Polar regions are warming at twice the global average resulting to large hydrologic and biogeochemical changes. In the Antarctic, glacial melt is releasing iron, the element most noted for limiting primary production in the region. This addition of iron and the corresponding increasing sea surface temperatures are leading to shifts in the dominant late season phytoplankton species and thus changes in the dissolved organic matter (DOM) that they produce. Changes in the source of DOM will impact heterotrophic growth and the respiration efficiency of the system. The Arctic has distinctly different characteristics than the Antarctic. Its shallow nature makes the Arctic more similar to large estuaries than to other oceans. In addition to expected increases in phytoplankton growth, warming in the Arctic is also leading to substantial increases in permafrost thawing, discharge of terrestrially-derived DOM (tDOM) from rivers, and coastal erosion. The tDOM released from Arctic rivers is carbon (C)-rich and dark in color, allowing it to serve as a C source for bacterial growth and to attenuate light required for photosynthesis. Once thought to be highly refractory, at least a portion of Arctic tDOM is bioavailable to coastal microorganisms on time scales of days to months. However, due to tDOM’s high C:N ratio, additional nitrogen (N) is required for microorganisms to utilize all of the labile C. I will discuss a series of experiments conducted in coastal Antarctic and Arctic waters to explore how changes in DOM sources will affect C and N cycling in polar regions.