How does eutrophication cause fish kills?

One of the negative impacts of eutrophication and increased algal growth is a loss of available oxygen, known as anoxia. These anoxic conditions can kill fish and other aquatic organisms such as amphibians.

Eutrophication reduces the clarity of water and underwater light. In eutrophic water, algae are starved for light. When algae don’t have enough light they stop producing oxygen and in turn begin consuming oxygen. Moreover, when the large blooms of algae begin to die, bacterial decomposers further deplete the levels of oxygen. As a result, eutrophication can quickly remove much of the oxygen from the water, leading to an anoxic — and lethal — underwater environment. Source

How does eutrophication cause dead zones?

Dead zones are low-oxygen, or hypoxic, areas in the world’s oceans and lakes. Because most organisms need oxygen to live, few organisms can survive in hypoxic conditions. That is why these areas are called dead zones.

Dead zones occur because of a process called eutrophication, which happens when a body of water gets too many nutrients, such as phosphorus and nitrogen. At normal levels, these nutrients feed the growth of an organism called cyanobacteria, or blue-green algae. With too many nutrients, however, cyanobacteria grows out of control, which can be harmful. Human activities are the main cause of these excess nutrients being washed into the ocean. For this reason, dead zones are often located near inhabited coastlines. Source

How does eutrophication cause harmful algae blooms?

Nitrates and phosphates are nutrients that plants need to grow. In small amounts they are beneficial to many ecosystems. Excessive amounts, however, lead to a process called eutrophication, which stimulates an explosive growth of algae (algal blooms) that depletes the water of oxygen when the algae die and are eaten by bacteria. Estuarine waters may become hypoxic (oxygen poor) or anoxic (completely depleted of oxygen) from algal blooms. Hypoxia may cause animals in estuaries to become physically stressed. Anoxic conditions can kill them

Eutrophication is devastating to animals and plants living in affected water bodies (lakes, rivers, springs, estuaries, even coral reefs), and often devastating to the economies of the surrounding communities as well. Toxic algal blooms disrupt tourism due to foul odors and unsightly views, and poisoned fish and shellfish adversely affect recreational and commercial fisheries

How does eutrophication cause red tide?

In the ocean, as on the land, photosynthesis combines energy from the Sun, carbon dioxide, and nutrients such as nitrogen and phosphorus to produce carbon-rich plant material. This natural process is called primary production and forms the base of the marine food chain. It also provides most of the oxygen in the atmosphere. Without primary production, the world would be a much different (and a good deal less pleasant) place.

But every silver lining has a cloud. Of the thousands of species of algae, perhaps only a hundred are toxic. When these species occur in high concentrations, they can color the water and produce what are popularly referred to as “red tides” or “brown tides.” Scientists prefer to call these outbreaks harmful algal blooms or HABs.

Toxic algae enter the marine food chain when they are consumed by small marine animals called zooplankton and by fish or shellfish. The toxins that accumulate in these consumers are then passed up the food chain to marine mammals, seabirds, and humans, where they can cause illness or even death.

Blooms of some non-toxic species of algae can also cause problems. For example, the North Atlantic right whale is in grave risk of extinction. This species feeds seasonally off Cape Cod on concentrated patches of zooplankton called copepods. In some years, an algal species called Phaeocystis blooms in Cape Cod Bay. Although Phaeocystis is not toxic, large blooms essentially clog surface waters and right whales cannot find the copepod patches they need to eat.

Non-toxic HABs include large blooms of seaweed or macroalgae that can coat beaches, interfering with recreational activities. Other HABs clog seagrass beds and coral reefs, which provide nurseries for commercially important fish and support high levels of biological diversity necessary for a healthy environment.

It is difficult to assess the precise way in which human activities influence the occurrence and severity of HABs. The physical and biological processes involved are not well understood, and long-term observations are sorely lacking. To complicate matters, HABs can and do occur in relatively pristine conditions. But there is a clear connection between nutrient levels and primary production, and there is general agreement among scientists that, other factors being equal, the conditions that favor high levels of primary production also favor HABs. Source