Online

Kirol, Ashton P.

Geology

2023

M.S.

The eutrophication of freshwater lakes from excessive nutrient runoff leads to decreased water quality and worsening cyanobacteria blooms. Water quality improvements in shallow eutrophic lakes can be delayed by decades due to the seasonal recycling of legacy phosphorus (P) enriched lake sediments, even when external nutrient loads are addressed. It is critical to understand the drivers of internal P loading to suppress this source of P through intervention to meet water quality goals. This study contrasts two shallow eutrophic systems, Lake Carmi and Missisquoi Bay in Lake Champlain, impacted by legacy P loading driven by the occurrence of low dissolved oxygen and associated reductive dissolution of minerals that bind P in the sediment. Legacy P dynamics in these systems in their unmanipulated states were compared to Lake Carmi over four years of whole-lake aeration designed to suppress internal P loading. We employed in-situ high frequency water column monitoring along with water and sediment sampling to assess the distribution of P in response to changing lake conditions and aeration. In Lake Carmi, sustained anoxia during summer stratification causes significant internal P loading focused in the deep central trough that is then mixed throughout the water column during fall turnover. The large surface area of Missisquoi Bay leads to spatially heterogeneous legacy P distribution and conditions that promote internal loading. Frequent mixing brings P from internal loading to the surface earlier in the year and in pulses related to the impacts of changing wind speed and orientation on transient stratification vs. mixing. Despite both systems exhibiting P mobility coupled with iron redox reactions, we observed distinct differences in the spatial extent and drivers of internal P loading. Aeration altered the mixing regime of Lake Carmi and shifted P mobility from seasonal turnover to frequent wind-driven pulses of benthic legacy P to surface waters, while remaining tightly coupled to Fe redox driven cycling in bottom water. Anoxia developed during periods of stratification, and increased concentrations of bottom water total P were observed within 24-48 hours. Summer total P in Lake Carmi increased with aeration and exceeded the TMDL target of 22 [mu]g/L earlier in the season. P dynamics in Lake Carmi began to more closely match Missisquoi Bay which naturally mixes frequently. High frequency monitoring captured changes in P over short time scales in response to conditions that changed naturally and through aeration. This research demonstrates the highly dynamic nature of legacy P behavior within shallow lakes. The drivers of legacy P loading and distribution can vary substantially between shallow lakes in the same geographic area and are critical to understand when assessing management options.