Heterotrophic Nanoflagellates, Bacteria, and Labile Organic Compounds in Continental Shelf and Deep-Sea Sediments of the Eastern Mediterranean

Abstract Here we report on new data on plutonium (Pu) isotopes to elucidate activity concentrations, inventories, sources and their transport from the ocean surface to the sea floor from a collection of deep-sea sediment cores (depths ranging from 257 to 3739 m) in the Gulf of Mexico (GoM). Sediment cores collected from the continental shelf and upper slope region of the GoM consistently showed 240Pu/239Pu ratios of 0.15 to 0.26 and Pu-inventories ranging from 15 to 35 Bq m− 2. Inventories and ratios are consistent with global fallout Pu for this tropical region. In the continental shelf and upper slope regions, higher particle concentrations close to the margins favor significant scavenging and removal of Pu from the water column; in contrast with the deep-sea cores that show low 240Pu/239Pu ratios (0.07–0.13) and a much lower Pu inventory (< 7 Bq m-2) implying a small fraction of the expected global fallout inventory has reached into the lower slopes and abyssal plain of the GoM. Low values and a progressive decrease of 240Pu/239Pu ratios and Pu inventories with increasing water depth have been previously reported for the GoM. The low Pu ratios indicate that Nevada tests fallout was an important source of Pu to deep-sea sediments, and that this source was likely more efficiently removed from the water column than global fallout Pu. Analysis of Pu isotopes in two sediment traps from the upper slope regions show 240Pu/239Pu ratios comparable to the ones observed in the global fallout. These results indicate that global fallout Pu is currently the main source of Pu in water column particles. Therefore, a significant fraction of global fallout Pu must still be present; either in a dissolved phase, or as biologically recycled material in the water column, or scavenged on the shelf and shelf break. Our results bring to light important questions on the application of Pu isotopes to establish sediment chronologies, since these radionuclides are shown to be tracers of bioturbation rather than accumulation processes in deep-sea sediments of the GoM, similar to previously reported results from excess 210Pb.

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Deep-sea heterotrophic nanoflagellates of the Eastern Mediterranean Sea: qualitative and quantitative aspects of their pelagic and benthic occurrence

Marine Ecology Progress Series ◽

10.3354/meps256045 ◽

2003 ◽

Vol 256 ◽

pp. 45-56 ◽

Cited By ~ 41

Author(s):

H Arndt ◽

K Hausmann ◽

M Wolf

Keyword(s):

Mediterranean Sea ◽

Deep Sea ◽

Eastern Mediterranean ◽

Heterotrophic Nanoflagellates ◽

Eastern Mediterranean Sea ◽

Qualitative And Quantitative

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Picoplankton Distribution and Activity in the Deep Waters of the Southern Adriatic Sea

Water ◽

10.3390/w11081655 ◽

2019 ◽

Vol 11 (8) ◽

pp. 1655 ◽

Cited By ~ 5

Author(s):

Danijela Šantić ◽

Vedrana Kovačević ◽

Manuel Bensi ◽

Michele Giani ◽

Ana Vrdoljak Tomaš ◽

...

Keyword(s):

Deep Sea ◽

Large Scale ◽

Heterotrophic Bacteria ◽

Eastern Mediterranean ◽

Environmental Parameters ◽

Anoxygenic Phototrophic Bacteria ◽

Heterotrophic Nanoflagellates ◽

Nucleic Acid Content ◽

Viral Lysis ◽

Deep Waters

Southern Adriatic (Eastern Mediterranean Sea) is a region strongly dominated by large-scale oceanographic processes and local open-ocean dense water formation. In this study, picoplankton biomass, distribution, and activity were examined during two oceanographic cruises and analyzed in relation to environmental parameters and hydrographic conditions comparing pre and post-winter phases (December 2015, April 2016). Picoplankton density with the domination of autotrophic biomasses was higher in the pre-winter phase when significant amounts of picoaoutotrophs were also found in the meso-and bathy-pelagic layers, while Synechococcus dominated the picoautotrophic group. Higher values of bacterial production and domination of High Nucleic Acid content bacteria (HNA bacteria) were found in deep waters, especially during the post-winter phase, suggesting that bacteria can have an active role in the deep-sea environment. Aerobic anoxygenic phototrophic bacteria accounted for a small proportion of total heterotrophic bacteria but contributed up to 4% of bacterial carbon content. Changes in the picoplankton community were mainly driven by nutrient availability, heterotrophic nanoflagellates abundance, and water mass movements and mixing. Our results suggest that autotrophic and heterotrophic members of the picoplankton community are an important carbon source in the food web in the deep-sea, as well as in the epipelagic layer. Besides, viral lysis may affect the activity of the picoplankton community and enrich the water column with dissolved organic carbon.

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