Profile of Monomixis and Meromixis
Mixing regimes of Mono Lake
Click on footnotes -- 1 -- to see the notes at the bottom of the profiles. Click on words in italics to see the definition in the glossary.
The aquatic ecology and limnology of Mono Lake are very closely tied together, and it is difficult to discuss one without discussing the other. Since this profile is focusing on meromixis, the discussion will be limited as much as possible to the limnology (physical cycles), with minimal discussion of the ecology (biological cycles).
The mixing regime of a lake is referred to as either dimixis (mixing twice a year, in spring and autumn), monomixis (mixing once a year, from November through February), or meromixis (not mixing). Most temperate lakes are dimictic, but the denser water of Mono Lake leads to a monomictic heating cycle. Mono Lake does not experience dimixis.1 In heavier runoff years, freshwater mixes with the saline surface waters of Mono Lake and produces meromixis, a condition in which lighter less saline water overlies heavier more saline water throughout the year.
Monomixis is Mono Lake's normal mixing regime. Beginning in March, Mono Lake's water column begins stratifying into a warmer, lighter, upper layer, known as the epilimnion, and a colder, darker, lower layer, known as the hypolimnion. The temperature gradient between these layers is known as the thermocline.2 Once the lake is thermally stratified in the spring, mixing occurs only in the epilimnion, and high algae production depletes the nutrients there.
Plants such as algae need sunlight, water, and nutrients -- mainly phosphorous and nitrogen. Phosphorous is abundant because it has accumulated in Mono Lake for thousands of years. Nitrogen, however, is in low supply because nitrogen-fixing algae, present in less saline lakes, cannot tolerate Mono Lake water.3
During the summer, settling of Brine Shrimp cysts and fecal pellets and sinking algae removes nitrogen from the epilimnion (top of lake) and enriches the hypolimnion (bottom of lake). Decomposition in the latter depletes the dissolved oxygen, creating anoxic conditions unfit for Brine Shrimp.4
In autumn, the Brine Shrimp population declines, algal biomass increases, and algae becomes nitrogen limited. As the surface mixed layer deepens, ammonium nitrogen from decay of algae and Brine Shrimp fecal pellets in the hypolimnion becomes the principal source of nitrogen.5
In November, when the water is about 9° C (48° F), complete mixing of the lake, or holomixis, occurs. The hypolimnion (bottom) is reoxygenated and resupplies nutrients to the epilimnion (top). The influx of nutrients, particularly ammonium nitrogen, leads to a winter-spring algal bloom.6 As long as a circulation pattern occurs consisting of one period of complete mixing (in winter) and one period of thermal stratification (in summer), Mono Lake is monomictic.7
In years with large freshwater inflows, Mono Lake sometimes becomes meromictic. A fresher water layer, or mixolimnion (still twice as saline as the ocean), floats on top of the more saline water layer, or monimolimnion. The chemical gradient between these layers is the chemocline. The usual autumn mixing only occurs in the mixolimnion (upper layer), allowing this condition to persist. The first time it was recorded was in 1983, and it lasted, thanks to another wet year (1986), until 1988.8 Meromixis occurred again in 1995, although not quite as pronounced as it was in 1983.9 It is uncertain how long it will last (2003 update: nearly complete mixing!).
The lake surface west of Paoha Island froze in the winter of 1982-83, an unusual occurrence for a saline lake.10 This, contrary to the common misconception, had nothing to do with meromixis. "There are almost always periods during the winter in which a significant layer of ice occurs in the western basin," according to Dr. Jellison, UCSB Researcher. "This occurs whenever you have a slight winter thaw and calm wind followed by colder temperatures."
If additional freshwater inflows do not occur, concentration of salts due to evaporation will cause the mixolimnion (upper layer) to become more saline, the chemical gradient will lessen, and meromixis will slowly break down. Ammonium will also become more concentrated in the monimolimnion (lower layer), and as meromixis weakens, the amount of nitrogen infused into the mixolimnion (upper layer) will be increased, allowing more primary production (algal growth).11 Eventually the usual holomixis (complete mixing) will occur in autumn, causing the lake to become monomictic again. There are lakes, however, that are permanently meromictic.12 Big Soda Lake, in Nevada, a lake very similar to Mono Lake but much smaller, has been meromictic for 90 years.13
Meromixis is less likely at higher lake levels because the volume of freshwater inflow is a smaller percentage of the volume of the lake, thus having less of an impact on the change in salinity and depth of stratification. Therefore it probably occurred much less frequently before 1941.14 Jellison and Dr. Dean Blinn (Univ. Arizona) have found evidence from the diatom record in Mono Lakes sediment of two additional periods of meromixis in the past 170 years, each probably lasting less than 7 years and centered around 1910 and 1930.
After meromixis broke up in late 1988, the early spring 1989 oxygen levels and temperatures were low, and concentrations of toxic compounds such as hydrogen sulfide and ammonia may have been high. As a possible consequence, the spring 1989 brine shrimp population was low.15
The productivity of Mono Lake's aquatic ecosystem is strongly influenced by variables such as temperature, salinity, circulation patterns, and freshwater inflow. Therefore, when meromixis replaces the normal monomixis, it affects the aquatic ecosystem in as yet not fully understood ways. It probably has little effect on the littoral zone, or nearshore areas, and thus little effect on alkali fly productivity.16 However, in the pelagic zone, or deeper waters, the seasonal mixing regime is altered, therefore affecting productivity of algae there. But effects of meromixis on algal production don't necessarily affect the brine shrimp population.17 The long term effect of meromixis on Mono Lake's aquatic productivity is uncertain.
Dr. Bob Jellison produced a model of meromixis in Mono Lake that initially showed over 40 years of meromixis, then as more was learned, in 1999 showed that the lake would be meromictic for "10 years and possibly longer," and that at the 6,392 management level, meromixis will reestablish approximately every decade.18 In June 2001 Jellison said the model now showed Mono Lake was likely to be meromictic for less than 10 years. In 2000 only 1/3 of the lake area and 15% of the lake volume was below the chemocline, and primary productivity was back at 1994 levels. 25% of the freshening of the hypolimnion is believed to be due to spring inputs around Paoha Island.19 Click here for a Sept. 2002 Update. In the spring of 2003, mixing was nearly complete and primary productivity reached record levels. Studies on this topic continue, and as new information is discovered it will be presented here. Click here for a diagram of mixing, and a chart of chemical stratification since 1994 (553K Powerpoint).
(1)Fax from Dr. Jellison, UCSB
Researcher, February 1996
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