Pelagic phytoplankton community change-points across nutrient gradients and in response to invasive mussels

Phytoplankton communities can experience nonlinear responses to changing nutrient concentrations, but the nature of species shifts within phytoplankton is not well understood and few studies have explored responses of pelagic assemblages in large lakes. Using pelagic phytoplankton data from the Great Lakes, we assessed phytoplankton assemblage change-point responses to nutrients and invasive Dreissena, characterising community responses in a multi-stressor environment and determine whether species responses to in situ nutrients can be approximated from nutrient loading. We demonstrate assemblage shifts in phytoplankton communities along major stressor gradients, particularly prominent in spring assemblages, providing insight into community thresholds at the lower end of the phosphorus gradient and species-stressor responses in a multi-stressor environment. We show that responses to water nutrient concentrations could not be estimated from large-scale nutrient loading data likely due to lake-specific retention time and long-term accumulation of nutrients. These findings highlight the potential for significant accumulation of nitrates in ultra-oligotrophic systems, nonlinear responses of phytoplankton at nutrient concentrations relevant to current water quality standards and system-specific (e.g. lake or ecozone) differences in phytoplankton responses likely due to differences in nutrient co-limitation and effects of dreissenids.