From Land to Lake: Contrasting Microbial Processes Across a Great Lakes Gradient of Organic Carbon and Inorganic Nutrient Inventories

Freshwater ecosystems have strong linkages to the terrestrial landscapes that surround them, and contributions of carbon and inorganic nutrients from soil, vegetation and anthropogenic sources subsidize autochthonous water body productivity to varying degrees. Abundant freshwater phytoplankton and bacterioplankton are key to linking the planet\u27s geosphere and atmosphere to the food webs in the hydrosphere through their growth and respiration. Rich resources that move through land margin waterways make them active sites for cycling organic carbon and thus important, but understudied, contributors to global climate. 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 West 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 NO3 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 all other sites. Muskegon Lake dominated GPP among the sites reaching a peak of \u3e1000 μg carbon/L/d during a large fall Microcystis bloom. Offshore Lake Michigan had less variation in GPP and R than the other sites with GPP:R ratio close to 1 in all seasons but spring. 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, whereas both autotrophy and heterotrophy were minimal but in near-balance in offshore waters receiving little subsidy from the land. Along this land-to-lake gradient terrestrial subsidies combined with a host of other factors making conditions “just right” for a hot-spot to emerge, highlighting Muskegon Lake estuary a “Goldilocks Zone” of net biological productivity