem. The majority of fishes
sampled (both species) were likely to have originated as juveniles from lower salinity water
environments such as estuaries fringing the Gulf. For snapper, our data suggest that only a
small percentage of juveniles derive from reefs themselves. However, greater sampling
replication is now required across a range of reef sites to better define the ratio of reef- versus
estuary-derived juveniles, given the low percentage of reef-derived snapper.
A qualitative model of northeast New Zealand rocky reef ecosystems was developed to explore
the complexity of interactions amongst New Zealand rocky reef species and the impacts of
exploitation. This model was developed on the basis of a review and summary of interactions
among reef components. A key modelling outcome was the highly predictive but opposite
responses by small lobsters and large predatory invertebrates to changes in the abundance of a
range of other groups. This suggests that these two groups are ideal candidates as variables for
monitoring reef ecosystem responses to perturbations. The modelling agreed with a well
documented example of responses to a perturbation in fishing pressure in the Leigh Marine
Reserve. However, the predictability was low for all responses. This implies, for example, that
the reduction of kina in the Leigh Marine Reserve and the subsequent increase in macro-algae
consequent to an increase in lobster abundance may not necessarily occur in another area.
Field sampling at ten rocky reef sites across the Hauraki Gulf revealed differences among sites
in community structure of macroalgae and invertebrates within all habitat strata. Of the
environmental factors available, depth followed by a measure of water clarity (mean secchi)
explained the most variation in the dependent variables (invertebrate taxa) from the quadrat
data. Fish abundance data showed a similar, though weaker, trend across sites with depth,
distance across the Gulf, and water clarity being the most important factors. The strong
association between depth and water clarity and abundances of key taxa was expected and is
similar to that found in earlier studies. With the exception of crayfish, there was no apparent
overall relationship between invertebrate and fish abundances and marine reserve status of
study sites, though the baited underwater video data showed snapper to be significantly larger
within marine reserve sites than at fished sites.
Stable isotope analysis of tissue samples collected from key species from all study sites allowed
insight into the functional relationships among species as well as dietary sources of carbon.
Many of the study taxa, from the primary producers through to the predators, had the most
depleted δ 13 C values at the furthest inshore and offshore sites (e.g. Poor Knights and Long Bay)
and the highest δ 13 C values at the coastal sites (e.g. Leigh, Tawharanui and Kawau). Without
direct modelling of end point source signatures we cannot definitively determine the percentage
contribution of each carbon source. However, we suggest that the depleted δ 13 C of taxa from
offshore sites is the result of a pelagic source of C and the enriched δ 13 C at coastal sites is the
result of a more benthic input of C than at offshore sites, with sources including kelp detritus.
Taxa at the inner gulf sites are also likely to be subjected to a proportion of benthicaly-derived
enriched δ 13 C. There were no obvious effects of marine reserve status on the isotopic signatures
of study taxa with the exception of slightly enriched δ 13 C of kina and snapper at Leigh, and of
kina at Tawharanui.
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