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has an extensive historical macrophyte record available both from literature
sources (e.g. Hooker 1821; Balfour & Sadler 1863; West 1910; Spence 1964;
Jupp & Spence 1977a, b) and contemporary monitoring data (e.g. Robson 1986,
1990; Murphy & Milligan 1993; Griffin & Milligan 1999; Carvalho et al . 2004).
Plant macrofossils have been analysed in sediment cores from Groby Pool
(Davidson et al . 2005) and Loch Leven (Salgado 2006; Salgado et al . 2009) and
the records compared with the old plant survey data. Hence, the data sets for
these two lakes provide a rare opportunity for a comparative study of the botanical
record with the aquatic plant history as revealed by the sediment record.
At both sites, there was good agreement in the timing and nature of changes in
the selected aquatic taxa between the two data sources, with both data sets
suggesting a clear succession in the aquatic flora over approximately the last 150
years (Figs. 9.2 and 9.3). However, both methods are affected by inherent bias.
Old plant records are generally patchy and in the Groby Pool dataset, a gap in the
historical record between 1747 and 1835 gives a false impression of species
absence (Fig. 9.2), whilst at Loch Leven there is a paucity of survey data for the
period 1910-66 (Fig. 9.3). Furthermore, the botanical surveys are likely to be
skewed towards rarities which the sediment record is less likely to capture (e.g.
Zhao et al . 2006). Macrofossils generally underestimate past species richness,
with approximately 40% of the historically recorded aquatic taxa in Groby Pool
represented by macro-remains, and pondweeds being particularly poorly
represented (Davidson et al . 2005). Nevertheless, when comparing the macrofossil
remains against the historical species described for the eastern side of St Serf
Island, the coring location in Loch Leven, the fossil record captured 79% of the
historically recorded species. Conversely, several taxa were absent from the old
plant surveys but were present in the macrofossil data including, at Groby Pool,
Utricularia vulgaris agg., Myriophyllum alterniflorum (Fig. 9.2) and Characeae
(Fig. 9.4) and, at Loch Leven, Isoetes lacustris, Lobelia dortmanna and Elatine
hexandra (Fig. 9.3).
The Groby Pool example also highlights the complementarity of the pollen
record which adds information on species that leave few macro-remains such as
Littorella uniflora and can provide more accurate information on the occurrence
of Potamogeton spp. than seeds and leaves (Fig. 9.2). Given that historical
records are lacking for most sites, these two case studies illustrate that macrofossils
can be used on their own to provide a reliable record of the dominant components
of the plant community. Additionally, these studies demonstrate the value of
using a combination of methods (here historical plant records and macrofossils)
to describe the ecological reference condition (albeit not the reference condition
sensu the European Water Framework Directive definition of 'no, or only very
minor' evidence of distortion (European Commission 2000), and indeed the use
of multiple groups of organisms to establish reference conditions is gaining in
popularity.
Multiple (palaeo) indicators
The sediment record contains the remains of a range of biotic components
which can be examined individually or in combination (multi-proxy study) to
assess changes in biological structure. Although much early work focused on
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