|Lakes are highly valued freshwater ecosystems which provide many goods and services upon which humans are reliant. Eutrophication of waterways, driven by the over-enrichment of nutrients such as phosphorus and nitrogen, is a threat to the future quality of water resources globally. Lake restoration methods are increasingly being employed to improve eutrophic waterways, via numerous catchment derived and in-lake approaches. However, spatio-temporal heterogeneity in physicochemical and biogeochemical conditions within lakes may restrict the efficacy of restoration methods. This thesis explores in-stream alum dosing as a lake remediation strategy for the purpose of dissolved phosphorus inactivation, and its physicochemical and biogeochemical interaction and fate within Lake Rotoehu, New Zealand.
Laboratory and field measurements were undertaken at Lake Rotoehu, on New Zealand’s North Island. The shallow, polymictic lake resides in an active volcanic area with geothermal inputs, and is subject to several management issues including elevated nutrient concentrations, invasive macrophytes, and frequent cyanobacteria blooms. The geothermal Waitangi Springs, which discharge into Lake Rotoehu, contribute ~69 % of the lake’s total ionic content, and are responsible for enhanced concentrations of biologically limiting nutrients including phosphorus, silicon, nitrate, ammonium, and iron. In an effort to curb in-lake phosphorus levels, in-stream alum-dosing has been employed in order to floc out dissolved reactive phosphorus (DRP), through chemical adsorption and sedimentation.
The results presented here, culminate from an investigation of the physicochemical and geochemical dynamics across the mixing zone from the Waitangi Springs geothermal stream outlet across Te Wairoa Bay to the main lake body. A combination of approaches was used: two field experiments with fixed location and transect measurements, laboratory analysis and geochemical speciation modelling with PHREEQC.
Results show sharp changes in physicochemical water properties across the mixing zone within the bay: pH, O2 and dissolved reactive phosphorus values increased with distance from the stream outlet, whereas major ion concentrations, temperature and conductivity values decreased. Initial in-stream phosphorus stripping through alum dosing is effective in reducing the DRP load by ~50 % of background concentration. As alum is introduced to the stream water (~pH 6) it precipitates to form amorphous aluminium hydroxide (Al(OH)3(am)) and adsorbs phosphorus via hydroxyl ligand exchange. However, elevated levels of iron in amorphous hydrous ferric hydroxides Fe(OH)3(am) are also likely to be contributing to natural phosphorus binding capacity. Sediment core data also indicated that settled Al(OH)3(am) floc and Fe(OH)3(am) particulates were primarily concentrated within the inner portion of Te Wairoa Bay near the Waitangi Springs outlet. Surface water physicochemical and geochemical concentrations were spatially resolved and indicated distinct mixing boundaries (pH, DO, temp) and patchiness (Al, Fe, DRP) within Te Wairoa Bay associated with the locations and configuration of dense submerged macrophytes (Ceratophyllum demersum). Geochemical speciation modelling also indicated that the primary dissolved Al species was Al(OH)4- under the observed daytime conditions, and that Fe was primarily in colloidal form Fe(OH)3(am), which was confirmed through diffusive gradients in thin films (DGT) measurements.
A diel sampling experiment also confirmed that as alum-dosed water enters the lake, daytime biogeochemical conditions driven by C. demersum alter physicochemical water properties from ~pH 6 to ~pH 9, and DO (supersaturation) via increased photosynthesis. Diel-fluxes in geochemical solubility (Al, Fe, and DRP) responded to physicochemical shifts (pH, O2,) and demonstrate that C. demersum have the capacity to influence Al solubility and DRP availability within Te Wairoa Bay.
This work highlights the complexity of biogeochemical processes within aquatic freshwater ecosystems. Moreover, the results emphasise the need to account for the significant spatial and temporal heterogeneity of physicochemical parameters in the development of effective lake remediation strategies.