Glacial fjord circulation determines the import of oceanic heat to the Greenland Ice Sheet and the export of ice sheet meltwater to the ocean. However, limited observations and the presence of both glacial and coastal forcing - such as coastal-trapped waves - make uncovering the physical mechanisms controlling fjord-shelf exchange difficult. Here we use multi-year, high-resolution, realistically forced numerical simulations of Sermilik Fjord in southeast Greenland to evaluate the exchange flow. We compare models, with and without a plume, to differentiate between the exchange flow driven by shelf variability and that driven by subglacial discharge. We use the Total Exchange Flow framework to quantify the exchange volume fluxes. We find that a decline in offshore wind stress from January through July drives a seasonal reversal in the exchange flow increasing the presence of warm Atlantic Water at depth, that the exchange flux in the summer doubles with the inclusion of glacial plumes, and that the plume-driven circulation is more effective at renewal with a flushing time 1/3 that of the shelf-driven circulation near the fjord head.