Water quality

The north-flowing St. Johns is one of the laziest rivers in the world. The total drop of the river from its source in the marshes south of Melbourne to its mouth in Mayport is less than 30 feet — or about one inch per mile. In addition, the incoming tide from the Atlantic Ocean causes the river to reverse its flow twice a day. In periods of low water, tides may cause a reverse flow as far south as Lake Monroe — 161 miles upstream from the river’s mouth. High and sustained northeasterly winds can result in many days of reversed flow. Because the river flows slowly and reverses course regularly, it is difficult for the river current to flush pollutants.

When sediments, sewage or fertilizers are introduced into a waterway, increases in concentrations of nitrogen or phosphorous may trigger algal blooms.

The pollution causing the most harm to the river is often referred to as nutrient pollution. It comes from an overabundance of nitrogen and phosphorus entering the waterway from our homes and yards and farm fields, from our cars, from the places we work, and from the production of goods we use in our everyday lives.

Nitrogen can be found in nature as a gas, dissolved in water or as small particles of organic matter. Nitrogen pollution comes from many sources. Nitrogen, in the form of nitrates, originates from fertilizers used in row crop agriculture and landscaping, septic tank seepage, discharges of treated wastewater (sewage), and even rainfall that has become enriched with nitrate from fossil fuel burning. This nitrate-rich water makes its way to surface waters as direct discharge and runoff during rain showers, or it may drain slowly from the soil over time.

Nitrogen and phosphorus themselves are natural, but an overabundance causes significant imbalances in the river’s natural ecology. Nutrient pollution feeds harmful algal blooms, which block sunlight from reaching aquatic plants and can produce toxins.

Algal blooms

Too much nitrogen and phosphorus — nutrients — spur the growth of too many algae, which itself is a photosynthetic organism. While algae are the vital base of the aquatic food chain, too many algae can crowd together, and the resulting mass (often called blooms) blocks sunlight from reaching underwater plants. As for most plants, sunlight is vital for the growth of aquatic vegetation, which provides food and a place to live and grow for fish and animals.

Dying algae also lower levels of dissolved oxygen in the water, which fish and other aquatic animals breathe. This can result in fish kills.

Blooms can last for months at a time, which can harm a waterway’s ecosystem by causing declines in dissolved oxygen, underwater plant growth and fish populations.

Harmful algal blooms also may impact recreational activities and commerce due to the unsightly green scum and accompanying unpleasant odor, and the public health concerns about toxins associated with some algal species.