Export of dissolved organic carbon from peatlands under elevated carbon dioxide levels Crystal Lebreton

Peatlands A Peatland has a naturally accumulated type of soil which contains high proportions of dead organic material. Mainly plants, which have accumulated over thousands of years. Dead plants in peatlands do not decompose due to its waterlogged conditions which cause a lack of oxygen. This prevents bacteria and fungi from rapidly decomposing the dead plants. The formation of peat is a very slow process. (~10 years for 1cm of peat.)

Peatlands Peatlands have the ability to store huge amounts (~1/3 global soil carbon stock) of organic carbon. Unbalanced ecosystems . The rate of photosynthetic production of organic matter > the rate of decomposition. It has been observed that the levels of dissolved organic carbon (DOC) are increasing in rivers flowing from these ecosystems.

Concerns??? Over the last 39 years a trend has appeared, showing fast increases in the DOC concentrations in the aquatic ecosystems draining from peatlands. Causes some concern that the C store are becoming unstable. When unstable these ecosystems release their stored C in either gaseous or aqueous forms.

Potential mechanisms? Warming Increased river discharge Increased temperatures cause an increase in decomposition. Increased river discharge Shifting trends in annual summer rainfall. However none of these reasons offer a good enough reason so a fourth mechanism has been proposed

Fourth Mechanism Carbon dioxide mediated stimulation of primary productivity. Under elevated CO2 levels, DOC proportions derived from recently assimilated CO2, was ten times higher than that of the control cases. Concentrations of dissolved organic carbon appear far more sensitive to environmental drivers that affect net primary productivity than those affecting decomposition alone.

Why not warming? Studies show that DOC concentrations do not respond adequately enough to increased temperatures in order to explain the 65% increase in DOC. Experimental testing have shown that in order to achieve such a high DOC yield, more than 10 °c of additional warming would be necessary. Since there are no models that predict these types of temperature rises in the future, this is unlikely. There is also the lack of studies to provide a positive correlation between DOC and temperature.

Why not increased discharge? Although some studies have found evidence of increasing DOC correlating with increasing river discharge, there have also been some studies which show that DOC increases can also occur when there has been no increase in discharge. Decomposition has been seen to increase when conditions are warmer and drier. When wetter conditions return it is assumed that soluble carbon compounds are mobilized. Therefore proposed an alternative hydrological theory that suggests that discharge alone is not responsible; but also a decrease in the amount of annual summer rain.

Why not increased summer precipitation? In order to rule out this mechanism they needed to compared the effects of both drought-associated hydrological changes and the previously ignored change in atmospheric CO2 on DOC release from peatlands.

How? Simulated repeated summer droughts in a series of peatlands in mid-Wales. They built a dam which allowed the diversion of flowing water around an ‘experimental’ wetland. Experiment ran for three years (1992– 94) and then another two years (1999– 2000). During this time the wetland was subjected to simulated droughts over 16–20 weeks between late spring and early autumn.

Results Therefore this mechanisms seems to be unlikely. Each drought showed DOC concentrations that where considerably lower than those seen in the control (average 44%), with no evidence that DOC release increased above that of the control. Although DOC decrease was expected under drought conditions, it was seen that gaseous CO2 was the major end product of decomposition. After five drought simulations, there was no evidence that DOC release would increase above that of the control. Solid line = control Dashed line = simulations Therefore this mechanisms seems to be unlikely.

Fourth Mechanism?? Carbon dioxide mediated stimulation of primary productivity. Theorized that elevated CO2, would cause the mobilization of the stored Carbon from peatlands.

Studies using elevated co2 levels. Compared three Welsh peatlands to ambient and elevated levels of CO2. Bog Occur where water in ground surface is acidic, accumulates acidic peat. Fen Usually neutral or alkaline. Riparian Interface between land and stream, characterized by hydrophilic plants.

Results Found that there was significantly more DOC release from bog (14%), fen (49%) and riparian (61%) peatlands. bog < fen < riparian Not a short termed response. These types of results are not easily explained by mobilization carbon stocks alone. Although decomposition is affected by the availability of nutrients, it does not directly respond to elevated CO2.

This led to the proposal that increases in DOC were caused by increased primary production and DOC exudation from plants. Bogs, being nutrient poor, are more likely to be inhibited by nutrient like nitrogen and phosphorus than CO2, therefore bogs are more likely to show a more modest response to increased CO2 levels. Elevated treatment = increase DOC export Ambient treatment However, when nutrient concentrations increases, net primary productivity (NPP) becomes more responsive to increases in CO2.

Testing NPP driven DOC release Used a 13C tracer. Pealands that had been exposed to elevated CO2 levels were pulse labelled with 13CO2.

Results Found that peatlands with elevated CO2 levels contained more 13C (35.8mg). The amount of DOC in the soil with the 13C was 10 times higher then that of the control.

Conclusion Therefore since there is such a large change, it suggests that DOC release is more sensitive to increases in CO2 levels, then to the other proposed mechanisms such as warming or hydrological changes.

Trophic cascades promote threshold-like shifts in pelagic marine ecosystems

Fisheries impacts Due to the large impacts that fisheries have on trophic levels, this paper looks at data collected over the last 30 years to show that the shift in trophic cascades in the Baltic Sea was due to the loss of the top predator. Determined that there were two main alternative ecosystem configurations which are separated by an ecological threshold.

Trophic levels of the Baltic Sea Over the last thirty years, the Baltic Sea ecosystem has had a food web containing four trophic levels. Top predator fish (the gadoid cod), zooplanktivorous fish (the clupeid sprat), zooplankton, and phytoplankton. Fisheries caused a huge drop in cod biomass. This in addition to changes in salt and oxygen conditions triggered a trophic cascade.

Results After loss of cod, there was a huge increase in the amount of sprat. This changed the zooplankton community. Decrease in total zooplankton biomass. Decrease in amount cladocerans. Due to their low escape response and the conspicuousness of egg-carrying individuals. (easier to see and catch.)

Results Found evidence of a shift in way the central Baltic Sea ecosystems functiones which appears to be related to an ecological threshold. An ecological threshold separates 2 different ecosystem configurations, which are characterized by differences in trophic interactions. The ecological threshold is associated with the total sprat abundance (17, 1010). Determined the different configurations: Cod-dominated configuration. Low sprat abundance and an independence between zooplankton and sprat variations. Sprat-dominated configuration. Low cod biomass and zooplankton are strongly controlled by sprat predation.

Sprat Control Configuration Sprat control on zooplankton biomass and species composition is higher. This suggests a change in the sprat predation pressure on zooplankton. Since Sprats eat the larger copepods, they are selective for the later larger stages, decreasing these later stages copepods. Therefore a higher proportion of their biomass occurred in the deeper layers of the water column in daytime .

Cod control configuration Sprat and zooplankton are clearly uncoupled. Because sprat abundance not high enough to regulate the zooplankton. Since there are no sprat predation pressures, zooplankton communities are controlled by hydrological conditions. Suggests that cod acts as an ecological attractor. Control sprat abundance and buffer high-sprat recruitment events.

When the Sprat population is high, they cause a change from cod being the main force controlling zooplankton, to hydrology being the main force.

Conclusion Therefore it is humans causing change in how an ecosystem works which causes a change in trophic level interactions, instead of climate change.