Signs of Summer 11: Summer Algal Blooms

Lake Erie, Public Domain

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News reports in mid-July of this year talked about the likelihood of another algal bloom in Lake Erie this summer. This is becoming an annual event that poses serious health risks to not only the fish, waterfowl  and invertebrates that live in the lake but also to people (and their pets!) who get their drinking water from the lake or use the lake for recreational purposes.

The National Oceanic and Atmospheric Administration (NOAA) predicts that this year’s bloom will rise to a severity of 7.5 out of a possible 10. The severity index is based on the biomass of the algae over a sustained period of time, and any index value over 5 is considered serious. The bloom last year was rated a 3.6. In 2017, an article in The New York Times (October 3, 2017) described a 700 square mile bloom of bright green algae covering the western end of Lake Erie (see Signs of Fall 13, November 30, 2017). This bloom was rated 8.0 on the NOAA severity index. In 2011 an even more massive Lake Erie algal bloom was rated a full 10.0 on the NOAA index, and then in 2015 a bloom occurred that actually exceeded the index’s upper value and was rated a 10.5!

These algal blooms are primarily caused by the inflow of phosphate-rich, fertilizer and animal waste runoff from surrounding agricultural lands into the warm, shallow waters of western Lake Erie. Lake Erie is the shallowest of the five Great Lakes (its average depth is only 62 feet, and its western basin is especially shallow with average depths between 25 and 32 feet). The consequences of Lake Erie’s shallow depth are multifold: 1. It has the smallest volume of the five Great Lakes, 2. It has the shortest “water residence time” of any of the Great Lakes (only 2.6 years), 3. It is the first of the Great Lakes to freeze in the winter, and 4. It is the warmest of the Great Lakes in the summer. The shallowness and warmth of Lake Erie makes it a particularly ideal habitat for many species of green algae and plankton which serve as the base for a diverse food web of fish. It is estimated that half of all of the fish in the Great Lakes live in Lake Erie!

Photo by C. Fischer, Wikimedia Commons

Unfortunately, these phosphate-fed, summer-warmed algal blooms do not just involve the green algae that are components of the lake’s food webs.  Cyanobacteria (also called “blue-green algae”) also are found, although usually in small numbers, as a part of the rich, complex bacterial community of Lake Erie and most other freshwater ecosystems all over the Earth.

Many species of these cyanobacteria produce complex protein toxins. These chemicals were probably weapons used by the cyanobacteria in the intense, on-going chemical battles with other bacteria in their crowded, resource limited, aquatic habitats. The cyanobacteria have existed for nearly three billion years and, as a result of competition and evolution, have generated an incredibly diverse array of survival strategies. As a result they can synthesize three to four hundred different, toxic peptides. Usually, cyanobacteria are a small, non-threatening part of the freshwater microflora, but they  respond quite vigorously to added phosphates! Also, cyanobacteria need water temperatures above 20 degrees C (64 degrees F). Temperatures that warm can be generated in small shallow ponds fairly easily but will occur in larger (especially shallow) lakes (like Lake Erie!) toward the end of a summer season. The annual , late summer, Lake Erie algae blooms, then, almost always involve significant numbers of toxin-producing cyanobacteria.

Cyanobacteria. Photo by NASA, Wikimedia Commons

There are a few other interesting features of cyanobacteria also come into play during a bloom: 1. Cyanobacteria contains gas filled, intracellular vacuoles (therefore they float!),  2. Cyanobacteria require high levels of sunlight to run their photosynthesis (therefore they NEED to be on the top of the water),  3. Cyanobacteria are not very high quality food sources for zooplankton (they tend to get left behind after feeding and grazing … maybe the peptides have something to do with this, too?), 4. Cyanobacteria clump together into large colonies (which may be too large for most zooplankton to easily consume), and 5. Cyanobacteria can obtain the nitrogen they need to grow directly from the nitrogen gas of the atmosphere (they are “nitrogen fixers”).

So, the floating, clumped together masses of cyanobacteria when they get their population boost from the phosphates and the warm temperatures of the water and since they are already making the nitrogen that they need, tend to survive and accumulate in their ecosystems and crank out their hundreds of different toxic peptides! These toxins then can sicken and/or kill fish, waterfowl, livestock, dogs, and even people! There is also a concern that chronic, low level ingestion of these toxins can lead to liver damage and even liver cancer! Drinking water taken from surface water sources needs to be checked for these dangerous bacteria!

A recent New York Times article (August 12, 2019) described deaths of dogs all across the country who swam in cyanobacteria contaminated ponds and lakes. Dogs when they swim tend to ingest and swallow great quantities of water making them particularly vulnerable to the toxic impacts of the cyanobacteria’s secretions. Some dogs may only have skin rashes when exposed to cyanobacteria contaminated water but neurological damage, respiratory arrest and liver failure are also possible consequences of exposure to the cyanobacteria toxins.

The sizes and durations of these later summer blooms in Lake Erie have been increasing since 1985 as have the levels of dissolved phosphorus in the waters of the lake. Ongoing climate change with its increase in rainfall (and greater volume of phosphate rich river runoff) and elevated summer temperatures is only expected to make these blooms even more extensive and potentially even more toxic. I talked about the biology of these blooms back in 2014 (see Signs of Summer 1, June 12, 2014). On a small scale these algae blooms are dangerous, on a large scale, as we are now seeing in Lake Erie, they are potentially calamitous.

Sargassum on beach. Photo by P. Bourjon, Wikimedia Commons

Another algae that is excessively blooming in our climate changed world is sargassum. As I wrote about back in Signs of Winter 3 (December 20, 2018) great mounds of sargassum are accumulating on the beaches of Mexico, Texas, Louisiana, Mississippi, Florida and nearly two dozen islands of the Caribbean. Since 2011, the amount of sargassum in the Caribbean and Atlantic has greatly increased. A recent paper in Science (July 5, 2018) describes a 8850 kilometer long, continuous mass of sargassum that extends from West Africa across the Atlantic Ocean into the Caribbean and the Gulf of Mexico. It is the largest macroalgal bloom ever recorded.

Causes of this continuing sargassum explosion, like the summer algal blooms in Lake Erie are thought to be the inflow of nutrient-rich (especially phosphate and nitrogen rich) waters into the Atlantic via the Amazon River as it drains the increasingly deforested and agriculturally manipulated Amazon Basin. Fed by these nutrients, the sargassum steadily increases and dominates its open ocean and shoreline ecosystems.

Algae blooms, heavy rains and warmer summer temperatures (and all of their un-intended consequences!)! Welcome to the “new normal!”