Role of zooplankton dynamics for Southern Ocean phytoplankton biomass and global biogeochemical cycles

Global ocean biogeochemistry models currently employed in climate change projections use highly simplified representations of pelagic food webs. These food webs do not necessarily include critical pathways by which ecosystems interact with ocean biogeochemistry and climate. Here we present a global biogeochemical model which incorporates ecosystem dynamics based on the representation of ten plankton functional types (PFTs); six types of phytoplankton, three types of zooplankton, and heterotrophic bacteria. We improved the representation of zooplankton dynamics in our model through (a) the explicit inclusion of large, slow-growing zooplankton, and (b) the introduction of trophic cascades among the three zooplankton types. We use the model to quantitatively assess the relative roles of iron vs. grazing in determining phytoplankton biomass in the Southern Ocean High Nutrient Low Chlorophyll (HNLC) region during summer. When model simulations do not represent crustacean macrozooplankton grazing, they systematically overestimate Southern Ocean chlorophyll biomass during the summer, even when there was no iron deposition from dust. When model simulations included the developments of the zooplankton component, the simulation of phytoplankton biomass improved and the high chlorophyll summer bias in the Southern Ocean HNLC region largely disappeared. Our model results suggest that the observed low phytoplankton biomass in the Southern Ocean during summer is primarily explained by the dynamics of the Southern Ocean zooplankton community rather than iron limitation. This result has implications for the representation of global biogeochemical cycles in models as zooplankton faecal pellets sink rapidly and partly control the carbon export to the intermediate and deep ocean

Major role of nutrient supply in the control of picophytoplankton community structure.

abstractThe Margalef´s mandala (1978) is a simplified bottom-up control model that explains how mixing and nutrient concentration determine the composition of marine phytoplankton communities. Due to the difficulties of measuring turbulence in the field, previous attempts to verify this model have applied different proxies for nutrient supply, and very often used interchangeably the terms mixing and stratification. Moreover, because the mandala was conceived before the discovery of smaller phytoplankton groups (picoplankton <2 μm), it describes only the succession of vegetative phases of microplankton. In order to test the applicability of the classical mandala to picoplankton groups, we used a multidisciplinary approach including specifically designed field observations supported by remote sensing, database analyses, and modeling and laboratory chemostat experiments. Simultaneous estimates of nitrate diffusive fluxes, derived from microturbulence observations, and picoplankton abundance collected in more than 200 stations, spanning widely different hydrographic regimes, showed that the contribution of eukaryotes to picoautotrophic biomass increases with nutrient supply, whereas that of picocyanobacteria shows the opposite trend. These findings were supported by laboratory and modeling chemostat experiments that reproduced the competitive dynamics between picoeukaryote sand picocyanobacteria as a function of changing nutrient supply. Our results indicate that nutrient supply controls the distribution of picoplankton functional groups in the ocean, further supporting the model proposed by Margalef.RADIALES (IEO

Maximal feeding with active prey-switching: A kill-the-winner functional response and its effect on global diversity and biogeography

Predators’ switching towards the most abundant prey is a mechanism that stabilizes population dynamics and helps overcome competitive exclusion of species in food webs. Current formulations of active prey-switching, however, display non-maximal feeding in which the predators’ total ingestion decays exponentially with the number prey species (i.e. the diet breadth) even though the total prey biomass stays constant. We analyse three previously published multi-species functional responses which have either active switching or maximal feeding, but not both. We identify the cause of this apparent incompatibility and describe a kill-the-winner formulation that combines active switching with maximal feeding. Active switching is shown to be a community response in which some predators become prey-selective and the formulations with maximal or non-maximal feeding are implicitly assuming different food web configurations. Global simulations using a marine ecosystem model with 64 phytoplankton species belonging to 4 major functional groups show that the species richness and biogeography of phytoplankton are very sensitive to the choice of the functional response for grazing. The phytoplankton biogeography reflects the balance between the competitive abilities for nutrient uptake and the degree of apparent competition which occurs indirectly between species that share a common predator species. The phytoplankton diversity significantly increases when active switching is combined with maximal feeding through predator-mediated coexistence

Phytoplankton functional diversity increases ecosystem productivity and stability

International audienceThe effect of biodiversity on ecosystem functioning is one of the major questions of ecology. However, the role of phytoplankton functional diversity in ecosystem productivity and stability under fluctuating (i.e. non-equilibrium) environments remains largely unknown. Here we use a marine ecosystem model to study the effect of phytoplankton functional diversity on both ecosystem productivity and its stability for seasonally variable nutrient supply and temperature. Functional diversity ranges from low to high along these two environmental axes independently. Changes in diversity are obtained by varying the range of uptake strategies and thermal preferences of the species present in the community. Species can range from resource gleaners to opportunists, and from cold to warm thermal preferences. The phytoplankton communities self-assemble as a result of species selection by resource competition (nutrients) and environmental filtering (temperature). Both processes lead to species asynchrony but their effect on productivity and stability differ. We find that the diversity of temperature niches has a strong and direct positive effect on productivity and stability due to species complementarity, while the diversity of uptake strategies has a weak and indirect positive effect due to sampling probability. These results show that more functionally diverse phytoplankton communities lead to higher and more stable ecosystem productivity but the positive effect of biodiversity on ecosystem functioning depends critically on the type of environmental gradient. Previous article in issu

SPEAD 1.0 -- A model for simulating plankton evolution with adaptive dynamics in a two trait continuous fitness landscape applied to the Sargasso Sea

Diversity plays a key role in ecosystem adaptive capacities. However, modeling phytoplankton trait diversity remains challenging due to the strength of the competitive exclusion of sub-optimal phenotypes. A recent approach to sustain diversity, called "trait diffusion", consists in allowing evolution to occur at contemporary timescales. In this study, we present a model for Simulating Plankton Evolution with Adaptive Dynamics (SPEAD), where phenotypes characterized by two traits, nitrogen half-saturation constant and optimal temperature, can mutate at each generation. SPEAD does not resolve all phenotypes, computing instead six aggregate properties: biomass, mean traits, trait variances and inter-trait covariance. The adaptive dynamics are driven by vertical and seasonal variations in water temperature, irradiance, vertical mixing, and nutrient concentration. The bulk properties are validated by observations from station BATS in the Sargasso Sea. We find only minor discrepancies between SPEAD and a similar model that represents the full phenotype distribution, but SPEAD has a lower computational cost by two orders of magnitude. Moderate mutation rates are shown to sustain trait diversity at decadal timescales and to soften the almost total inter-trait covariance induced by the environment alone, without reducing the annual primary production or promoting permanently maladapted phenotypes, as occurs with high mutation rates. The response to environmental changes is faster than in single-trait models. Future axes of improvement include increasing the number of traits, beginning with optimal irradiance, refining the description of the phytoplankton community by resolving several functional groups, and coupling SPEAD with a general circulation mode

Disentangling environmental effects in microbial association networks

Ecolocial interctions among microorganisms are fundamental for ecosystem function, yet they are mostly unknown or poorly understood. High-throughput-omics can indicate microbial interactions by associations across time and space, which can be represented as association networks. Links in these networks could result from either ecological interactions between microorganisms, or from environmental selection, where the association is environmentally-driven. Therefore, before downstream analysis and interpretation, we need to distinguish the nature of the association, particularly if it is due to environmental selection or not.S

Phytoplankton functional diversity increases ecosystem productivity and stability

13 pages, 7 figures, 3 tables, 2 appendices, supplementary data https://doi.org/10.1016/j.ecolmodel.2017.06.020The effect of biodiversity on ecosystem functioning is one of the major questions of ecology. However, the role of phytoplankton functional diversity in ecosystem productivity and stability under fluctuating (i.e. non-equilibrium) environments remains largely unknown. Here we use a marine ecosystem model to study the effect of phytoplankton functional diversity on both ecosystem productivity and its stability for seasonally variable nutrient supply and temperature. Functional diversity ranges from low to high along these two environmental axes independently. Changes in diversity are obtained by varying the range of uptake strategies and thermal preferences of the species present in the community. Species can range from resource gleaners to opportunists, and from cold to warm thermal preferences. The phytoplankton communities self-assemble as a result of species selection by resource competition (nutrients) and environmental filtering (temperature). Both processes lead to species asynchrony but their effect on productivity and stability differ. We find that the diversity of temperature niches has a strong and direct positive effect on productivity and stability due to species complementarity, while the diversity of uptake strategies has a weak and indirect positive effect due to sampling probability. These results show that more functionally diverse phytoplankton communities lead to higher and more stable ecosystem productivity but the positive effect of biodiversity on ecosystem functioning depends critically on the type of environmental gradientThis work was funded by national research grants CGL2013-41256-P (MARES) and CTM2014-54926-R (SUAVE) from the Spanish government. S.M.V. and P.C. are supported by «Ramon y Cajal» (RyC) contracts from the Spanish government. S.D. was supported by NSF (grant 1434007) from the United States government. M.L. and J.M.M. are supported by the French Laboratory of Excellence project «TULIP» (ANR-10-LABX-41; ANR-11-IDEX-0002-02). M.L. was also supported by the BIOSTASES Advanced Grant, funded by the European Research Council under the European Union's Horizon 2020 research and innovation programme (grant agreement No 666971) and J.M.M. by the Region Midi- Pyrenees project (CNRS 121090)Peer Reviewe

Examining the generalizability of research findings from archival data.

This initiative examined systematically the extent to which a large set of archival research findings generalizes across contexts. We repeated the key analyses for 29 original strategic management effects in the same context (direct reproduction) as well as in 52 novel time periods and geographies; 45% of the reproductions returned results matching the original reports together with 55% of tests in different spans of years and 40% of tests in novel geographies. Some original findings were associated with multiple new tests. Reproducibility was the best predictor of generalizability-for the findings that proved directly reproducible, 84% emerged in other available time periods and 57% emerged in other geographies. Overall, only limited empirical evidence emerged for context sensitivity. In a forecasting survey, independent scientists were able to anticipate which effects would find support in tests in new samples