Dynamics of baseline stable isotopes within a temperate coastal ecosystem: Relationships and projections using physical and biogeochemical factors

Measurements of carbon (13C/12C; δ13C) and nitrogen (15N/14N; δ15N) stable isotope ratios have become important tools for: estimating energy flow and trophic positions in aquatic foodwebs; comparing food webs; and aiding in the tracking of wide-ranging consumers. However, each of these applications requires accurate measurements of isotopic signatures in organisms at or near the base of the food web (e.g. autotrophs and their consumers), which act as basal reference points from which to calibrate inferences. Therefore, understanding variations in isotopic baselines, and the mechanisms leading to their variability, is crucial for food web ecology. Using the shallow temperate reefs along the lower west coast of Australia as a test case, the broad aim of this thesis was to determine isotopic variations in a coastal food web and their relationship with surrounding environmental factors or food sources, to determine the suitability and issues for using such baselines in interpreting trophic links and positions in food webs and predicting shifts in isotopic signatures across broad spatial and temporal scales. To achieve this, I have determined the critical scales for accurately capturing baseline isotopic variation in ecologically important autotrophic and consumer taxa (e.g. macroalgae, particulate organic matter and suspension and grazing consumers), and then related isotopic variation in macroalgae to properties of their surrounding physical and biogeochemical environment. I have then used these relationships to project and forecast baseline values in space and time. Lastly, I have tested the suitability of different primary consumers, representing suspension and grazing functional groups to capture spatial and temporal isotopic variation of their respective diets, to act as invariable proxies of δ13C and δ15N baselines. Isotopic variability of basal resources differed between autotrophs and consumers, and among species within these groups, and between isotopes (i.e. δ13C and δ15N), demonstrating the difficulty in capturing accurate and representative values. Spatial patterns for the δ13C and δ15N in the kelp Ecklonia radiata, the δ15N in the calcareous red alga Amphiroa anceps, and the δ13C in the ascidian Herdmania momus and the bivalve Septifer bilocularis, were influenced by variation at replicates (10s of m) and sites (1s of km) over regions (10s of km). Whereas, the reverse was true for δ13C and δ15N in the foliose red alga Plocamium preissianum, δ15N in S. bilocularis, and δ13C in the gastropod Turbo torquatus, with regional differences being the greatest source of influence. Temporally, patterns of δ13C and δ15N variation in each taxon and trophic group were more comparable, with seasonal variation eliciting a weak or negligible effect, but monthly variation often resulting in a strong effect. Temperature, light, water motion, and measurements of dissolved inorganic carbon, nitrogen and phosphorous correlated with spatial and temporal variation in the stable isotopes of macroalgae, but these relationships varied with taxa and isotope. Surprisingly, water temperature was the best single explanatory variable, accounting for ~50-60% of variation in the δ13C and δ15N of P. preissianum, and the δ15N of E. radiata and A. anceps. From the above relationships, spatial predictions of δ13C and δ15N values in macroalgae showed clear latitudinal patterns, which covered a far wider range of values than temporal predictions, over a 12-month period. This illustrates the potential scale in the shift of isotopic baseline food sources over broad scales, and its implications for food web studies. Primary consumers, particularly the bivalve Septifer bilocularis and the gastropod Turbo torquatus, generally mitigated a large proportion of the autotrophic δ15N variation and displayed relatively stable δ15N over time, and appeared to time-integrate the δ15N of their diet. Further, by conducting a controlled feeding study, I showed that S. bilocularis exhibited slow δ15N turnover estimates (e.g. half-life of 56 days), which from simulations, negated and “dampened” the effect of fluctuating δ15N values of food sources. This suggests that, owing to their high abundances and wide distributions, these species can be used to compare δ15N baselines over large spatial scales. However, this was not the case for δ13C, where primary consumers were as variable as those autotroph(s) they are assumed to proxy. Therefore, consistency in consumer δ15N does not necessarily equate to consistency in δ13C. The fact that different autotrophs and consumers elicit different patterns of variation, show that sampling designs need to be compatible to the research questions of interest. Otherwise, improper allocation of sampling effort may decrease the probability of detecting ecologically important differences. Consequently, my results provide a reference from which to determine the appropriate sampling design to capture variation in ecologically important taxa, to help inform future studies. Further, the models I have developed to predict isotopic values for important autotrophic sources, and the identification of reference taxa of baseline δ15N (bivalves and gastropods), could be used by ecologists to remove a large proportion of unexplained variation, thus facilitate the interpretation of variation in stable isotopes of consumers in food webs, to help answer important questions in food web ecology