Investigating internal nutrient loading from sediments of the Qu'Appelle lakes, Saskatchewan, Canada
Date
2024-10-04
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0002-6069-5436
Type
Thesis
Degree Level
Doctoral
Abstract
Nutrients can be recycled from sediments to the water column in a process known as internal loading, which can be a major contributor to the nutrient budget in lakes, influencing phytoplankton biomass and community structure and delaying the recovery of lakes from degradation even after external loads are reduced. This process can be important in lakes in the Canadian Prairies, helping to fuel recurring harmful algal blooms. The Ph.D. work focused on four eutrophic hardwater lakes in southern Saskatchewan, Canada, to better understand internal loading. First, sediment phosphorus (P) forms were characterized at different depths in the lakes using solution phosphorus-31 nuclear magnetic resonance (P-NMR) spectroscopy of NaOH-EDTA extracts and geochemical factors influencing sediment P forms were examined using single-step sediment metal extractions. Second, rates of internal P loading were quantified using laboratory sediment core incubations and compared to geochemical factors controlling sediment P fluxes across the four lakes using sediment P fractionations and P-NMR spectroscopy. Finally, internal nitrogen (N) and P loads were compared to external N and P loads to understand the importance of internal nutrient loads in the context of nutrient budgets in the lakes, and how the stoichiometry of sediment nutrient fluxes and long-term in-lake nutrient mass accumulation affect the water column nutrient stoichiometry and phytoplankton biomass.
Results showed that sediment N and P release contributed substantially (69 to 99 %) to the annual in-lake N and P budget under drier (more normal) conditions in Pasqua and Echo lakes. External nutrient loads in wet periods appear to minimize the importance of internal nutrient loads to the nutrient budget in these lakes on an annual scale, but internal nutrient loading may be the primary driver of increases in summertime in-lake nutrient mass accumulation among lakes in drier periods. Sediment P release was significantly higher under hypoxic conditions than under oxic conditions. Phosphate bound in the redox-sensitive pool in sediments (9 to 16 % of total P in sediment) was the only sediment pool that significantly differed among lakes. Principal component analysis results indicated that the redox-sensitive P pool was inversely related to sediment P fluxes under oxic and hypoxic conditions, suggesting that redox plays an important role in sediment P fluxes. Calcium dominated all but three study lakes and had an overarching effect on sediment P flux in these lakes, masking the effects of redox control on internal P loading.
Sediment N fluxes did not significantly differ between oxic and hypoxic conditions, while sediment P fluxes were affected by hypoxia, suggesting that changes in oxygen concentrations at the sediment surface may drive changes in N:P stoichiometry of sediment nutrient fluxes. Low N:P ratios in fluxes during the summer months can influence the water column N:P stoichiometry and ultimately the ecological function of the lakes. Strong relationships exist between sediment dissolved inorganic N (DIN) and dissolved molybdate-reactive P (DMRP) fluxes under hypoxic and oxic conditions, with the slopes (5.93 and 6.60 by mass, respectively) near the Redfield ratio (7.23 by mass), suggesting that organic matter (OM) decomposition and organic P mineralization are key processes driving sediment DIN and DMRP fluxes. Results that show positive DMRP fluxes when DIN fluxes equal zero suggest that an imbalance in these nutrient fluxes from sediments may be related to geochemical controls on sediment P release or biological controls on N release. Orthophosphate was the dominant inorganic P form, with concentrations increasing with sediment depth. Orthophosphate diesters, typical of living biota, were the dominant organic P forms, with the highest concentrations at the sediment surface, suggesting that organic matter (OM) decomposition and organic P mineralization are key processes controlling sediment P release. Elevated concentrations of internally generated total organic P forms and OM in surface sediments play a key role in internal P loading in these hardwater lakes, with implications for P management.
Description
Keywords
Sediment P flux, Sediment cores, P-NMR, Sediment P fractions, Biogeochemical processes, Calcium
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
School of Environment and Sustainability
Program
Environment and Sustainability