Quantitative Ecological Impact Assessments using Natural Abundance Carbon and Nitrogen Stable Isotope Signatures
Authors:   , Dudley Bruce David 
Published: November 2021

<p>The use of delta15N and delta13C signatures to infer sources of enrichment in ecological systems relies on predictability in the transfer of delta15N and delta13C ratios. This thesis examines patterns of delta15N and delta13C change as pools of nitrogen and carbon move from a sewage effluent discharge into organisms in an adjacent coastal rocky reef community (Titahi Bay, New Zealand). These changes and their mechanisms are examined in the broader context of current research using carbon and nitrogen stable isotope ratios in marine ecology, with particular reference to impact assessment. Firstly this thesis examines the assimilation of nitrogen and carbon isotopes in Ulva sp. under varying light conditions and nitrogen source (e.g., nitrate or ammonium). In a field study, algae grown at depth and under lower light conditions showed comparatively lighter nitrogen isotope signatures relative to the predicted concentration of available 15N-enriched sewage nitrogen. In a complementary laboratory experiment, results from manipulated light availability and N source (either nitrate or ammonium, in equivalent molar concentrations) suggest that: 1) low-light conditions can produce algae with lighter nitrogen isotope signatures; and 2) this effect was more pronounced for ammonium (3.7 per mil difference between high light and low light treatments) than for nitrate (0.6 per mil difference between high light and low light treatments) sources. Stable carbon isotope ratios (delta13C) of Ulva sp.grown in conditions of low nitrogen availability were shown to be generally lower than those grown in nitrogen rich conditions in both field and laboratory studies. Where nitrogen supply was sufficient for growth, low light conditions also produced generally lower delta13C signatures than high light conditions. Experimental trials with a uniform dissolved inorganic carbon source and altered light and nitrogen enrichment levels produced delta 13C levels in Ulva sp. tissue that spanned the recorded delta13C ranges of many common algal species; -5.99 per mil (high light, with added ammonium and phosphate) to -17.61 per mil (high light without nutrient additions). Chapter 3 of this study examines the growth response of Ulva sp. to surplus nitrate and ammonium (the two most common forms of nitrogen available to plants in seawater), under light limited conditions. Ulva sp. experienced a temporary reduction in growth rate and nitrogen assimilation capacity (shown in tissue nitrogen indices) when grown on nitrate, relative to ammonium. The magnitude and the temporary nature of these results suggest that in natural populations the relative proportion of nitrate or ammonium available is unlikely to significantly affect the growth capacity of Ulva sp. In chapter 4, I use delta13C and delta15N signatures to separately trace the dissolved and particulate fractions of sewage effluent dispersal onto a rocky reef community. Delta15N signatures from tissue of the macroalga Carpophyllum maschalocarpum, and the herbivorous isopod Amphoroidea media tracked the distribution and signature of DIN from a sewage treatment plant that generated heavy delta15N signatures. Delta13C signatures from tissue of the filter-feeding half-crab Petrolisthes elongatus tracked the distribution and signature of suspended sewage particulate organic matter.</p>