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Muskegon Lake Estuary: A Goldilocks Zone of Net Biological Productivity in a Great Lakes Watershed

by Bopi Biddanda, Annis Water Resources Institute, GVSU, and Deb Dila, School of Freshwater Sciences, UW-Milwaukee

Once upon a time, there was a little girl named Goldilocks.  She went for a walk in the forest.  Pretty soon, she came upon a house.  She knocked, and when no one answered, she walked right in.  At the table in the kitchen, there were three bowls of porridge.  Goldilocks was hungry.  She tasted the porridge from the first bowl.  "This porridge is too hot!" she exclaimed.  So, she tasted the porridge from the second bowl.  "This porridge is too cold," she cried.  So, she tasted the last bowl of porridge.  "Ahhh, this porridge is just right," she said happily and she ate it all up.......Goldilocks and the Three Bears (A Fairy Tale, circa 1813).

Photosynthesis and respiration fuel the cycle of life in the biosphere.  Freshwater systems that cover ~1% of our planet are characterized by strong linkages to the surrounding land from which they receive significant inputs of terrestrial carbon, such as soil carbon and plant material, and inorganic nutrients. Natural and anthropogenic sources of runoff from the watershed create a strong terrestrial contribution to aquatic productivity.  Tiny but abundant freshwater plankton link the planet's geosphere and  atmosphere to the food webs in the hydrosphere through their growth and respiration – collectively called metabolism (Figure 1).  Because of their intimate connection to the land, freshwater lakes are particularly active sites for the cycling of  organic carbon and inorganic nutrient inputs. Lakes contribute disproportionately to global cycling of elements, more than their relatively small surface area would otherwise suggest.

Microbial Plankton

Figure 1.  Microbial Plankton:  Microbes from Muskegon Lake illuminated by epifluorescence microscopy of Sybr Green-stained water samples (stains all nuclear material in cells) highlighting a 30 µm long photosynthetic Diatom, some >2 µm round photosynthetic

The Laurentian Great Lakes of North America contain ~ 20% of Earth’s surface freshwater and drain more than 200,000 square miles of land, resulting in significant terrestrial loadings.  The 10,000 miles of Great Lakes’ shoreline are intersected by almost 3,000 tributaries.  Many complex processes occur from the headwaters of these tributaries all the way to their river mouths and into receiving basins of the five Great Lakes.  Muskegon River – a drowned river mouth estuary, which empties into Muskegon Lake followed by Lake Michigan, is the second longest river in Michigan and drains an area of ~ 6,000 km2 (Figure 2).  In this study, we summarize the results of a 2-year examination of plankton biomass and metabolism from 1. Cedar Creek, a tributary of Muskegon River, 2. the Muskegon River, 3. Muskegon Lake, a Lake Michigan estuary, to 4. Lake Michigan, one of the Laurentian Great Lakes. 

Figure 2.  The watershed:  Map of the southwest portion of the Muskegon River watershed with four study sites marked.  Inset map shows the location of sites, in Lake Michigan and the State of Michigan, with the larger watershed area outlined in black

The watershed

We examined seasonal changes in biogeochemical inventories, microbial community metabolism and the general composition of the phytoplankton communities along a land-to-lake gradient in this major western Michigan watershed. Our objective was to describe concurrent seasonal changes in environmental gradients, ecosystem production-respiration processes and broad categories of associated microbes (such as autotrophs and heterotrophs) along the sub-ecosystems of a Lake Michigan watershed.  Our results show that site associated photosynthesis and respiration are determined by both the type of phytoplankton and bacterioplankton present and by the inventories of nutrients and carbon available to them along this land-to-lake gradient.

At the land end, as represented by Cedar Creek surface water, photosynthesis occurred at comparatively low levels while respiration and secondary production were at their highest. Creek heterotrophs were dependent on rich terrestrial subsidies. At the lake end, 8 km offshore in Lake Michigan where light is abundant but nutrients and DOC are scarce, phytoplankton appear to make more efficient use of Chl a and heterotrophic respiration was tightly coupled to primary production.   However, throughout the year (but especially during spring and fall), production was consistently higher than respiration in the intermediate coastal estuary – Muskegon Lake – a “Goldilocks Zone” along this land-to-lake transect.  This net autotrophy resulted in a positive buildup of phytoplankton biomass (Figure 3) and sustained a level of productivity that supports a highly productive estuarine food web, leading to a viable nursery for riverine and lake species and sustainable fisheries.  We identify a host of factors – such as terrestiral subsidies of plant nutrients, intermediate water residence time (15-21 days) that allows riverine plarticles to settle and increase water clarity allowing greater light penetration for enhanced photosysnthesis and give additional time for invertebrate life-cycles to complete, and for phytoplankton biomass to build up and persist enabling in turn the sustainance of a robust estuarine food web – that converge to make Muskegon Lake estuary into a “Goldilocks Zone” of net biological productivity.  Thus, our study provides insights into how resource inventories drive community level processes, and how autotrophic and heterotrophic aquatic microbes link terrigenous nutrients and carbon to aquatic food.  It is conceivable that every watershed in the world (there are millions of them) has a “Goldilocks Zone” where things are “just right” such that they are biogeochemical hot-spots of ecological hot-moments in the landscape. 

Phytoplankton Peak

Figure 3.  Phytoplankton Peak:  Relative annual Chlorophyll a levels – an index of phytoplankton abundance in the watershed (results for net biological production, not shown here, are similar to this), showing how plant biomass and therefore plankton productivity peaks at the the land-water interface ecosystem – the Muskegon Lake estuary.

Such findings are useful to students, researchers and policy makers.  Students can use this information in classes to study the systematic and seasonal changes within and across a gradient of ecosystems and make comparisons to other ecosystems.  Researchers can use this data as a baseline in comparison to similar systems in other coastal areas, and to track relative changes through time.  Policy makers can use such information to formulate regulations regarding land-use development and nutrient use in the watershed and to combat emerging issues such as eutrophication, bottom water hypoxia, harmful algal blooms and climate change mitigation.

In the language of Astronomy, the “circumstellar habitable zone”, or simply the “habitable zone” is the region around the star within which planetary objects with adequate atmospheric pressure can support the presence of liquid water at their surface.  The “habitable zone” is also refered to as the “Goldilocks Zone”, a metaphore for the childrens’s fairy tale of Goldilocks and the Three Bears, in which a little girl chooses from sets of three items, ignoring the ones that are too extreme (such as too hot or too cold, too large or too small, etc.), settling on the one in the middle – which is “just right”.  Earth, the 3rd planet from the Sun, is fortunately located in this “Goldilocks Zone” where conditions are “just right” to sustain life.  At the watershed scale, it appears land-margin lakes and estuaries straddling riversheds and receiving lakeshed/seasheds act as hotspots of intense net biological productivity.  Such “Goldilocks Lakes” occurring at the land’s edge deserve greater study and conservation focus due to their value not only as biogeochemical hotspots but also for their potential role as sentinels of change in the world’s watersheds undrgoing major transformations during the current Anthropocene.

This research effort was supported by a NASA Michigan Space Grant Consortium Seed Grant and an EPA Great lake Restoration Initiative Grant to BB, and a MSGC Graduate Fellowship and a GVSU Presidential Research Grant to DD.

Reference:

Dila, D.K., Biddanda, B.A. (2015).  From land to lake: Contrasting microbial processes across a Great Lakes gradient of organic carbon and inorganic nutrient inventories. J. Great Lakes Research. Advance Open Access published on August 21, 2015. Article