Te Waihora/Lake Ellesmere is a shallow, eutrophic, intermittently open lake/lagoon in Canterbury, New Zealand. It is considered one of the most polluted lakes in New Zealand due to high nutrient loading from its catchment. Efforts are underway to improve water quality, water clarity and reduce phytoplankton development. It is essential to understand the response of phytoplankton to nutrients and light, in order to guide management of phytoplankton. The focus of this study was to determine how light and nutrients control phytoplankton growth. Nutrient limitation was determined using nutrient-addition mesocosm bioassays. 40 cm tall mesocosms were established five times through a year to which nitrate (N), phosphate (P), both nitrate and phosphate (NP) or no nutrients (control) were added to freshly collected lake water. Phytoplankton responses were followed using changes in chlorophyll a, quantum yield of photosynthesis and cell numbers. Results indicated that phytoplankton in Te Waihora are predominantly limited by nitrogen, but can at times of high ambient nitrate concentration become phosphorus-limited. Quantum yield responses indicated nutrient limitations did not usually affect photosystem II photochemical efficiency of phytoplankton cells. Nutrient additions commonly had no measureable effect on cell density, and community composition remained unchanged, with single-cell picocyanobacteria numerically dominant throughout. Combined effects of light and nutrients were also determined using mesocosm experiments. No nutrients, or both nitrogen and phosphorus, were added to mesocosms of 80 cm, 40 cm, and 20 cm depth. Chlorophyll a responses indicated phytoplankton biomass in the 80 cm mesocosms were frequently unable to respond to nutrient enrichment, whereas the shorter mesocosms tended to show enhanced chlorophyll a after enrichment. The 20 cm mesocosms always had more chlorophyll a after enrichment, though sometimes reduced in overall chlorophyll a over time. Quantum yield decreased in the 20 cm mesocosms relative to both 40 and 80 cm, likely due to downregulation of the photosystem II protein complex under the higher irradiance prior to measurement. There was no clear effect of nutrients nor light on cell density, and single-celled picocyanobacteria was the dominant algae in these experiments, with neither light nor nutrient addition resulting in a community shift. Light limitation of photosynthesis was explored by measuring light, photosynthesis, and respiration both in Te Waihora and in 80 cm mesocosms. Light is rapidly attenuated in Te Waihora, with both a shallow euphotic depth (0.5 m) and critical mixing depth (0.6 m). These confirmed that rate processes of phytoplankton can be severely light limited, however during calm weather and near the margins net growth can occur. Whole-mesocosm photosynthesis and respiration showed phytoplankton growth was likely light limited in the deepest mesocosms, confirming observations based on biomass accrual that light limitation prevented a response to nutrient enrichment. The results suggest that phytoplankton growth and biomass in Te Waihora is controlled by both light and nutrient availability, and management actions for the reduction of phytoplankton need to focus on these key factors. Whole-catchment dual-nutrient control is highly recommended, as a reduction in nitrogen alone may allow potentially toxic picocyanobacteria to persist. Internal management of nutrients would likely be expensive or inefficient due to the size of the lake. Controlling phytoplankton by reducing light availability would be contradictory to the current management objective of increasing water transparency. However, increasing water column depth or reducing wave action may reduce sediments resuspension and internal nutrient loading, therefore reducing phytoplankton biomass.