Confounding factors in algal phosphorus limitation experiments

Abstract

Assessing algal nutrient limitation is critical for understanding the interaction of primary production and nutrient cycling in streams, and nutrient diffusing substrate (NDS) experiments are often used to determine limiting nutrients such as nitrogen (N) and phosphorus (P). Unexpectedly, many experiments have also shown decreased algal biomass on NDS P treatments compared to controls. To address whether inhibition of algal growth results from direct P toxicity, NDS preparation artifacts, or environmental covariates, we first quantified the frequency of nutrient inhibition in published experiments. We also conducted a meta-analysis to determine whether heterotrophic microbial competition or selective grazing could explain decreases in algal biomass with P additions. We then deployed field experiments to determine whether P-inhibition of algal growth could be explained by P toxicity, differences in phosphate cation (K vs. Na), differences in phosphate form (monobasic vs. dibasic), or production of H2O2 during NDS preparation. We found significant inhibition of algal growth in 12.9% of published NDS P experiments as compared to 4.7% and 3.6% of N and NP experiments. The meta-analysis linear models did not show enhanced heterotrophy on NDS P treatments or selective grazing of P-rich algae. Our field experiments did not show inhibition of autotrophic growth with P additions, but we found significantly lower gross primary productivity (GPP) and biomass-specific GPP of benthic algae on monobasic phosphate salts as compared to dibasic phosphate salts, likely because of reduced pH levels. Additionally, we note that past field experiments and meta-analyses support the plausibility of direct P toxicity or phosphate form (monobasic vs. dibasic) leading to inhibition of algal growth, particularly when other resources such as N or light are limiting. Given that multiple mechanisms may be acting simultaneously, we recommend practical, cost-effective steps to minimize the potential for P- inhibition of algal growth as an artifact of NDS experimental design.

Experimental nitrogen and phosphorus enrichment stimulates multiple trophic levels of algal and detrital‐based food webs: a global meta‐analysis from streams and rivers

“…2E, F, Table 3). Similar to previous studies (Beck & Hall, 2018), P enrichment treatments in NDS experiments produced consistently low responses, as neither rate nor state LRR distributions differed from zero (Fig. 2E, F).…”

“…Our analyses revealed contrasts in biotic responses in NDS and non‐NDS enrichment experiments, corroborating the conclusions of previous meta‐analyses that called for caution when interpreting the results of NDS experiments in assessing nutrient limitation in streams (Keck & Lepori, 2012; Beck et al ., 2017; Beck & Hall, 2018). Although we do not know the reasons for the higher (2–5 times) LRR values obtained in flume and whole‐stream experiments compared to NDS experiments (Fig.…”

“…2), we postulate that: ( i ) NDS may not be suitable for natural levels of algal establishment and growth, for example Klose, Cooper & Leydecker (2012) found that Cladophora that were abundant in the surrounding stream did not colonize NDS; ( ii ) NDS experiments are much smaller than flume and stream experiments (0.01 versus 10–100 m 2 , respectively) and NDS often diverge from natural stream bottoms in physical and chemical characteristics (e.g. flow, primary succession, habitat characteristics); and ( iii ) P addition in commonly used NDS designs often elicit inhibitory responses (Sanderson et al ., 2009; Klose et al ., 2012; Beck et al ., 2017; Beck & Hall, 2018). The latter could be an artefact owing to the production of toxic hydrogen peroxide when agar is autoclaved together with phosphate (Tanaka et al ., 2014; Beck & Hall, 2018), a commonly used procedure when preparing NDS experiments that is now discouraged (Tank, Reisinger & Rosi, 2017), or may be due to differences in pH that result from different phosphate salts (Beck & Hall, 2018).…”

Seasonal shifts in the importance of bottom–up and top–down factors on stream periphyton community structure

“…We constructed NDS (Tank et al 2017) by filling 30 ml plastic vials with either 2% agar (control treatment), agar + 0.5 M NaNO 3 (N treatment), agar + 0.05 M KH 2 PO 4 + 0.05 M K 2 HPO 4 (P treatment), or agar + all three nutrient chemicals (NP treatment). Two types of phosphate were used to create an agar pH that was close to neutral, to avoid any confounding influences of pH alterations on periphyton (Beck and Hall 2018). Individual vials were capped with fritted glass discs (5.7 cm 2 ) and randomly attached to three plastic L-bars so that each plot contained three parallel L-bars holding six replicates of each of the four nutrient treatment types (Fig.…”

Increasing the reproductive function of Large White boars

“…The most important condition for increasing the productivity of pigs in industrial complexes is the intensification of their reproductive function. At present, animals with high genetic productivity potential have been created in our country and abroad [7,12,15,16,17]. Nevertheless, in the production conditions that have been created in our country, a sufficiently large number of animals cannot realize their genetic potential.…”