Nutrient supply does play a role on the structure of marine picophytoplankton communities

Conference communicationThe 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.Spanish Governmen

What controls CCN seasonality in the Southern Ocean? A statistical analysis based on satellite-derived chlorophyll and CCN and model-estimated OH radical and rainfall

13 pages, 4 figuresA 3-year time series set (from January 2002 to December 2004) of monthly means of satellite-derived chlorophyll (CHL) and cloud condensation nuclei (CCN), as well as model outputs of hydroxyl radical (OH), rainfall amount (RAIN), and wind speed (WIND) for the Southern Ocean (SO, 40°S–60°S) is analyzed in order to explain CCN seasonality. Chlorophyll is used as a proxy for oceanic dimethylsulfide (DMS) emissions since both climatological aqueous DMS and atmospheric methanesulfonate (MSA) concentrations are tightly coupled with chlorophyll seasonality over the Southern Ocean. OH is included as the main atmospheric oxidant of DMS to produce CCN, and rainfall amount as the main loss factor for CCN through aerosol scavenging. Wind speed is used as a proxy for sea salt (SS) particles production. The CCN concentration seasonality is characterized by a clear pattern of higher values during austral summer and lower values during austral winter. Linear and multiple regression analyses reveal high significant correlations between CCN and the product of chlorophyll and OH (in phase) and rainfall amount (in antiphase). Also, CCN concentrations are anticorrelated with wind speed, which shows very little variability and a slight wintertime increase, in agreement with the sea salt seasonality reported in the literature. Finally, the fraction of the total aerosol optical depth contributed by small particles (ETA) exhibits a seasonality with a 3.5-fold increase from austral winter to austral summer. The biogenic contribution to CCN is estimated to vary between 35% (winter) and 80% (summer). Sea salt particles, although contributing an important fraction of the CCN burden, do not play a role in controlling CCN seasonality over the SO. These findings support the central role of biogenic DMS emissions in controlling not only the number but also the variability of CCN over the remote oceanThis work is part of the AMIGOS project, funded by the Spanish Ministerio de Ciencia y TecnologíaPeer reviewe

Global relationship between phytoplankton diversity and productivity in the ocean

10 pages, 10 figuresThe shape of the productivity-diversity relationship (PDR) for marine phytoplankton has been suggested to be unimodal, that is, diversity peaking at intermediate levels of productivity. However, there are few observations and there has been little attempt to understand the mechanisms that would lead to such a shape for planktonic organisms. Here we use a marine ecosystem model together with the community assembly theory to explain the shape of the unimodal PDR we obtain at the global scale. The positive slope from low to intermediate productivity is due to grazer control with selective feeding, which leads to the predator-mediated coexistence of prey. The negative slope at high productivity is due to seasonal blooms of opportunist species that occur before they are regulated by grazers. The negative side is only unveiled when the temporal scale of the observation captures the transient dynamics, which are especially relevant at highly seasonal latitudes. Thus selective predation explains the positive side while transient competitive exclusion explains the negative side of the unimodal PDR curve. The phytoplankton community composition of the positive and negative sides is mostly dominated by slow-growing nutrient specialists and fast-growing nutrient opportunist species, respectively. © 2014 Macmillan Publishers LimitedThis work was supported by a Marie Curie Fellowship (IOF—FP7) to S.M.V. from the European Union and was performed within the MIT’s Darwin Project (http://darwinproject.mit.edu/). M.J.F. and S.D. are grateful for support from the >Gordon and Betty Moore Foundation>. M.L. was supported by the TULIP Laboratory of Excellence (ANR-10-LABX-41). S.M.V., P.C. and J.M.M. are currently supported by >Ramon y Cajal> contracts from the Spanish Ministry of Economy and Competitiveness (MINECO)Peer Reviewe

Phytoplankton size-diversity mediates an emergent trade-off in ecosystem functioning for rare versus frequent disturbances

Biodiversity is known to be an important determinant of ecosystem-level functions and processes. Although theories have been proposed to explain the generally positive relationship between, for example, biodiversity and productivity, it remains unclear which mechanisms underlie the observed variations in Biodiversity-Ecosystem Function (BEF) relationships. Using a continuous trait-distribution model for a phytoplankton community of gleaners competing with opportunists, and subjecting it to differing frequencies of disturbance, we find that species selection tends to enhance temporal species complementarity, which is maximised at high disturbance frequency and intermediate functional diversity. This leads to the emergence of a trade-off whereby increasing diversity tends to enhance short-term adaptive capacity under frequent disturbance while diminishing long-term productivity under infrequent disturbance. BEF relationships therefore depend on both disturbance frequency and the timescale of observation

SPEAD 1.0 - Simulating Plankton Evolution with Adaptive Dynamics in a two-trait continuous fitness landscape applied to the Sargasso Sea

37 pages, 10 figures, 3 tables, 2 appendices, supplement https://doi.org/10.5194/gmd-14-1949-2021-supplementDiversity plays a key role in the adaptive capacity of marine ecosystems to environmental changes. However, modelling the adaptive dynamics of phytoplankton traits remains challenging due to the competitive exclusion of sub-optimal phenotypes and the complexity of evolutionary processes leading to optimal phenotypes. Trait diffusion (TD) is a recently developed approach to sustain diversity in plankton models by introducing mutations, therefore allowing the adaptive evolution of functional traits to occur at ecological timescales. In this study, we present a model called Simulating Plankton Evolution with Adaptive Dynamics (SPEAD) that resolves the eco-evolutionary processes of a multi-trait plankton community. The SPEAD model can be used to evaluate plankton adaptation to environmental changes at different timescales or address ecological issues affected by adaptive evolution. Phytoplankton phenotypes in SPEAD are characterized by two traits, the nitrogen half-saturation constant and optimal temperature, which can mutate at each generation using the TD mechanism. SPEAD does not resolve the different phenotypes as discrete entities, instead computing six aggregate properties: total phytoplankton biomass, the mean value of each trait, trait variances, and the inter-trait covariance of a single population in a continuous trait space. Therefore, SPEAD resolves the dynamics of the population's continuous trait distribution by solving its statistical moments, wherein the variances of trait values represent the diversity of ecotypes. The ecological model is coupled to a vertically resolved (1D) physical environment, and therefore the adaptive dynamics of the simulated phytoplankton population are driven by seasonal variations in vertical mixing, nutrient concentration, water temperature, and solar irradiance. The simulated bulk properties are validated by observations from Bermuda Atlantic Time-series Studies (BATS) in the Sargasso Sea. We find that moderate mutation rates sustain trait diversity at decadal timescales and soften the almost total inter-trait correlation induced by the environment alone, without reducing the annual primary production or promoting permanently maladapted phenotypes, as occur with high mutation rates. As a way to evaluate the performance of the continuous trait approximation, we also compare the solutions of SPEAD to the solutions of a classical discrete entities approach, with both approaches including TD as a mechanism to sustain trait variance. We only find minor discrepancies between the continuous model SPEAD and the discrete model, with the computational cost of SPEAD being lower by 2 orders of magnitude. Therefore, SPEAD should be an ideal eco-evolutionary plankton model to be coupled to a general circulation model (GCM) of the global oceanThis work was funded by national research grant CTM2017-87227-P (SPEAD) from the Spanish government. We acknowledge support for the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI). The Institute of Marine Sciences (ICM – CSIC) is supported by a “Severo Ochoa” Centre of Excellence grant (CEX2019-000928-S) from the Spanish governmentPeer reviewe

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

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

How to Consistently Include both Laboratory Results and Oceanic Observations into Size-Based Models of Planknton Ecosystems?

2018 Ocean Sciences Meeting, 11-16 February, in Portland, OregonOur recent size-based studies of phytoplankton communities and plankton ecosystems reveal an apparent inconsistency between laboratory-based results and oceanic observations. Uni-modal distributions of maximum phytoplankton growth rate over cell size have been reported from laboratory experiments using single-species cultures. Our models, formulated based on size-scalings derived from laboratory results, tend to predict that medium-sized (nano-) phytoplankton dominate in the most productive regions of the ocean where chlorophyll concentrations are highest. However, oceanic observations of size-fractionated chlorophyll consistently reveal that the largest size fraction (micro-size) phytoplankton increases steadily with increasing total chlorophyll and dominates at the highest chlorophyll concentrations. Including size-selective grazing with decreasing preference for larger prey has been proposed as one possible means to reconcile laboratory-based size-scalings, which do not account for the effects of grazing, with oceanic observations, which reflect the net effect of both bottom-up and top-down processes. However, including size-selective grazing has not allowed our model to reproduce consistently the observed patterns of size-fractionated chlorophyll, primary production, and specific growth rate. Indeed, ship-board experiments reveal that micro-sized diatoms (> 20 μm) tend to have the fastest growth rates. This suggests that laboratory datasets, although valuable for providing information from controlled experiments, do not represent the full range of species and sizes present in the ocean. We will briefly present other recent observation-based results constituting targets for future modeling studies. For example, the interaction of predator and prey size diversities has been reported to affect trophic transfer, suggesting a mechanistic link between size structure and an important ecosystem functionPeer Reviewe