Notes on CO2 in Water, Temperature Effects, and Turbidity

Carbon Dioxide in Water: Sources, Levels, and Effects

• Carbon dioxide (CO₂) is a clear, colourless, odourless, tasteless gas. It is the “fizz” in soda pop.

• Purpose of discussion: how CO₂ enters water and what it does to aquatic organisms.

Carbon Dioxide from the Atmosphere

• Air composition: CO₂ is about 0.034% by volume.

• If air were the only source of CO₂, very little would dissolve in water.

• Solubility at 0°C: CO₂ solubility in water is approximately
  $ext{solubility} \approx 1\,\mu g/g.$
  Thus, a fast clean stream would likely contain no more than about $1\ \mu g/g$ of CO₂ when air is the only source.

Carbon Dioxide from Respiration

• In most natural waters, respiration by living organisms is the major CO₂ source, especially in lakes and slow or polluted rivers.

• Surface water CO₂: up to about $10\ \mu g/g$; bottom ooze can have more due to decomposers respiring CO₂.

• Harmful levels to gill-breathers:

  • Water containing > $25\ \mu g/g$ CO₂ is harmful to most gill-breathers.

  • Concentrations of $50-60\ \mu g/g$ can kill many species.

CO₂ from Rain and Groundwater

• Rain dissolves CO₂ from the air; typical rain CO₂ dissolution is up to about $0.6\ \mu g/g$.

• When rain infiltrates soil, CO₂ from soil respiration (micro-organisms, decomposers) raises the CO₂ level in groundwater.

• Groundwater discharging into a lake can raise the lake’s CO₂ concentration.

Factors Removing CO₂

• Atmospheric exchange (gas exchange with air): fast-moving rivers lose CO₂ to the atmosphere.

  • Example: a marsh draining into a river could have CO₂ up to $30\ \mu g/g$ at 20°C, but a fast river with no production of CO₂ holds only about $0.5\ \mu g/g$ at 20°C. The excess CO₂ (≈ 29.5 μg/g) is removed in turbulent regions.

• Photosynthesis (in standing waters): photosynthesis removes CO₂, while respiration increases CO₂ at night.

• Overall, daytime photosynthesis reduces CO₂, while nighttime respiration raises CO₂.

Carbon Dioxide, Photosynthesis, and Dissolved Oxygen

• CO₂ quickly combines with water to form carbonic acid, a weak acid:
  $\mathrm{CO<em>2 + H</em>2O \rightleftharpoons H<em>2CO</em>3 \rightleftharpoons H^+ + HCO_3^-}$

• Carbonic acid in waterways can be beneficial or harmful depending on pH and alkalinity:

  • In alkaline (high pH) water, CO₂/H₂CO₃ can neutralize excess base.

  • In already acidic (low pH) water, carbonic acid can worsen acidity.

• Photosynthesis and respiration interplay:

  • Green plants photosynthesize in light, removing CO₂ from the water.

  • At night, respiration dominates, increasing CO₂ and reducing O₂ availability.

• Free CO₂ levels: rarely exceed about $20\ \mathrm{mg/L}$ in most waters, which is a level many fish tolerate without severe effects.

• During prolonged cloud cover, photosynthesis is reduced; in ponds with abundant plant life, low O₂ and high CO₂ can stress fish.

• When CO₂ levels are high and O₂ is low, fish respiration is compromised, and the problem worsens as temperature rises.

CO₂ and Fish Health: Thresholds and Effects

• Table (summary) of CO₂ effects on fish (CO₂ in mg/L):

  • $1.0-6.0$ mg/L: Fish avoid these waters.

  • $12$ mg/L: (high CO₂ levels begin to affect some species; exact effects vary by species).

  • $30$ mg/L: (further adverse effects; specific impacts depend on species).

  • $45$ mg/L: Trout eggs may fail to hatch.

  • >50 mg/L: Most sensitive fish die immediately.

• Practical implication: even modest increases in CO₂ can stress fish, especially when paired with reduced O₂ or higher temperatures.

Practical and Ecological Implications

• Seasonal and weather effects:

  • Cloudy days reduce photosynthesis, potentially raising CO₂ and lowering O₂ in water bodies with substantial plant life.

• Management relevance:

  • Monitoring CO₂, O₂, pH, and alkalinity is crucial for fish health, especially in ponds, ponds with heavy plant cover, and waters influenced by respiration and decomposition.

Carbon Dioxide, Photosynthesis, and Respiration: Detailed Concepts

• Photosynthesis (in light, with chlorophyll):

  • General formula in the lecture notes:
    $\text{CO}<em>2 + \text{H}</em>2O \rightarrow \text{O}<em>2 + \text{C}</em>6\text{H}<em>{12}\text{O}</em>6$

  • Green plants perform photosynthesis to produce oxygen and carbon-rich foods.

• Respiration (by plants, animals, bacteria):

  • General formula in the lecture notes:
    $\text{C}<em>6\text{H}</em>{12}\text{O}<em>6 + \text{O}</em>2 \rightarrow \text{CO}<em>2 + \text{H}</em>2\text{O}$

  • All animals and many bacteria respire, releasing CO₂.

• Night/day cycle:

  • At night, plants respire and burn stored carbohydrates, increasing CO₂ and consuming O₂.

  • At day, photosynthesis dominates, removing CO₂ and releasing O₂.

• Oxygen-CO₂ balance and fish health:

  • When CO₂ is high and O₂ is low, fish respiration is hindered.

  • Small CO₂ increases can affect fish, and higher CO₂ with higher temperatures worsens the problem.

• Practical note: CO₂ is a driver of water chemistry, influencing pH via carbonic acid formation, and interacts with alkalinity to determine buffering capacity.

Temperature and Aquatic Life: Factors, Behavior, and Growth

Temperature as a Key Environmental Factor

• Temperature influences metabolism: cold-blooded (ectothermic) organisms have metabolic rates that rise with temperature.

• Optimum temperature: each species has its own optimum; deviations can reduce growth, reproduction, or survival.

• Critical limits:

  • Cold temperatures can be lethal (some organisms cannot survive below 0°C / 32°F).

  • Very warm temperatures can be lethal for many species; rough tolerance exists (e.g., carp tolerating somewhat warmer water than many salmonids).

• Temperature and gas exchange:

  • Warmer water holds less dissolved oxygen (DO); high temperature + low DO can be stressful or lethal.

• Behavioral responses:

  • Fish migrate to zones with temperatures closer to their preferred range (e.g., seeking warmer water during fall/winter and cooler water in summer).

  • Small temperature differences (as little as 1–3°C) can cause movement between habitat patches.

• Temperature-driven ecological processes:

  • Fish migration and spawning runs are often cued by rising spring temperatures and autumn cooling.

  • Diatoms, green algae, and blue-green algae have their own temperature preferences that influence community composition.

• Algal temperature preferences (summarized):

  • Diatoms: best at ≈ 15–25°C

  • Green algae: ≈ 25–35°C

  • Blue-green algae: ≈ 30–40°C

• Toxicity interactions:

  • Warm water can increase the toxicity of certain substances (e.g., cyanides, phenol, xylene, zinc), especially when DO is low.

Temperature Effects on Fish and Aquatic Life: Quick References

• Fish are attracted to warm water when seeking food/work or during certain seasons and to cooler water when seeking refuge from heat.

• Fish can sense small temperature differences and move accordingly.

• Temperature shifts influence growth, spawning, and survival across species; table data illustrate species-specific responses (e.g., optimal ranges vs. lethal thresholds).

• Seasonal and regional climate differences shape community structure and population dynamics via temperature effects.

Water Temperature and Fish Behavior: Table Highlights (Overview)

• The notes include a table linking temperature to fish behavior, spawning, embryo growth, and survival for various species. While exact numeric entries vary by species, the core concepts are:

  • Each species has an optimum temperature range for growth and reproduction.

  • Temperatures above certain thresholds cause reduced spawning, slowed growth, or mortality.

  • Some species tolerate higher temperatures better than others; others are fragile to temperature increases.

• Practical takeaway: monitor water temperature and maintain ranges suitable for local species, particularly during sensitive life stages (spawning, egg development).

Temperature Effects on Primary Producers

• Diatoms, green algae, and blue-green algae have distinct temperature preferences, influencing which algal groups dominate at given temperatures:

  • Diatoms: optimal around 15–25°C

  • Green algae: 25–35°C

  • Blue-green algae: 30–40°C

• Water temperature can influence not only algal growth but also the overall oxygen balance in water bodies through photosynthesis rates and DO consumption.

Turbidity: Definition, Measurement, and Ecological Implications

What Is Turbidity?

• Definition (APHA): turbidity is the optical property of a water sample that causes light to be scattered and absorbed rather than transmitted in a straight line through the sample.

• Practical meaning: turbidity is a measure of how cloudy the water is.

• Causes of turbidity:

  • Suspended materials like silt, microorganisms, plant fibers, sawdust, wood ashes, chemicals, and coal dust.

  • Common causes in lakes/rivers: plankton and soil erosion from logging, mining, and dredging.

How to Measure Turbidity

• Primary method: electronic turbidimeter

  • Light source and photoelectric cell measure light scattered by suspended particles.

  • Result expressed in Nephelometric Turbidity Units (NTU or NTUs).

• Alternative method: filter-based comparison

  • Filter water through a white membrane filter and compare the color to a turbidity color chart.

  • Equipment needed: Gelman (or equivalent) filter apparatus, white filters, color chart.

Gelman Filter Apparatus: Procedure (Summary of Steps)

1) Place a white gridded filter on the filter apparatus.
2) Collect a water sample (avoid sediment); use a clean container.
3) Pour 100 mL of sample into the top of the filter apparatus.
4) Filter the sample (may require a hand vacuum pump).
5) Remove the filter and allow it to dry.
6) Estimate turbidity by comparing the filter’s appearance to the provided color chart.
7) Interpret turbidity values using the provided guidelines.

Industrial and Drinking Water Turbidity Standards

• Table (summary of turbidity limits by industrial use):

  • Beverages: 1–2 NTU

  • Food products: 10 NTU

  • Boilers: 1–20 NTU (varies with boiler type)

  • High-grade paper: 5–25 NTU

  • Rayon production: 1 NTU

  • Cotton production: 25 NTU

  • Baking: 10 NTU

  • Cooling water: 50 NTU

  • Ice making: (not specified in this extract)

  • Tanning leather: 0.5 NTU (same as drinking water)

• Drinking water standard: turbidity shall not exceed 0.5 NTU (some scientists advocate a more stringent target, e.g., 0.1 NTU).

Turbidity and Aquatic Life

• Turbidity interferes with sunlight penetration, reducing photosynthesis and oxygen production by aquatic plants.

• When light penetration is reduced, photosynthesis may be diminished, lowering oxygen production and increasing CO₂ accumulation.

• High turbidity can clog fish and shellfish gills and may reduce visibility for feeding.

• Turbidity can provide surfaces for harmful microorganisms to lodge and may serve as a breeding ground for bacteria.

• Fish perception in turbid water is affected: they may have reduced food-finding ability, but turbidity can also provide shelter from predators.

Plankton Density as a Function of Turbidity (Pond Types)

• Table (summary): Plankton density and fish production vary with turbidity.

  • Clear ponds: low turbidity (less than 25 NTU) with relatively high plankton density and higher potential fish biomass per acre.

  • Intermediate ponds: turbidity around 25–100 NTU with reduced plankton density and lower fish biomass per acre.

  • Muddy ponds: turbidity over 100 NTU with the lowest plankton density and the smallest fish biomass per acre.

• Example values (from the notes):

  • Clear ponds: fish in pounds per acre ≈ 162; plankton caught ≈ 12.8 (units not fully clear in the transcript).

  • Intermediate ponds: fish ≈ 94; plankton ≈ 1.6.

  • Muddy ponds: fish ≈ 46; plankton data less clear in the transcription.

• Key takeaway: Pristine (clear) waters tend to support a healthier plankton population and higher potential fish production than turbid waters, highlighting the ecological importance of water clarity.

References and Notes

• The material includes references to standard water-quality literature and regulatory guidance:

  • Quality Criteria for Water, U.S. EPA, 1976 and subsequent updates.

  • California Water Quality Resources Board criteria.

  • U.S. Geological Survey, WSP 1473 (1970) and related compilations.

  • Various reviews and data on ammonia, nitrate tolerance, and other toxicity considerations.

Connections to Foundational Principles and Real-World Relevance

• Gas exchange and dissolved gas balance (CO₂, O₂) are fundamental to aquatic chemistry and ecosystem respiration–photosynthesis dynamics.

• Water temperature governs metabolic rates, ecological interactions, and species distributions; climate and effluent management have direct implications for aquatic health.

• Turbidity links physics (light attenuation) with biology (photosynthesis, plankton, fish feeding) and public health (drinking water clarity).

• Understanding these factors supports environmental management, habitat restoration, and the protection of fish populations and water-quality standards.