The abundance and decomposition of coarse particulate organic matter (CPOM) in man-made ponds in central Virginia. Leanna R. Tacik, Annie Choi, Andreas N. Gregoriou, Carly Martin, Kaitlyn D. Peters, Kenneth Fortino Longwood University, Farmville Va Abstract Globally, man-made ponds and reservoirs approach the abundance of natural lentic systems. Furthermore, in regions that lack natural lakes, man-made ponds tend to be the dominant lentic habitat. However, our knowledge of the ecology and biogeochemistry of man-made ponds lags behind that of natural systems. In natural ponds allochthonous detritus input can represent a substantial proportion of the total organic matter budget. The decomposition of this detritus supports macroinvertebrates and fuels production across trophic levels. Our project investigates whether the role and regulation of allochthonous litter decomposition in man-made ponds differs from this model. Spring sampling of 4 man- made ponds in central Virginia shows that the median coarse particulate organic matter (CPOM) pool, mainly comprised of allochthonous detritus, ranged g m -2 and was variable among and within ponds. Comparison of CPOM density and the percent organic matter of the sediment, determined as loss on ignition at 550 o C, shows a complex relationship between detritus input and sediment organic matter content. Direct measurement of decomposition using litter bags in 3 ponds during the fall and winter months showed similar mass losses across ponds. Unlike natural ponds, detrital mass loss appears to be due to microbial activity because virtually no shredding macroinvertebrates were recovered from the litter bags or the CPOM samples. These results suggest that the decomposition of allochthonous detritus is an important component of organic matter cycling in man-made ponds but that its regulation may rely on factors different from those identified from natural ponds. Pond littoral zones contained more CPOM than open habitats. Figure 3. CPOM density in the littoral and open (near the pond center) habitats of 4 man-made ponds in Central Va. The littoral sediments contained greater amounts of CPOM than the open sediments; however, the open sediments still had an average of 29 g AFDM CPOM m -2. These results suggest limitations to the leaf litter redistribution in the ponds after initial deposition, but that CPOM can serve as a resource outside of the littoral zone of man-made ponds. CPOM varied within and among small ponds. Figure 2. CPOM density in the sediments of four man-made ponds in Central Va. All of the pond sediments contained considerable CPOM, but there was variation in the amount of CPOM among and within ponds. These results indicate that man- made pond sediments contain CPOM that likely serves as a resource for pond food webs. There is no relationship between CPOM density and fine sediment organic matter. Figure 4. There was no correlation between the percent organic matter of the fine sediments and the density of CPOM (plotted as the ln of CPOM density). This result indicates that there is a decoupling of the bulk sediment organic matter production and the input of CPOM. This decoupling may be due to: 1)limited conversion of CPOM to fine particulate organic matter (FPOM), 2)decomposition of FPOM that is unrelated to inputs, which could mean that sediment organic matter content is determined more by FPOM decomposition than by input, or 3)input of FPOM that is unrelated to CPOM deposition or processing, such as organic sediment input from the watershed. CPOM decomposition is slow after initial leaching Figure 5. The percent AFDM remaining in litter bags deployed in October 2013 in 3 man-made ponds in Central Va. All of the ponds show initial rapid mass loss, which likely represents the leaching of soluble organic compounds. Following the initial mass loss, the leaf litter shows very little mass loss for the remaining period of available data. Few of the leaf bags contained shredders, despite abundant macroinvertebrate colonization, which suggests that decomposition is mainly microbial. Decay constants were calculated as d -1 for Daulton Pond, d -1 for Lancer Park Pond, and d -1 for Campus Pond, although in general the models were either not significant or provided a poor fit to the data. The ponds represent a gradient of use. Daulton is the most “natural”. Lancer Park and Campus both receive stormwater runoff, and Campus is the most artificial with a concrete walled margin and stand-pipe drainage. There is more variation in the mass remaining in replicate leaf bags from the stormwater ponds than in the farm pond, which suggests that these ponds are a more heterogeneous environment. What is the contribution of leaf litter to sediment organic matter in man-made ponds? Does leaf litter decomposition in man-made ponds follow the predictions developed in natural systems? Conclusions What is the contribution of leaf litter to sediment organic matter in man- made ponds? Man-made ponds in Central Va contain sufficient leaf litter in the sediments to suspect that CPOM would contribute to pond food webs. Littoral sediments have more CPOM than open-water sediments, suggesting that variation in leaf litter input affects the CPOM density in the pond. CPOM density was variable among ponds suggesting that landscape factors affect CPOM inputs into man-made ponds. The percent organic matter in the sediment was uncorrelated with CPOM density, indicating a decoupling of fine and course organic matter dynamics. Does leaf litter decomposition in man-made ponds follow the predictions developed in natural systems? Leaf mass loss was slow after an initial leaching period in all 3 ponds. Leaf mass loss was primarily the result of microbial activity because very few shredding macroinvertebrates were found associated with the leaf packs. The two stormwater ponds (Lancer Park and Campus) showed more variation in mass loss than the farm pond (Daulton), suggesting that pond use can affect organic matter cycling. CPOM density was calculated as the ash-free dry mass (AFDM) of particles greater than 1 mm collected in Ekman samples from the littoral and open portions of the ponds. Prior to ashing, the sediments were washed through a 250 μm sieve to remove fine sediments and then preserved in 70% ethanol. Percent sediment organic matter was measured as the mass lost from 20 ml sample of homogenized sediments after ashing at 550 o C. Litter decomposition was measured as the loss of AFDM of 5 g tulip poplar (Liriodendron tulipifera) leaf packs incubated in each pond in 5 mm mesh litter bags. The incubations began in late October On each sample date 5 replicate leaf bags were randomly harvested from each pond and the leaves were rinsed of fine sediment over a 1 mm mesh sieve. The AFDM of the remaining leaf litter was determined by ashing at 550 o C. Methods Acknowledgements We would like to thank Longwood University, the Longwood Real Estate Foundation, the Town of Farmville, and Daulton Farm for permission to sample the ponds. We would also like to thank the Longwood University PRISM program, the Longwood University Department of Biological and Environmental Sciences and the Cook Cole College of Arts and Sciences for funding. Literature Cited Gessner, M. O., Chauvet, E., and Dobson, M A perspective on leaf litter breakdown in streams. Oikos 85: Figure 1. Leaf litter that falls into ponds becomes coarse particulate organic matter (CPOM) in the sediments. The organic matter in the CPOM can be used as a food resource by shredding macroinvertebrates, and microbial (fungi and bacteria) communities (red arrows). The organic matter in the CPOM can also serve as a source of resources for organisms in the pond that do not use the CPOM directly (blue arrows). Soluble organic compounds leach from CPOM and produce dissolved organic matter. The consumption and metabolic activity of organisms using CPOM convert the CPOM mass into inorganic nutrients and CO 2, as well as fine particulate organic matter. (figure adapted from Gessner et al. 1999). Leaf litter provides resources for ponds. Study Sites Daulton Pond SA = 0.6 ha Z max = 3.2 m Farm Pond Woodland Ct. Pond SA = 0.3 ha Z max = 1.6 m Stormwater Pond Lancer Park Pond SA = 0.06 ha Z max = 1.5 m Stormwater Pond Wilke’s Lake SA = 19.5 ha Z max = 2 m Quarry Pond Campus Pond SA = 0.3 ha Z max = 1.5 m Storwater Pond / Const. Wetland