EA winter 2003
Meromixis terminology.
A meromictic lake has some water which partly or wholly unmixed with the main water
mass at circulation periods.
Hutchinson defined meromictic stability as the amount of work needed to mix a
meromictic lake to a uniform concentration if closed or to mix the monimolimnion into
the perennially freshened mixolimnion if open.
Hutchinson also defined 3 categories of meromixis.
Ectogenic meromixis.
This type is due to bringing an external source of saline water into a fresh water lake, or
fresh water into a saline lake. This produces a dense saline layer below a fresh layer.
Mixing at the chemocline will reduce the meromictic stability.
Examples include lakes with an episodic input of marine water and saline lake that
episodically receive fresh water. Soap Lake is probably of this type. Little Goose lake in
eastern Washington is an example. Here fresher, less dense irrigation waste water was
introduced over a saltier, denser, old enclosed saline lake. A well studied lake, Big Soda
Lake in Nevada is of this type. Irrigation water inundated a very saline older isolated
lake. Mono Lake in California has recently turned meromictic by the reintroduction of
fresh water as part of an agreement between the City of Los Angeles and Mono and Inyo
Counties. The following abstract is quite interesting. Note the non-positive effects of
curtailing diversion. But, the second message shows the condition is likely to change.
Jellison, R. Marine Science Institute, jellison@lifesci.ucsb.edu
Melack, J. Bren School of Environmental Science and Management,
melack@lifesci.ucsb.edu
Jehl, J. R.. Hubbs-Sea World Institute, jjehl@hswri.org
ECOLOGICAL CONSEQUENCES OF THE ONSET OF MEROMIXIS DURING
RESTORATION OF MONO LAKE, CALIFORNIA, USA
Hypersaline Mono Lake constitutes a productive ecosystem. High rates of pelagic
primary productivity (ca. 500 g C m-2) support a large brine shrimp population that is a
major food source for a significant portion of North America's breeding California Gulls
(>20%) and migrating Eared Grebes (ca. 75%). Diversions of freshwater streams out of
the Mono Lake Basin since 1941 led to a gradual decrease in size, an approximate
doubling of lake-water salinity, and associated environmental impacts. To partially
restore pre-diversion conditions, the State of California decided to restrict diversions until
the lake rises 6 m above its 1982 low stand. Curtailment of diversions and above average
runoff in the mid-1990s caused a large rise in surface elevation and the onset of persistent
chemical stratification (meromixis). The episode of meromixis is expected to last several
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decades given the current management policy. The absence of a winter period of
holomixis in which nutrients are recycled to the euphotic zone has caused declines in
primary and secondary productivity and altered the temporal pattern of brine shrimp
(Artemia) abundance. This has contributed to low productivity of the gulls and reduced
body weight of grebes.
September 2002 Notes on Meromixis
From Bob Jellison, UCSB Researcher
The secchi depths, a measure of transparency and algal abundance, were about 2 m on
September 17 and the Artemia while declining markedly are still quite abundant. We
won’t have exact numbers for comparison to other years until the samples are counted.
The Secchi is unusually low (phytoplankton abundant) for September and reflects the
breakdown of meromixis and increased nutrient fluxes upward from the bottom where
they have been accumulating the past eight years. Secchi depths have only been this low
in September one other year and that was 1988 when the previous 6-yr period of
meromixis broke down. We projected the evaporative concentration of the surface water
observed thus far this summer to December 1st and the lake will be very close to turning
over this year. If it does not turn over this winter, it certainly will next year. Even if it
does not completely mix this year, we can expect next year to have very high levels of
primary productivity as nutrient concentrations increase throughout the water column due
to the entrainment of nutrient-rich monimolimnetic water.
Crenogenic meromixis.
This type is due to saline springs delivering denser water to a fresher water lake. Lakes
Nyos and Monoun in Cameroon are examples. This type might include examples like Hot
Lake where salts go into solution from the bottom.
Biogenic meromixis.
Here dissolved and particulate material accumulates in the monimolimnion.
Decomposition can add dissolved material over time, especially from the sediments. Lake
Mary in Wisconsin, Fayetteville Green Lake in New York, and Hall Lake in Snohomish
County are excellent examples. All are in glacial drift. I think Ward Lake and the eyelet
pond at Kings Lake Bog might be meromictic and of the biogenic type. These lakes
usually have very meager stability, so they might mix occasionally to frequently.
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