Global biodiversity patterns of marine phytoplankton

One of the most pressing tasks faced by environmental scientists and society is to predict how climate and other environmental factors shape the distribution of biological species at the global scale. Whether a region houses many or few species affects ecosystem functions, including productivity and stability, through complementarity in species’ niches. Phytoplankton have evolved ecological niches that fill the entire near-surface global ocean. Despite being some of the tiniest organisms on Earth, they drive as much as half of global primary production. However, the functional and ecological consequences of phytoplankton species diversity have been poorly known, as global diversity patterns of these microbes have been among the least characterized throughout the history of ecology. To determine global patterns and drivers of phytoplankton diversity, this thesis combines ocean-sourced data with statistical models, developed specifically to address data sparseness

Shaping species richness: ecological filtering in native and non-native plants along elevation gradients in the Swiss Alps

Ecological filtering, imposed by climatic constraints or competitive interactions, might shape distribution patterns of native and non-native plants differently along elevation gradients. However, patterns of species richness that might reflect these processes have rarely been compared between native and non-native plants. All angiosperms and gymnosperms were recorded along and away from three road-corridors that vertically extended >1100 m in a major valley of the Swiss Alps. Non-native species richness declined with elevation and distance from roadsides. Native richness was humped across the elevational range and varied little away from the road. Conversely, richness of native ruderals was distributed similarly as for the non-native plants whereas mountain species numbers strongly increased with elevation. Consistent with directional ecological filtering, non-native species either dropped out or exhibited progressively wider elevational tolerances if recorded at higher altitudes. A similar pattern was found for native ruderals and the opposite pattern for mountain specialist species, i.e. widening ranges in top-down direction. Furthermore, elevational limitations of species within these groups were mostly relaxed along the road-corridor compared to adjacent habitats. These results led to the following conclusions: (i) elevational richness patterns among the native plants strongly differ based upon ecological functional groups, (ii) richness patterns potentially reflect directional ecological filtering both in bottom-up (native ruderals, non-natives) and top- down (mountain plants) direction, (iii) species among groups with asymmetric richness distributions across elevation seem to be inhibited from laterally invading non-roadside habitats, especially towards their elevational range limits, (iv) understanding the processes of directional dispersal and biotic/abiotic components of ecological filters will help towards predictions of future distributions of plant groups, including the non-native group

Nitrogen fixation increases with diazotroph diversity in the global ocean

Marine diazotrophs convert atmospheric nitrogen gas into bioavailable nitrogen that can fuel up to 50% of the primary productivity in oligotrophic subtropical and tropical seas. Despite their importance, little is known about their global biogeography and diversity since global studies have been hampered by scarce data observations within the marine environment. To analyse the correlation between global richness and nitrogen fixation rates, we integrate -omics and traditional microscopic based observations and make use of Species Distribution Models that have been developed to cope with datasets that suffer from uneven sampling efforts and scarce observations. Global diazotroph richness is generally high in subtropical and tropical marine regions declining polewards with hotspots of diversity found in the Pacific and Indian Ocean. We provide the first global biogeographic pattern of non-cyanobacterial diazotrophs that show increased probability of occurrences within upwelling regions when contrasted to cyanobacterial diazotrophs. Finally, to assess the relationship between global diazotroph richness and ecosystem function, we analysed the correlation between our global richness projections and global nitrogen fixation rates from two separate publications and found a positive relationship between global diazotroph richness and nitrogen fixation rates supporting the resource use efficiency hypothesis

PHYTOBASE: A global synthesis of open ocean phytoplankton occurrences

Marine phytoplankton are responsible for half of the global net primary production and perform multiple other ecological functions and services of the global ocean. These photosynthetic organisms comprise more than 4300 marine species, but their biogeographic patterns and the resulting species diversity are poorly known, mostly owing to severe data limitations. Here, we compile, synthesize, and harmonize marine phytoplankton occurrence records from the two largest biological occurrence archives (Ocean Biogeographic Information System; OBIS, and Global Biodiversity Information Facility; GBIF) and three recent data collections. The resulting PhytoBase data set contains over 1.36 million phytoplankton occurrence records (1.28 million at the level of species) for a total of 1711 species, spanning the principal groups of the Bacillariophyceae, Dinoflagellata, and Haptophyta as well as several other groups. This data compilation increases the amount of marine phytoplankton records available through the single largest contributing archive (OBIS) by 65 %. Data span all ocean basins, latitudes and most seasons. Analyzing the oceanic inventory of sampled phytoplankton species richness at the broadest spatial scales possible, using a resampling procedure, we find that richness tends to saturate in the pantropics at ~ 93 % of all species in our database, at ~64% in temperate waters, and at ~ 35 % in the cold Northern Hemisphere, while the Southern Hemisphere remains underexplored. We provide metadata on the cruise, research institution, depth and date of collection for each record, and we include cell-counts for 195 339 records. We strongly recommend consideration of global spatiotemporal biases in sampling intensity and varying taxonomic sampling scopes between research cruises or institutions when analyzing the occurrence database. Including such information into statistical analysis tools, such as species distribution models may serve to project the diversity, niches, and distribution of species in the contemporary and future ocean, opening the door for a quantification of macroecological phytoplankton patterns.ISSN:1866-359

Biome partitioning of the global ocean based on phytoplankton biogeography

Biomes are geographical units that can be defined based on biological communities sharing specific environmental and climatic requirements. Contemporary ocean biomes have been constructed based on various approaches. These included the biogeographic patterns of higher trophic level organisms, physical and biogeochemical properties, or bulk biological properties such as chlorophyll-a, but none considered the biogeographic patterns of the first trophic level explicitly, i.e. phytoplankton biogeography. A global description of marine biomes based on phytoplankton and defined in analogy to terrestrial vegetation biomes is still lacking. A bioregionalization based on phytoplankton appears particularly timely, as phytoplankton have a high sensitivity to climatic changes and fuel marine productivity. Here, we partition the global ocean into biomes by using self-organizing maps and hierarchical clustering, drawing on the biogeographic patterns of 536 phytoplankton species predicted from empirical evidence. Our approach reveals eight different biomes at the seasonal scale, and seven at the annual scale. The biomes host characteristic phytoplankton species compositions, and differ in their prevailing environmental conditions. The largest differences in phytoplankton composition are found between a Pacific equatorial biome and other tropical biomes, and between subtropical and high latitude biomes. The Pacific equatorial biome is characterized by species with narrower ecological niches, the tropical and subtropical biomes by cosmopolitan generalists, and the high latitudes by species with a heterogeneous biogeography. The strongest differences between biomes are found along gradients of temperature and macronutrient availability, associated with latitude. We test whether our biomes can be reproduced based on indicator species, or potential co-occurrence networks of species determined from the predicted species distributions that are wide-spread in some but rare in other biomes. We find that our biomes can be reproduced by the 51 species identified, which together form significant species co-occurrences. This suggests that species co-occurrences, rather than individual indicator species drive oceanic biome partitioning at the first trophic level. Our biome partitioning may be especially useful for comparative analyses on the functional implications of phytoplankton organization, and impacts on zoogeographical partitionings. Furthermore, it provides a framework for predicting large-scale changes in phytoplankton community structure due to anthropogenic climate and environmental change.ISSN:0079-661

Phytoplankton based biome identity at sea surface at 1° latitude x 1° longitude resolution, for the annual, seasonal, and monthly scale

These data contain the annual, seasonal and monthly biome partitioning of the global surface ocean on a 1° latitude x 1° longitude resolution for the ocean mixed layer. The biomes were defined using self-organizing maps and hierarchical clustering, drawing on the biogeographic patterns of 536 phytoplankton species, projected based on global in situ presence observations and biogeographic extrapolation using statistical species distribution modeling (Righetti et al., 2019). This approach resulted in nine global clusters at the monthly scale. The clusters were used to define monthly biomes as spatially coherent units with consistent phytoplankton community composition that cover at least 0.5% of the surface ocean area. We define the seasonal and annual biomes as those biomes most frequently occurring during the months of each season and during the full year for each 1°-pixel, respectively

Global pattern of phytoplankton diversity driven by temperature and environmental variability

Despite their importance to ocean productivity, global patterns of marine phytoplankton diversity remain poorly characterized. Although temperature is considered a key driver of general marine biodiversity, its specific role in phytoplankton diversity has remained unclear. We determined monthly phytoplankton species richness by using niche modeling and >540,000 global phytoplankton observations to predict biogeographic patterns of 536 phytoplankton species. Consistent with metabolic theory, phytoplankton richness in the tropics is about three times that in higher latitudes, with temperature being the most important driver. However, below 19°C, richness is lower than expected, with ~8°– 14°C waters (~35° to 60° latitude) showing the greatest divergence from theoretical predictions. Regions of reduced richness are characterized by maximal species turnover and environmental variability, suggesting that the latter reduces species richness directly, or through enhancing competitive exclusion. The nonmonotonic relationship between phytoplankton richness and temperature suggests unanticipated complexity in responses of marine biodiversity to ocean warming.ISSN:2375-254

Do functional groups of planktonic copepods differ in their ecological niches?

International audienceAim: To assess the degree of overlap between the environmental niches of marine planktonic copepods and test if the distribution of copepod functional groups differs across environmental gradients. Location: The Mediterranean Sea. Methods: Functional groups were defined based on clustering of functional traits in 106 marine copepod species using a multivariate ordination analysis. Functional traits included maximum body length, feeding mode, spawning strategy and trophic group. Simultaneously, the global distribution of the species was used to model their environmental niches with six environmental variables. For each of these predictors, four niche parameters were derived from the univariate response curve of each species to summarise their environmental preferences and ordinate the species in niche space through a PCA. Finally, the differences in the position in niche space of functional groups were tested with variance analysis. Results: We identified seven copepod functional groups with different distributions along the environmental gradients covered by our study. While carnivorous functional groups were affiliated with oligotrophic and tropical conditions, large and small current-feeding herbivores are associated with colder, more seasonally varying and productive conditions. Small cruising detritivores and other small current-feeding herbivores were not affiliated with specific conditions as their constituting species were scattered in niche space. Main conclusions: Since copepod functional groups occupy distinct ecological niches, ecosystem processes related to these groups are expected to vary across environmental gradients. Conditions favouring large current-feeding herbivores should allow for enhanced fluxes of energy and nutrients through Mediterranean Sea ecosystems, while such fluxes should be weakened where large carnivores and small passive ambush-feeding copepods dominate. Our study supports the development of trait-based zooplankton functional groups in marine ecosystem models

PhytoBase: A global synthesis of open-ocean phytoplankton occurrences

Marine phytoplankton are responsible for half of the global net primary production and perform multiple other ecological functions and services of the global ocean. These photosynthetic organisms comprise more than 4300 marine species, but their biogeographic patterns and the resulting species diversity are poorly known, mostly owing to severe data limitations. Here, we compile, synthesize, and harmonize marine phytoplankton occurrence records from the two largest biological occurrence archives (Ocean Biogeographic Information System, OBIS; and Global Biodiversity Information Facility, GBIF) and three independent recent data collections. We bring together over 1.36 million phytoplankton occurrence records (1.28 million at the level of species) for a total of 1704 species, spanning the principal groups of the diatoms, dinoflagellates, and haptophytes, as well as several other groups. This data compilation increases the amount of marine phytoplankton records available through the single largest contributing archive (OBIS) by 65 %. Data span all ocean basins, latitudes, and most seasons. Analyzing the oceanic inventory of sampled phytoplankton species richness at the broadest spatial scales possible using a resampling procedure, we find that richness tends to saturate at ∼93 % of all species in our database in the pantropics, at ∼64 % in temperate waters, and at ∼35 % in the cold Northern Hemisphere, while the Southern Hemisphere remains under-explored. We provide metadata on the cruise, research institution, depth, and date for each data record, and we include phytoplankton cell counts for 193 763 records. We strongly recommend consideration of spatiotemporal biases in sampling intensity and varying taxonomic sampling scopes between research cruises or institutions when analyzing the occurrence data spatially. Including such information into predictive tools, such as statistical species distribution models, may serve to project the diversity, niches, and distribution of species in the contemporary and future ocean, opening the door for quantitative macroecological analyses of phytoplankton. PhytoBase can be downloaded from PANGAEA: https://doi.org/10.1594/PANGAEA.904397 (Righetti et al., 2019a).ISSN:1866-3516ISSN:1866-350