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Ecological Survey of Hobson Bay and Orakei Basin,

Auckland, New Zealand

Fadhil Sadooni & Bob Creese


1. Sixty samples were collected from 12 locations in Purewa Creek, Hobson Bay, Orakei Creek and Orakei Basin during December 97-January 98. Within each location, five quadrats (0.25 m2) were randomly placed on the sediment surface and all organisms counted. Sediment cores (15cm diameter, 15-cm depth) were taken adjacent to these quadrats and organisms extracted by washing through a 1-mm mesh sieve.

2. The only previous quantitative survey by Bioresearchers (1996) had used 3 replicate core samples washed through a 0.5-mm sieve. Their samples were characterised by large numbers of organisms (often numbering several hundred) but low species richness. In particular, they noted high abundances of polychaetes and amphipods.

3. Our samples were characterised by much lower abundances, possibly due to the larger mesh size, which could have resulted in poor retention of small worms and amphipods. Many of our samples from Orakei Creek, for instance, contained empty tubes (likely to have been from corophiid amphipods).

4. Both studies of Orakei Creek revealed average infaunal species richness in the order of 4-8 species per core. Two additional species were found in our quadrat sampling (which covers a much larger surface area than the cores).

5. Our study, however, found a higher average richness of infauna in Orakei Basin (6-10 species) compared to the earlier study (3.7 species). Of note was the fact that we found native cockles, wedge shells and nut shells in the Basin. These had not been found by Bioresearchers, probably because they had not sampled close to the floodgate area.

6. Because of the low level of sampling by Bioresearchers in the Basin (only 6 core samples) it is not realistic to directly compare the two data sets to ascertain changes in recent years.

7. Surface algal mats were a feature of all 4 sites sampled in Orakei Basin. These mats have the potential to eliminate native epifaunal species while encouraging introduced species such as Asian date mussels. However, the mats were not as thick nor as foul-smelling as reported previously. This may have contributed to the results obtained.

8. Our survey confirmed by Hayward's sampling, suggests that many species from the general Hobson Bay area are already present in at least some parts of Orakei Basin, but in low numbers.

9. Species likely to return to the Basin should it become fully tidal are truly intertidal species that require regular exposure to air (e.g., Diloma, Turbo, Amphibola, Xenostobus, and of course mangroves). In addition, largely intertidal bivalves such as cockles and wedge shells should increase in numbers.

10. Further sampling in the area (planned in April 1998) will give a better indication of the distribution and abundance patterns of the organisms present. It appears based on present information, however, that the benthic ecology of Orakei Basin is constrained by two main features - lack of regular tidal flushing and the buildup of algal mats.