In my recent post on Ennerdale Water I referred to the interaction between geology and man in shaping the characteristics of a lake (see “A lake of two halves …”). As I was writing, I had in mind some famous early work on this topic by Harold (“W.H.”) Pearsall, a botanist who made some of the first tentative steps towards linking patterns and processes in lake ecosystems, whilst working at the universities of Leeds and Sheffield. He had visited many of the lakes since boyhood and co-opted his father as a field assistant to cycle around the Lake District performing the surveys that formed the basis of this paper.
Pearsall had noted differences in the types of plants growing in the various lakes in the region, and attributed these differences to the geology of the surrounding land. He took this idea one step further by also suggesting that the lakes became modified as they increased in age, illustrating this by arranging the English Lakes into an “evolutionary sequence”, with Wastwater and Ennerdale Water representing the least evolved, and Windermere and Esthwaite Water representing the most advanced. His first proposition is now well-established amongst those who study lakes; the second is also generally accepted (I remember writing an essay entitled “Lakes are temporary features of the landscape” as part of my A-level Geography course), although his use of the English Lakes to illustrate this is not.
The lakes of the English Lake District, arranged in the evolutionary sequence proposed by Pearsall: 1: Wastwater; 2: Ennerdale Water; 3: Buttermere; 4: Crummock Water; 5: Hawes Water; 6: Derwent Water; 7: Ullswater; 8: Bassenthwaite Lake; 9: Coniston Water; 10: Windermere; 11: Esthwaite Water.
The graph below makes Pearsall’s case, using his own data (note that his records for Hawes Water refer to the small natural lake that was submerged to form the current Haweswater Reservoir). The left hand axis shows the proportion of land in the catchment of each lake which was under cultivation (at the time of his study) steadily increasing as we move through his evolutionary sequence. The right hand axis shows how proportion of the shoreline of each lake that was rocky (down to a depth of 30 feet – 9.2 metres) steadily decreases through the sequence. He pointed out that both the amount of cultivatable land and the character of the shoreline depended largely on the character of the surrounding country.
A graphical representation of Table 1 in Pearsall (1921): “Effects of erosion”. Lakes are arranged in order of Pearsall’s “evolutionary sequence”.
The next graph shows the same sequence of lakes (excluding Hawes Water) but with the average values of the Lake Trophic Diatom Index (TDI) plotted on the Y axis, and with lakes sub-divided into those with low alkalinity (deriving most of their runoff from the Borrowdale Volcanics and associated hard rocks, including the Ordovician granite discussed in the post about Ennerdale) and those with moderate alkalinity (associated with softer rocks to the north and south of the Borrowdale Volcanics). This confirms the primary role of geology, with Pearsall’s “primitive” lakes underlain by the Borrowdale Volcanics whilst the more “evolved” are associated with the softer rocks. Within each category there is an upward trend, rather more pronounced in the moderate alkalinity lakes, as we move through Pearsall’s sequence. I suspect that this represents the interaction between geology and man, with higher TDI values associated with lakes where there is more agriculture and greater population density. These factors may, in turn, combine to affect the physical factors within the lake over time, but the implication that a “primitive” lake such as Ennerdale Water might one day “evolve” to have characters similar to those of Windermere is no longer accepted. On the other hand, he did set up some testable hypotheses that kept freshwater ecologists occupied for a long time subsequently. As Lao Tzu reminded us: “a journey of a thousand miles begins with a single step”…
Average lake TDI values (using data from Bennion et al., 2014) for Lake District water bodies, arranged by Pearsall’s evolutionary sequence (no data for Hawes Water). Open circles are low alkalinity lakes; closed circles are moderate alkalinity lakes.
Bennion, H., Kelly, M.G., Juggins, S., Yallop, M.L., Burgess, A., Jamieson, J. & Krokowski, J. (2014). Assessment of ecological status in UK lakes using benthic diatoms. Freshwater Science 33: 639-654.
Clapham, A.R. (1971). William Harold Pearsall. 1891-1964. Biographical Memoirs of Fellows of the Royal Society 17: 511-540.
Pearsall, W.H. (1921). The development of vegetation in the English Lakes, considered in relation to the general evolution of glacial lakes and rock basins. Proceedings of the Royal Society of London Series B 92: 259-285.