On the sensitivity of plankton ecosystem models to the formulation of zooplankton grazing

Type Article
Date 2021-05
Language English
Author(s) Chenillat FannyORCID1, Rivière Pascal3, Ohman Mark D.2
Affiliation(s) 1 : Laboratoire des Sciences de l’Environnement Marin (LEMAR), Université de Brest, Ifremer, IRD, IUEM, Brest, France
2 : Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, United States of America
3 : Laboratoire des Sciences de l’Environnement Marin (LEMAR), Université de Brest, Ifremer, IRD, IUEM, Brest, France
Source Plos One (1932-6203) (Public Library of Science (PLoS)), 2021-05 , Vol. 16 , N. 5 , P. e0252033 (27p.)
DOI 10.1371/journal.pone.0252033
Abstract

Model representations of plankton structure and dynamics have consequences for a broad spectrum of ocean processes. Here we focus on the representation of zooplankton and their grazing dynamics in such models. It remains unclear whether phytoplankton community composition, growth rates, and spatial patterns in plankton ecosystem models are especially sensitive to the specific means of representing zooplankton grazing. We conduct a series of numerical experiments that explicitly address this question. We focus our study on the form of the functional response to changes in prey density, including the formulation of a grazing refuge. We use a contemporary biogeochemical model based on continuum size-structured organization, including phytoplankton diversity, coupled to a physical model of the California Current System. This region is of particular interest because it exhibits strong spatial gradients. We find that small changes in grazing refuge formulation across a range of plausible functional forms drive fundamental differences in spatial patterns of plankton concentrations, species richness, pathways of grazing fluxes, and underlying seasonal cycles. An explicit grazing refuge, with refuge prey concentration dependent on grazers’ body size, using allometric scaling, is likely to provide more coherent plankton ecosystem dynamics compared to classic formulations or size-independent threshold refugia. We recommend that future plankton ecosystem models pay particular attention to the grazing formulation and implement a threshold refuge incorporating size-dependence, and we call for a new suite of experimental grazing studies.

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S1 Fig. Schematic representation of (a) ingestion rate and (b) clearance rate for a classic sigmoidal functional response (solid line) vs. a response considering a feeding threshold Ti and Tc, ... 132 KB Open access
S2 Fig. Seasonal variation of detailed pfts accounting for emergent phytoplankton size classes (in mmol C m-3). 174 KB Open access
S3 Fig. Seasonal variation of (column 1) the 4 plankton functional types (mmol C m-3) and (column 2) detailed size classes of zooplankton (mmol C m-3). 196 KB Open access
S4 Fig. Seasonal variation of grazing fluxes (averaged on the last year of simulation, integrated over depths and averaged over longitude and latitude, excluding boundary conditions). 251 KB Open access
S5 Fig. (column 1) Annual mean surface chlorophyll-a concentration (mg Chl-a m-3); (column 2) Vertical sections of annual mean chlorophyll-a concentration across CalCOFI line 70 (mg Chl-a m-3)... 257 KB Open access
S6 Fig. Z:P ratio. (column 1) Seasonal variation of total phytoplankton biomass (blue), total zooplankton biomass (red) and the associated Z:P ratio (black). The number in the upper-left corner... 236 KB Open access
S7 Fig. (columns 1 and 2) Richness and (column 3 and 4) niches of phytoplankton (columns 1 and 3) and zooplankton (columns 2 and 4) for grazing in cases 3 and 4. 515 KB Open access
S8 Fig. Comparison of phytoplankton concentrations between coastal (0-100km) and offshore region (300-400km), in the Southern CCS (+/- 0.5° around line 90 of CalCOFI): Seasonal variation of detailed.. 406 KB Open access
S9 Fig. Comparison of zooplankton concentrations between coastal (0-100km) and offshore region (300-400km), in the Southern CCS (+/- 0.5° around line 90 of CalCOFI): Seasonal variation of detailed... 416 KB Open access
S10 Fig. Comparison of Z:P ratio between coastal (0-100km) and offshore region (300-400km), in the Southern CCS (+/- 0.5° around line 90 of CalCOFI): Seasonal variation of total phytoplankton ... 378 KB Open access
S1 Table. Various mathematical formulations of the grazing rate R, in their general formulation, i.e., for several prey (in biomass, mmolC m-3). 14 KB Open access
S2 Table. Biomasses (mmolC m-3). Biomasses are averaged over the 6 last years of the numerical experiments and spatially computed over the full depth, excluding up to 450 km (30 grid points) at ... 15 KB Open access
S3 Table. Grazing fluxes (mmolC m-3 d-1). Grazing fluxes are averaged over the 6 last years of the numerical experiments and spatially computed over the full depth, excluding up to 450 km (30 grid .. 15 KB Open access
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