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Effect of Pulsing on Macrophyte Productivity and Nutrient Uptake: A Wetland Mesocosm Experiment

Posted on: Wednesday, 26 October 2005, 09:01 CDT

By Anderson, Christopher J; Mitsch, William J

ABSTRACT.-

A study was conducted to evaluate the effect of a pulsing hydrology on the productivity and nutrient uptake of an herbaceous riverine wetland. Pulsing effects were evaluated using 20 0.9-m^sup 2^ wetland mesocosms: 10 planted with Schoenoplectus tabernaemontani (C.C. Gmel) Palla and the other 10 planted with Typha angustifolia L. For each species, half the mesocosms were subjected to a 3-mo pulsing regime while the others were subjected to steady-flow conditions. Hydrology parameters were selected to approximate a pulsing experiment being carried out concurrently at two 1-ha wetlands at the research site. Typha wetlands were significantly more productive than Schoenoplectus wetlands; however no significant differences in productivity or morphology were observed between pulsed or steady-flow wetlands among species groups. No significant differences in nutrient concentrations, uptake or uptake efficiency were detected among species groups either, however hydrology did influence plant tissue N:P ratios. For all wetland mesocosms, the mean N:P ratio was 9.2 0.6 for steady flow wetlands and 11.7 0.5 for pulsed, suggesting that the steady flow wetlands were more N limited than pulsed wetlands. The potential applications and limitations of applying these results to the 1-ha wetlands study are discussed.

INTRODUCTION

Wetland plant productivity and nutrient uptake can be influenced by hydrologie parameters including flood depth (Waters and Shay, 1992; Newman et al., 1998; Kellogg et al, 2003), flood duration (Newman et al, 1998) and flooding frequency (Giovannini and Da Motta Marques, 1998; Tanner, 1999; Casanova and Brock, 2000). Further, water level fluctuations associated with a pulsing hydrology may increase wetland productivity and nutrient uptake (Mitsch and Ewel, 1979; Mitsch et al, 1979; Junk et al, 1989; Odum et al, 1995; Day et al, 2000). Inflowing flood pulses often contribute higher than normal concentrations of nutrients thus providing a fertilizing effect for plant growth (Spink et al., 1998). Increased nutrient availability can also occur in wetland sediments by the release of phosphorus during anaerobic conditions and the increase in nutrient mineralization caused by the fluctuation of wet-dry soil conditions. However, prolonged wetland inundation increases the potential for anoxic conditions that can be detrimental to macrophyte vegetation (van der VaIk and Davis, 1978).

Pulsing water regimes are characteristic of riparian wetlands adjacent to flashy streams or rivers in the Midwest United States (Baker et al., 2004). The temporary nature of river flood pulses often leads to rapidly dropping water levels, thus decreasing the chance for soil anoxia while increasing nutrient mineralization and the potential for greater productivity (Mitsch and Rust, 1984). The influence of a pulsing hydrology on increased productivity has been well demonstrated for wetland forests (Mitsch et al, 1991; Brown, 1981) and planktonic communities (Hein et al, 1999), but has been less predictable for herbaceous wetlands. Herbaceous wetland vegetation may be more sensitive to different flood conditions (depth, duration, etc.) that may make a pulsing effect more difficult to detect.

The purpose of this study was to determine if a pulsing hydrologie regime would elicit morphological and functional responses by two common wetland plants in experimental mesocosms. This project was conducted in conjunction with a multi-year experiment using two 1-ha created wetlands to examine the effects of a pulsing regime. We hypothesized that productivity and nutrient uptake would be higher in the pulsed water regime and that pulsing river water into wetlands would result in greater N uptake by vegetation compared to a steady flow regime.

FIG. 1.-Experimental wetland mesocosms used in this study. Cross- section view and dimensions of two mesocosms and associated French drain system. Water depth was controlled by the height of the connected stand pipe extending from the mesocosm tub. Bleed-down orifices were installed at the soil elevation of each mesocosm soil elevation

METHODS

The project was conducted at the 12-ha Olentangy River Wetland Research Park (ORW) at The Ohio State University in Columbus, Ohio, USA (latitude 40.02N, longitude 83.017E). A total of 20 mesocosm tubs (Fig. 1; area = 0.9 m^sup 2^, volume = 540 liters) were used. In each mesocosm a total of 8-10 cm of pea-size gravel was placed in the bottom to allow seepage. Approximately 30 cm of on-site alluvial, upland soil (Ross and Eldean series, consisting of silt loam, silt clays and clay loams; Mcloda and Parkinson, 1980) was placed on top of the gravel. Water levels were controlled with a French drain system established for each mesocosm using 5-cm diameter pvc pipes (Fig. 1). The control structure (top of the stand pipe) was established 5 cm above the soil layer. To provide gradual water subsidence, a bleed down orifice (2 mm hole) was drilled into the side of each pipe at the mesocosm soil elevation.

To evaluate the effect of pulsing, mesocosms were randomly assigned to one of four treatments based on macrophyte species and hydrology (Table 1 ). In March 2002, a total of 30 rhizomes each of Typha angustifolia L. (Cooperrider et al., 2001) (narrow-leaved cattail, hereafter Typha) and Schoenoplectus tabernaemontani (C.C. Gmel) Palla (Cooperrider et al, 2001) (soft-stemmed bulrush, a.k.a. Scirpus validus, hereafter Schoenoplectus) from a Midwestern nursery were trimmed to approximately 25 5 g in size and planted 3 cm below the soil surface. Three rhizomes (with no inter-species mixing) were planted in each mesocosm. To expedite growth of the planted rhizomes, wetlands were kept in moist condition throughout the summer and autumn 2002 with groundwater equally distributed to all mesocosms. At the end of August 2002, initial condition aboveground biomass was non-destructively estimated in each mesocosm by relating various morphological parameters to reference plants. For Schoenoplectus, stem density, mean stem height, number of flowers and mean maximum stem height were recorded. For Typha, the number of leaves per ramet, ramet density, mean ramet height, number of flower spikes and mean maximum ramet height were recorded. Over the winter of 2002-2003, the mesocosms were not watered; a layer of snow and ice covered them throughout most of the season.

TABLE 1.-Species and hydrology prescribed for wetlands during the experimental hydrology period (April, May and June) in 2003. Actual hydrology period was extended approximately 2 wk during pump repair. The use of 'Schoenopkctus (1-y)' mesocosms was necessary due to muskrat damage during winter 2002

In March 2003 it was discovered that muskrats (Ondatra zibethicus) excavated three Schoenoplectus wetlands of nearly all its rhizomes during winter 2002-03. In April 2003, sod containing Schoenoplectus rhizomes were collected from control wetlands used in another mesocosm experiment, and transplanted into the disturbed mesocosms. Each disturbed mesocosm received five pieces of Schoenoplectus sod that were approximately 15 20 cm wide and 4 cm thick. All measurements from these mesocosms were analyzed separately and designated as 'Schoenoplectus (1-y)' wetlands, compared to 'Schoenoplectus (2-y)' for the undisturbed wetlands (Table 1).

During the experimental hydrology period (April through June 2003), water was pumped from the Olentangy River based on the pulsing schedule in Table 1. By design, all mesocosms received a near equivalent monthly hydrologie load of 160 cm month^sup -1^. Water was pumped directly from the Olentangy River to the mesocosms via a low-pressure pump and an elevated reservoir tank system. A garden hose was used for each water regime and extended along each mesocosm. At each nexus point, the hose was attached to the mesocosm rim and a volume-adjustable irrigation sprayer (Raindrip R180C) was used to control the amount of water pumped into each mesocosm. In mid-May 2003, the pumping schedule was postponed for 11 days to repair the pumping system.

Pumping volumes and water depths were recorded for each water regime. Three mesocosms were omitted from the study in 2003 because of faulty drainage systems that developed and could not be repaired. During the experimental hydrology period, a 250-ml sample of the river water was collected weekly and analyzed for NO^sub 3^-N and total P using a Lachat QuikChem IV automated system and Lachat methods (U.S. EPA, 1983). Total P was analyzed using the ascorbic acid and molybdate color reagent methods after digesting with 0.5 ml of 5.6N H^sub 2^SO^sub 4^ and 0.2 g NH^sub 3^SO^sub 4^ to 25ml of sample and exposing the samples to a heated and pressurized environment for 30 minutes in an autoclave. Using the Lachat automated system, nitrate was analyzed using the cadmium reduction method. Nutrient concentrations were used with pumping rates to estimate weekly NO^sub 3^-N and total P nutrient loads into the mesocosms (converted to g m^sup -2^ for comparison).

Plant morphology was measured on 18 June, 23 July and 28 August 2003 using the same parameters measured in September 2002. After the August 2003 measurements, all aboveground vegetation in ea\ch mesocosm was harvested and the mesocoms were covered until belowground biomass was harvested. Harvested aboveground material was air-dried and a subsample was oven dried for 3 d (or until constant mass was achieved) at 80 C. Total aboveground biomass was calculated and converted to g m^sup -2^ for comparison.

In September 2003, soil was extracted from each mesocosm and carefully washed from all root material. Root material was weighed for each mesocosm and a subsample was air dried for 5 d at 105 C or until constant mass was achieved. Total belowground biomass was estimated for each mesocosm and converted to g m^sup -2^ for comparison. Belowground biomass data was combined with aboveground data in August 2003 to calculate total biomass and the mean root:shoot ratio.

To estimate tissue nutrient concentrations for each mesocosm, 10 mature stems were randomly selected and analyzed for nutrient content. Tissue specimens were air dried, ground to pass through a 2 mm sieve and mixed to make a homogenous sample. In mesocosms where significant plant senescence occurred (>50% of the plant surface yellowed), 10 random senescent stems were analyzed separately from the living stems and because of the potential for nutrient translocation, the belowground tissue was analyzed separately by randomly selecting five 5-cm root/rhizome sections. All tissue specimens were sent to Service Testing and Research (STAR) Laboratory, Ohio Agricultural Research and Development Center, Wooster, Ohio and analyzed for total N and P. Samples were digested with HClO^sub 4^/HNO^sub 3^ and analyzed for total P by inductively coupled plasma emission spectrometry (Isaac and Johnson, 1985). Samples were analyzed for total N through combustion analysis (AOAC, 1989). Using mean nutrient concentrations, the mean N:P ratio of the aboveground tissue was calculated for all treatment types and compared to thresholds developed by Koerselman and Meuleman (1996) to detect possible community level N or P limitations. As an indication of site fertility, tissue nutrient concentrations were used with peak biomass measurements in August 2003 to calculate total nutrient uptake for each wetland.

Statistical analyses.-For each species group [Typha, Schoenoplectus (2-y) and Schoenoplectus (1-y)], a two-way, repeated measure analysis of variance (ANOVA) was used to examine for differences in mean macrophyte morphological measurements between pulsed and steadyflow wetlands. An independent t-test was used to compare mean productivity, nutrient uptake and nutrient loads (mg total P and NO^sub 3^-N wk^sup -1^) between the pulsed and steady- flow mesocosms. In the case of root:shoot ratios and leaf-tissue N and P concentrations, Schoenoplectus (2-y) and (1-y) data were comparable (P > 0.05, Mest) and therefore pooled. A two-way ANOVA was conducted to compare the factors of hydrology (pulsed and steady flow wetlands), species and species hydrology interaction. Data were analyzed and transformed when necessary to meet assumptions for parametric statistics. For all comparative tests, P-values <0.05 were considered significant and P-values <0.01 were considered highly significant. Tests for parametric statistic assumptions and t- tests were conducted using Minitab Release 14 (Minitab, Inc., 2003), and two-way ANOVAs were conducted using Systat v.10.2 (Systat Software Inc., 2002).

FIG. 2.-Weekly hydrologie loading rate of pulsed and steady flow mesocosms. Total loading input includes pumped river water and rainfall. Pulsed and steady-flow mesocosms received 698 and 694 cm of river water, respectively, during the 2003 experimental period (April-mid July). Arrows indicate the weeks in which pulsing occurred

RESULTS

Hydrology and water depths.-River water was pumped into the wetland mesocosms for 4.5 mo in 2003. Three week-long pulses of river water occurred during the experimental hydrology period (between 3 April and 21 July 2003, Fig. 2) to simulate spring and early summer flood pulses typical of the region. From 22 July and 18 August 2003, all mesocosms received an equivalent load (approximately 32 cm wk^sup -1^) (Fig. 2). Water levels fluctuated based on the prescribed loading rates and other environmental factors including rainfall, evapotranspiration and apparent differences in permeability. In the pulsed wetlands, weekly mean water depths were 6.7 2.4 cm during the non-pulsing weeks and rose to 15.3 0.8 cm during the three pulsing weeks. The steady flow wetlands maintained a more consistent mean water level of 11.2 1.1 cm. Based on the measurements taken during the experimental hydrology period, water levels were at or below the surface (<1 cm) 16.4% of the time for pulsed wetlands and 6.9% of the time for steady-flow wetlands. Mean water depths of all mesocosms progressively decreased throughout the growing season as temperatures increased and as evapotranspiration increased.

Plant morphology and primary productivity.-There was no significant difference in peak dryweight biomass between the pulsed and steady-flow mesocosms for Typha or Schoenoplectus (1-y and 2-y) (Fig. 3). The highest mean aboveground biomass was recorded in the steady-flow Typha wetlands (1032 53 g m^sup -2^) while pulsed Schoenopkctus (1-y) wetlands had the lowest (296 58 g m^sup -2^). Pulsed Typha wetlands had the highest belowground biomass (1551 160 g m^sup -2^) and highest overall (above and belowground) biomass (2557 184 g m^sup -2^) while pulsed Schoenoplectus (1-y) wetlands had the lowest overall biomass (800 69 g m^sup -2^).

FIG. 3.-Mean (1 SE) total aboveground and belowground biomass (g m^sup -2^) for Schoenoplectus- ly, Schoenopkctus- 2-y and Typha mesocosms at August 2003. Aboveground standard error represents total (live and dead) biomass

By early August, all Schoenoplectus wetlands had some senescence of mature culms, particularly in the 2-y, steady-flow wetlands (Fig. 3). By the end of August, the mean dry weight of senescent biomass in the 2-y, steady-flow wetlands was 234 89 g m^sup -2^, or approximately 38% of the live biomass dry weight. A comparable distribution (32%) occurred in the pulsed Schoenoplectus (2-y) wetlands. The Schoenoplectus (1-y) wetlands had lower percentages of senescent biomass for both steady and pulsed wetlands (14% of the live dry weight for both wetland types). Between July and August new Schoenoplectus shoots had emerged; however a substantial amount of the aboveground stems had yellowed by the end of August 2003.

The mean root:shoot ratios for all species groups ranged between 1.4 (steady-flow Typha wetlands) and 1.7 (steady-flow Schoenoplectus [1 y] wetlands) in August 2003. No differences in rootshoot ratios were detected when the factors hydrology (F^sub 1,3^ = 0.021, P = 0.89), species (F^sub 1,3^ = 0.934, P = 0.35) or species X hydrology (F^sub 1,3^ = 0.800, P = 0.39) were analyzed.

No significant differences were detected in plant morphology measurements between pulsed and steady-flow wetlands for all three species groups. Mean Typha ramet density was highest for pulsed wetlands in August 2003 with 97 4 ramets m^sup -2^ however, both pulsed and steady-flow wetlands averaged over 95 ramets m^sup -2^ by mid-July (Fig. 4). Pulsed and steady-flow Typha wetlands also had similar monthly trajectories in mean ramet height (Fig. 4), mean number of flower spikes, mean number of leaves/ramet and mean maximum ramet heights. Schoenoplectus (1-y and 2-y) wetlands were more variable than Typha between pulsed and steady-flow wetlands, but no significant differences were detected (Fig. 4). Senescence of mature stems between July and August reduced mean stem density and stem height of Schoenolectus (2-y) wetlands, but not steady-flow Schoenoplectus (1-y) wetlands. Despite the higher percentage of senescence, Schoenoplectus (2-y) wetlands still had greater mean stem length than the Schoenoplectus (1-y) wetlands.

Plant tissue nutrient concentration.-Nutrient concentrations were comparable between Schoenoplectus (1-y) and (2-y) and, therefore, were combined to analyze hydrological regime effects on plant tissue nutrient concentrations. Analysis of plant tissue indicated that there were no significant differences in N or P concentrations for pulsed and steady-flow Typha or Schoenoplectus wetlands. However, when N:P ratios were analyzed, a significant difference was detected for hydrology (pulsed and steady-flow wetlands) (F^sub 1,13^ = 9.95, P < 0.01). No differences in N:P ratios were detected when species (F^sub 1,13^ = 0.021, P = 0.89) or species hydrology interaction (F^sub 1,13^ = 0.463, P = 0.51) were analyzed. For all mesocosms combined, the mean N:P ratios were 9.2 0.6 for steady flow wetlands (9.1 0.7 for Schoenoplectus and 9.5 l.Ofor Typha) and 11.7 0.5 pulsed wetlands (12.1 0.5 for Schoenoplectus and 11.5 0.8 for Typha). For senescent Schoenoplectus tissue, no difference was detected in N and P concentrations between steady-flow (11.0 0.5 mg N g^sup -1^ and 0.9 0.1 mg P g^sup -1^) and pulsed (12.2 0.9 mg N g^sup -1^ and 1.0 0.0 mg P g^sup -1^) wetlands. Using the N:P ratios calculated for each hydrology and species type, a comparison was made to threshold values determined by Koerselman and Meuleman (1996) (Fig. 5). In all cases, the N:P ratios were less than the 14:1 ratio which indicates that the wetlands were nitrogen limited.

Nutrient loading and uptake.-A weekly mean nutrient loading rate was estimated for pulsed and steady flow wetlands using weekly hydrologie loading rates and river water nutrient concentrations. The mean nutrient loading rates to each mesocosm were similar: 1509 349 mg NO^sub 3^-N wk^sup -1^ and 53 19 mg total P wk^sup -1^ for pulsed wetlands and 1418 187 mg NO^sub 3^-N wk^sup -1^ and 57 14 mg P wk^sup -1^ for steady flow wetlands.

FIG. 4.-Mean (1 SE) ramet height (cm) and density of Typhaand mean ( 1 SE) stem length and density of Schoenople\clus (1-y) and (2- y) and for pulsed and steady-flow wetlands in September 2002, June 2003, July 2003 and August 2003

Using the N and P tissue concentrations and plant biomass in August 2003 to compare treatments, pulsed Typha wetlands accumulated the most nitrogen (35.9 4.3 g N m^sup -2^, Fig. 6) and the most phosphorus (3.13 0.18 g P m^sup -2^, Fig. 7). No significant differences were detected among hydrological regimes for any of the species groups (Figs. 6, 7).

DISCUSSION

Plant morphology and primary productivity.-The pulsing hydrology did not have a substantial effect on Typha or Schoenoplectus primary productivity when compared to the steady-flow wetlands in this mesocosm study. Typha had rapid growth during the first 2 mo of the 2003 growing season, accumulating over 60% of its 2003 aboveground peak biomass by midjune. These observations are consistent with other Typha spp. studies (Martin and Fernandez, 1992; Garver et al, 1988). The range of Typha aboveground dry weight biomass was also consistent with other wetland studies (Dubbe et al, 1988; Mason and Bryant, 1975). The rootshoot ratios calculated in this study were within the range (0.9-1.2) found by Kvet and Husak (1978) for T. angustifolia in pond littoral zones in Czechoslavakia. However, Farnsworth and Meyerson (2003) found that T. angustifolia in Connecticut tidal marshes had lower aboveground biomass (>800 g m^sup -2^), but greater rootshoot biomass (2.5 0.1).

FIG. 5.-Mean N and P concentrations (1 SE) of aboveground plant tissue of wetland mesocosms in August 2003. N:P ratios of <14:1 and >16:1 are indications of N and P limitations, respectively (Koerselman and Meuleman, 1996)

Schoenopkctus productivity was low compared to literature ranges for natural wetlands, but comparable to other mesocosm studies. In a literature review conducted by Tanner (2001), peak biomass of S. tabernaemontani ranged widely between 400-1400 g m^sup -2^ for natural marshes in North America. However, in mesocosm studies, the ranges found in this study are more typical. Ahn and Mitsch (2002) evaluated S. tabernaemontani productivity in a two-year mesocosm study, maintaining water levels at 10 cm depth and a hydrologie loading rate of 51.1 cm wk^sup -1^ and 37.1 cm wk^sup -1^ during the 2 y, respectively. The mean net annual primary productivity of aboveground Schoenopkctus estimated by peak biomass was 425 33 g m^sup -2^ (prior to an imposed correction factor). In another 2-y mesocosm study conducted by Svengsoukand Mitsch (2001), mean aboveground Schoenopkctus biomass was greater than 350 g m^sup -2^ for control wetlands and between 450 and 600 g m^sup -2^ for wetlands fertilized with N, P and both. For their study, groundwater was used and depth was maintained at the soil surface.

The early senescence by Schoenopkctus by mid-August was unexpected. In their review of seed bank and vegetation dynamics in prairie pothole wetlands, van der VaIk and Davis (1978) reported that the rhizomes of Schoenopkctus may not tolerate prolonged high water levels and anoxic/anaerobic conditions. They partially contributed this condition and the resulting decline of Schoenopkctus to the degenerating vegetation phase of the prairie pothole marsh cycle (van der VaIk and Davis, 1978). In our study, there was evidence (e.g., the flocculation of Fe along the outflow stand pipes) that soils were highly reduced. However, this condition may have been partially caused by the sunken position of the mesocosm wetlands. It has been demonstrated that mesocosm wetlands can be susceptible to stagnation and less exposed to wind fetch because of their small size and sunken position (Ahn and Mitsch, 2002).

FIG. 6.-Mean N retention (1 SE) for a) Typha and b) Schoenoplectus (2-y) wetland mesocosms in August 2003. Inflow of NO^sub 3^-N is based on river water concentrations and pumping rates recorded during the 2003 experimental period (April-August 2003). No significant differences were detected among hydrological regimes for either of the vegetation groups

FIG. 7.-Mean P retention (1 SE) for a) Typha and b) Schoencrplectus (2-y) wetland mesocosms in August 2003. Inflow of P is based on river water concentrations and pumping rates recorded during the 2003 experimental period (April-August 2003). No significant differences were detected among hydrological regimes for either of the species groups

Plant tissue nutrient concentrations and uptake.-No significant differences between the hydrological regimes were detected for N and P concentrations separately, but based on N:P ratios, steady-flow wetlands were more N-limited than their pulsed counterparts. It is possible that hydrology affected the availability of both N and P. Steady-flow wetlands remained inundated longer and had the potential to lose more N through denitrification than pulsed wetlands, but based on Schoenopkctus tissue nutrient data, it appeared that P was the more responsive nutrient to hydrology (Fig. 5). The longer periods of anoxic/anaerobic conditions in the steady-flow wetlands may have released more P from the sediment. Evidence of the highly reduced conditions suggests that P (associated with Fe) may have been released during the experimental period, enhancing biomass production. Bayley et al. (1985) found that merely the presence of standing water, regardless of its nutrient content, can influence marsh primary productivity and credited this process to the release of P during anoxic conditions. It is possible that the vegetation in steady-flow wetlands had further uptake of P relative to N which would explain their slightly lower N:P ratio compared to pulsed wetlands. While the difference between N:P ratios may or may not be biologically significant, the statistical difference is noteworthy considering the high N loading rate during much of the experimental period.

When N:P ratios were examined using the thresholds developed by Koerselman and Meuleman, (1996) vegetation was considered N limited in both water regimes. These values are consistent with the results of other N:P ratios from North American marsh studies cited from the literature by Bedford et al. (1999), although their mean N:P ratio was less (7.5 0.7) than either the mean pulsed (11.7 0.5) or steady-flow (9.2 0.6) ratios calculated in this study. The individual ranges of N and P concentrations and uptake observed in this study are consistent with similar studies. Cronk and Fennessy (2001) reported Typha spp. leaf tissue concentrations can range from 5-32 mg N g^sup -1^ and 1-5 mg P g^sup -1^. For Schoenopkctus, it is likely that nutrient translocation to belowground tissue occurred prior to the onset of senescence and that N and P levels were even higher in July when most Schoenopkctus wetlands were at peak biomass. However, nutrient proportions between aboveground and belowground plant parts were comparable among Typha and Schoenopkctus (2-y) (Figs. 6, 7). Tissue concentrations for Schoenolpectus were also comparable to literature ranges. Tanner (1996) reported that for second-year Schoenopkctus growth treated with wastewater effluent, N levels were between 9.2 and 12.9 mg g^sup -1^ and P levels between 2.2 and 3.5mg g^sup -1^. These N levels are comparable to those found in this study with the P levels slightly higher.

IMPLICATIONS FOR THE FULL-SIZED WETIAND PULSING STUDY

The negligible effect of pulsing on most measured parameters was unexpected and may have been diminished by high hydrologie loading rates which were designed to be comparable to the full-sized ORW wetlands. Even during the later weeks of the pulsed months when hydrologie input was low, there was still enough water to keep the wetlands well saturated even though water levels were below the ground surface. This condition may have reduced some of the prescribed effects associated with pulsing (for instance, enhanced mineralization though drying and rewetting).

The results of this study suggest that for a full-sized wetlands pulsing experiment as now underway at the ORW, pulsing may only show a negligible effect on Typha and Schoenopkctus productivity, given the high hydrological loading rate. However, there are several issues regarding scale that need to be considered before the results of this study are extrapolated to the full-size wetlands. First, the full-sized wetlands encompass a large elevation range and will provide a greater range of inundated conditions. second, even in areas that remain inundated, the greater exposure to wind fetch and water movement should alleviate the highly reduced conditions that may have negatively affected the Schoenoplectus in this study. Finally, the mesocosms had vegetation and soils that had only existed for one or two growing seasons while the full-sized wetlands have established vegetation and soils that have become depleted of Fe and available P and, therefore, may be more tied to fluctuating water levels to release organic-bound nutrients. Nevertheless, this study has demonstrated that Typha was more resilient to prolonged hydroperiods and may be better equipped physiologically than Schoenoplectus to withstand anoxic periods that may occur during steady-flow hydrological regimes.

Acknowledgments.-W. Dick and two anonymous reviewers provided helpful comments that improved this manuscript. Funding for this project came from The Ohio State University, School of Natural Resources and OARDC Grant No. 2002-079 and USDA Grant No. 2002- 35102-13518. Publication number 05-009 of the Olentangy River Wetland Research Park.

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SUBMITTED 20 DECEMBER 2004 ACCEPTED 11 APRIL 2005

CHRISTOPHER J. ANDERSON AND WILLIAM J. MITSCH

The Olentangy River Wetland Research Park, School of Natural Resources, The Ohio State University, 352 W. Dodndge Street, Columbus 43202

Copyright American Midland Naturalist Oct 2005

Source: American Midland Naturalist, The

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