Modelling key variables for understanding the effects of grazing and nutrient recycling by zooplankton on the freshwater microbial loop

SCHENONE, L.; B. MODENUTTI,; MARTYNIUK, N; BASTIDAS NAVARRO, M.,; LASPOUMADERES, C; BALSEIRO, E

Freshwater Biology

John Wiley and Sons Inc

Año: 2021 vol. 66 p. 2322 - 2337

1. Pelagic microbial food webs are structured by zooplankton through grazing andnutrient recycling. Cladocerans and copepods are assumed to have different effectson the microbial loop by grazing on different prey sizes and releasing phosphorus(P) differentially. Here, we assessed this effect of differential zooplanktongrazing and nutrient recycling on microbial loop dynamics using a combination ofexperimental and modelling approaches.2. We performed field incubation experiments in an oligotrophic mountain lake(north-PatagonianAndes) using the natural microbial community and the twodominant zooplankton taxa: a cladoceran (Diaphanosoma chilense) and a copepod(Boeckella gibbosa). The effect of zooplankton grazing and nutrient recycling wereassessed separately in different treatments with direct and indirect zooplanktonpresence, respectively. We built a mechanistic model to estimate zooplanktongrazing and P recycling and prey P quotas. The model was parameterised withthe results from our field experiment and with prior information from size-basedtraits and zooplankton C:P using a Bayesian approach. Laboratory experiments forzooplankton P excretion were also performed to test the predictive accuracy ofour model.3. Our model showed that copepods and cladocerans have contrasting effects onthe microbial loop. Diaphanosoma chilense grazed mainly on picoplankton while B.gibbosa grazed on nanoflagellates and algae. Diaphanosoma chilense reduced thebiomass and increased P quota of picoplankton, and reduced the P quota of nanoflagellates.In contrast, B. gibbosa released more P, increasing the picoplanktonbiomass and reducing the biomass of nanoflagellates, but increasing its P quota.4. Based on our experimental and model results, copepod grazing favours higher Pacquisition rates for cladocerans by releasing more P for picoplankton. By contrast,cladocerans would have a mixed effect on the main food items of copepodsby increasing P quotas of the strict osmotrophic algae but decreasing P quotas ofnanoflagellates.5. Our mechanistic model is useful to quantitatively assess key planktonic variables,which are usually difficult to measure in the field, such as zooplankton P excretion