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

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.

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

“…Carbon and nutrient runoff from terrestrial sources, load streams, rivers, wetlands and small lakes with intermittent but consistent subsidies to the metabolic processes that take place in these varied aquatic environments (Beman et al, 2005;Weinke et al, 2014). Our results show planktonic autotrophs and heterotrophs perform biogeochemical processes at increased levels in these tributary waterways before nutrient loads have a chance to enter larger receiving basins, such as the great lakes.…”

“…The biogeochemical processes and the environmental conditions that are associated with land-to-lake gradients are locally relevant to restoration and future management of large lake ecosystems (Allen et al, 2013;Larson et al, 2013) and, on a larger scale, to global carbon cycling (Cole et al, 2007;McClain et al, 2003;Weinke et al, 2014). The knowledge gained from this research project will be a useful reference source for ongoing studies in this watershed and an important comparative source for any biogeochemical land-margin study of tributaries and their receiving waters.…”

“…In Weinke et al (2014) the authors summarized data from 2003 through 2013 monitoring the rates of carbon metabolism in summer surface waters collected at multiple sites in the estuary and out into Lake Michigan. They tracked changes in GPP, R and NPP from year to year and found a systematic decrease in metabolism in surface waters extending from Muskegon Lake to offshore Lake Michigan (Figure 1).…”

“…Inserting the upstream element to the watershed study area adds a missing piece to the gradient puzzle and expands on the Weinke et al (2014) Muskegon Lake conceptual diagram (Figure 8),…”

“…Productivities peak in many estuaries and nearshore coastal zones around the world as exemplified by the Mississippi River estuary and the dramatic increase in phytoplankton growth and primary production measured from point of discharge to near-and mid-field plume . In fact, land margin coastal ecosystems are recognized as key hotspots with hot moments in the global carbon cycle (Cole et al, 2007;McClain et al, 2003;Weinke et al, 2014). Cole and others argue that freshwater ecosystems are not merely "passive pipes", but are highly "reactive sites" of global carbon cycling.…”