Heterosiphonia japonica is a species of red seaweed from the western Pacific Ocean that has recently invaded the Massachusetts shoreline, damaging native ecosystems and emitting an unpleasant rotten-egg stench when it washes ashore. This project seeks to determine the effects of Heterosiphonia on biodiversity in nearshore subtidal habitats. We will quantify the direct and indirect effects of Heterosiphonia in comparison with other drivers of biodiversity change in order to understand the seasonal dynamics and abundance of this pest species. Effects of Heterosiphonia will be evaluated based on surveys of subtidal seaweed diversity at sites within and north of the invader’s current range. Impacts on ecosystem functioning (i.e., nutrient uptake, growth, herbivory) will be assessed in laboratory and outdoor mesocosms. We expect that this study will fundamentally advance our understanding of the consequences of invasion-mediated declines in biodiversity for coastal ecosystem functions.