Benthic Nitrogen excretion by the Invasive Quagga Mussel Dreissena bugensis offshore gradients in Lake Michigan: A Time Series Approach

Benthification processes due to Quagga mussel (QM) colonization and establishment have changed the biogeochemistry of the Laurentian Great Lake Michigan. The Mid-Lake Reef Complex experienced a zero mussel environment, to few mussels, to a full QM colonization that reached extraordinary densities higher than 50,000/sqmtr. QM excretion rates were measured with freshly collected mussels from north basin, plateau, and south slope stations of the plateau. The spatial distribution of QM biomass specific excretion rates was repeatedly assessed from 2006-2018. Quagga mussel dry body weight, N content, and ammonium excretion rate are a cubic function of shell length. Only biomass specific and areal excretion varied systematically with location and density of mussels. Mussel-covered benthos is a source of ammonium that can escape surface sediment nitrification. The 0.5-3 mmol N/sqmtr/day flux is a substantial return of resources to the water column. As the flux increases by larger aging individuals, further flux may now be increasing to the ecosystem. On the upstream side of Northeast Reef, where freshly advected bottom water flows across mussel communities, excretion rates are about 0.8 nmol/animal/hr for early adult quagga mussels 15-20 mm in length. Excretion decreased along the top of the plateau, where there is less turbulence to reduce benthic boundary layer effects, and on the downstream slope, where food particles are poorer in nutritional quality. Inshore shallow stations have similar rates to upstream mid-lake reef populations. N:P excretion ratios (30-40) are greater than Redfield, and consistent with growing animals nearing their late summer spawning effort.