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ECOLOGY, THREATS AND MANAGEMENT OPTIONS FOR SMALL ESTUARIES AND ICOLLS environments (Kitting, Fry & Morgan 1984; Miller, Geider & MacIntyre 1996; Middelburg, Barranguet, Boschker, Herman, Moens & Heip 2000). In addition to the influence of sewage discharges on community composition, it is likely that hydrology (entrance status) may have also influenced the sampled diversity of Tallows Creek in this study. For example, the presence of some taxa with strong freshwater affinities (e.g. Corixids) in our samples reflects the closed entrance status of Tallows Creek at the beginning of our sampling. As Griffiths (1999) and Dye and Barros (2005a) observed, time since connection with the ocean is likely to be a key driving force in determining the structure of communities in intermittently open estuaries. Our collections, although restricted in space and time, appear to support these conclusions, although more detailed studies that follow a series of opening and closing events are required to independently quantify the influence of entrance status on community structure and composition. Patterns of Fish Movement Inferred from Nitrogen Isotope Signatures Many researchers have used stable isotopes to assess movements of biota within and between aquatic ecosystems (Hobson 1999; Rubenstein & Hobson 2004; Herzka 2005). Whilst most studies use δ 13 C signatures to examine patterns of movement of consumers between two or more habitats with different source δ 13 C signatures (Cunjak, Roussel, Dietrich, Cartwright, Munkittrick & Jardine 2005), Hansson et al. (1997) showed that enriched δ 15 N signatures from sewage effluent can also provide useful information relating to residency and movement of fish between sites. On the basis of our findings, we propose that the greatly enriched nitrogen isotope signatures of biota in Tallows Creek can provide useful information on the movement of commercially important fish species into and out of this system during the intermittent periods of connectance with the ocean (i.e. when the entrance is artificially or naturally opened). Schlacher et al. (2005) proposed the use of fish δ 15 N signatures as an indicator in estuarine environments receiving wastewater. They provided evidence of high site fidelity for a range of estuarine fish species and claim this residency facilitates their use as an indicator of nutrient inputs. In our study, we confirmed that resident taxa have elevated δ 15 N signatures in Tallows Creek, a system receiving substantial sewage inputs. However, the presence of comparatively unenriched individuals in Tallows Creek following an entrance opening event during our sampling period revealed that recent recruits into the system could be discriminated on the basis of their comparatively lower δ 15 N signatures. This discrimination between residents and recent immigrants provides evidence for movements of fish species into Tallows Creek and can provide useful insights into increasing our understanding of movements between habitats with vastly different nitrogen isotope baseline signatures. Knowledge of patterns of movement into and out of estuaries is scant for most species. For Mugil cephalus, there is conflicting evidence for the site fidelity proposed by Schlacher et al. (2005). While Field (1987) reported that juvenile mullet (including Mugil cephalus) show a high level of site fidelity and tend not to mix with neighbouring groups or move between neighbouring estuaries, Pusey, Kennard and Arthington (2004) report that at various points in their life cycle, substantial movements within and between estuaries is a common trait of this species (particularly on the east coast of Australia). Specifically, individuals have been reported to re-enter the marine environment (from estuaries) during periods of high rainfall (due to washing out of their detrital food resources). Pusey, Kennard and Arthington (2004) note that when such movements occur, M. cephalus individuals tend not to feed and this has important implications for the use of elevated nitrogen isotope signatures (like those observed in resident biota in Tallows Creek) as potential indicators of movements of M. cephalus individuals caught offshore. We propose that these characteristics of M. cephalus movements facilitate their use as indicators of emigration and immigration patterns into and out of neighbouring estuarine habitats, particularly when heavily 15 N-enriched systems (like Tallows Creek) can leave such a strong and lasting mark (especially during periods of non-feeding movements) on individual δ 15 N signatures. Pusey, Kennard and Arthington (2004) indicated that following these ‘wash out’ runs, M. cephalus individuals tend to re-enter the original estuary or move to a neighbouring estuary (typically to the north). Monitoring stable isotope signatures of immigrants in neighbouring estuaries (like Belongil Creek in this study) might therefore aid in assessing movement patterns in this species and the contribution of sewage effluent across broader spatial scales. For the other species for which we identified new recruits into Tallows Creek (Ambassis marianus), there is very little information on movement patterns into and between estuaries. However, Pusey, Kennard and Arthington (2004) indicate that substantial upstream and downstream migrations occur for many freshwater ambassid species and Broadfoot, Berghuis and Heidenreich (2000) observed upstream migrations of Ambassis spp (thought to be mostly A. marianus) through a tidal barrage fishway in the Kolan River (in south-east Queensland). Given that A. marianus inhabits brackish waters along the Queensland and New South Wales coasts (Grant 2002), it is not surprising that movements within and between estuaries would occur. We propose that the enriched isotopic signature of Tallows Creek and its resident biota may be able to provide ideal 34
ECOLOGY, THREATS AND MANAGEMENT OPTIONS FOR SMALL ESTUARIES AND ICOLLS opportunities for gathering more information of patterns of movements for species like A. marianus where existing data is scant. Whilst we did not sample nearshore environments in the current study, we propose that fish emigrating from Tallows Creek will maintain these enriched δ 15 N signatures for a reasonable period of time (at least weeks), particularly for species that undertake non-feeding movements in response to environmental cues (e.g. Mugil cephalus – Pusey, Kennard & Arthington 2004). These enriched isotope signatures in emigrants might be useful in determining patterns of movement into and out of neighbouring offshore and estuarine habitats (Hansson et al. 1997; Herzka 2005). Given that many of the species that inhabit intermittently open estuaries are of marked commercial and recreational value (Griffiths 1999; Griffiths 2001b), the use of these stable isotope markers to improve our understanding of recruitment and movement patterns deserves further attention. We have shown in this study that in small intermittently open estuaries, sewage effluent can enrich all components of the food web. The enriched δ 15 N signatures of resident biota can be used to assess patterns of movement by mobile taxa like fish (Hansson et al. 1997; Herzka 2005). We propose that heavily enriched systems like Tallows Creek can provide excellent opportunities for assessing patterns of fish recruitment and ecosystem processes (like nutrient cycling), without the inherent costs and impacts associated with adding 15 Ntracers and nutrients to an otherwise undisturbed site (Peterson 1999; Bedard-Haughn, van Groenigen & van Kessel 2003). Studies of this nature are likely to be particularly relevant to coastal resource and fisheries managers, as the findings will enable the evaluation of the role that intermittently open estuaries play in sustaining the viability of commercially and recreationally valuable fish populations (Griffiths 1999; Watts & Johnson 2004). 35
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