Polychaetes Associated with a Tropical Ocean Outfall: Synthesis of a Biomonitoring Program Off O'ahu, Hawai'f

Polychaetes Associated with a Tropical Ocean Outfall: Synthesis ofa Biomonitoring Program off O'ahu, Hawai'F J. H. Bailey-Brock,2,3,4,5 B. Paavo,3,4 B. M. Barrett,3,4 and J. Dreyer3,4 Abstract: A comparison of benthic polychaete communities off the Sand Island Wastewater Outfall was undertaken to recognize organic enrichment indicator species for Hawaiian waters. Primary-treatment sewage is discharged off the south shore of O'ahu at 70 m depth. A historical data set spanning 9 yr for seven sites at 70 m and two recent studies at 20, 50, and 100 m depths were analyzed. Geochemical data did not support the assumption that the outfall is an im portant source of organic enrichment in nutrient-poor sandy sediments within oligotrophic tropical waters. Five polychaete species, however, appeared partic ularly sensitive, positively or negatively, to environmental conditions near the outfall. Neanthes arenaceodentata (Nereididae) and Ophryotrocha adherens (Dor villeidae) have been dominant at sites within the outfall's zone of initial dilution (ZID). Since 1993, N arenaceodentata has virtually disappeared, and 0. adherens concurrently became abundant and continued to flourish at ZID sites. Well known indicators within the Capitella capitata complex (Capitellidae) were pres ent at ZID and control (far field) sites though their ZID abundance was greater. Two sabellids, Euchone sp. Band Augeneriella dubia were inversely distributed, the smaller Euchone sp. B at far field sites and larger A. dubia within ZID sta tions. The former was most likely restricted to a greater proportion offine sed iment particles at two far field sites. The most abundant and widespread polychaete off O'ahu's south shore was Pionosyllis heterocirrata (Syllidae), which does not seem to represent a sensitive indicator species. Ophryotrocha adherens was the most abundant indicator species within the ZIDj P. heterocirrata was the most ubiquitous species at all sites and should always be expected in these sedi ments. Traditional measurements ofnumerical abundance, species richness, and diversity (H') have not shown a clear distinction between ZID and far field sites in annual analyses. An examination of composited data over an 11-yr period does support such a distinction. Multidimensional scaling (MDS) analyses clearly delineate different assemblages. We suggest that MDS analyses are sen sitive to the community differences present near the outfall. The ZID commu nity is clearly contained within the Environmental Protection Agency-approved ZID boundary. Because each ZID and far field site supports a diverse and coarsely similar polychaete fauna, no pollution level effects seem to be present. I This study was supported by the Deparnnent of ENVIRONMENTAL MEASUREMENT and mon Environmental Services, City and County of Honolulu, itoring methods have been debated by ecolo Hawai'i, under contract no. C54997. This is Water Re gists for decades. Defining and monitoring sources Research Center contracted paper CP-2002-04. indicator species, determining the propor Manuscript accepted 19 February 2002. 2 Department of Zoology. tions of nematodes to copepods, monitoring 3 Water Resources Research Center. physical parameters such as granulometry and 4 University of Hawai'i at Manoa, 2538 McCarthy total organic carbon, and calculating species Mall, Honolulu, Hawai'i 96822. richness and abundal1ces are widely accepted 5 Cotreslfortdence toJ. H. Bailey-Brock. methods in marine pollution research (Pesch Pacific Science (2002), vol. 56, no. 4:459-479 et al. 1987, Jones and Kaly 1996, Krassulya © 2002 by University of Hawai'i Press 2001). More recently, ecologists have de All rights reserved veloped benthic indices for comparative pur- 459 460 PACIFIC SCIENCE· October 2002 poses. Benthic fauna are regularly used be and evenness of abundance with increasing cause they often represent a substantial por distance from the input until the usual abun tion of the biomass or supply much of the dance, evenness, and species richness for in food to the more visible and sometimes com digenous balanced communities are reached. mercially valuable water column inhabitants. Numerically dominant polychaete species The methods and models chosen for a par play a major role in shaping benthic com ticular study often depend upon the in munities and are regarded as indicators vestigator's knowledge of the local habitat, of environmental quality (Pocklington and trained personnel available for field and lab Wells 1992). Changes in species richness, di oratory work, or government regulations and versity, community evenness, and movement approved protocols. within dominance rankings suggest biological The University of Hawai'i Water Re responses to environmental changes and may sources Research Center began monitoring be reason for concern unless the change can environmental conditions surrounding the be attributed to a known and benign factor. Sand Island Wastewater Outfall in 1986. This Graded changes may reflect the dynamic study presents a synthesis of data compiled nature of benthic assemblages and provide from the Water Resources Research Center's valuable, time-sensitive baseline data. Rapid annual reports from 1990 to 2000 when sam and/or inexplicable changes may suggest that pling and reporting practices were stan a closer examination is called for and that dardized as presented in Nelson et al. continued monitoring is necessary. In addi (1992a,b, 1994, 1995, 1996, 1997) and Swartz tion to community-level analyses, we examine et al. (1998, 1999). The program has been the 10 numerically dominant species that designed to examine the abundance of ex typically represent 70 to 90% of collected pected polychaete species and other faqual specimens. By characterizing the life histories components within the area defined as the of these species and the fluctuations in their zone of initial dilution (ZID) when compared abundance from 1990 to the present we hope with similar control sites (far field). Poly to gain a better understanding of these ben chaetes are particularly useful because of the thic assemblages and assess the impacts of relative ease of identification, a trophic posi organic enrichment by comparing sites within tion that may respond to the organic enrich the ZID boundary with those outside it. ment produced by the outfall, their ubiquity Recognizing a species as a "pollution in in Hawaiian sediments, and densities that dicator" can have several meanings. Various often approach 100,000 individuals per square species have been labeled as indicators for meter. The binary sampling design, ZID reasons not necessarily consistent among im versus non-ZID, was chosen to provide ready pact studies (Jones and Kaly 1996). Every statistical comparison and to develop a reli region is likely to present a different series of able characterization of the ZID-associated positive and negative organic enrichment in community in preparation for more regional dicators. Positive indicators may be ecological and gradient-based studies currently under opportunists whose feeding style, reproduc way. tion, and growth allow the population to in We base our fundamental assessments on crease concomitantly with organic input over one of the most broadly applied models of a relatively short period of time (Davis 1987, biological response to organic enrichment Bailey-Brock 1996). Negative indicators are used in many studies from North America, those sensitive species that die directly as a northern Europe, Australia, and India: the result of conditions related to habitat alter Pearson-Rosenberg model (Pearson and ation or are excluded by competitive inter Rosenberg 1978). In essence, the benthic actions--- -- within the altered habitat.- community composition is measured along a When comparing invertebrate assemblages gradient from a source of organic enrichment between any two sites, within the context of to areas outside the zone of influence. The the Pearson-Rosenberg model, it is important model predicts an increase in species richness to consider influences besides organic en- Polychaetes Associated with a Tropical Ocean Outfall . Bailey-Brock et at. 461 Airport Area of Detail Honolulu C1• ........70m---a4G E1• 85 N , 1 km FIGURE 1. Study area and sample sites in relation to the O'ahu shoreline and 70-m isobath. Open circles denote 70 m sites sampled from 1990 to 1998. Solid circles indicate 1999 and 2000 study sites at 20 m (C), 50 m (D), and 100 m (E) depths. WWTP, Sand Island Wastewater Treatment Plant. The border surrounding the outfall's seaward end marks the zm boundary. richment that can vary between sites. For MATERIALS AND METHODS benthic polychaetes, many of which are mi Sampling Sites and Collection Procedures croscopic, the sediment grain size distribution is one such important consideration because The Sand Island Wastewater Treatment space between the grains defines the physical Plant on the south shore of O'ahu, Hawai'i, nature of the habitat. The physical composi discharges 60-78 million gallons per day tion of sediments broadly affects the available (2.6-3.4 m 3/sec) of primary-treatment efHu habitat microtopography, oxygen and waste ent at a depth of 70 m over a sandy bottom. exchange, building material availability, and Seven stations at approximately 70 m depth, interstitial stability. Each species' motility and four around the diffuser within the ZID, and feeding methods may be more or less suc three approximately 1, 1.5, and 2 Ian distant cessful within

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