No abstract

A strategy has been developed to permit the in situ calibration of measurements made with benthic chambers of chemical fluxes across the sediment-water interface. The resistance to chemical transport posed by the boundary layer is estimated using chemical transport techniques in benthic flux chamber mock-ups and on the sea floor. Dissolution from gypsum (alabaster) plates is measured both inside and outside the flux chambers. CaCOo dissolution and radioisotope uptake techniques are suitable for laboratory and deep-sea calibration of benthic fluxes. Broadly concordant resistance estimates have been achieved within flux chambers with these three quite differentapproaches. An estimate of 470 p for the equivalent stagnant boundary film thickness has been made by the alabaster method on the sea floor at MANOP site H in the eastern Pacific. The transport of Mn2' from the bottom waters to site H nodules, if controlled by diffusion through a film of this thickness, would be sufficient to explain their growth rates.

1986. Response of radioactive trace metals to acid-base titratisns i w controlled experimental ecosystems: evaluation of transport parameters for application to whole-lake radiotracer experiments. Can. ). Fish. Radiotracer experiments were carried out in 20 enclosures located in two lakes at the Experimental Lakes Area (ELM, northwestern Ontario, to study pathways of trace metal removal from the water column of shallow lakes. Two removal mechanisms were characterized: (1 j sorption to and subsequent transport with falling particles and ( 2 ) direct adsorption to surface sediments. Our approach was to measure independently the kinetics of radiotracer sorption, fluxes and concentrations for particles, particle settling velocities, and the "equivalent stagnant bsundary film." Our radistracer results enabled us to test the sensitivity of the tracer removal rates on these ratedetermining processes using a numerical transport model. Acid titrations of whole ecosystems revealed that some trace metals (e.g. Mn, Co, and Zn) can diffuse back to the water column as the pH is lowered from 6.5 to 4.8 after 18 d, while others remain tightly bound (e.g. Sn, Fe, Se, Cr, Ag, and Hg isotopes). Subsequent CaC03 additions to bring back the pH to its original value restored the initial removal conditions for acid-sensitive radiotracers, indicating that the pH sensitivity is reversible. Transport parameters for particle-related pathways or diffusive pathways across the sediment-water interface obtained from our enclosure experiments were used to predict the removal rates of "particle-reactive" ' OCo and the "diffusive" pathway tracer lJ4Cs observed in earlier experiments where radiotracers were added to whole lakes or to larger enclosures.On a realise des experiences avec des traceurs radioactifs dans 20 enclos situes dans deux lacs de la region des lacs experimentaux (RLE), dans le nord-ouest de I'Ontario, pour etudier Bes processus de B'6lirnination dans la colonne d'eau des metaux qui se trouvent a I'etat de trace dans les lacs peu profonds. Deux mecanismes d'elimination sont decrits : (1) sorption au niveau des particules qui tombent et transport ulterieur avec ces dernieres et ( 2 ) adsorption directe au niveau des sediments de surface. Notre approche consistait 2 mesurer de facon independante la cinetique de la sorption des traceurs radioactifs, les mouvements et 86s concentrations des particules, les vitesses de skdimentation des particules et la (( couche limite inactive equivalente )). Les resultats que nous avsns obtenus avec les radiotraceurs nous ont permis de verifier, en faisant appel au modPle de transport numerique, I'effet qu'ont les taux d'elirnination des traceurs sur ces processus qui determinent les taux. Le dosage a I'acide d'6cosyst&mes entiers a revel6 que certains metaux a ['eta? de trace (p. ex. Mn, Co, Zn) peuvent retsurner dans la colonne d'eau lorsque re pH tombe de 6,s 2 4,8 apres 18 j, tandis que d'autres demeurent fortement lies (p. ex. isotopes de Sn, Fe, Se, Cr, Ag et Hg). L'addition ultkrieure de C...