The H in HABs (Harmful Algal Blooms)

HAB in Lake Michigan. Photo Credit: Zachary Haslick

Ranging from microscopic, single-celled organisms to large seaweeds, algae are aquatic photosynthesizers that form the base of food webs. Sometimes, however, their roles are more detrimental to ecological communities. When given intense sun, high nutrients, and warm water, algae often grow out of control. Harmful algal blooms (HABs, also commonly called “red tides”) refer to the rapid and unchecked proliferation of algal colonies (conglomerates of photosynthesizers) capable of overwhelming ecosystems with lethal effects on fish, shellfish, marine mammals, birds, and people. A few of these blooms (dense concentrations of algal cells) produce toxins that can cause illnesses and even death; other varieties are nontoxic, but deplete oxygen when they decay, clog organisms’ gills, smother benthic organisms, contaminate drinking water, and block light from penetrating the water. Aesthetically, effects are often equally displeasing. Discolored water, sticky algal piles, smelly sand, and closed beaches decorate coasts.

Cyanobacteria (Blue-Green Algae). Photo Credit: NJDEP

Innumerable algae species act as producers in aquatic ecosystems feeding a host of organisms like zooplankton, fish, snails, insects, frogs, and more. Surprisingly, very few algal families can become harmful, when given the chance. The species that do make the jump are hard to stop: multiplying uncontrollably to form malignant masses. HABs can cover massive areas, often stimulated by the right combination of high temperatures salinity, nutrients, sunlight, and opportune tidal conditions. While HABs are an ancient phenomenon, human choices are capable of increasing their severity. Recent records show that excess nutrients from human’s agricultural and fertilized properties, sewage runoff, redirected waterways, tree-clearing, and unusually high temperatures drive microscopic algae to congregate in blooms that can be seen from space. Although causal factors of HABs are well known, the contribution and placement of these initiators are less understood. It is unclear, for instance, if HABs need high-nutrient environments followed by direct sunlight, or if chronology is reversed, or even irrelevant.

HAB Bloom in Lake Eerie. Photo Credit: NOAA

HABs are divided into two predominant causes of destruction: biotoxins and hypoxic dead zones. The harmful effects of many HABs are the result of natural toxin production and the consequent disruption of physiological function in exposed organisms. The majority of the toxic species are normally occurring members within the phytoplankton community, and reside at low concentrations with not evident health impacts, outside of blooms. In other words, toxic effects are primarily dependent on higher-than-normal algal cell densities. The more algae there are packed together, the more toxic the body of water becomes. Many of the biotoxin algal species have potent effects on their victims. Neurotoxins, for example, target ion channels and their supporting components, interfering with the body’s ability to send messages and move. Such poisons are deadly to fish and marine mammals, but provide a competitive advantage over competing algal species. Algae utilize these chemicals to compete for resources against intruding photosynthesizers. As an added benefit, biotoxins behave as antipredation mechanisms— targeting zooplankton and small herbivores to avoid being eaten. Although intended for the former groups, toxic algal colonies’ impact on higher trophic level species, like marine mammals and humans, is probably incidental.

Red Tide HAB off the Coast of San Diego. Photo Credit: Kai Schumann

Unlike their chemical relatives, some algae cause havoc by simply growing. Dead zones are areas of water where aquatic life cannot survive due to low oxygen levels. Generally caused by nutrient pollution, algae proliferate as they intake excess nitrogen and phosphorous. The overgrowth of algae blocks sunlight from underwater photosynthesizers. With no light to grow, plants die off. Bacteria quickly decompose the plant remains, taking in extensive dissolved oxygen (gas present in the water column) to undergo decomposition. The algae at the surface continue to bloom, fed by the additional nutrients from the decay. This bloom causes a negative feedback loop, as they increasing block out light energy and kill off all remaining sunlight-dependent producers. The bacteria in turn, remove the remaining oxygen. With no oxygen available, fish, shellfish, and other organisms die. The body of water becomes uninhabitable, plagued by hypoxia, low light, and excessive nutrients.

The largest dead zone in the world occurs in the Arabian Sea and encompasses nearly 63,700 square miles! Much larger than any biotoxin event, this dead zone, and dead zones all over the world, are possibly the greatest risk HABs pose for the biosphere. Growing dead zones suggest bad news for climate regulation. When oxygen is absent in the water column, chemical nitrogen cycling is dramatically altered. Anoxia (no oxygen) spurs elevated nitrous oxide levels– a greenhouse gas that is about 300 times more potent than carbon dioxide! The more nitrous oxide, the higher earth’s temperatures will go, and the more likely HABs will become.

HABs pose more than environmental challenges; economic losses, unusable resources, and medical expenses pose long-term issues for communities with recurrent HABs. The diversity of HAB species and their impacts present significant and dynamic challenges for freshwater and coastal management systems. Scientific study and targeted responses require interdisciplinary knowledge of complex processes, ranging in subject from oceanography, to limnology, to molecular and cell biology, to mathematical modelling, and remote sensing. Our understanding of these phenomena continues to progress, allowing technologies and management tools to constantly change in an effort to reduce HABs. Current research involves monitoring systems, control and management plans, cell and toxin detection kits, remote sensing and tracking software, bloom control and mitigation strategies, and the use of large-scale models to analyze past blooms and forecast future blooms. Nothing less than ingenuity is needed to solve HABs.

Have you seen an HAB? Leave your thoughts in the comments below.