Affects of Temperature on Density. As warm water cools it becomes more dense. The max density for H2O occurs at 4⁰C. As the water continues to cool it actually becomes lighter. The body of water forms a thin ice layer on top and will build from there. If water acted like most other liquids the density would continue to increase as the temperature decreased, causing the body of water to freeze from the bottom up!
Overturn of a body of water Overturn of the water is mostly fueled by the temperature/density relationship As the temp drops, the stratification that separated the water breaks down The cooler temp causes the surface water to become slightly denser and sink until it reaches water of the same density. As the season progresses this process repeats until the density and temp of the body of water is the identical from top to bottom.
Overturn Once the temp and density throughout the lake are even, overturn can occur As much as a gentle wind running across the surface of the lake can create this top-to-bottom turnover, causing a complete circulation of nutrients and oxygen. The lake is now saturated with oxygen, plus the colder temperature gives the water to hold more oxygen. (oxygen in water @ 5 ⁰C = 12.37 ppm) vs. (oxygen in water @ 20 ⁰C = 8.84 ppm) The oxygen is a finite supply under the ice.
Winter for Aquatic Life Since the lake is @ max density, the colder temp causes the surface water to become lighter This is a reverse stratification effect A skim of ice forms over the top of the water This is the official start of winter in the aquatic world. The thickening ice cover is like a cap on the underwater ecosystem Light levels diminish with the build up snow on top of the ice, the exchange of gasses between the lake and atmosphere are shut off, and the cycles of aquatic life shift to winter mode
Life Under the Ice The productivity of phytoplankton declines, and minute zooplankton drift freely without the difference in water densities Some amplify their vertical migrations in the water column, responding to light cues Others move their daily excursions into the upper waters
Winter Time Woes Slowly continuing respiration, aquatic organism’s (plant and animal) oxygen availability gradually diminish Waste products are not recycled as quickly; carbon dioxide, ammonia, and hydrogen sulfide levels gradually increase Less tolerant fish migrate to inlets or drift to the upper waters Temperature levels below the ice usually reside between 4 ⁰C and the freezing point, but not below the freezing point The strain of a long northern winter will take a toll on some of the aquatic life
Freezing Around the Edges In times of little snow cover and extreme cold, the surface ice can extend well into the shorelines, stream bank sediments, also bottom mud Under these circumstances benthic (bottom dwelling) invertebrates will likely experience sub-freezing temps In the streambed invertebrates may experience temps a fraction of a degree below zero, getting trapped in the anchor ice Invertebrates in shallower sediments could experience temps several degrees below zero, these benthic fauna must migrate or develop some type of freeze tolerance
Aquatic Insects A number of aquatic insects do regularly withstand subfreezing temps Some insects have the ability to undergo supercooling to the same temps as freeze- tolerant terrestrial insects Stoneflies, caddis flies, and mayflies are freeze-tolerant insects, whose supercooling points range from -3 ⁰C to -7 ⁰C ( although most try to avoid freezing) The length exposed to the subfreezing temperatures is critical to the survival of these insects
Fish Under Ice Water temps often fall below -1.9 ⁰C in high lattitudes, the freezing point of a normal fish is -0.8 ⁰C, this could be a problem. Marine fishes of cold regions have to prepare for these conditions, they have two options: 1) Migrate to warmer waters or go to deeper water, where existence in a super cooled state is possible because of no ice 2) Develop freezing resistance in ice laden shallow waters
Freeze Resistance in Fish A large number of polar and north-temperate fishes acquire some degree of freeze resistance They produce antifreeze molecules that depress their freezing point to just below the freezing point of sea-water High concentrations of NaCl account for most of the depression, although other salts and metabolites ( K, Ca, amino acids, glucose, and urea) do help the cause NaCl dissociates in water into separate Na and Cl ions, each interacting w/ H2O molecules. Some coldwater fish produce antifreeze proteins (peptides+ glycopeptides) that are very effective against freezing
Compensating for the Cold All aquatic invertebrates and fish are poikilotherms Zero degree temps pose two serious problems: 1. Fluidity of membrane proteins and lipids becomes greatly reduced, impairing membrane functions 2. Rates of chemical reactions and the equilibria between formation and breakdown of reaction products are strongly temp dependent For every 10 ⁰C drop in temp, the chemically meditated metabolic rate of the poikilotherm could be expected to decrease by a factor of two or slightly more. Some aquatic organisms go into a “semihibernation” in which they are slow breathing and lethargic, but still respond to physical stimuli
Compensating for the Cold Not all poikilotherms show a depressed metabolic rate, some have the ability to adjust their metabolic rate over time as the acclimatized to the cold water A winter acclimatized organism can maintain the same activity levels as it did in much higher temperatures For example a salamander that is used to 15 ⁰C, experiences a sudden drop in temp, and its metabolic rate will fall, but as the colder temp remains, its metabolic rate will soon be back to where it was @ 15 ⁰C This has been seen in newts, mollusks, crayfish, and sand crabs
Oxygen Depletion! All aquatic organisms utilize oxygen throughout the winter Mud-dwelling decomposers are the most numerous and heaviest users of oxygen, causing depletion of O at the bottom of the lake first Oxygen slowly diffuses from upper waters It is not uncommon to see a near absence of oxygen at the bottom of the lake come midwinter Fish move toward the tops of the lake, less tolerant organisms may die More tolerant organisms can switch to an anaerobic state using glycolysis to get by
Aquatic Plants Known for causing stress on oxygen levels while using it to complete photosynthesis at low light and temperature levels. Minute plants such as motile algae, remain suspended and stay photosynthetically active through winter Elodea canadensis is a submerged aquatic plant common in northern lakes This plant has been known to have a pay back of 75% the oxygen used in late winter.
Life Under the Ice When it comes down to it,surviving under the ice is pretty much a game of endurance, especially for a long winter.