Nutrients

The nutrients in a watercourse are fundamental to its health and the life it supports, but in excess they result in pollution. In a healthy watercourse, a balanced mix of macronutrients (N, P, C, Si) and micronutrients (Fe, Zn, Cu, etc.) supports a thriving and diverse food web. The central challenge to water quality currently is the levels of nitrogen and phosphorus which are causing destructive eutrophication.

Macronutrients (primary nutrients): the nutrient elements required in the largest quantities by aquatic plants and algae.

• Nitrogen (N): exists in several forms, each with different implications:
  • Nitrate (NO₃⁻): the most common and stable form of nitrogen in well-oxygenated water. It is essential for plant growth but is also the primary culprit in cultural eutrophication (explained below)
  • Ammonium/Ammonia (NH₄⁺/NH₃): found in water from decomposing organic matter and direct waste (e.g., fish, fertilizer runoff); toxic to aquatic life at high concentrations.
  • Nitrite (NO₂⁻): an intermediate form during the conversion of ammonia to nitrate; toxic to fish, even at low levels.
• Phosphorus (P):
  • Phosphate (PO₄³⁻): often the "limiting nutrient" in freshwater ecosystems, meaning its availability controls the growth of algae and aquatic plants. Even a small increase can lead to massive algal blooms. Sources include agricultural runoff, sewage, and detergents.
• Carbon (C):
  • Dissolved Organic Carbon (DOC): from the decay of plants and soil matter (e.g., leaves, wood) washed into the water. It gives some rivers and streams a brown, "tea-stained" color.
  • Inorganic Carbon: exists as Carbon Dioxide (CO₂) dissolved in water and as Bicarbonate (HCO₃⁻) and Carbonate (CO₃²⁻) ions, which are critical for maintaining pH and are used by plants for photosynthesis.
• Silicon (Si):
  • Found as **Silicate (SiO₄⁴⁻), a crucial nutrient for certain types of algae, particularly diatoms, which use it to build their glass-like cell walls.
• Potassium (K), Calcium (Ca), Magnesium (Mg): essential for general plant and animal physiology. They are usually abundant in water and are less frequently a cause of pollution.

Micronutrients (trace elements), required in very small amounts but still vital for enzyme function and other biological processes.

• Iron (Fe): important for photosynthesis and oxygen transport in invertebrates.
• Manganese (Mn): involved in photosynthesis and nitrogen metabolism.
• Zinc (Zn), Copper (Cu), Molybdenum (Mo), Boron (B): act as cofactors for various enzymes.
• Cobalt (Co): especially important for nitrogen-fixing bacteria.

Eutrophication, the most important ecological process related to nutrients in water:

• Natural Eutrophication: a slow, natural process where a water body gradually becomes enriched with nutrients and sediments over centuries.
• Cultural Eutrophication: the human-caused acceleration of this process. When excess nutrients—primarily Nitrogen and Phosphorus — from sources like agricultural fertilizer, sewage, and urban runoff enter a watercourse, they act like a super-fertilizer.

Negative Consequences of Excess Nutrients:

1. Algal Blooms: explosive growth of algae, creating dense mats on the surface.
2. Oxygen Depletion: when the algae die, they sink and are decomposed by bacteria, a process that consumes dissolved oxygen.
3. Dead Zones: The resulting low oxygen levels (hypoxia) or no oxygen (anoxia) can kill fish and other aquatic life.
4. Blocked Sunlight: Dense algal blooms block sunlight from reaching submerged aquatic plants, which then die and decompose, further depleting oxygen.
5. Toxin Production: Some algal blooms (e.g., cyanobacteria, or blue-green algae) can produce potent toxins harmful to wildlife, pets and humans.

Nutrient Sources: