Award: OCE-1658390

The primary objective of this project was to understand how invasive quagga mussels (Dreissenia rostriformis bugensis) have altered carbon and phosphorus dynamics in a Great Lake (Lake Michigan), and how the effect of these mussels is modulated by mixing dynamics within the lake. This understanding can then be used to improve mathematical models used to guide decisions related to water quality and fisheries management. The research was conducted using a variety of methods. We used in-lake measurements and lab experiments to quantify vertical mixing rates in the lake at scales ranging from millimeters to the entire water column, and quantify feeding and nutrient excretion rates by quagga mussels. Vertical mixing rates were measured using acoustic Doppler current profilers (ADCPs), temperature loggers positioned throughout the water column, a microCTD that measures vertical profiles of temperature with high spatial resolution, and a novel particle image velocimetry (PIV) system that allows for measurements of water movement at mm scales immediately above the mussel bed on the lake bottom. Experiments with mussels were designed to measure the rate at which mussels release dissolved and particulate carbon and phosphorus, and the fate of particulate carbon and phosphorus (biodeposits) following release. The measurements of mixing rates and mussel carbon / phosphorus recycling were then combined to develop mathematical models that can be used to explore how quagga mussels affect the lake?s carbon and phosphorus cycles under different physical conditions. The main findings of this project support our two general hypotheses: 1. Quagga mussels have dramatically altered carbon and phosphorus dynamics in Lake Michigan, and by extension, likely the other lower Great Lakes. 2. The impact of dreissenid mussels is strongly dependent on lake mixing dynamics, including vertical mixing in the offshore zone, and horizontal mixing between the nearshore and the offshore zone. Specific findings include: In the deep parts of the lake, mussels create near-bottom mixing by pumping water in and out of the inhalant and exhalant siphons. This mixing allows them to increase their grazing efficiency by promoting a continuous supply of plankton to the near-bottom boundary layer. Model results indicate the optimal pumping rate is 1 to 5 liters per day, depending on ambient mixing conditions. Quagga mussels have dramatically increased (~12 X) the rate at which particles are transferred from the water column to the lake bottom. This is because mussels are able to scavenge these particles from the water as it mixes from shallow depths to the near-bottom layer, whereas in the past this delivery depended primarily on passive sinking of particles. This scavenging fundamentally alters the ways in which energy and nutrients are cycled within the lake. Mussels have lowered the total phosphorus concentration in Lake Michigan, primarily by grazing out particulate phosphorus. They have a much smaller effect on dissolved phosphorus because much of the particulate phosphorus they consume is rapidly recycled into the dissolved phosphorus pool. The implication is that phytoplankton growth rates (which are proportional to the dissolved P concentration) have not changed, and that the decline in phytoplankton abundance (and the abundance of higher trophic level organisms) since the dreissenid invasion is due primarily to mussel grazing, not a decline in the availability of phosphorus to phytoplankton. Dreissenid mussels have greatly increased the efficiency with which the nearshore zone retains particulate phosphorus. The results of this research indicate that the fundamental structure and function of a large lake have been significantly altered due to a major change in the biotic community. From a practical perspective, this means that some of the statistical and mechanistic models that were previously used to guide management decisions related to phosphorus loading and fisheries are no longer applicable. Previous models assumed a relatively constant relationship between external phosphorus loading, internal total phosphorus concentration, and phytoplankton abundance. This is no longer the case, and phosphorus loading targets that were originally set by the 1972 Great Lakes Water Quality Agreement are no longer valid. Scientists and managers in the Great Lakes community are currently struggling to determine what the optimal phosphorus loads should be for each of the Laurentian Great Lakes. The data and models resulting from this research are allowing us to determine how both the nearshore zone (where nuisance algal growth is a serious management problem) and the offshore zone (where declines in plankton abundance have affected the entire food web) will respond to any changes in phosphorus inputs to Lake Michigan. This project supported the training of three PhD and two MS students. Last Modified: 08/29/2022 Submitted by: Harvey A Bootsma