FLUCTUATING SILICATE NITRATE RATIOS AND COASTAL PLANKTON FOOD WEBS/

Marine diatoms require dissolved silicate to form an external shell. a nd their growth becomes Si-limited when the atomic ratio of silicate t o dissolved inorganic nitrogen (SI:DIN) approaches 1:1, also known as the ''Redfield ratio.'' Fundamental changes in the diatom-to-zooplankt on-to-higher trophic level food web should occur when this ratio falls below 1:1. and the proportion of diatoms in the phytoplankton communi ty is reduced. We quantitatively substantiate these predictions by usi ng a variety of data from the Mississippi River continental shelf, a s ystem in which the SI:DIN loading ratio has declined from around 3:1 t o 1:1 during this century because of land-use practices in the watersh ed, We suggest that, on this shelf, when the Si:DIN ratio in the river decreases to less than 1:1, then (i) copepod abundance changes from > 75% to <30% of the total mesozooplankton, (ii) zooplankton fecal pelle ts become a minor component of the in situ primary production consumed , and (iii) bottom-water oxygen consumption rates become less dependen t on relatively fast-sinking (diatom-rich) organic matter packaged mos tly as zooplankton fecal pellets. This coastal ecosystem appears to be a pelagic food web dynamically poised to be either a food web compose d of diatoms and copepods or one with potentially disruptive harmful a lgal blooms. The system is directed between these two ecosystem states by Mississippi River water quality, which is determined by land-use p ractices far inland.