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Effects of salinity changes on growth of Ruppia maritima L.
Aquatic Botany 77 (2003) 235–241
Short communication
Effects of salinity changes on growth of Ruppia maritima L. Megan K. La Peyre∗ , Sheryl Rowe U.S.G.S. Louisiana Fish and Wildlife Cooperative Research Unit, School of Renewable Natural Resource, Louisiana State University Agricultural Center, Baton Rouge, LA 70803, USA Received 28 January 2002; received in revised form 2 May 2003; accepted 18 July 2003
Abstract The ability of Ruppia maritima L. to tolerate moderate salinity changes was determined in a greenhouse study. While R. maritima has been shown to survive in salinities from 0 to 70 ppt, it has been suggested that changes in salinity alone may be detrimental. We tested the hypothesis that along the northern shore of the Gulf of Mexico, R. maritima may be limited not by salinity averages, but by salinity changes which occur. In a 9-week experiment, in summer 2001, relative growth rate of R. maritima was compared between salinity treatments which included constant salinity (10 g l−1 ), one salinity change (±10 g l−1 ), and two salinity changes (±10 g l−1 ). Relative growth rate was highest under constant salinity, and significantly lower than the control (constant salinity) when salinity was reduced by 10 g l−1 over 48 h at week 3, and increased by 10 g l−1 at week 6 suggesting that short-term freshening events may negatively impact R. maritima growth. Relative growth rate was also significantly lower when salinity was raised twice, once at week 3, and again at week 6 to reach 30 g l−1 . Pulsed salinity changes may limit R. maritima growth and distribution in this region. Published by Elsevier B.V. Keywords: Submerged aquatic vegetation; Louisiana; Greenhouse study; Salinity change; Variation
1. Introduction Fluctuations in salinity are common in coastal waters along the northern shore of the Gulf of Mexico (Mac et al., 1998; Wiseman and Swenson, 1988). While some variation is natural, resulting from rain or storm events, other variation results from the extensive marsh/water management projects along this shoreline (Wiseman and Swenson, 1988). As these are discrete events, their effects on salinity may occur relatively rapidly, and their ∗ Corresponding author. Tel.: +1-225-578-4180; fax: +1-225-578-4144. E-mail address: [email protected] (M.K. La Peyre).
0304-3770/$ – see front matter. Published by Elsevier B.V. doi:10.1016/S0304-3770(03)00109-8
236
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impact on salinity may last from days to weeks. Numerous researchers have hypothesized that fluctuations in salinity may be an important factor in structuring brackish wetland plant communities (i.e. Chabreck and Palmisano, 1973; Brewer and Grace, 1990; Flynn et al., 1995; Gunterspergen et al., 1995; Howard and Mendelssohn, 2000), but few have examined the impact of changing salinities on submerged aquatic vegetation communities. Ruppia maritima, a submerged angiosperm, occurs world-wide and appears to thrive in diverse habitats (e.g. Davis et al., 1985; Kantrud, 1991). Along the northern shore of the Gulf of Mexico, R. maritima is pervasive, existing in fresh to brackish water areas (Chabreck, 1971). It is generally thought that turbidity and salinity may be the two dominant abiotic factors restricting R. maritima growth (Verhoeven, 1979; Bonis et al., 1993; Adair et al., 1994; Schutten et al., 1994), while competition from other species may limit R. maritima outside its ideal environment (Verhoeven, 1979; Pulich, 1985). R. maritima is known to survive in salinities ranging from 0 to 70 gl−1 (Kantrud, 1991), suggesting that the actual salinity averages may not limit its growth along this coast. Instead, it is hypothesized that salinity fluctuations may be limiting R. maritima growth and distribution. Previous studies are unclear as to the ability of R. maritima to withstand salinity changes. Several studies have tested the effects of various levels of salinity held constant on R. maritima growth (e.g. Bird et al., 1993; Bonis et al., 1993). However, to our knowledge, no controlled studies exist testing the effects of salinity fluctuations on R. maritima growth. In Europe, R. maritima died when salinity increased over 18 g l−1 in a few weeks, and salinity fluctuations of less than 18 g l−1 over a year resulted in the best stands of R. maritima (Verhoeven, 1979). In contrast, other field observations indicate that salinity changes greater than 14 g l−1 over 24 h and 44 g l−1 over several months failed to have negative effects on R. maritima (Richardson, 1980; Van Vierssen, 1982). This initial greenhouse study was designed to evaluate the growth of R. maritima in response to salinity changes similar to ones that occur along the north shore of the Gulf of Mexico. Specifically, we asked three questions about the effects of salinity changes on R. maritima growth. First, how is growth affected by a freshening event from rain or freshwater diversion events? Second, how is growth affected by increased salinity events? Third, what are the impacts if the increase or decrease in salinity is pulsed?
2. Materials and methods 2.1. Plant and sediment material Individual ramets of R. maritima were collected from the northern shore of Lake Pontchartrain, LA, USA, in early May 2001. At the same time and location, sediment was collected for use in the experiment. Plants and sediment were placed in bins, covered with wet towels and immediately transported to a greenhouse located at Louisiana State University, Baton Rouge, LA, USA. In the greenhouse, 500 individual ramets with roots were selected that were approximately 10 cm in length. Periphyton attached to the stems of all ramets was removed by running a closed, gloved hand up each stem through the leaves. Ramets were then placed in a
M.K. La Peyre, S. Rowe / Aquatic Botany 77 (2003) 235–241
237
freshwater bath and shaken clean according to the procedure in Edgar and Robertson (1992). Initial propagules were chosen so that plants were approximately the same size at the beginning of the experiment. Initial fresh weights and blade lengths of the cleaned R. maritima were measured. A regression using extra plants was used to obtain estimates of initial dry weights. The regression was highly significant (P < 0.001) and predicted biomass well (r 2 = 0.90). Initial dry weight (DW) of the experimental plants was 0.06 ± 0.01 g DW (mean 1 ± S.E.; N = 250), and measured stem height was 10.95 ± 2.40 cm (N = 250). Sediment used for the experiment was sieved to pass through 1 mm mesh. 2.2. Experimental design and procedure Ten ramets of R. maritima L. were placed in small plastic containers holding 450 ml of the collected and sieved sediment for a total of 20 containers (200 ramets). Following planting, clean sand was added to the surface of each container to a depth of 1 cm to minimize suspension of sediments and retard nutrient loss (Smart and Barko, 1995). Containers were placed in individual plastic tanks (0.7 m × 0.35 m × 0.4 m), and water maintained at 2 cm below the top of the bin for the duration of the experiment. Baseline conditions were 10 g l−1 salinity with water depth 25 cm above the substrate surface. Initial salinity was adjusted to 10 g l−1 using artificial seawater (Hawaiian Marine Imports, Inc., Houston, TX). Salinity treatments were applied to each tank, and consisted of a control and five different treatments (Fig. 1). Treatments consisted of no salinity change (CTL), one salinity change (LO1, HI1) or two salinity changes (LO2, HI2, HI3). The experiment was run over a 9-week period. All bins were maintained at 10 g l−1 for the first 3-week period, allowing the plants time to acclimate to the new environment. After 3 weeks, over a 48 h period, water was changed in all the tanks in order to achieve the target salinities of 10 g l−1 (CTL), 0 g l−1 (LO), or 20 g l−1 (HI). After three more weeks, water was again changed in all the tanks over a 48 h period in order to achieve the final target salinities of 10 g l−1 (CTL, LO2, HI2), 0 g l−1 (LO1), 20 g l−1 (HI2) and 30 g l−1 (HI3). Bins that did not require a salinity change 30
CTL
LO1
LO2
HI1
HI2
HI3
Salinity (g L-1)
20 10 0 30 20 10 0 0
3
6
9
0
3
6
9
0
3
6
9
Time (weeks) Fig. 1. Salinity treatments applied to experimental tank units. Each salinity level was maintained for a 3-week period. Changes in salinity were carried out over 48 h periods.
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were treated similarly to bins requiring a salinity change with water being siphoned out, and new water placed in the bin. These salinity treatments were selected to mimic the two types of salinity changes that occur frequently along the northern coast of the Gulf of Mexico: (1) freshening events from rain or freshwater diversion events; and (2) increased salinities from storm events pushing Gulf waters inshore. Salinity, water temperature and light levels were measured daily throughout the experiment. Nitrates, pH and DO were measured initially and 6 weeks into the experiment. Stem lengths were measured every 3 weeks, just prior to salinity adjustments. Roots and shoots of all plants were harvested on 31 July 2001, dried to a constant weight, and total biomass determined. Relative growth rates (RGR) in terms of biomass and height were calculated as RGR = {ln([yf ]) − ln([yi ])}/time, where [yf ] is final biomass (g DW), [yi ] is initial biomass (g DW) or initial height (cm), and time is the length of the experiment in days. 2.3. Analysis Data were analyzed as a randomized block design. Data were tested for assumptions of normality (Shapiro–Wilk test statistic) and homogeneity of variance. One-way analysis of variance followed by Dunnett’s test (Snedecor and Cochran, 1989; Day and Quinn, 1989) were used to compare relative growth rates of biomass and stem length to the treatment control.
3. Results
0.04 0.03
*
*
0.02 0.01
H I3
H I2
2 LO
H I1
LO
1
0 C TL
(g g-1 d-1)
Relative Growth Rate
Relative growth rate of biomass of R. maritima ranged from 0.018 g g−1 per day ±0.004S.D. in the LO2 treatment to 0.029 g g−1 per day ±0.006S.D. in the CTL treatment (Fig. 2). The one-way ANOVA for biomass was significant (P = 0.03). Dunnett’s test indicated that LO2 and the HI3 treatments were significantly differently from the control.
Salinity Treatment
Fig. 2. Relative growth rate (g g−1 per day) of R. maritima as influenced by salinity changes and direction of salinity change (±10 g l−1 ) calculated over a 9-week growth period (Fig. 1). ANOVA tested for the effect of salinity treatment on relative growth rate (P = 0.03), followed by contrasts comparing the control to the treatments. Error bars represent standard deviations. Asterisk represents significant difference from the control treatment.
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Fig. 3. Relative stem length increase rate (mm mm−1 per day) of R. maritima as influenced by salinity changes and direction of salinity change (±10 g l−1 ) calculated over a 9-week growth period (Fig. 1). ANOVA tested for the effect of salinity treatment on relative growth rate (P = 0.12). Error bars represent standard deviations.
Relative growth rate of length of R. maritima stems ranged from a low of 0.006 cm cm−1 per day ±0.008 in the LO2 treatment to a high of 0.016 cm cm−1 per day ±0.003 in the CTL treatment (Fig. 3). The one-way ANOVA for length was not significant (P = 0.12).
4. Discussion Past field observations that the healthiest (i.e. greatest relative growth rate) R. maritima occur in areas with a more constant salinity regime (e.g. Verhoeven, 1979; Kantrud, 1991) were supported by our study with several of the treatments resulting in significantly lowered relative growth rates as compared to the control (constant salinity). Relative growth rate was significantly lowered with a short freshwater pulse, and with a pulsed salinity increase. R. maritima growth was half that of the control (constant salinity) when salinity was reduced by 10 g l−1 over 48 h at week 3, and increased by 10 g l−1 at week 6 (LO2 treatment) suggesting that short-term freshening events may negatively impact R. maritima growth. At the other end of the spectrum, relative growth rate was also reduced by about half when salinity was raised twice, once at week 3, and again at week 6 to reach 30 g l−1 (HI3 treatment), suggesting that pulsed increases in salinity may negatively impact R. maritima growth. The lowered growth rates for the LO2 and HI3 treatments might suggest that either (1) changed salinity, per se, negatively impacted R. maritima growth, or (2) pulsed salinity changes negatively affected R. maritima relative growth rate. Numerous studies document that R. maritima can survive in salinities ranging from 0 to 70 gl−1 (Kantrud, 1991). In Louisiana, one study found no differences in R. maritima growth in salinity ranging from 3.7 to 33.4 g l−1 (Joanen and Glasgow, 1965). We tested growth in salinity ranging from 0 to 30 g l−1 . Although R. maritima is known to grow in freshwater, it is possible that some of the reduced growth in the LO2 treatment may be due to this freshwater treatment. Longer term effects of low salinity have been documented, but were not addressed in this study. For example, Bonis et al. (1993, 1995) found that lowered salinity alone resulted in reduced reproductive effort of R. maritima, suggesting that lower salinity
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may have more long-term impacts on R. maritima by impacting reproduction, and thus potentially the future structure, density and longevity of R. maritima communities. However, since the LO1 treatment was not significantly different from the control, the reduced growth observed in the LO2 treatment is more likely explained by the short freshwater flushing event, similar to what might happen after a heavy rain, or from some of the freshwater diversion projects occurring in this region. The pulsed salinity changes (10 g l−1 over 48 h) provide a more likely explanation for the lowered relative growth rates of R. maritima observed in this study. Both a pulsed increase (HI3) and decrease (LO2) in salinity resulted in lowered relative growth rates suggesting that both freshening events and pulsed high salinity events (e.g. storms) may negatively affect R. maritima growth over the short-term. The lack of a significant reduction in growth for the HI2 treatment may be due to the fact that the range of salinity covered by this treatment falls squarely within what most researchers agree is the optimum salinity range for R. maritima (10 to 20 gl−1 ). While the importance of other factors such as turbidity and competition also need to be examined, R. maritima distributions may be affected by repeated salinity changes, or possibly by the less frequent changes of a greater magnitude and rate of change than were tested in this study. We limited our study to the magnitude and rate of change tested as they represent the common rates of change in fresh to brackish water areas along the northern coast of the Gulf of Mexico. Salinity changes of a greater magnitude and/or rate may be more likely to impact R. maritima communities through impacts on relative growth rates, however, these types of changes are far less frequent. Regardless, the success of R. maritima is most likely related to the interaction between its life history traits and environmental conditions, of which the local salinity regime (range and change) is critical.
Acknowledgements The authors thank Chi-Howe Teo for help in the field and Ron Boustany for advice on greenhouse space set-up. We also thank Dr. Vermaat for his very helpful comments, as well as two anonymous reviewers for helpful comments on an earlier version. This research was supported by the Louisiana Department of Wildlife and Fisheries and the U.S.G.S. Louisiana Fish and Wildlife Cooperative Research Unit.
References Adair, S.E., Moore, J.L., Onuf, C.P., 1994. Distribution and status of submerged vegetation in estuaries of the upper Texas coast. Wetlands 14, 110. Bird, K.T., Cody, B.R., Jewett-Smith, J., Kane, M.E., 1993. Salinity effects on Ruppia maritima L. cultured in vitro. Bot. Mar. 36, 23–28. Bonis, A., Grillas, P., Vanwijck, C., Lepart, J., 1993. The effects of salinity on the reproduction of coastal submerged macrophytes in experimental communities. J. Veg. Sci. 4, 461–468. Bonis, A., Lepart, J., Grillas, P., 1995. Seed bank dynamics and coexistence of annual macrophytes in a temporary and variable habitat. Oikos 74, 81–92. Brewer, J.S., Grace, J.B., 1990. Plant community structure in an oligohaline tidal marsh. Vegetatio 90, 93–107.
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Chabreck, C.H., 1971. Ponds and lakes of the Louisiana coastal marshes and their value to fish and wildlife. In: Proceedings of the 25th Annual Conference of the SE Association of Game and Fish Commissioners. pp. 206–215. Chabreck, R.H., Palmisano, A.W., 1973. The effects of Hurricane Camille on the marshes of the Mississippi River Delta. Ecology 54, 1118–1123. Davis, G.J., Bradshaw, H.D., Harlan, S.M., 1985. Submersed macrophytes in Jacks and Jacobs Creeks. J. Elisha Mitchell Sci. Soc. 101, 125–129. Day, R.W., Quinn, G.P., 1989. Comparisons of treatments after an analysis of variance in ecology. Ecol. Monogr. 59, 433–463. Edgar, G.J., Robertson, A.I., 1992. The influence of seagrass structure on the distribution and abundance of mobile epifauna: pattern and process in a Western Australian Amphibolis bed. J. Mar. Biol. Ecol. 160, 13–31. Flynn, K.M., McKee, K.L., Mendelssohn, I.A., 1995. Recovery of freshwater marsh vegetation after a saltwater intrusion event. Oecologia 103, 63–72. Gunterspergen, G.R., Cahoon, D.R., Grace, J., Steyer, G.D., Fournet, S., Townson, M.A., Foote, A.L., 1995. Disturbance and recovery of the Louisiana coastal marsh landscape from the impacts of Hurricane Andrew. J. Coastal Res. S121, 324–339. Howard, R.J., Mendelssohn, I.A., 2000. Structure and composition of oligohaline marsh plant communities exposed to salinity pulses. Aquat. Bot. 68, 143–164. Joanen, T., Glasgow, L.L., 1965. Factors influencing the establishment of wigeongrass stands in Louisiana. Proc. Southeast Assoc. Game Fish Comm. 19, 78–92. Kantrud, H.A., 1991. Wigeongrass (Ruppia maritima L.): A literature Review. US Fish and Wildlife Survey, Fish and Wildlife Research, No. 10. p. 58. Mac, M.J., Opler, P.A., Puckett Haecker, C.E., Doran, P.D., 1998. Status and trends of the nation’s biological resources. vol. 2. US Department of the Interior, US Geological Survey, Reston, VA, p. 964. Pulich, W.M., 1985. Seasonal growth dynamics of Ruppia maritima L. s.l. and Halodule wrightii aschers. In southern Texas and evaluation of sediment fertility status. Aquat. Bot. 23, 53–66. Richardson, F.D., 1980. Ecology of Ruppia maritima in New Hampshire (USA) tidal marshes. Rhodora 82, 403– 439. Schutten, J., Van der Velden, J.A., Smit, H., 1994. Submerged macrophytes in the recently freshened lake system Vokerak-Zoom (The Netherlands), 1987–1991. Hydrobiologia 276, 207–218. Smart, R.M., Barko, J.W., 1995. Laboratory culture of submersed freshwater macrophytes on natural sediments. Aquat. Bot. 21, 251–263. Snedecor, G.W., Cochran, W.G., 1989. Statistical Methods. Iowa State University Press, IA, USA Van Vierssen, W., 1982. The ecology of communities dominated by Zannichellia taxa in Western Europe. II. Distribution, synecology and productivity aspects in relation to environmental factors. Aquat. Bot. 13, 385– 483. Verhoeven, J.T.A., 1979. The ecology of Ruppia-dominated communities in Western Europe. I. Distribution of Ruppia representatives in relation to their autoecology. Aquat. Bot. 6, 197–268. Wiseman, W.J., Swenson, E.M., 1988. Long-term salinity trends in Louisiana estuaries. In: Turner, R.E., Cahoon, D.R. (Eds.), Causes of Wetland Loss in the Coastal Central Gulf of Mexico, vol. 2. Technical Narrative, OCS Study MMS 87–0120, Minerals Management Service, New Orleans, LA, pp. 1017–1021.
Short communication
Effects of salinity changes on growth of Ruppia maritima L. Megan K. La Peyre∗ , Sheryl Rowe U.S.G.S. Louisiana Fish and Wildlife Cooperative Research Unit, School of Renewable Natural Resource, Louisiana State University Agricultural Center, Baton Rouge, LA 70803, USA Received 28 January 2002; received in revised form 2 May 2003; accepted 18 July 2003
Abstract The ability of Ruppia maritima L. to tolerate moderate salinity changes was determined in a greenhouse study. While R. maritima has been shown to survive in salinities from 0 to 70 ppt, it has been suggested that changes in salinity alone may be detrimental. We tested the hypothesis that along the northern shore of the Gulf of Mexico, R. maritima may be limited not by salinity averages, but by salinity changes which occur. In a 9-week experiment, in summer 2001, relative growth rate of R. maritima was compared between salinity treatments which included constant salinity (10 g l−1 ), one salinity change (±10 g l−1 ), and two salinity changes (±10 g l−1 ). Relative growth rate was highest under constant salinity, and significantly lower than the control (constant salinity) when salinity was reduced by 10 g l−1 over 48 h at week 3, and increased by 10 g l−1 at week 6 suggesting that short-term freshening events may negatively impact R. maritima growth. Relative growth rate was also significantly lower when salinity was raised twice, once at week 3, and again at week 6 to reach 30 g l−1 . Pulsed salinity changes may limit R. maritima growth and distribution in this region. Published by Elsevier B.V. Keywords: Submerged aquatic vegetation; Louisiana; Greenhouse study; Salinity change; Variation
1. Introduction Fluctuations in salinity are common in coastal waters along the northern shore of the Gulf of Mexico (Mac et al., 1998; Wiseman and Swenson, 1988). While some variation is natural, resulting from rain or storm events, other variation results from the extensive marsh/water management projects along this shoreline (Wiseman and Swenson, 1988). As these are discrete events, their effects on salinity may occur relatively rapidly, and their ∗ Corresponding author. Tel.: +1-225-578-4180; fax: +1-225-578-4144. E-mail address: [email protected] (M.K. La Peyre).
0304-3770/$ – see front matter. Published by Elsevier B.V. doi:10.1016/S0304-3770(03)00109-8
236
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impact on salinity may last from days to weeks. Numerous researchers have hypothesized that fluctuations in salinity may be an important factor in structuring brackish wetland plant communities (i.e. Chabreck and Palmisano, 1973; Brewer and Grace, 1990; Flynn et al., 1995; Gunterspergen et al., 1995; Howard and Mendelssohn, 2000), but few have examined the impact of changing salinities on submerged aquatic vegetation communities. Ruppia maritima, a submerged angiosperm, occurs world-wide and appears to thrive in diverse habitats (e.g. Davis et al., 1985; Kantrud, 1991). Along the northern shore of the Gulf of Mexico, R. maritima is pervasive, existing in fresh to brackish water areas (Chabreck, 1971). It is generally thought that turbidity and salinity may be the two dominant abiotic factors restricting R. maritima growth (Verhoeven, 1979; Bonis et al., 1993; Adair et al., 1994; Schutten et al., 1994), while competition from other species may limit R. maritima outside its ideal environment (Verhoeven, 1979; Pulich, 1985). R. maritima is known to survive in salinities ranging from 0 to 70 gl−1 (Kantrud, 1991), suggesting that the actual salinity averages may not limit its growth along this coast. Instead, it is hypothesized that salinity fluctuations may be limiting R. maritima growth and distribution. Previous studies are unclear as to the ability of R. maritima to withstand salinity changes. Several studies have tested the effects of various levels of salinity held constant on R. maritima growth (e.g. Bird et al., 1993; Bonis et al., 1993). However, to our knowledge, no controlled studies exist testing the effects of salinity fluctuations on R. maritima growth. In Europe, R. maritima died when salinity increased over 18 g l−1 in a few weeks, and salinity fluctuations of less than 18 g l−1 over a year resulted in the best stands of R. maritima (Verhoeven, 1979). In contrast, other field observations indicate that salinity changes greater than 14 g l−1 over 24 h and 44 g l−1 over several months failed to have negative effects on R. maritima (Richardson, 1980; Van Vierssen, 1982). This initial greenhouse study was designed to evaluate the growth of R. maritima in response to salinity changes similar to ones that occur along the north shore of the Gulf of Mexico. Specifically, we asked three questions about the effects of salinity changes on R. maritima growth. First, how is growth affected by a freshening event from rain or freshwater diversion events? Second, how is growth affected by increased salinity events? Third, what are the impacts if the increase or decrease in salinity is pulsed?
2. Materials and methods 2.1. Plant and sediment material Individual ramets of R. maritima were collected from the northern shore of Lake Pontchartrain, LA, USA, in early May 2001. At the same time and location, sediment was collected for use in the experiment. Plants and sediment were placed in bins, covered with wet towels and immediately transported to a greenhouse located at Louisiana State University, Baton Rouge, LA, USA. In the greenhouse, 500 individual ramets with roots were selected that were approximately 10 cm in length. Periphyton attached to the stems of all ramets was removed by running a closed, gloved hand up each stem through the leaves. Ramets were then placed in a
M.K. La Peyre, S. Rowe / Aquatic Botany 77 (2003) 235–241
237
freshwater bath and shaken clean according to the procedure in Edgar and Robertson (1992). Initial propagules were chosen so that plants were approximately the same size at the beginning of the experiment. Initial fresh weights and blade lengths of the cleaned R. maritima were measured. A regression using extra plants was used to obtain estimates of initial dry weights. The regression was highly significant (P < 0.001) and predicted biomass well (r 2 = 0.90). Initial dry weight (DW) of the experimental plants was 0.06 ± 0.01 g DW (mean 1 ± S.E.; N = 250), and measured stem height was 10.95 ± 2.40 cm (N = 250). Sediment used for the experiment was sieved to pass through 1 mm mesh. 2.2. Experimental design and procedure Ten ramets of R. maritima L. were placed in small plastic containers holding 450 ml of the collected and sieved sediment for a total of 20 containers (200 ramets). Following planting, clean sand was added to the surface of each container to a depth of 1 cm to minimize suspension of sediments and retard nutrient loss (Smart and Barko, 1995). Containers were placed in individual plastic tanks (0.7 m × 0.35 m × 0.4 m), and water maintained at 2 cm below the top of the bin for the duration of the experiment. Baseline conditions were 10 g l−1 salinity with water depth 25 cm above the substrate surface. Initial salinity was adjusted to 10 g l−1 using artificial seawater (Hawaiian Marine Imports, Inc., Houston, TX). Salinity treatments were applied to each tank, and consisted of a control and five different treatments (Fig. 1). Treatments consisted of no salinity change (CTL), one salinity change (LO1, HI1) or two salinity changes (LO2, HI2, HI3). The experiment was run over a 9-week period. All bins were maintained at 10 g l−1 for the first 3-week period, allowing the plants time to acclimate to the new environment. After 3 weeks, over a 48 h period, water was changed in all the tanks in order to achieve the target salinities of 10 g l−1 (CTL), 0 g l−1 (LO), or 20 g l−1 (HI). After three more weeks, water was again changed in all the tanks over a 48 h period in order to achieve the final target salinities of 10 g l−1 (CTL, LO2, HI2), 0 g l−1 (LO1), 20 g l−1 (HI2) and 30 g l−1 (HI3). Bins that did not require a salinity change 30
CTL
LO1
LO2
HI1
HI2
HI3
Salinity (g L-1)
20 10 0 30 20 10 0 0
3
6
9
0
3
6
9
0
3
6
9
Time (weeks) Fig. 1. Salinity treatments applied to experimental tank units. Each salinity level was maintained for a 3-week period. Changes in salinity were carried out over 48 h periods.
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were treated similarly to bins requiring a salinity change with water being siphoned out, and new water placed in the bin. These salinity treatments were selected to mimic the two types of salinity changes that occur frequently along the northern coast of the Gulf of Mexico: (1) freshening events from rain or freshwater diversion events; and (2) increased salinities from storm events pushing Gulf waters inshore. Salinity, water temperature and light levels were measured daily throughout the experiment. Nitrates, pH and DO were measured initially and 6 weeks into the experiment. Stem lengths were measured every 3 weeks, just prior to salinity adjustments. Roots and shoots of all plants were harvested on 31 July 2001, dried to a constant weight, and total biomass determined. Relative growth rates (RGR) in terms of biomass and height were calculated as RGR = {ln([yf ]) − ln([yi ])}/time, where [yf ] is final biomass (g DW), [yi ] is initial biomass (g DW) or initial height (cm), and time is the length of the experiment in days. 2.3. Analysis Data were analyzed as a randomized block design. Data were tested for assumptions of normality (Shapiro–Wilk test statistic) and homogeneity of variance. One-way analysis of variance followed by Dunnett’s test (Snedecor and Cochran, 1989; Day and Quinn, 1989) were used to compare relative growth rates of biomass and stem length to the treatment control.
3. Results
0.04 0.03
*
*
0.02 0.01
H I3
H I2
2 LO
H I1
LO
1
0 C TL
(g g-1 d-1)
Relative Growth Rate
Relative growth rate of biomass of R. maritima ranged from 0.018 g g−1 per day ±0.004S.D. in the LO2 treatment to 0.029 g g−1 per day ±0.006S.D. in the CTL treatment (Fig. 2). The one-way ANOVA for biomass was significant (P = 0.03). Dunnett’s test indicated that LO2 and the HI3 treatments were significantly differently from the control.
Salinity Treatment
Fig. 2. Relative growth rate (g g−1 per day) of R. maritima as influenced by salinity changes and direction of salinity change (±10 g l−1 ) calculated over a 9-week growth period (Fig. 1). ANOVA tested for the effect of salinity treatment on relative growth rate (P = 0.03), followed by contrasts comparing the control to the treatments. Error bars represent standard deviations. Asterisk represents significant difference from the control treatment.
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Fig. 3. Relative stem length increase rate (mm mm−1 per day) of R. maritima as influenced by salinity changes and direction of salinity change (±10 g l−1 ) calculated over a 9-week growth period (Fig. 1). ANOVA tested for the effect of salinity treatment on relative growth rate (P = 0.12). Error bars represent standard deviations.
Relative growth rate of length of R. maritima stems ranged from a low of 0.006 cm cm−1 per day ±0.008 in the LO2 treatment to a high of 0.016 cm cm−1 per day ±0.003 in the CTL treatment (Fig. 3). The one-way ANOVA for length was not significant (P = 0.12).
4. Discussion Past field observations that the healthiest (i.e. greatest relative growth rate) R. maritima occur in areas with a more constant salinity regime (e.g. Verhoeven, 1979; Kantrud, 1991) were supported by our study with several of the treatments resulting in significantly lowered relative growth rates as compared to the control (constant salinity). Relative growth rate was significantly lowered with a short freshwater pulse, and with a pulsed salinity increase. R. maritima growth was half that of the control (constant salinity) when salinity was reduced by 10 g l−1 over 48 h at week 3, and increased by 10 g l−1 at week 6 (LO2 treatment) suggesting that short-term freshening events may negatively impact R. maritima growth. At the other end of the spectrum, relative growth rate was also reduced by about half when salinity was raised twice, once at week 3, and again at week 6 to reach 30 g l−1 (HI3 treatment), suggesting that pulsed increases in salinity may negatively impact R. maritima growth. The lowered growth rates for the LO2 and HI3 treatments might suggest that either (1) changed salinity, per se, negatively impacted R. maritima growth, or (2) pulsed salinity changes negatively affected R. maritima relative growth rate. Numerous studies document that R. maritima can survive in salinities ranging from 0 to 70 gl−1 (Kantrud, 1991). In Louisiana, one study found no differences in R. maritima growth in salinity ranging from 3.7 to 33.4 g l−1 (Joanen and Glasgow, 1965). We tested growth in salinity ranging from 0 to 30 g l−1 . Although R. maritima is known to grow in freshwater, it is possible that some of the reduced growth in the LO2 treatment may be due to this freshwater treatment. Longer term effects of low salinity have been documented, but were not addressed in this study. For example, Bonis et al. (1993, 1995) found that lowered salinity alone resulted in reduced reproductive effort of R. maritima, suggesting that lower salinity
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may have more long-term impacts on R. maritima by impacting reproduction, and thus potentially the future structure, density and longevity of R. maritima communities. However, since the LO1 treatment was not significantly different from the control, the reduced growth observed in the LO2 treatment is more likely explained by the short freshwater flushing event, similar to what might happen after a heavy rain, or from some of the freshwater diversion projects occurring in this region. The pulsed salinity changes (10 g l−1 over 48 h) provide a more likely explanation for the lowered relative growth rates of R. maritima observed in this study. Both a pulsed increase (HI3) and decrease (LO2) in salinity resulted in lowered relative growth rates suggesting that both freshening events and pulsed high salinity events (e.g. storms) may negatively affect R. maritima growth over the short-term. The lack of a significant reduction in growth for the HI2 treatment may be due to the fact that the range of salinity covered by this treatment falls squarely within what most researchers agree is the optimum salinity range for R. maritima (10 to 20 gl−1 ). While the importance of other factors such as turbidity and competition also need to be examined, R. maritima distributions may be affected by repeated salinity changes, or possibly by the less frequent changes of a greater magnitude and rate of change than were tested in this study. We limited our study to the magnitude and rate of change tested as they represent the common rates of change in fresh to brackish water areas along the northern coast of the Gulf of Mexico. Salinity changes of a greater magnitude and/or rate may be more likely to impact R. maritima communities through impacts on relative growth rates, however, these types of changes are far less frequent. Regardless, the success of R. maritima is most likely related to the interaction between its life history traits and environmental conditions, of which the local salinity regime (range and change) is critical.
Acknowledgements The authors thank Chi-Howe Teo for help in the field and Ron Boustany for advice on greenhouse space set-up. We also thank Dr. Vermaat for his very helpful comments, as well as two anonymous reviewers for helpful comments on an earlier version. This research was supported by the Louisiana Department of Wildlife and Fisheries and the U.S.G.S. Louisiana Fish and Wildlife Cooperative Research Unit.
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