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MEPS 211:1-13 (2001)
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Abstract
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Fluid motion and solute distribution around sinking aggregates. I. Small-scale fluxes and heterogeneity of nutrients in the pelagic environment
Thomas Kiørboe1,*, Helle Ploug2, Uffe H. Thygesen1
1Danish Institute for Fisheries Research, Kavalergården 6, 2920 Charlottenlund, Denmark
2Marine Biological Laboratory, Strandpromenaden 5, 3000 Helsingør, Denmark
*E-mail: tk@dfu.min.dk
ABSTRACT: Marine snow aggregates are sites of elevated biological activity. This activity depends on the exchange of solutes (O2, CO2, mineral nutrients, dissolved organic material, etc.) between the aggregate and the environment and
causes heterogeneity in the distribution of dissolved substances in the ambient water. We described the fluid flow and solute distribution around a sinking aggregate by solving the Navier-Stokes' equations and the advection-diffusion equations
numerically. The model is valid for Reynolds numbers characteristic of marine snow, up to Re = 20. The model demonstrates the importance of a correct flow environment when making biological rate-measurements on aggregates (e.g., oxygen
consumption/production, growth rates of bacteria and phytoplankton) because both solute fluxes and internal solute concentrations depend strongly on the flow environment. Observations of flow and oxygen-concentration fields in the vicinity of both
artificial and natural oxygen-consuming aggregates that are suspended in a flow compare well with model predictions, thus suggesting that our set-up is suitable for making biological rate measurements. The sinking aggregate leaves a long slender plume in
its wake, where solute concentration is either elevated (leaking substances) or depressed (consumed substances) relative to ambient concentration. Such plumes may impact the nutrition of osmotrophs. For example, based on published solubilization rates of
aggregates we describe the amino acid plume behind a sinking aggregate (0.1 to 1.0 cm radius). The volume of the plume with amino acid concentrations high enough to significantly affect bacterial uptake rates is ca 100x the
volume of the aggregate itself. Thus, sinking aggregates may create significant microniches also for free-living bacteria.
KEY WORDS: Marine snow · Mass transport · Bulk Sherwood number · Local Sherwood number · Aggregate respiration rate
Full text in pdf format
Published in MEPS Vol.
211
(2001) on February 14
Print ISSN: 0171-8630; Online ISSN: 1616-1599.
Copyright © Inter-Research, Oldendorf/Luhe, 2001
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