Harsh environments are typically characterized by some dominant variable limiting diversity, making them interesting systems for studying how species diversity patterns change with abiotic conditions. Several environmental factors with the potential to influence phytoplankton diversity in hypersaline lakes were examined with microcosm experiments using inoculum from the Great Salt Lake, Utah. Experimental combinations of salinity and nutrient supply were run at three different temperatures. Results confirm salinity as a strong determinant of phytoplankton diversity, while also demonstrating the importance of nutrient supply, where species richness decreased with increasing salinity and increased with nutrient enrichment. Community evenness decreased with nutrient enrichment, indicating few species were favored by nutrient enrichment, becoming very abundant. Community biomass was positively correlated with richness and negatively correlated with evenness. Additionally, the abundance of particular species, most notably Dunaliella sp., was strongly affected by salinity, temperature and nutrient enrichment following patterns observed in the Great Salt Lake, where its abundance increased with salinity, cooler temperatures and increased nutrient enrichment. These results add to growing evidence that while salinity is a dominant factor influencing diversity in hypersaline lakes, other abiotic factors are also important and can interact with salinity to influence phytoplankton communities.