Effects of different macrophyte growth forms on sediment and P resuspension in a shallow lake

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1 Hydrobiologia (25) 545: Ó Springer 25 DOI 1.17/s Primary Research Paper Effects of different macrophyte growth forms on sediment and P resuspension in a shallow lake Jukka Horppila*, & Leena Nurminen Department of Biological and Environmental Sciences/Aquatic Sciences, University of Helsinki, P.O. Box 65 (Viikinkaari 1), FIN-14, Finland (*Author for correspondence: jukka.horppila@helsinki.fi) Received 1 24; in revised form 15 December 24; accepted 19 February 25 Key words: sediment resuspension, macrophyte growth forms, internal phosphorus loading Abstract The effects of floating-leaved, submerged and emergent macrophytes on sediment resuspension and internal phosphorus loading were studied in the shallow Kirkkoja rvi basin by placing sedimentation traps among different plant beds and adjacent open water and by sediment and water samples. All the three life forms considerably reduced sediment resuspension compared with non-vegetated areas. Both among submerged (Ceratophyllum demersum, Potamogeton obtusifolius, Ranunculus circinatus) and emergent (Typha angustifolia) plants, resuspension rate was on average 43% of that in the adjacent open water, while within floating-leaved plants (Nuphar lutea) the corresponding value was 87%. The effects of submerged and emergent vegetation increased in the course of the growing season together with increasing plant density. Among floating-leaved vegetation, such seasonal trend in resuspension effects was not observed. Compared with the non-vegetated area, floating-leaved, submerged and emergent plants reduced internal phosphorus loading on average by 21, 12 and 26 mg m )2 d )1, respectively. The effects of floating-leaved plants on resuspension-mediated internal phosphosrus loading were thus comparable to the effects of the other two life forms. Introduction In many lakes, sediment resuspension is one of the main processes regulating water turbidity, with consequences for physicochemical water quality and biotic communities (Bruton, 1985; Hellstro m, 1991; Kirk, 1991; Kristensen et al., 1992; Evans, 1994). Aquatic macrophytes may substantially prevent sediment resuspension and erosion, thus reducing the concentration of inorganic suspended solids in the water (Jackson & Starret, 1959; Dieter, 199; James & Barko, 199; Horppila & Nurminen, 21). Macrophyte abundance is also an important source of error in models that aim to predict suspended sediment concentrations from wave effects and more field measurements are needed to improve model formulation (Hamilton & Mitchell, 1996; Teeter et al., 21). Different macrophyte species have variable effects on water column hydrodynamics and consequently sediment resuspension (Vermaat et al., 2). This is due to the architecture and distribution of plant tissue over the water column varying substantially among growth form and species (Duarte & Kalff, 199). Most studies on resuspension effects of macrophytes have been focused on submerged species (Jackson & Starret, 1959; James & Barko, 199; Petticrew & Kalff, 1992), and the resuspension effects of emergents have been more rarely investigated (Dieter, 199; Horppila & Nurminen, 21). The third main growth form, floating-leaved species, has been ignored. This is a shortcoming,

2 168 since in many turbid-water lakes where submerged plants are sparse, floating-leaved species are dominant and cover large water areas. In the eutrophic Kirkkoja rvi basin of Lake Hiidenvesi (southern Finland), the effects of both emergent and submerged macrophytes on sediment resuspension have been studied and the data have been published in two separate papers (Horppila & Nurminen, 21, 23). In the present paper these data are compared with new data on floating-leaved plants in order to evaluate the effects of different macrophyte growth forms on sediment and P resuspension in this lake. Methods Study area The study was conducted in the Kirkkoja rvi basin (1.6 km 2, mean depth 1.1 m, max depth 3.5 m), which is the most eutrophic part of Lake Hiidenvesi in southern Finland (6 24 N, E) (Fig. 1). The average summertime total phosphorus concentration is 8 12 lg l )1 and the total nitrogen concentration 1 15 lg l )1 (Tallberg et al., 1999; Nurminen, 23). Due to resuspended sediments and runoff from agricultural areas, the concentration of suspended solids exceeds 2 mg l )1 and Secchi depth remains below.5 m during the open water season (Tallberg et al., 1999). Dense beds of emergent macrophytes (Typha angustifolia., T. latifolia L., Glyceria maxima (Hartm.) Holmb.) exist in up to m depth and zones of floating-leaved species (Nuphar lutea (L.) Sibth. and Sm., Nymphaea alba L.) exist between the emergent plants and the open water. Due to the high turbidity of the water, the biomass of submerged species is low. Stands of species such as Myriophyllum verticillatum L. and Ceratophyllum demersum L. exist only in the shallow water (Nurminen, 23). The lake is ice-covered from November December to April, and the senescence of the macrophyte stands takes place during September and October. Sampling In the stand of emergent macrophytes (Typha angustifolia), the study was conducted in May August 1999, while the studies on submerged and floating-leaved species were performed in May August 2. The stand of submerged plants included Ranunculus circinatus Sibth., Potamogeton obtusifolius Hert. and Koch and Ceratophyllum demersum, while the effects of floating-leaved plants were studied in a stand of Nuphar lutea. The study sites for emergent (EMER station) and submerged (SUB station) species were situated in the southwestern end of the Kirkkoja rvi basin, the highest open fetch (18 m) being to the northeast (Fig. 1). The studies on floating-leaved species (FLOAT station) took place in a bay situated in the eastern side of Kirkkoja rvi, the open fetch thus being highest (13 m) to the west (Fig. 1). The abundance of floating-leaved and emergent species was studied by measuring the changes in their stem density. The density of submerged species was Lake Hiidenvesi River Vanjoki River Vihtijoki Kirkkojärvi Kirkkojärvi Kiihkelyksenselkä Mustionselkä EMER FLOAT 1 km Nummelanselkä SUB 5 m > 1 m > 2 m Figure 1. The location of the three sampling stations in Lake Hiidenvesi.

3 169 estimated as per cent volume infested (PVI), calculated as the product of percentage coverage and plant height divided by water depth. Sediment resuspension rate was calculated using the method by Gasith (1975), which is applicable in shallow water bodies. The method uses the equation R ¼ S ðf S f T Þ (Gasith, 1975; Bloesch, 1994); ðf R f T Þ where R = resuspended bottom sediment (dry weight) S = entrapped settling flux (dry weight), f S = organic fraction of S, f R = organic fraction of R (surface sediment), f T = organic fraction of seston (T) suspended in the water. For each growth form, gross sedimentation rate (S) was determined by placing five sediment traps among the plants (5 1 m from the edge, termed inner zone) and five traps in the adjacent open water (5 3 m from the edge, termed outer zone). Water depth in the FLOAT station was 9 cm in the inner zone and 11 cm in the outer zone. In the SUB station, the corresponding depths were 9 and 1 cm and in the EMER station 1 and 11 cm, respectively. The traps were constructed from a cylindrical plastic pipe and had an inside diameter of 54 mm. The length: width ratio of the traps was 6:1, which is appropriate in relatively calm conditions (Bloesch & Burns, 198). The tops of the traps were 4 cm above the sediment. No floats were used, but each trap was equipped with a buoyant collar keeping the trap in the upright position. The traps were lifted at 14-day intervals. In the laboratory, the contents of the traps were dried in 15 C and weighed. The values of gross sedimentation rate (S) were corrected by subtracting the dry weight of suspended matter, contained by the water volume in each trap, from the gross dry weight per trap (Gasith, 1975). The loss on ignition (organic fraction) of the entrapped material (f S ) was determined by ignition at 55 C. In each sampling date, three replicate surface sediment samples were lifted both from the inner zone and the outer zone with a corer and analyzed for loss on ignition (f R ) and for total P concentration after wet combustion with sulphuric acid and hydrogen peroxide. Seston samples from each zone were taken with a tube sampler, filtered through a GF/C filter and analyzed for suspended solids (SS) and loss on ignition (f T ). Additionally, total P and SRP (soluble reactive phosphorus) concentrations in the water were measured using the methods by Murphy & Riley (1962) and Koroleff (1979). The rate of P resuspension in each zone was calculated using the calculated resuspension rates and P contents of surface sediment. In each station, sediment resuspension rate, concentration of suspended solids and P content of surface sediment in the inner zone and in the outer zone were statistically compared with analysis of variance for repeated measurements. The analyses were performed with the GLM procedure of SAS statistical software (SAS Institute Inc., 1989). The method of Gasith (1975) is usable if the organic content of seston (f T ) is significantly different from that of surface sediment (f R ) (Blomqvist & Ha kanson, 1981). To test the method, the values of f R were compared with f T within each station and zone with analysis of variance for repeated measurements. Additionally, to reduce the effects of variations in the environmental conditions between the study sites and years, analysis of covariance was used to compare sediment resuspension rate, and phosphorus resuspension rate in the inner zones of the three different macrophyte stands. Water depth, and average and maximum wavelengths at each site were used as covariates. To describe the wave environment in the different stations, the theoretical wavelengths were calculated using equations by Carper & Bachmann (1984): L ¼ gt2 2p where L is the wavelength (m), g is the gravitational constant and T the wave period (s). The value of T is given by " gt ¼ 1:2 tanh :77 gf # :25 2pU U 2 where U is the wind velocity (m s )1 ) and F is the effective fetch (m). Data on wind velocity and direction (8 measurements per day) were obtained from Helsinki-Vantaa airport situated 4 km east of Kirkkoja rvi. The effective fetch for each wind direction was calculated according to Beach Erosion Board (1972):

4 17 P xi cos k i F ¼ s 13:5 where x i is the distance from the sampling site to land for every deviation angle c i (up to±42 ) and s is a scale constant. The daily average and maximum wavelengths at the different stations were compared with analysis of variance. Results During the study periods, wind velocities were moderate and only occasionally exceeded 1 m s )1. Maximum wavelength mostly remained below 15 cm (Fig. 2). The average daily wavelengths were higher in the FLOAT station than in the SUB and EMER stations although the difference was not statistically significant (p >.5). Maximum daily wavelengths were also higher in the FLOAT station than in the other two stations, the difference between the FLOAT and EMER stations being significant (p <.5). In each station, macrophytes were absent when the sampling was started in the spring. In the inner zone of FLOAT station, the stem density of Nuphar lutea increased steadily and reached 18 Wave length (cm) Wave length (cm) Maximum Average May July August Sept. Figure 2. The maximum (top) and average (bottom) theoretical wavelengths at the three sampling stations in Kirkkojärvi during the study periods. stems m )2 on 26 July. In the SUB station, the density of macrophytes reached a maximum PVI of 3% in July (biomass percentages 4 % C. demersum, 4% R. circinatus, 2% P. obtusifolius). In the EMER station, the stem density reached the maximum of 22 stems m )2 by 7 July. In the FLOAT station, the rate of sediment resuspension in the inner zone fluctuated between 24 and 67 g DW m )2 d )1 (Fig. 3). In outer zone, Resuspension rate (g DW m -2 d -1 ) Resuspension rate (g DW m -2 d -1 ) Resuspension rate (g DW m -2 d -1 ) May May July 9-23 Inner zone July July July 23-7 July Outer zone 26 July July July July - 3 Figure 3. The average daily rate of sediment resuspension (with 95% confidence limits) in the inner zone and in the outer zone of floating-leaved (top), submerged (middle) and emergent (bottom) macrophytes in Kirkkojärvi.

5 171 with the exception of 1 14, resuspension rate was higher than among the plants, and fluctuated between 35 and 58 g DW m )2 d )1. The difference between the two zones was statistically significant (Table 1). In the inner zone of the SUB station, in early July, the rate of sediment resuspension fluctuated between 1 and 2 g DW m )2 d )1, while in the outer zone the stand it varied from 2 to 25 g DW m )2 d )1 (Fig. 3). In August, the resuspension rate was g DW m )2 d )1 in the outer zone and below 5 g DW m )2 d )1 in the inner zone (Fig. 3). The difference between the inner zone and the outer zone was statistically significant (Table 1) and increased in the course of the summer. In the inner zone of the EMER station, the rate of sediment resuspension remained below 12 g DW m )2 d )1, with the exception of the period 9 23 (2.7 g DW m )2 d )1 ) (Fig. 3). In the outer zone, resuspension rate was above 2 g DW m )2 d )1, except in May, which was significantly higher than among the plants (Table 1). In the analysis of covariance, resuspension rate in the FLOAT station was significantly higher than in the SUB and EMER stations (df=2, F=18.85, p=.2). The effect of the covariates was insignificant (average wavelength p=.2522, maximum wavelength p=.657, water depth p=.187). No difference in the resuspension rate between the SUB and EMER stations was found. In both zones of all three sampling stations, the values of f T were statistically significantly higher (p <.1) than those of f R, suggesting that resuspension rate could be reliably calculated using the method by Gasith (1975). The concentration of suspended solids (SS) in the FLOAT station, was mg l )1 in May, after which it varied mostly between 15 and 25 mg l )1 (Fig. 4). There was no significant difference between the inner zone and the outer zone (Table 1). In the SUB station, the initial concentration of SS was 1 mg l )1 both in the inner zone and in the outer zone (Fig. 4). Thereafter, the SS concentration decreased in both zones, but was 1 4 mg l )1 higher in the outer zone compared with the inner zone (Table 1). In the EMER station, in May early concentration of SS was below 1 mg l )1 in both zones (Fig. 4). Thereafter, SS concentration remained below 12 mg l )1 in the inner zone, while in the outer zone it reached 27 mg l )1. The difference between the inner zone and the outer zone was statistically significant (Table 1). In the different stations, concentration of total phosphorus in the surface sediment fluctuated mostly between.9 and 1.4 mg g )1 DW (Table 2). In all three stations, total P concentration in the sediment was statistically significantly higher in the outer zone than in the inner zone (p <.1). Table 1. Results from the analyses of variance for repeated measurements for the differences in the concentration of suspended solids, sediment resuspension, and phosphorus resuspension Inner vs. Outer Time Interaction df F p df F p df F p Suspended solids < Sediment resuspension < <.1 P resuspension < Suspended solids Sediment resuspension P resuspension < < <.1 Suspended solids Sediment resuspension < < <.1 P resuspension

6 172 Suspended solids (mg l -1 ) Suspended solids (mg l -1 ) Suspended solids (mg l -1 ) Inner zone May July August May July August May July August Outer zone Figure 4. The concentration of suspended solids (with 95% confidence limits) in the inner zone and in the outer zone of floating-leaved (top), submerged (middle) and emergent (bottom) macrophytes in Kirkkojärvi. In all the three stations, P resuspension rate was significantly higher in the outer zone than in the inner zone (Table 1). The rate of phosphorus resuspension in the inner zone of the FLOAT station varied between 2 and 6 mg P m )2 d )1, while in the outer zone it increased steadily reaching 75 mg P m )2 d )1 in August (Fig. 5). In the inner zone of the SUB station, P resuspension rate decreased from 2 25 mg P m )2 d )1 to below 5mgPm )2 d )1. In the outer zone, P resuspension rate decreased as well, but never fell below 15 mg P m )2 d )1. In the inner zone of the EMER station, P resuspension decreased from 2 to 6mgPm )2 d )1, while in the outer zone it increased in the course of the summer and reached 63 mg P m )2 d )1 at highest (Fig. 5). In the analysis of covariance, P resuspension rate in the FLOAT station was significantly higher than in the SUB and EMER stations (df=2, F=15.7, p=.5). The effect of the covariates was insignificant (average wavelength p=.49, maximum wavelength p=.3842, water depth p=.81). No difference in the P resuspension rate between the SUB and EMER stations was found. Discussion A deepwater wave feels the bottom, when it moves into water where depth is less than one-half of its wavelength (Carper & Bachmann, 1984). Such situations were rare in the sampling stations of Kirkkoja rvi. However, resuspension was a continuous process, as is common in shallow lakes (Evans, 1994). No dependence between wavelength and rate of resuspension was observed, suggesting that even during moderate wind velocities, water currents were strong enough to cause sediment redistribution and/or that biological factors, such as benthivorous fish, also contributed to resuspension (Bengtsson et al., 199; Breukelaar et al., 1994). In Kirkkoja rvi, dense assemblages of benthivorous cyprinids (roach Rutilus rutilus (L.), bream Abramis brama (L.) and white bream Abramis bjo rkna (L.)) exist (Vinni et al., 2). Many earlier studies have suggested that both emergent and submerged species considerably reduce sediment resuspension and erosion (Dieter, 199; Van den Berg et al., 1997; Barko & James, 1998; Vermaat et al, 2; James & Barko, 24). For instance, Barko & James (1998) found very low resuspension rates (Marsh Lake, Minnesota) even during high wind speeds, when submerged macrophytes (Potamogeton pectinatus) were abundant and high resuspension values in the absence of macrophytes. Dieter (199) reported that in shallow marsh lakes (South Dakota) sediment resuspension rate in areas protected by emergent vegetation was only one-third of the resuspension rate in the open water. Our findings

7 Table 2. The concentration of total phosphorus (mg g )1 DW) in surface sediment (±95 % confidence limits) in the inner zone and in the outer zone of different macrophyte stands July 26 July 9 August 23 August Inner zone.88 ±.6.98 ± ± ± ±.1 1. ±.13 Outer zone 1.6 ± ± ± ± ± ±.1 Inner zone 1.37 ± ±.7.96 ± ± ± ±.2 Outer zone 1.3 ± ± ± ± ± ±.1 26 May July 21 July 3 August Inner zone 1.55 ± ± ± ± ± ±.3 Outer zone 1.26 ± ± ± ± ± ± were consistent with such results. Both in SUB and EMER stations, resuspension rate in the inner zone was on average 43% of that in the outer zone. However, the effects of floating-leaved plants on sediment resuspension seemed to be weaker than those of submerged and emergent species. Resuspension rate in the inner zone was on average 87% of that in the outer zone. Resuspension rate in the inner zone of the FLOAT station was also absolutely higher than in the inner zone of the other two stations, which could not be explicitly attributed to differences in either water depth or wave environment. Benthivorous fish were neither the main cause for the higher resuspension rates in the FLOAT station compared with the other two stations. Gillnet catches in the inner zone of the FLOAT station are dominated by small planktivorous fish (mostly roach and perch (Perca fluviatilis L.)) (Pekcan-Hekim et al., 25). The effects of submerged and emergent species increased substantially in the course of the growing season. The effects of submerged species on resuspension increased considerably, when PVI approached 3%. In the stand of emergents, resuspension was reduced, when stem density reached 22 stems m )2. Among the floating-leaved plants, increasing stem density had no strong effects on resuspension. Such difference between the life forms is understandable due to the limited underwater structures of Nuphar lutea compared with the other studied species. It may be that the resuspension effects of nymphaeids were mostly due to their large underground structures rather than their stems in the water column. Nuphar lutea has a higher percentage of biomass in underground structures than the other species studied (Wetzel, 1983). Such effect could also explain, why there was no difference in SS concentration between the inner and outer zones in the FLOAT station, while the sediment resuspension rate was greater in the outer zone. Both in SUB and EMER stations, SS concentration was considerably higher in the open water than among the plants. The tops of the sediment traps were 4 cm above the sediment, while water samples for SS analyses were taken from the uppermost 5 cm layer. Among submerged and emergent species, resuspended sediment was lifted high enough to be collected by the sediment traps, but did not reach the top of the water due to high coverage of plant structures. In the FLOAT station, resuspended sediment could be freely mixed to the whole water column. In the FLOAT station, the stand of floatingleaved plants reduced internal phosphorus loading on average by 21 mg m )2 d )1. In the SUB station, the plant-induced reduction in the internal P loading was on average 12 mg m )2 d )1, and in the EMER station 26 mg m )2 d )1. The effects of floating-leaved plants on P resuspension were thus as strong as the effects of the two other life forms. This was since the amount of phosphorus brought back to the water column did not depend only on sediment resuspension rate, but also on sediment quality. In all three sampling stations, total P concentration in the surface sediment was lower among the plants compared with the sediment in the open water. Rooted macrophytes use sediment as their main source of phosphorus, which may

8 174 P resuspension (mg m -2 d -1 ) Presuspension (mg m -2 d -1 ) P resuspension (mg m -2 d -1 ) May May July July 9-23 Inner zone July July 23-7 July Outer zone 26 July-9 26 July July July - 3 Figure 5. The average daily rate of phosphorus resuspension (with 95% confidence limits) in the inner zone and in the outer zone of floating-leaved (top), submerged (middle) and emergent (bottom) macrophytes in Kirkkojärvi. result in lowered P concentration of the sediment (Carignan & Kalff, 198; Chen & Barko, 1988). The difference in sediment P content between the inner and the outer zone was highest in the FLOAT station, resulting in a reduced P resuspension rate in the inner zone without comparable reduction in sediment resuspension. The result is in accordance with the findings that in floatingleaved plants phosphorus uptake by roots is of more importance than in most other macrophytes (Carignan, 1982). Additionally, part of the plants in the SUB station were bentho-pleustophytic (Ceratophyllum demersum, Ranunculus circinatus), relying less on sediment nutrients than truly rooted species. Accordingly, the effect of submerged plants on phosphorus resuspension was relatively much weaker than their effect on overall sediment resuspension. Considering the resuspension effects of different macrophyte species, another important factor is the area of stands. In eutrophic, clay-turbid waters, floating-leaved nymphaeids are often able to colonize large areas, while submerged species and emergents are severely limited by light availability and looseness of bottom substrate (Chambers & Kalff, 1985; Weisner, 1991). Accordingly, in Kirkkoja rvi, the area inhabited by floating-leaved plants is c. 3 ha, while the areas of submerged and emergent plant beds are 15 and 17 ha, respectively. All three growth forms had a considerable reductive effect on sediment resuspension, the effect of floating-leaved species being weaker than those of submerged and emergent macrophytes. Due to their reductive effect on the phosphorus concentration of the sediment, however, the effects of floating-leaved plants on resuspension-mediated internal phosphorus loading were comparable to the effects of the other two life forms. Acknowledgements The study was funded by the Academy of Finland (project 532) and by Alfred Kordelin Foundation. Jouko Sare n and Raija Mastonen helped with the laboratory analyses. Wind data were provided by the Finnish Meteorological Institute. References Barko, J. W. & W. F. James, Effects of submerged macrophytes on nutrient dynamics, sedimentation, and resuspension. In Jeppesen, E, Søndergaard, Ma, Søndergaard, Mo, Christoffersen, K (eds). The Structuring role of Macrophytes in Lakes. Ecological studies. Springer Verlag, New York: Beach Erosion Board, Waves in inland reservoirs. Technical Memoir 132, Beach Erosion Board Corps of Engineers. Washington DC. Bengtsson, L., T. Hellstro m & L. Rakoczi, 199. Redistribution of sediment in three Swedish lakes. Hydrobiologia 192: Bloesch, J., A review of methods used to measure sediment resuspension. Hydrobiologia 284:

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