I have written previously in this blog and elsewhere about the costs and damage associated with excessive fertilization of our waters. Too much plant 'food' in the form of nitrogen phosphorous and dissolved organic carbon (DOC) from septic systems, farm fields, and other sources can result in too much plant growth. When the plants die and rot, oxygen depleted “dead zones” result where sometimes botulism toxins amplify the already grim picture for fish and fish eaters.
Previous studies have indicated that methane production from over fertilized fresh waters may be an order of magnitude higher than previously thought. (Methane is a potent though relatively short lived green house gas). In the photo above taken a few years ago the gray spot is a methane bubble 'eruption' in the lake shallows off Blind Creek.
I recently learned of another reason to be concerned about excessive input of nutrients into our near shore waters- it may be directly contributing to and accelerating climate change through huge CO2 emissions.
It has been established by numerous studies that microbes (such as cyanobacteria, heterotrophic bacteria, one celled algae and viruses) control the rate at which CO2 moves through the global ecosystem. Much attention has been focused on the microbiology of marine exchange rates. However, freshwater bacteria and their interaction with viruses may be far more significant than previously suspected.
In late October a visiting Australian scientist sampled the inshore waters of Lake Ontario and Fair Haven Bay. Dr. Peter Pollard, from Griffith University in Brisbane, came here to measure bacterial activity here as part of a world wide study he has undertaken of CO2 emissions from fresh water.
Pollard has a chemistry and microbiology background and has developed methods for measuring bacterial growth and respiration rates in their natural environment. This “in situ” sampling of aquatic bacteria has not been widely replicated. But Pollard's preliminary data from tropical, subtropical and temperate waters is intriguing.
He has found that bacteria interacting with viruses in warmer freshwater shallows are cycling DOC in the water at rates far more rapid than anyone suspected. Under summer temperatures in three meter deep shallows he has confirmed (using radioactive isotopes) that bacteria populations can double every twenty minutes. As their numbers increase, the amount of CO2 produced by their respiration also increases. Though some of the carbon is taken up by photosynthesis, much of it escapes into the air as a green house gas.
Past studies have focused on measuring the total amount of dissolved organic carbon in the water. Surface freshwater is a tiny percentage of earth's total and so has been largely ignored as a source of CO2 emissions.
The graphic above shows a tiny dot representing lakes and rivers compared to larger area of oceans and ice/groundwater.
But, Pollard says, it's the turn over of carbon that is key to understanding the global scheme of things. Measuring the dissolved organic carbon in water simply quantifies the 'pool' of carbon. It doesn't measure the flux (turnover) of carbon. The rate at which carbon enters and leaves the “pool” is key. Because of incredible bacteria population growth rates under good conditions, they emit far more CO2 from the water than measurements of DOC in a given sample of water would suggest.
In fact, Dr. Pollard believes that freshwater bacteria may be emitting as much CO2 as the worldwide burning of fossil fuels!
There are a couple of factors that have generally been over looked by researchers that make this a possibility. One is the interaction of viruses and explosive bacterial growth that causes a more rapid breakdown of dissolved organic carbon into CO2. As bacteria increase in numbers viruses attack them. The bacteria cells break open and release more viruses and dissolved organic carbon. The carbon then fuels the rapid population growth of other bacteria.
The other factor is the impact of summer time temperatures on emissions from temperate waters. Nearly all of earth's surface freshwater is in the northern hemisphere above the 43rd parallel. Fair Haven is right at the southern boundary of all this watery abundance and for a couple months of the year inshore water here gets warm enough to support lots of rapid bacterial growth. Yet almost no measurements of bacterial growth in such waters have been made. And more prolonged hot spells will increase CO2 production from inshore waters as bacteria then grow more rapidly,
Pollard was on his way to the Jet Propulsion Labs of NASA to try for some data on surface CO2 levels from a new satellite, OCO-3, due to be launched next year. He hopes to confirm his own limited data on emissions from temperate waters if the new hardware is sensitive enough to pick up high CO2 emissions during the summer from the Great Lakes region.
In the meantime he concluded in a presentation made to the Hardware Store Cafe regulars of Fair Haven we must identify diffuse sources of DOC entering our waters. These “non point” sources are key to understanding global carbon cycling. As temperatures rise and waters get warmer, the rates of release will continue to rise.
When asked what we can do, he suggested along with burning less fossil fuel, we should do all we can to reduce inputs of organic carbon into our water. Eat less meat, prevent soil erosion, and don't over fertilize your lawn!