| dc.description.abstract | Northern peatlands are a globally important soil carbon (C) store, and aquatic systems draining
peatland catchments receive a high loading of dissolved and particulate forms of C from the
surrounding terrestrial environment. Once incorporated into the aquatic environment, internal
processes occur to modify the C pool. Of these, photo-processing preferentially targets
terrestrially derived C and therefore might have a significant effect on the C budget of peatland
draining aquatic systems. The overarching aim of this study was to investigate photochemical
processing of C in Scottish peatland draining aquatic systems in order to determine the
importance of this pathway in aquatic biogeochemical cycles.
For initial laboratory experiments, water samples from a peatland headwater stream
(Auchencorth Moss, SE Scotland) were collected. Laboratory based irradiation experiments
were conducted at a range of temperatures, and different filtration treatments, including
unfiltered samples, were employed to understand the fraction of C most susceptible to photo-processing.
UV irradiation and temperature had a significant effect on DOC and gas headspace
concentrations, with Q10 values of ~1.42 and ~1.65 derived for CO2 and CO photoproduction
in unfiltered samples, respectively. However, filtration treatment did not induce significant
changes in gaseous C production between light and dark samples, indicating that the
experimental conditions favoured breakdown of DOC rather than POC to CO2 and CO. In all
light treatments a small but significant increase in CH4 concentration was detected. These data
were compared to results from experiments conducted in ambient light and temperature
conditions. DOC normalised CO2 photoproduction was an order of magnitude lower than in
laboratory conditions, although relative abundances of C species within overall budgets were
similar and these experiments demonstrated that ambient exposure is sufficient to generate
photo-processing of aquatic peatland C. Overall these data show that peatland C, particularly
the < 0.2 μm fraction, is highly photoreactive and that this process is temperature sensitive.
Further laboratory irradiation experiments were conducted on filtered water samples collected
over a 13-month period from two contrasting aquatic systems. The first was the headwater
stream draining Auchencorth Moss peatland with high DOC concentrations. The second was
a low DOC reservoir (Loch Katrine, C Scotland) situated in a catchment with a high percentage
peat cover. Samples were collected monthly from May 2014 to May 2015 and from the stream
system during two rainfall events. Significant variation was seen in the photochemical
reactivity of DOC between the two systems, with total irradiation induced change typically
two orders of magnitude greater and DOC normalised CO2 production a factor of two higher
in the headwater stream samples. This is attributed to longer water residence times in the
reservoir rendering a higher proportion of the DOC recalcitrant to photo-processing. Overall
the magnitude of photo-induced C losses was significantly positively correlated with DOC
concentration in the headwater stream, which varied seasonally with highest concentrations
detected in late autumn and winter. Rainfall events were identified as important in replenishing
the stream system with photoreactive material, with lignin phenol data indicating mobilisation
of fresh DOC from woody vegetation in the upper catchment during a winter rainfall event.
Whilst these data clearly demonstrate that peatland catchments generate significant volumes
of photoreactive DOC, the degree to which it is processed in the aquatic environment is
unclear.
Field investigations were undertaken to address this uncertainty. In-situ experiments with
unfiltered water samples in light and dark conditions were conducted in two contrasting open
water peatland pool systems. At the high DOC site (Red Moss of Balerno, SE Scotland), DOC
concentrations in surface light exposed samples decreased by 18% compared to dark controls
over 9 days and light treatments were enriched in CO2 and CH4. Photochemical processing
was evident in δ13C-DOC and δ13C-DIC signatures of light exposed samples, which were
enriched and depleted, respectively, relative to dark controls (+0.23 ‰ and -0.38 ‰) after 9
days of surface exposure. At the low DOC site (Cross Lochs, Forsinard, N Scotland) net
production of DOC occurred in both light and dark samples over the experiment duration, in
part due to POC breakdown. δ13C-DIC signatures indicated photolysis had occurred in light
exposed samples (-1.98 ‰), whilst δ13C-DOC data suggest an absence of photo-processing, as
the signatures in both treatments were similar. Accounting for light attenuation through the
water column, 46 ± 4.9 and 8.7 ± 0.5 g C-CO2 eq m−2 yr−1 was processed by photochemical
and microbial activity in peatland pools within the catchments at the high and low DOC sites,
respectively. At both sites, light driven processing was responsible for a considerable
percentage (34 and 51%) of gaseous C production when compared to equivalent estimates of
microbial C processing and thus should be considered a key driver of peatland pool
biogeochemical cycles.
It is clear from this study that temperature, seasonal cycles, rainfall events and water residence
time provide strong controls on the photoreactivity of aquatic C in Scottish peatland systems.
The photo-processing pathway has the potential to alter the C balance of peatland catchments
with a high percentage coverage of aquatic systems. Under climate change scenarios where
light, temperature and rainfall conditions are expected to change, this process may become
increasingly important in aquatic C cycling, particularly if the upward trend in DOC
concentrations in northern aquatic systems continues. | en |
