1. Bio- and Photodegradation of DOM from Lakes, Streams, and Rivers within the Kolyma River Watershed, Northeast Siberia
Lydia Russell-Roy Carleton College, Northfield Minnesota
P. J. Mann, E. B. Bulygina, J. Schade, W. V. Sobzcak, N. Zimov, R. M. Holmes
Summary
Sampling Sites and Methods
Inland aquatic systems are often an overlooked portion of the global carbon cycle, however, lakes, streams and rivers actively process organic matter. In Arctic ecosystems, like the Kolyma River watershed in Northeastern Siberia, underlain with large stores of DOC in permafrost and vulnerable to warming temperatures, understanding inland carbon processing will help to monitor and anticipate the effects of climate change. As dissolved organic matter (DOM) is transported through waterways it is altered by microbial and photodegradation. These degradation pathways can compete for the same DOM, and work in concert mineralizing dissolved organic carbon (DOC), and depositing refractory DOM to the Arctic Ocean. The objectives of this project were to:
1. Determine how much DOM was being removed by bacterial and photodegradation in the lakes, streams and rivers of the Kolyma Watershed.
2. Observe the effect of light on the quality, the type of DOM.
3. Determine if the photodegradation increases the bio-availability of DOM.
DOM Degradation
Figure 2. Experimental set-up to measure the bio and photoreactivity of Kolyma DOM..
Samples were collected throughout the Kolyma River watershed. Water from two streams, six rivers one ocean and two lakes was collected. To one of the lake samples, an extract of DOM-rich litter was added to mimic the spring flush addition to the lake.
The DOC loss from bacterial degradation was calculated from the oxygen demand from 5 day incubations. The change in [O2] was multiplied by to estimate the DOC consumed. The DOC loss from photodegradation was calculated by subtracting the [DOC] measured on a Shimadzu TOC analyzer, from the light exposed and dark controlled samples after 12 days of irradiation. The percentages were calculated using the intial [DOC].
To determine the quality of the DOM absorbance measurements were made from nm before, after 3 and 12 days of irradiation. The spectral slope ratio (Sr), a proxy measurement for the humic concentration in DOM, was calculated in the same manner as Helms et al.(2008).
Shuchi Lake
Flood Plain Stream
Figure 1. Map of sampling locations in Kolyma river basin, Siberia. Produced by Blaize Denfled.
Sukharnaya
Arctic Ocean
Photodegradation alters the Bioavailability of DOM
Effect of Photodegradation on DOM Quality
Figure 3. The percent DOC loss from bacterial degradation (left), photodegradation (middle), and photodegradation followed by bacterial degradation (right) for 11 locations in the Kolyma watershed.
Figure 4. The spectral slope ratio (Sr) after 0 (left), 3(middle), and 12 (right) days of light exposure. The spectral slope was calculated from the ratio of 2 spectral slopes (S /S ). Smaller numbers indicate DOM molecules with higher molecular weight, usually terrestrially derived, while larger numbers is small MW and typical of marine DOM.
Figure 5. The percent of DOC loss from bacterial degradation of samples not exposed to light (left) and bacterial degradation after photodegradation (right). DOC loss was calculated from O2 consumption in 5-day incubations. The right bars are the samples that were exposed to the sun for 12 days and inoculated with reserved bacteria.
In the Kolyma river watershed, bacteria and light are processing DOM. In most locations the bio- and photodegradation both remove DOC. The largest loss in DOC was 1.81 mg/l from photodegradation, the most DOC consumed by bacteria was 1.2 mg/l. Combined the two processes removed 0-26% of DOC which is distributed throughout the watershed, DOM continues to be processed at the Arctic Ocean.
In many locations the photodegradation removes more DOC than bacteria. Microbes are only able to consume certain types DOM, and in many cases the small DOC loss from bio-degradation is probably due to the remaining DOM being biologically refractory. Photolysis is less discriminating and can process most DOM. For many of the locations bacterial degradation is much smaller which indicates that only a small pool of DOM is biologically labile. However, in Airport Lake 36, Medvezka, Sukharnaya and Arctic Ocean, the two processes removed similar amounts of DOC, bacteria and light are competing for the same DOM.
For most locations photodegradation caused a larger decrease in DOC. Light is powerful in degrading DOM, however these result may not be realistic in situ. To measure photodegradation, the samples were kept on the surface of a pond and light easily penetrated throughout. Often DOM is colored and light does not reach beyond a few meters. Although photodegradation can remove a larger pool of DOC it only occurs in the top few meters, while bacteria are degrading DOM throughout the water column. However, there is only a certain pool of biologically available DOM, and photolysis can break down some of the larger DOM molecules and make the DOM available to bacteria.
Photodegradation can create more biologically labile DOM. In some cases the shift in DOM quality from photolysis breaks apart large, refractory particles into digestible pieces. For six locations in the Kolyma watershed, the bacterial DOC consumption was larger after the water was irradiated for 12 days; the photodegradation increased the bio-degradation. These locations are the same rivers and ocean that had the largest increase in Sr.
Light’s ability to increase bio-availability complicates the relationship between photo- and bacterial degradation. The two processes can work on separate components of DOM, compete for the same pool, and work in concert to increase total degradation; and this relationship is not consistent even within a watershed. More work should be done to try and tease apart this relationship in hopes of understanding the processing of DOC in streams and rivers and getting a more complete picture of the carbon cycle, especially in high-latitude ecosystems.
In addition to removing DOC, light can also alter the quality of DOM. Photolysis breaks apart larger DOM molecules without decreasing the concentration of DOC. In order to observe this shift in quality, the spectral slope ratio (Sr) was calculated from the absorbance curve from nm.
Initially the DOM for each sample had an Srbelow 1, which means the DOM is mostly high molecular weight humics and terrestrially derived particles. During the 12 days of irradiation the Sr increased to 1-1.5, a small change in ratio, but indicative of a large shift in quality towards smaller, protein-like molecules. Bacteria consume smaller DOM particles, and this shift in quality from photodegradation could make more DOM biologically available. The largest changes in the quality of DOM occurred in the rivers and ocean location.
References
Helms, J. R., A. Stubbins, J. D. Ritchie, E. C. Minor, D. J. Kieber, and K. Mopper (2008), Absorption spectral slopes, and slope ratios as indicators of molecular weight,
source, and photobleaching of chromophoric dissolved organic matter. Limnol.
Oceanogr., 53,