Forms of phosphorus and phosphorus-iron relationships in the suspended matter of the St. Lawrence ~stuary' MARC LUCOTTE AND BRUNO D'ANGLEJAN Institute of Oceanography, McGill University, Eaton Bldg., 3620 University St., Montreal, P.Q., Canada H3A 2B2 Received October 6, 1982 Revision accepted June 14, 1983 The partition of phosphorus between the organic fraction (PoRG) and both the detrital (apatite rich) and non-detrital (adsorbed) fractions (PAP and PCDB)was studied in the particulate matter of the St. Lawrence Estuary and two of its tributaries. The adsorption of the PCDB fraction on particulate iron was examined. PoRG makes up about one third of the total particulate P, which ranges betweeen 44 and 64 pg . at. /g. Everywhere in the estuary, and regardless of salinity, the concentration of PAP represents twice that of PCDB. The hydroxide fraction (FecDB) is on the average 25 and 29% of the total particulate iron in May and September, respectively, but is much higher in the tributaries. The atom ratio (F~/P)CDB of hydroxide Fe to adsorbed P fluctuates around 16 for May and 19 for September. Low values (13-16) are characteristic of the turbidity maximum, whereas higher ones are found in the deeper, more saline waters (16-26) and in the tributaries (17-26). Although any iron precipitation in the low-salinity turbid zone is masked by sediment resuspension, the low values of the ratio there suggest some P uptake on Fe-rich particles. Dissolved phosphate being conservative with respect to the salinity, desorption does not take place, or is only minor seaward of the turbidy maximum. On a CtudiC la distribution du phosphore entre sa fraction organique et ses deux fumes inorpaniques. I'une detritique et riche en apatite (PAP), l'autre non dttritique et adsorbCe (PCDB), dans la matiere particulaire de l'estuaire du St. Laurent et deux de ses affluents. L'adsorption de la forme PCDB sur le fer particulaire a it6examinie. Le phosphore particulaire total reprCsente de44 h 64 pg at. /g et est form6 d'environ un tiers de PoRG. Quant au PAP, sa concentration est i peu pks double de celle du PCDB. ceci tout au long de l'estuaire, et indkpendamment de la salinitC. La fraction oxyhydratke repdsente respectivement 25 et 29% en mai et en septembre du fer particulaire total, et beaucoup plus dans les affluents. Le rapport atomique {Fe/P) des hydroxydes de fer sur le phosphore adsorb6 oscille autour de 16 en mai et de 19 en septembre. De faibles valeurs ( 13-16) sont charactCristiques de la zone turbide, alors que d'autres plus ClevCes ont CtC trouvCes dans les eaux plus salCes et plus profondes (16-26) et dans les affluents (17-26). Bien que la moindre precipitation de fer dans la zone turbide et peu sake soit masquCe par la resuspension de sCdiments du fond, les faibles valeurs du rapport qui y ont Ct6 relevees suggbrent une certaine adsorption de phosphate sur les particules riches en fer. En aval de cette zone, aucune dCsorption n'est dtcelable si on considbre les propriCtCs conservatives du phosphore dissout avec la salinitC. Can. 1. Earth Sci. 20, 1880-1890 (1983) Introduction Recent studies have shown that a significant fraction of the biologically available inorganic phosphate present in lakes or in coastal marine waters is held in association with particulate suspended matter (seston) or surface sediments. The main mode of association has been established to be surface adsorption on hydrous ferric oxide precipitates (Leckie and Stumm 1970; Upchurch et al. 1974; Williams et al. 1976; Filipek and Owen 1981). These precipitates form coating on clay minerals and other detrital particles. The nature and the kinetics of the adsorption mechanism are not entirely resolved. Although it is generally proved to be rapid and reversible (Pomeroy et al. 1965), Carritt and Goodgal (1954) have indicated that some of the phosphate seems to be more strongly bound to the adsorbing substrate, perhaps as a result of solid diffusion. Once the particulate matter is deposited and buried, 'A contribution to the program of GIROQ (Groupe Interuni- versitaire de Recherche OcCanographique du Quebec). the prevailing oxygen-poor environment leads to a reduction of the iron, which may control the release of adsorbed phosphorus to the overlying waters (Mortimer 1971), or its diagenesis and accumulation as the mineral vivianite (Nriagu 1972; Williams et al. 1976; Filipek and Owen 1981). Williams et al. (1971) also postulated a strongly adsorbing reduced iron gel under such conditions. Carritt and Goodgal (1954) and Jitts (1959) have experimentally established that phosphate adsorption is inhibited by the salinity or the presence of organic matter, but that it increases with rise in temperature. They also determined an optimum uptake in the pH range 5-7. In an estuary, the above parameters are highly variable, and consequent uptake or release of orthophosphate from suspended or surface sediments should be observed. Indeed, an inverse relationship between the salinity and the fraction of P sorbed on the Fe-rich phase of the sediments has been reported (Upchurch et al. 1974; Stirling and Wormald 1977). The flocculation and deposition of ferric colloids are Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by 184.162.93.143 on 06/25/13 For personal use only.