A vertical model of particle size distributions and fluxes in the midwater column that includes biological and physical processes—Part II: application to a three year survey in the NW Mediterranean Sea

• Published in: Deep-sea research. Part I, Oceanographic research papers Vol. 51; no. 7; pp. 885 - 908
• Main Authors: Stemmann, Lars, Jackson, George A., Gorsky, Gabriel
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.07.2004; Elsevier
• Subjects: Coagulation; Earth Sciences; Earth, ocean, space; Exact sciences and technology; External geophysics; Marine; Marine snow; Mediterranean Sea; Mesopelagic; Midwater zone; Modeling; Oceanography; Particle flux; Physics of the oceans; Sciences of the Universe; MED, Western Mediterranean; Particle flux; Midwater zone; Marine snow; Mesopelagic; Coagulation; Modeling; Mediterranean Sea; Particle size; Particle size distribution; zooplankton; plankton; Particulate carbon; Dynamic model; Dimension spectrum; organic carbon
• ISSN: 0967-0637; 1879-0119
• DOI: 10.1016/j.dsr.2004.03.002

The largest decrease in the particle vertical flux occurs in the mesopelagic zone where particles are transformed by biological and physical mechanisms. Particle size distributions provide important clues into those processes affecting particle transformations in this region. We have studied them using an inter-annual data set showing the evolution of particle size distributions between 100 and 1000 m, comparing them to results from a series of size-resolved models of particle dynamics that include physical coagulation and biological remineralization. The formulation that best fits the observations consists of a combination of settling, microbial activity and zooplankton feeding. The calculated particulate organic carbon losses to microbial activity and zooplankton feeding are consistent with independent estimates of these rates. The model shows that it is possible to predict the particle size distribution at 1000 m depth knowing the particle size distribution at 100 m depth and the rates of transformation in the mesopelagic. The mesozooplankton appears to be important in decreasing the high flux of large particles in the upper midwater zone, microbes becomes more important in the deeper midwater zone as zooplankton become rarer. The results suggest that the mesozooplankton have a much greater effect on particle flux than the macrozooplankton. Their importance requires the mesozooplankton to feed preferentially on large settling particles, probably using remote detection. The present work shows that using particle size spectra is a useful way to understand the transformation of the vertical flux of element in the midwater zone. However, most of the assumptions on particle properties and processes are based on surface studies and more data from the midwater zone are needed to confirm the hypotheses. The model allows us to formulate crucial questions regarding particle dynamics in the midwater zone.