Abstract
The intensification of western boundary currents in the global ocean will potentially influence meso-scale eddy generation, and redistribute microbes and their associated ecological and biogeochemical functions. To understand eddy-induced changes in microbial community composition as well as how they control growth, we targeted the East Australian Current (EAC) region to sample microbes in a cyclonic (cold-core) eddy (CCE) and the adjacent EAC. Phototrophic and diazotrophic microbes were more diverse (2–10 times greater Shannon index) in the CCE relative to the EAC, and the cell size distribution in the CCE was dominated (67%) by larger micro-plankton n}{}(geq 20lrm{mu }mathrm{m}), as opposed to pico- and nano-sized cells in the EAC. Nutrient addition experiments determined that nitrogen was the principal nutrient limiting growth in the EAC, while iron was a secondary limiting nutrient in the CCE. Among the diazotrophic community, heterotrophic NifH gene sequences dominated in the EAC and were attributable to members of the gamma-, beta-, and delta-proteobacteria, while the CCE contained both phototrophic and heterotrophic diazotrophs, including Trichodesmium, UCYN-A and gamma-proteobacteria. Daily sampling of incubation bottles following nutrient amendment captured a cascade of effects at the cellular, population and community level, indicating taxon-specific differences in the speed of response of microbes to nutrient supply. Nitrogen addition to the CCE community increased picoeukaryote chlorophyll a quotas within 24 h, suggesting that nutrient uplift by eddies causes a ‘greening’ effect as well as an increase in phytoplankton biomass. After three days in both the EAC and CCE, diatoms increased in abundance with macronutrient (N, P, Si) and iron amendment, whereas haptophytes and phototrophic dinoflagellates declined. Our results indicate that cyclonic eddies increase delivery of nitrogen to the upper ocean to potentially mitigate the negative consequences of increased stratification due to ocean warming, but also increase the biological demand for iron that is necessary to sustain the growth of large-celled phototrophs and potentially support the diversity of diazotrophs over longer time-scales.
Highlights
There are two broad nutrient limitation regimes for phytoplankton growth in the contemporary ocean, whereby iron (Fe) limitation occurs across ∼30% of the ocean’s surface area where high macronutrient concentrations occur, and nitrogen (N) limitation occurs across most of the oligotrophic low-latitude systems (Moore et al, 2013)
We focused on communities from the subsurface chlorophyll-a maximum to simulate the impact of moderate nutrient uplift into the euphotic zone, and advance knowledge about responses of microbial communities that are difficult to detect using satellites
The East Australian Current (EAC) surface velocity was 1.2 m s−1 estimated from altimetry (Fig. 1B, arrows) with the current separating from the coast at ∼30◦S, forming the Tasman Front
Summary
There are two broad nutrient limitation regimes for phytoplankton growth in the contemporary ocean, whereby iron (Fe) limitation occurs across ∼30% of the ocean’s surface area where high macronutrient concentrations occur (high latitudes, upwelling and some coastal areas), and nitrogen (N) limitation occurs across most of the oligotrophic low-latitude systems (Moore et al, 2013). Global Climate Model (GCM) projections indicate warming and increased stratification of the upper ocean over the coming decades, limiting the upwards delivery of nitrogen (e.g., ‘‘new’’ N) into the euphotic zone, and potentially leading to increased reliance on a smaller pool of regenerated forms or N fixation to support primary production (Behrenfeld, 2011) These models typically do not consider the influence of smaller scale oceanographic features such as meso-scale eddies, which could act as a compensatory mechanism and enrich the upper ocean with new nutrients delivered from deeper ocean waters, potentially mitigating the negative consequences of climate change (Matear et al, 2013). The role eddies play in regulating internal nutrient inputs from the deep ocean is likely to be regionally dependent
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