microbial activity and the biofilms of a wetland stream. C. FREEMAN. Institute of Terrestrial Ecology and School of Biological Sciences, University of. Wales ...
Man's Influence on Freshwater Ecosystems and Water Use (Proceedings of a Boulder Symposium, July 1995). IAHS Publ. no. 230, 1995.
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Climate change: Man's indirect influence on wetland microbial activity and the biofilms of a wetland stream C. FREEMAN Institute of Terrestrial Ecology and School of Biological Sciences, University of Wales, Bangor LL57 2UP, UK
R. GRESSWELL, M. A. LOCK, & C. SWANSON School of Biological Sciences, University of Wales, Bangor LL57 2UW, UK
H. GUASCH, F. SABATER & S. SABATER Department of Ecology, University of Barcelona, Diagonal 645, 08028 Barcelona, Spain
J. HUDSON Institute of Hydrology, Staylittle, Powys SY19 7DB, UK
S. HUGHES & B. REYNOLDS Institute of Terrestrial Ecology, University of Wales, Bangor LL57 2UP, UK
Abstract Certain mathematical models suggest droughts may occur more frequently as a consequence of an enhanced greenhouse effect. We have studied the effects of a simulated drought upon the microorganisms of a wetland, and the microbial biofilms of a stream flowing from that wetland. The activity of the wetland population was found to be 36% lower during drought conditions, while in contrast, that of the outflow biofilm community was 85 % higher. The release of inorganic nutrients from the wetland increased by 57% while organic nutrient release fell by 26%. During the drought, in the biofilm, the autotrophic chlorophyll content was 145% higher, while the bacterial reserves of poly-beta-hydroxy alkanoate were 51% lower than the control. These changes may also influence higher trophic levels in the ecosystem.
INTRODUCTION Appreciation of the need to conserve our wetland resources is increasing rapidly as a result of the realization that wetlands represent not only a wildlife habitat and recreational resource, but also a powerful tool for the amelioration of water quality (Richardson, 1994). It is becoming clear that Man's overall impact upon wetlands and their associated freshwater ecosystems may include direct effects such as drainage for agriculture and peat cutting for fuel, and also indirect effects such as global warming (Immirzi et al., 1992). Mathematical models suggest that the long-term trend towards a warmer climate could lead to an increased frequency of short-term extreme events (Katz, 1992). And in wetlands, events such as droughts represent a particularly serious threat as a consequence of the destabilizing effects of water table drawdown (Freeman etal, 1993a).
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The results of recent studies suggest that such drier (more aerobic) conditions could stimulate microbial activity (and hence mineralization processes) leading to greater inorganic nutrient release into wetland drainage streams (Freeman et al., 1993c). As a test of that hypothesis, we have conducted a field-based experiment to investigate the effects of drought upon (a) the activity of a wetland microbial community, and (b) the release of nutrients into a stream flowing from that wetland. Finally, we have related the hydrochemical responses to changes in the attached micro-organisms growing within biofilms in the outflow stream waters. MATERIALS AND METHODS The field site (Fig. 1) Cerrig-yr-Wyn, Plynlimon, mid-Wales (UK National Grid Reference SN 820 866) was described in Freeman et al. (1993b), and consists of a series of wetlands at the base of a small gully. The uppermost wetland has been retained as a control, while just below the control, a sheet aluminium (duralumin) barrier has been installed, from which plastic pipes divert incoming water around the outside of an experimentally "droughted" wetland. Stream water from the two wetlands was supplied to river biofilms (Fig. 1), via constant head tanks which were continuously overfilled
control —5, channe
Fig. 1 Schematic diagram of the field manipulation illustrating sources of water from the control and treated wetlands.
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with a direct feed of water from the control or experimental areas. Water passed from the tanks through duplicate channels of 3 m length, constructed from longitudinal half sections of 20 cm diameter PVC drainage piping, which were placed immediately below the experimental site. The channels contained biofilm-coated stones which had been collected from the stream that drains the two wetlands. Samples of peat (10 cm3) were collected from each of the wetlands on a weekly basis over a total period of 12 weeks and returned to the laboratory for measurements of microbial activity. On each occasion, a sample of water from the outflow of each wetland was collected for analysis of organic and inorganic nutrients. For the biofilms, sampling involved stones being removed from the experimental channels every second week of the study. Metabolic activity in both peat and biofilm samples was measured as electron transport system (ETS) activity according to the protocols of Gammelgaard et al. (1992) and Blenkinsopp & Lock (1990), respectively. For the peat, four 10 x 1 x 1 cm3 (vertical) sub-samples were taken from each peat block. To each was added 20 cm3 0.4% 2-(piodophenyl)-3-(p-nitrophenyl)-5-phenyltetrazolium chloride (INT). All samples were homogenized in a Seward Colwarth model 80 Stomacher homogenizer for 1 min and then incubated at 11 °C for 48 h. Twenty cubic centimetres of formaldehyde were added to terminate the reaction, followed by a further 1 min homogenization. The homogenates were then transferred to open Petri dishes and freeze dried for 24 h at —40°C. Following transfer of the dried material to a 50 cm3 centrifuge tube, 20 cm3 methanol were added, and the samples maintained for 24 h in a freezer for INT-formazan extraction. The extracts were then centrifuged for 15 minutes at 2270 x g, and filtered through Whatman GF/C filters prior to measurement of absorbance at 480 nm. In the biofilm assay, the biofilm-coated stones were placed in 100 cm3 beakers and incubated with 30 cm3 0.02% INT at 11°C for 18 h. The incubation was terminated by discarding the INT and replacing it with 30 m3 of 2% formalin. The formalin was then replaced with 30 cm3 methanol, and the biofilm disrupted by sonication. INT-formazan from within the biofilm was extracted into methanol over a 1 hour period at -20°C. The absorbance of the solution was measured spectrophotometrically at 480 nm. Formalin-killed controls were also included in each batch of assays. ETS activities were expressed as y.g INT formazan cm"2 h"1 (biofilm) or fig INT formazan g"1 h"1 (peat). Poly-beta-hydroxy alkanoate (PHA), a bacterial storage compound, was analysed as an indicator of the nutritional status of the biofilm bacteria, using a modification of the ion exclusion HPLC technique of Karr etal. (1983). This involved conversion of the PHA to crotonic acid during a sulphuric acid digestion at 90°C. The HPLC system included an Aminex HPX 87H column with 0.028 M H2S04 eluent, and photometric detection set at 214 nm (Freeman et al., 1990). PHA was expressed in terms of /*g crotonic acid (Karr et al., 1983). The chlorophyll content of the biofilm was determined by first removing the community from the stones by gentle abrasion with a toothbrush over a period of 5 min. Thirty cubic centimetres of acetone were added, and the chlorophyll extracted over a period of 24 h in darkness at 40°C. Chlorophyll-a concentrations were determined according to APHA procedures (APHA, 1989). Areas of the stones used in the biofilm assays were estimated by covering the stones with aluminium foil (Shelley, 1979), removing the excess (folds) and then tracing the outline of the unfolded foil template on to paper of known surface area to weight ratio. The outline was then cut out and weighed
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to determine its surface area {sensu Doeg & Lake, 1981). Organic and inorganic nutrient concentrations were determined in the stream draining the wetlands. A DIONEX 2000i ion chromatograph was used to determine NH4, K, Mg, Ca and N0 3 (P0 4 was not detectable, < 0.02 mg l' 1 ), while a Skalar autoanalyser with UV digestion/colorimetric detection system was used to determine dissolved organic carbon (DOC). RESULTS The simulated drought induced substantial changes both within the drought-impacted wetland and within the outflowing stream waters. Measurement of microbial activity (as electron transport system activity) showed the micro-organisms in the wetland peat to respond to the drought simulation in a completely different manner to the microorganisms of the outflow waters (Fig. 2). Within the peat, microbial activity levels were 36% (p < 0.05) lower in the treated wetland soil than in the control. In contrast, the activity of the attached microbial biofilms were 85% (p < 0.05) higher in the waters flowing from the treated wetland than those flowing from the control (Fig. 2). The activity of the wetland peat microorganisms was inversely correlated with activity of the biofilms in the outflow stream (Fig. 3, R = -0.58, p < 0.05). Several substantial changes were also noted in the waters flowing from the wetland (Fig. 4). Inorganic nutrient release was estimated as the sum of ammonium, potassium, magnesium, calcium and nitrate, expressed as mg l"1 (phosphate was not detectable). During the simulated drought, the release of inorganic nutrients from the treated wetland increased by 57% (p < 0.001). In contrast, organic nutrient release fell by 26% although this was only significant at the 10% level (p = 0.09). Within the biofilm, the autotrophic chloro-
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