Andrew D. Barton

Patterns of Diversity in Marine Phytoplankton

Diversity Gradients Latitudinal gradients in species abundance, with relatively few occurring at the poles and many at the equator, are well known for macroorganisms. It is a matter of controversy, fueled by a lack of observational data, whether such gradients also occur among microorganisms. Barton et al. (p. 1509, published online 25 February) have built on a global marine circulation model to predict the dynamics of phytoplankton populations. In silico, they obtain patterns of latitudinal gradation for plankton that are interspersed with hotspots of amplified diversity, which point to plausible natural explanations for the phenomenon that can be tested in the future by systematic metagenomic surveys. Highest diversity occurs in physically dynamic mid-latitude zones, and lowest diversity and highest biomass occur toward the poles. Spatial diversity gradients are a pervasive feature of life on Earth. We examined a global ocean circulation, biogeochemistry, and ecosystem model that indicated a decrease in phytoplankton diversity with increasing latitude, consistent with observations of many marine and terrestrial taxa. In the modeled subpolar oceans, seasonal variability of the environment led to competitive exclusion of phytoplankton with slower growth rates and lower diversity. The relatively weak seasonality of the stable subtropical and tropical oceans in the global model enabled long exclusion time scales and prolonged coexistence of multiple phytoplankton with comparable fitness. Superimposed on the decline in diversity seen from equator to pole were “hot spots” of enhanced diversity in some regions of energetic ocean circulation, which reflected lateral dispersal.

The biogeography of marine plankton traits

Changes in marine plankton communities driven by environmental variability impact the marine food web and global biogeochemical cycles of carbon and other elements. To predict and assess these community shifts and their consequences, ecologists are increasingly investigating how the functional traits of plankton determine their relative fitness along environmental and biological gradients. Laboratory, field and modelling studies are adopting this trait-based approach to map the biogeography of plankton traits that underlies variations in plankton communities. Here, we review progress towards understanding the regulatory roles of several key plankton functional traits, including cell size, N2 -fixation and mixotrophy among phytoplankton, and body size, ontogeny and feeding behaviour for zooplankton. The trait biogeographical approach sheds light on what structures plankton communities in the current ocean, as well as under climate change scenarios, and also allows for finer resolution of community function because community trait composition determines the rates of significant processes, including carbon export. Although understanding of trait biogeography is growing, uncertainties remain that stem, in part, from the paucity of observations describing plankton functional traits. Thus, in addition to recommending widespread adoption of the trait-based approach, we advocate for enhanced collection, standardisation and dissemination of plankton functional trait data.

Biophysical Aspects of Resource Acquisition and Competition in Algal Mixotrophs

Mixotrophic organisms combine autotrophic and heterotrophic nutrition and are abundant in both freshwater and marine environments. Recent observations indicate that mixotrophs constitute a large fraction of the biomass, bacterivory, and primary production in oligotrophic environments. While mixotrophy allows greater flexibility in terms of resource acquisition, any advantage must be traded off against an associated increase in metabolic costs, which appear to make mixotrophs uncompetitive relative to obligate autotrophs and heterotrophs. Using an idealized model of cell physiology and community competition, we identify one mechanism by which mixotrophs can effectively outcompete specialists for nutrient elements. At low resource concentrations, when the uptake of nutrients is limited by diffusion toward the cell, the investment in cell membrane transporters can be minimized. In this situation, mixotrophs can acquire limiting elements in both organic and inorganic forms, outcompeting their specialist competitors that can utilize only one of these forms. This advantage can be enough to offset as much as a twofold increase in additional metabolic costs incurred by mixotrophs. This mechanism is particularly relevant for the maintenance of mixotrophic populations and productivity in the highly oligotrophic subtropical oceans.

Anthropogenic climate change drives shift and shuffle in North Atlantic phytoplankton communities

Significance Phytoplankton play essential roles in marine food webs and global biogeochemical cycles, yet the responses of individual species and entire phytoplankton communities to anthropogenic climate change in the coming century remain uncertain. Here we map the biogeographies of commonly observed North Atlantic phytoplankton and compare their historical (1951–2000) and projected future ranges (2051–2100). We find that individual species and entire communities move in space, or shift, and that communities internally reassemble, or shuffle. Over the coming century, most but not all studied species shift northeastward in the basin, moving at a rate faster than previously estimated. These pronounced ecological changes are driven by dynamic changes in ocean circulation and surface conditions, rather than just warming temperatures alone. Anthropogenic climate change has shifted the biogeography and phenology of many terrestrial and marine species. Marine phytoplankton communities appear sensitive to climate change, yet understanding of how individual species may respond to anthropogenic climate change remains limited. Here, using historical environmental and phytoplankton observations, we characterize the realized ecological niches for 87 North Atlantic diatom and dinoflagellate taxa and project changes in species biogeography between mean historical (1951–2000) and future (2051–2100) ocean conditions. We find that the central positions of the core range of 74% of taxa shift poleward at a median rate of 12.9 km per decade (km⋅dec−1), and 90% of taxa shift eastward at a median rate of 42.7 km⋅dec−1. The poleward shift is faster than previously reported for marine taxa, and the predominance of longitudinal shifts is driven by dynamic changes in multiple environmental drivers, rather than a strictly poleward, temperature-driven redistribution of ocean habitats. A century of climate change significantly shuffles community composition by a basin-wide median value of 16%, compared with seasonal variations of 46%. The North Atlantic phytoplankton community appears poised for marked shift and shuffle, which may have broad effects on food webs and biogeochemical cycles.

The impact of fine‐scale turbulence on phytoplankton community structure

We examined the effect of fine-scale fluid turbulence on phytoplankton community structure in an idealized, size-structured community model. It has been shown that turbulence can enhance nutrient transport toward a cell, particularly for larger cells in highly turbulent conditions. Our model suggests that under weak grazing pressure the effect of this mechanism on relative phytoplankton fitness and community structure is negligible. Under these conditions, the high nutrient affinity of small cells dominates relative fitness and allows them to outcompete larger cells. In contrast, when grazing pressure is strong, the turbulent enhancement of nutrient uptake and fitness for larger cells can become ecologically significant. Here, increasing turbulence broadens the size range of coexisting phytoplankton and increases the size of the dominant cell type at equilibrium. We also estimate and map open ocean turbulent dissipation rates as a function of climatological surface wind stresses. The turbulent enhancement of nutrient uptake is most likely to be ecologically significant in regions with low nutrient levels, strong grazing pressure, and relatively high turbulence, such as in windier portions of the subtropical gyre or post-bloom conditions at higher latitudes. In these regions, turbulence may help sustain larger cell populations through otherwise unfavorable environmental conditions.

What drives seasonal change in oligotrophic area in the subtropical North Atlantic

The oligotrophic regions of the subtropical gyres cover a significant portion of the global ocean, and exhibit considerable but poorly understood intraseasonal, interannual, and longer-term variations in spatial extent. Here using historical observations of surface ocean nitrate, wind, and currents, we have investigated how horizontal and vertical supplies of nitrate control seasonal changes in the size and shape of oligotrophic regions of the subtropical North Atlantic. In general, the oligotrophic region of the subtropical North Atlantic is associated with the region of weak vertical supply of nitrate. Though the total vertical supply of nitrate here is generally greater than the total horizontal supply, we find that seasonal expansion and contraction of the oligotrophic region is consistent with changes in horizontal supply of nitrate. In this dynamic periphery of the subtropical gyre, the seasonal variations in chlorophyll are linked to variations in horizontal nitrate supply that facilitate changes in intracellular pigment concentrations, and to a lesser extent, phytoplankton biomass. Our results suggest that horizontal transports of nutrient are crucial in setting seasonal cycles of chlorophyll in large expanses of the subtropical North Atlantic, and may play a key and underappreciated role in regulating interannual variations in these globally important marine ecosystems.