harmful algal blooms


Red Tides

Red tides, a common term for the more proper name of “harmful algal blooms” (HABs), occur when colonies of algae—simple ocean plants that live in the sea—grow out of control while producing toxic or harmful effects on people, fish, shellfish, marine mammals and birds. The human illnesses caused by HABs, though rare, can be debilitating or even fatal. As the name suggests, the bloom of algae often turns the water red.

But not all algal blooms are harmful. Most blooms, in fact, are beneficial because the tiny plants are food for animals in the ocean. In fact, they are the major source of energy that fuels the ocean food web.


Eutrophication is when there are too many nutrients released into the water system.  The opposite of that is oligotrophication, or nutrient poor.

Coral: grows in oligotrophic waters.  DIES by being overgrown by algae (which thrive in eutrophic waters), which aren’t controlled by overfishing herbivores (small fishes and urchins).

Contributions to Eutrophication:  

  • Agriculture/Aquaculture: releases a high amount of ammonia (N2 + H2 = ammonia) from fertilizers and CAFOs (Concentrated Animal Feeding Operations) –> cheap/subsidized! –> enters coastal waters
  • Hurricanes: destruction of waste holders in CAFOs, ex. Hurricane Floyd

Results of Eutrophication:

  • Harmful Algal Blooms (HAB): Red tides–> have toxins, creates anoxic zones –> INCREASING
  • Toxins: bioaccumulation, infected shellfish –> paralysis, neurotoxins, diarrhetic, amnesiac/domoic acid poisonings
  • Anoxic zones: kills fish, shellfish, whales, seabirds, manatees, humans –> spring/summer because of increased light, warmer watter, strong waters (little mixing), decaying/accumulation of organic matter, and ecosystem loading –> Mostly affects: benthic organisms (can’t migrate), Demersal (benthic) fisheries (loss of habitat/prey), long-term ecosystem effects

Examples of Eutrophic Systems:

  • Chesapeake Bay: Largest/best studied US estuary, degraded system –> psychocline (increased density) of nutrients
  • Gulf of Mexico Dead Zone: Almost no O2, heavy plankton decay

Prevention of Eutrophication:

  • Elimination of Point Source Pollution –> Farms (regulation, cooperative efforts between states/nations)
  • Elimination/Reduction of Non-Point Source Polution –> phosphate-free soaps, over fertilization, treatment of waste water, over consumption
  • Prevent Overharvesting of Filter Feeders