Date of Award

December 2014

Degree Type

Thesis

Degree Name

Master of Science

Department

Freshwater Sciences and Technology

First Advisor

Harvey Bootsma

Committee Members

Val Klump, Laodong Guo

Keywords

Lake Michigan, Nutrient Cycling, Quagga Mussel

Abstract

Quagga mussels (Dreissena rostiformis bugensis) act as ecosystem engineers in the southern basin of Lake Michigan, altering physical habitats and biogeochemical processes. Adapted to cold and oligotrophic conditions, profunda quagga mussels thrive on the soft substrate of deeper depths. At a 55 m site (10,000 mussels m-2) offshore from Milwaukee, WI, profunda mussel biomass (g m-2) was 1/3 of biomass (g m-2) measured at a 10 m comparison site (5,000 mussels m-2). Higher densities but less biomass is due to profunda mussels having less tissue for a given length and the population per m2 comprising of mostly small mussels (< 8 mm). Cold temperatures (≤ 6 ºC) and oligotrophy in the hypolimion (≤ 0.10 µmol P L-1) limit profunda quagga mussel grazing, excretion, and egestion. Profunda mussels ≥ 8 mm excrete at similar rates (0.004 to 0.010 µmol SRP mgDW-1 d-1) that are nearly a magnitude lower than the nearshore phenotype. Past studies have focused on mussel excretion, but these experiments suggest the excretion : egestion ratio is 3 : 2, and mussel biodeposits could a critical factor in the amount of P sequestered by mussels. Mussel effective clearance rates (0.19 to 3.65 L mgDW-1 d-1) increased with decreasing mussel dry tissue weight and were dependent on particulate P concentrations above the mussel bed. In one year, nutrient cycling by profunda quagga mussels is 100 times greater than nutrients tied up in biomass, highlighting how the filter feeders increase the cycling rate.

Internal recycling is a critical component of Lake Michigan's P cycle. In the pre-dreissenid period, P was utilized in the hypolimnion by phytoplankton, settled passively to the lake bottom, and largely returned to the system via resuspension. Profunda quagga mussel grazing has altered P fluxes by reducing sediment resuspension and increasing nutrient cycling in the benthos. Mussel grazing rates are higher than passive settling rates due to vertical mixing replenishing the food supply above mussel beds. Dreissenids act as both nutrient recyclers and sinks, but in the deeper depths of Lake Michigan, profunda quagga mussels filtering year-round appear to have altered P residence time in the benthos.

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