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ECOLOGY, THREATS AND MANAGEMENT OPTIONS FOR SMALL ESTUARIES AND ICOLLS Chapter 3 SEWAGE EFFLUENT SIGNIFICANTLY INCREASES THE δ 15 N SIGNATURES OF RESIDENT BIOTA IN AN INTERMITTENTLY OPEN ESTUARY Despite their appeal as sites for tourism and recreation, many ICOLLS have been deleteriously impacted by coastal development. The key impacts to date have often manifested as deteriorated water quality. This is generally the result of eutrophication or excessive delivery of nutrients from industry, agriculture and urban sources. Conditions in ICOLLS range from freshwater-dominated when closed, to estuarine and marine when open to the ocean. This variability in physical and chemical attributes ensures that ICOLLs have variable biotic community composition in time and space. When closed to the ocean, nutrient added to ICOLLs can lead to reduced water quality and increased frequency, duration and intensity of algal blooms. One of the major point source inputs of nutrients comes from STPs and these sources can be traced using stable isotopes of carbon and nitrogen. Nutrient inputs from industrial and urban sources have been shown to increase the δ 15 N isotope signatures of biota in other types of receiving waterbodies. In this study, we used stable isotopes of carbon and nitrogen to examine food web structure and function in two ICOLLs in northern New South Wales, Australia. Specifically, our focus was on examining the effects of sewage effluent inputs on food web structure and function. To do this, we compared the nitrogen isotope signatures of resident biota, as one of our study systems (Tallows Creek) has a history of secondary treated sewage inputs (McAlister et al. 2000). We predicted that this system would have significantly higher δ 15 N signatures than the neighbouring, less impacted system (Belongil Creek). We also predicted that the community composition and food web structure of Tallows Creek would differ from that of Belongil Creek, due to the influence of sewage discharge and possibly, entrance status. The composition of the food web in Tallows Creek appeared to be influenced both by the sewage effluent (and increased nutrient availability) and the entrance status at the time of sampling. Freshwater taxa collected from this system revealed a strong riverine contribution to community composition, in contrast to the estuarinemarine characteristics of the biota in Belongil Creek. As expected, all samples forming the Tallows Creek food web were enriched in δ 15 N signatures relative to biota sampled from Belongil Creek. However, despite these differences, mixing model analyses revealed that both systems were primarily driven by autochthonous (algal) sources of carbon. Interestingly, some of the organisms collected immediately following an entrance opening event in Tallows Creek had δ 15 N signatures more typical of unenriched estuaries. We suggest these isotopically light individuals were recent migrants into Tallows Creek and that δ 15 N signatures can act as a guide to the importance of heavily polluted systems as nursery grounds for mobile taxa, owing to this distinction between resident and migrant δ 15 N signatures. Introduction Eutrophication is one of the greatest threats to aquatic ecosystem health and integrity around the globe (Balls et al. 1995; Braga, Bonetti, Burone, & Bonetti Filho 2000). Whilst urban stormwater, industrial and agricultural inputs can represent substantial sources of nutrients to receiving waters (Bowen & Valiela 2001; Anderson, Glibert & Burkholder 2002; Nedwell, Sage & Underwood 2002), STP discharges often represent the most significant point source of nutrients and contaminants to waterways, particularly in populated regions (Cromey, Black, Edwards & Jack 1998; Alonso-Rodriguez, Paez-Osuna & Cortes-Altamirano 2000; Costanzo, O'Donohue & Dennison 2003). Numerous studies in coastal environments have demonstrated the extent and effects of sewage discharges on water quality and organism health (Hall, Frid & Gill 1997; Seguel, Mudge, Salgado & Toledo 2001; Costanzo, O’Donohue & Dennison 2003). Significantly, many of these studies use nitrogen isotope signatures in biota to assess the impacts of sewage discharge, as STP effluent typically has an enriched δ 15 N signature relative to that of the receiving waters (Waldron, Tatner, Jack & Arnott 2001; Parnell 2003; Gaston, Kostoglidis & Suthers 2004). Several species-focused studies have also used δ 15 N signatures to provide evidence of the spread and trophic implications of sewage in coastal waters (Risk & Erdmann 2002; Gaston, Kostoglidis & Suthers 2004). 26
ECOLOGY, THREATS AND MANAGEMENT OPTIONS FOR SMALL ESTUARIES AND ICOLLS Whilst techniques for examining the impacts of sewage discharges are applicable to a wide range of aquatic environments, the effects of STP effluent discharge have most often been examined in large macrotidal estuaries (Hall, Frid & Gill 1997; Leeming & Nichols 1998; Costanzo, O’Donohue & Dennison 2003), bays and open coastal waterways (Tucker, Sheats, Giblin, Hopkinson & Montoya 1999; Seguel et al. 2001; Rogers 2003) and more recently, coral reefs (Risk & Erdmann 2000; Umezawa, Miyajima, Yamamuro, Kayanne & Koike 2002; Yamamuro, Kayanne & Yamano 2003). However, given the number and distribution of ICOLLS in Australia (Roy et al. 2001), it is timely that an investigation of the effects of STP inputs be conducted in this subset of estuaries. Materials and Methods Sampling Sites This study was conducted in Tallows and Belongil Creeks; two ICOLLS in northern New South Wales, Australia (Figure 3). These systems are typical of many of the small ICOLLS in eastern Australia and are characterised by having relatively small catchments with a diverse range of land uses. Australia Figure 3: Map of study ICOLLs in Byron Bay, New South Wales, Australia Both Belongil Creek and Tallows Creek have received large amounts of nutrients over the past 50 years, principally from STPs that discharge into their waters (McAlister et al. 2000). However, the Belongil Creek system receives sewage from the West Byron Treatment Plant, which discharges treated wastewater into an extensive treatment wetland system before it enters the waters of the creek. The success of this treatment is reflected in the nutrient concentrations in this system. For example, McAlister et al. (2000) reported that Belongil Creek nutrient concentrations are within ANZECC water quality guidelines during wet weather conditions, but may partially exceed the guidelines during dry periods. In contrast, Tallows Creek receives effluent directly from the South Byron Treatment Plant, which uses comparatively old treatment technologies. McAlister et al. (2000) investigated the fate and effects of these direct inputs and found that nutrient concentrations in Tallows Creek consistently exceeded the ANZECC water quality guidelines (ANZECC 1999), with total nitrogen ranging from 1 – 6 mg/L and total phosphorus from 0.2 – 1.8 mg/L over the course of their study. Sampling Methods NSW A Samples for stable isotope analysis of food web structure were collected in both systems in early May 2003. Significantly, sampling occurred immediately following an artificial opening event in Tallows Creek, whereas Belongil Creek had been open and subject to tidal influences for the proceeding two months. In both systems, samples were collected from two sites less than 500 m from the entrance. Where possible, replicate samples of key taxa were processed separately. The reported mean (± S.E.) stable isotope signatures consequently revealed A Belongil Creek Tallows Creek Taylor’s Lagoon Cape Byron 27
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