Toilets: Then and Now, Part 09: Excessive Human Wastes in Coastal Waters, Page 2 of 4

Areas Known as Dead Zones

• Not too long ago, the oceans were thought to be immune to the combined forces of nutrient enrichment and oxygen depletion, which were then commonly observed at work in lakes.
• After all, the seas are vast and restless; the waste discharged from land seemed just a drop in a giant, sloshing bucket.
• Scientists now know this assumption was wrong.
• The fertilization of coastal waters constitutes a major environmental threat to the Baltic Sea, the Gulf of Mexico, Chesapeake Bay, the Lagoon of Venice, the North Sea, and a great many other estuaries, bays and lagoons in the industrial world.
• Most at risk are sheltered regions that do not experience winds or tides strong enough to keep the sea thoroughly mixed the whole year around.
• For just like nutrient-rich lakes, polluted bays and estuaries can become starved of oxygen when their bottom waters are cut off from the atmosphere.
• Coastal areas are especially vulnerable to oxygen depletion because freshwater draining into the ocean from rivers and streams; often laden with nutrients, tends to float on top of denser saltwater.
• In the summer, the surface layer becomes even more buoyant as it warms in the sun.
• Unless some energetic mixing takes place, the lighter, oxygen-rich veneer will remain isolated from the denser water below.
• In areas of weak wind and tide, such stratification can last an entire summer.
• When a polluted bay or estuary remains relatively still for weeks, months or whole seasons, the difference between life at the top and life at the bottom becomes extreme.
• The surface waters, rich in nutrients and bathed in sunlight, teem with phytoplankton and other forms of floating plant life.
• The bottom layers become choked with dead plant matter, which consumes more and more oxygen as it decomposes.
• Below the surface, entire bays can suffocate. And the problem is not necessarily limited to protected waters near the shore.
• For instance, oxygen deprivation cuts a lethal swath through some 18,000 square kilometers (7,000 square miles) of the deep waters of the Gulf of Mexico every summer, creating a barren region called the “dead zone”.

The effects of eutrophication trickle up into human affairs in various ways

• Bays and estuaries provide some of the richest fishing grounds, yet oxygen depletion kills fish, and human-waste nutrients from toilets may cause certain toxic varieties of phytoplankton to bloom, contaminating the shellfish that feed on them.
• Picturesque shores are sullied by dead fish and rotting plant waste, and the water may reek of rotten eggs as bacteria on the ocean floor spew out hydrogen sulfide.
• Fertilization of coastal waters also changes life underwater in more subtle ways.
• For example, as the balance of nutrients changes, the mix of phytoplankton may shift in response.
• In particular, diatoms, which need about as much silicon as nitrogen, cannot benefit.
• Because pollution increases the supply of nitrogen but not the amount of silicon, these important organisms may be crowded out by other species of phytoplankton that are less useful to feeding fish and shellfish.
• In addition, sunlight does not penetrate deeply into water clouded by blooms.
• Thick layers of phytoplankton may shade out the sea grasses and seaweeds that typically grow in coastal waters and shelter vulnerable creatures such as crabs and young fish.
• As a result, complex aquatic food chains may be broken apart.