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Factors influencing aboveground production of Scirpus marshes in the St. Lawrence estuary, Québec, Canada
Aquatic Botany, 29 (1987) 195-204
195
Elsevier Science Publishers B.V., Amsterdam w Printed in The Netherlands
FACTORS I N F L U E N C I N G A B O V E G R O U N D P R O D U C T I O N OF S C I R P U S M A R S H E S IN THE ST. LAWRENCE ESTUARY, QUI~BEC, CANADA
JEAN-FRAN(~OIS GIROUX 1 and JEAN BI~DARD
Ddparternent de Biologie, Universitd Laval, Ste. Foy, Qudbec, GI K 7P4 (Canada) (Accepted for publication 13 July 1987)
ABSTRACT Giroux, J.-F. and B~lard, J., 1987. Factors influencing aboveground production of Scirpus marshes in the St. Lawrence estuary, Quebec, Canada. Aquat. Bot., 29:195-204. Aboveground production of macrophytes in tidalmarshes was studiedfor 3 years at Montmagny and Cap St. Ignace on the south shore of the St. Lawrence River in Quebec, Canada. Year-to-year variation of production of the dominant species, Scirpus americanus Pets., was observed and relatedto variationin weather during the growing season. With the exception of Zizania aquatica L., relativeproduction of the macrophytes for differentparts of the marshes remained constant in successiveyears.A multipleregressionprocedure was used to establishthe relationshipbetween production and a seriesof independent variablesincluding biotic (goose use and plant competition) and abiotic factors (submersion, substrate hardness, soiltexture and sedimentation). A lower percentage of organic matter in the soiland a firm substratewere associatedwith a greater production of S. americanus. Accretion of sediments was positivelycorrelatedwith the production ofS. americanus and Sagittariaspp.,whereas the percentage of time of submersion was negatively relatedwith production ofSagittariaspp.,Eleocharisspp. and Scirpus torreyiOlney. Competitive interferencewas apparent between S. arnericanusand S. torreyiand between Eleocharisspp. and Zizania spp. N o measured abioticcharacteristicof the marsh explained the variationof Zizania production.
INTRODUCTION
The importance of elevation (submersion) and salinity as controls on plant distribution in intertidal marshes has been demonstrated along the St. Lawrence River estuary ( Lacoursi~re and Grandtner, 1971; Gauthier, 1982; Desch~nes and Sdrodes, 1985) as well as along the Pacific coast (Disraeli and Fonda, 1979; Hutchinson, 1982; Ewing, 1983, 1986). Except for the work of Ewing 'Present address:CultertyField Station,University of Aberdeen, Newburgh, Aberdeenshire, A B 4 0AA, Gt. Britain.
0304-3770/87/$03.50
© 1987 Elsevier Science Publishers B.V.
196 (1986), these studies were more concerned with the zonation of the plants from a phytosociological point of view than with the variation in production of the different macrophyte species. Giroux and B~dard (1988a) have described the primary production of intertidal marshes of the St. Lawrence estuary. One major characteristic of these marshes was the large intrinsic variability of the production. Giroux and B& dard (1987) have also shown that grubbing by snow geese (Chen caerulescens atlantica Kennard) can influence production of the different species, but the importance of other factors such as weather, soil texture, sediments and submersion, etc. remain unknown. The objective of this study was to look at some factors that may influence production of macrophytes i n intertidal marshes of the St. Lawrence River estuary. In this paper, we first present the variation of production between years and sites and then look at the relative importance of several biotic and abiotic factors in controlling production of the dominant plant species. STUDY SITES Our study was conducted at Montmagny and Cap St. Ignace on the south shore of the St. Lawrence River, Canada, at approximately 70 and 80 km northeast of Qudbec city, respectively. The marshes are slightly brackish (0.2-1.5%o) and characterized by a mixed semi-diurnal tide of 4-6 m amplitude. The dominant macrophyte is Scirpus americanus Pers. but Zizania aquatica L. var. brevis Fassett, Eleocharis spp., Sagittaria spp. ( mostly S. latifolia Willd. ) and Scirpus torreyi Olney are also frequently encountered. The marshes selected for this study cover 102 and 73 ha at Montmagny and Cap St. Ignace, respectively. In winter, the marshes are covered by a 1.0-1.5-m layer of ice. During spring break-up, large ice floes drift over the marshes plowing and scouring the surface (Dionne, 1984). This part of the estuary is also characterized by high turbidity that results in accretion of sediments within the marshes during the summer (Sdrodes and Troude, 1984). During their spring and a u t u m n migrations, 15-25 thousand greater snow geese spend 5-7 weeks feeding in this region. Rhizomes of S. americanus are the main food item, but the shoots of S. americanus, the tubers of Sagittaria, the rhizomes of Eleocharis and the seeds of Zizania are also consumed (J. B~dard and J.-F. Giroux, unpublished data). Additional description of the areas are presented by Giroux and Bddard (1988a). METHODS
Experimental design and vege'tation sampling We first systematically distributed 30 30 × 30-m stations at each location and then randomly established three permanent vegetation sampling plots (1.25 X 5 m) per station. Geese were allowed to feed in these plots.
197 Aboveground biomass of vegetation was measured in late August from 1983 to 1985 by double sampling (Lieffers, 1983). This non-destructive technique consisted of counting the number of stems of each species in 25 × 25-cm quadrats and measuring the height of a sample of stems. Using equations relating height and mass, the average stem mass was estimated for each species and then multiplied by the number of stems in the quadrat to get the estimates of aboveground biomass. The allometric equations are presented in Giroux and B~dard (1988b) and they were developed using between 72 and 1398 individual stems for each species. The r values of these equations varied between 0.59 and 0.95 and averaged 0.85 _+0.03 ( n = 11 ). Peak aboveground biomass is considered to be a good approximation of aboveground primary production in these Scirpus marshes ( Giroux and B~dard, 1988b). We sampled three quadrats per plot and calculated an average production for each station. The same 540 quadrats ( 2 sites X 30 stations/site X 3 plots/station X 3 quadrats/plot) were studied each year.
Independent variables Bird use was estimated by daily observations conducted from an elevated tower at each site, alternating between a morning (dawn-noon) and an afternoon (noon-dusk) period. The number of geese feeding in each station was recorded every 30 min using a spotting scope. The number of goose-hours/ station was computed by summing the total number of geese counted during one season divided by two (half-hourly scans) and corrected with a sampling fraction. This fraction was the number of hours of observation divided by the total number of hours of daylight during the staging period. The mean altitude of each station corner was measured using a theodolite and telemeter. The measurements were first recorded according to the International Great Lakes Datum (IGLD) system and then converted to a maregraphic scale using the appropriate factor from the nearest reference harbour. Because submersion time is biologically more significant than altitude, these elevations were thus converted to a percentage of time of submersion using Fig. 2 of S~rodes et al. (1985). We used the method of Sdrodes and Troude (1984) to evaluate accretion of sediments in each station. Fifty-cm steel rods were pushed into the substrate until a 10 X 10 cm × 2.5 m m plate soldered at one end was flush with the surface. The plates were placed in mid-June 1984 and measurements taken in midAugust. One plate was placed in each station at Cap St. Ignace and two at Montmagny. Penetration resistance of the substrate and the abundance of rocks was estimated at 30 points systematically distributed 2 m apart along two 30-m lines within each station. The hardness of the substrate was measured with a penetrometer made of a 1.6-m long rod with a diameter of 1.3 cm and a weight of
198
1600 g. The rod was dropped from a height of 30cm into a 1-m pipe held at the surface of the marsh. The depth at which the rod penetrated into the substrate was recorded and averaged for each station. We also noted the number of times that the rod hit a rock and used the percentage frequency as an index of rock abundance. These measurements were made in mid-June 1983 and 1984 at Cap St. Ignace and Montmagny, respectively. The texture of the soil was determined with a hydrometer using the method of Bouyoucous (1962) for the percentage of sand, silt and clay and by sieving for the gravel. A composite sample was made by taking four subsamples per station at the rhizome level, about 10 cm below the surface. The organic matter expressed on a dry weight basis was determined by combustion of about 20 g of soil at 4000 C for 24 h in a muffle furnace. The soil was collected in September 1983 and 1984 at Cap St. Ignace and Montmagny, respectively. We assumed that no significant changes in soil texture and penetration resistance occurred between years. Weather data were provided by the Service de Mdt~orologie of the Minist~re de l'Environnement du Quebec. Maximum and m i n i m u m temperatures were recorded at Montmagny and the mean daily temperature was obtained by averaging these two figures. The number of hours of bright sunshine per day was measured with an heliograph at Lauzon, about 65 km from Montmagny.
Statistical analyses We used a series of multiple regressions to determine which independent variables explained the variation in the production of the five dominant species measured in 1984. Each station represented a sampling unit and separate analyses were conducted for each location. The production of the plant species not considered as dependent variables was inclu'ded in the set of independent variables to consider the possible effect of competition. Angular transformations were applied to percentage data while the production values were transformed with loglo (y + 1 ). Stepwise procedures were employed using the SPSSX package ( SPSS, 1983 ). We first examined the correlation matrices and no problem of multicollinearity ( r > 0.75) was detected. The assumptions of the models were verified through analyses of the residuals. Wilcoxon matched-pairs signed-ranks tests were used to compare vegetation and bird use between years because data were not distributed normally. Means + 1 s.e. are presented throughout and the statistical level of significance was established at 5%. RESULTS AND DISCUSSION At Cap St. Ignace, primary production measured in 1984 was higher than in 1983 for the five species studied (Table I). Scirpus americanus showed the
199 TABLE I Annual variation in the aboveground production of macrophytes in intertidal marshes at Montmagny and Cap St. Ignace, Quebec, 1983-85 Species
Scirpus americanus Zizania aquatica Sagittaria spp. Eleocharis spp. Scirpus torreyi Other 2 Total
Montmagny
Cap St. Ignace
1983
1984
1985
1983
1984
135 _ 23a 1 10 _ 2a 9 _ 2a 5 _ 2a 11 _ 3ab 2 _ la 172 ± 22a
162 _ 26b 12 _ 2a 13 _ 3a 5 _+2a 16 _ 6a 1 _+la 209 + 25b
152 ___24ab 26 _ 5b 11 _+2a 4 _+la 22 _ 8bc 2 _ la 217_+ 23b
69 ± 14a 12 _+3a 13 _ 5a 5 ± 2a 1 ± la 1 ± la 101 ± 15a
120_+20b 20 _ 6a 16 _ 5b 8 _ 5a 2 _ la 3 _ 2a 169 ± 24b
1Values are means ___1 s.e. g m -2 year -1 ash-free dry mass (n=30). For each location and each species, means followed by the same letter are not significantly different (Wilcoxon matchedpairs signed ranks tests, P > 0.05). 2Includes less common species such as Scirpus validus Vahl, Cyperus rivularis Kunth, Juncus spp., Sium suave Walt., Equisetum fluviatile L., Sparganium spp., Bidens cernua L., Isoetes sp., LimoseUa subulata Ives and Lindernia sp. g r e a t e s t i n c r e a s e in p r o d u c t i o n a n d t h i s w a s r e l a t e d to b o t h t h e n u m b e r of s h o o t s p e r s q u a r e m e t e r (360 _+87 vs. 518 + 110; P = 0.0001 ) a n d t h e m e a n bio m a s s o f i n d i v i d u a l s h o o t s ( 0.198_+ 0.012 vs. 0.246 _+0.018; P = 0.0002). T h i s d i f f e r e n c e c o u l d n o t be a t t r i b u t e d to a r e d u c e d goose u s a g e b e c a u s e t h e s a m e n u m b e r of g o o s e - h o u r s { a u t u m n p l u s s p r i n g ) w a s r e c o r d e d in 1982-83 as in 1983-84 ( 1 1 9 5 + 2 4 0 vs. 1 0 2 3 + 181; P - - 0 . 9 4 3 ) . M e a n a i r t e m p e r a t u r e in t h e M o n t m a g n y / C a p St. I g n a c e a r e a f r o m April 15 to A u g u s t 15 w a s 12.8 a n d 14.7°C in 1983 a n d 1984, r e s p e c t i v e l y , r e s u l t i n g in 1005 a n d 1188 d e g r e e - d a y s > 5 ° C . T h e n u m b e r o f h o u r s o f b r i g h t s u n s h i n e r e c o r d e d d u r i n g t h e s a m e p e r i o d w a s 765 in 1983 a n d 780 in 1984. A l t h o u g h m e a s u r e m e n t s were n o t t a k e n d i r e c t l y in t h e m a r s h , t h e s e r e s u l t s suggest t h a t t h e m o r e f a v o r a b l e g r o w i n g c o n d i t i o n s p r e v a i l i n g in 1984 m a y h a v e s t i m u l a t e d p r o d u c t i o n . T u r n e r (1979) h a s s h o w n t h a t s e a s o n a l b i o m a s s of S p a r t i n a sp. w a s s t r o n g l y r e l a t e d to m e a n a i r t e m p e r a t u r e d u r i n g t h e g r o w i n g season. A t M o n t m a g n y , t h e p r o d u c t i o n w a s also h i g h e r in 1984 c o m p a r e d to 1983 b u t t h e r e l a t i v e i m p o r t a n c e o f goose use c o u l d n o t b e d i s t i n g u i s h e d f r o m t h e i n f l u e n c e o f w e a t h e r b e c a u s e n o b i r d s u r v e y w a s c o n d u c t e d in 1982-83. I n 1985, t h e p r o d u c t i o n o f S. americanus did n o t differ f r o m t h e p r e v i o u s 2 y e a r s , although that of Zizania increased considerably. The number of goose-hours per s t a t i o n did n o t differ b e t w e e n 1983-84 a n d 1984-85 ( 2792 _+305 vs. 2166 _+202; P - - 0 . 0 6 3 ) . M e a n air t e m p e r a t u r e w a s 13.9°C w i t h 1112 d e g r e e - d a y s a n d 820 h o f s u n s h i n e in 1985. T h e p r o d u c t i o n o f S. americanus a t M o n t m a g n y a v e r a g e d 150 g m -2 y e a r -1
200 TABLE II Pearson correlation coefficients between successive years for the aboveground production of five macrophyte species in intertidal marshes at Montmagny and Cap St. Ignace, 1983-85 Species
Montmagny
Scirpus americanus Zizania aquatica Sagittaria spp. Eleocharis spp. Scirpus torreyi
Cap St. Ignace
1983 vs. 1984
1984 vs. 1985
1983 vs. 1984
0.92* 0.33 0.69* 0.79* 0.90*
0.93* 0.20 0.76* 0.83* 0.99*
0.89* 0.27 0.95* 0.79* 0.92*
*P< 0.001, (n=30). d u r i n g t h e s t u d y c o m p a r e d to o n l y 95 a t C a p St. Ignace. T h i s c o u l d n o t r e s u l t f r o m goose g r a z i n g b e c a u s e m o r e g o o s e - h o u r s ( P < 0 . 0 5 ) w e r e r e c o r d e d at M o n t m a g n y t h a n a t C a p St. I g n a c e ( G i r o u x a n d B d d a r d , 1988c). C a p St. Ign a c e is l o c a t e d 10 k m d o w n s t r e a m f r o m M o n t m a g n y a n d t h i s region is c h a r a c t e r i z e d b y a t r a n s i t i o n f r o m f r e s h to o l i g o h a l i n e water. M o r r i s et al. (1978) h a v e n o t e d d r a m a t i c b i o g e o c h e m i c a l c h a n g e s in s u c h t r a n s i t i o n zones a n d t h e s e c h a n g e s m a y be r e l a t e d to t h e d i f f e r e n c e in S. americanus p r o d u c t i o n b e t w e e n o u r t w o s t u d y areas. A l t h o u g h a n n u a l v a r i a t i o n of t h e p r i m a r y p r o d u c t i o n was o b s e r v e d ( T a b l e I ), t h e r e l a t i v e p r o d u c t i o n o f p e r e n n i a l s a t e a c h s a m p l i n g s t a t i o n r e m a i n e d t h e s a m e , y e a r a f t e r y e a r , as s h o w n b y t h e s i g n i f i c a n t p o s i t i v e c o r r e l a t i o n s b e t w e e n t h e p r o d u c t i o n m e a s u r e d in successive y e a r s ( T a b l e I I ) . T h i s suggests t h a t TABLE III Variables that best explained variation in aboveground production of five macrophyte species in 30 stations at Montmagny, 1984 ( + = positive influence; - = negative influence) Species
Independent variables ( % of variation explained)
Scirpus americanus Zizania aquatica Sagittaria spp. Eleocharis spp. Scirpus torreyi
+SED (42)***-PEN (37)*** + SAG (36) *** +ZIZ (36)***+SED (12)*+SILT (8)* SPUSE ( 23 ) ** - SUBM (13) ** - S U B M (53)***-SCAM (15)** -
R2 0.79 0.36 0.56 0.36 0.68
F 49.5*** 15.4"** 11.1"** 7.7** 28.2***
1SED=accretion of sediments (cm); PEN=penetration depth of the penetrometer (cm); SAG=production of Sagittaria spp. (gm-2year-~); ZIZ=production of Zizania aquatica (g m r2 year-1); SILT = % of silt in the soil; SPUSE = number of goose-hours/station in spring; SUBM = % of time of submersion; SCAM = production of Scirpus americanus ( g m -2 year-~ ). *P< 0.05; **P< 0.01; ***P< 0.001.
201 TABLE IV Variables that best explained variation in aboveground production of five macrophyte species in 30 stations at Cap St. Ignace, 1984 ( + --positive influence; - = negative influence) Species
Independent variables (To of variation explained)
R2
F
Scirpus americanus Zizania aquatica Sagittaria spp. Eleocharis spp. Scirpus torreyi
- P O M (28)** + SAG (33) *** + SCAM (14) * - SUBM (36) *** + SED (18)** - SUBM (52) ***-ZIZ (10) * + SPUSE (38) ***
0.28 0.47 0.54 0.62 0.38
11.1"* 12.1"** 15.8"** 21.9"** 16.8"**
~POM= To of organic matter in the soil; SAG=production of Sagittaria spp. (g m -2 year-X); SCAM = production of Scirpus americanus ( g m -2 year- ~); SUBM = % of time of submersion; SED=accretion of sediments (cm); ZIZ=production of Zizania aquatica (gm-2year-1); SPUSE = number of goose-hours/station in spring. *P < 0.05; **P< 0.01; ***P < 0.001.
some intrinsic characteristics of the marsh could be associated with the variation of production among the stations. On the other hand, the production of the annual Zizania could not be predicted in successive years, possibly because the abundance of this species was first influenced by the seed rain which is itself a function of factors such as wind, tide, currents, etc.. At Montmagny, the production of S. americanus was higher in stations with greater accretion of sediments and with a firm substrate (Table III). Sediments found in Scirpus marshes of the St. Lawrence estuary are rich in nutrients (Deschfines and Sdrodes, 1986) and the greater abundance of sediments in some parts of the marsh may thus result in a greater availability of nutrients for the plants. On the other hand, sites with a firm substrate are used less by geese for feeding (Giroux and Bddard, 1988c). Considering the impact of geese on plant production (Giroux and Bddard, 1987 ), decrease of use due to a firm substrate may result in greater production of S. americanus. The percentage of organic matter in the soil was the only variable that influenced production of S. americanus at Cap St. Ignace (Table IV). It varied between 1.8 and 5.5% among the stations and was inversely related to production. Hogeland and Killingbeck (1985) reported a similar inverse relationship between organic matter and productivity of Juncus militaris Bigel. Accumulation of organic matter may indicate lower rates of decomposition and lower availability of nutrients. In Spartina marshes, DeLaune et al. (1979) found that the percentage of organic matter was higher in interior sites characterized by lower primary productivity than along the stream banks. Moreover, reducing conditions in inland sites were associated with lower redox potentials resulting in higher concentrations of hydrogen sulfide (DeLaune et al., 1983). Sulfide can inhibit nutrient uptake, especially nitrogen and can thus be toxic
202 to the plants (King et al., 1982). However, the specific processes involved in Scirpus marshes require more work. Abiotic characteristics of the marsh did not influence the production of Zizania (Tables III and IV). This was expected considering the lack of correlation between the production in successive years (Table II). The positive relationship between the production of Zizania, Sagittaria and S. americanus does not necessarily represent a causal relationship; we suggest that the most productive stations were suitable for these three major species. Greater production of Sagittaria was related to a greater accumulation of sediments at both locations ( Tables III and IV). In our marshes, it is difficult to discern if greater accretion of sediments is the result or the cause of greater production of Sagittaria. Possibly, initial growth is promoted by the presence of sediments (e.g. depressions) but once established, the large leaves of this plant can trap a large amount of sediment. At Cap St. Ignace, the production of Sagittaria was negatively affected by submersion. The reasons for decreased productivity resulting from inundation are not clear but several have been proposed: restricted photosynthesis, decreased gas exchange and leaching of dissolved organic carbon and of substances important to growth (Gallagher et al., 1976; Mahall and Park, 1976). Finally, the production of Sagittaria was positively related to the percentage of silt in the soil. A greater proportion of silt may represent a greater abundance of nutrients at the root level. The production of Eleocharis was negatively associated with submersion at both sites (Tables III and IV). In fact, it was found only at higher elevations within the marshes. This plant was also negatively affected by geese at Montmagny (Table III) and by Zizania at Cap St. Ignace (Table IV). The adverse effect of goose grubbing on Eleocharis has been experimentally shown by preventing geese from feeding in fenced plots (Giroux and B~dard, 1987). The interference of Zizania on the other hand, may be related to competition for light as Zizania grows much taller t h a n Eleocharis. At Montmagny, the production of S. torreyi was inversely related to submersion and to S. americanus (Table III). The effect of submersion has already been discussed and the relationship with S. americanus suggests a competitive interference (Harper, 1977 ). The resource (s) that may be limiting and competed for have not yet been identified. Both species have a very similar aboveground morphology but the rhizomes of S. torreyi are smaller that those of S. americanus. The positive relationship with use by geese at Cap St. Ignace (Table IV) suggests that preferential feeding by geese on S. americanus may allow S. torreyi to colonise the feeding sites. This contention needs to be substantiated, however. CONCLUSION Multiple regression is a good tool to establish the relationships between a series of independent descriptors and a dependent variable. However, it rep-
203
resents only a preliminary step because it has some limitations, the most obvious being the impossibility of establishing causal relationships. Experimental work is thus required to study the r81e and the mechanisms of action of the factors that we have found to be related to production of the different macrophytes in two tidal marshes of the St. Lawrence estuary. A large proportion of the variation remained unexplained in some of our models, suggesting that other factors were influencing plant production. The soil and water chemical characteristics are the most likely among those and further investigations should include them. However, it would be essential to design a sampling scheme that would take into account the dynamic changes occurring during a tidal cycle and during a complete growing season. We do not believe that one short-term sampling, as often conducted in such studies, would be adequate. Ecological conditions in tidal marshes are among the most complex that can be encountered in any ecological system ( Ewing, 1986). Besides general growing conditions which may account for considerable differences in production from year to year, several abiotic factors, namely submersion, accretion of sediments and percentage of organic matter in the substrate seem to exert a considerable influence upon macrophyte production. The mechanisms through which these factors operate, however, are largely unknown. ACKNOWLEDGEMENTS
We thank G. Fillion, D. Fiset, A. Lamontagne, E. Landa, G. Rochette, G. Picard and J. Turcotte for valuable field and laboratory assistance. This research was supported by Ducks Unlimited Canada, the Canadian Wildlife Service (contract OSD82-00009 with the Ministry of Supply and Services), the Universitd Laval, the Natural Sciences and Engineering Research Council of Canada (operating grant to J.B.) and the programme La Formation de Chercheurs et l'Aide h la Recherche of the Qudbec government (scholarship to J.-F.G.). J. Huot and R.L. Jefferies commented on early versions of the manuscript. The Culterty Field Station of the University of Aberdeen provided word processing facilities.
REFERENCES Bouyoucos, G.J., 1962. Hydrometer method improved for making particle size analyses of soils. Agron. J., 54: 464-465. DeLaune, R.D., Buresh, R.J. and Patrick Jr., W.H., 1979. Relationship of soil properties to standing crop biomass of Spartina alterniflora in a Louisiana marsh. Estuarine Coastal Mar. Sci., 8: 477-487. DeLaune, R.D., Smith, C.J. and Patrick Jr., W.H., 1983. Relationship of marsh elevation, redox potential, and sulfide to Spartina alterniflora productivity. Soil Sci. Soc. Am. J., 47: 930-935.
204 Desch~nes, J. and Sdrodes, J.-B., 1985. The influence of salinity on Scirpus americanus tidal marshes in the St. Lawrence River estuary, Quebec. Can. J. Bot., 63: 920-927. Desch~nes, J. and S~rodes, J.-B., 1986. Recyclage des m$taux et du phosphore par Scirpus americanus et Spartina alterniflora dans l'estuaire moyen du Saint-Laurent (Quebec). Nat. Can. (Qu&), 113: 143-151. Dionne, J.-C., 1984. An estimate of ice-drifted sediments based on the mud content of the ice cover at Montmagny, Middle St. Lawrence Estuary. Mar. Geol., 57: 149-166. Disraeli, D.J. and Fonda, R.W., 1979. Gradient analysis of the vegetation in a brackish marsh in BeUingham Bay, Washington. Can. J. Bot., 57: 465-475. Ewing, K., 1983. Environmental controls in Pacific Northwest intertidal marsh plant communities. Can. J. Bot., 61: 1105-1116. Ewing, K., 1986. Plant growth and productivity along complex gradients in a Pacific Northwest brackish intertidal marsh. Estuaries, 9: 49-62. Gallagher, J.L., Pfeiffer, W.J. and Pomeroy, L.R., 1976. Leaching and microbial utilization of dissolved organic carbon from leaves of Spartina alterniflora. Estuarine Coastal Mar. Sci., 4: 467-471. Gauthier, B., 1982. L'dtagement des plantes vasculaires en milieu saum~tre, estuaire du SaintLaurent. Nat. Can. (Qua.), 109: 189-203. Giroux, J.~F. and B~dard, J., 1987. The effects of grazing by greater snow geese on the vegetation of tidal marshes in the St. Lawrence estuary. J. Appl. Ecol., 24: in press. Giroux, J.-F. and B~dard, J., 1988a. Above- and belowground macrophyte production in Scirpus tidal marshes of the St. Lawrence estuary, Quebec. Can. J. Bot., in press. Giroux, J.-F. and B~dard, J., 1988b. Estimating above- and belowground macrophyte production in Scirpus tidal marshes. Can. J. Bot., in press. Giroux, J.-F. and BSdard, J., 1988c. Use of bulrush marshes by greater snow geese during the spring and fall staging periods. J. Wildl. Manage., in press. Harper, J.L., 1977. Population Biology of Plants. Academic Press, London, 892 pp. Hogeland, A.M. and KiUingbeck, K.T., 1985. Biomass, productivity and life history traits of Juncus militaris Bigel. in two Rhode Island (U.S.A.) freshwater wetlands. Aquat. Bot., 22: 335-346. Hutchinson, I., 1982. Vegetation-environment relations in a brackish marsh, Lulu Island, Richmond, B.C. Can. J. Bot., 60: 452-462. King, G.M., Klug, M.J., Wiegert, R.G. and Chalmers, A.G., 1982. Relation of soil water movement and sulfide concentration to Spartina alterniflora production in a Georgia salt marsh. Science, 218: 61-63. Lacoursi~re, E. and Grandtner, M.M., 1971. Contribution ~ l'6tude ~cologique de la v~g~tation riparienne de i'~le d'Orlgans. Nat. Can. (Qu$.), 98: 443-459. Lieffers, V.J., 1983. Growth of Typha latifolia in boreal forest habitats, as measured by double sampling. Aquat. Bot., 15: 335-348. Mahall, B.E. and Park, R.B., 1976. The ecotone between Spartina [oliosa Trin. and Salicornia virginica L. in salt marshes of northern San Francisco Bay. III. Soil aeration and tidal immersion. J. Ecol., 64: 811-819. Morris, A.W., Mantoura, R.F.C., Bale, A.J. and Howland, R.J.M., 1978. Very low salinity regions of estuaries: important sites for chemical and biological reactions. Nature, 274: 678-680. S~rodes, J.-B. and Troude, J.-P., 1984. Sedimentation cycle of a freshwater tidal flat in the St. Lawrence Estuary. Estuaries, 7: 119-127. S~rodes, J.-B., Desch~nes, J. and Troude, J.-P., 1985. Temps de submersion des marais ~ Scirpe (Scirpus americanus) de l'Estuaire du Saint-Laurent. Nat. Can. (Qua.), 112: 119-129. SPSS, 1983. SPSSX Users' Guide. McGraw-Hill, New York, 806 pp. Turner, R.E., 1979. A simple model of the seasonal growth of Spartina alterniflora and Spartina patens. Cont. Mar. Sci., 22: 137-147.
195
Elsevier Science Publishers B.V., Amsterdam w Printed in The Netherlands
FACTORS I N F L U E N C I N G A B O V E G R O U N D P R O D U C T I O N OF S C I R P U S M A R S H E S IN THE ST. LAWRENCE ESTUARY, QUI~BEC, CANADA
JEAN-FRAN(~OIS GIROUX 1 and JEAN BI~DARD
Ddparternent de Biologie, Universitd Laval, Ste. Foy, Qudbec, GI K 7P4 (Canada) (Accepted for publication 13 July 1987)
ABSTRACT Giroux, J.-F. and B~lard, J., 1987. Factors influencing aboveground production of Scirpus marshes in the St. Lawrence estuary, Quebec, Canada. Aquat. Bot., 29:195-204. Aboveground production of macrophytes in tidalmarshes was studiedfor 3 years at Montmagny and Cap St. Ignace on the south shore of the St. Lawrence River in Quebec, Canada. Year-to-year variation of production of the dominant species, Scirpus americanus Pets., was observed and relatedto variationin weather during the growing season. With the exception of Zizania aquatica L., relativeproduction of the macrophytes for differentparts of the marshes remained constant in successiveyears.A multipleregressionprocedure was used to establishthe relationshipbetween production and a seriesof independent variablesincluding biotic (goose use and plant competition) and abiotic factors (submersion, substrate hardness, soiltexture and sedimentation). A lower percentage of organic matter in the soiland a firm substratewere associatedwith a greater production of S. americanus. Accretion of sediments was positivelycorrelatedwith the production ofS. americanus and Sagittariaspp.,whereas the percentage of time of submersion was negatively relatedwith production ofSagittariaspp.,Eleocharisspp. and Scirpus torreyiOlney. Competitive interferencewas apparent between S. arnericanusand S. torreyiand between Eleocharisspp. and Zizania spp. N o measured abioticcharacteristicof the marsh explained the variationof Zizania production.
INTRODUCTION
The importance of elevation (submersion) and salinity as controls on plant distribution in intertidal marshes has been demonstrated along the St. Lawrence River estuary ( Lacoursi~re and Grandtner, 1971; Gauthier, 1982; Desch~nes and Sdrodes, 1985) as well as along the Pacific coast (Disraeli and Fonda, 1979; Hutchinson, 1982; Ewing, 1983, 1986). Except for the work of Ewing 'Present address:CultertyField Station,University of Aberdeen, Newburgh, Aberdeenshire, A B 4 0AA, Gt. Britain.
0304-3770/87/$03.50
© 1987 Elsevier Science Publishers B.V.
196 (1986), these studies were more concerned with the zonation of the plants from a phytosociological point of view than with the variation in production of the different macrophyte species. Giroux and B~dard (1988a) have described the primary production of intertidal marshes of the St. Lawrence estuary. One major characteristic of these marshes was the large intrinsic variability of the production. Giroux and B& dard (1987) have also shown that grubbing by snow geese (Chen caerulescens atlantica Kennard) can influence production of the different species, but the importance of other factors such as weather, soil texture, sediments and submersion, etc. remain unknown. The objective of this study was to look at some factors that may influence production of macrophytes i n intertidal marshes of the St. Lawrence River estuary. In this paper, we first present the variation of production between years and sites and then look at the relative importance of several biotic and abiotic factors in controlling production of the dominant plant species. STUDY SITES Our study was conducted at Montmagny and Cap St. Ignace on the south shore of the St. Lawrence River, Canada, at approximately 70 and 80 km northeast of Qudbec city, respectively. The marshes are slightly brackish (0.2-1.5%o) and characterized by a mixed semi-diurnal tide of 4-6 m amplitude. The dominant macrophyte is Scirpus americanus Pers. but Zizania aquatica L. var. brevis Fassett, Eleocharis spp., Sagittaria spp. ( mostly S. latifolia Willd. ) and Scirpus torreyi Olney are also frequently encountered. The marshes selected for this study cover 102 and 73 ha at Montmagny and Cap St. Ignace, respectively. In winter, the marshes are covered by a 1.0-1.5-m layer of ice. During spring break-up, large ice floes drift over the marshes plowing and scouring the surface (Dionne, 1984). This part of the estuary is also characterized by high turbidity that results in accretion of sediments within the marshes during the summer (Sdrodes and Troude, 1984). During their spring and a u t u m n migrations, 15-25 thousand greater snow geese spend 5-7 weeks feeding in this region. Rhizomes of S. americanus are the main food item, but the shoots of S. americanus, the tubers of Sagittaria, the rhizomes of Eleocharis and the seeds of Zizania are also consumed (J. B~dard and J.-F. Giroux, unpublished data). Additional description of the areas are presented by Giroux and Bddard (1988a). METHODS
Experimental design and vege'tation sampling We first systematically distributed 30 30 × 30-m stations at each location and then randomly established three permanent vegetation sampling plots (1.25 X 5 m) per station. Geese were allowed to feed in these plots.
197 Aboveground biomass of vegetation was measured in late August from 1983 to 1985 by double sampling (Lieffers, 1983). This non-destructive technique consisted of counting the number of stems of each species in 25 × 25-cm quadrats and measuring the height of a sample of stems. Using equations relating height and mass, the average stem mass was estimated for each species and then multiplied by the number of stems in the quadrat to get the estimates of aboveground biomass. The allometric equations are presented in Giroux and B~dard (1988b) and they were developed using between 72 and 1398 individual stems for each species. The r values of these equations varied between 0.59 and 0.95 and averaged 0.85 _+0.03 ( n = 11 ). Peak aboveground biomass is considered to be a good approximation of aboveground primary production in these Scirpus marshes ( Giroux and B~dard, 1988b). We sampled three quadrats per plot and calculated an average production for each station. The same 540 quadrats ( 2 sites X 30 stations/site X 3 plots/station X 3 quadrats/plot) were studied each year.
Independent variables Bird use was estimated by daily observations conducted from an elevated tower at each site, alternating between a morning (dawn-noon) and an afternoon (noon-dusk) period. The number of geese feeding in each station was recorded every 30 min using a spotting scope. The number of goose-hours/ station was computed by summing the total number of geese counted during one season divided by two (half-hourly scans) and corrected with a sampling fraction. This fraction was the number of hours of observation divided by the total number of hours of daylight during the staging period. The mean altitude of each station corner was measured using a theodolite and telemeter. The measurements were first recorded according to the International Great Lakes Datum (IGLD) system and then converted to a maregraphic scale using the appropriate factor from the nearest reference harbour. Because submersion time is biologically more significant than altitude, these elevations were thus converted to a percentage of time of submersion using Fig. 2 of S~rodes et al. (1985). We used the method of Sdrodes and Troude (1984) to evaluate accretion of sediments in each station. Fifty-cm steel rods were pushed into the substrate until a 10 X 10 cm × 2.5 m m plate soldered at one end was flush with the surface. The plates were placed in mid-June 1984 and measurements taken in midAugust. One plate was placed in each station at Cap St. Ignace and two at Montmagny. Penetration resistance of the substrate and the abundance of rocks was estimated at 30 points systematically distributed 2 m apart along two 30-m lines within each station. The hardness of the substrate was measured with a penetrometer made of a 1.6-m long rod with a diameter of 1.3 cm and a weight of
198
1600 g. The rod was dropped from a height of 30cm into a 1-m pipe held at the surface of the marsh. The depth at which the rod penetrated into the substrate was recorded and averaged for each station. We also noted the number of times that the rod hit a rock and used the percentage frequency as an index of rock abundance. These measurements were made in mid-June 1983 and 1984 at Cap St. Ignace and Montmagny, respectively. The texture of the soil was determined with a hydrometer using the method of Bouyoucous (1962) for the percentage of sand, silt and clay and by sieving for the gravel. A composite sample was made by taking four subsamples per station at the rhizome level, about 10 cm below the surface. The organic matter expressed on a dry weight basis was determined by combustion of about 20 g of soil at 4000 C for 24 h in a muffle furnace. The soil was collected in September 1983 and 1984 at Cap St. Ignace and Montmagny, respectively. We assumed that no significant changes in soil texture and penetration resistance occurred between years. Weather data were provided by the Service de Mdt~orologie of the Minist~re de l'Environnement du Quebec. Maximum and m i n i m u m temperatures were recorded at Montmagny and the mean daily temperature was obtained by averaging these two figures. The number of hours of bright sunshine per day was measured with an heliograph at Lauzon, about 65 km from Montmagny.
Statistical analyses We used a series of multiple regressions to determine which independent variables explained the variation in the production of the five dominant species measured in 1984. Each station represented a sampling unit and separate analyses were conducted for each location. The production of the plant species not considered as dependent variables was inclu'ded in the set of independent variables to consider the possible effect of competition. Angular transformations were applied to percentage data while the production values were transformed with loglo (y + 1 ). Stepwise procedures were employed using the SPSSX package ( SPSS, 1983 ). We first examined the correlation matrices and no problem of multicollinearity ( r > 0.75) was detected. The assumptions of the models were verified through analyses of the residuals. Wilcoxon matched-pairs signed-ranks tests were used to compare vegetation and bird use between years because data were not distributed normally. Means + 1 s.e. are presented throughout and the statistical level of significance was established at 5%. RESULTS AND DISCUSSION At Cap St. Ignace, primary production measured in 1984 was higher than in 1983 for the five species studied (Table I). Scirpus americanus showed the
199 TABLE I Annual variation in the aboveground production of macrophytes in intertidal marshes at Montmagny and Cap St. Ignace, Quebec, 1983-85 Species
Scirpus americanus Zizania aquatica Sagittaria spp. Eleocharis spp. Scirpus torreyi Other 2 Total
Montmagny
Cap St. Ignace
1983
1984
1985
1983
1984
135 _ 23a 1 10 _ 2a 9 _ 2a 5 _ 2a 11 _ 3ab 2 _ la 172 ± 22a
162 _ 26b 12 _ 2a 13 _ 3a 5 _+2a 16 _ 6a 1 _+la 209 + 25b
152 ___24ab 26 _ 5b 11 _+2a 4 _+la 22 _ 8bc 2 _ la 217_+ 23b
69 ± 14a 12 _+3a 13 _ 5a 5 ± 2a 1 ± la 1 ± la 101 ± 15a
120_+20b 20 _ 6a 16 _ 5b 8 _ 5a 2 _ la 3 _ 2a 169 ± 24b
1Values are means ___1 s.e. g m -2 year -1 ash-free dry mass (n=30). For each location and each species, means followed by the same letter are not significantly different (Wilcoxon matchedpairs signed ranks tests, P > 0.05). 2Includes less common species such as Scirpus validus Vahl, Cyperus rivularis Kunth, Juncus spp., Sium suave Walt., Equisetum fluviatile L., Sparganium spp., Bidens cernua L., Isoetes sp., LimoseUa subulata Ives and Lindernia sp. g r e a t e s t i n c r e a s e in p r o d u c t i o n a n d t h i s w a s r e l a t e d to b o t h t h e n u m b e r of s h o o t s p e r s q u a r e m e t e r (360 _+87 vs. 518 + 110; P = 0.0001 ) a n d t h e m e a n bio m a s s o f i n d i v i d u a l s h o o t s ( 0.198_+ 0.012 vs. 0.246 _+0.018; P = 0.0002). T h i s d i f f e r e n c e c o u l d n o t be a t t r i b u t e d to a r e d u c e d goose u s a g e b e c a u s e t h e s a m e n u m b e r of g o o s e - h o u r s { a u t u m n p l u s s p r i n g ) w a s r e c o r d e d in 1982-83 as in 1983-84 ( 1 1 9 5 + 2 4 0 vs. 1 0 2 3 + 181; P - - 0 . 9 4 3 ) . M e a n a i r t e m p e r a t u r e in t h e M o n t m a g n y / C a p St. I g n a c e a r e a f r o m April 15 to A u g u s t 15 w a s 12.8 a n d 14.7°C in 1983 a n d 1984, r e s p e c t i v e l y , r e s u l t i n g in 1005 a n d 1188 d e g r e e - d a y s > 5 ° C . T h e n u m b e r o f h o u r s o f b r i g h t s u n s h i n e r e c o r d e d d u r i n g t h e s a m e p e r i o d w a s 765 in 1983 a n d 780 in 1984. A l t h o u g h m e a s u r e m e n t s were n o t t a k e n d i r e c t l y in t h e m a r s h , t h e s e r e s u l t s suggest t h a t t h e m o r e f a v o r a b l e g r o w i n g c o n d i t i o n s p r e v a i l i n g in 1984 m a y h a v e s t i m u l a t e d p r o d u c t i o n . T u r n e r (1979) h a s s h o w n t h a t s e a s o n a l b i o m a s s of S p a r t i n a sp. w a s s t r o n g l y r e l a t e d to m e a n a i r t e m p e r a t u r e d u r i n g t h e g r o w i n g season. A t M o n t m a g n y , t h e p r o d u c t i o n w a s also h i g h e r in 1984 c o m p a r e d to 1983 b u t t h e r e l a t i v e i m p o r t a n c e o f goose use c o u l d n o t b e d i s t i n g u i s h e d f r o m t h e i n f l u e n c e o f w e a t h e r b e c a u s e n o b i r d s u r v e y w a s c o n d u c t e d in 1982-83. I n 1985, t h e p r o d u c t i o n o f S. americanus did n o t differ f r o m t h e p r e v i o u s 2 y e a r s , although that of Zizania increased considerably. The number of goose-hours per s t a t i o n did n o t differ b e t w e e n 1983-84 a n d 1984-85 ( 2792 _+305 vs. 2166 _+202; P - - 0 . 0 6 3 ) . M e a n air t e m p e r a t u r e w a s 13.9°C w i t h 1112 d e g r e e - d a y s a n d 820 h o f s u n s h i n e in 1985. T h e p r o d u c t i o n o f S. americanus a t M o n t m a g n y a v e r a g e d 150 g m -2 y e a r -1
200 TABLE II Pearson correlation coefficients between successive years for the aboveground production of five macrophyte species in intertidal marshes at Montmagny and Cap St. Ignace, 1983-85 Species
Montmagny
Scirpus americanus Zizania aquatica Sagittaria spp. Eleocharis spp. Scirpus torreyi
Cap St. Ignace
1983 vs. 1984
1984 vs. 1985
1983 vs. 1984
0.92* 0.33 0.69* 0.79* 0.90*
0.93* 0.20 0.76* 0.83* 0.99*
0.89* 0.27 0.95* 0.79* 0.92*
*P< 0.001, (n=30). d u r i n g t h e s t u d y c o m p a r e d to o n l y 95 a t C a p St. Ignace. T h i s c o u l d n o t r e s u l t f r o m goose g r a z i n g b e c a u s e m o r e g o o s e - h o u r s ( P < 0 . 0 5 ) w e r e r e c o r d e d at M o n t m a g n y t h a n a t C a p St. I g n a c e ( G i r o u x a n d B d d a r d , 1988c). C a p St. Ign a c e is l o c a t e d 10 k m d o w n s t r e a m f r o m M o n t m a g n y a n d t h i s region is c h a r a c t e r i z e d b y a t r a n s i t i o n f r o m f r e s h to o l i g o h a l i n e water. M o r r i s et al. (1978) h a v e n o t e d d r a m a t i c b i o g e o c h e m i c a l c h a n g e s in s u c h t r a n s i t i o n zones a n d t h e s e c h a n g e s m a y be r e l a t e d to t h e d i f f e r e n c e in S. americanus p r o d u c t i o n b e t w e e n o u r t w o s t u d y areas. A l t h o u g h a n n u a l v a r i a t i o n of t h e p r i m a r y p r o d u c t i o n was o b s e r v e d ( T a b l e I ), t h e r e l a t i v e p r o d u c t i o n o f p e r e n n i a l s a t e a c h s a m p l i n g s t a t i o n r e m a i n e d t h e s a m e , y e a r a f t e r y e a r , as s h o w n b y t h e s i g n i f i c a n t p o s i t i v e c o r r e l a t i o n s b e t w e e n t h e p r o d u c t i o n m e a s u r e d in successive y e a r s ( T a b l e I I ) . T h i s suggests t h a t TABLE III Variables that best explained variation in aboveground production of five macrophyte species in 30 stations at Montmagny, 1984 ( + = positive influence; - = negative influence) Species
Independent variables ( % of variation explained)
Scirpus americanus Zizania aquatica Sagittaria spp. Eleocharis spp. Scirpus torreyi
+SED (42)***-PEN (37)*** + SAG (36) *** +ZIZ (36)***+SED (12)*+SILT (8)* SPUSE ( 23 ) ** - SUBM (13) ** - S U B M (53)***-SCAM (15)** -
R2 0.79 0.36 0.56 0.36 0.68
F 49.5*** 15.4"** 11.1"** 7.7** 28.2***
1SED=accretion of sediments (cm); PEN=penetration depth of the penetrometer (cm); SAG=production of Sagittaria spp. (gm-2year-~); ZIZ=production of Zizania aquatica (g m r2 year-1); SILT = % of silt in the soil; SPUSE = number of goose-hours/station in spring; SUBM = % of time of submersion; SCAM = production of Scirpus americanus ( g m -2 year-~ ). *P< 0.05; **P< 0.01; ***P< 0.001.
201 TABLE IV Variables that best explained variation in aboveground production of five macrophyte species in 30 stations at Cap St. Ignace, 1984 ( + --positive influence; - = negative influence) Species
Independent variables (To of variation explained)
R2
F
Scirpus americanus Zizania aquatica Sagittaria spp. Eleocharis spp. Scirpus torreyi
- P O M (28)** + SAG (33) *** + SCAM (14) * - SUBM (36) *** + SED (18)** - SUBM (52) ***-ZIZ (10) * + SPUSE (38) ***
0.28 0.47 0.54 0.62 0.38
11.1"* 12.1"** 15.8"** 21.9"** 16.8"**
~POM= To of organic matter in the soil; SAG=production of Sagittaria spp. (g m -2 year-X); SCAM = production of Scirpus americanus ( g m -2 year- ~); SUBM = % of time of submersion; SED=accretion of sediments (cm); ZIZ=production of Zizania aquatica (gm-2year-1); SPUSE = number of goose-hours/station in spring. *P < 0.05; **P< 0.01; ***P < 0.001.
some intrinsic characteristics of the marsh could be associated with the variation of production among the stations. On the other hand, the production of the annual Zizania could not be predicted in successive years, possibly because the abundance of this species was first influenced by the seed rain which is itself a function of factors such as wind, tide, currents, etc.. At Montmagny, the production of S. americanus was higher in stations with greater accretion of sediments and with a firm substrate (Table III). Sediments found in Scirpus marshes of the St. Lawrence estuary are rich in nutrients (Deschfines and Sdrodes, 1986) and the greater abundance of sediments in some parts of the marsh may thus result in a greater availability of nutrients for the plants. On the other hand, sites with a firm substrate are used less by geese for feeding (Giroux and Bddard, 1988c). Considering the impact of geese on plant production (Giroux and Bddard, 1987 ), decrease of use due to a firm substrate may result in greater production of S. americanus. The percentage of organic matter in the soil was the only variable that influenced production of S. americanus at Cap St. Ignace (Table IV). It varied between 1.8 and 5.5% among the stations and was inversely related to production. Hogeland and Killingbeck (1985) reported a similar inverse relationship between organic matter and productivity of Juncus militaris Bigel. Accumulation of organic matter may indicate lower rates of decomposition and lower availability of nutrients. In Spartina marshes, DeLaune et al. (1979) found that the percentage of organic matter was higher in interior sites characterized by lower primary productivity than along the stream banks. Moreover, reducing conditions in inland sites were associated with lower redox potentials resulting in higher concentrations of hydrogen sulfide (DeLaune et al., 1983). Sulfide can inhibit nutrient uptake, especially nitrogen and can thus be toxic
202 to the plants (King et al., 1982). However, the specific processes involved in Scirpus marshes require more work. Abiotic characteristics of the marsh did not influence the production of Zizania (Tables III and IV). This was expected considering the lack of correlation between the production in successive years (Table II). The positive relationship between the production of Zizania, Sagittaria and S. americanus does not necessarily represent a causal relationship; we suggest that the most productive stations were suitable for these three major species. Greater production of Sagittaria was related to a greater accumulation of sediments at both locations ( Tables III and IV). In our marshes, it is difficult to discern if greater accretion of sediments is the result or the cause of greater production of Sagittaria. Possibly, initial growth is promoted by the presence of sediments (e.g. depressions) but once established, the large leaves of this plant can trap a large amount of sediment. At Cap St. Ignace, the production of Sagittaria was negatively affected by submersion. The reasons for decreased productivity resulting from inundation are not clear but several have been proposed: restricted photosynthesis, decreased gas exchange and leaching of dissolved organic carbon and of substances important to growth (Gallagher et al., 1976; Mahall and Park, 1976). Finally, the production of Sagittaria was positively related to the percentage of silt in the soil. A greater proportion of silt may represent a greater abundance of nutrients at the root level. The production of Eleocharis was negatively associated with submersion at both sites (Tables III and IV). In fact, it was found only at higher elevations within the marshes. This plant was also negatively affected by geese at Montmagny (Table III) and by Zizania at Cap St. Ignace (Table IV). The adverse effect of goose grubbing on Eleocharis has been experimentally shown by preventing geese from feeding in fenced plots (Giroux and B~dard, 1987). The interference of Zizania on the other hand, may be related to competition for light as Zizania grows much taller t h a n Eleocharis. At Montmagny, the production of S. torreyi was inversely related to submersion and to S. americanus (Table III). The effect of submersion has already been discussed and the relationship with S. americanus suggests a competitive interference (Harper, 1977 ). The resource (s) that may be limiting and competed for have not yet been identified. Both species have a very similar aboveground morphology but the rhizomes of S. torreyi are smaller that those of S. americanus. The positive relationship with use by geese at Cap St. Ignace (Table IV) suggests that preferential feeding by geese on S. americanus may allow S. torreyi to colonise the feeding sites. This contention needs to be substantiated, however. CONCLUSION Multiple regression is a good tool to establish the relationships between a series of independent descriptors and a dependent variable. However, it rep-
203
resents only a preliminary step because it has some limitations, the most obvious being the impossibility of establishing causal relationships. Experimental work is thus required to study the r81e and the mechanisms of action of the factors that we have found to be related to production of the different macrophytes in two tidal marshes of the St. Lawrence estuary. A large proportion of the variation remained unexplained in some of our models, suggesting that other factors were influencing plant production. The soil and water chemical characteristics are the most likely among those and further investigations should include them. However, it would be essential to design a sampling scheme that would take into account the dynamic changes occurring during a tidal cycle and during a complete growing season. We do not believe that one short-term sampling, as often conducted in such studies, would be adequate. Ecological conditions in tidal marshes are among the most complex that can be encountered in any ecological system ( Ewing, 1986). Besides general growing conditions which may account for considerable differences in production from year to year, several abiotic factors, namely submersion, accretion of sediments and percentage of organic matter in the substrate seem to exert a considerable influence upon macrophyte production. The mechanisms through which these factors operate, however, are largely unknown. ACKNOWLEDGEMENTS
We thank G. Fillion, D. Fiset, A. Lamontagne, E. Landa, G. Rochette, G. Picard and J. Turcotte for valuable field and laboratory assistance. This research was supported by Ducks Unlimited Canada, the Canadian Wildlife Service (contract OSD82-00009 with the Ministry of Supply and Services), the Universitd Laval, the Natural Sciences and Engineering Research Council of Canada (operating grant to J.B.) and the programme La Formation de Chercheurs et l'Aide h la Recherche of the Qudbec government (scholarship to J.-F.G.). J. Huot and R.L. Jefferies commented on early versions of the manuscript. The Culterty Field Station of the University of Aberdeen provided word processing facilities.
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