Eight lake sites in central and south-west Scotland, north-west England and north
Wales, forming part of the UK Acid Waters Monitoring Network (UKAWMN), have been studied
with regard to the influence of marine ions on surface water chemistry. Since monitoring
began in 1988 these sites have exhibited large and long-term variation in Cl concentration,
which are consistent between regions and can be linked to inter-annual variations
in wet deposition. Through regression analysis against Cl, the response of other solutes
to these fluctuations has been assessed. Sites show a highly consistent pattern of
Na, and Mg retention during periods of high Cl, in accordance with the 'sea-salt'
mechanism of marine cation adsorption onto soil exchange sites following large marine
inputs. An associated displacement of cations with non-marine sources is also observed,
with one or more of non-marine Ca, labile Al and hydrogen ions exhibiting a positive
relationship with Cl at all sites. The relative extent to which these are released
appears not to follow a simple relationship to site acidity, and may be linked to
site/region-specific geology and soil characteristics. In addition, an inverse relationship
between non-marine SO4 and Cl is observed at five of the sites, and the possibility
is considered that a sea-salt related process, with soil retention and subsequent
release, may also operate for SO4. A mechanism that might explain this process is
suggested. The impact of marine inputs on non-marine solutes, including important
indicators of acidification such as pH, labile Al and non-marine SO4, has clear implications
for the detection of long-term trends in acidity status and is, therefore, of particular
relevance to the UKAWMN. Due to their unpredictability, and the long timescale over
which they operate, fluctuations caused by marine inputs may be difficult to separate
from acid deposition related long-term trends. Evidence from a longer Cl time series
from mid-Wales shows that fluctuations in concentration could be linked to the North
Atlantic Oscillation and might therefore be expected to exhibit a similar, decal periodicity.
Currently, the UKAWMN dataset only appears long enough to represent one climatic cycle.
Consequently, and since few surface water chemistry datasets in the UK extend over
more than a decade, it is important that: (a) trend analyses of current data from
marine-impacted areas take account of possible marine input cycles; and (b) long-term
monitoring is maintained into the future so that the impact of these cycles can be
better quantified, and distinguished from anthopogenically-induced long-term changes.