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Seagrass species are widely distributed along all major coasts of Australia and provide a number of crucial ecological functions in which they grow, but also to surrounding ecosystems. During the monsoon season along the Great Barrier Reef (GBR) increased rainfall causes significant freshwater runoff from adjacent agricultural land, transporting suspended sediment, nutrient and contaminants into nearby rivers and ultimately coastal inshore waters. Photosystem-II (PSII) herbicides are the most commonly applied herbicides along the Queensland coast, and diuron is among the most commonly detected PSII herbicides in this marine environment. Recent risk assessments have highlighted the need for new tools to assess: (i) the toxicity of alternative PSII herbicides to seagrass, (ii) the combined effects of mixtures of PSII herbicides on seagrass and (iii) the combined effects of PSII herbicides in the presence of other stressors. A miniature bioassay methodology was developed and validated in Chapter 2 to measure effects of photosystem-II herbicide exposure on the seagrass, Halophila ovalis. Three exposure types: (i) potted, intact plants, (ii) hydroponic and (iii) isolated leaves, were experimentally exposed to a range of concentrations of diuron for a 24 h experimental period. The effects of diuron on photosynthetic activity in PSII (the target activity of PSII herbicides) was measured, including the inhibition of effective quantum yield (ΔF/Fₘ') and maximum quantum yield (Fᵥ/Fₘ) using pulse amplitude modulation (PAM) fluorometry. Results from the Imaging-PAM were compared directly to the widely used mini-PAM to evaluate consistency between …