Compound Specific Isotope Analysis of Invertebrate Tissues: Determining Nitrogen Variability in Coastal Nova Scotia

Abstract

Anthropogenic nitrogen (N) loading in coastal marine environments causes eutrophication, alters food web structures, and degrades water quality. In Nova Scotia, anthropogenic N sources are introduced to coastal waters through multiple sources including sewage wastewater outfalls, seafood processing, aquaculture, and agricultural runo . Di erent N sources have characteristic isotopic ( 15N) signatures that can be traced using bioindicator organisms to map the spatial variability of 15N along the coastline. Organisms such as the blue mussel (Mytilus edulis) integrate the isotopic 15N composition of their local environment by feeding on particulate organic matter and integrating it into their tissues over several months. Mussels are ideal bio-indicators for this study because they are sessile organisms that are found along the rocky intertidal zone of the province's coastline, preserving the biogeochemical variability of their collection sites. This allows us to map the spatial variability of 15N to help identify sources of N in a region with few previous N measurements. Specimens of M. edulis were collected in triplicate from 21 sites across the province's Atlantic and Northumberland coasts. The adductor muscle of M. edulis was isolated from the rest of the tissue, removed, and freeze-dried for 15N analysis. Within-site variability was considerably low (mean of standard deviation = 0.56‰), indicating narrow variability within specimens at each site. There was, however, a wide range of mean 15N values between sites, ranging from 6.80 ± 0.27 ‰ to 11.08 ± 1.00 ‰ (n=21). To re ne these baseline 15N estimates independent of trophic fractionation, 8 sites with the largest 15N ranges were selected for compound speci c isotope analysis (CSI-AA). Amino acids were isolated for 15N analysis through acid hydrolysis, cation exchange and isopropyl TFAA derivatization. Individual amino acid values ranged from -7.5 ‰ to 18.1 ‰ (SD=5.5; n=250). Phenylalanine (Phe) is the best amino acid proxy for baseline 15N Phe values and ranged from 2.4 ± 1.2 ‰ to 7.8 ± 0.9 ‰; n=22. This variability re ects similar trends in the bulk 15N values, indicating that the local variability seen in M. edulis 15N likely re ects inputs of di erent nitrogen sources speci c to each sample site. This study pairs the novel CSI-AA approach with traditional nutrient data and bulk 15N analysis to generate estimates of baseline 15N variability for the Northumberland and Atlantic coasts of Nova Scotia.