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Environmental stoichiometry mediates phytoplankton diversity effects on communities’ resource use efficiency and biomass

Citation

Gerhard, Miriam; Schlenker, Alexandra; Hillebrand, Helmut; Striebel, Maren (2021), Environmental stoichiometry mediates phytoplankton diversity effects on communities’ resource use efficiency and biomass, Dryad, Dataset, https://doi.org/10.5061/dryad.s1rn8pk8w

Abstract

Positive biodiversity-ecosystem functioning (BEF) relationships are predicted to increase in strength when high environmental variability allows for complementarity between resource use strategies in diverse communities. This environmental variability can be represented by spatial or temporal variation in nutrient ratios, but resource use efficiency (RUE) and therefore biomass build up of primary producers might be restricted when nutrient ratios are highly imbalanced (i.e., limitation by one nutrient and beyond optimal ratios for growth). Whereas the linkages between ecosystem functioning, diversity and nutrient availability are theoretically well understood, we lack experimental evidence on how phytoplankton diversity affects resource use and biomass under variable nutrient ratios (N:P ratios).

Combining a mesocosm and a microcosm experiment we tested diversity effects on ecosystem functioning by exposing a species diversity gradient generated by the loss of rare species in a natural community to different N:P ratios (uniform vs a gradient). The N:P supply ratio gradient also allowed us to evaluate responses across balanced and imbalanced ratios.

We found that increased species diversity led to increased community RUE when supplied a gradient of N:P ratios; but restricted to the highest diversity level. However, diversity did not affect RUE under uniform nutrient ratios. The overall phytoplankton biomass and carbon:nutrient ratios responses to diversity reflected the patterns detected for RUE. Contrary to theoretical predictions, RUE was maintained under highest N:P supply ratios (extreme phosphorous limitation) suggesting that imbalanced N:P ratios do not necessarily decrease function. Thus, we showed that the nutrient context influences diversity effects on RUE and biomass.

Synthesis. Overall, our results suggest that the effect of rare phytoplankton species losses on community RUE and biomass can be compensated by the persistent species when nutrient ratios are uniform, but leads to decreases in ecosystem functioning under variable nutrient ratios. This work provides a first attempt for testing interactions between the nutrient context (including concentrations and ratios) and the diversity of (natural) communities experimentally, which is conceptually understood but poorly tested for phytoplankton.

Methods

General experimental setup

A mesocosm and a microcosm experiment were conducted using the same initial phytoplankton diversity gradient and different nutrient treatments.

For generating the diversity gradient, a natural phytoplankton community was collected from a lake (Grafschaftsee, Germany, 53°330 05″ N; 7°580 49″ E) at the end of the summer (2017). Grazers were excluded by filtering the water through a 53μm mesh and six diversity levels were generated by dilution of the natural community and thus, removal of rare-species. Nutrient treatments were performed by manipulating N and P concentrations while other elements were supplied in excess according to WC phytoplankton growth medium (Guillard & Lorenzen, 1972). Mesocosm experimental setup

The six diversity treatments were inoculated under two nutrient levels (low and high) in 600L indoor mesocosms comprising 12 experimental units. The phytoplankton inoculum (corresponding to the six diversity levels) was added in small volumes (between 80 and 150mL) to the mesocosms assuring the same initial biomass amount in all treatments. For the low nutrient treatments, 0.65μmolL-1 P and 14.2μmolL-1 N were added, and to simulate nutrient enrichment, but maintaining the same N:P ratios, 1.9μmolL-1 of P and 42.6μmolL-1 of N were added in the high nutrient treatments. The mesocosms were sampled every four days from day 7 to day 27 comprising a total of six samplings. The experiment was continued until the phytoplankton communities reached the stationary growth phase. In every sampling event, 10% (60L) of water was replaced with water containing the design nutrient concentrations.

Microcosm experimental setup

The microcosm experiment was carried out using a factorial design where each of the six diversity levels was incubated under 25 different nutrient conditions leading to a total of 150 experimental units. For that, cell culture flaks (250mL polystyrene, Sarstedt Ltd. Leicester, UK) were filled with 200mL water taken from the six diversity treatments of the mesocosms (from the low nutrient treatment) on day 7. The nutrient conditions were based on combinations of 0.1, 0.7, 1.3, 1.9 and 2.6µmol P L-1 and 3, 19, 35, 48 and 62µmol N L-1 and added in each diversity level treatment. Samples were taken when the stationary growth phase was reached.

Laboratory analysis

Water samples for particulate organic carbon (POC), nitrogen (PON) and phosphorus (POP) were filtered onto acid-washed pre-combusted glass-fibre filters (Whatman GF/C). Filters for POC and PON were measured using an elemental analyser (Flash EA 1112, Thermo Scientific). Water samples for dissolved nitrogen fractions (NOx- and NH4+) were filtered and determined following the method described by Schnetger & Lehners (2014) for NOx- (NO3- and NO2), and a modified version of Benesch & Mangelsdorf (1972) method for ammonium (NH4+). POP and dissolved phosphorus (PO43-) were measured by molybdate reaction of filtered water samples for the dissolved fraction and after digestion with potassium peroxydisulfate (K2S2O8) solution for the particulate fraction (Wetzel & Likens, 2013). Phytoplankton samples corresponding to the initial communities of each experiment and the end of the mesocosm experiment were preserved in lugol solution until counted and identified under inverted microscope based on Utermöhl's method (Utermöhl, 1958). Phytoplankton was identified to the species level and morphospecies were used when clear assignment of a species name was not possible.

Calculations

We used richness and inverse Simpson diversity as diversity indices for estimating realized initial diversity in the experiments and at the end of the mesocosm experiment. The standing biomass was measured as POC, and RUE was calculated as unit of biomass per unit of total nutrient in the system (RUEP= POC/total P or RUEN= POC/total N, respectively), where total nutrients represent the sum of dissolved and particulate N or P, respectively.

All methods are described in detail in Gerhard et al. 2021. Environmental stoichiometry mediates phytoplankton diversity effects on communities’ resource use efficiency and biomass.

Funding

Uruguayan Agency of Investigation and Innovation (ANII), Award: POS_EXT_2015_1_122989

Deutscher Akademischer Austauschdienst, Award: 91645020

Deutsche Forschungsgemeinschaft, Award: STR 1383/6-1, SPP 1704