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Barton, AD, Irwin AJ, Finkel ZV, Stock CA.  2016.  Anthropogenic climate change drives shift and shuffle in North Atlantic phytoplankton communities. Proceedings of the National Academy of Sciences of the United States of America. 113:2964-2969.   10.1073/pnas.1519080113   AbstractWebsite

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 kmper 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.

Dave, AC, Barton AD, Lozier MS, McKinley GA.  2015.  What drives seasonal change in oligotrophic area in the subtropical North Atlantic? Journal of Geophysical Research-Oceans. 120:3958-3969.   10.1002/2015JC010787   AbstractWebsite

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.

Barton, AD, Lozier SM, Williams RG.  2015.  Physical controls of variability in North Atlantic phytoplankton communities. Limnology and Oceanography. 60:181-197.   10.1002/lno.10011   Abstract

The structure of marine phytoplankton communities in the North Atlantic Ocean varies considerably on seasonal, interannual, and longer timescales in response to environmental change. However, the causes of ecological variability on interannual and longer timescales remain uncertain. Here, using a half-century of observations, we compare changes in atmospheric forcing (surface wind speed and heat fluxes) and ocean surface properties (sea surface temperature, mixed layer depth, thermal stratification, and turbulent kinetic energy) with variability in total phytoplankton biomass and the abundances of diatoms and dinoflagellates, as measured by the Continuous Plankton Recorder survey. On seasonal timescales, there is a clear connection between observed changes in the physical environment and the phytoplankton assemblages. Strong turbulence, deep mixed layers, and weak stratification decrease diatom abundance in the subpolar gyre, but increase diatoms in the subtropical gyre, a pattern broadly consistent with growth limitation of phytoplankton in high and low latitudes by light and nutrients, respectively. In contrast, dinoflagellates prosper in stratified, weakly turbulent conditions in sampled portions of the subpolar and subtropical gyres. On interannual to multidecadal timescales, however, the links between observed ecological and physical changes are much weaker. The physical mechanisms that differentiate the fates of diatoms and dinoflagellates on seasonal timescales do not appear to control their longer-term variability, perhaps because year-to-year variability in the phytoplankton assemblages is greater than in the physical drivers. This suggests that other biological (e.g., zooplankton grazing, chaos in the plankton) or physical mechanisms (e.g., changes in ocean circulation) may play important regulatory roles.

Barton, AD, Ward BA, Williams RG, Follows MJ.  2014.  The impact of fine-scale turbulence on phytoplankton community structure. Limnology and Oceanography: Fluids and Environments. 4:34-49.   10.1215/21573689-2651533   Abstract

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.

Barton, AD, Pershing AJ, Litchman E, Record NR, Edwards KF, Finkel ZV, Kiorboe T, Ward BA.  2013.  The biogeography of marine plankton traits. Ecol Lett. 16:522-34.   10.1111/ele.12063   Abstract

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.

Mariani, P, Andersen KH, Visser AW, Barton AD, Kiørboe T.  2013.  Control of plankton seasonal succession by adaptive grazing. Limnology and Oceanography. 58:173-184.   10.4319/lo.2013.58.1.0173   Abstract

The ecological succession of phytoplankton communities in temperate seas is characterized by the dominance of nonmotile diatoms during spring and motile flagellates during summer, a pattern often linked to the seasonal variation in the physical environment and nutrient availability. We focus on the effects of adaptive zooplankton grazing behavior on the seasonal succession of temperate plankton communities in an idealized community model consisting of a zooplankton grazer and two phytoplankton species, one motile and the other nonmotile. The grazer can switch between ambush feeding on motile cells or feeding-current feeding on nonmotile cells. The feeding-current behavior imposes an additional mortality risk on the grazer, whereas ambush feeding benefits from small-scale fluid turbulence. Grazer-phytoplankton feeding interactions are forced by light and turbulence and the grazer adopts the feeding behavior that optimizes its fitness. The adaptive grazing model predicts essential features of the seasonal plankton succession reported from temperate seas, including the vertical distribution and seasonal variation in the relative abundance of motile and nonmotile phytoplankton and the seasonal variation in grazer abundance. Adaptive grazing behavior, in addition to nutrient and mixing regimes, can promote characteristic changes in the seasonal structure of phytoplankton community observed in nature.

Barton, AD, Finkel ZV, Ward BA, Johns DG, Follows MJ.  2013.  On the roles of cell size and trophic strategy in North Atlantic diatom and dinoflagellate communities. Limnology and Oceanography. 58:254-266.   10.4319/lo.2013.58.1.0254   Abstract

We have examined the inter- and intra-group seasonal succession of 113 diatom and dinoflagellate taxa, as surveyed by the Continuous Plankton Recorder (CPR) in the North Atlantic, by grouping taxa according to two key functional traits: cell size (µg C cell−1) and trophic strategy (photoautotrophy, mixotrophy, or heterotrophy). Mixotrophic dinoflagellates follow photoautotrophic diatoms but precede their obligate heterotrophic counterparts in the succession because of the relative advantages afforded by photosynthesizing when light and nutrients are available in spring. The mean cell size of the sampled diatoms is smallest in the summer, likely because of the higher specific nutrient affinity of smaller relative to larger cells. Contrastingly, we hypothesize that mixotrophy diminishes the size selection based on nutrient limitation and accounts for the lack of a seasonal size shift among surveyed dinoflagellates. Relatively small, heterotrophic dinoflagellates (µg C cell−1 < 10−3) peak after other, larger dinoflagellates, in part because of the increased abundance of their small prey during nutrient-deplete summer months. The largest surveyed diatoms (µg C cell−1 > 10−2) bloom later than others, and we hypothesize that this may be because of their relatively slow maximum potential growth rates and high internal nutrient storage, as well as to the slower predation of these larger cells. The new trait database and analysis presented here helps translate the taxonomic information of the CPR survey into metrics that can be directly compared with trait-based models.

Ward, BA, Dutkiewicz S, Barton AD, Follows MJ.  2011.  Biophysical aspects of resource acquisition and competition in algal mixotrophs. Am Nat. 178:98-112.   10.1086/660284   Abstract

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.

Barton, AD, Dutkiewicz S, Flierl G, Bragg J, Follows MJ.  2010.  Patterns of diversity in marine phytoplankton. Science. 327:1509-1511. Abstract

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.

Barton, AD, Casey KS.  2005.  Climatological context for large-scale coral bleaching. Coral Reefs. 24:536-554.   10.1007/s00338-005-0017-1   Abstract

Large-scale coral bleaching was first observed in 1979 and has occurred throughout virtually all of the tropics since that time. Severe bleaching may result in the loss of live coral and in a decline of the integrity of the impacted coral reef ecosystem. Despite the extensive scientific research and increased public awareness of coral bleaching, uncertainties remain about the past and future of large-scale coral bleaching. In order to reduce these uncertainties and place large-scale coral bleaching in the longer-term climatological context, specific criteria and methods for using historical sea surface temperature (SST) data to examine coral bleaching-related thermal conditions are proposed by analyzing three, 132 year SST reconstructions: ERSST, HadISST1, and GISST2.3b. These methodologies are applied to case studies at Discovery Bay, Jamaica (77.27°W, 18.45°N), Sombrero Reef, Florida, USA (81.11°W, 24.63°N), Academy Bay, Galápagos, Ecuador (90.31°W, 0.74°S), Pearl and Hermes Reef, Northwest Hawaiian Islands, USA (175.83°W, 27.83°N), Midway Island, Northwest Hawaiian Islands, USA (177.37°W, 28.25°N), Davies Reef, Australia (147.68°E, 18.83°S), and North Male Atoll, Maldives (73.35°E, 4.70°N). The results of this study show that (1) The historical SST data provide a useful long-term record of thermal conditions in reef ecosystems, giving important insight into the thermal history of coral reefs and (2) While coral bleaching and anomalously warm SSTs have occurred over much of the world in recent decades, case studies in the Caribbean, Northwest Hawaiian Islands, and parts of other regions such as the Great Barrier Reef exhibited SST conditions and cumulative thermal stress prior to 1979 that were comparable to those conditions observed during the strong, frequent coral bleaching events since 1979. This climatological context and knowledge of past environmental conditions in reef ecosystems may foster a better understanding of how coral reefs will respond in future, ocean warming scenarios.

Barton, AD, Greene CH, Monger BC, Pershing AJ.  2003.  The Continuous Plankton Recorder survey and the North Atlantic Oscillation: Interannual- to Multidecadal-scale patterns of phytoplankton variability in the North Atlantic Ocean. Progress in Oceanography. 58:337-358.   10.1016/j.pocean.2003.08.012   Abstract

At interannual to multidecadal time scales, much of the oceanographic and climatic variability in the North Atlantic Ocean can be associated with the North Atlantic Oscillation (NAO). While evidence suggests that there is a relationship between the NAO and zooplankton dynamics in the North Atlantic Ocean, the phytoplankton response to NAO-induced changes in the environment is less clear. Time series of monthly mean phytoplankton colour values, as compiled by the Continuous Plankton Recorder (CPR) survey, are analysed to infer relationships between the NAO and phytoplankton dynamics throughout the North Atlantic Ocean. While a few areas display highly significant (p < 0.05) trends in the CPR colour time series during the period 1948–2000, nominally significant (p < 0.20) positive trends are widespread across the basin, particularly on the continental shelves and in a transition zone stretching across the Central North Atlantic. When long-term trends are removed from both the NAO index and CPR colour time series, the correlation between them ceases to be significant. Several hypotheses are proposed to explain the observed variability in the CPR colour and its relationship with climate in the North Atlantic.