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Lakes contribute disproportionately to global cycling of elements, more than their relatively small surface area would otherwise suggest. Natural and anthropogenic sources of runoff from the watershed create a strong terrestrial contribution to aquatic productivity. Tiny but abundant plankton link the water and airsheds to the food webs in the the receiving lake through their growth and respiration – collectively called metabolism. Because of their intimate connection to the land, freshwater lakes are particularly active sites for the cycling of organic carbon and inorganic nutrient inputs – and serve as sensitive sentinels of change in their collective water and airsheds.
The 10,000 miles of Great Lakes’ shoreline are intersected by almost 3,000 tributaries. One of these intersections is Muskegon Lake. It is fed by the Muskegon River, the second longest river in Michigan and drains an area of ~ 6,000 km2 (Figure 1) into Lake Michigan. As the receiving basin for a large watershed, Muskegon Lake is a concentration point for vast amounts of terrestrial organic carbon and inorganic nutrients delivered by the Muskegon River. Thus, variable river discharge can potentially fuel highly variable rates of productivity and respiration by in-lake phytoplankton and bacteria, respectively.
Figure 1. Map of Muskegon Lake study area and sample locations: Muskegon River, Muskegon Channel and Muskegon Deep, with Lake Michigan in the foreground. Inset: Laurentian Great Lakes basin, with boxed area highlighting location of Muskegon Lake located at the terminus of Muskegon River watershed (gray shaded area in the state of Michigan).
Figure 2a. Graduate Student, Angela Defore sampling Muskegon Lake water and incubating lake water-filled bottles underwater through an ice hole during the winter. Winter is the most understudied portion of the annual cycle in temperate latitude lakes.
Figure 2b. Site averaged Chlorophyll a (an index of phytoplankton biomass) in the surface waters of Muskegon Lake from February 4, 2009 – February 11, 2010. Error bars represent 1 standard error.
We examined monthly changes in biological inventories and community metabolism in Muskegon Lake – including during the ice-covered winter season and found that there was a positive buildup of phytoplankton biomass during spring through fall (Figure 2).
Similarly, rate measurements indicated that net production dominates (Production > Respiration) during the spring, summer, and fall seasons, whereas net respiration is only slightly dominant during the winter months (Figure 3). This results in net annual autotrophy (i.e., surplus production) in the ecosystem as evidenced by the accumulation of phytoplankton biomass. It additionally suggests, that the surface waters of Muskegon Lake may be a “carbon sink”, which buries carbon in its sediments, or exports it to Lake Michigan supporting neashore metabolism.
Figure 3. A. Conceptual diagram of trends in the seasonal metabolism (Gross Primary Production, GPP and Respiration, R; A) and represented as the ratio of GPP:R (B). The 1:1 line indicates the zone of carbon balance above which net production (autotrophy) prevails and below which net respiration (heterotrophy) prevails.
These findings also shed light on a seriously understudied component of the annual cycle of lakes: the key role of winter months in nutrient regeneration that subsequently fuel spring production. Future studies should reassess the critical role of winter months as an annual “reset button” for the lake ecosystem as whole. The in-lake regeneration of nutrients during winter, spring-time nutrient loading from the Muskegon River watershed, increased summer-time sunlight availability, and favorable residence time (2-4 weeks) enable optimal retention of resources and completion of plankton and fish life cycles within Muskegon Lake (Figure 4). All these factors converge to make this Great Lakes estuary a zone of net annual primary productivity in the watershed, supporting one of the most productive fisheries in the state. Similar phenomena may be at play in inland waters and coastal estuaries everywhere where food web productivity is tied to peaks of primary production. Therefore, understanding the delicate dance between autotrophy (Production > Respiration) and heterotrophy (Respiration > Production) in lake and estuarine ecosystems that are sentinels of change in their air and watersheds will be key to their wise management in an uncertain future.
For further details, please see (below) the peer-reviewed journal article that is openly accessible.
This research effort was supported by a NASA Michigan Space Grant Consortium Seed Grant and an EPA Great Lakes Restoration Initiative Grant to BB, and a MSGC Graduate Fellowship and a GVSU Presidential Research Grant to Angela Defore.
Defore, A. L., A. D. Weinke, M. M. Lindback, and B. A. Biddanda (2016). Year-round measures of planktonic metabolism reveal net autotrophy in surface waters of a Great Lakes estuary. Aquatic Microbial Ecology 77: 139-153. http://www.int-res.com/articles/ame_oa/a077p139.pdf
Figure 4. Schematic illustration highlighting the seasonal succession of life in temperate lakes. Life awakens from the winter’s nap into spring growth, followed by summer-fall’s peak biomass, ending in the cold and slow holiday season towards the year end. Winter months serve as a system reset wherein inorganic plant nutrients are regenerated from net respiration of organic matter. Spring through fall, these plant nutrients are consumed – resulting in net production of biomass containing organic carbon that sustains a diverse and productive food web.