From land to lake: Contrasting microbial processes across a Great Lakes gradient of organic carbon and inorganic nutrient inventories

Freshwater aquatic biota receive carbon and nutrients from within the system as well as from the terrestrial environment in varying proportions. During 2010-2011, we examined seasonal changes in carbon and nutrient inventories, plankton community composition and metabolism along a land-to-lake gradient in a major western Michigan watershed at four interconnected habitats ranging from a small creek to offshore Lake Michigan. In all seasons Lake Michigan had significantly lower concentrations of CDOM and DOC than any of the other sites. Lake levels of nitrate were not significantly lower than tributaries other than Cedar Creek, and SRP was not measurable in any of the sites other than Cedar Creek. Bacterial production as % of GPP revealed a distinct land-to-lake gradient from an average of 448% in Cedar creek to 5% in Lake Michigan. Microbial activity in Cedar Creek (bacterial production 3-93 μg C/L/d, and plankton respiration 9-193 μg C/L/d) was generally higher than other sites. Muskegon Lake dominated GPP among the sites reaching a peak of > 1000 μg C/L/d during a fall Microcystis bloom. Offshore Lake Michigan had less variation in GPP and R than other sites, with GPP:R ratio close to 1 in all seasons but spring. Aquatic metabolism appears to be substantially subsidized by terrigenous inputs in the creek/river ecosystem with heterotrophy dominant over autotrophy. Autotrophy was maximized in the coastal/estuary “Goldilocks Zone” with longer residence times, whereas both autotrophy and heterotrophy were minimal but in near-balance in offshore waters receiving little subsidy from the land.