Phosphorus biogeochemistry in meltwater ponds of Victoria Land, Antarctica
Thesis DisciplineAntarctic Studies
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Phosphorus plays an essential role in the biochemistry of all living organisms, and understanding factors controlling its availability in an ecosystem can provide insight into how the ecosystem will respond to change. Freshwater ecosystems in Antarctica are important biodiversity elements, containing vibrant microbial communities dominated by benthic cyanobacterial mats. Productivity in meltwater ponds can become limited by nutrient availability, and inland aquatic systems are typically P deficient. In order to understand phosphorus (P) biogeochemistry in meltwater ponds in Victoria Land, the distribution and speciation of P was determined in ponds at 7 locations, and processes which influence P concentration in the water column were investigated. The biogeochemical cycle concept has been applied to the results as a tool to interpret P behaviour. This involved identification of the key reservoirs that hold P within the pond ecosystem, and the processes which can transfer P between these reservoirs. Sediment, soil, water and benthic microbial mats were identified as important reservoirs in the ponds. Microbial mats can accumulate P to concentrations over 2 g/kg, and often had higher concentrations of P than soils and sediments. Soils consistently had higher P concentrations than sediments, and comparatively little P was present in pond waters. Concentration and pond structure data were used to create a conceptual model of P distribution in meltwater ponds, which revealed sediments are the major reservoir of P in these systems. Saturated soils are the next largest reservoir of P, followed by microbial mats then the pond water. Sediments are the major source of P to meltwater ponds. This is demonstrated by low total P concentrations in sediments relative to adjacent soils. Transects from pond shorelines reveal that both reactive and apatite P fractions in sediments are transported into ponds, and that soils within 2 m from pond edges can also act as a P source. It is not clear whether P from marginal soils is provided to the pond during inundation. Wind-blown dust contains high concentrations of labile P relative to soils, and is an intermittent source of P to ponds. Significant quantities of P are lost from pond ecosystems via wind transport of foam and desiccated mats. There is significant biotic control over DRP concentrations in pond water columns. Benthic microbial mats are the main consumers of DRP in most ponds, and are capable of sequestering DRP from the water column in lit conditions. In the dark, mats act as a source of DRP to pond water. Circadian cycles drive changes in physico-chemical conditions present in ponds, but have little effect of DRP concentrations. Biological activity seems to drive short term fluctuations in DRP concentrations occur throughout the day. A systematic pattern is apparent between pond location and P abundance. High P concentrations exist in all of the reservoirs of low elevation coastal ponds, while inland pond systems at high elevation generally contained very low P concentrations. Soil P concentrations correlate inversely with pond elevation, as do inorganic N:P ratios. Soil P composition is constrained by the composition of its parent material, and is thus a product of bedrock geology and landscape history. Sediment P composition derives from an initial composition similar to the soil, which is altered by weathering and accumulation of biological and inorganic debris. Therefore the bedrock geology and landscape history of an area are major determinants of P abundance in ponds.