Interactive effects of rising temperature and nutrient enrichment on aquatic plant growth, stoichiometry, and palatability

The abundance and stoichiometry of aquatic plants are crucial for nutrient
cycling and energy transfer in aquatic ecosystems. However, the
interactive effects of multiple global environmental changes, including
temperature rise and eutrophication, on aquatic plant stoichiometry and
palatability remain largely unknown. Here, we hypothesized that (1) plant
growth rates increase faster with rising temperature in nutrient-rich than
nutrient-poor sediments; (2) plant carbon (C): nutrient ratios (nitrogen
(N) and phosphorus (P)) respond differently to rising temperatures at
contrasting nutrient conditions of the sediment; (3) external nutrient
loading to the water column limits the growth of plants and decreases
plant C:nutrient ratios; and that (4) changes in plant stoichiometry
affect plant palatability. We used the common rooted submerged plant
Vallisneria spiralis as a model species to test the effects of temperature
and nutrient availability in both the sediment and the water column on
plant growth and stoichiometry in a full-factorial
experiment. The results confirmed that plants grew faster in
nutrient-rich than nutrient-poor sediments with rising temperature,
whereas external nutrient loading decreased the growth of plants due to
competition by algae. The plant C: N and C: P ratios responded differently
at different nutrient conditions to rising temperature. Rising temperature
increased the metabolic rates of organisms, increased the nutrient
availability in the sediment and enhanced plant growth. Plant growth was
limited by a shortage of N in the nutrient-poor sediment and in the
treatment with external nutrient loading to the water column, as a
consequence, the limited plant growth caused an accumulation of P in the
plants. Therefore, the effects of temperature on aquatic plant C:nutrient
ratios did not only depend on the availability of the specific nutrients
in the environment, but also on plant growth, which could result in either
increased, unaltered or decreased plant C:nutrient ratios in response to
temperature rise. Plant feeding trial assays with the generalist consumer
Lymnaea stagnalis (Gastropoda) did not show effects of temperature or
nutrient treatments on plant consumption rates. Overall, our results
implicate that warming and eutrophication might interactively affect plant
abundance and plant stoichiometry, and therefore influence nutrient
cycling in aquatic ecosystems.