Homeostasis and non‐linear shift in the stoichiometry of P‐limited planktonic communities

Abstract

Planktonic 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 (PO 3À 4) and N (NH þ 4 or NO À 3) 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.

Episodic nutrient enrichments stabilise protist coexistence in planktonic oligotrophic conditions

“…The enclosures consisted of 20‐m‐long transparent cylindrical polythene bags with an upper open polyvinyl chloride tube enclosed in a float and a lower closed tube that acted as a sediment trap. For deployment, the bags were initially folded in the tubes and allowed to sink until a depth of 20 m, where the lacing that held the tubes together untied, and the upper tube started to float up towards the lake surface, thus gently filling the bag with water with little disturbance to the plankton community (see Giménez‐Grau et al., 2020 for further details). We performed the experiment at the middle of the lake stratification period when the epilimnion usually shows the lowest chlorophyll concentration (Figure ; Felip & Catalan, 2000), allowing the species to grow with less competition when limiting nutrients were added.…”

“…Details on productivity and seston stoichiometry from the experiment can be found in the study by Giménez‐Grau et al. (2020). Nutrients were not exhausted in any enclosure during the experiment.…”

Self-filling enclosures to experimentally assess plankton response to pulse nutrient enrichments

“…If strong currents are expected, more buoys and anchor weights interspersed between every few enclosures can be added. In Lake Redon, winds can be relatively strong, but the fetch is short, we were using only mooring at the extremes, and the line sustained 22 enclosures ( Giménez-Grau et al ., 2020 ).…”

“…The planktonic community in Lake Redon is P-limited due to the long-range atmospheric transport of reactive nitrogen ( Camarero and Catalan, 2012 ). The primary objective of the ENEX experiment was to evaluate the effects of the fluctuations in P availability and N:P imbalance on the structure of the planktonic protist community ( Zufiaurre et al ., 2021 ) and its stoichiometry ( Giménez-Grau et al ., 2020 ). We first describe the enclosure design and use as performed in the ENEX experiment, and, in the discussion, we comment on alternative options and designs according to other objectives and lake characteristics.…”

Biological Microbial Interactions from Cooccurrence Networks in a High Mountain Lacustrine District

“…The metacommunity was composed of lake surface plankton samples that were taken during summer of 2011 along the Pyrenees mountain range ( 4 ). Most of these lakes lie within ultraoligotrophic/oligotrophic ranges ( 52 ), where nitrate-phosphate imbalances caused by atmospheric depositions ( 53 ) may alter significantly the phytoplanktonic behavior and trophic status ( 54 ). However, pH is often the strongest segregator of community types ( 55 – 57 ).…”