Trophic interactions in warming waters: Aquatic plant-consumer interactions under climate change

Anthropogenic activities have led to global environmental change, resulting in the deterioration of shallow aquatic ecosystems. This includes the decline of submerged aquatic plant communities, which provide vital functions in shallow aquatic ecosystems by maintaining a clear water state and sustaining high biodiversity. Eutrophication has been perceived as the main driver of these plant declines. However, the hypothesis that climate change might further exacerbate the decline of aquatic plant abundance remains largely untested. In this thesis, I aimed to clarify the effects of global warming on aquatic plants. I studied the effects of warming on aquatic plants with regard to four aspects: the effects of warming on plant growth, plant stoichiometry, plant-herbivore interactions, and plant-omnivore interactions. By manipulating temperature, environmental nutrient availability or both in experiments, I investigated their effects on aquatic plant growth, stoichiometry, and palatability to the generalist consumer Lymnaea stagnalis. I also tested the effects of temperature on plant consumption rates of aquatic herbivores and omnivores.
First, I found that temperature rise increases plant growth when within the physiological thermal tolerance range of the plant species. I also found that the plants grew faster in nutrient-enriched sediment, whereas the plant growth was inhibited by external nutrient loading to the water layer. Hence, warming and eutrophication can interactively influence the growth of aquatic plants. Secondly, I found that warming effects on aquatic plant stoichiometry depend on the environmental nutrient conditions. Warming could either decrease, not change or increase the plant C:nutrient ratio. Thirdly, I found that, from the perspective of aquatic plants, warming might decrease plant palatability in some species, but not in others. From the perspective of ectothermic herbivores, warming will lead to an increased plant consumption and further impose top-down grazing pressure on aquatic plant species. Similarly, I also found that plant palatability increased under eutrophication, which could lead to enhanced top-down grazing pressure on some aquatic plant species as well. Therefore, I conclude that both warming and eutrophication could lead to a stronger top-down grazing pressure on aquatic plants by herbivory, but this varies among plant species. Finally, I found that multiple taxa of aquatic omnivores (including fish, tadpole, crayfish and snail) increased the proportion of plants in their diet under warming. In addition, I found that aquatic omnivores increased aquatic plant consumption as plant quality increased with eutrophication. Therefore, I conclude that warming might increase herbivory of aquatic ectothermic omnivores, and eutrophication might strengthen this effect.
Both warming and eutrophication could lead to an increased grazing pressure on aquatic plants by ectothermic herbivores and omnivores. Therefore, the combination of eutrophication and warming will first cause aquatic plant communities to shift in species composition, followed by a decline in their overall abundance. Furthermore, both warming and eutrophication could lead to stable states shift from an aquatic plant-dominated clear system to a phytoplankton-dominated turbid system.