The effect of pH, dissolved humic substances, and ionic composition on the transfer of iron and phosphate to particulate size fractions in epilimnetic lake water

Transformations of dissolved iron and phosphate to particulate material (>0.2 pm) were investigated by adding 55FeC1, and 32P0,3to pH adjusted epilimnetic lake water samples of contrasting ionic strengths and dissolved humic substances (DHS) concentrations. The distributions of these tracers between particulate size fractions in samples without appreciable DHS after 24 h of dark incubation varied markedly over the pH range 4-8. The effect of pH on the movement of 55Fe and 32P to particulate size fractions in clear-water samples depended on the ionic strength. Over the same pH range, the presence of DHS suppressed the movement of 55Fe and 32P to particulate size fractions. There was little difference between low and high ionic strength DHS-rich samples. The capability of DHS to maintain added 55Fe and 32P in solution (co.2 pm) was confirmed by simultaneous measurements of precipitation losses. The presence of appreciable quantities of dissolved organic C (DOC) appears to be an important factor in determining responses of lacustrine ecosystems to inputs of inorganic acids from anthropogenic sources. Recent investigations have shown that DOC is an important energy source and system regulator in surface lake waters (Salonen et al. 1992). Dissolved humic substances (DHS) are a significant component of the DOC pool in lakes. The influence of DHS on biotic and abiotic interactions in lake water can be modified by changes in pH and other environmental factors, such as UV irradiation and trophic status (De Haan 1992). Differences between the ecological responses of clear-water and humic lakes to acidification may thus be due in part to the influence of pH-sensitive abiotic processes involving DHS. It has been suggested that the presence of DHS in lakes modifies the speciation and subAcknowledgments Thanks to H. De Haan and R. I. Jones for critically reviewing the manuscript. This work was supported by contract CT90-0 112 under the STEP Research Program of the Commission of European Communities. sequent bioavailability of key nutrients (Francko and Heath 1979; De Haan et al. 1990). There is substantial evidence for an association of phosphate and iron with DHS, and the quantities of iron and phosphate apparently associated with DHS in lake waters also vary with pH (Shaw et al. 1992; Jones et al. 1993). The association is reversible, and the release of bound phosphate can be stimulated by exposure to light (Francko and Health 1979, 1982; Jones et al. 1988). Investigations of abiotic transformations involving iron and phosphate in epilimnetic waters to date have been largely concerned with interactions in the dissolved or colloidal phase (co.2 pm). Fewer studies of iron and phosphate interactions involving particulate size fractions ~0.2 pm have been conducted. Tipping et al. (1982) showed that particulate iron exists in natural surface waters but largely as centrifugable particles of submicron size. Phosphorus and humic substances were also identified as important constituents of these particles. Buffle et al. (1989) reported that the size of most particulate iron at the oxic-anoxic boundary of a eutrophic lake was < 0.2 pm and found a correlation between the iron and phosphorus contents of the particles. Experimental studies have also highlighted the significance of interactions between dissolved colloidal and particulate material in natural surface waters. For example, Carpenter and Smith (1984) demonstrated that when phosphate was added to suspensions of different composition, the fraction of phosphate retained by filters of 0.2-pm pore size depended on salinity, pH, and the presence of amorphous iron hydroxide. Tipping (198 1) showed that adsorption of DHS by submicron hydrous iron oxide particles (goethite and hematite) depended on pH and the ionic composition of the electrolyte. Interactions between ferric iron and humic substances are capable of maintaining iron in “dissolved” form (co.45 pm) in oxygenated water (Cameron and Liss 1984).