Freshwater Abiotic Factors and Limnology Vocabulary

Administrative Announcements and Field Work Logistics

• There are six choices provided throughout the semester for coursework and participation.

• The first quiz is upcoming. It is a low-stakes assessment worth $5$ points, intended to incentivize students to keep up with the course material throughout the semester. The quiz may be assigned as a take-home or an in-class assessment.

• Fieldwork requires appropriate attire for safety and comfort. This includes closed-toed shoes with good grip, as students will be visiting streams. Recommended items include water, sunscreen, and bug spray. Some activities may involve getting into the water.

• Students will be assigned to "Stream Teams" consisting of four individuals for fieldwork projects.

• Carpooling for off-site trips will be organized in advance to ensure all students have transportation.

Fundamental Chemical Properties of Water

• The chemical formula of water is $H_2O$, consisting of two hydrogen atoms and one oxygen atom.

• According to the periodic table, oxygen is grouped in a series with sulfur ($S$), selenium ($Se$), and tellurium ($Te$). Ordinarily, elements in the same vertical series exhibit very similar chemical properties based on their shared valence electron patterns.

• Oxygen is considered a "weirdo" or an exception to the patterns of the periodic table. Based on its position, water should behave like other hydrides (compounds linked to hydrogen), but it does not.

• Comparison of Hydrides:

  • Hydrogen Sulfide ($H_2S$): Melting point is $-85.5\,^{\circ}C$; boiling point is $-60.7\,^{\circ}C$.

  • Hydrogen Selenide ($H_2Se$): Similar to $H_2S$, it is a gas at room temperature.

  • Hydrogen Telluride ($H_2Te$): Also a gas at room temperature.

• At room temperature ($19\,^{\circ}C$ to $\approx 21\,^{\circ}C$ or $70\,^{\circ}F$), these substances are gases because the ambient temperature is well above their boiling points.

• Predicted vs. Actual Values for Water: Based on periodic trends, water was expected to have a melting point of $-100\,^{\circ}C$ and a boiling point of $-80\,^{\circ}C$, meaning it should be a gas under almost all natural conditions on Earth. Instead, its melting point is $0\,^{\circ}C$ ($32\,^{\circ}F$) and its boiling point is $100\,^{\circ}C$ ($212\,^{\circ}F$). This uniqueness allows water to support life by remaining liquid in most terrestrial environments.

Specific Heat Capacity and Thermal Stability

• Definition: Specific heat capacity is the amount of energy required to raise the temperature of one gram of a substance by one degree Celsius ($1\,^{\circ}C$).

• Water has an exceptionally high specific heat capacity, meaning it requires a significant input of energy to change its temperature.

• Biological Significance: This property makes water a very stable environment for organisms. While air temperature may fluctuate wildly within a single day, water temperatures change much more slowly. This stability reduces the need for extreme physiological adaptations (like thick fat layers or specialized enzymes) for organisms living in aquatic systems.

• Practical Example: During extreme heat, people swim to cool off. Even though the water is exposed to the same solar energy as the air, it remains cooler because it takes much more energy to raise the temperature of the water than the air.

Polarity, Solubility, and Surface Tension

• Water is a polar molecule, meaning it shares electrons between atoms through covalent bonds, but the sharing is unequal.

• Electronegativity: Oxygen is a larger atom and more electronegative than hydrogen. In the "tug of war" for electrons, oxygen wins, pulling the negatively charged electrons closer to itself. This results in the oxygen side having a partial negative charge and the hydrogen side having a partial positive charge.

• Hydrogen Bonding: Because opposites attract, the partial positive hydrogen of one water molecule is attracted to the partial negative oxygen of another, creating weak interactions called dipole-dipole interactions or hydrogen bonds.

• Solvency: Water is an excellent solvent, particularly for polar solutes. It dissolves substances by surrounding polar molecules. This is why water is used to clean spills rather than non-polar substances like oils or milk.

• Adhesion and Cohesion:

  • Cohesion: Water is attracted to itself (it is "sticky"). Droplets placed near each other will merge into one because of this attraction.

  • Adhesion: Water is attracted to other polar surfaces. For example, water clings to human skin after a shower because the skin is polar; it must be physically rubbed off with a towel.

• Surface Tension: Because water is polar and air is non-polar, they repel each other. This creates a physical "shield" at the water's surface. Small organisms like water striders can walk on water because their weight is not sufficient to break this surface tension. For larger mammals, surface tension is negligible, but for the majority of life on the planet (small insects and microbes), it is a major physical force.

• Biological Adaptation: Mosquito larvae utilize "butt snorkels" to break the surface tension and breathe air while remaining submerged.

Thermal Density and the Anomaly of Ice

• General Rule of Matter: Usually, as molecules lose energy (get colder), they slow down and condense, becoming more dense.

• The Molecular Analogy:

  • Solid (Ice): Like people in deep, slow, long-lasting conversations. Highly organized and rigid.

  • Liquid: Like people at a party, schmoozing and mingling, moving around but staying close.

  • Gas: Like a mosh pit. Molecules are flying around, bouncing off walls and each other with high energy and no organization.

• Water’s Unique Density: Water increases in density as it cools, but only until it reaches approximately $4\,^{\circ}C$ ($3.94\,^{\circ}C$). Below this point, the density begins to decrease.

• Crystallization: As water freezes ($0\,^{\circ}C$ or $32\,^{\circ}F$), it forms rigid crystals. This crystallization forces the molecules into an organized lattice that actually creates tiny pockets of space, making ice less dense than liquid water.

• Insulation and Survival: Because ice is less dense, it floats. In a lake, the floating ice acts as an insulator, sealing in the heat of the liquid water below and preventing the entire water column from freezing solid. This allows fish and other organisms to survive through winter.

Lake Stratification and Mixing Regimes

• When a lake is the same temperature from top to bottom, it is called isothermal ("iso" meaning equal). This usually occurs at $4\,^{\circ}C$ (maximum density), making the water easy to mix via wind energy.

• Thermal Stratification (Summer): In summer, surface waters warm and become less dense. The water column separates into three distinct layers:

  1. Epilimnion: The warm, less dense surface layer, frequently mixed by wind.

  2. Metalimnion: The middle layer characterized by a sharp decrease in temperature and a sharp increase in density.

  3. Hypolimnion: The bottom layer of cold, dense water.

• Thermocline: The specific depth within the metalimnion where the rate of temperature change is greatest (the inflection point). It acts as a physical barrier to the transport of solutes (nutrients) and oxygen between the surface and the bottom.

• Inverse Stratification (Winter): In winter, the coldest water ($0\,^{\circ}C$) floats on top of the slightly warmer, denser water ($4\,^{\circ}C$).

• Lake Mixing Types:

  • Amictic: Lakes that never mix, typically found in polar regions where they are perennially covered by ice.

  • Polymictic: Shallow lakes (usually < 3\,m or $10\,ft$ deep) that mix constantly because they lack the depth to maintain a stable thermocline.

  • Dimictic: Lakes that mix twice a year (spring and fall). Common in temperate regions.

  • Monomictic: Lakes that mix once a year.

  • Cold Monomictic: Mix during the summer; found in high latitudes (e.g., Minnesota, Wisconsin).

  • Warm Monomictic: Mix during the winter; found in more southern temperate regions (e.g., Dalton, Georgia).

  • Meromictic: Rare lakes that do not mix due to chemical density gradients rather than temperature.

  • Example: Green Lake in Fayetteville, NY. Groundwater pumps in salts/ions (magnesium chloride, etc.), making the bottom layer (monimolimnion) so salty and dense that it never interacts with the upper layers.

Physics of Movement in Water

• All organisms live in a fluid medium (air or water).

• Viscosity: The resistance of a fluid to a change in form; essentially its "thickness." Water is much more viscous than air.

• Inertia: The resistance of a body to a change in its state of motion.

• Reynolds Number: A concept used to describe how organisms navigate these forces.

• Practical Comparison: A punch thrown in the air has less resistance and travels faster than a punch thrown underwater because the water’s viscosity slows the movement. For large animals, inertia is the primary force, but for microscopic organisms, viscosity is the dominant force they must overcome to move.