The fractionation of phosphorus in some modern and late-Holocene calcareous tufas in North Yorkshire, UK - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 235

The fractionation of phosphorus in some modern and late-Holocene
calcareous tufas in North Yorkshire, UK

ALLAN PENTECOST 1,2
1 The Freshwater Biological Association, Ferry Landing, Far Sawrey, Ambleside, Cumbria,
UK LA22 0LP (e-mail: [e-mail])
2 King's College, Strand, London UK WC2R 2LS

Abstract: Phosphorus fractionation studies were undertaken on seven UK tufas: three modern,
and four old. The phosphorus in the carbonate fraction of the tufa averaged 19% of the total phos-
phorus in the modern material, and from 48 - 64% in the 4000 year-old deposits and the increase
attributed to the mineralization of the contained organic matter. Two further phosphorus fractions
contained significant amounts of P. The dithionite fraction ranged from 10 - 27% of the total, and
most of this fraction was probably associated with detrital iron minerals. The alkali-soluble fraction
which removed most of the organically-bound P was highest in the modern bryophyte tufas.
Total P levels in the tufas ranged from 33 - 119 ppm. These are low values for stream sediments.
Phosphorus uptake rates into tufa were estimated for these sites. They were less than 5% of the
incoming P and deemed to have a negligible effect on the aquatic biota.

Actively-forming deposits of calcareous tufa are
often festooned with aquatic plants such as algae
and bryophytes. While our knowledge of the flora,
and to some extent, the fauna of these deposits in
Europe is good (Durrenfeldt 1978), the nutritional
ecology of the streams in which the deposits and
the associated biota occur has received little atten-
tion. This is in contrast with lake and stream systems
in general where a large amount of information
exists on the relationships between nutrient levels
and biota (e.g. Wetzel 2001; Prior & Johnes 2002).
Phosphorus is one of the most important micronutri-
ents for aquatic organisms, as it is often in such short
supply that it limits the growth rates of commu-
nities. When present in excess through human activ-
ity it can lead to dramatic changes in the biomass
and composition of aquatic ecosystems (Dodds
2002). The low solubility of most inorganic phos-
phorus compounds contrasts strongly with another
important nutrient, nitrogen, and results in the depo-
sition of phosphorus in freshwater sediments. These
sediments, under certain conditions, can release
phosphorus back into water where it becomes avail-
able to plants, and freshwater sediments can provide
a major reservoir for this element. Tufa, as a fresh-
water sediment, may also behave in this manner.
Phosphorus fractionation studies, using a range of
extractants, permit some understanding of the asso-
ciation of this element with the different mineralogi-
cal and organic components of sediments. From this
it is often possible to predict the mobility of this
element within the sediment and also understand
the phosphorus pathway from water to sediment.
In order to provide some preliminary data that
will help answer these questions, a small number

of samples from modern and late Holocene tufas
have been investigated, to determine their total
phosphorus content, and the fractionation of the
element within them.

Materials and methods

Seven samples of tufa were collected from deposits
in the Yorkshire Dales, UK. They consisted of two
modern bryophyte-dominated samples, two
modern alga-dominated and three inactive deposits
dating from 0 - c. 4000 years in age (Table 1).
They were chosen to provide a representative
sample of the dominant plant associations, modern
and fossil within this area. Five separate sub-
samples from each site were pooled to provide a
single sample for analysis. All samples were
retained moist for the fractionation studies, since
anomalous results have been reported from
samples that have been dried (Golterman 1996).
To determine gross composition, sub-samples
were first dried at 105 8C for 2h then lightly
ground and weighed, and the calcium carbonate
removed by dissolution in M HCl. The residues
were washed in distilled water, ashed at 550 8C for
2h and the organic and acid-insoluble inorganic
residue determined by difference.
For the phosphorus fractionation about 260 mg
of material was used and the P extracted according
to the method of Golterman (1996) for lake sedi-
ments. This method is based upon a series of chemi-
cal extractions to remove the phosphorus associated
with the main mineral and organic components. An
initial cold water extraction of the material (25 ml)