Concentrate (when discharged to the ocean) may have chronic/acute impacts on marine ecosystems, particularly in the mixing zone around outfalls. The environmental impact of the desalination plant discharges is very site- and volumetric specific, and depends to a great extent on the salinity tolerance of the specific marine microbial communities as well as higher order organisms inhabiting the water column in and around this extreme discharge environment. Scientific studies that aim to grant insight into possible impacts of concentrate discharge are very important, in order to understand how this may affect different marine species when exposed to elevated salinity levels or residual chemicals from the treatment process in the discharge site. The objective of this PhD research was to investigate the potential environmental effects of the concentrate discharge in the near-field area around the submerged discharge of the King Abdullah University of Science and Technology (KAUST) seawater reverse osmosis (SWRO) plant by a combination of biological and hydrological studies. Possible changes in microbial abundance were assessed by using flow cytometric (FCM) analysis on a single-cell level in 107 samples, taken from the discharge area, the feed-water intake area and two control sites. Results indicate that changes in microbial abundance in the near-field area of the KAUST SWRO outfall are minor and appear to bethe result of a dilution effect rather than a direct impact of the concentrate discharge. In order to also investigate potential impacts on higher order organisms, a longterm in-situ salinity tolerance test at the discharge site was conducted on the coral Fungia granulosa and its photophysiology. The corals were exposed to elevated levels of salinity as a direct result of concentrate discharge. Their photosynthetic response after exposure to extreme salinity conditions around the full-scale operating SWRO desalination discharge was measured. A pulse amplitude modulated (PAM) fluorometer was used to assess photochemical energy conversion in photosystem II (PSII) measured under constant concentrate discharge conditions. Based on a literature review, we anticipated distinct impairment of photosynthetic characteristics as a response to elevated salinity levels. We also expected particularly quick indications of bleaching for the specimens exposed to the highest salinity levels. The hypothesis was strongly rejected as symbiotic dinoflagellates of Fungia granulosa demonstrated high tolerance to hyper saline stress as measured by effective quantum yield of PSII (ΔF/Fm’) during this study. A series of propulsion driven autonomous underwater vehicle (AUV) missions with velocity and salinity measurements were used for possible plume detection and evaluation of the discharge. The Cornell Mixing Zone Expert System (CORMIX) was additionally utilized in order to assess discharge performance under different ambient velocity magnitudes. Results show that AUV missions could provide significant insight with regards to plume identification and effluent discharge environmental impact studies. Combined with robust in-situ field measurements, models and expert systems were used to evaluate possible impacts on the marine environment in comparison with regulatory mixing zones and dilution criteria. Based on the findings and existing environmental governance (national and international), a revised regulatory framework for mixing zones within the Kingdom of Saudi Arabia is recommended.