Abstract
During the summers of 2002–2013, we measured rates of carbon metabolism in surface waters of six sites across a land-to-lake gradient from the upstream end of drowned river-mouth Muskegon Lake (ML) (freshwater estuary) to 19 km offshore in Lake Michigan (LM) (a Great Lake). Despite considerable inter-year variability, the average rates of gross production (GP), respiration (R) and net production (NP) across ML (604 ± 58, 222 ± 22 and 381 ± 52 µg C L−1 day−1, respectively) decreased steeply in the furthest offshore LM site (22 ± 3, 55 ± 17 and −33 ± 15 µg C L−1day−1, respectively). Along this land-to-lake gradient, GP decreased by 96 ± 1%, whereas R only decreased by 75 ± 9%, variably influencing the carbon balance along this coastal zone. All ML sites were consistently net autotrophic (mean GP:R = 2.7), while the furthest offshore LM site was net heterotrophic (mean GP:R = 0.4). Our study suggests that pelagic waters of this Great Lakes coastal estuary are net carbon sinks that transition into net carbon sources offshore. Reactive and dynamic estuarine coastal zones everywhere may contribute similarly to regional and global carbon cycles.
Highlights
During the summers of 2002 – 2013, we measured rates of carbon metabolism in surface waters of six sites across a land-to-lake gradient from the upstream end of drowned river-mouth Muskegon Lake (ML) to km offshore in Lake Michigan (LM)
Our study suggests that pelagic waters of this Great Lakes coastal estuary are net carbon sinks that transition into net carbon sources offshore
Studies suggest that the annual stream and river discharge into southern Lake Michigan (LM), equaling 1% of its total volume, have terrestrial subsidies of dissolved organic carbon (DOC) and phosphorus 10 and 20 times higher than LM, that support up to 20% of the phytoplankton production and up to 10% of heterotrophic bacterial production in southern LM (Biddanda and Cotner, 2002)
Summary
During the summers of 2002 – 2013, we measured rates of carbon metabolism in surface waters of six sites across a land-to-lake gradient from the upstream end of drowned river-mouth Muskegon Lake (ML) (freshwater estuary) to km offshore in Lake Michigan (LM) (a Great Lake). Our study suggests that pelagic waters of this Great Lakes coastal estuary are net carbon sinks that transition into net carbon sources offshore. Reactive and dynamic estuarine coastal zones everywhere may contribute to regional and global carbon cycles. Measurements that enable such assessments, coupled measurements of photosynthesis and respiration, are severely lacking in aquatic ecosystems (del Giorgio et al, 1997; Karl et al, 2003; Serret et al, 2001)
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