Responses of summer phytoplankton biomass to changes in top-down forcing: Insights from comparative modelling

Abstract

The present study describes the responses of summer phytoplankton biomass to changes in top-down forcing (expressed as zooplankton mortality) in three ecosystems (the North Sea, the Baltic Sea and the Nordic Seas) across different 3D ecosystem models. In each of the model set-ups, we applied the same changes in the magnitude of mortality (±20%) of the highest trophic zooplankton level (Z1). Model results showed overall dampened responses of phytoplankton relative to Z1 biomass. Phytoplankton responses varied depending on the food web structure and trophic coupling represented in the models.Hence, a priori model assumptions were found to influence cascades and pathways in model estimates and, thus, become highly relevant when examining ecosystem pressures such as fishing and climate change. Especially, the different roles and parameterizations of additional zooplankton groups grazed by Z1, and their importance for the outcome, emphasized the need for better calibration data. Spatial variability was high within each model indicating that physics (hydrodynamics and temperature) and nutrient dynamics also play vital roles for ecosystem responses to top-down effects. In conclusion, the model comparison indicated that changes in top-down forcing in combination with the modelled foodweb structure affect summer phytoplankton biomass and, thereby, indirectly influence water quality of the systems.3

Stratification, nitrogen fixation, and cyanobacterial bloom stage regulate the planktonic food web structure

“…The mean trophic position of zooplankton is routinely used in biogeochemical models (Butenschön et al., ; Daewel & Schrum, ; Hjøllo, Huse, Skogen, & Melle, ; Neumann, Fennel, & Kremp, ), and yet, the degree of complexity of planktonic food webs under different environmental conditions that can lead to changes in phytoplankton biomass is still not well understood (Maar et al., ; Peck et al., ). Our examination of the mean trophic position of field mesozooplankton using amino acid δ 15 N revealed how the structure of planktonic food webs can alter with changing environmental conditions.…”

Food web structure and intraguild predation affect ecosystem functioning in an established plankton model

“…Food web complexity (hence omnivory) is seen as one cause for the weaker evidence for cascades in marine compared to freshwater systems (Hessen and Kaartvedt 2014;Pershing et al 2015), and omnivory is common in marine food webs (Thompson et al 2007). The prominence of trophic cascades will be highly relevant for model projections of future ecosystem changes and effects of fisheries (Bascompte et al 2005;Frank et al 2005;Maar et al 2018).…”

“…These presumably dominant trophic interactions then define the model's food web structure that is employed to simulate dynamics of the different plankton groups as biomass change over time. The food web structure of plankton and the number and type of trophic interactions are expected to affect ecosystem functions such as mesozooplankton production available to fish or carbon export (Steinberg and Landry 2017; Maar et al 2018).…”

Environmental Regulation of the Nitrogen Supply, Mean Trophic Position, and Trophic Enrichment of Mesozooplankton in the Mekong River Plume and Southern South China Sea

“…Despite their importance, the mean trophic position (TP) of marine mesozooplankton remains a poorly constrained variable in biogeochemical models (Butenschön et al., 2016; Daewel & Schrum, 2013; Neumann et al., 2002). It is still not well understood how the structure of planktonic food webs responds to changing environmental conditions (Maar et al., 2018; Peck et al., 2018), which has impacts on carbon fluxes, nitrogen (N) cycling, and net primary production (Steinberg & Landry, 2017).…”