Benthic Faunal Sampling Adjacent to the Wai’anae Ocean Outfall, O’ahu, Hawai’i, June 2008

Benthic Faunal Sampling Adjacent to the Wai’anae Ocean Outfall, O’ahu, Hawai’i, June 2008

Benthic Faunal Sampling Adjacent to the Wai’anae Ocean Outfall, O’ahu, Hawai’i, June 2008

Anthony R. Russo, Julie H. Bailey-Brock, William J. Cooke, Regina K. Kawamoto

December 2008, xii + 188 pp.


In June 2008 City and County of Honolulu Department of Environmental Services Oceanographic Team divers collected bottom-sediment samples for biological and geochemical analyses at a depth of approximately 34 m in the vicinity of the Wai’anae Wastewater Treatment Plant ocean outfall on the leeward coast of O’ahu, Hawai’i.

The six stations sampled were 1) ZID station Z, located in the zone of initial dilution (ZID) at the diffuser; 2) ZID station ZE, located on the southeast boundary of the ZID, about 30 m from the diffuser; 3) ZID station ZW, located about 60 m southwest of the diffuser; 4) reference station W1, located about 2.5 km southeast of the diffuser; 5) reference station W2, located about 1.0 km southeast of the diffuser; and 6) reference station W9, located about 2.5 km northwest of the diffuser.

All stations had sediment fractions with >90% coarse, medium, and fine sand. Stations W1 and ZE had the lowest percentages of coarse and medium sediments and the highest percentage of fine sands, whereas station ZW had the highest percentage of medium sand. Station W9 had a relatively high percentage of coral rubble. All stations had very low percentages (<4.0%) of silt. Oxidation-reduction-potential and total-volatile-solid measurements indicated a nonreducing benthic environment at all stations. In 2008 the sediments around the outfall were rich in crustacean (including copepods), molluscan, nematode, oligochaete, and polychaete faunas. A total of 11,405 nonmollusk individuals of 217 taxa were collected. Of this total 2,559 were copepods. These crustaceans were very abundant this year, especially at stations W1 and W9. In contrast, in 2007, a total of 5,335 nonmollusk individuals of 188 taxa were recorded. Copepods were far less abundant last year (2007). Crustaceans represented 30.0% and polychaetes 21.3% of total nonmollusk abundance. Crustaceans had a taxa richness of 71 and polychaetes had a taxa richness of 123. The highest mean abundance of nonmollusks was recorded at reference station W1 and the lowest at ZID station Z. Mean nonmollusk taxa richness was greatest at station W2 and least at station Z. The nonmollusk taxa composition similarity was shown by station group clusters: stations W1, W2, and W9, and stations Z, ZE, and ZW. Stations W1 and W9 were the most similar to one another; stations W1 and ZE were the most dissimilar. In 2008, 9,462 mollusk individuals of 188 taxa were recorded. In 2007 over 10,000 individuals of 210 taxa were collected. The highest mean abundance of mollusks was recorded at station ZE and the lowest at stationW2. Mean taxa richness for the mollusk component was highest at ZID station ZE and lowest at ZID station Z. Gastropods comprised >90% of the total molluscan fauna. Stations ZW and W2 were most similar in mollusk taxa composition. Grouping by “species in common” resulted in two clusters; one group containing stations ZW, W1, and W2 and another composed of stations Z, ZE, and W9.

Cluster analysis, based on taxa composition, indicated no clear pattern of interaction with the effluent discharge for either the mollusks or nonmollusks.

Near the outfall discharge no large increase in abundance was found among some taxa relative to others; the equitability of relative abundance was high at all stations for mollusks and for nonmollusks (all values above 0.60). Mean ZID diversity index (2.82) did not differ significantly from the non-ZID (2.96) mean diversity index. There was no large decrease in taxa diversity at stations near the diffuser, as would be predicted by the Pearson-Rosenberg pollution model had significant pollution been present.

Annual averages for effluent flow rates and chemical parameter concentrations in the outfall discharge were essentially the same in 2008 as in 2007. At all stations the sediment was oxygen-rich, as indicated by positive oxidation-reduction-potential and relatively low total-organic-carbon and total-volatile-solid readings. Thus the results of the 2008 Wai’anae benthic study, along with results from previous years, suggest that no deleterious effects from treated effluent discharge occur to the biologically indigenous populations near the outfall.