Applied Vegetation Science 9: 223-230, 2006
© IAVS; Opulus Press Uppsala.
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223
1*
1,2
3
1 Environmental Microbiology Research Unit, Department of Microbiology, National University of Ireland Galway,
Ireland; 2 E-mail emer.colleran@nuigalway.ie;
3 Martin Ryan Marine Science Institute, National University of Ireland Galway, Ireland; robin.raine@nuigalway.ie;
* Corresponding author; Fax +353 91512510, E-mail t.higgins1@nuigalway.ie
Abstract
Question: Which nutrient limits primary production in a lake created by flooding industrial cutaway peatland?
Location : Clongawny Lake (53
°
10' N, 07
°
53' W), County
Offaly, Ireland
Methods: Nutrient concentrations in lake water and the dynamics of phytoplankton populations were monitored over a
38-month period. The ratio of dissolved inorganic nitrogen to total phosphorus (DIN:TP) and nutrient enrichment bio-assays were used to investigate temporal changes in nutrient limitation.
Results: Primary production in the new lake was phytoplankton-driven due to the scarcity of recolonizing macrophytes. Phytoplankton growth was initially phosphorus-limited. The runoff of phosphate fertilizer from an adjacent coniferous forestry plantation raised the TP concentration of lake water 5.5-fold. Consequently, the biovolume of phytoplankton increased 30-fold, and chlorophyll-a concentrations increased eightfold, reaching hyper-eutrophic levels. A concurrent depletion of nitrogen in lake water reduced the DIN:TP ratio from 17.8 to 0.6, and phytoplankton growth rapidly became nitrogen-limited. Phytoplankton composition shifted from dinoflagellates to minute, unicellular chlorophytes, with a coincident decline in species diversity. Cyanobacteria did not proliferate, most likely due to the acidic nature of the lake.
Conclusions: Results illustrated the vulnerability of newly created cutaway peatland lakes to developing severe phytoplankton blooms and coincident secondary nitrogen limitation in the presence of moderate external phosphorus inputs.
Keywords: Bog; Chlorophyte; Lake; Nitrogen; Nutrient limitation; N/P ratio; Phosphorus; Restoration.
Abbreviations : DIN = Dissolved inorganic nitrogen; TP =
Total phosphorus.
Introduction
Oceanic raised bogs originally covered an area of
311 300 ha, or almost 5% of the total land surface, in the
Republic of Ireland, representing one of the highest proportions of raised peatland in the world. Large-scale mechanical extraction of peat has occurred on Ireland’s raised bogs since the establishment of the Bord na
Móna, the National Peat Board, in 1946. Vast expanses of exhausted peatland are progressively coming out of production and, by 2030, over 80 000 ha of industrially milled peatland will have become ‘cutaway’ in Ireland.
Of this, more than half has been assigned for amenity and nature conservation uses (Egan 1998). Drier areas will be allowed to recolonize naturally into a diverse semi-natural wilderness, while areas of impeded draining will be flooded to create 20 000 ha of shallow lakes
(Rowlands & Feehan 2000). These restoration projects represent one of the largest habitat creation opportunities ever experienced in Europe in modern times. Since
1991, Bord na Móna has created a series of experimental shallow lakes by blocking drains and creating embankments within a 2000-ha cutaway site in County
Offaly (Egan 1998). 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.
Lakes created by flooding industrial cutaway peatland have a number of unusual characteristics. They are generally shallow (< 1.5 m), well-oxygenated systems containing a high degree of dissolved colour. Lake substrates comprise a highly variable mix of ombrotrophic and minerotrophic peat types and inorganic subsoils (McNally 1999). Their pH ranges from < 5 to
> 8, depending on the depth and type of residual peat, the exposure of sub-peat, calcareous sediments and the presence of hard- versus soft-water inflows (Wheeler &
Shaw 1995; Higgins & Colleran 2006). Nutrient concentrations are characteristically low and any increase in nutrient levels tends to be allochthonous and anthro-

224 H
IGGINS
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ET AL
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pogenic in origin, rather than due to internal nutrient mineralization from the peat sediment (O’Connor 2000;
Higgins & Colleran 2006). The trophic organization is typically rudimentary, with microbiota assuming a greater importance in the food web than in natural lakes
(van Duinen et al. 2003). Primary production is frequently phytoplankton-driven, due to a complete absence of higher vegetation at recently abandoned peatfields and the hampering of natural plant recolonization by a lack of viable seed banks (Curraun &
MacNaeidhe 1986) and the extreme physical conditions, which severely retard the establishment of propagules (Lavoie & Rochefort 1996).
Phytoplankton photosynthesis in shallow aquatic systems is typically rate-limited by nutrient availability, due to a usually ample availability of light and carbon dioxide. Phytoplankton populations respond vigorously to nutrient additions by increasing in number and biomass, with associated negative, ecosystem-scale impacts such as reduced water transparency, depleted hypolimnetic dissolved oxygen concentrations and problems of toxicity and odour associated with certain algal taxa. Understanding phytoplankton nutrient limitation has, as a result, become a central focus of modern limnology.
Phosphorus has traditionally been regarded as the primary nutrient limiting phytoplankton growth in freshwaters (Schindler 1977), and conventional lake trophic classification models have thus placed primacy on phosphorus loading (Carlson 1977; Vollenweider &
Kerekes 1982). Recent studies, however, have uncovered numerous exceptions to the paradigm of universal freshwater phosphorus limitation and a new understanding has emerged that oscillations between phosphorus and nitrogen limitation occur in many systems. Although nitrogen is rarely the natural limiting nutrient in temperate lakes, exclusive nitrogen limitation sometimes develops as an ‘unnatural’ secondary condition in artificially enriched temperate lakes containing exceptionally high concentrations of phosphorus (Vrede et al.
1999), while moderately eutrophic lakes frequently become seasonally co-limited by phosphorus and nitrogen, as ambient nitrogen supplies become depleted in summertime (Hameed et al. 1999). Nevertheless, nitrogen limitation continues to be underestimated as a factor affecting algal growth in temperate lakes (Elser et al.
1990).
In light of the elevated importance of phytoplankton as the sole primary producers in many flooded cutaway peatland lakes, an assessment of phytoplankton community response to changing nutrient concentrations in these unique, pioneering systems was warranted. To address this requirement, this research aimed to identify the key limiting nutrient(s) in an experimental cutaway lake over a 38-month study period and to assess the significance of these findings for future cutaway lake creation in Ireland and elsewhere.
Fig. 1.
Location and major characteristics of Clongawny Lake. Inset map shows location of County Offaly in Ireland.

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Study site
Clongawny Lake (53
°
10' N, 07
°
53' W) is located in mid-west County Offaly, central Ireland (Fig. 1), 10 km southwest of the main Lough Boora Parklands development. The peatland at Clongawny forms part of an expansive raised bog complex, generically referred to as the Bog of Allen, which stretches over large parts of the
Irish Midlands.
Bord na Móna harvested peat from Clongawny using the Peko milled peat production system. Harvesting ceased prematurely in 1993 before all of the commercial Sphagnum peat was removed due to the exposure of obstructive fossil tree stumps. The cutaway lay redundant until Autumn 2001 when lake construction began. The artificial drainage system was blocked by infilling drainage ditches with peat. Unable to drain by gravity, the low-lying site quickly flooded under the influence of precipitation and associated surface runoff, to create a lake 12 ha in area. A narrow outflow channel constructed at the northwest corner of the lakes maintained an average water depth of 1.5 m. Lake substrates comprised a thin, upper layer of unconsolidated acid
S phagnum peat overlying deep, compacted layers of fen peat of woody and reed swamp ( Phragmites ) origin.
Physicochemical data presented in Table 1 indicate that
Clongawny was an acidic, base-poor, highly coloured lake. Very low levels of conductivity, alkalinity and inorganic carbon reflected the absence of mineral influences at the site.
Natural plant recolonization at Clongawny has proceeded very slowly since peat harvesting ceased, and both terrestrial and aquatic vegetation remain sparse.
There was evidence of Campylopus introflexus and
Potamogeton polygonifolius regeneration in some of the waterlogged drainage ditches, while small stands of
Phragmites australis have developed at the southern end of the lake. The periphery of the lake was largely bare peat, interspersed with small patches of pioneering
Eriophorum vaginatum , Juncus effusus , Calluna vulgaris and Erica tetralix , and scrub vegetation, such as
Ulex europaeus and Betula spp. Actively milled peatfields are situated to the south of the lake, while cutaway peatlands to the west and north of the lake have been planted with commercial coniferous tree species.
225
Material and Methods
Clongawny Lake was sampled between August 2001 and September 2004 at two-week intervals for the first year and monthly thereafter. Near-surface (0.5 m) water samples were collected at sampling stations at the northern and southern ends of the lake. Sub-samples for phytoplankton analysis were preserved in Lugol’s iodine. Samples for chlorophyll-a analysis were filtered immediately through GF/C filters, extracted using a mixture of 90% acetone and dimethly sulfoxide (1:1 v/v)
(Burnison 1980), and measured spectrophotometrically, with correction for phaeopigments. The filtrate was frozen immediately for dissolved nutrient analyses.
Soluble reactive phosphorus (SRP), total soluble phosphorus (TSP) and total phosphorus (TP) were analysed using the ascorbic acid reduction method of Murphy
& Riley (1962), involving persulphate digestion for
TSP and TP determinations. SRP and TSP was analysed on filtered samples, while unfiltered samples were used for TP analysis. Ammonium was determined on filtered samples according to the indophenol blue method
(Chaney & Morbach 1962). Nitrite and nitrate were measured spectrophotometrically on filtered samples using Hach diazotization (Method 8507: range 0-0.3
mg/l NO
NO
3
–
2
– -N, precision
±
0.0006 mg/l NO
2
– -N) and cadmium reduction (Method 8192: range 0 - 0.5 mg/l
-N, precision
±
0.01 mg/l NO
3
– -N) methods, respectively, which involved custom low-range calibrations. The sum of nitrate, nitrite and ammonium is reported hereafter as dissolved inorganic nitrogen (DIN).
Nutrient analyses were performed for both sampling stations in triplicate; data presented are overall means.
The ratio of DIN to TP (DIN:TP) was used as a predictive index of nutrient limitation. DIN:TP approximates the supply of nitrogen and phosphorus available to phytoplankton and has been demonstrated to be accurate in predicting nutrient limitation in a range of lakes
(Morris & Lewis 1988; Lafrancois et al. 2003).
Phytoplankton cells 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
(Rott 1981). Phytoplankton species diversity was assessed using Simpson’s index of diversity. Relation-
Table 1. Principal physico-chemical properties of Clongawny Lake, between August 2001-September 2004 ( n = 54). Values shown are mean
±
SE ; ranges are given in parentheses.
pH
µ
S.cm
–1
4.64
±
0.04
(4.11 - 5.35)
Conductivity mg.l
–1
72
±
3.84
(57 - 87)
Dissolved O
2 mg.l
–1
11.3
±
0.25
(8.6 - 16.4)
Alkalinity mg.l
–1 CaCO
3
1.6
±
0.21
(0.0 - 7.5)
True Colour mg.l
–1 Pt. Co.
156
±
3
(132 - 255)
Organic C mg.l
–1
29.3
±
0.9
(19.6 - 52.2)
Inorganic C mg.l
–1
0.5
±
0.2
(0.0 - 5.2)

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, T.
ET AL
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ships between phytoplankton and nutrient variables were determined on log
10
-transformed data using Pearson’s coefficients of determination ( r 2 ) and paired t -tests.
Nutrient enrichment bio-assays, involving natural phytoplankton populations, were performed on eight occasions between December 2003 and September 2004.
Five treatments were performed in duplicate, in addition to an untreated control, by incrementing nutrient concentrations in 200-ml samples of lake water with K
2
HPO
4 and NH
4
Cl spike solutions as follows: (1) low P (0.1
mg/l P); (2) high P (0.5 mg/l P); (3) low N (0.3 mg/l N);
(4) high N (1.5 mg/l N), and (5) high P plus N (0.5 mg/ l P and 1.5 mg/l P). Samples were incubated outdoors at ambient light and temperature conditions for two weeks.
Flasks were swirled twice a day and re-randomized daily.
Chlorophyll-a was measured in duplicate at the beginning, middle and end of the incubation. Only end-point chlorophyll-a results, which represented the maximum response in all treatments, are presented. During a separate experiment in February 2004, the DIN:TP ratio in a series of duplicate flasks was increased in an incremental fashion from 0.7 to 14.7 by adding successively higher concentrations of nitrogen (as NH
4
Cl), while maintaining phosphorus additions at 0.1 mg/l P.
Fig. 2.
Nutrient concentrations in Clongawny: A.
SRP (soluble reactive phosphorus); TSP (total soluble phosphorus); TP
(total phosphorus); B.
Dissolved inorganic nitrogen levels, taken as the sum of NH
4
-N, NO
3
-N and NO
2
-N, (C) DIN:TP, the ratio of dissolved inorganic nitrogen to total phosphorus.
Results
Nutrient conditions
Mean total phosphorus (TP) concentrations in
Clongawny increased 5.5-fold over the course of the study period, from 11.8
±
0.8
µ g/l P in the first year of sampling to 65.3
±
3.1
µ g/l P in year three (Fig. 2A). The northern sampling station exhibited consistently higher
(2.2-fold) levels of soluble reactive phosphorus (SRP) than the southern station, indicating the source of the phosphorus was phosphate fertilizer runoff from adjacent, heavily afforested cutaway peatland. Incoming phosphate was quickly assimilated by the plankton, as evidenced by the observed increase only in TP and not in SRP or total soluble phosphorus (TSP). Dissolved inorganic nitrogen (DIN) showed a reverse trend to phosphorus, being high for the first eight months of sampling (261.1
±
26.7
µ g/l N) and then declining rapidly from spring 2002 (Fig. 2B). DIN was undetectable in Clongawny from March 2002 until November 2003 when DIN levels increased to 25
µ g/l N. For the remainder of the sampling period, levels of DIN ranged between 5 and 106
µ g/l N. The DIN/TP ratio (Fig. 2C) highlights the temporal shift that occurred in the relative concentrations of nitrogen and phosphorus; mean
DIN:TP declined from 17.78
±
2.98 in year one to 0.59
±
0.14 in year three of sampling.
Phytoplankton abundance and community structure
Phytoplankton levels were low in Clongawny early on in the sampling period (Fig. 3A, B). In autumn 2002, a very steep increase in phytoplankton abundance occurred, as reflected in both the chlorophyll-a concentration (Fig. 3A) and biovolumes (Fig. 3b), which increased eight- and 30-fold, respectively. The very high phytoplankton concentrations persisted until summer
2004, when substantial declines in chlorophyll-a and biovolume were observed. The increase in phytoplankton abundance was due to a surge in the number of chlorophytes (Fig. 3B), which replaced an earlier sparser, but more diverse, phytoplankton flora dominated by dinoflagellates (Fig. 3C). Two small, unicellular chlorophyte species, Chlorella spp. and Cosmarium pygmaeum , dominated the phytoplankton assemblage in a sequential fashion (Fig. 3E). The small dimensions of both species effected a marked change in the size profile of the phytoplankton community (Fig. 3D). There was a steep decline in phytoplankton species diversity (Fig.
3F), concomitant with the explosion in the number of minute chlorophyte cells. Species diversity increased notably towards the end of the sampling period, as the population of C. pygmaeum collapsed.
Relationships between nutrient concentrations and descriptors of the phytoplankton community (Table 2)

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227
Fig. 3.
Temporal changes in the phytoplankton community in
Clongawny between August 2001 and September 2004. A.
chlorophyll-a concentrations; B.
Biovolumes of the major phytoplankton groups; C.
Relative biovolumes of the major phytoplankton groups; D.
Biovolumes of dominant species;
E.
Relative size fractions of phytoplankton cells; F.
Species diversity of the phytoplankton community according to
Simpson’s Index D .
Fig. 4.
Phytoplankton responses to nutrient treatments in eight bioassays ( A-H ). Chlorophyll-a values shown are mean changes in chlorophyll-a ( n = 4), relative to an untreated control, after two-week incubation at ambient temperature and light. Initial
DIN:TP ratios of lake samples are shown in parentheses.
indicate that phosphorus was the primary nutrient responsible for shaping phytoplankton populations in
Clongawny. Indeed, TP correlated more strongly than
DIN with all variables describing standing crop and community structure, with the exception of dinoflagellate abundance.
Results from the nutrient enrichment bio-assays conducted between December 2003 and September 2004 are illustrated in Fig. 4. Over the course of the experiments, the natural DIN:TP ratio in the lake ranged between 1.74 to 0.09. When DIN:TP was naturally high
(> 1) during December 2003 and January 2004, phytoplankton in the bio-assays responded positively only to singular nitrogen additions, while additions of phosphorus, either alone or in combination with nitrogen, stimulated negative responses relative to the control (Fig. 3A, B). In March 2004, DIN:TP in the lake decreased to approximately 1, as ambient nitrogen levels declined while phosphorus levels remained
Table 2.
Relationships between nutrient variables and phytoplankton abundance and composition in Clongawny.
Values shown are coefficients of determination ( r 2 ) between log
10
-transformed data sets ( n = 53); inverse relationships are indicated by a minus symbol. TP = total phosphorus; DIN = dissolved inorganic nitrogen; DIN:TP = ratio of dissolved inorganic nitrogen to total phosphorus.
TP DIN
Chlorophyll-a
Biovolume
Diversity ( D )
% Chlorophyte
% Dinoflagellate
% Nanoplankton
0.671 ***
0.772 ***
–0.545 ***
0.761 ***
0.002 ns
0.801 ***
–0.399 ***
–0.475 ***
0.175 *
–0.436 ***
–0.160 *
–0.430 ***
*** = p < 0.0001; * = p < 0.005; ns = not significant.
DIN:TP
–0.542 ***
–0.650 ***
0.297 ***
–0.601 ***
0.103 ns
–0.606 ***

228
Fig. 5.
Chlorophyll-a responses to increasing DIN:TP ratios.
CHL-WK1, change in chlorophyll-a, relative to the control, after one-week incubation; CHL-WK2, change in chlorophyll-a, relative to the control, after two-week incubation;
NH
4
-WK2, residual ammonium concentrations, shown on secondary axis, after two-week incubation.
high. Singular phosphorus enrichments in the bio-assays continued to exert a strong negative effect on chlorophyll-a levels, while additions of nitrogen alone and in combination with phosphorus stimulated comparable positive responses (Fig. 4C). As the natural DIN/TP ratio in Clongawny further declined with depleting nitrogen supplies between April and August 2004, simultaneous enrichments of both nitrogen and phosphorus in the bio-assays elicited a much stronger positive response than adding nitrogen singularly (Fig. 4D-G).
With the exception of June 2004, the addition of phosphorus to these samples no longer produced a substantial reduction in chlorophyll-a (Fig. 4D-G). With a decrease in ambient phosphorus concentrations to 46.2
µ g/ l in September 2004, the lowest phosphorus level recorded in the lake in 2004, a different picture emerged in the bio-assays. Singular phosphorus enrichment stimulated a mild positive response, the positive effect of adding nitrogen alone declined in magnitude while combined nitrogen plus phosphorus enrichments elicited a strong positive response (Fig. 4h).
Chlorophyll-a responses to incremental increases in the DIN/TP ratio were consistent in both weeks of incubation (Fig. 5). Incremental increases in DIN stimulated an almost linear increase in chlorophyll-a up to
DIN:TP 4. Above this ratio, chlorophyll-a levels remained constant up to DIN:TP 10. Residual ammonium became increasingly detectable above DIN:TP 5. At the highest ratio of 15, a significant decline in chlorophylla was recorded, coincident with very high levels of residual ammonium (Fig. 5).
H
IGGINS
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Discussion
Naturally occurring acidic, highly stained bog lakes are typically unproductive systems, due to a combination of low nutrient concentrations and a thin euphotic zone. Initially mesotrophic, Clongawny evolved into a hypereutrophic lake over the course of the 3-year sampling period as phytoplankton production surged.
The changes in Clongawny were stimulated by a
5.5-fold increase in phosphorus in the lake. The likely source of the enrichment was phosphate fertilizer runoff from young cutaway forestry plantations adjacent to the lake, as identified from spatial differences in phosphate levels between sampling stations, together with soil and foliar nutrient analysis from within the coniferous forestry plantations (Renou & Farrell 2004a).
Cutaway peatland is particularly susceptible to phosphorus leaching due to the naturally poor sorption capacity of peat (Renou & Farrell 2004b), coupled with the high runoff and erosion risk of cutaways
(Collins & Cummins 1996).
Increased levels of phosphorus in Clongawny led to a massive proliferation in phytoplankton, indicating phosphorus was initially the limiting nutrient in the lake. The phytoplankton growth surge was enhanced by the paucity of higher vegetation (Higgins & Colleran
2006), a reflection of the lag-time involved in the natural recolonization of industrially harvested peatlands.
The availability of incoming phosphorus exclusively for phytoplankton uptake in Clongawny resulted in chlorophyll-a increases well in excess of those predicted by empirical chlorophyll-phosphorus loading models
(Carlson 1977; Vollenweider & Kerekes 1982). Negligible ambient concentrations of minerals such as calcium carbonate, iron and aluminium, which co-precipitate phosphate and make it unavailable for biological uptake (House 1990; Deppe & Benndorf 2002), may have contributed to the high chlorophyll response to phosphorus loading in Clongawny.
Rapid phytoplankton growth in Clongawny depleted nitrogen supplies to undetectable levels (< 0.6
µ g/l N) and the DIN/TP ratio declined in a similar manner to that reported from other artificially enriched lakes (Vrede et al. 1999; Matthews et al. 2002). Axler et al. (1994) determined that DIN:TP > 4 indicated phosphorus limitation, while nitrogen limitation was associated with ratios < 1.3 and intermediate ratios indicated co-limitation by nitrogen and phosphorus. Bio-assay results from
February 2004 (Fig. 5) supported the prediction of Axler et al. (1994) that DIN:TP 4 marked the threshold ratio at which phosphorus became limiting. Based on these criteria, Clongawny was phosphorus-limited prior to
April 2002 and nitrogen-limited thereafter, except for short periods in winter 2003 when nitrogen and phos-

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phorus were co-limiting .
The development of secondary nitrogen limitation in response to excessive phosphorus enrichment typically favours the growth of bloom-forming heterocystous cyanobacteria, which can mitigate the effects of nitrogen depletion through their nitrogen-fixing capabilities.
This trend was not observed in Clongawny in spite of the favourable nutrient conditions, probably because cyanobacterial growth is suppressed in very acidic (pH
< 5.1) waters (Moss 1973). The proliferation of nitrogen-fixing cyanobacterial populations is also thought to be inhibited by low light transmission, a high frequency of sediment resuspension, water column instability and high shearing stress (Zhang & Prepas 1996; Moisander et al. 2002; Havens et al. 2003), all of which are characteristic of the physical environment in cutaway lakes.
Instead, two small chlorophyte species, Chlorella spp.
and subsequently Cosmarium pygmaeum , dominated the phytoplankton assemblage in Clongawny in a sequential fashion. High intrinsic growth rates and generation times of < 24 hour enabled these minute, highly buoyant chlorophytes to respond quickly to the phosphorus inputs and rapidly out-compete the large sized dinoflagellates which previously prevailed in the lake.
These trends mirror those observed by Findlay et al.
(1999) on the effects of adding nutrients in conjunction with acidification.
Predictions of exclusive nitrogen limitation in
Clongawny were supported by the nutrient-enrichment bio-assay results for December 2003 and January 2004, while from March 2004 onwards, bio-assay results indicated increasing co-limitation by nitrogen and phosphorus. When nitrogen was exclusively limiting, phosphorus additions in the bio-assays induced a negative response relative to the control. Apparent inhibition of phytoplankton growth by nutrient additions has previously been recorded in other small-volume enrichment experiments (Hameed et al. 1999). Reasons proposed include short-term competition between CO
2
and NH for ATP and reducing electrons during the initial period
4 of ammonium uptake (Healey 1979), and a re-allocation of energy in nutrient-replete cells from photosynthesis to nutrient uptake and assimilation (Lean et al. 1983). In their review, Gerhart & Likens (1975) concluded that such short-term observations of inhibition may not be relevant to the growth of phytoplankton in nature. As co-limitation by phosphorus and nitrogen developed, the inhibitory effect of adding phosphorus in the bioassays gradually waned and, as phosphorus levels exhibited the appearance of a downward trend in September 2004, singular additions of phosphorus elicited a positive response for the first time. Predictions from the bio-assays of a transition from phosphorus limitation to co-limitation by nitrogen and phosphorus correlated with the collapse in the C. pygmaeum bloom in the lake and a recovery in phytoplankton species diversity. These trends may suggest the beginning of a gradual return to naturally phosphorus-limiting conditions in the lake, although further monitoring is required to ascertain whether this transition is sustained in the longer-term.
Nutrient analysis of peat cores determined that no significant accumulation of phosphorus had occurred in the lake sediments (T. Higgins unpubl. data), indicating that the problem of internal phosphorus loading, widely reported elsewhere, is unlikely to pose a future problem for Clongawny.
Conclusions
229
Results highlighted the critical impact of watershed land uses on the trophic status of a cutaway lake. The trophic status of Clongawny shifted from mesotrophic to hypereutrophic in the presence of moderate phosphorus leaching from adjacent cutaway forestry plantations, with a coincident transition from phosphorus limitation to secondary nitrogen limitation. Phytoplankton responses to phosphorus loading were likely to have been amplified by the absence of recolonist vegetation at the site and the vulnerability of cutaway peatlands to runoff and erosion. Findings emphasized the paramount need for an integrated, catchment-based approach to designating post-harvesting land uses, particularly in the watersheds of proposed new lakes.
Acknowledgements. This research was funded by Bord na
Móna. The authors also wish to thank the Environmental
Change Institute, NUI Galway, for facilitating this research.
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Received 2 February 2005;
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Co-ordinating Editor: R. van Diggelen.