Eddies: hotspots of prokaryotic activity
A review of Baltar, F et al (2010), Mesoscale eddies: hotspots of prokaryotic activity and differential community structure in the ocean, The ISME journal, Vol. 4, pg 975-988.
Mesoscale eddies, including anticyclonic (AE) and cyclonic eddies (CE), are a universal feature of all oceans and have strong influences on regional communities and biogeochemistry. Cyclonic eddies are known to enhance nutrient inputs and increase chlorophyll concentrations whilst anticyclonic eddies cause accumulation of organic matter and increase heterotrophic activity, all of which will affect the productivity of heterotrophic prokaryotes and their community structure. It is important to investigate these effects as prokaryotes have important functions within the marine biogeochemical cycles.
The study looks at four island-induced eddies (two CE and two AE), compared with two unaffected (far-field) sites, both in an oligotrophic region of the Canary archipelago. Samples were taken at depths ranging from 5-2000m and analysed using flow cytometry, PCR and CARD-FISH to establish prokaryotic abundance (PA), heterotrophic activity (PHA), viability, single-cell activity and the bacterial and archaeal community structures. Their hypothesis stated that eddies could generate oceanic ‘hotspots’ of activity and shifts in prokaryote assemblage composition.
Effects of eddies on prokaryotic communities were observed, as they generated hotspots of significantly increased PA, PHA and fraction of living cells compared to that of the FF sites. However, these differences were only mainly apparent in the epipelagic layer (0-200m). Similarly in the mesopelagic layer (200-1000m), there were significant differences in PA and PHA between the eddy and FF sites, but there were no distinct differences in community composition or single-cell activity.
It was observed that in all sites, bacterial abundance decreased with depth and instead Crenarchaeota (Archaea) abundance significantly increased. In the bathypelagic layer, no significant differences were found, suggesting these waters are not influenced by eddies.
AE were also found to have a greater effect on prokaryotic activity and community structure compared to CE, but this effect was also reduced as the depth increased.
They concluded that both types of eddies had distinct community compositions within them, compared to the FF sites and do in fact represent oceanic ‘hotspots’ of prokaryotic activity, but mainly above 1000m.
Despite results showing that eddies provide favourable conditions increasing prokaryote activity, they also found extremely high prokaryotic heterotrophic activity in the surface layer of FF1. It would be interesting to explain unexpected results like these. Temperature was also not the main factor in controlling communities as first thought because bacterial assemblages varied between CE1 and CE2 despite similar temperatures, and did not vary much between AE1 and AE2 which had contrasting temperatures. They suggested that processes such as grazing, organic/inorganic matter supply or phytoplankton community structure influenced them instead, but only theoretical explanations were provided. Further research into these would be good and also on the maturity of eddies as recent studies have found that this also has an influence. The authors say more research should focus on eddies as further data would allow ‘constraining the fate of carbon in the ocean’ and would help build more accurate models of global biogeochemical cycles. As 50% of global new primary production is caused by eddy-induced nutrient fluxes, I would agree!
2 comments:
Jelena - this is very interesting. I don't know enough oceanography, but my understanding is that these eddies can be short or long lived, big ans small. Theye were described as "mesoscale" - do the authors give any idea of the scale? Are they hundreds or thousands of klometres, and are they permanent or temporary?
Yes your right Colin, eddies can vary greatly in size and duration. They can be as small as a few centimetres in diameter, in which case they only tend to last a few seconds. Instead they can span over 500km and these can last months or even years!
Unfortunately, the paper only mentions that they are mesoscale eddies and doesn’t give any specific details about their size or age. I only managed to find out that the study lasted approximately 1 month, from them locating and sampling the eddies and far-field sites (so they were at least a month old). It also later mentions that anti-cyclonic eddy site 1 (AE1) was a mature eddy and (AE2) was a newly formed eddy, so perhaps only a month old. They suggested this due to the varying temperature of the two sites, as more mature eddies will hold water masses with higher temperatures. Also, the older eddies are, the more layers of water they will affect. In AE1 the warm water reached down to 200m, whereas in AE2, the warm water only mixed down to 60m with a strong thermocline underneath (an abrupt change in temperature after the 60m).
The two cyclonic eddies had similar temperatures, suggesting they were of similar age.
I also found that generally, ’mesoscale eddies’ refer to eddies that are between 10 and 500km in diameter and tend to last several months.
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