Discussion

3.7.1 L ake environm ents

morphometry and site, climate and water chemistry (Figure 3.1). All the lakes are glacial

in origin, small (<4.3 hectares) and generally take the form of a central trough surrounded by a sub-lacustrine shelf 1-2 metres deep formed by moraine damming. All

the lakes are freshwater, even those close to the coast. With the exception of Pumphouse

Lake, which has undergone some anthropogenic modification of its shoreline, all lakes

are natural systems. Strong seasonal gradients produce high interannual variability in

physical, chemical and biological processes but the synthesis of several years data as

mean values to match the resolution of the surface-sediment samples (Chapter 4)

smooths interannual variability, so emphasizing spatial effects (site differences) over

temporal effects in data analysis. Lake water chemistry, especially nutrient status,

strongly determines differences in the lake environments in PCA and this gradient is also

reflected in the results of cluster analysis. Site groupings closely match those of Jones,

Juggins & Ellis-Evans (1993). The major environmental gradient contrasts indicators of

high trophic status (total phosphate, soluble reactive phosphate, total dissolved nitrogen,

chlorophyll-fl and phaeopigments) and associated ions (calcium and magnesium) with

nutrient-poor, higher-altitude inland sites with low productivity but high benthic biomass.

The marine influence is ubiquitous and all lakes receive some nutrient and ion inputs

from precipitation and snowmelt (Allen, Grimshaw & Holdgate, 1967; Hawes, 1983).

Naturally, these effects are greatest near the coast. Eutrophication is most enhanced in

lakes which are affected by biotic agents; seals are positively correlated with nutrients (TON and TP) and productivity indicators, supporting the causal link for enhanced

eutrophication identified in Heywood Lake by Ellis-Evans (1990). Shallow lakes o f the

coastal zone also have higher summer water temperatures and longer periods of open

water to sustain a seasonal phytoplankton population. Phytoplankton growth in summer

is responsible for the poor clarity of these eutrophic lakes (Heywood, Knob, Amos and

Bothy Lakes). Wind exposure additionally increases turbidity of the lake waters.

Branchinecta positively correlate (r=0.56,/7<0.05) with shallow, productive sites as they

require algae as food sources and longer summer seasons to complete their life cycle

(Bjorck et a l, 1996). The presence of Branchinecta in Lake 13 is interesting since it has

not been recorded at this site before; shallowing of the lake profile by silting and

reduced meltwater inputs with a shift in the ice-front has created a more favourable

environment for their existence. Pseudoboeckella is less clearly related to lake trophy

Most lakes are ice-free for 3-4 months per year and ice-cover duration has been

decreasing over the past 25 years, probably in response to climatic change (Smith, 1990). Unfortunately regression reconstructions with air temperature records have failed

to find good agreement (R. Thompson, pers.comm.). Maximum summer water

temperatures reach 5-6°C. A seasonal temperature component is found in the second

ordination gradient; the three temperature variables are the only determinants in the data­

set which directly integrate seasonal lake climate. The water temperature gradient

reflects lake morphometry, especially basin depth, as well as altitude, exposure, length

of the open water period, the influence of influent meltwaters and bottom warming.

Lowland eutrophic lakes, such as Heywood, Amos and Bothy, typically have high

summer surface water temperatures; longer open water periods allow more warming and

the number of open days is positively correlated with high summer water temperatures

(/^0.68, /7<0.01). Winter temperatures are lower due to heat conduction through the lake

ice (Heywood, 1968) with minimal insulation by irregular snow-cover; minimum winter

temperatures are close to zero and are observed in Heywood, Knob, Spirogyra, Tioga

and Bothy Lakes. The opposite trend is observed in inland oligotrophic lakes. High

through-flow of meltwater in summer depresses lake water temperatures, especially in

glacier-fed lakes such as Emerald, Tranquil, Sombre, Pumphouse, Moss and Changing;

winter temperatures are higher because longer periods of lake ice-cover coupled with a

seasonal snow-cover, encouraged at these sites by topographic shelter, insulates the lakes

from heat losses by conduction. Data-set bias also enhances these temperature effects

since water temperature readings are surface values which show greater variability than

bottom waters. Unfortunately, lack of detailed records of snow-cover over the lake ice

led to its preclusion as a variable in the analysis.

Morphometric variables have significant collinear relationships (Table 3.8) and

morphometry is an important determinant of between-lake variation in PCA, separating

lakes on the basis of their overall size and depth. Maximum depth was a highly

significant determinant of lake character in a previous lake classification scheme

(Heywood, Dartnall & Priddle, 1980). There is a clear gradient in ordination of

vegetation type in lakes of contrasting trophies and morphometries. Only the deeper

lakes with larger volumes, low concentrations of suspended materials, good light

transmission and quiescent conditions favour the support of perennial phytobenthos

(Light & Heywood, 1973; Hawes, 1990). A large influx of summer runoff from the

Wynn-W illiams, 1985) and short residence times in lakes with a high catchmentilake

area ratio (Hawes, 1990) bring sufficient nutrient supplies into even the most

oligotrophic lakes to maintain a high benthic biomass in the form of filamentous algae

and freshwater mosses which are extremely efficient nutrient sequesters (Hawes, 1988).

Therefore, oligotrophic lakes (e.g. Moss Lake) can support complex perennial

communities despite their isolation from marine nutrient sources and short periods of

open water. Lakes with small basin volumes are prone to winter anoxia (Hawes, 1983),

significantly affecting algal mat growth (Gallagher, 1985) and when extreme, causing

complete degradation of the phytobenthos (Hawes, 1988). In the deeper lakes, anoxia

is restricted to greater depths: for example, below 9 metres in Sombre Lake.

3.7.2 Catchment environments

Fourteen variables in the data-set characterise the catchment environment including

measures of site, exposed bedrock, land-cover and fauna (Figure 3.1). Some variables

do not possess the same temporal precision owing to lack of recent survey data (e.g.

bedrock and vegetation) but their values are still sufficiently representative of the

adopted 1993 control year to provide a meaningful addition to the analysis. Catchments

vary in complexity from domination by permanent ice-cover to extensive areas of moss

vegetation, often frequented by birds and seals. Catchment development is matched by

characteristic lake trophies in PCA (cf. Priddle & Heywood, 1980). The principal bedrock - quartz-mica-schist - shows maximal exposure in the deglaciated coastal

lowlands. The interplay of % marble and % amphibolite in the second PCA gradient

suggests some source-1 ithology effects on lake characteristics, associated with

oligotrophic waters, ice-cover and altitude (Moss, Pumphouse, Light, Gneiss Lakes).

Smith (1990) noted the alkaline reactions of morainic materials and cations (Ca^"^, Mg^"^,

K^) in these lakes probably derive from sub-glacial materials independent of marine

sources. Mackereth (1966) regarded these cations as glacial indicators in lake sediment

sequences.

The overall island altitude is not particularly high (maximum 279 metres) but still

provides an environmental gradient affecting lake character. This is strongly associated

with distance from the sea and catchment ice-cover. Altitude is a surrogate variable for

accessibility by marine organisms to the lakes. Thus lakes at high altitude are too remote

for marine organisms and are largely excluded from nutrient-enrichment. Altitude is too

climatic component since permanent ice cover is largely restricted to higher ground. Ice

cover in PCA has a strong effect on site groupings. Moss and lichen vegetation commonly occur together sharing a significant relationship (r=0.67, /?<0.01). In this

analysis, the lakes are more sensitive to the presence or absence of vegetation rather

than actual vegetation type. The gross categories of ’m oss’ and ’lichen’ lack a

significant response to other catchment variables unlike the more detailed study by

Jeffers (1977) who identified several significant environmental variables explaining

terrestrial vascular plant distributions on Signy Island. The mesotrophic status of Light

Lake may be partly explained by percolation through moss stands leading to higher

nutrient loadings (Collins, Baker & Tilbrook, 1975).