Ranking distribution reveals opposite shifts in evenness and survival thresholds of phytoplankton under environmental stress.

Evaluating phytoplankton diversity in response to environmental stress is important in the context of climate change and increasing anthropogenic activities. Here, we investigated the response of the phytoplankton diversity indices to temperature and nutrient gradients. Combining microcosm experiments and field investigations, we found that the minimum percentage threshold required for a taxon to survive (a) first increased and then decreased with increasing temperature, but decreased with increasing nutrient levels. In contrast, the evenness of the taxon distribution (k) showed an opposite trend to a. We identified that the index a showed a significant negative correlation with positive cohesion, the absolute value of negative cohesion (|negative cohesion|), and total cohesion under non-stress conditions for algal growth, whereas k showed the opposite pattern. However, the relationships between a, k, and the cohesion values were not significant under stress conditions. In addition, closeness centrality was positively correlated with inhibition rate, whereas weighted degree, eigenvector centrality, positive connectivity, and |negative connectivity| were negatively correlated with it under eutrophication at moderate temperatures. Moreover, a change in biomass was the dominant initial response of phytoplankton to short-term environmental stress. Our results also indicated that the proliferation ability of the algal community was inversely related to its evenness (k and Pielou index), and vice versa. These findings clarify how phytoplankton diversity indices respond to environmental stress and how this response is associated with community structure under climate change and anthropogenic activities. This understanding is critical for assessing the health of global aquatic ecosystems.