This thesis describes an investigation into the impacts on charophytes of four selected aquatic herbicide and a mycoherbicide products currently used or under development in New Zealand for alien invasive aquatic plant management. Of particular interest was the charophyte response with respect to oospore germination, germling susceptibility and species response. In New Zealand, charophytes are native submerged aquatic plants which are recognized as beneficial components of lake ecosystems. They form dense meadows on the lake sediment and are rapid colonisers as they are the first plant to recolonise a lake after a disturbance event. Charophytes produce oospores (seeds) which can remain dormant in seed banks until suitable germination conditions are met. New Zealand lake sediments contain a seed bank which is dominated by charophyte oospores. Alien invasive plants severely impact charophytes by rapidly forming tall, dense monospecific stands which can displace and completely replace the native vegetation. Chemical weed control in New Zealand is limited to two aquatic herbicides, diquat and endothall, which are currently registered for use on submerged aquatic weeds in lakes and waterways. Fluridone, which is widely used in the USA, is not currently registered for commercial use in New Zealand but has been used in several New Zealand studies. More recently, the development of a mycoherbicide, an inundative biological control, using a formulated naturally occurring aquatic fungus has been trialed in the USA and New Zealand. The effects of these four products on charophytes were investigated in this study. Chelated copper was included as a control, as it is known to control algae, including charophytes overseas. For this research, lake sediment was collected from three New Zealand lakes and combined to give one mixed seed bank material of known oospore density and composition. There were two types of charophyte experiments; germination experiments and germling experiments. Two germination experiments examined different scenarios for herbicide treatments under controlled temperature and light conditions. Oospores were either retained in sediment or directly exposed to treatment. Two germling experiments examined germling susceptibility to herbicide treatments under outdoor conditions as well as controlled temperature and light conditions. The second germling experiment included a known charophyte control treatment (chelated copper compound) and a known target invasive plant (Lagarosiphon major) control treatment for the herbicides. Herbicide treatment doses started at the maximum label rate and decreased in concentration across a dilution series. Results from this research indicate that oospore germination was not negatively impacted by any of the herbicide treatments or doses although some species-specific sensitivity was evident. However, further research into species sensitivity is required to ascertain if the sensitivity was due to herbicidal effects or a combination of naturally occurring factors and what the implications of sensitivity are for weed management. For the germling experiments no negative effects were observed for the duration of the study. These results have positive implications for field application of herbicides, indicating that the younger charophyte growth stages (oospores and germlings) were unaffected by the type of herbicide used at any potential field application rate.