The Power of Ponds? Quantifying sediment carbon stocks within and fluxes from, small ponds

Gilbert, Peter (2016) The Power of Ponds? Quantifying sediment carbon stocks within and fluxes from, small ponds. Doctoral thesis, Northumbria University.

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Abstract

The role of ponds within the terrestrial carbon cycle has been receiving increasing interest. Existing evidence suggests that they have substantial global coverage, with ecosystem function rates disproportionately intense for their size making them significant cyclers of atmospheric carbon. This project aims to: (1) provide a comprehensive survey and quantification of carbon stocks within lowland ponds from a diverse range of ecological pond types; (2) provide a comparison of carbon stocks from pond sediments across significantly different biogeographical regions across England; and (3) monitor the temporal and spatial variability of carbon fluxes from ponds.

Carbon stocks were surveyed in 40 ponds across Druridge Bay, Northumberland. These ponds were selected for their distinct plant communities and hydrological patterns that form four broad pond types: dune-slack ponds; arable field ponds; pasture field; and classically vegetated ponds. High measures of percentage carbon were found within the sediments, however, when quantified in terms of carbon (C) stock, (kg-1 C m-2 < upper 10 cm), little difference was observed among classically vegetated, arable, and pasture pond types (means = 3.14, 3.17, 4.94 kg-1 C m-2 < upper 10 cm respectively); only sediment C stocks of dune-slack ponds (6.18 kg-1 C m-2 < upper 10 cm) were significantly different from other pond types. Equally, the heterogeneity of C stocks among dune-slack ponds varied markedly, with ponds in arable fields being fairly consistent. No significant difference was observed between C stocks in the pond sediments compared to those in surrounding soil. This does not mean that they play a similar role in the carbon cycle, but highlights the importance of acquiring sediment burial rates within these systems in order to quantify their role as C stores.

To test if the patterns of C storage could be generalised beyond the Northumberland ponds to other regions in the England, 15 ponds were surveyed, 5 each from 3 separate regions of England with differing climatic influences and biogeographical characteristics: temporary ponds on the Lizard Peninsula, Cornwall, with Mediterranean climate; pingo ponds of Thomspon Common, Norfolk; and peat excavation ponds at Askham Bog, Yorkshire. Sediment C stocks of ponds sampled in Cornwall (mean = 2.6 kg-1 C m-2 < upper 10 cm), were > 43 % lower compared to those in Yorkshire (6.0 kg-1 C m-2 < upper 10 cm) and Norfolk (7.7 kg-1 C m-2 < upper 10 cm). However, cumulatively, the variation observed among all sites was comparable to the high level of variation observed in the comprehensive survey of ponds at Druridge.

The absence of detailed C flux rates from small water bodies, especially from desiccated sediments during summer dry phases, is a key factor constraining their inclusion in terrestrial carbon budgets. Thus, CO2 fluxes were monitored from 26 neighbouring experimental ponds of known age, history and ecology, focusing on short-term hydrological changes over two, two-week periods, comprising a drying phase and re-wetting phase. During the drying phase flux rates exhibited a 9-fold increase resulting in a shift from a net intake of CO2 to a net site emission whilst the reverse was observed during the rewetting phase. Moreover, significant variability in fluxes of CO2 were observed among ponds on individual sampling days; the highest range observed was -2154 to 10658 mg m-2 d-1. The result is marked spatial variability in CO2 processing.

The large degree of temporal and spatial heterogeneity repeatedly observed throughout this study, both in sediment carbon stocks and CO2 fluxes, highlights the complexity of carbon processing within small aquatic systems such as ponds. This study specifically highlights the need for accurate measures of burial rates within pond systems in order to fully assess their carbon capture capability.

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