Estuaries are increasingly becoming eutrophic due to anthropogenically induced nutrient enrichment. Presently, 24% of global anthropogenically derived nutrients released into coastal catchments are delivered directly into coastal waters such as estuaries. Thirty five percent of New Zealand’s estuaries are susceptible to eutrophication and some are already severely degraded. In-situ macroalgal bioremediation – the cultivation of live macroalgae directly in nutrient-enriched waters for the primary purpose of nutrient assimilation – could help prevent and reduce estuarine eutrophication. The indigenous red macroalga Gracilaria transtasmanica has been identified as a potential target species for bioremediation in New Zealand estuaries. Samples of G. transtasmanica were collected from estuaries in the Bay of Plenty region of New Zealand, and were isolated, formally identified and scaled up into stock cultures. Controlled experiments were then used to assess the bioremediation potential of this species in winter and summer conditions under ambient and extreme nutrient and sediment levels. Biomass productivity, photosynthetic health, and N removal were assessed to quantify performance. Biomass productivity and total N removed were 45 and 50% higher respectively in summer compared to winter. Biomass productivity ranged from 0.4 – 4.3 g DW m² in summer and 0.6 – 1.8 g DW m² in winter, and total N removed ranged from 6.0 – 30.7 mg N in summer and 6.1 – 9.1 mg N in winter. In contrast, standardised N removal rates were higher in winter (37.8 mg g DW biomass growth⁻¹ ± 0.6 S.E.) compared to summer (22.3 mg g DW biomass growth⁻¹ ± 1.2 S.E.), suggesting that the biomass was storing N and therefore that N uptake was independent of productivity under sub-optimal growing conditions. Productivity was significantly affected by sediment treatment, however this effect differed seasonally, with higher productivity in the high sediment treatments in summer, and lower productivity in the high sediment treatments in winter. These results demonstrate that G. transtasmanica can assimilate nutrients under a range of environmental conditions that are representative of both local background and high levels of suspended sediment and nutrient concentrations, and therefore is a viable target species for the bioremediation of nutrient enriched estuaries.In addition to nutrient uptake and productivity, target species for in-situ estuarine bioremediation need to maintain productivity under a range of challenging abiotic conditions. In chapter three of this thesis, a series of independent experiments assessed the tolerance of G. transtasmanica to ambient and extreme levels of salinity, exposure, and light limitation that are likely to be experienced in an estuary. Photosynthetic functioning and growth were used to quantify the tolerance range of G. transtasmanica in each experiment. Specific Growth Rate (SGR) was significantly affected by salinity, exposure, and light limitation. Gracilaria transtasmanica was able to grow in salinities ranging from 5 to 35 ppt, but growth rates decreased with decreasing salinity. Exposure periods of up to 9 hours were tolerated, but growth rates were decreased as exposure period increased. Gracilaria transtasmanica was able to maintain growth with a loss of up to 75% of ambient light and was also able to tolerate short periods of no light. Measurements of optimal quantum yields showed photosynthetic function was unaffected by salinity, exposure, or light limitation. These results demonstrate the high tolerance and adaptability of G. transtasmanica to a broad range of challenging abiotic estuarine conditions. In combination, these findings demonstrate that Gracilaria transtasmanica is well suited to estuarine conditions. However, selection of habitats with optimal conditions for growth will maximise productivity. Moreover, as nutrient uptake appears to be independent of growth under challenging conditions, low productivity may not result in reduced bioremediation capability. These characteristics make G. transtasmanica an ideal species for in-situ bioremediation in New Zealand estuaries. Additionally, G. transtasmanica biomass is suitable for use in a range of bioproducts. Therefore, there is potential for a bioremediation based circular economy approach with the use of harvested G. transtasmanica biomass following bioremediation. In-situ trials are required to verifying the findings reported here and enable optimisation of both bioremediation efficiency and biomass productivity.