Edited by: James Joseph Elser, University of Montana, USA
Reviewed by: Evelyn Elaine Gaiser, Florida International University, USA; Paul Frost, Trent University, Canada
*Correspondence: Mandy Velthuis
This article was submitted to Functional Plant Ecology, a section of the journal Frontiers in Plant Science
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
Human activity is currently changing our environment rapidly, with predicted temperature increases of 1–5°C over the coming century and increased nitrogen and phosphorus inputs in aquatic ecosystems. In the shallow parts of these ecosystems, submerged aquatic plants enhance water clarity by resource competition with phytoplankton, provide habitat, and serve as a food source for other organisms. The carbon:nutrient stoichiometry of submerged aquatic plants can be affected by changes in both temperature and nutrient availability. We hypothesized that elevated temperature leads to higher carbon:nutrient ratios through enhanced nutrient-use efficiency, while nutrient addition leads to lower carbon:nutrient ratios by the luxurious uptake of nutrients. We addressed these hypotheses with an experimental and a meta-analytical approach. We performed a full-factorial microcosm experiment with the freshwater plant
Human activity has led to rapid environmental changes on our planet (Vitousek et al.,
Changes in plant abundances and growth rates can have effects on their nutrient demand and uptake and as such can influence their carbon:nutrient stoichiometry (Sterner and Elser,
However, contrasting hypotheses exist on how temperature and nutrient availability may affect carbon:nutrient ratios in aquatic plants. Elevated temperature can lead to an increase in plant biomass and a biomass dilution effect, where increased growth rates are accompanied by reduced tissue content (per unit of biomass) of a particular element (Taylor et al.,
Similarly, nutrient addition can positively affect the nutritional quality of aquatic plants (e.g., lower carbon:nutrient ratios; Burkholder et al.,
Here, we aimed to quantify the effects of temperature and nutrient addition on the carbon:nutrient stoichiometry of submerged aquatic angiosperms. We hypothesized that (1) both elevated temperature and nutrient addition lead to enhanced growth rates of submerged angiosperms, (2) if the biomass-dilution effect applies, elevated temperature will lead to higher carbon:nutrient ratios, whereas (3) nutrient addition is expected to lead to decreased carbon:nutrient ratios. These hypothesized changes in carbon:nutrient ratios are expected to be driven by changes in nutrient contents as opposed to carbon (4). Furthermore, we hypothesized that elevated temperature and nutrient addition are antagonists in their combined effect on carbon:nutrient ratios (5).
We tested these hypotheses using two complementary approaches. First, we performed a full-factorial experiment on the effects of temperature and sediment nutrient content (and their interaction) on the growth and carbon:nutrient stoichiometry of the submerged freshwater angiosperm
To test the effect of temperature and sediment nutrient content on
The experiment was carried out in 4 L plastic microcosms (14 × 14 × 21 cm), which contained 1.1 L of sediment and 2.7 L of water. Nutrient treatments were achieved by mixing artificial pond sediment (20% organic matter, Velda, Enschede, The Netherlands) with sand and consisted of 12.5, 25, 50, and 100% (v/v) of pond sediment (
After 58 days the experiment was terminated. From the middle of each microcosm, water samples were taken to determine dissolved nutrient concentrations, filtered over prewashed GF/F filters (Whatman, Maidstone, U.K.) and stored at −20°C until further analysis. Samples for pore water nutrients were taken in each microcosm through a 10 cm Rhizon SMS (Rhizosphere, Wageningen, the Netherlands) and stored at −20°C until further analysis. Plants were cut at the sediment level, and the above- and belowground biomass was harvested and rinsed with demi-water. All plant materials (above- and belowground) were dried at 60°C until constant dry mass and weighed. During the harvest, basic parameters were measured that describe the growing conditions (pH, alkalinity and seston chlorophyll-a). Methods and results of these measurements can be found in Supplementary Material
Plant material was grinded to a fine powder on a microfine grinder (MF 10 basic, IKA-werke, Staufen, Germany) or in test tube with a 1/8” ball bearing (Weldtite, Lincolnshire, UK) on a Tissuelyser II (QIAGEN, Germantown, USA). For nitrogen (N) and carbon (C) content, 0.2–2 mg dry mass was analyzed on a NC analyser (FLASH 2000 NC elemental analyser, Brechbueler Incorporated, Interscience B.V., Breda, The Netherlands). For phosphorus (P) content, 1–4 mg dry mass was combusted in a Pyrex glass tube at 550°C for 30 min. Subsequently, 5 mL of persulfate (2.5%) was added and samples were autoclaved for 30 min at 121°C. Digested P (as
Plant specific growth rate (SGR) was calculated with the following formula:
Where
Data on above- and belowground parameters (specific growth rate, above- and belowground biomass, carbon:nutrient stoichiometry and elemental contents) and dissolved nutrient concentrations (DIN and DIP) in the water column and the pore water were tested for effects of temperature, nutrients and their interaction with generalized linear models (function
A systematic literature review was carried out in Web of Science based on the guidelines described by the Collaboration for Environmental Evidence (
Control and elevated treatments were defined for both temperature and nutrient addition for each experiment separately. The lowest water temperature reported was defined as the control temperature treatment and 3–6°C above that temperature [equivalent to RCP scenario 8.5 from IPCC (
Delta response ratios and their variances were calculated for each separate study according to Lajeunesse (
Where X denotes mean of the fixed factor of interest [C:N and C:P ratio, growth rate (μ) and C, N and P contents], SD the standard deviation of that mean and N the sample size.
All statistics were carried out in R (R Core Team,
Temperature affected the specific growth rate and above- and belowground biomass of
Specific growth rate | day−1 | 7.3 | ||
Aboveground biomass | mg DW | 13.1 | ||
Belowground biomass | mg DW | |||
Aboveground C:N | mol:mol | 3.5 | ||
Aboveground C:P | mol:mol | 7.7 | 8.2 | |
Belowground C:N | mol:mol | 4.7 | ||
Belowground C:P | mol:mol | 6.0 | 7.5 | |
Aboveground C | mol g DW−1 | 5.1 | 0.7 | 1.3 |
Aboveground N | mol g DW−1 | 1.4 | 3.1 | |
Aboveground P | mol g DW−1 | 7.6 | ||
Belowground C | mol g DW−1 | 9.8 | ||
Belowground N | mol g DW−1 | 7.4 | 0.9 | 14.3 |
Belowground P | mol g DW−1 | 6.1 | 1.2 | |
Pore water DIN | μM | |||
Pore water DIP | μM |
Sediment nutrient content affected the specific growth rate and the above- and belowground biomass of
Temperature negatively affected aboveground C:N ratios (Table
Sediment nutrient content affected aboveground C:P ratio, while no effect on aboveground C:N ratios was observed (Table
Accompanied by the changes in aboveground C:N ratio, temperature seemed to affect aboveground N content (Table
Sediment nutrient content affected aboveground P content, with a 3-fold increase over the entire range of nutrient treatments (
Temperature affected dissolved nutrient concentrations in the pore water, and this effect interacted with nutrient treatment (Table
No significant effects of elevated temperature were observed on aboveground C:N and C:P ratios (Figure
Nutrient addition significantly decreased aboveground carbon:nutrient ratios, with 24.7 and 21.9% for C:N and C:P ratios, respectively (Figure
Aboveground carbon:nutrient stoichiometry of marine and freshwater plants responded qualitatively similar to nutrient addition, though the number of studies in the latter group was far lower (Figure
To address the impacts of temperature and nutrient availability on the growth and carbon:nutrient stoichiometry of aquatic plants, we performed a microcosm experiment and a meta-analysis. In line with our first hypothesis, elevated temperatures led to higher growth rates and standing stock biomass of the freshwater plant
In the meta-analysis, elevated temperature did not lead to enhanced growth rates or increased carbon:nutrient ratios of submerged aquatic plants in general, in contrast to our first and second hypotheses. However, it should be noted that overall sample sizes were very low (
Aboveground C:N ratio of
In our meta-analysis, we observed no overall effect of an 3–6°C elevated temperature on carbon:nutrient ratios of submerged aquatic plants, which contradicts findings in other groups of primary producers, such as phytoplankton (Toseland et al.,
Aboveground C:P ratios decreased about 4-fold with increasing sediment nutrient content in the
Increased plant nutrient content as observed in our meta-analysis and
Our analysis indeed indicated qualitatively similar responses to nutrient addition in both marine and freshwater submerged plants, though the responses were stronger in the latter group. Sample sizes for freshwater plants were far lower than for marine plants, highlighting the potential for freshwater research to learn from physiological studies on marine plants. Mean C:N ratios of freshwater plants are lower than marine plants (Bakker et al.,
Changes in plant carbon:nutrient stoichiometry in aquatic systems can have consequences for carbon cycling. In our meta-analysis, nutrient addition tended to increase plant growth rates, with positive (Murray et al.,
We conclude that nutrient (e.g., nitrogen and phosphorus) addition decreases carbon:nutrient stoichiometry in submerged aquatic plants, while no consistent effects of elevated temperature on these ratios were observed. The latter could be an effect of low sample size or could indicate species-specific responses in carbon:nutrient stoichiometry to global warming, which is an interesting avenue for future research. Furthermore, our experiment shows that the impact of temperature on aquatic plant stoichiometry depends on the availability of nutrients for plant growth, which is seldom taken into account. The impact of temperature may thus be modified by nutrient availability. The observed decline in carbon:nutrient stoichiometry of aquatic plants in response to nutrient addition can stimulate the further energy transfer to herbivores and decomposers, leading to reduced carbon stocks. With ongoing global warming, the knowledge gap of temperature effects on carbon:nutrient stoichiometry of submerged aquatic plants is in urgent need for further investigation.
MV, EB, and EvDo conceived and designed the experiments. EvDe performed the experiments. MV, EvDe, and PZ analyzed the data. MV and EB wrote the manuscript; all other authors provided editorial contributions.
The work of MV is funded by the Gieskes-Strijbis Foundation and the work of PZ by the China Scholarship Council (CSC).
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The authors would like to thank Dennis de Raaij and Arjan Wiersma for their help with extracting papers and setting up the database and Nico Helmsing for the chemical analyses during the experiment. Furthermore, we would like to thank Joost Keuskamp for the development of statistical tools for the meta-analysis, Sven Teurlincx for his help with the harvest together with fruitful discussions and Inés Castejón, Kim Holzer and Marion Cambridge for contributing unpublished data to the meta-analysis database. This is NIOO publication number 6275.
The Supplementary Material for this article can be found online at: