Coastal Restoration Trust of New Zealand

Coastal Dune Ecosystem Reference Database

Effect of storm drain discharge on the soft shore ecology of Porirua Inlet, New Zealand. Technical Report

Botherway, K.J. and Gardner, J.P.A.
Journal / Source
New Zealand Journal of Marine and Freshwater Research.
Porirua Inlet, biological communities, water quality, heavy metals, contaminants, sediments, estuarine, discharge, stormwater, sandy habitat, outfall
The effect of storm drain discharge on the ecology of the soft shore community near the Semple Street outfall in Porirua Inlet, New Zealand, was investigated from December 1998 to April 1999. Biological community structure, sedimentary properties, and heavy metal concentrations of surficial sediments were examined at increasing distance (5, 90, 140 m) from the storm drain at two (upper and lower) shore heights (total of six sites). Concentrations of copper (Cu), lead (Pb), and zinc (Zn) at nine sites from the storm drain to the mouth of the inlet were estimated in April 1999. Typically, mean numbers of individuals decreased, whereas mean numbers of taxa, and the Shannon-Wiener diversity index increased with increasing distance from the storm drain. Analysis of variance revealed that the 5, 90, and 140 m sites at both tidal heights exhibited small but significant differences in community structure which were attributable to differences in species abundance, rather than to differences in the suite of species which characterise these sites. It was not possible to identify temporal trends in the data set. Analysis of similarities revealed significant differences in species abundance among the three sites for the high shore, the low shore, and both tidal heights combined. Sites furthest apart (5 versus 140 m) exhibited the greatest, and sites closest together (90 versus 140 m) exhibited the least, difference in biological community structure, suggesting that storm drain outflow modifies benthic community composition immediately in front of the drain and up to <100 m away. Non-metric multidimensional scaling (MDS) ordination of taxon abundance showed community structure at the two 5 m sites to be dissimilar from that at the other four sites, where it was similar. Despite this, however, changes in taxon abundance with increasing distance from the storm drain were minimal, with the result that indicator taxa could not be identified which explained significant differences in community structure on the scale of this study. A survey of heavy metal concentrations along the length of the inlet indicated that Cu, Pb, and Zn concentrations decreased in a linear manner with distance away from the storm drain and that biological communities in at least the southern half of the inlet are exposed to elevated levels of Cu, Pb, and in particular, of Zn, which exceed internationally recognised threshold, and sometimes, probable effect levels.