Invertebrate Food Web Structure and Methylmercury Accumulation Across a land-to-Sea Gradient: Implications for Ecosystem Connectivity and Contaminant Risk

Master of Science (Environmental Science) candidate: Gretchen McPhail

4 December 2026

1:00 PM Atlantic

Hybrid Defence

Fountain Commons Alumni Boardroom

Join: https://teams.microsoft.com/meet/2388448714969494?p=F0dfKROrr45Arxmy5F
Meeting ID: 238 844 871 496 949 4
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Thesis Committee

Dr. Nelson O’Driscoll, Supervisor
Dr. Mark Mallory, Internal Examiner
Dr. Joshua Kurek, Mount Allison Univ., External Examiner
Dr. Deanne Van Rooyen, Head of Earth and Environmental Science
Dr. Alex Marland, Chair of the defence

Abstract

The land-to-sea gradient is a highly dynamic transition zone characterized by continuous shifts in physical and biogeochemical conditions between terrestrial and marine environments. These transitions drive considerable change in biological community structure and mediate the production, bioavailability, and trophic transfer of methylmercury (MeHg), a neurotoxin that bioaccumulates in consumers and biomagnifies in the food web, posing significant risk to wildlife. Despite growing recognition of the role riverine systems play in delivering mercury (Hg) to coastal ecosystems, how food web dynamics and MeHg accumulation shift across the land-to-sea gradient remains poorly understood. A total of 64 macroinvertebrate taxa were collected from freshwater, brackish, and coastal ecosystems along the Jijuktu’kwejk river and mudflats surrounding its outflow to the Minas Basin, Bay of Fundy, and grouped into functional feeding groups (FFGs). Stable isotopes of 13C and 15N were used to characterize resource use, trophic niche space, and trophic position, while MeHg concentration and the proportion THg as MeHg (%MeHg) were used to quantify bioaccumulation and model trophic magnification within each invertebrate community across the land-to-sea gradient. A complete regime shift in invertebrate food web structure was observed across the gradient, with all three ecosystems occupying entirely separate isotopic spaces. 13C values became significantly enriched downstream across all FFGs, reflecting the transition from terrestrial to marine-derived energy sources. In contrast, 15N values declined significantly from freshwater to coastal ecosystems across all FFGs, likely representing the combined influence of shifting food web structure and nitrogen enrichment along the study system. The freshwater community showed the broadest trophic range (3.38) and highest trophic-level variance (0.416), while the coastal community showed the smallest isotopic niche (SEAc= 8.51‰2) and most tightly clustered niche structure. The brackish community had the largest isotopic niche (SEAc = 19.01‰2) and carbon range (14.2‰), representing a structurally unique transitional habitat. The freshwater community showed elevated MeHg concentrations, with individuals from predator-engulfer and collector-filterer groups exceeding the MeHg Canadian tissue residue guideline of 137.62 ngg-1 dry weight (dw) for that ecosystem. All FFGs from the marine-influenced ecosystems were roughly an order of magnitude below the respective guidelines. Both FFG identity and ecosystem type were significant determinants of invertebrate MeHg dynamics, with their interaction explaining significantly more variance in baseline MeHg concentration than ecosystem type alone (R2adj = 0.844, p < 0.0001). The freshwater ecosystem showed the most elevated and differentiated MeHg baseline structure, with extensive separation among FFGs including significantly lower MeHg in grazers and significantly higher concentrations in piercing predators. Few significant differences were observed in the among FFGs in the brackish ecosystem, and none were observed in the coastal ecosystem. Trophic magnification slopes (TMS) were positive but non-significant across all three ecosystems (TMS: 0.063 [–0.010, 0.135], p = 0.086; 0.014 [–0.249, 0.275], p = 0.917; and 0.071 [–0.127, 0.268], p = 0.480, respectively), demonstrating that FFG identity was a more consistent and meaningful predictor of invertebrate MeHg concentrations than trophic position. Divergence between MeHg concentration and %MeHg patterns across FFGs and ecosystem highlight that Hg exposure, and MeHg assimilation are independent aspects of invertebrate MeHg dynamics, and that assessing both metrics together provides a more complete picture of MeHg trophic transfer risk than concentration alone. Together, these findings improve our understanding of how habitat connectivity along the land-to-sea gradient shapes invertebrate food webs dynamics and MeHg trophic transfer, and demonstrate that functional identity is a valuable and transferable framework for assessing contaminant exposure and risk across aquatic ecosystems.

About Gretchen…

Gretchen McPhail is an M.Sc. student in Environmental Science at Acadia University, studying mercury food web dynamics in aquatic ecosystems under the supervision of Dr. Nelson O’Driscoll. Her research focuses on how resource use and methylmercury accumulation differ among invertebrate feeding groups across a land-to-sea gradient, to better understand how exposure risk to wildlife shifts across interconnected ecosystems. Gretchen completed a B.Sc. in Marine Biology at Memorial University, where her Honours research examined seabird mortality patterns from the highly pathogenic avian influenza outbreak on the island of Newfoundland. She hopes to continue contributing to marine science and conservation. In her free time, Gretchen enjoys drawing and scientific illustration, and her defence will feature much of her artwork.