Year-round measures of planktonic metabolism reveal net autotrophy in surface waters of a Great Lakes estuary

Abstract

During 2009 and 2010, we quantified monthly changes in plankton metabolism and environmental variables in the surface waters of Muskegon Lake, a Great Lakes estuary connected to Lake Michigan. Muskegon Lake's mean (±SE) annual gross plankton primary production (GPP) and respiration (R) rates were 46 ± 9 and 23 ± 4 mg C l-1 yr-1, respectively. GPP:R ratios of 0.6 to +4.8 with a yearly mean of 2.0 ± 0.3 indicated that the surface water of Muskegon Lake was net autotrophic during all but the winter months under ice cover, when it was in a near carbon balance to slightly heterotrophic state. Approximately 5% of GPP and 12% of R occurred during the winter months, highlighting winter's potential role in nutrient regeneration. An overall positive annual net community production (NCP) rate of 28 ± 6 mg C l-1 yr-1 makes Muskegon Lake's surface waters a net sink for carbon on an annual basis. Annual heterotrophic bacterial production (BP) rates were 5 ± 3 mg C l-1 yr-1, suggesting a substantial fraction of GPP was likely processed through the microbial food web (2 to 76%). A stepwise multiple linear regression model revealed the plausible drivers of GPP (temperature [T], photosynthetically active radiation [PAR], total phosphorus [TP], dissolved oxygen [DO], chlorophyll a [chl a]), NCP (T, PAR, TP), R (T, DO, ammonium [NH3], soluble reactive phosphorous [SRP], dissolved organic carbon [DOC]) and GPP:R (T, PAR, SRP, DOC). Year-round measurements inform us of the strong seasonality in the carbon cycle of temperate lakes.