Date of Award

August 2020

Degree Type

Thesis

Degree Name

Master of Science

Department

Freshwater Sciences and Technology

First Advisor

Harvey A Bootsma

Committee Members

Qian Liao, Laodong Guo

Keywords

Lake Michigan, Nutrient Dynamics, Phosphorus, Profundal, Quagga Mussels

Abstract

Quagga mussels (Dreissena rostriformis bugensis) are an invasive ecosystem engineer that have successfully colonized both profundal and nearshore regions in Southern Lake Michigan. Quagga mussels directly altered the flow of nutrients by filtering particles, excretion of soluble forms of nutrients in the benthos, and through the production of biodeposits. Mussel excretion, egestion, and capture rates of P were compared between a 25 meter and a 55 meter deep site near Milwaukee Harbor to determine the retention of nutrients in these regions. The capture rate of particulate phosphorus at the 25m site (278 ± 388 μmol m-2) was similar to that at the 55m site (324 ± 245 μmol m-2). When these values are compared with estimates for 10m depth nearshore Milwaukee (Bootsma et al. 2012), the highest capture rates occur nearshore but remain relatively high through mid-depth regions compared to phytoplankton growth meaning the benthos in this region has a high capacity to retain nutrients. However, a large portion of captured phosphorus is recycled in the dissolved form and in the form of biodeposits.

Approximately 40-50% of recycled phosphorus was in the form of biodeposits. However, there is limited understanding if biodeposits represent a sink or source of nutrients. A series of incubations were conducted to determine the fate of biodeposits and their effect on dissolved nutrients over time scales of 2-3 weeks. Biodeposits promoted bacteria growth, which in turn led to an uptake of dissolved forms of C, N, and P. The bacteria depleted dissolved phosphorus concentration to below detection limit, 0.016 μmol/L, within ten days of incubation. After 12-15 days, dissolved P concentration increased to levels higher than the starting conditions, indicating that biodeposits were a net source of dissolve P over longer time scales.

Mussels may further alter phosphorus cycles through indirect processes such as interfering with apatite dissolution, the promotion of bacterial growth, and increasing the effective settling rate of phosphorus by filtering small particles. The passive settling rates of phosphorus was determined at the 55 meter site using sediment traps. Sediment cores were also taken from the 55 meter site as well as a 100 meter site offshore of Muskegon, MI to determine the phosphorus burial rate. By comparing sedimentation and sediment burial rates, approximately 60% of recently deposited phosphorus at the sediment-water interface is recycled back into the water column. Measurements of sediment fluxes, including sedimentation, accumulation, and permanent burial rate, were combined with mussel fluxes to create a new conceptual model of internal P cycling in Lake Michigan. Before the mussel invasion, zooplankton within the water column were responsible for the bulk of phosphorus recycling. In the new model, the majority of phosphorus cycling occurs in the benthos with heterotrophic bacteria playing a larger role.

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