Abstract
AbstractPlanktonic communities are naturally subjected to episodic nutrient enrichments that may stress or redress the imbalances in limiting nutrients. Human‐enhanced atmospheric nitrogen deposition has caused profound N:P imbalance in many remote oligotrophic lakes in which phosphorus has largely become limiting. These lakes offer an opportunity to investigate the relationship between the changes in plankton stoichiometry, productivity, and community structure occurring during nutrient fluctuations in P‐limited conditions. We performed P () and N ( or ) pulse additions to the summer epilimnetic community of an ultraoligotrophic lake using self‐filling ~100‐L enclosures and analyzed the response to varying P availability, N:P imbalance, and N source. Seston C:N:P proportions remained fairly unchanged to P additions that were within the range of values seasonally found in the lake. However, the seston N:P ratio abruptly shifted and approached Redfield’s proportions at P additions typical of mesotrophic conditions that provided non‐limiting conditions. N surplus did not affect seston C:N:P proportions. The patterns of seston N:P stability and shift were similar for both N sources. In contrast, productivity was highly sensitive to low and medium P additions and decelerated at high P additions. Phytoplankton biomass dominated particulate organic matter. The autotrophic community differentiated almost linearly across the P gradient. Chrysophytes' dominance decreased, and diatoms and cryptophytes relative abundance increased. Nonetheless, the stoichiometry stability and non‐linear shift involved large biomass proportions of the same species, which indicates that the bulk stoichiometry was related to similar physiological behavior of phylogenetically diverse organisms according to the biogeochemical context. The C:N:P seston stability in P‐limited conditions—with loose coupling with productivity, nutrient supply ratios, and species dominance—and the sudden shift to Redfield proportions in P‐repleted conditions suggest a complex regulation of P scarcity in planktonic communities that goes beyond immediate acclimation growth responses and might include alternative physiological and biogeochemical states.
Highlights
The biomass of all living organisms consists of more than 20 essential elements in quite defined proportions, and whichever of these elements is in shortest supply than demand may be limiting the growth (Hessen et al 2013)
Accepting that the observed seston stoichiometry changes in our experiment were mainly driven by phytoplankton, our results indicate C: N:P homeostasis in the range of P limitation naturally occurring in the lake
The C:N:P seston stability in P-limited conditions found in our experiment—with loose coupling with productivity, nutrient supply ratios and species dominance—and the sudden shift to Redfield proportions in P-repleted conditions suggest a complex regulation of P scarcity in planktonic communities, which may go beyond immediate acclimation growth responses and might include alternative physiological and biogeochemical states
Summary
The biomass of all living organisms consists of more than 20 essential elements in quite defined proportions, and whichever of these elements is in shortest supply than demand may be limiting the growth (Hessen et al 2013). Demand for nitrogen (N) and phosphorus (P) is high in all organisms (e.g., synthesis of proteins and nucleic acids) and, typically limits productivity in many aquatic ecosystems (Elser et al 2007). In P-limited conditions, strong negative relationships between growth and N:P have been suggested based on observations (Hillebrand et al 2013) and model approaches that consider the protein: RNA ratio. According to the GRH, the N:P molar ratio of 16 (Redfield 1963) had been suggested to have no intrinsic optimality significance (Klausmeier et al 2004a) and, quite the opposite, being the optimal stoichiometry in nutrient-replete conditions when the feedbacks between the protein and rRNA synthesis processes are considered (Loladze and Elser 2011). Different stoichiometry states across the oceans are fostering a system-wide view in which many unknowns remain concerning physiological and biogeochemical processes (Martiny et al 2013)
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