The Short Answer: Mark, I think I can give you a general answer, but a specific answer would require some investigation at the site. What you saw is probably a cyanobacterial slime or mat that has trapped bubbles of gas, produced by the cyanobacteria. The bubbles might be oxygen, carbon dioxide, or some mixture of the two. Cyanobacteria often dominate in waters that are over-enriched with nutrients. Mark June-Wells, a certified lake manager, and founder of Aquatic Ecosystem Research, in Connecticut, has not seen this bubble phenomenon before, but he suggested “That is most likely cyanobacteria that are at the surface in the early morning. The reason they are bubbling is likely due to their density and their metabolic byproduct (oxygen) which forms the bubbles. An alternative hypothesis is that the bubbling is due to respiration, which yields carbon dioxide. The algae may be modifying their buoyancy to optimize light harvesting.”
More Information: Cyanobacteria used to be called “blue-green algae.” They aren’t really what we usually think of as algae, however. They are actually bacteria and exist in virtually every body of surface water in the world. About three billion years ago, they evolved the kind of photosynthesis that releases oxygen. The cyanobacterial cells capture sunlight as a source of energy to split carbon dioxide (CO2) into carbon and O2. The carbon is used to build organic compounds, leaving the O2 free to join the atmosphere as free oxygen. In today’s world, most oxygen is produced by true algae and other plants, but cyanobacteria probably created the oxygen-rich atmosphere that allows oxygen-loving creatures like us to exist. So although cyanobacteria have gotten a bad name these days – for good reason – we should probably also remember that we owe them for every breath we take.
Cyanobacteria have gotten a bad name because many species release multiple toxins that can kill fish, birds and other organisms. According to the World Health Organization, “The only documented and scientifically substantiated human deaths due to cyanobacterial toxins have been due to exposure during dialysis.” Of course, in the very next sentence, WHO says, “People exposed through drinking-water and recreational-water have required intensive hospital care,” so cyanotoxin poisoning can be a pretty severe thing.
We don’t yet know exactly why cyanobacteria are sometimes, but not always, toxic, but it doesn’t seem likely that the cyanobacteria are “trying” to poison us. Recent research suggests the toxins help protect cyanobacteria from reactive and destructive forms of oxygen such as superoxide and hydrogen peroxide, which are often produced in warm summertime surface waters. The toxins may also be effective against tiny zooplankton grazers like daphnia, rotifers, and copepods and reduce parasitism by certain types of fungus. Toxins are probably the weapons the cyanobacteria use to protect themselves from all these dangers.
In aquatic ecosystems with normal levels of phosphorus, cyanobacteria never become numerous enough for their toxins to affect vertebrates. But lakes, ponds, rivers, and ocean bays that have been over-fertilized by agricultural and lawn runoff, or by nitrogen and phosphorus released from sewage systems, can provide a habitat where cyanobacteria “bloom,” multiplying far beyond the ability of grazers and parasites to keep them in check.
And in some highly fertilized waters, cyanobacteria slime or mats form on the surface or on the bottom. Dr. Hans Paerl, Kenan Professor of Marine and Environmental Sciences, who studies cyanobacteria at the University of North Carolina Institute of Marine Sciences, thinks that’s what is going on here. “Fascinating shots! It looks like an algal bloom, most probably cyanobacterial, that may have started on the lake bottom (as mats) and then made its way to the surface due to oxygen bubbles (formed by algal photosynthesis) that gave the mat some buoyancy and made it float. I can’t be sure what the organisms are, but cyanobacteria in the genera Nostoc, Lyngbya and Oscillatoria are known to form floating mats that often start on the lake bottom.”
If You See This Phenomenon: As questioner Mark noted, this phenomenon was very brief and the bubbles disappeared once the sun came up. Probably the warmth of the sun on the bubbles expanded the gas inside and they popped. Or Dr. Paerl suggests the gasses were reabsorbed, causing the cyanobacteria mat to lose buoyancy and sink again. Neither of the experts I contacted had actually seen this particular phenomenon themselves, and in my own search of the internet I was only able to find one other example, a photo taken of an irrigation pond in California’s wine growing region by George Rose (http://www.gettyimages.com/license/458453830), so if anyone else observes these green bubbles on a lake and can take a picture, I’d love to see it. Email them to email@example.com.
Sources: Paerl H W, Gardner W S, Havens K E, Joyner A R, McCarthy M J, Newell S E, Qin B, Scott J T. 2016. Mitigating cyanobacterial harmful algal blooms in aquatic ecosystems impacted by climate change and anthropogenic nutrients. Harmful Algae 54. 213–222.
Paerl, H. W., & Otten, T. G. (2013). Blooms bite the hand that feeds them. Science, 342(6157), 433-434.
Rohrlacka T, Christiansenb G, Kurmayerb R. 2013. Putative Antiparasite Defensive System Involving Ribosomal and Nonribosomal Oligopeptides in Cyanobacteria of the Genus Planktothrix. Applied and Environmental Microbiology. Vol. 79 no. 8. 2642-2647.
Wilson A E, Sarnelle O, Tillmanns A R. 2006. Effects of cyanobacterial toxicity and morphology on the population growth of freshwater zooplankton: Meta-analyses of laboratory experiments. Limnology and Oceanography., 51(4), 1915–1924.
Water-related diseases: Cyanobacterial Toxins. Prepared for World Water Day 2001. Reviewed by staff and experts at the Federal Environmental Agency, Germany, and the Water, Sanitation and Health Unit (WSH), World Health Organization (WHO), Geneva. Viewed on June 27, 2016 at: http://www.who.int/water_sanitation_health/diseases/cyanobacteria/en/.