Lecture 11 - Physicochemical Parameters of Streams and Rivers

Overview of Physicochemical Parameters in Streams and Rivers

• Longitudinal Changes in Water Quality:     * Rivers change significantly from their source to the sea.     * Blue Springs (Putararu): Exemplifies incredibly clean, clear water with a striking blue color near the source.     * Waihou River (near Thames): The main river downstream looks very different compared to the source due to materials picked up along its progression.

Classification of Materials Transported in River Water

• Main Categories:     * Dissolved vs. Suspended:         * Amazon River: Drains from the Andes Mountains; carries a very high suspended sediment load of fine rock material ground down by glaciers, resulting in a "chocolate milk" consistency.         * Rio Negro: A lowland system heavily stained by tannins from organic matter (leaves), resulting in "black water" similar to tea. This represents high dissolved organic carbon.     * Organic vs. Inorganic:         * Inorganic: Rocks, minerals, and metals.         * Organic: Tannins, biofilms, and decaying matter.

• Specific Measuring Constituents:     * Suspended inorganic matter.     * Suspended and dissolved organic matter ($DOM$).     * Dissolved Major Ions: Calcium ($Ca^{2+}$), Sodium ($Na^+$), Bicarbonate ($HCO_3^-$), and Sulfates or Sulfites ($SO_4^{2-}$ or $SO_3^{2-}$).     * Dissolved Nutrients: Nitrogen ($N$), Phosphorus ($P$), and Silicon/Silica ($Si$).     * Metals: Both dissolved and suspended; particularly important in urban or mining-impacted areas.     * Gases: Dissolved oxygen ($O_2$) and Carbon Dioxide ($CO_2$).

Dissolved Gases: Oxygen and Carbon Dioxide

• Equilibration and Solubility Drivers:     * Gases move between the atmosphere and water to reach equilibrium.     * Altitude: As the atmosphere thins at higher elevations, the partial pressure decreases, reducing the water's capacity to hold gases.     * Temperature: Solubility of oxygen decreases as water temperature increases (inducing outgassing).     * Salinity: Higher salinity (e.g., increased concentrations of $Na^+$ and $Cl^-$) reduces oxygen solubility due to chemical effects with ions.

• Measurement of Dissolved Oxygen ($DO$):     * Concentration: Measured in $\text{mg/L}$.     * Percentage Saturation: Relates the concentration to the maximum possible $DO$ at a specific temperature. "Supersaturated" refers to values over $100\%$, typically driven by high biological production.

• Abiotic Controls and Channel Morphology:     * Turbulent Flow: Mountain rivers with large boulders and high turbulence have high rates of aeration/diffusion, leading to high $DO$ and low $CO_2$.     * Slow-Moving Flow: Turbid, slow rivers (e.g., the Waipa River) have limited re-aeration and high rates of respiration, leading to low $DO$ and high $CO_2$.     * Groundwater Infiltration: Groundwater is typically low in $DO$ and high in $CO_2$ due to microbial processing of organic matter in the soil. Upon reaching the surface, it equilibrates with the atmosphere quickly.

• Biological Processes (The "Heartbeat of the River"):     * Photosynthesis: Plants, algae, and biofilms produce oxygen and carbohydrates using light:         * $CO_2 + H_2O + ext{light} \rightarrow ext{carbohydrates} + O_2$     * Respiration: All organisms (plants, bacteria, etc.) consume oxygen and produce $CO_2$.     * Diurnal Cycles: $DO$ rises during the day due to photosynthesis and falls at night when only respiration occurs.

• Ecological Status and Standards:     * Oxic/Aerobic: Water with sufficient oxygen.     * Anoxic/Anaerobic: Water with very low oxygen levels.     * National Policy Statement (NPS) for Freshwater Management: Sets a national bottom line for $DO$ at $5\,\text{mg/L}$ (seven-day mean during summer).     * Lethal and Stress Thresholds:         * $\ge 7\,\text{mg/L}$: Comfortable for most fish.         * $< 5\,\text{mg/L}$: Induces stress effects and impairs functioning (feeding, etc.).         * $\le 3\,\text{mg/L}$: Lethal for sensitive species like brown trout.

Dissolved Ions and Water Hardness

• Dominant Ions: Eight ions dominate the ionic composition of freshwaters:     * Cations (+): Sodium ($Na^+$), Potassium ($K^+$), Calcium ($Ca^{2+}$), Magnesium ($Mg^{2+}+$).     * Anions (-): Bicarbonate ($HCO_3^-$), Carbonate ($CO_3^{2-}$), Sulfate ($SO_4^{2-}$), Chloride ($Cl^-$).

• Sources of Ions:     * Rainwater: Typically very low in ion concentration. Marine salts ($NaCl$) are more prevalent in coastal rain.     * River Water: Generally $20$ times more concentrated than rainwater. While evaporation accounts for a three-fold increase (based on $30\%$ of precipitation becoming runoff), the remainder comes from rock weathering and human sources (e.g., fertilizer).

• Conductivity:     * Measures electrical conductance to estimate total dissolved ions.     * Specific Conductivity: Compensates for temperature, as conductivity increases as water warms. Measured in microsiemens per centimeter ($\mu\text{S/cm}$).

• Water Hardness and Buffering:     * Hardness: Measured as an equivalent quantity of Calcium Carbonate ($CaCO_3$). Hard water has high concentrations of $Ca^{2+}$ and $Mg^{2+}$.     * New Zealand Context: Waters are generally "soft" due to high water yields (high rainfall) preventing long contact times with rock. Hamilton supply (Waikato River) is approximately $40\,\text{mg/L}$, whereas Waitomo (limestone areas) ranges from $100\text{ to }150\,\text{mg/L}$.     * The Carbonate Buffer: Carbonic acid ($H_2CO_3$) is formed when $CO_2$ dissolves in water, lowering pH. This is buffered by dissolved calcium carbonate reacting with the acid to form bicarbonate, maintaining a circumneutral pH.

Nutrients and the Nitrogen Cycle

• Anthropogenic Impacts: Nutrient runoff is a major concern, particularly from the dairy industry. The number of dairy cattle and nitrogen fertilizer use have increased drastically over recent decades.     * Urine Patches: High nitrogen content in cattle urine often exceeds the grass's uptake capacity, leaching into groundwater and surface streams.

• Forms of Nitrogen:     * Gaseous Nitrogen ($N_2$): Highly abundant in the atmosphere but relatively unavailable for biological uptake without processing.     * Dissolved Inorganic Nitrogen (DIN): Includes Nitrate ($NO_3^-$), Nitrite ($NO_2^-$), and Ammonium ($NH_4^+$).     * Ammonia Toxicity: In alkaline water (high pH), ammonium ($NH_4^+$) converts to toxic ammonia gas ($NH_3$).

• Nitrogen Transformations:     * Ammonification/Mineralization: Decay of organic matter into ammonia ($NH_3$).     * Nitrification: Aerobic oxidation of ammonium to nitrate ($NO_3^-$) via chemoautotrophic bacteria.     * Denitrification: The conversion of nitrate back into $N_2$ gas. This requires anoxic conditions.

• Management and Mitigation:     * Bioreactors: Experiments in Canterbury used stacks of wood-chip bags in channelized streams to create anoxic pockets for denitrification. Effectiveness is limited by the need to balance flow with anoxia.     * Constructed Wetlands: Nature-based solutions that utilize the natural denitrification capacity of wetland soils.

Phosphorus, Stoichiometry, and Spiraling

• Phosphorus ($P$):     * Major limiting nutrient for ecosystem growth. A famous Canadian whole-lake experiment showed that adding phosphorus (along with carbon and nitrogen) caused massive algal blooms compared to lakes without added phosphorus.     * Forms: Soluble Reactive Phosphorus ($SRP$) is the inorganic form. Total Phosphorus ($TP$) = $SRP$ + Particulate Phosphorus + Dissolved Organic Phosphorus.     * Sources: Rock weathering is the primary natural source. In New Zealand's North Island, volcanic geology leads to naturally high background levels of phosphorus.

• Nutrient Spiraling Concept: Nutrients are taken up by organisms, then remobilized (released), all while moving downstream due to gravity. The cycle of uptake and release forms a "spiral."

• Stoichiometry (Redfield Ratio):     * The optimal ratio for growth is $C:N:P$ at approximately $106:16:1$; specifically, an $N:P$ ratio of $16:1$.     * New Zealand streams are often considered phosphorus-limited, particularly in the South Island.

Temperature Regimes and Ecological Impacts

• Drivers of Temperature:     * Latitude: New Zealand shows a latitudinal gradient (warmer in the North, cooler in the South).     * Elevation: Higher altitudes are cooler.     * Local Factors: Topography, channel incision, aspect (orientation to the sun), and riparian shading (planting trees on stream banks helps cool water by blocking solar radiation).     * Groundwater: Provides stable, cool temperature inputs that buffer extremes.

• Physiological Effects:     * Metabolic Theory of Ecology: Temperature is a primary driver of metabolic rates, fitness, and performance.     * "Double Whammy": High temperatures increase the metabolic oxygen demand of organisms while simultaneously reducing the water's oxygen-holding capacity.

• New Zealand Species Thermal Tolerance:     * Sensitive: Common smelt ($Retropinna\,retropinna$) and Banded kokopu ($Galaxias\,fasciatus$) (forest specialists).     * Mid-range: Torrent fish and Common bully.     * Tolerant: Longfin and Shortfin eels ($Anguilla$ spp.); Salmonids (trout/salmon) are Northern Hemisphere imports and are relatively sensitive to high temperatures, showing significant stress at critical thermal maxima.

Questions & Discussion

• Field Trip Logistics:     * Location: Rangatukee Stream.     * Meeting Point (Hamilton): Gate 9, tomorrow. Bring food and water.     * Meeting Point (Tauranga): Durham Lane, by the Mr G mural.     * Methods: Students will practice electrofishing, collecting macroinvertebrates, and measuring physicochemical parameters.     * Timing: Anticipated return around $03:30 \,\text{PM}$.

• Assessments:     * The online test is currently open and closes tonight. It is worth $10\%$ of the final grade.