Undergraduate Dissertation
My undergraduate dissertation titled ‘On the Biotic Enhancement of Weathering by Lichens’ consisted of three experiments:
1. An in situ study to examine the affect of lichens on the weathering of concrete
2. An ex situ study examining the affect of lichens on the weathering of concrete and natural rock
3. A study to assess whether or not silicon in run-off waters increases the Si loading of a nearby lake and river.
All experiments were conducted on the UEA campus in Norwich, eastern England. The UEA is well known for its fine 1960s architecture constructed of, predominately, concrete!
The in situ study was similar in design to that of Aghamiri & Schwartzman, 2002 .
Ultra pure (Milli-Q) water was applied to the concrete wall surface using a handheld water mister, and the resulting run-off collected in guttering, and transferred to centrifuge tubes. Milli-Q water has approximately the same pH as rainwater in a de-industrialised city like Norwich. All equipment was decon-washed prior to use, using the procedure of Spokes .
The ionic composition of the run-off water was analysed using ICP-OES. Silicon was by far the most abundant element in run-off waters (NB the difference in ionic composition between lichen colonised and control sites was insignificant.
2. Ex Situ Weathering StudyThe ex situ involved a more intense weathering regime using lichenised and unlichenised rocks of turbidite, ultramafic, and sandstone type(rocks supplied by Colin Goldblatt!) For this experiment I designed and built a Re-circulating Rainwater Simulator (RRS)
Rocks were placed into the RRS and ‘watered’ for 24 hours, or until the sump ran dry (see photo). Samples were taken from the sump on the hour, and frozen at -20C. Samples were later defrosted, and analysed for silicon using the Strickland and Parsons (1965) ammonium molybdate/photometric method. Once again, there was no significant difference in silicate weatheing between lichenised and non-lichenised rocks.
The graph above shows silicate (SiOx) weathering over time from the slab of lichenised concrete shown in the previous photo (UNICAM and TECHCOMP are two different spectrophotometers). Note how the weathering rate is linear, with no sign of saturation/thermodynamic equilibrium between sump [SiOx] and concrete [SiOx].
3. Run-off StudyWater samples were taken from a lake known as UEA Broad (where storm water from the study sites, and, indeed, the whole of UEA campus, discharges to) and the adjacent River Yare. All samples were taken approx. 10cm below water level, with some kick samples (K) being taken (kicking bottom sediment, and then sampling). Samples were filtered using a 2.2um filter to remove any organisms such as diatoms which may utilise the silicate, leading to erroneous concentrations being recorded. Samples were frozen at -20C and allowed to thaw prior to analysis, the Strickland and Parsons method being used (Refer to 2). The map below shows the sites where samples were taken:
And here is a graph of the results: 
NB that the results for the River Yare show a gradient down the river.
Note that the [SiOx] of the River Yare is higher than UEA Broad (P<0.01). Also, kick samples (K) appear to have a lower [SiOx] than their ‘normal’ counterparts (though this is insignificant after t-test). This was an unexpected result.
4. Conclusion/SummaryThe results appeared to show that lichens do not significantly contribute to the weathering of rocks (be they natural, or anthropogenic in nature, such as concrete). It is possible that lichen-enhanced weathering affects are being ‘drowned out’ by the huge Si signal.
Silica dissolution from concrete weathering was found to be vastly greater than any previously recorded values. Such a flux of silicon could have a major impact on ecosystems, providing silicon for structural components in a diverse range of organisms (diatoms, siliceous plants etc.) Increasing the Si loading of a water body can also ameliorate eutrophication problems by bringing the N:P:Si ratios more in line with the natural Redfield ratio Link.
However, no evidence was found that Si in runoff waters is causing a measurable affect on the [Si] of UEA Broad waters.
Michael Crouch, School of Environmental Sciences, UEA, March 2007. Last Updated: November 2009