Wednesday 4 October 2017

Algal Bloom covers much of western Lake Erie.

Much of the western end of Lake Erie in North America is currently covered by an Algal Bloom, which estimated to cover about 1800 square kilometres of the lake's surface by the end of September 2017. This is the latest in a series of such late-summer blooms that have been recorded since the early 2000s, causing problems for communities and wildlife around the lake, as the Algae are both toxic and cause problems by absorbing oxygen from the water, causing other aquatic organisms to asphyxiate. These Blooms are thought to be the result of eutrophication (excess nutrient loading) caused by excessive use of fertilisers on farmland in the basins of rivers which drain into the lake. Authorities in the US have responded to these Blooms by introducing stringent rules on fertiliser use in these river basins, yet the Blooms have continued to grow larger each year, causing many people to question whether fertiliser use is truly the cause of the problem.

MODIS/Terra satellite image of an Algal Bloom on Lake Erie taken on 24 September 2017. NASA/Earth Observatory.

In a paper published in the Journal of Great Lakes Research on 12 May 2017, Jeff Ho of the Department of Civil & Environmental Engineering at Stanford University and the Department of Global Ecology at the Carnegie Institution for Science, and Anna Michalak, also of the Department of Global Ecology at the Carnegie Institution for Science, present the results of a study in which they combine data from the current MERIS spectroradiometer on the European Space Agency's Envisat satellite and MODIS spectroradiometer on NASA's Terra satellite with data collected by the historic Landsat 5 satellite to produce a longer-term analysis of Algal Blooms on Lake Erie.

The combination of the Terra, Envist and Landsat 5 data enabled Ho and Michalak to build up a series of images of the lake dating back to 1984. In addition, since different types of Algae refract light at different wavelengths, it was possible to determine the makeup of these Blooms.

Ho and Michalak found that Blooms were present in the earliest images, with the summers of 1984-87 dominated by Diatom and Chlorophyte (Green Algae) growth. In 1988-92 there was a significant decrease in the overall biomass of the Blooms, with the Diatoms being replaced incrementally by Chrysophyte (Golden) Algae, which are largely inedible to Zooplankton, beginning a breakdown in food-chains in the lake. From 1993 to 1996 the Chlorophyte component of the Blooms was progressively replaced by Cyanobacteria of the genus Microcystis, which produce a range of harmful toxins, further excluding larger organisms from the lake. These Microcystis-dominated Blooms grew in size, recovering the 1984-87 biomass over the period 1997-2000, then increasing further in size from 2000 onwards, eventually producing the large and alarming Blooms of recent years.

Microcystis aeruginosa, a highly toxic species of Cyanobacteria associated with harmful Algal Blooms. Sergei Shalygin/AlgaeBase.

Ho and Michalak suggest that while excess nutrients from farm runoff may have been the original cause of the crisis, triggering a change in the makeup of the Blooms, the increase in the size of the Blooms is a result of this change, with an explosion in Bloom size occurring after the Blooms became dominated by species which excluded secondary consumers. Once this occurred the original cycle, in which nutrients are observed by Algae, then passed to secondary consumers (Zooplankton and small Fish), then larger Fish, and eventually taken out the lake by terrestrial predators (Humans, Birds, etc.), with one in which Algae died and sank to the bottom, where they decompose and release nutrients back into the lake, fuelling larger Blooms each year.

Ho and Michalak still believe that reducing the amount of nutrient runoff reaching the lake is the key to solving the problem, but that this will now take much longer to accomplish, as rather than relying on food chains to remove the excess nutrients, it will now be necessary to wait for them to be washed out via the River Niagara or be incorporated into permanent sediments at the bottom of the lake.

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