Interactive Effects of Rising Temperature and Nutrient Enrichment on Aquatic Plant Growth, Stoichiometry, and Palatability

Abstract

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.

Keywords: Lymnaea stagnalis; Vallisneria spiralis; herbivore; macrophyte; nitrogen; phosphorus; plant quality; warming.

Figure 1

Schematic graph of hypothesized temperature effects on aquatic plant growth rate (A) and plant C:nutrient ratio (B) at different sediment nutrient conditions.

Figure 2

Temperature effects on plant growth parameters indicated per nutrient treatment. (A) Plant shoot biomass, (B) root biomass, (C) relative growth rate, and (D) root:shoot ratio. S1 indicates nutrient-rich sediment, S0 indicates nutrient-poor sediment, W1 indicates with external nutrient loading to the water, and W0 indicates without external nutrient loading. A solid line indicates p < 0.05, and no line is drawn when p > 0.05. Vertical bars are standard errors (n = 6).

Figure 3

The relationship between algae growth and plant shoot biomass at the end of the experiment. (A) Periphyton biomass density (dry weight) and plant shoot biomass (dry weight per vase); (B) Seston concentration (dry weight) and plant shoot biomass. Linear regression test results are shown in the figures. See caption of Figure 2 for an explanation of the abbreviations of the nutrient treatments.

Figure 4

Temperature effects on plant elemental composition (C, N, and P contents) and stoichiometry (C:N, C:P, and N:P ratio) in dry weight indicated per nutrient treatment. (A) Plant C content, (B) N content, (C) P content, (D) C:N ratio, (E) C:P ratio, and (F) N:P ratio. Nutrient treatments are as indicated in Figure 2. A solid line indicates p < 0.05, and vertical bars are standard errors (n = 6).

Figure 5

The relationship between sediment porewater nutrient concentrations and plant nutrient contents. (A) porewater DIN concentration and plant N content, DIN indicates total dissolved inorganic nitrogen (including N from NH₄⁺, NO₂⁻, and NO₃⁻). (B) porewater P-PO₄³⁻ concentration and plant P content. Linear regression test results are shown in the figures. See caption of Figure 2 for an explanation of the abbreviations of the nutrient treatments.

Figure 6

Temperature effects on plant palatability to the pond snail L. stagnalis expressed as relative consumption rate (RCR), indicated per nutrient treatment. Nutrient treatments are as indicated in Figure 2. Vertical bars are standard errors (n = 6).

Figure 7

Structural equation model (SEM) of temperature, sediment, and external nutrient loading treatment effects on the growth and elemental compositions of the plant. Exogenous variables are indicated by rounded rectangles, and endogenous variables are represented by ovals. Coefficients of determination (r²) are shown for all endogenous variables. Numbers adjacent to arrows are standardized path coefficients and indicative of the effect of the relationship. Positive and negative effects among variables are depicted by green solid and red long-dashed arrows, respectively, with arrow thicknesses proportional to the strength of the relationship. Covariance between the plant elements are depicted by dashed double-headed arrows. The covariance between N and P content of the plant marginally significant at p = 0.06, all other relationships in the model are significant at p < 0.01. The model satisfied each of the three model fit criteria with significant χ² of p = 0.35, standardized root mean squared residuals of 0.04, and comparative fit index values of 0.997.