Date of Award

August 2023

Degree Type

Thesis

Degree Name

Master of Science

Department

Freshwater Sciences and Technology

First Advisor

Russell L Cuhel

Second Advisor

Carmen Aguilar

Committee Members

Russell L Cuhel, Carmen Aguilar, James T Waples

Keywords

biogeochemistry, dredging, eutrophication, freshwater, nitrogen, phosphorus

Abstract

This study examined the nutrient environment of sediments in the Milwaukee River estuary and the dynamics of those nutrients during simulated disturbance experiments within the context of large-scale dredging remediation. Surface sediments were collected from throughout the Milwaukee estuary (including river, harbor, and nearshore stations) by PONAR, centrifuged to separate porewater (interstitial water) from solid material, and filtered to further isolate and stabilize dissolved material. Porewaters were analyzed for dissolved nutrients including ammoniacal nitrogen (AN), nitrate, nitrite, and soluble reactive phosphorus (SRP). Surface sediment porewaters within the estuary were often highly enriched in AN and SRP, which were often 10-2000x more concentrated in estuarine surface sediment porewaters than their overlying water columns, with AN ranging from 150-2000 µM and SRP ranging from 0.1-70 µM. Nitrate concentrations in surface sediment porewaters were strongly depleted relative to overlying waters—between 0.01-0.40x. Simulated disturbance experiments designed to approximate dredging-induced disturbances were performed on sediment samples, which involved continuously mixing whole sediments with filtered harbor water. In these experiments, more AN and SRP were released into the receiving waters than was expected based on the concentrations present in the sediment porewaters initially, with AN reaching concentrations 4-10x higher than expected within 4-5 hours. These dynamics suggest organic decomposition within the first 4-5 hours after the initial disturbance while the sediments were still resuspended. In most samples, AN was depleted to very low concentrations within 2 days of the initial disturbance. Nitrite, the intermediate product of complete ammonia oxidation, rose to moderate concentrations before also being depleted to low concentrations for the remaining 1-3 weeks of the experiments. Nitrate, the end product of complete ammonia oxidation, rose gradually throughout the remaining weeks of the experiments to concentrations 1-25x higher than predicted. These dynamics strongly suggest nitrification. Soluble reactive phosphorus also reached concentrations 2-12x higher than predicted, rising quickly in the first 4-5 hours after mixing then more slowly over the following weeks. Alongside decomposition, this may also be the result of particulate inorganic phosphorus (PIP) dissolution due to acidification by nitrification. The concentrations and forms of eutrophicating nutrients released by dredge plumes or effluent discharged from confined disposal facilities are determined not only by the initial dissolved nutrient contents of sediment porewaters but by biogeochemical processes at the sediment-water interface such as those described here. The nature of these biogeochemical processes as they relate to dredging will influence the ecological effects of dredging on the surrounding environment.

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