Journal of Environmental Science and Health Part A, 41:849–863, 2006
C Taylor & Francis Group, LLC
Copyright ISSN: 1093-4529 (Print); 1532-4117 (Online)
DOI: 10.1080/10934520600614462
Trophic Status of Experimental
Cutaway Peatland Lakes in
Ireland and Implications for
Future Lake Creation
Tara Higgins and Emer Colleran
Environmental Microbiology Research Unit, Department of Microbiology, National
University of Ireland, Galway, Ireland
Of the 80,000 hectares of Bord na Móna owned peatland coming out of industrial
production in Ireland approximately the next 25 years, over 20,000 hectares has been
designated for shallow lake creation. Four experimental lakes created by flooding areas
of redundant cutaway peatland in Co. Offaly were monitored over a 3-year period in
order to obtain baseline information on their water quality and trophic status. Results
indicate that water chemistry in the constructed lakes was predominantly influenced
by the depth and type of the residual peat layers at the sites, the degree of exposure of
underlying inorganic subsoils and the type of hydrological regime. Nutrient status was
strongly governed by catchment land-uses. Lack of recolonising vegetation at recently
abandoned cutaway peatland sites made some new lakes particularly vulnerable to
nutrient runoff and algal bloom development. Biologically, the embryonic lakes were
characterised by rudimentary food chains, in which higher trophic levels were absent
and where the microbiota played an elevated role.
Key Words: Cutaway peatland; Lake creation; Water quality; Trophic status; Phytoplankton.
INTRODUCTION
By 2030, over 80,000 hectares of industrially milled peatlands will have come
out of production in Ireland. The Irish Peat Board, Bord na Móna, has been
investigating alternative post-harvesting land-uses for cutaway peatland since
the 1970s. Research has identified serious problems associated with the two
conventional and economically attractive after-uses of commercial forestry
Address correspondence to Tara Higgins, Environmental Microbiology Research Unit,
Department of Microbiology, National University of Ireland, Galway, Ireland; E-mail:
t.higgins1@nuigalway.ie
849
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Higgins and Colleran
and agricultural grassland, including nutrient deficiencies, water-logging, soil
subsidence, frost damage and pest control.[1,2] As a result, more than 50% of
all future cutaway peatlands in Ireland will be designated as non-commercial
semi-natural wilderness areas.[3] Much of this low-lying land will be allowed to
flood, creating approximately 20,000 hectares of shallow lakes. Adjacent drier
areas will be allowed to revegetate naturally to create a diverse mosaic of
grassland, bog, fen and scrub habitats surrounding the new water bodies.[4]
The scale of the proposals is hugely significant, representing one of the largest
habitat creation opportunities to emerge in modern Europe.
Bord na Móna began creating lakes by flooding redundant cutaway
peatlands in 1991.[5] The focus in Ireland has been on the creation of large
(4–60 ha) and reasonably deep (>1 m) water bodies, designed principally
for conservation and amenity purposes.[6] This approach differs fundamentally from cutaway peatland rehabilitation projects in the UK,[7] Holland,[8]
Germany,[9] Scandinavia[10] and North America,[11] where the focus is on
shallow re-wetting, with the aim of promoting the re-growth of Sphagnum
peat-forming communities. Over 400 ha of experimental lakes have been
created in Ireland to date, centred on a 2,000 ha cutaway site in Co. Offaly. This
pilot project, called the Lough Boora Parklands, is being viewed as a blueprint
for future large-scale development of integrated land-uses on Ireland’s cutaway
peatlands.[3]
Early cutaway lake creation projects in Ireland involved a considerable
level of on-site development work. Prior to flooding, much of the residual
peat substrate was removed from the cutaway peatland to create a lake
basin. Underlying mineral sub-soils were typically exposed during this process,
including a mixture of silty clays, glacial till soils, gravel and calcareous shell
marl.[6] Embankments were formed around the lake basin using the excavated
peat, artificial drainage channels were in-filled and the basin was allowed to
flood to a depth of 1–2 m from a combination of precipitation, groundwater
spring discharges, and surface drainage.[12] In some cases, water levels in the
new lakes were supplemented by an introduced piped inflow, diverted from a
nearby natural stream. A small number of lakes were designed specifically for
angling purposes and pioneering aquatic plants such as charophytes, together
with a variety of macroinvertebrate species, were introduced to initiate and
assist the natural colonisation of the lake.[13] In more recent years, less sitepreparation has been conducted prior to flooding. Shallow lake creation has
concentrated on cutaway sites that are naturally low-lying. Peat excavation is
minimal and sites are flooded by simply blocking the on-site drainage network
which was constructed prior to the commencement of peat harvesting. Lakes
created according to this strategy are allowed to recolonise naturally, with
minimum human interference.
Lakes created by reflooding areas of industrial cutaway peatland represent
new and essentially artificial phenomena. Very little is understood about the
Industrial Cutaway Peatland and Lake Creation in Ireland
ecology of these systems and no true basis in experience exists on which
to predict their development. At the same time, these water bodies provide
a unique and exciting opportunity to trace the course of development of
embryonic shallow lake ecosystems as they evolve and mature from the time
of their inception. As part of wider research in Ireland into the ecological
value of lakes created on industrial cutaway peatlands, this research aimed
to investigate the baseline physicochemical properties and nutrient status of
a selection of representative cutaway lake types; to assess the abundance and
community structure of phytoplankton assemblages in the new lakes, which
serve as valuable bio-indicators of water quality; and to make recommendations regarding the future creation of lakes on cutaway peatland which will
optimise their water quality and, in turn, conservation potential.
MATERIALS AND METHODS
Physicochemical Parameters
Four cutaway peatland lakes, Finnamore, Tunduff, Turraun, and Clongawny, were sampled between August 2001 and September 2004 at 2-week
intervals for the first year and monthly thereafter. Near surface (1 m) water
samples were collected at two opposite sampling stations at each lake. pH,
conductivity, dissolved oxygen and temperature were determined on site
using a portable field kit (WTW P4 Multiline). Dissolved colour was read
spectrophotometrically at 465 nm on samples filtered through GF/C filters.[14]
Alkalinity and turbidity were calculated using a standard H2 SO4 titration and
nephelometric (WTW Turb 565) methods, respectively.[15]
Nutrient Variables
Soluble reactive phosphorus (SRP), total soluble phosphorus (TSP) and
total phosphorus (TP) were analysed using the ascorbic acid reduction
method,[16] involving persulphate digestion for TSP and TP determinations.
SRP and TSP were analysed on samples filtered through GF/C filters, while
unfiltered samples were used for TP analysis. Ammonium was determined
on filtered samples according to the indophenol blue method.[17] Nitrite and
nitrate were measured spectrophotometrically on filtered samples using the
Hach low range diazotization (Method 8507: range 0–0.3 mg NO2 − -N L−1 ,
precision ± 0.0006 mg NO2 − -N L−1 ) and cadmium reduction methods (Method
8192: range 0–0.5 mg NO3 − -N L−1 , precision ± 0.01 mg NO3 − -N L−1 ), using
custom-devised low range calibrations.[14] The sum of nitrate, nitrite and
ammonium is reported, hereafter, as dissolved inorganic nitrogen (DIN).
Soluble organic and inorganic carbon was measured on filtered samples using a
851
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Higgins and Colleran
Shimadzu TOC-5000A analyser.[15] Silica was determined on filtered samples
using a low-range heteropoly blue method.[14] All nutrient analyses were
performed in triplicate for both sampling stations at each lake; data presented
are overall means for each lake, ± standard error of mean (SEM).
Phytoplankton Analysis
Samples for chlorophyll-a analysis were filtered immediately on return to
the laboratory through GF/C filters, extracted using a mixture of 90% acetone
and dimethly sulfoxide (1:1 v/v)[18] and measured spectrophotometrically with
correction for phaeopigments.[19] Phytoplankton cells preserved in Lugol’s
iodine were identified, measured and counted with an inverted microscope.
Biovolumes were calculated by comparing individual cells to simple geometric
shapes and applying relevant standard formulae.[20] Cells of less than 10 µm
in size were classed as nanoplankton. Phytoplankton species diversity was
assessed using Simpson’s index of diversity,[21] which ranks samples on a scale
of 0 to 1, where 0 denotes no diversity (i.e., a community comprised of a single
species) and 1 represents maximum diversity (all species in a community
are present in equal proportion). Relationships between chlorophyll-a and
total phosphorus was determined on log10 -transformed data using Pearson
correlation coefficients.
RESULTS
Study Sites
The four cutaway lakes included in this study, Finnamore, Tumduff,
Turraun and Clongawny, are situated in the vicinity of the Lough Boora
Parklands, County Offaly, in the heart of the Irish Midlands. The principal
characteristics of the four study lakes are listed in Table 1. Table 2 presents
the physico-chemical results for the four study lakes. Finnamore, Tumduff and
Turraun had a similarly alkaline pH (8.1–8.2), high levels of conductivity and
alkalinity and moderately low amounts of colour and turbidity. In contrast,
Clongawny lake was very acidic (mean pH 4.6 ± 0.04, n = 54), highly
coloured and turbid, with concurrently low concentrations of conductivity and
alkalinity. Dissolved oxygen levels were high in all four lakes (>10 mg L−1 ).
The observed temperature ranges (<1◦ to >20◦ C) represented large seasonal
fluctuations in water temperature.
Nutrient concentrations in the four study lakes are presented in Table 3.
Total phosphorus levels were low in Finnamore and Tumduff (<16 µg L−1 ),
moderately high in Turraun (mean 26.7 ± 1.5 µg L−1 , n = 54) and very high in
Clongawny (39.1 ± 3.5 µg L−1 , n = 54). Soluble reactive phosphorus levels were
consistently low (<3 µg L−1 ) in all four lakes. Dissolved inorganic nitrogen
853
Late-1980s
Mid-1970s
Tumduff
(N18 18)
Turraun
(N17 23)
Clongawny 1993/94
(N07 13)
Mid-1980s
Finnamore
(N21 20)
2001
1991
1995
1996
Harvesting Lake
ceased created
12
60
6
4.8
1
0.5
1
1.5
Surface Mean
area, depth,
m
ha
>1
Phragmites
peat
Precipitation, Sphagnum,
runoff
Woody &
(ClonPhragmites
gawny)
peats
(Clongawny)
Typha latifolia ,
Chara spp.
Juncus spp.
Glyceria
fluitans
Vegetation
263
Post flooding
management
71 Environs
planted;
invertebrates,
fish &
Charopytes
introduced
21 Environs planted
Wet Dry
57
Eriophorum
anguslifolium,
Agrostis
stolonifera,
Carex spp.
0–0.5 Carex spp. Typha 122 38 Environs planted
latifolia,
Phragmites
australis
>1.5 Phragmites
6 0.6 None
australis, Juncus
effusus
0
Silty clays
(Finnamore)
gravel,
glacial till
Sediments
Precipitation, Phragmites
peat, Marl
gw springs
(Turraun)
Piped inflow
Piped inflow
Inflows
Peat
depth,
m
Plant
biomass
kg m−2
Table 1: Characteristics of the four Co. Offaly, Ireland cutaway lakes (Finnamore, Tumduff, Turraun and Clongawny).
854
Higgins and Colleran
Table 2: Physico-chemical characteristics of the four study lakes.∗
pH
Conductivity, µS cm−1
Dissolved oxygen, mg L−1
Temperature, ◦ C
Alkalinity, mg L−1 CaCO3
Turbidity, NTU
Colour, mg L−1 Pt. Co.
∗ Values
Finnamore
Tumduff
Turraun
Clongawny
8.1 ± 0.02
(7.8–8.4)
429 ± 0.89
(204–613)
11.7 ± 0.23
(7.5–16.1)
11.7 ± 0.71
(3.2–20.7)
176 ± 6.83
(107–256)
2.0 ± 0.1
(1.1–8.1)
18 ± 1
(10–52)
8.1 ± 0.02
(7.5–8.6)
365 ± 5.11
(299–446)
10.8 ± 0.26
(7.5–16.0)
11.4 ± 0.75
(1.2–20.6)
126 ± 1.79
(104–146)
2.3 ± 0.3
(0.8–15.9)
69 ± 3
(45–143)
8.2 ± 0.03
(7.6–8.6)
299 ± 5.48
(240–416)
10.7 ± 0.25
(7.6–15.8)
11.6 ± 0.81
(0.7–22.3)
136 ± 2.79
(99–179)
6.3 ± 0.8
(1.19–32.0)
47 ± 1
(29–85)
4.6 ± 0.04
(4.1–5.4)
72 ± 13.84
(57–87)
11.3 ± 0.25
(8.6–16.4)
11.4 ± 0.81
(0.5–23.5)
1.6 ± 0.21
(0.0–7.5)
12.9 ± 1.2
(1.9–36.6)
156 ± 3
(132–255)
shown are means ± SEM (n = 54). Ranges are given in parentheses.
levels were considerably higher in Finnamore (mean 1,560 ± 169 µg L−1 , n =
54) than the other study lakes (<250 µg L−1 ). Clongawny exhibited negligible
levels of silica and dissolved inorganic carbon.
Table 4 characterises the phytoplankton populations in the four lakes.
Phytoplankton productivity, estimated as chlorophyll-a and phytoplankton biovolume, was considerably higher in Clongawny than in Finnamore, Tumduff or
Turraun. The phytoplankton assemblage in Clongawny was simple, dominated
by chlorophytes (mean 58%, n = 54) and dinoflagellates (mean 36%, n =
54). Overall species diversity was very low in Clongawny and a considerable
fraction of the phytoplankton assemblage was comprised of nanoplankton
(cells <10 µm). Lower phytoplankton abundances in the other three lakes
Table 3: Nutrient concentrations in the four study lakes.∗
Finnamore
Total phosphorus, µg L−1
Tumduff
12.2 ± 0.5
15.6 ± 0.5
(6.4–19.3) (10.6—24.9)
Soluble reactive phosphorus, 1.90 ± 0.2
1.59 ± 0.2
µg L−1
(0.00–6.84) (0.00–4.95)
Dissolved inorganic nitrogen, 1,560 ± 169
243 ± 49
µg L−1
(43–4,213) (0.7–1,691.9)
Inorganic carbon, mg L−1
26.6 ± 1.3
21.9 ± 0.6
(6.5–46.7)
(11.6–28.5)
−1
Organic carbon, mg L
17.6 ± 1.0
28.2 ± 0.7
(7.2–34.1)
(17.2–36.3)
Silica, mg L−1
1.33 ± 0.17 0.49 ± 0.06
(0.10–3.84) (0.005–1.70)
∗ Values
Turraun
Clongawny
26.7 ± 1.5
39.1 ± 3.4
(12.2–61.3)
(6.3–98.8)
2.28 ± 0.2
2.74 ± 0.3
(0.04–5.70) (0.57–13.25)
153 ± 26
99 ± 17
(1.7–666)
(0.1–502)
23.8 ± 0.6
0.55 ± 0.2
(11.6–33.1)
(0.0–5.2)
20.7 ± 0.9
29.3 ± 1.0
(10.6–33.5) (19.6–52.2)
0.36 ± 0.05 0.08 ± 0.01
(0.03–1.52) (0.00–0.50)
shown are means ± SEM (n = 54). Ranges are given in parentheses.
855
5.2 ± 0.4
(0.9–13.0)
3.3 ± 0.2
(1.3–9.2)
12.7 ± 1.4
(1.9–49.1)
1,101 ± 130
(28–3,901)
608 ± 83
(7–2,682)
3,057 ± 395
(79–11,446)
Tumduff
Turraun
Taxonomic
composition,
%
0.78 ± 0.02 Diatoms: 49
(0.35–0.90) Chlorophytes: 21
Cyanophytes: 14
Chrysophytes: 13
0.75 ± 0.02 Diatoms: 39
(0.32–0.89) Chlorophytes: 35
Cryptophytes: 8
Chrysophytes: 7
0.69 ± 0.04 Chlorophytes: 39
(0.10–0.94) Diatoms: 28
Cyanophytes: 18
Dinophytes: 7
0.51 ± 0.04 Chlorophytes: 58
(0.07–0.89) Dinophytes: 36
Diatoms: 2
Species
diversity D∗∗
39 ± 6
(0–100)
13 ± 2
(0–55)
15 ± 2
(1–70)
11 ± 2
(1–52)
Nanoplankton
%
Scenedesmus quadricauda ,
Pediastrum boryanum, Navicula
tripunctata , Coelosphaerium
kuetingianum
Cosmarium pygmaeum, Chlorella spp.,
Peridinium umbonatum
Cyclotella spp., Navicula radiosa ,
Synedra spp., Cosmarium
bioculatum, Merismopedia
elegans
Achnanthes exigua , Chlamydomonas
spp., Cryptomonas ovata , Dinobryon
sertularia
Characteristic species
∗ Values shown are means (n = 53) with ranges given in parentheses. Taxonomic compositions and nanoplankton (cells <10 µm) fractions are
expressed as percentage contributions to total phytoplankton biovolume.
∗∗ Diversity scale is from 0 to 1, with 0 representing minimum diversity and 1 representing maximum diversity.
Clongawny 33,641 ± 4,740 52.5 ± 6.2
(10–120,386) (2.0–154.9)
Finnamore
Chl-a
µg l−1
Biovolume
mm3 m−3
Table 4: Phytoplankton abundance and community structure in the four study lakes.∗
856
Higgins and Colleran
Figure 1: (A) Temporal variation in total phosphorus (TP) and chlorophyll-a (Chl-a) in
Clongawny over the course of 3-year study period; (B) Relationship between chlorophyll-a
and total phosphorus (n = 58; ∗ p < 0.0001).
were complemented by higher species diversities, larger cell sizes and greater
abundances of diatoms, cyanophytes and chrysophytes.
Mean total phosphorus concentrations in Clongawny exhibited a distinctive temporal trend (Figure 1A). Total phosphorus levels increased 5.5-fold
over the course of the study period, from 11.8 ± 0.8 µg P L−1 (n = 24) in
the first year of sampling to 65.3 ± 3.1 µg P L−1 (n = 22) in year three.
The northern sampling station exhibited consistently higher (2.2-fold) levels
of soluble reactive phosphorus than the southern station, indicating the source
of the phosphorus to be runoff from adjacent, heavily afforested cutaway peatland. Chlorophyll-a levels closely mirrored total phosphorus concentrations
(Figure 1A), increasing 8-fold over the course of the monitoring period, from
a mean of 11.2 ± 1.6 µg L−1 (n = 24) in year 1 of sampling to a mean value of
89.4 ± 8.9 µg L−1 (n = 22) in year 3 and declining markedly from spring 2004
onwards. An inverse pattern was observed for phytoplankton species diversity,
which declined rapidly as phosphorus levels increased.[22] Figure 1B indicates
that the relationship between chlorophyll-a and total phosphorus levels in
Clongawny was highly significant (P < 0.0001). The outlying value in Figure
1B represents the point at which chlorophyll-a in the lake crashed to 3 µg L−1
in July 2004, before recovering to 50.7 µg L−1 in September 2004.
DISCUSSION
The cutaway lakes were well-oxygenated systems, due to a combination of their
shallow depth (<2 m), large surface-to-volume ratios and the flat, exposed
nature of the sites which resulted in high levels of wind-induced wave action.
These physical characteristics also made the cutaway lakes prone to large
Industrial Cutaway Peatland and Lake Creation in Ireland
seasonal variations in water temperature. Marked differences in the physicochemical characteristics of the cutaway lakes related to the varied nature of
the sediments at the sites, coupled with their contrasting hydrological regimes.
Finnamore, Tumduff and Turraun were alkaline, with high ionic contents, high
buffering capacities and low to moderate levels of dissolved colour. These properties reflect the presence of external hardwater influxes into the lakes, either
in the form of piped inflows from nearby streams or spring discharges from the
limestone aquifer, coupled with the minerotrophic nature of the sediments at
these sites, which comprised a mixture of mineral-rich fen peats, calcite-rich
marls and blue silty clays. The presence of these inorganic, minerotrophic
sediments served as a reservoir of bases, offsetting any incoming acidic
drainage water and maintaining an alkaline pH in the flooded cutaways.[23]
The chemical characteristics of the youngest cutaway lake, Clongawny,
contrasted sharply with the other three lakes. Clongawny was very acidic and
highly stained, with a weak ionic content. These characteristics were direct
consequences of the exclusively peaty nature of the substrate at Clongawny,
which included considerable amounts of residual ombrotrophic Sphagnum
peat, coupled with the fact that the lake was fed entirely by rainwater and
associated surface drainage from the surrounding actively milled peatfields.
Nutrient levels in the four lakes reflected, to a large extent, land-uses in
the immediate catchment areas. Elevated levels of dissolved inorganic nitrogen
in Finnamore reflected the presence of nitrate fertiliser runoff. The piped
inflow at Finnamore lake is diverted from a nearby stream which drains
intensively grazed agricultural grassland. It is well established that the high
mobility and export rate of nitrates in arable land[24] contribute to the high
inorganic nitrogen levels observed in streams affected by agriculture.[25] These
findings give a clear indication of the impact of stream quality on the nutrient
status of cutaway lakes containing an introduced inflow.
Clongawny lake experienced a very significant 5.5-fold increase in phosphorus levels over the course of the 3-year sampling period. This increase
was the result of phosphate fertiliser runoff from adjacent coniferous forestry
plantations.[22,26] Cutaway peatland is particularly susceptible to phosphorus
leaching, due to the high runoff and erosion risk of cutaways,[27] coupled
with the naturally poor sorption capacity of peat particles arising from low
levels of chelating aluminium and iron ions.[28] Current results highlight the
vulnerability of cutaway lakes to nutrient enrichment arising from watershed
land-uses.
Finnamore and Tumduff lakes were characterised by very low phytoplankton biomasses, complemented by generally high values of species diversity. This pattern was concomitant with the low phosphorus levels in both
Finnamore and Tumduff, supporting the premise that phosphorus generally
regulates phytoplankton growth in freshwaters.[29] Two main algal groups, the
857
858
Higgins and Colleran
diatoms and the Chlorophytes, characterised the phytoplankton populations
in Finnamore and Tumduff. Individual phytoplankton cell sizes tended to
be medium to large, and included a diverse range of desmids, which are
characteristic of lakes in peaty catchments.
Phytoplankton abundances in Turraun were higher than in Finnamore or
Tumduff, corresponding with the higher phosphorus levels recorded in this
lake. The phytoplankton community in Turraun was generally highly diverse,
and this site recorded the highest species richness of all the lakes (Simpson’s D
= 0.94). Turraun exhibited a high proportion of Cyanophytes, which, at certain
times, dominated the phytoplankton assemblage in the lake.
Phytoplankton concentrations were particularly high in Clongawny. As the
sampling period proceeded, chlorophyll-a concentrations increased to levels
defined by the OECD[30] as hypertrophic. This finding was unexpected, given
that heavily stained, acidic bog lakes, such as Clongawny, are generally
very unproductive systems, due to a combination of rapid underwater light
attenuation[31] and naturally low availabilities of inorganic nutrients.[32] The
increase in phytoplankton productivity in Clongawny was stimulated by the increased availability of phosphorus (Figure 1B), indicating that phytoplankton
productivity in the lake was hitherto strongly phosphorus limited. Nitrogen
limiting conditions subsequently developed in Clongawny in response to the
elevated phosphorus availability,[22] mirroring trends from other artificially
enriched lakes worldwide.[33,34]
The simple composition of the phytoplankton, comprising a dual dominance of motile dinoflagellates and highly buoyant, minute Chlorophyte
species, reflected physiological adaptations to the limited light regime[35] and
the very low pH[36] in Clongawny lake. The absence of diatoms in Clongawny
related to the silica deficiency in this lake, a nutrient essential for the
construction of the diatom frustule.[37] Cyanophytes, which frequently develop
bloom populations in response to phosphorus enrichment,[38,39] are likely to
have been suppressed in Clongawny by the low pH.[36,40]
The very strong response of the phytoplankton population in Clongawny
to the increased input of phosphorus was enhanced by the paucity of recolonising higher vegetation at Clongawny. On recently abandoned cutaway
peatland, such as Clongawny Bog, plant recolonisation is hampered by a lack
of viable seed banks[41–43] and the inhospitable physical environment (bare,
unconsolidated peat surface, extreme temperature fluctuations) which retards
the establishment of propagules.[44,45] Aquatic and terrestrial macrophytes,
such as those abundant in Finnamore, Tumduff and Turraun, can effectively
buffer lakes from excessive nutrient loading by filtering nutrients from
surface runoff,[46,47] competing directly with phytoplankton within lakes for
nutrients[48] as well as providing habitats and refugia for grazing zooplankton.
Submerged macrophytes have the added benefit of reducing water column
turbulence and increasing water clarity through a reduction of sediment
Industrial Cutaway Peatland and Lake Creation in Ireland
resuspension and an increase in nanoplankton sedimentation rates.[49,50] In
the absence of higher vegetation, primary productivity in Clongawny was overwhelmingly phytoplankton-driven. The availability of incoming phosphorus in
Clongawny solely for phytoplanktonic primary production elicited chlorophylla responses (Figure 1B) of more than double the magnitude predicted by either
empirical lake chlorophyll-phosphorus loading models[30,51,52] or regression
lines measured for other three cutaway lakes in this study (data not shown).
The high chlorophyll values observed in Clongawny appear to signify a
major deficiency of top-down (i.e., grazer) control in the lake. Analyses of
the zooplankton assemblage in Clongawny revealed high numbers (up to 305
animals l−1 ) of the large-bodied predacious cyclopoid copepod Tropocylops prasinus (T.Higgins, unpublished data). It is possible that invertebrate predation
by T. prasinus influenced the unusually low numbers of small herbivorous
cladocerans and rotifers[53,54] recorded in Clongawny and, subsequently, the
lake’s high phytoplankton biomass. The success of T. prasinus in Clongawny
is likely to reflect the rudimentary nature of the food web in the lake,
characterised by a notable absence of higher predators, such as fish or
macroinvertebrates, which normally suppress the proliferation of predatory
copepods.[55] In contrast to Clongawny, the longest established cutaway lake,
Turraun, had a rich and well developed macroinvertebrate population. These
findings emphasise the importance of age in the ecology of cutaway lakes. Unlike populations of phytoplankton, microinvertebrates[57] and protozoans,[58]
which establish very quickly after cutaway peatland is flooded, higher trophic
groups require a considerable length of time to colonise new water bodies.[59]
Age increases both the length of time available for colonisation and also
influences sediment characteristics, food availability, vegetation cover and
plant species richness. It can be expected that younger cutaway lakes such
as Clongawny will become colonised by larger invertebrates in time; further
monitoring would be highly desirable to assess how this pioneering ecosystem
develops.
CONCLUSIONS
The cutaway lakes were atypical aquatic systems. Water chemistry was
determined by a combination of the underlying lake substrates, in particular
the type and depth of the residual peat layer, and the nature of the inflows
into the lake. The findings of the present study, although somewhat limited
due to the anthropogenic enrichment of the only acidic study lake, Clongawny,
illustrated a positive impact on water quality and overall biological species
diversity of introducing some inorganic mineral influence when creating lakes
on cutaway peatland. This could be achieved either by excavating sufficient
residual peat to expose the calcareous subsoils or by introducing a mineral
859
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Higgins and Colleran
surface or groundwater inflow. Nutrient status was found to be predominantly
influenced by watershed land-uses such as agriculture and forestry,
highlighting the need to take an integrated, catchment-based approach to the
future designation of post-harvesting uses of cutaway peatland in Ireland.
Biologically, the cutaway lakes were characterised by a rudimentary food
chain in which the higher trophic levels were absent and the microbiota
assumed an elevated importance. Length of time in existence had a crucial
impact on cutaway lake ecology, influencing both the degree of revegetation
by macrophytes, which provided a valuable buffering effect in older lakes
against external nutrient inputs and excessive phytoplankton growth, and
also influenced food web dynamics through increasing the colonisation time
for macroinvertebrates and higher trophic groups. Longer-term monitoring of
cutaway lakes is required in order to ascertain the processes and time scales
involved in the establishment, development and eventual stabilisation of these
unique ecosystems.
ACKNOWLEDGMENTS
The authors wish to acknowledge Bord na Móna for funding this research
and the Environmental Change Institute, NUI Galway for providing research
facilities.
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