WRRCPR No.2002-07 Regional Monitoring of Benthic Fauna in Mamala Bay, Oahu, Hawaii, August 2001

Abstract

Benthic fauna in Mamala Bay was sampled on 8-17 August 2001 at 22 stations with a modified van Veen grab sampler and at 18 stations with diver-operated sediment corers. Station locations were selected according to a random probabilistic sampling design. The depth range of the stations was 0.9 to 79.6 m. Baseline conditions in Mamala Bay in 2001 are described with respect to the range in sediment and biological parameters; the spatial distribution of samples with minimal values of taxa richness; cluster analysis of stations based on faunal similarity; dominant species composition; quantitative changes in the abundance and taxa richness of nonmollusks, crustaceans, and mollusks in relation to water depth; and the frequency distribution of areal taxa richness. Sediments were predominantly (>85%) sand at all stations. Total organic carbon in the sediments ranged from 0.21% to 0.76%. Total Kjeldahl nitrogen ranged from 59 to 665 mg/dry kg. Values for oxidation-reduction potential showed no evidence of reducing conditions at the surface of sediments at any station. A total of 7,053 nonmollusk individuals from 234 taxa were collected. Nematodes represented 29.6%, polychaetes 28.4%, crustaceans 24.0%, oligochaetes 7.9%, and nemerteans 3.6% of total nonmollusk abundance. Total nonmollusk abundance ranged from 3 individuals/sample (661/m^2, at Station 62) to 594 individuals/sample (130,939/m^2, at Station 92). The number of nonmollusk taxa ranged from 2 (at Stations 62, 74, and 88) to 67 (at Station 67). Crustacean abundance ranged from 0 (at Station 98) to 215 (47,394/m^2, at Station 77). The number of crustacean taxa ranged from 0 (at Station 98) to 29 (at Station 67). Mollusks were analyzed separately because they represent time-averaged collections of live and dead shells. Mollusk abundance ranged from 30 individuals/15 cm^3 (at Station 95) to 798 individuals/15 cm^3 (at Station 82). The number of mollusk taxa per 15 cm^3 ranged from 9 (at Station 96) to 64 (at Station 100), Index values for diversity and evenness were quite variable for both nonmollusks and mollusks. Correlation and cluster analyses indicated that the differences in the nonmollusks of Mamala Bay were associated primarily with depth. The data were therefore divided according to eight 10-m depth ranges. The abundance and taxa richness of both nonmollusks and the crustacean component of the nonmollusks were highest at depth ranges between 30 and 60 m and lower in deeper and shallower water. Most low values of nonmollusk taxa richness were recorded for shallow waters and were widely distributed along the bay. The frequency distribution of nonmollusk taxa richness reflected the dichotomy between the taxa-rich sites of intermediate depths and the taxa-poor sites in shallow and deep water. The relation to depth was less obvious for mollusks, which were more evenly distributed in the bay, especially in terms of taxa richness. However, the highest mean abundance for mollusks was recorded for the two deepest depth ranges between 60 and 80 m. Most low values of mollusk taxa richness were recorded at sites with rocks or thin sand layers. The frequency distribution for mollusk taxa richness reflected the relatively uniform distribution of mollusks in the bay. The results of this study establish a baseline for benthic conditions in Mamala Bay in 2001. This baseline was used to assess previously reported conditions at the zone of initial dilution (ZID) of the Sand Island and Barbers Point ocean outfalls the last time they were surveyed in 1998 and 2001, respectively. Nonmollusk and mollusk abundance and taxa richness at the outfall ZIDs were close to expected values for comparable depths in Mamala Bay. Crustacean abundance and richness at the ZIDs were somewhat less than expected, a conclusion consistent with the historic evidence for a slightly diminished crustacean assemblage at the ZIDs. The frequency distributions for mollusk taxa richness for the ZID surveys fell within the frequency distribution for the bay survey. The frequency distributions for nonmollusk taxa richness for the ZID surveys followed the taxa-rich segment of the distribution for the bay, i.e., they did not include taxa-poor samples found inshore and offshore of the ZIDs. Comparison with the Mamala Bay 2001 baseline confirms the presence of a diverse and abundant macrobenthos in the immediate vicinity of the Sand Island and Barbers Point ocean outfalls.