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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Seasonal growth stimulation of sub-temperate estuarine phytoplankton to nitrogen and phosphorus: An outdoor microcosm experiment
Auteurs :	D. A. Flemer, Auteur ; R. J. Livingston, Auteur ; S. E. McGlynn, Auteur
Type de document :	Article
Dans :	Estuaries (21(1) MAR 1998)
Article en page(s) :	145-159
Résumé :	A study of nutrient limitation of phytoplankton biomass production with emphasis on nitrate-nitrogen (NO(3)(-)) and ortho-phosphate-phosphorus (PO(4)(3-)) was conducted in Perdido Bay, Alabama-Florida. The experimental design employed 18-1 outdoor microcosms operated in a static renewal mode. Phytoplankton growth responses (i.e., growth stimulation) measured as chlorophyll a (chi a) fell into three principal categories: primary P stimulation occurred mostly during the cooler months at the upper bay (tidal brackish) and mid bay (lower mesohaline) stations; a total of 12 out of 36 experiments; primary N stimulation occurred mostly during the warmer months primarily at the mid-bay station and infrequently at the upper and lower bay stations (upper mesohaline); a total of 7 out of 36 experiments; and N+P costimulation occurred primarily during the warmer months in the upper bay and mid bay and during both warmer and cooler months in the lower bay, a total of 17 out of 36 experiments. Primary P stimulation was generally associated with high ratios of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphate (DIP) (ratio range: 18 to 288). Conversely primary N stimulation was associated with decreasing DIN:DIP ratios (range 8-46). Redfield ratios of particulate organic N (PON) to particulate organic P (POP) often indicated N limitation (i.e., values often less than 10). PON:chl a ratios often indicated N sufficiency, but three occasions were noted where PON:POP and PON:chl a ratios were not congruent It is difficult to reconcile the inorganic and organic N and P ratios with the relatively low DIP and DIN concentrations. The phytoplankton assemblage appeared not to be strongly nutrient-limited but, given a nutrient increase, responded differentially to N and P, both seasonally and along the longitudinal salinity gradient. Grazing pressure in concert with nutrient limitation was advanced as an hypothesis to explain N+P co-limitation.

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