Seasonal Shifts: Lake Stratification Quiz

If a scientist specializes in the ecosystems of lakes and rivers, then they are studying freshwater systems; if the specific field for this study is named, then it is called limnology.

If water freezes, then its molecules form a lattice that takes up more space; if it takes up more space for the same mass, then it is less dense; if it is less dense than the liquid water below, then it must float.

If the sun heats the surface, then the top layer (epilimnion) is the warmest; if cold water is denser, then it stays at the bottom (hypolimnion); therefore, the profile shows a decrease in temperature as depth increases.

If the lake is layered, then the bottom water cannot touch the air; if bacteria at the bottom consume oxygen while decomposing matter, then the oxygen is not replaced; therefore, the hypolimnion becomes oxygen-depleted.

If the sun adds thermal energy (spring) or the air removes it (fall), then the water's temperature changes; if temperature changes lead to density changes, then gravity causes the water to move; therefore, thermal energy transfer is the driver of the physical movement.

If the temperature of water changes, then its density also changes; if water of different densities meets, then the denser water sinks below the less dense water; therefore, density differences create the distinct layers in a lake.

If water cools from room temperature, then its molecules move closer together and it becomes denser; if it reaches 4∘C, then it reaches maximum density; if it cools further toward 0∘C, then it becomes less dense again; therefore, the statement is true.

If a lake is heated by the sun, then the top layer becomes warm and less dense; if this layer floats on top of the cooler layers, then it is called the epilimnion; therefore, the epilimnion is the correct term for the upper layer.

If you move from the warm top layer to the cold bottom layer, then you pass through a zone of transition; if this zone is defined by a steep temperature gradient, then it is known as the thermocline.

If the air temperature drops in autumn, then the lake's surface water loses heat; if the surface water cools to 4∘C, then it becomes denser than the water below it; if it is denser, then it sinks and forces the bottom water up, causing turnover.

If a lake is stratified, then oxygen at the surface cannot reach the bottom; if turnover occurs, then the oxygen-rich surface water mixes with the bottom water; if the bottom water receives this oxygen, then deep-dwelling fish can survive.

If the air temperature changes, then the surface water temperature changes; if the water is deep enough to stratify, then there are layers to mix; if density changes cause sinking, then turnover occurs; therefore, temperature, depth, and density are necessary.

If water molecules are cooled, then they pack tighter until they reach a specific point; if they cool below 4∘C, then they begin to form a crystalline structure that occupies more space; therefore, 4∘C is the temperature of peak density.

If water is cold, then it is denser; if it is the densest water in the lake during summer, then it stays at the bottom; if the bottom layer is the coldest and densest, then it is called the hypolimnion.

If ice forms on the surface, then the water touching it must be at the freezing point; if the freezing point of water is 0∘C, then the water directly beneath the ice is at that temperature.

If a lake mixes its water twice a year (spring and fall), then it is following a specific cycle; if "di-" means two and "mictic" refers to mixing, then the term for this two-cycle mixing is dimictic circulation.

If the top layer is very warm and the bottom is very cold, then there is a large density gap; if the top layer is much lighter than the bottom, then it floats effectively; therefore, the density "barrier" prevents mixing without a significant energy input.

If surface water sinks, then it carries oxygen down; if bottom water rises, then it carries decomposed nutrients and CO2​ up; if these are redistributed, then the whole lake is replenished; therefore, oxygen, nutrients, and CO2​ are the correct choices.

If the sun warms the ice and the cold 0∘C water, then the temperature rises; if the water reaches 4∘C, then it becomes denser than the 1−3∘C water below it; if it is denser, then it sinks and initiates spring turnover.

If the entire volume of water in the lake is circulating, then it moves material from the bottom; if nutrients and sediments are lifted from the lake bed, then the water becomes murky; therefore, the statement is true.