The Characterization and Lability of Dissolved Organic Carbon (DOC) in Two Environmentally Varied Mesocosms and a Vernal Pool at the VCU Rice Center

Ann Benjamin, Emily Hannah, Luke Evancoe, Mariah Davis, Eric Hall, Rachel Cooper, Lindsey Koren, Anne Wright

IntroductionCarbon cycles through three reservoirs: the atmosphere, the ocean, and the terrestrial environment. (Post et al. 2009). Inland water systems have historically been absent from the global carbon budget (Battin et al. 2009). Recent findings suggest that inland waters can act as a conduit in addition to significantly acting as a fourth reservoir which physically transports solid particulates. Inland waters also support phase changes of carbon via photosynthesis and respiration (Cole et al. 2007). Carbon lability is an important factor in the amount of carbon potentially processed in fluvial systems. Lability is often thought to be connected to whether the carbon is autochthonous or allochthonous in nature. (Wikner 1999; Krtizberg et al. 2004). Scientists traditionally thought that autochthonous carbon was more labile than allochthonous carbon because a greater ratio of carbon is accessible for biomass assimilation (Wikner 1999; Kim et al. 2012).

Each of three environments; a forested mesocosm, a mesocosm exposed to the sun, and a vernal pool were explored to determine if they were dominated by allochthonous or autochthonous carbon input. Vernal pools are described as wetlands that allow seasonal plant growth, followed by a brief waterlogged-terrestrial stage and ending in dry soil conditions over an extended period of time (Keeley et al. 1998).To explore the effects of carbon lability on carbon cycling in inland water systems, carbon consumption by bacteria was measured and compared between the three environments.

H1: The vernal pool samples will exhibit the highest DOC consumption due to its high biodiversity, which above certain temperatures positively correlates with net primary production (Costanza 2007). H2: The sunny mesocosm will exhibit moderate DOC consumption due to highly labile autochthonous carbon presence (Eiler 2004).H3: The forested mesocosm will exhibit less DOC consumption due high leaf litter input and reduced sunlight exposure. Sun exposure accelerates the breakdown of carbon and facilitates increased lability (Molot 2005; Fellman 2012).

MethodsAs part of a Carbon Capstone/Service Learning Course (BIOL 491), undergraduate students from Virginia Commonwealth University collaborated with local high school students in the greater Richmond area to conduct carbon characterization and consumption experiments. These experiments were done to gain a better understanding of how carbon is cycled throughout inland water systems. Dissolved oxygen (DO), carbon dioxide (CO2), pH, and temperature were measured in three different environments: a sunny mesocosm, a forested mesocosm, and a natural vernal pool located at the VCU Rice Center. On March 9, 2013, DO, CO2, and pH levels were taken using test kits. An environmental gas monitor (EGM) was used to measure carbon dioxide in the atmosphere and in the water (sampled on March 26th). Chlorophyll a was also tested in all three environments. A dissolved organic carbon (DOC) incubation experiment was setup to study how bacteria process DOC in these respective systems. Water samples were taken from the three different environments and filtered through 0.7 μm filter. Filtrate was placed in triplicate in dark incubation bottles and subsampled (treated with 200 μL hydrochloric acid) weekly over a 28 day period. At the conclusion of the incubation period, all samples were sent to VCU’s Environmental Analysis Laboratory for DOC analysis. Results were analyzed using an analysis of variance (ANOVA).

ResultsThe vernal pool had the highest concentration of pCO2, followed by the forested mesocosm, and then the sunny mesocosm. Concentrations of dissolved oxygen ranged from a low in the forested mesocosm, to a high in the sunny mesocosm. The sunny mesocosm had the highest amount of Chlorophyll a, while the vernal pool had the lowest (Table 1).

Initially, the sunny mesocosm had the highest amount of DOC followed by the forest and the vernal pool (Figures 1A-C). The most carbon consumption during the incubation period occurred in the vernal pool. Further, the vernal pool also had the most negative slope (-0.109), indicating the fastest rate of consumption (Figure 1C) The sunny mesocosm consumption curve had a slope of -0.0381 and the forested mesocosm exhibited a slope of -0.046 (Figures 1A-B,2).

An ANOVA and Tukey-Kramer post hoc test revealed significant differences in carbon consumption between the vernal pool sample and samples from both the forested and sunny mesocosms. Carbon consumption in the two mesocosm samples was not significantly different (F(2,5 d.f.) =174.9, p<0.001).

Conclusions• The sunny mesocosm was dominated by photosynthesis and autochthonous carbon production. The forested mesocosm was dominated by respiration and supported by allochthonous

materials. The vernal pool contained a mix of allochthonous and autochthonous carbon. • Although the sunny mesocosm had more DOC and was dominated by autochthonous carbon there was no difference in carbon consumption when compared to the forested mesocosm.

This result contradicts literature that states autochthonous carbon and increased DOC concentrations lead to more consumption (Wikner 1999; Kim et al. 2012).• Bacterial carbon consumption in the vernal pool was significantly higher than the mesocosm samples. Little research has been conducted on vernal pool carbon cycling; however, our

results suggest they may play a critical role.

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DO (ppm) 10 5.2 6.2

pCO2 (ppm) 435 1752 2500

pH 6.5 6.0 5

Chl. a (µg/L) 71.9 2.1 18.2

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Figure 1A

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Figure 1C

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Figure 2

Sunny Mesocosm

Forested Mesocosm

References: Boykoff, M. T., and J. M. Boykoff. 2004. Balance as bias: global warming and the US prestige press. Global Environmental Change-Human and Policy Dimensions 14: 125-136. Cole, J. J. and others 2007. Plumbing the global carbon cycle: Integrating inland waters into the terrestrial carbon budget. Ecosystems 10: 171-184. Eiler, A., S. Langenheder, S. Bertilsson, and L. J. Tranvik. 2003. Heterotrophic bacterial growth efficiency and community structure at different natural organic carbon concentrations. Applied and Environmental Microbiology 69: 3701-3709. Fellman, J. B., K. C. Petrone, and P. F. Grierson. 2013. Leaf litter age, chemical quality, and photodegradation control the fate of leachate dissolved organic matter in a dryland river. Journal of Arid Environments 89: 30-37. Keeley, J.E. and P.H. Zedler. 1998. Characterization and global distribution of vernal pools, pp. 1-14. In C.W. Witham, E.T. Bauder, D. Belk, W.R. Ferren, Jr., and R. Ornduff (eds), Conservation and Management of Vernal Pool Ecosystems - Proceedings from a 1996 Conference. California Native Plant Society, Sacramento. Molot, L. A., J. J. Hudson, P. J. Dillon, and S. A. Miller. 2005. Effect of pH on photo-oxidation of dissolved organic carbon by hydroxyl radicals in a coloured, softwater stream. Aquatic Sciences 67: 189-195. Wikner, J., R. Cuadros, and M. Jansson. 1999. Differences in consumption of allochthonous DOC under limnic and estuarine conditions in a watershed. Aquatic Microbial Ecology 17: 289-299.

Broader ImpactsWith inland waters being overlooked by the Intergovernmental Panel on Climate Change, researchers lack a complete comprehension of which factors are essential for cycling carbon across the globe. Previously understudied systems, like vernal pools and inland waters, are important in carbon processing. Further, understanding how lability influences consumption is critical to developing a broader understanding of how these systems work.

Misconceptions concerning the carbon cycle detract from the public’s understanding. For example, The Washington Post reported biased coverage on the causes and problems of global warming (Boykoff et al. 2004), which in turn left readers misinformed. Educating local area high school students and their teachers, through service learning courses like this one, prepares an emerging generation of students who understand carbon cycling and take ownership of global climate change in order to mitigate this pressing issue.

Raw Data

Vernal Pool

Acknowledgements Dr. S. Leigh McCallister; VCU Environmental Analysis Lab; Tucker High School; Highland Springs High Schools; Powhatan High Schools,; Funded by Dominion Higher Educational Partnership