WRRCPR No.2004-06 Regional Monitoring of Benthic Fauna in Mamala Bay, Oahu, Hawaii, August 2003

Swartz, Richard C.
Bailey-Brock, Julie H.
Cooke, William J.
Kay, E. Allison
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Water Resources Research Center, University of Hawaii at Manoa
Benthic fauna in Mamala Bay was sampled on 6-14 August 2003 at 10 stations with a modified van Veen grab sampler and at 30 stations with diver-operated sediment corers. Station locations were selected according to a random probabilistic sampling design. The depth range of the stations was 1.2 to 108.8 m. Baseline conditions in Mamala Bay in 2003 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 (>66%) sand at all stations. Total organic carbon in the sediments ranged from 0.26% to 0.94%. Total Kjeldahl nitrogen ranged from 60 to 929 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 6,908 nonmollusk individuals from 226 taxa were collected. Polychaetes represented 33.4%, crustaceans 24.7%, nematodes 22.8%, oligochaetes 8.4%, and nemerteans 3.8% of total nonmollusk abundance. Total nonmollusk abundance ranged from 9 individuals/sample (l,984/m^2, at Station 52) to 1,091 individuals/sample (240,496/m^2, at Station 57). The number of nonmollusk taxa ranged from 4 (at Stations 55 and 61) to 75 (at Station 47). Crustacean abundance ranged from 0 (at Stations 32, 49, and 70) to 432 (95,228/m^2, at Station 57). The number of crustacean taxa ranged from 0 (at Stations 32, 49, and 70) to 27 (at Station 44). Mollusks were analyzed separately because they represent time-averaged collections of live and dead shells. Mollusk abundance ranged from 42 individuals/15 cm^3 (at Station 55) to 898 individuals/15 cm^3 (at Station 41). The number of mollusk taxa per 15 cm^3 ranged from 17 (at Station 52) to 89 (at Station 70). 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 eleven 10-m depth ranges and three depth zones (shallow, 0 to 29.9 m; mid-depth, 30.0 to 69.9 m; and deep, ≥70.0 m). The abundance and taxa richness of both nonmollusks and the crustacean component of the nonmollusks were highest in the mid-depth zone. 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 in intermediate-depth water 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, cluster analysis showed that stations with the highest mean mollusk abundance and taxa richness were located in the deep-water zone. Several of the lowest values of mollusk taxa richness were recorded at sites in the surf zone or under the influence of ocean swells. The frequency distribution for mollusk taxa richness reflected the relatively uniform distribution of mollusks in the bay. The results of this study, together with the 2001 survey results, establish a baseline for benthic conditions in Mamala Bay. 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 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. Relatively few crustaceans were collected at Station 64, which is located near the Sand Island ocean outfall in the 2003 survey. Station 64 was also characterized by the presence of the indicator species Ophryotrocha adherens and Neanthes arenaceodentata. The frequency distributions for mollusk taxa richness for the outfall ZID surveys were similar to those for the two bay surveys. The frequency distributions for nonmollusk taxa richness for the outfall 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. Comparisons with the Mamala Bay 2001 and 2003 baseline surveys confirm the presence of a diverse and abundant macrobenthos in the immediate vicinity of the Sand Island and Barbers Point ocean outfalls.
wastewater outfall, benthic fauna, water pollution, mollusks, polychaetes, oligochaetes, crustaceans, amphipods, isopods, marine sediments, statistical analysis, Mamala Bay, benthic sampling, impacts to benthic community, Oahu, Hawaii
Swartz RC, Bailey-Brock JH, Cooke WJ, Kay EA. 2004. Reginal monitoring of benthic fauna in Mamala Bay, Oahu, Hawaii, August 2003. Honolulu (HI): Water Resources Research Center, University of Hawaii at Manoa. WRRC project report, 2004-06.
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