Collaborative Research: Regulation of plankton and nutrient dynamics by hydrodynamics and profundal filter feeders

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Benthic filter feeders such as clams and mussels influence plankton and nutrient dynamics in shallow marine and freshwater systems, but their role is generally considered to be minor in large, deep systems. However, recent evidence indicates that profundal quagga mussels (Dreissena rostriformis bugensis) have dramatically altered energy flow and nutrient cycling in the Laurentian Great Lakes and other large, deep aquatic systems, with negative consequences for many other components of the food web. Previously, management strategies for these lakes relied on numerical models that predicted the lake's response to phosphorus loading. The proliferation of benthic filter feeders in the lakes appears to have fundamentally altered the dynamics of phosphorus cycling and energy flow, so that these models are no longer valid. This project measures energy flow and nutrient cycling processes in Lake Michigan, with a focus on the role of quagga mussels in these processes, and uses these measurements to develop numerical models to guide management decisions with regard to nutrient loading and fish stocking. In addition to supporting the education of three Ph.D. students and providing research training for undergraduate students, the project promotes education of future aquatic scientists by hosting a workshop on interdisciplinary research and modeling for students and making workshop lectures available through an open-access project website.This collaborative biophysical project is structured around two primary questions: 1) What role do profundal dreissenid mussels play in large lake carbon and nutrient cycles? 2) How are mussel grazing and the fate of nutrients recycled by mussels modulated by hydrodynamics at scales ranging from mm (benthic boundary layer) to meters (entire water column)? The investigators hypothesize that the apparent enhanced particle deliver rate to the lake bottom results from high filtration capacity combined with vertical mixing processes that advect phytoplankton from the euphotic zone to the near bottom layer. However, the role of hydrodynamics is unclear, because these processes are poorly characterized both within the hypolimnion as a whole and within the near-bottom layer. In addition, the implications for phytoplankton and nutrient dynamics are unclear, as mussels are both grazers and nutrient recyclers. State-of-the-art instruments and analytical tools, including particle image velocimetry, acoustic Doppler current profilers, and nutrient micro-profilers, are being deployed in Lake Michigan to quantify these critical dynamic processes, including boundary layer turbulence, mussel grazing, excretion and egestion, and benthic fluxes of carbon and phosphorus. Resulting data are used to calibrate a 3D hydrodynamic-biogeochemical model, which is used to address the overarching question of how plankton and nutrient dynamics in large, deep lakes with abundant profundal filter feeders differ from the conventional paradigm described by previous models. The project also provides insight into bottom boundary layer physics, with applicability to other large lakes, atidal coastal seas, and the deep ocean.