A tale of two stressors: Nitrogen, microplastics, and their influence on estuarine organic matter degradation

Estuaries are highly dynamic systems with strong physicochemical and biological gradients that drive ecosystem functions. Increasing anthropogenic pressures have altered carbon cycling and degradation processes and reduced key ecosystem functions, leading to a marked decline in global estuarine health. This study investigates the individual and combined effects of two common anthropogenic stressors (microplastics and nitrogen) across a gradient of soft-sediment habitats with contrasting infaunal communities that reflect dominant functional traits: head-down deposit-feeding polychaetes, deep-dwelling facultative-feeding bivalves, and a mix of both. In situ rapid organic matter assays (ROMA) were used to assess whole-community organic matter degradation using media with different stressor combinations (nitrogen addition, microplastics, or both). Separate models were developed for each treatment, with predictors selected using the Akaike Information Criterion (AIC) to achieve model parsimony without compromising model fit. Our results clearly demonstrate that, in this system, single stressor models may not adequately capture organic matter cycling in sediments following exposure to multiple stressors. In single-stressor treatments, the role of sedimentary organic matter content on organic matter degradation increased significantly in plastic-treated media, and the density of a head-down deposit feeding polychaete was significantly related to the extinction rate of organic matter with sediment depth in nitrogen treated media. These relationships were decoupled when a secondary stressor was added in the multiple-stressor treatment. While direct effects of nitrogen and microplastic addition were not detected, the treatment-specific models indicate that environmental drivers of degradation vary across stressor contexts, highlighting nuanced estuarine responses to anthropogenic pressures.