Impacts of microplastics and nitrogen on invertebrate-mediated biogeochemical cycling in estuarine ecosystems

Estuarine ecosystems are hotspots for biogeochemical cycling yet are increasingly threatened by multiple anthropogenic stressors. Among these stressors, microplastic pollution and nitrogen enrichment are known to affect ecosystem functioning individually, but there is a paucity of research on their cumulative impacts. This thesis experimentally evaluates how these two stressors interact to influence processes mediated by benthic macrofauna in temperate estuaries, using a combination of controlled mesocosms and in situ field assays. In laboratory mesocosms, bivalve-mediated nitrogen cycling was assessed under single- and multiple-stressor conditions using porewater nutrient analysis, sediment–water flux incubations, sediment profile imaging (a proxy for redox conditions) and fatty acid analysis of sediment microphytobenthos and bivalve tissue. Results revealed that nitrogen addition alone elevated ammonium porewater concentrations by ~250%, but this effect was not observed in the multiple stressor treatment. Conversely, sediment–water nitrogen effluxes under light conditions were enhanced with combined stressors, while sediment redox conditions declined significantly. Fatty acid concentrations indicated a higher proportion of DHA in bivalves exposed to multiple stressors reflecting potential changes to food quality. The in situ field experiment used rapid organic matter assays (ROMA) deployed across contrasting macrofaunal communities to assess organic matter degradation rates under single- and multiple-stressor conditions. Environmental characteristics (e.g., chlorophyll-a, organic matter content, sediment grain size, apparent redox potential discontinuity depth, macrofaunal community) were assessed and related to the breakdown of carbon-based agar with different stressor combinations and control media. Multiple linear regression models showed that organic matter degradation in microplastic-media was related to more environmental characteristics than controls, whereas multiple-stressor treatments were linked to fewer predictors, indicating a decoupling of environmental drivers from organic matter degradation processes. Trait-based analyses of infaunal communities revealed that functional dispersion was positively correlated with organic matter degradation, underscoring the role of biodiversity and functional traits in mediating organic matter cycling and carbon dynamics under stress. This thesis provides the first experimental evidence of interactions between microplastics and nitrogen in estuarine ecosystems, demonstrating cumulative impacts on nutrient fluxes, sediment biogeochemistry, and organic matter degradation rates. By integrating mesocosm manipulations, field assays, and trait-based ecology, the work advances the mechanistic understanding of estuarine vulnerability to global change and emphasizes the importance of incorporating multi-stressor interactions and biodiversity into predictive models of ecosystem functioning.