BIO 206 — Hydrobiology: Lecture Note

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BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Course code: BIO 206 Course title: Hydrobiology Course credit load: 2 Units Series 1: Physical Properties and Dynamics of Water • Part 1.1: Unique Physical and Chemical Properties of Water o Sub Part 1.1.1: Molecular structure and hydrogen bonding o Sub Part 1.1.2: Density, viscosity, and surface tension o Sub Part 1.1.3: Thermal properties and their biological significance • Part 1.2: Light and Sound in Aquatic Environments o Sub Part 1.2.1: Penetration and attenuation of light in water o Sub Part 1.2.2: Photic, aphotic, and compensation zones o Sub Part 1.2.3: Transmission of sound and pressure in water • Part 1.3: Water Movements: Waves and Currents o Sub Part 1.3.1: Waves and their effects on the substratum o Sub Part 1.3.2: Currents in rivers, lakes, and oceans o Sub Part 1.3.3: Biological responses to water movement • Part 1.4: Dissolved Gases and Inorganic Ions in Freshwater o Sub Part 1.4.1: Dissolved oxygen and its biological significance o Sub Part 1.4.2: Carbon dioxide, the carbonate-bicarbonate system, and pH o Sub Part 1.4.3: Major inorganic ions and their roles

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Introduction Water is the medium of life for aquatic organisms and the substrate of the discipline of hydrobiology. Its physical and chemical properties are unique among common substances and are directly responsible for the structure, distribution, and functioning of aquatic ecosystems. Understanding why water behaves as it does provides the foundation for interpreting every aspect of lake, river, estuarine, and marine biology covered in this course. This Series covers the molecular basis of water's anomalous properties, the behaviour of light and sound in aquatic systems, the forces generating waves and currents and their ecological consequences, and the dissolved gases and inorganic ions that determine water chemistry and biological productivity. These physical and chemical parameters recur throughout all subsequent series of this course. Series Learning Outcomes Upon completing this Series, you should be able to: • SLO 1.1 explain how the molecular structure of water gives rise to its unique physical and chemical properties (Linked to Part 1.1), • SLO 1.2 describe the density anomaly of water and its ecological consequences for aquatic organisms (Linked to Part 1.1), • SLO 1.3 explain the penetration and attenuation of light in water and define the photic, aphotic, and compensation zones (Linked to Part 1.2), • SLO 1.4 describe the formation of waves and currents and explain their effects on aquatic substrata and organisms (Linked to Part 1.3), • SLO 1.5 explain the factors controlling dissolved oxygen concentration in freshwater and state its biological significance (Linked to Part 1.4), • SLO 1.6 describe the carbonate-bicarbonate buffering system and explain how it regulates pH in freshwater (Linked to Part 1.4), and • SLO 1.7 name the major inorganic ions in freshwater and describe their ecological roles (Linked to Part 1.4).

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS 1.1 Unique Physical and Chemical Properties of Water 1.1.1 Molecular structure and hydrogen bonding Water (H2O) is a polar molecule: the two hydrogen atoms are bonded to one oxygen atom at an angle of approximately 104.5 degrees. The oxygen atom is strongly electronegative, drawing the shared electrons toward itself and creating a partial negative charge (delta-) on the oxygen and partial positive charges (delta+) on the hydrogens. This polarity enables water molecules to form hydrogen bonds with each other and with other polar or charged molecules: the delta+ hydrogen of one molecule is attracted to the delta- oxygen of a neighbouring molecule. Hydrogen bonds (individually weak, collectively very strong) are responsible for most of water's anomalous properties: its high boiling point (100 degrees Celsius, far higher than predicted for a molecule of its size); high specific heat capacity; high latent heat of vaporisation; high surface tension; and the expansion of water upon freezing. Water is also a universal solvent: its polarity enables it to dissolve ionic compounds (which dissociate into ions surrounded by water molecules) and polar organic molecules. This solvent power underlies the chemical richness of natural waters. Fill in the blank: Water molecules form ________ bonds with each other because the electronegative oxygen atom creates a partial negative charge that attracts the partial positive charge on the hydrogen atoms of neighbouring molecules. 1.1.2 Density, viscosity, and surface tension The density of water reaches its maximum at 4 degrees Celsius (1,000 kg/m^3) and decreases both above and below this temperature. As water cools below 4 degrees Celsius, the hydrogen bond network becomes more ordered and open, causing ice to be less dense than liquid water; this is why ice floats. In lakes, this density anomaly drives seasonal turnover (mixing) and determines the structure of thermal stratification (covered in Series 2). Freshwater is less dense than seawater (approximately 1,025 kg/m^3) due to dissolved salts; the density difference affects stratification at river mouths and estuaries.

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Viscosity is the resistance of a fluid to flow. Water's viscosity decreases with increasing temperature; warm water flows more freely than cold water. Viscosity is ecologically important because it governs the drag experienced by swimming organisms, the sinking rate of particles and plankton, and the delivery of nutrients to cell surfaces. Surface tension arises from the cohesive hydrogen bonds between surface water molecules, which have no neighbours above; the result is a film strong enough to support small organisms (e.g. water striders, Gerris) that exploit the water surface as a habitat. Surface tension also drives capillary rise in soils and plant cells. Fill in the blank: Water reaches its maximum density at ________ degrees Celsius; below this temperature it becomes less dense, causing ice to float on liquid water, which insulates aquatic organisms from freezing in cold climates. 1.1.3 Thermal properties and their biological significance The specific heat capacity of water (4,182 J/kg/K) is exceptionally high compared to other common liquids. It takes a large input of heat energy to raise water temperature and a large loss of energy to lower it; water bodies therefore heat and cool slowly, buffering aquatic organisms against rapid temperature fluctuations. This thermal stability is critical in tropical African lakes such as Lake Chad and the Niger Delta wetlands, where diurnal air temperature swings are large but water temperature remains relatively stable. The high latent heat of vaporisation (2,260 kJ/kg) means that a large amount of energy is absorbed when water evaporates; this is the basis of evaporative cooling in organisms and in climate regulation. The latent heat of fusion (334 kJ/kg) means that freezing or thawing a water body requires the exchange of large amounts of heat, moderating temperature changes at the freezing point. Together, these thermal properties make aquatic habitats thermally stable relative to terrestrial environments, allowing ectothermic aquatic organisms to maintain enzyme function and metabolic activity within a narrow range. Fill in the blank: The high specific heat capacity of water means that a large input of ________ energy is required to raise its temperature; this causes water bodies to heat and cool slowly, buffering aquatic organisms against rapid temperature fluctuations.

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Figure 1.1: Molecular structure of water, the density-temperature relationship, and the comparative specific heat capacity of water Pilot questions 1.1 1. Explain how the polarity of the water molecule gives rise to hydrogen bonding. 2. Explain the density anomaly of water and state two ecological consequences. 3. Explain why viscosity is ecologically important for aquatic organisms. 4. State two ways in which the high specific heat capacity of water benefits aquatic organisms. 5. Name three properties of water that arise directly from hydrogen bonding. 1.2 Light and Sound in Aquatic Environments 1.2.1 Penetration and attenuation of light in water Light is rapidly attenuated (reduced in intensity) as it passes through water by three processes: absorption (conversion of light energy to heat by water molecules and dissolved substances); scattering (redirection of light by particles and molecules); and reflection at the water surface

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS (approximately 2 to 5% of incident light for calm conditions; up to 100% at shallow angles). The combined effect is described by the extinction coefficient (k): intensity at depth z = surface intensity x e^(-kz). A higher k (turbid water) means more rapid light attenuation with depth. Different wavelengths of light are attenuated at different rates. Red and infrared light are absorbed within the first few metres; blue-green light penetrates deepest in clear oceanic water. In turbid or coloured freshwater (stained by dissolved organic matter or suspended sediment), even blue-green light is attenuated quickly. Turbidity is measured using a Secchi disk: a white disk lowered until it disappears; the Secchi depth (typically 0.3 to 30 m depending on water clarity) provides a simple

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