Effects of canopy removal and nitrogen enrichment on a Distichlis spicata-edaphic diatom complex

Estuarbte, Coastal and Shelf Science (x98x) I3, xx9-xz9

Effects of Canopy Removal and Nitrogen Enrichment on a Distichlis spicataEdaphic Diatom Complex

Michael J. Sullivan Department of 11iological Sciences, P.O. Drazcer G Y, l~llssisslppi State University, l~Iississippi 39762, U.S.A. Received 26 November z979 and hz revlsed form 26June x98o

Keywords: diatoms; salt marshes; light intensity; nitrogen; community composition; species diversity; fertilization; Mississippi Edaphic diatoms were collected seasonally from a monotypic stand of

Distlchlls spicata (L.) Greene on Graveline Bay Marsh, Mississippi, in which the marsh surface had been enriched with NH4CI and exposed to high light intensity by clipping the grass shoots. Clipping greatly reduced species diversity (H') and the number of taxa in a sample (S) in all seasons except winter, but did not stimulate the growth of filamentous algae. Nitrogen enrichment increased H ' and S in spring. Of the I I I taxa encountered, clipping eliminated nine pre-existing tzxa and introduced three new taxa into the communitT. No such effect was induced by nitrogen enrichment. A 3-way ANOVA (light•215 of the relative abundances of the x6 most abundant taxa revealed that II taxa were characterized by a significant 3-way interaction term. As with community diversity, effects due to clipping were more prevalent than those due to nitrogen enrichment. When all responses were considered, the specific combination of clipping and NH~CI enrichment had for all practical purposes the same effects on community structure as did clipping alone. Nitrogen enrichment greatly stimulated the aerial yield of intact D. spicata stands and the regrowth of those clipped 3 months earlier.

Introduction Salt marshes are extremely productive ecosystems. It is now known that the edaphie or sediment-associated algae make a significant contribution to total marsh productivity on a year-round basis (Gallagher & Daiber, x974; Van Raalte et al., x976b). In those areas of the marsh shaded by a spermatophyte cover, the diatoms are usually dominant in the edaphie algal community, and most salt marshes are characterized by a rather extensive and often dense spermatophyte canopy. In recent years, increased nutrient loading of waters that flood salt marshes has occurred. In view of the possibility that such loading may increase sharply in the future, it is highly desirable to test experimentally the effects of nutrient enrichment of salt marsh sediments on the resident biota. A possible complicating effect upon nutrient enrichment is the removal 6f the spermatophyte canopy by ice rafting, deliberate oi; accidental burning, grazing activities of various animals and other events. Such events expose the marsh surface and its resident biota to a great increase in light intensity. The present paper thus reports on the effects of nitrogen enrichment and high light intensity, xI9 oz7z-7714/Sx/oSoix9+xI $oz.oo/o

9 x98i Academic Press Inc. (London) Ltd.

12o

3f. ft. Sullivan

both as single effects and in concert, on tile structure of an edaphic diatom community inhabiting the sediments of an irregularly flooded AIississippi salt marsh. Nitrogen is an essential element for plant growth and has been shown to limit the growth and production of the dominant marsh grasses, Spartina alterniflora Loisel., S. patens (/kit.) ~Iuhl. and Distichlis spicata (L.) Greene (e.g. Sullivan & Daiber, 1974; Valiela & Teal, 1974; Haines, 1979), as well as that of the edaphic algae (Sullivan & Daiber, i975; Van Raalte et al., 1976b) in Atlantic coastal salt marshes. Only taro studies have related changes in edaphic diatom community structure to nitrogen enrichment in Atlantic marshes. Sullivan (1976) and Van Raaltc et al. (I976a) found that nitrogen enrichment significantly decreased community diversity and had significant effects on the relative abundances of many of the more abundant diatom taxa in a Delaware and Massachusetts marsh, respectively. Of the two nutrient enrichment studies carried out in Gulf Coast marshes, only one was characterized by responses similar to those found for Atlantic coastal marshes. Patrick & Delaune (I976) showed the net aerial production of S. alterniflora to be significantly increased by nitrogen enrichment in a Louisiana salt marsh. Sullivan (1978/,), working in this same floral zone on a ~'Iississlppi marsh, could detect no effect on the production of the grass and virtually no effects on the resident edaphic diatom community in response to either nitrogen or phosphorus enrichment. Removal of the grass canopy by clipping has been shown to depress severely the community diversity of edaphic salt marsh diatoms (Sullivan, 1976) as well as greatly stimulate the productivity of the entire edaphic algal community (Sullivan & Daiber, i975; Van Raalte et al., 1976b). The first two studies also provided evidence for a shift in diatom community structure from that characteristic of an ,S'. alterMflora zone to that characteristic of a salt panne algal mat due to clipping. In a laboratory experiment, Admiraal (1977) evaluated the tolerance of xo species of estuarine benthic diatoms to high levels of different forms of inorganic nitrogen and only ammonia was found to inhibit diatom growth. Ammonia was also found to be inhibitory to photosynthesis, and this inhibitory effect was further enhanced by a high pH level in the culture medium or by high light intensity. High pH resulted in a shift in equilibrium towards the toxic N H a form of ammonia (see Warren, 1962), while high light intensity interacted with ammonia enrichment to stress greatly the photosynthetic machinery. Sullivan (1976) studied the interaction of ammonium nitrate enrichment and clipping the spermatophyte canopy on a Delaware salt marsh. Clipping, irrespective of whether nitrogen enrichment was an accompanying factor or not, favored the growth of filamentous green and blue-green algae over diatoms. The specific combination of ammonium nitrate enrichment and high light intensity inhibited the growth of the filamentous green alga Rhizoclonium riparium (Roth) Harvey during spring and greatly stimulated growth of blue-green algae during summer.

Methods and materials

FieM site All field work was carried out in Graveline Bay Marsh, Mississippi (3o~ 88~ an irregularly flooded marsh with a relatively diverse spermatophyte flora. The physical and biological characteristics of this salt marsh have already been described by Sullivan (I978a). Tile target area of the present investigation was a monotypic stand of Distichlis spicata (spike grass) on the marsh proper. This species is not only an important floral component of Atlantic and Gulf Coast salt marshes, but also of marshes on the Pacific coast as well. In all marshes studied to date, the edaphic diatom community beneath D. spicata

Light and nitrogen effects on an edaphle algal community

rat

has proved to be the most diverse one and therefore to contain more structural information than any other community (Sullivan, 1978a).

Experimental manipulations The experimental design employed two levels of light intensity in conjunction with two levels of nitrogen enrichment for a total of four treatment combinations. A single study area consisted of two replicates (each 1 • 2 m), at least I m apart, of each experimental treatment for statistical purposes. The marsh surface was subjected to high light intensity by clipping the D. spicata shoots at the mud-water interface. The first clipping was done on 19 July 1978. It was necessary to clip the regrowth of the grass shoots each time the marsh was subsequently visited and fertilized. Certain study areas received a natural light intensity, as the shading D. spicata canopy was left intact throughout the experiment. On 26 June 1979 measurements made with a Kahlsico No. 23A~!I2O phytophotometer showed that light energies in the blue, red and far-red bands at the marsh surface beneath an intact D. splcata canopy were 127, 80 and 7 I, respectively; whereas corresponding values were 5280, 39io and 3o60 gW cm -z, respectively, at the marsh surface devoid of a grass cover. Nitrogen enrichment consisted of application of NH4CI to the marsh surface at the rate of 2o g N m -'~ (3 months) -1- The amount of nitrogen added every 3 months is equivalent to that lost from the sediments of a Louisiana salt marsh through the process of denitrification over a x-year period (Delaune et aL, 1976). The NH~C1 was applied (i.e. broadcast by hand) during low slack water on 19 July, 25 October 1978, 8 January and 12 April i979. Removal of the fertilizer by tidal flushing is not a problem on Graveline Bay ~Iarsh, since the D. spicata zone has the highest elevation of the five floral zones studied by Sullivan (i978a) and the diurnal tides of Mississippi Sound rarely flood the marsh. Furthermore, two independent studies in regularly flooded Atlantic coastal marshes where organic sewage sludge was hand-broadcast into plots showed that salt marsh sediments have a tremendous capacity for retaining exogenously added nitrogen (Valiela et aL, 1973; Haines, i979). Although measurements of interstitial nutrient concentrations would have been desirable, lack of facilities and funds made such determinations impossible. In the results and discussion to follow the four experimental treatments are designated by two symbols. The first symbol refers to light intensity: N=natural light intensity (not clipped) and C = h i g h light intensity (clipped). The second symbol refers to nitrogen enrichment: o = n o nitrogen enrichment and N=NH4CI enrichment. The N--o study area represents the natural situation on the marsh and is therefore referred to as the natural marsh.

Sampling Edaphic diatoms were collected from each study area on 25 October 1978, 8 January, 1~ April and 26 June 1979 by taking sediment cores of the marsh. These dates were chosen to correspond to the mid-points of the traditional four seasons and allowed approximately 3 months be~veen an enrichment and subsequent collection. On a given date, two replicate samples were taken in each of four study areas for a total of 3 z samples over four dates. The techniques for separation of edaphie diatoms from the sediment core and preparation of permanent Hyrax mounts have been previously described (Sullivan, x975), i~licroscopie examination of the surface sediments beneath D. spicata revealed only clays and fine silt (mud), as well as considerable amounts of organic debris. Therefore, epipsammie diatoms may be assumed to be absent since sand particles were lacking.

xzz

M..7. Sullivan

Data analysis Exactly 500 diatom valves were identified and counted in each of the 32 samples, except for a single replicate from the N--o study area on 26 June in which 537 valves were counted. Thus, a total of i6 037 diatom valves represent the data base for all statistical analyses. After each of the 3 z diatom samples had been analysed taxonomically, community diversity statistics based on information theory were determined. T h e first of these was the information index (Shannon & Weaver, I949): H'=--~

S

nl

~-- logz

!--1

nl g

where H ' is expressed as bits individual -1, n~ is the number of valves of the flh taxon, N is the total number of valves in the sample and S is the total number of taxa in the sample. The computing formula and tables presented by Lloyd ct al. (I968) were used to calculate H ' to three decimal places. T o compare the structure of selected pairs of diatom communities the following similarity index proposed by Stander (197o) was employed: S

P u P~, 1--1

SIMI =

- - ,

where P u and P~. are the proportions of the ith taxon in t h e j t h and nth samples, respectively, and S is the total number of taxa. If the two samples being compared share no taxa in common, S I M I has a minimum value of o; whereas, if the taxa present and their relative abundances are identical in both samples, S I M I has a m~.ximum value of i.

Yield of Distichlis Immediately following the last collection of edaphie diatoms on z6 June, all shoots of D. spicata in each replicate were harvested from a representative o'x-mz quadrat. Upon return to the laboratory fresh weights were immediately obtained and then each sample was oven dried to constant weight. Results

Community diversity Species diversity (H') and the number of taxa in a sample (5') were calculated for all 3 z samples. Values of H ' and S for each study area are listed in Table i. The data bases for both H ' and S were subjected to a 3-way ANOVA (light • nitrogen X date) and tile results were identical. Tile 3-way interaction term was not significant (P>o.os) in either case, whereas the light • date (P < o.oo 5 for H ' and P < o'ooi for S) and nitrogen • date (P < o.oo 5 for H ' and P < 0.05 for S) interaction terms were significant. With regards to main effects only nitrogen was not significant. Responses of the edaphic diatom community resident in the sediments beneath D. spicata as measured by H ' and S were remarkably similar. Clipping of the grass canopy and exposure

Light attd nitrogen effects on an edaphlc algal community

Iz3

of the sediments to high light intensity significantly decreased b o t h H ' and S on all dates except t h e winter (8 J a n u a r y ) collection ( T a b l e z). T h e negative effect of clipping was most p r o n o u n c e d in the s u m m e r (26 June) collection w h e n a m b i e n t temperature a n d light i n t e n s i t y were greatest. I n t h e case of the second interaction term, n i t r o g e n e n r i c h m e n t significantly decreased H ' in the fall (z 5 October) collection a n d significantly increased b o t h H ' and S in the s p r i n g (IZ April) collection ( T a b l e 3). I t s h o u l d be noted, however, t h a t the difference b e t w e e n the control a n d n i t r o g e n e n r i c h m e n t m e a n s on 25 O c t o b e r was j u s t large e n o u g h to be significant in the case of H ' .

TABLE I. Species diversity (H') in bits individual-I and the number of diatom taxa in a sample (S) at each study area of Graveline Bay i~Iarsh (25 October x97826 June x979). H ' values are not enclosed by parentheses whereas S values are; -~t is the treatment mean (n=8), for all other values n = a Study area

25 October

8 January

x2 April

26 June

Rt

N--o N-N C-o C-N

4"006(44"5) 3"288 (37"5) 3"x62 (23"5) 3"o5I (22"5)

3"47z (36"5) 3"598 (39"o) 3"503 (35"0) 3"2x6 (32"o)

3"702(43"0) 4"336 (5t'o) 3"2x8 (27"5) 3"7x4 (34"5)

4"504(48"5) 4"2o4 (44"o) 3"x34 (20"5) 2"855 (2o'5)

3"92I (43"I) 3"857 (42"9) 3"254 (26"6) 3"209 (27"4)

TABLE Z. Comparison of H ' and S at each study area for the significant light xdate interaction by the least significant difference test (LSD=o'355 for H ' and 5"4 for o~ at P=o'o5). Algebraic symbols indicate either a significant difference or equivalence between the natural light intensity (N) and the high light intensity (C) on each date (n=4) H" Month

N

October January April June

3"647 3"535 4"ot9 4"354

S C

> = > >

3"xo6 3"359 3"466 z'994

N

C

4I'O > 37"8 = 47"o > 46"z >

23"o 33"5 3I"o 2o'5

TAnLE 3. Comparison of H ' and S at each study area for the significant nitrogen • date interaction by tile least significant difference test (LSD=o.355 for H ' and 5"4 for S at P=o'o5). Algebraic symbols indicate either a significant difference or equivalence between no nitrogen enrichment (o) and nitrogen enrichment (N) on each date (n=4) H" Month October January April June

o 3"584 > 3"487 = 3"46o < 3"819 =

S N 3"I7O 3"407 4"024 3"53o

o 34"0 = 35"8 = 35"2 < 34"5 =

N 30"0 35"5 42"8 3z'z

xz4

M..7. Sullivan

Diatom flora I t is appropriate in the present study to limit virtually all remarks to the resident edaphie diatom community, as none of the treatments on any date p r o d u c e d macroscopic growths of blue-green or green algae. Seraplngs of the marsh surface at the times of collection revealed a great n u m b e r o f viable, small pennate diatoms a n d very occasional colonies and filaments of blue-green algae. A total of i i I diatom taxa were encountered during the study, three of which could not be identified. F o r a complete listing of all taxa, see Sullivan (x979). A low of 6i taxa were identified from the C - o study area while a high of 91 were found in the natural marsh ( N - o study area). T a b l e 4 lists the x6 most abundant diatoms and their relative abundance at each study area and in the combined samples. Each of these taxa had an overall relative abundance greater than x . 2 5 ~ (i.e. N t > 2oo valves) and collectively they accounted for 8 5 . 3 ~ (i 3 676) o f the x6 o37 valves counted. Achnanthes lanceolata var. duma was most abundant in the two s t u d y areas shaded by an intact grass canopy (N--o and N - N ) while Navicula salinlcola was the single most abundant taxon at t h e two clipped study areas ( C - o and C - N ) . Navicula tripunctata and Nitzschia minutt,.la ( = N . frustulum var. tenella Grun.) were the second and third most abundant diatoms at all four study areas. TABLE 4. Relative abundance (expressed as number of valves) of the x6 most abundant diatom taxa at each study area of Graveline Bay Marsh (z5 October x97826 June x979). Nr=total number of valves counted Diatom taxon

11chnanthes haucklana Grun. tt. lanceoIatavar. dubia Grun. Amphora exigua Greg. tt. granulata Greg. 11. tcnerrima Hust. 11naulus balticus Simonsen Navicula binodulosa Sulliv. & Reim. IV. salhffcola Hust. IV. tripunctata (M011.) Bory N . sp. # 2

1Vitzschia gandersl,eimiensis Krasske IV. granulata Grun. N. mhzutula Grun. IV. obtusa var. nana Grun. IV. perversa Grun. Stauroneis amphioxys Greg.

N-o

N-N

C--o

C-N

Nt

xoo 840 x27 I4 5 x25 8I 294 486

83 97x 69 x4 z x38 78 3o2 5xx 23 I 2zo 384 84 248 x

x3 4z 21o 356 3x2 6 45 929 419 II2 x55 z5z 608 x22 x x57

3z 20 300 2o9 ' 3x7 x8 93 990 577 46 34 ~ 6x

228 1872 7o6 593 636 287 z97 2515 x993 2xz 505 6t6

52x

zzz8

36 7 xo5

322 402 264

31

9 x83 7x5 80 x46 x

Removal of the grass canopy appeared to have eliminated pre-existing taxa and introduced new taxa. I n the former category all the following taxa would seem to belong--~lnaulus balticus, Denticula subtilis Grun., Diploneis pseudovalis Hust., Fragilaria phmata Ehr., Nitzschia bilobata var. atnbigua Manguin, Nitz. brevissima Grun. (=Nits. parvula Lewis), Nitz. obsidialis Hust., Nirz. perversa and Nitz. pseudoamphio:,ys Hust. T a x a newly introduced into the c o m m u n i t y by clipping would appear to include Mastogloia exlgua Lewis, Nitz. gandershelmiensls (=Nitz. laevis Hust.) and Stauroneis amphioxys. T o satisfy oneself as to true introduction or elimination, the experiment performed here should be of longer duration. I t is significant that fertilization of the natural m a r s h with NH4CI neither introduced new taxa n o r eliminated pre-existing taxa. T h e relative abundances of the 16 most abundant diatoms were subjected to the same 3-way A N O V A (light • nitrogen • date) as were H ' and S, except that date was treated as

I25

Light a n d nitrogen effects on an edaphlc algal community

TABLE 5. Summary of 3-way ANOVA (lightxnitrogen• of the relative abundances of the x6 most abundant diatoms. (***=P o ' o 5 for calculated/;'-values associated with interaction terms) Diatom taxon Achnanthes hauckiana A . lameolata var. dubia Amphora exigua A . granulata A. tenerrima Anaulus baltlcus Navicula bhmdulosa N . salinicola iV. tripunctata

N. sp. # 2 Nitzschia gamtershclmiensis iV. granulata iV. mhzutula N . obtusa var. nana N. perversa Stauronels amphlox2;s

LxN

LxD

NxD

LxNxD

NS NS NS NS NS NS NS NS NS NS

** *** ** *** *** NS NS *** ** *

*** *** * *** * ** NS NS NS NS

** *** NS *** * ** * ** NS NS

*

***

*

**

NS NS NS

NS ** NS

NS * NS

* NS NS

**

***

***

***

NS

***

**

**

a repeated measure. These results are summarized in Table 5. Eleven taxa were characterized by a significant 3-way interaction term, four exhibited either one or two significant 2-way interaction terms, and a single taxon showed no significant effects whatsoever. Limitations of space prohibit a detailed discussion of treatment effects for each and every taxon. A detailed analysis of treatment effects for all i6 taxa after calculation of least significant differences and comparisons of results with previous work can be found in Sullivan (x979). However, some general patterns were discernible from an examination of the data. T h e majority of taxa were positively or negatively affected by removal of the grass canopy (see Table 4). Six increased in relative abundance in response to high light intensity while three were negatively affected by clipping. Nitrogen effects were m u c h less commonplace and consistent patterns did not emerge for most taxa. However, two taxa may have potential as bioindieator organisms on Graveline Bay Marsh. T h e specific combination of natural light intensity and nitrogen enrichment ( N - N ) greatly promoted the growth of N i t z . perversa in spring and summer, while that of high light intensity and nitrogen enrichment ( C - N ) greatly increased the relative abundance of N i t z . gandersheimiensis from fall through spring.

Communitysimilarity Table 6 lists S I M I values for comparisons of the four study areas. In all seasons, S I M I values for comparisons w i t h i n a given light intensity (i.e. N - o vs. N - N and C-o vs. C - N ) were relatively high and ranged from o.7z 7 to 0"952 ( , ~ = o ' 8 6 7 ; n = 8 ) . Furthermore, all comparisons between the two dipped study areas except one were greater than 0.900 , thus suggesting that the effects of high light intensity were largely independent of the presence or absence of nitrogen enrichment. Comparisons between different light intensities irrespective of whether the study area(s) received nitrogen enrichment (e.g., N--o vs. C - N ) were considerably lower and ranged from only o.2o8 to o.679 ( X = o - 4 5 7 ; n = i 6 ) . One should note that the two ranges given above do not overlap, and that S I M I comparisons between the natural marsh (N-o) and either the C--o or C - N study areas were for the most part very close to one another and always much less than comparisons between the natural marsh and N - N .

I26

A l . ff. S u l l i v a n

TABLE 6. ~Iatrix of similarity values* ( S I M I ) comparing study areas of Gravcline Bay Marsh (z 5 October 1978-26 June *979). All values • xo3 Study area pairs

25 October

8 January

*z April

26 June

N--ovs. N - N N--o vs. C--o N--o vs. C - N N - N vs. C-o N - N vs. C - N C - o vs. C - N

830 679 584 3Io no8 948

869 396 394 473 404 934

727 580 5*6

95z 4*0 47z 464 535 758

4 zz

46z 92*

*Each S I M I value calculated after two replicates pooled to yield a single sample.

TABLE 7- Aerial yield (g m -~) of Distichlis spkata expressed as dry and fresh weight following its harvest on 26 June x979 from Graveline Bay Marsh, Mississippi Study area

Replication

Dry weight

Fresh weight

Dry/Fresh

N--o

x z

I366 I318

x89o 175o

o'7z 0"75

N-N

I z

24z7 19IO

39oo a99o

o-6z 0'64

C-o

* z

*63

3*o

x58

300

0"53 0.53

r 2

380

720

0"53

349

650

0"54

C-N

YieM of Distiehlis Visual observations on 26 June indicated that grass shoots in the N-N and C-N study areas were noticeably taller, denser and greener than their counterparts in the N-o and C-o study areas, respectively, and for this reason the spike grass was harvested and its yield determined. Table 7 lists the yield of D. spicata as both dry and fresh weight in each replicate and their ratio. The yield for tile C-o and C-N study areas represents the regrowth of the shoots after they were clipped down to the mud-water interface following the previous collection on 12 April. A z-way ANOVA (light• revealed that only the main effects of light ( P < o.ool) and nitrogen ( P < 0"025) were significant as far as dry weight was concerned. Nitrogen enrichment increased the dry weight of D. spicata by 6z% in the unclipped study areas (N-o vs. N-N) and by I28% for the regrowth in the clipped study areas (C-o vs. C-N). Although a second ANOVA was not performed on the fresh weight data, it appears that the exact same conclusions could be drawn. The ratio of dry to fresh weight revealed differences in water content between shoots in the various study areas, but these differences were not statistically tested. The range in values for ~o dry weight listed in Table 7 are very similar to those obtained by Gallagher (I979) for different stands of Sporobolus virglnicus (L.) Kunth., a plant with a vegetative morphology nearly identical to that olD. spicata. Finally, the figure of 134z g m -~ (n=2) for tlle standing crop of control D. spicata shoots in Graveline Bay Marsh is higher than that measured in other Gulf Coast salt marshes. Kruczynski et aL (I978) reported a peak standing crop of D. spicata shoots of 965 while White et aL (x978) recorded 1164 g m -2 in Florida and Louisiana, respectively.

Light and nitrogen effects on an edaphic algal community

x27

Discussion

High light intensity and nitrogen enrichment (as NH,CI) had numerous significant effects on the structure of an cdaphic diatom community inhabiting the scdiments beneath the salt marsh spike grass, Distichlis spicata. Most significant effects resulted from the removal of the grass canopy by clipping, and with one exception these effects were independent of the presence or absence of nitrogen enrichment. Any deleterious effects caused by an interaction of the ammonium fertilizer and high light intensity, as found by Admiraal (x977) for laboratory cultures of marine benthic diatoms, were not detected in the present study. Although productivity measurements were not made, preparation of permanent slides suggested that diatoms were equally abundant in the C--o and C-N study areas. The results of the present study compare well with previous ones in regard to clipping effects, but differ with respect to nitrogen enrichment. In the D. spicata zone, clipping greatly decreased species diversity (H') and the number of diatom taxa in a sample (S). Clipping also raised salinity to values in the range of hypersalinity (> 45~oo). ~Iarsh surface salinity, as determined by a method described in Sullivan (1975), reached values as high as 80 and 7o~o in the C-o and C-N study areas, respectively. Sullivan (I976) recorded identical results after clipping a monotypic stand of Sparthza alterniflora in a Delaware salt marsh. Furthermore, both studies found that clipping eliminated pre-existing taxa and introduced new ta_xa into the community and that nitrogen enrichment in addition to clipping (C-N) had the same effect on H ' and S as did clipping without nitrogen enrichment (C--o). I-Iowever, in the Delaware study clipping greatly stimulated the growth of filamentous green and blue-green algae, which formed macroscopic mats in summer in the C--N treatment. Not only were such mats conspicuously absent from the clipped study areas of the present stud)', but for unknown reasons have never been found to occur in any open area of Graveline Bay Marsh. Therefore, the filamentous algae did not take up the 'slack' in productivity caused by removal of the highly productive grass canopy as is typical for Atlantic coastal marshes (Estrada et aL, I974; Sullivan & Daiber, x975). Evidently, factors other than light intensity and nitrogen supplies are limiting for filamentous algae on this marsh. No evidence for increased grazing levels in the clipped plots surfaced during the study. In contrast to other studies (Sullivan, x976; Van Raalte ct aL, 1976a), nitrogen enrichment had only positive effects on community diversity as measured by H ' and S, and did not eliminate diatom tara from the community. A somewhat similar result was obtained by Sullivan (x97Sb), who supplied nitrogen as NHaNO a in a monotypic stand of S. alterniflora on Graveline Bay Marsh. Only a single taxon, Aritzschia obtusa W.Sm., was affected by nitrogen enrichment (i.e. exhibited a positive increase in relative abundance) while H" was unaffected and S decreased only in winter. Therefore it would appear that the edaphic diatom communities of Graveline Bay Marsh are largely 'resistant' (see Christian et aL, 1978) to additions of inorganic nitrogen. The aerial yield of Distichlis spicata was greatly increased by nitrogen enrichment and its rcgrowth following clipping was similarly affected. In the only other enrichment study known involving this grass, Valiela & Teal (I974) fcrtilized a ~Iassachusetts salt marsh with urea and found that nitrogen enrichment significantly increased the aerial yield of high marsh D. spicata. However, NH4NO a enrichment of Graveline Bay ~1arsh had no effect on the aerial yield of Sparthza alterniflora (Sullivan, x978b). The literature is filled with reports on the 'universal' limitation of S. alterniflora by nitrogen supplies (e.g. Sullivan & Daiber, 1974; Valiela & Teal, x974; Patrick & Delaune, ~976; Haines, I979). Why one floral component should respond so dramatically and another be unaffected by the same perturbation

xz8

l~f. ft. Sullivan

cannot be answered at present, but the concept of resistance as put forth by Christian et al. (i978) deserves consideration here also. A recent report by Gallagher (I979) showed the net aerial production of high marsh Sporobohts virginlcus to be unaffected by NH~NO a enrichment. T h e growth patterns of D. splcata in the clipped study areas may have important implications for formulating marsh management policies. First, trimonthly clipping of the aerial shoots stimulated the underground rhizomes to produce green, healthy shoots over an entire yearly cycle. Normally D. spicata shoots are annual and dieback during winter, with the production of new shoots in early spring. Second, addition of NH4CI to the clipped plots increased aerial biomass by more than a factor of two over plots that were only clipped in a period of less than 3 months. Therefore, areas of high marsh supporting stands of D. spicata that have been burned or grazed by animals could be more quickly restored to their former condition by the addition of nitrogen fertilizer. T h e findings of the present study contribute to our knowledge of salt marsh ecosystems because the resident diatom communities were largely unaffected by additions of inorganic nitrogen. This is significant in view of the suggestion that eutrophication in salt marshes could be monitored and detected by following changes in edaphic diatom community structure over time (Sullivan, x976; Van Raalte et aL, x976a ). Therefore, although such communities in Atlantic and Gulf Coast salt marshes are structurally quite similar (Sullivan, I978a) their functioning appears to be characterized by some significant differences. The data available at present are too preliminary to formulate any hypotheses as to the nature of functional linkages between spermatophytes and edaphie diatoms.

Acknowledgements T h e author thanks L. N. Eleuterius for use of laboratory facilities, W. W. Sage and S. R. O'Quinn for technical assistance and hi. Morris for statistical help. Comments provided by two anonymous referees improved the manuscript. The work upon which this publication is based was supported in part by funds provided by the Office of Water Research and Technology (Project No. ~A-I24-MISS), U.S. Department of the Interior, Washington, D.C., as authorized by the Water Research and Development Act of 1978.

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Light and nitrogen effects on an edaphic algal comn, unity

z29

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