Phosphorus and arsenic distributions in a seasonally stratified, iron- and manganese-rich lake: microbiological and geochemical controls

Environmental context Despite being present at trace concentrations, arsenic and phosphorus are among the most important of freshwater contaminants. This research highlights the biogeochemical coupling of both elements in a New Zealand lake. We find that the mineralisation of organic residues coupled to the dissolution of colloidal iron and manganese hydroxides may be an important driver of the bioavailability of phosphorus and arsenic. Abstract Seasonal stratification in temperate lakes greater than a few metres deep provides conditions amenable to pronounced vertical zonation of redox chemistry. Such changes are particularly evident in eutrophic systems where high phytoplankton biomass often leads to seasonally established anaerobic hypolimnia and profound changes in geochemical conditions. In this study, we investigated the behaviour of trace elements in the water column of a seasonally stratified, eutrophic lake. Two consecutive years of data from Lake Ngapouri, North Island, New Zealand, demonstrate the occurrence of highly correlated profiles of phosphorus, arsenic, iron and manganese, all of which increased in concentration by 1-2 orders of magnitude within the anaerobic hypolimnion. Stoichiometric and mass-balance considerations demonstrate that increases in alkalinity in hypolimnetic waters were consistent with observed changes in sulfate, Fe and Mn concentrations with depth, corresponding to dissimilatory reduction of sulfate, Fe-III and Mn-IV hydroxides. Thermodynamic constraints on Fe, Mn and Al solubility indicate that amorphous Fe-III, Mn-IV hydroxides most probably controlled Fe and Mn in the surface mixed layer (0 to 8m) whereas Al-III hydroxides were supersaturated throughout the entire system. Surface complexation modelling indicated that iron hydroxides (HFO) potentially dominated As speciation in the lake. It is likely that other colloidal phases such as allophanic clays also limited HPO42- activity, reducing competition for HAsO42- adsorption to iron hydroxides. This research highlights the coupling of P, As, Fe and Mn in Lake Ngapouri, and the apparent role of multiple colloidal phases in affecting P and As activity within overarching microbiological and geochemical processes.