Species richness can be important for the functioning and resilience of marine ecosystems, with planktonic protists forming the base of the marine food web. Climate change-driven shifts in water mass distributions are projected to alter plankton diversity. This process is particularly relevant in Arctic fjords experiencing changes in coastal water inflow, affecting local hydrography, nutrients, and biodiversity. Yet, empirical data on the environmental effects on Arctic plankton diversity are scarce. In this study, we analyze a 15-year time series from Nuup Kangerlua, a West Greenland fjord, and report an abrupt decline in microplankton (> 20 μm) species richness following a high-richness period from 2009 to 2013. The high-richness period was also characterized by a period of substantial inflow of Atlantic type water (ATW). While ATW did not appear to directly introduce temperate species, it was associated with elevated surface temperatures and nitrate concentrations – conditions that may have supported elevated microplankton richness. Nitrate is typically the key limiting nutrient in Arctic fjords and may support higher microplankton species richness consistent with the Species Energy theory.