Turner, R. E.. Louisiana State University, turner@wr3600.cwr.lsu.edu
Qureshi, N. E.. University of Karachi, naureen@qureshi.khi.erum.com.pk
Rabalais, N. N.. Louisiana Universities Marine Consortium, nrabalais@lumcon.edu
Dortch, Q. Louisiana Universities Marine Consortium, qdortch@lumcon.edu
Justic, D. Louisiana State University, dubravko.justic@ri.tel.hr
Cope, J. Louisiana State University,

 
FLUCTUATING SILICATE:NITRATE RATIOS AND COASTAL PLANKTON FOOD WEBS
 
Marine diatom growth becomes Si-limited when the dissolved Si:DIN atomic ratio approaches 1:1, (the Redfield ratio). Fundamental changes in diatom-to-zooplankton-to-higher trophic level food webs should occur when this ratio falls below 1:1 and the proportion of diatoms in the phytoplankton community is reduced. We quantitatively substantiate these predictions using a variety of data from the Mississippi River continental shelf, a system whose Si:DIN loading ratio has declined from around 3:1 to 1:1 this century. We suggest that, on this shelf, when the Si:DIN ratio in the river decreases to less than 1:1, then: 1) copepod abundance changes from >75% to <30% of the total meso-zooplankton, 2) zooplankton fecal pellets become a minor component of the in situ primary production consumed, and, 3) bottom water oxygen consumption rates become less dependent on relatively fast sinking (diatom-rich) organic matter packaged mostly as zooplankton fecal pellets. This coastal ecosystem appears poised to be either a diatom-copepod food web or one with potentially disruptive harmful algal blooms. The system is directed between these two ecosystem states by Mississippi River water quality determined by land use practices far inland.
 
Day: Friday, Feb. 5
Time: 02:00 - 02:15pm
Location: Sweeney Center
 
Code: SS13FR0200S