Systematically variable planktonic carbon metabolism along a land-to-lake gradient in a Great Lakes coastal zone

Journal of Plankton Research, Nov 2014

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.

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Systematically variable planktonic carbon metabolism along a land-to-lake gradient in a Great Lakes coastal zone

J. Plankton Res. ( Systematically variable planktonic carbon metabolism along a land-to-lake gradient in a Great Lakes coastal zone ANTHONY D. WEINKE 2 SCOTT T. KENDALL 2 DANIEL J. KROLL 1 2 ERIC A. STRICKLER 0 2 MAGGIE E. WEINERT 2 THOMAS M. HOLCOMB 2 5 ANGELA A. DEFORE 2 4 DEBORAH K. DILA 2 3 MICHAEL J. SNIDER 2 LEON C. GEREAUX 2 BOPAIAH A. BIDDANDA 2 0 SALISH SEA EXPEDITIONS 1 MUNSON MEDICAL CENTER 2 ANNIS WATER RESOURCES INSTITUTE 3 SCHOOL OF FRESHWATER SCIENCES 4 USC BARUCH MARINE FIELD LAB 5 HERMAN MILLER 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 mg C L21 day21, respectively) decreased steeply in the furthest offshore LM site (22 + 3, 55 + 17 and 233 + 15 mg C L21day21, 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 ¼ 2.7), while the furthest offshore LM site was net heterotrophic (mean GP:R ¼ 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. coastal; pelagic; production; respiration; gradient - The exchange of carbon between atmospheric, terrestrial and aquatic pools is one of the largest biogeochemical pathways on Earth, second only to the water cycle (Schlesinger and Berhardt, 2013) . The dual processes of autotrophy and heterotrophy drive the biosphere’s carbon cycle, making their quantification essential for estimating the magnitude of carbon flux and quantifying the net carbon balance of ecosystems. However, 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) . Carbon and nutrients that run off or drain through the terrestrial environments of watersheds pass through the myriad of waterways consisting of streams, rivers, wetlands and lakes on to receiving basins such as the Great Lakes and the oceans (Beman et al., 2005; Biddanda and Cotner, 2002) . However, inland waters do not play a passive role as mere conduits of carbon and nutrients in this worldwide phenomenon. Indeed, recent studies have shown that inland waters of the world, representing only 1% of the Earth’s surface area, play a disproportionately large role in the global carbon cycle (Cole et al., 2007; Tranvik et al., 2009) . In particular, significant processing of terrigenous nutrients and carbon occurs in land-margin ecosystems such as streams, rivers, estuaries and coastal zones (Marko et al., 2013) . It is estimated that annually of the about 5 Pg of carbon entering the inland waters, 0.2 – 0.6 Pg of carbon is buried in freshwater sediments (twice the annual burial into oceanic sediments), and 0.7 – 4.0 Pg of carbon is respired to the air in freshwater systems (equal to the net uptake of carbon by the oceans). These estimates lend credence to the emerging notion that inland waters are highly reactive and play a crucial and hitherto underappreciated role in regional and global carbon cycles (Raymond et al., 2013; Wehrli, 2013) . The North American Great Lakes contain nearly 20% of Earth’s liquid surface freshwater. Though large, they are heavily human-impacted systems with stressors acting on multiple fronts (Cuhel and Aguilar, 2013; Evans et al., 2011; Rothlisberger et al., 2010) . Climate change, for example, is resulting in warmer temperatures occurring earlier in the year leading to earlier spring blooms followed by a cascade of ecosystem effects (Russ et al., 2004) . Furthermore, in recent years, invasive dreissenid mussels have nearly eliminated the winter and spring blooms (Kerfoot et al., 2010; Vanderploeg et al., 2010) . 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) . LM is currently on its way to becoming as oligotrophic as Lake Superior (Evans et al., 2011; Mida et al., 2010) , making terrestrial subsidies of nutrients and organic matter more vital than ever to its annual balance (Johengen et al., 2008) (...truncated)


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Anthony D. Weinke, Scott T. Kendall, Daniel J. Kroll, Eric A. Strickler, Maggie E. Weinert, Thomas M. Holcomb, Angela A. Defore, Deborah K. Dila, Michael J. Snider, Leon C. Gereaux, Bopaiah A. Biddanda. Systematically variable planktonic carbon metabolism along a land-to-lake gradient in a Great Lakes coastal zone, Journal of Plankton Research, 2014, pp. 1528-1542, 36/6, DOI: 10.1093/plankt/fbu066