The role of sea urchin grazing in regulating subtropical seagrass meadows: evidence from field...

J. Exp. Mar. Biol. EcoL, 154 (1991)215-230 © 1991 Elsevier Science Publishers B.V. All rights reserved 0022-0981/91/$03.50

215

JEMBE 01694

The role of sea urchin grazing in regulating subtropical seagrass meadows: evidence from field manipulations in

the northern Gulf of Mexico

John F. Valentine ! and Kenneth L. Heck, Jr. 1.2 ~ Marine Environmental Sciences Consortium. Dauphin Island Sea Laboratory, Dauphin Island, Alabama, USA" 2Department of Biology. Coastal Research and Development Institute, University of South Alabama,

Mobile, Alabama. USA

(Received 25 September 1990; revision received 16 August 1991; accepted 10 September 1991)

Abstract: Sea urchin herbivory has been demonstrated to limit kelp forest distributions but its role in influencing distribution of seagrass habitats is inadequately investigated. During two annual cycles, w e manipulated sea urchins [Lytechinus variegatus (Lamarck)] at densities ranging from 0 to 80 individuals/m 2, and documented their seasonal impact on turtlegrass Thalassia testudinum Banks ex Konig meadows in St. Joseph Bay, Florida. Results from these experiments indicate that the lowest densities of sea urchins required to overgraze (i.e., completely defoliate) turtlegrass occur during winter (approximately 20 individuais/m 2) while higher densities (approximately 40 individuals/m 2) are required for overgrazing during summer and fail. Field censuses frequently encountered sea urchin densities sufficient for overgrazing. We conclude that sea urchin herbivory is important in regulating subtropical seagrass meadow biomass and size particularly where sea urchin densities exceed 20 indlviduals/m 2.

Key words: Grazing; Herbivory; Seagrass habitat; Sea urchin

INTRODUCTION

Sea urchin herbivory has been demonstrated to be a controlling factor in limiting the distribution and abundance of marine macroalgae. For example, there is good obser- vational and experimental evidence to indicate that sea urchins are capable ofconverting kelp forests to coralline algal pavements (i.e., "barren grounds"). After such events, sea urchin densities often remain at levels sufficient to prevent kelp recolonization (Leighton, 1971; Lawrence, 1975a; Hagen, 1983; Dean et al., 1984; Dayton, 1985; Foster & Schiel, 1985; Harrold & Pearse, 1987).

Despite reports ofintense sea urchin grazing in seagrass habitats (Ogden et al., 1973; Lawrence, 1975a; Lilly, 1975; Ogden, 1976; Zimmerman & Livingston, 1976a; Bak & Nojima, 1980; Kirkman & Young, 1981 ; Verlaque & Nedelec, i983; Cambridge et al.,

Correspondence address: J.F. Valentine, Marine Environmental Sciences Consortium, Dauphin Island Sea Laboratory, PO Box 369, Dauphin Island, AL 36528, USA.

MESC Contribution to Marine Sciences 176.

216 J.F. VALENTINE AND K.L. HECK. JR.

1986), only one large scale overgrazing (i.e., the complete defoliation of seagrass habitat) event of comparable size to those reported in kelp forest habitats has been reported in seagrass meadows (Camp et al., 1973). In that case, intensive grazing by Lytechinus variegatus (Lamarck) defoliated large areas (approximately 20?/o) of seagrass along Florida's west coast. Smaller overgrazing events in seagrass habitats, by other sea urchins, have also been observed elsewhere (Ogden et al., 1973; Lilly, 1975; Ogden, 1976; Zimmerman & Livingston, 1976a; Bak & Nojima, 1980; Kirkman & Young, 1981; Verlaque & Nedelec, 1983; Cambridge et al., 1986). Mechanisms involved in seagrass overgrazing are not completely understood (Greenway, 1976; Ogden, 1976; Tribble, 1981; Tertschnig, 1984), but may be associated either with the immigration of dense aggregations (Ogden et al., 1973; Bak & Nojima, 1980), or the episodic recruitment of sea urchins (Camp et al,, 1973).

In many tropical regions herbivorous mammals, fishes and sea turtles can also impact seagrass meadows (Randall, 1965; Lipkin, 1975; Anderson & Birtles, 1978; Nietschmann & Nietschmann, 1981). In the northern Gulf of Mexico, the suite of herbivores is similar to that of the intensively fished tropics; herbivorous manatees and sea turtles are seldom seen in these seagrass habitats, and the diversity and abundance ofherbivorous fishes that feed on seagrasses (parrotfishes and surgeonfishes) is very low (Eidemiller, 1972; Livingston, 1984; Zieman & Zieman, 1989).

During the past 8 years we have observed the persistence oflarge unvegetated habitats isolated in otherwise stable stands of turtlegrass Thalassia testudinum Banks ex Konig in St. Joseph Bay, Florida (30°N, 85.5°W). Such habitats are commonly reported in the literature (Chesher, 1969, Mosher, 1980; Young & Young, 1982; Iverson & Bittaker, 1986; Telford et al., 1987; Walker et al., 1988; Duarte & Sand-Jensen, 1990a,b) but few attempts have been made to explain either their origin or persistence in seagrass dominated systems. Aerial photography (Williams, 1981) showed that these unvegetated habitats cover areas ranging from 5 to 35 ?/o of the total seagrass-sand flat mosaic in St. Joseph Bay. L. variegatus, the sea urchin previously observed defoliating large expanses of seagrass in the northern Gulf of Mexico (Camp et al., 1973), i~ commonly found within vegetated areas of St. Joseph Bay with population densities as high as 140 individuals/m 2.

Based on these observations, we hypothesized that sea urchin herbivory might be important in controlling local seagrass distribution and abundance in this subtropical setting. The research reported here examined the potential effectiveness of L. v,~riegatus in controlling the local distribution of T. testudinum in the northern Gulf of Mexico through the use of a series of short-term (18 day) experiments. In particular, we asked the following questions: (1)at what densities are urchins capable of overgrazing seagrass habitats; (2)are these densities seen in na, ture; and (3)are there seasonal differences in the impact of sea urchin grazing. In addition, we discuss the relative importance of sea urchin herbivory in regulating seagrass meadow patch size and above-ground biomass.

URCHIN HERBIVORY IN SUBTROPICAL SEAGRASS MEADOWS 217

MATERIALS AND METHODS

STUDY-SITE

The study-site in St. Joseph Bay, Florida, is located in the northeastern Gulf of Mexico (30°N, 85.5°W). St. Joseph Bay is a protected, shallow coastal embayment with no significant source of freshwater input. Salinities range from 30 to 36700 annually (Stewart & Gorsline, 1962; Folger, 1972; pers. obs.). Temperatures vary annually from approximately 8 to 30 °C (pers. obs.), and the mean tidal range is 0.5 m (Rudloe, 1985).

Enclosed within this bay is an extensive seagrass habitat that has been estimated to occupy some 26 km 2 of shallow bottom (McNulty et al., 1972). Seagrass production is highly seasonal with leafbiomass and density peaking near 150 g ash free dry weight/m 2 and approximately 3000 leaves/m 2 during summer months (lverson & Bittaker, 1986). This seagrass habitat is dominated by large monospecific stands of T. testudinum interspersed with smaller patches of Halodule wrightii, unvegetated sand flats and small amounts of Syringodium filiforme (Iverson & Bittaker, 1986). Only the shallowest portion of the seagrass habitat has been observed to be exposed during low tides.

EXPERIMENTAL AND SAMPLING PROCEDURES

Grazing experiments were conducted using 50 replicate 1 m 2 circular cages, made of 30 mm mesh Vexar plastic, which were placed in an array of 5 x 10 cages in a heavily vegetated portion of the turtlegrass meadow. Primary production varies seasonally in St. Joseph Bay (Iverson & Bittaker, 1986). Therefore, grazing experiments were conducted quarterly from fall 1988 to 1989. Sea urchins were. added to randomly selected cages at treatment densities of 0, 10, 20, 40 and 80 individuals/m 2. Each treatment density was replicated 10 times. These densities represent the range of densities observed and reported in the literature, excluding massive episodic recruitment events (Table I).

TABLE I

L. variegatus densities reported from seagrass habttats in the western North AtlanUc

Densities Location Source

20/m 2 0.4-15/m 2 3.-636/m 2 250/m 2 42.8-250/m 2 (juveniles) !.9-15.9/m 2 (adults) 0.8-10.8/m 2 4.92-30.70/m 2

Kingston, Jammca S.W. Puerto Rtco Steinhatchee, Florida Biscayne Bay, Florida Biscayne Bay, Florida Discovery Bay, Jamaica Miskito Cays, Nmcaragua Jobos Bay, Puerto Rico

Greenway (1976) 3 Engstrom t 1982) 3 Camp et al. (1973) 1"4 Moore et al. (1963) 2.4 Moore & McPherson (1965) 4 Keller (1983) Vadas et al. (1982) 2.3 Rivera (1978) 3

t Studies conducted during episodic recruitments. 2 Authors suggest high variance to mean ratio's indicated reported densiues are probably underestimated. ~ Mean densities. 4 Range of densities.

218 J.F. VALENTINE AND K.L. HECK, JR.

Ingestion rates of L. variegatus are known to be size dependent (Klinger, 1984). Therefore, the horizontal diameter of the first 10 sea urchins to be placed in each cage was measured to the nearest millimeter. Only mature urchins [i.e., greater than 30 mm (Serafy, 1979)] were used. Urchin size differences among treatments were analyzed using a one-way analysis of variance (ANOVA; Sokal & Rohlf, 1981).

Sea urchin treatment effects were compared by collecting three 0.01 m z clip samples (3% of cage area) of aboveground biomass at 6-day intervals for 18 days. Following clipping, engineering tape tied to a thin wire stake was placed in the center ofthe sample area to ensure the location was not clipped during subsequent samplings. Abovegroun~( biomass was determined by drying seagrass clippings at 90 °C for 24 h. All cages were censused for urchin densities on each sample date. Where losses occurred, new urchins were placed in cages to maintain treatment densities. All replacement urchins were measured to ensure size conformity. Differences among treatments on each sample date were analyzed using ANOVA or a KruskaI-Wallis test when normality was violated (detected using Hartley's Fma~ test). If significant differences among treatments were detected, a posteriori multiple comparisons were employed using a parametric least sigmficant difference test or non-parametric multiple comparisons test (Hollander & Wolfe, 1973; Sokal & Rohlf, ! 98 !).

Following completion of grazing experiments all cages were removed. The perimeter ofeach cage enclosing 80 sea urchins was marked with 2-inch PVC pipe and periodically resampled for aboveground biomass in order to estimate recovery times following overgrazing events. In addition, aboveground biomass samples were collected from an area directly adjacent to the experimental site (hereafter referred to as unmanipulated seagrass habitat) for comparative purposes. Each sampling consisted of three 0.01 m z clippings from experimentally grazed plots and five clippings from unmanipulated seagrass habitat. Comparisons of aboveground biomass between overgrazed sites and unmanipulated seagrass habitat were conducted using a two sample t-test, independent cases (Sokal & Rohlf, 1981). All statistical tests were considered significant at the preselected 5% level of significance.

To assess seasonal cycles in L. variegatus abundance, five 50 x I m transects were haphazardly placed monthly in the seagrass habitat. L. variegatus densities were counted in 10 randomly selected locations along each of the five transects using a I m e quadrat.

RESULTS

SEASONAL GRAZING EXPERIMENTS

Results from the grazing experiments indicate that seasonal differences in seagrass productivity shift the minimum sea urchin densities required for overgrazing. In fall 1988, aboveground seagrass biomass did not vary significantly among treatments on Day 0 indicating that initially seagrass biomass was homogeneous (F = 0.95, p > 0.44,


Fig. la). However, significant differences in aboveground seagrass biomass did exist among treatments during subsequent collections (Day 6 X z = 34.73, p < 0.0001; Day 12 X z = 40.70, p < 0.0001; Day 18 X: = 40.56, p < 0.0001). Pairwise comparisons of

aboveground seagrass biomass, found significantly less biomass in sea urchin enclosure treatments than in the control treatment (0 urchin treatment) after Day 0, indicating that

10.0 0tI 6.0

. . o

2.0

0.0 o i

m

Foil 1988 (o)

1'2 18

@ ~ 0 Control & ~ & lO/m 2

- - ,~ - - , 20/m 2 V ~ V 40/m 2

@ ~ @ 80/m 2

E c) d

E ol

, 4 J JC: ol

. ! Q)

4.0

3.0

2.0

1 . 0 "

Summer 1989

h,"

I

0.0 o

(b)

3.0

2.0

1.0-

0.0

Foll 1989 (c)

0 6 12 18

Days

Fig. 1. Sea urchin grazing experiments conducted in St. Joseph Bay, Florida, during (a) fall 1988, (b) summer 1989, and (c) fall 1989. Aboveground biomass per 0.01 m 2 vs. sample date (mean _+ 1 standard

deviation).

220 .l.F. VALENTINE AND K.L. HECK. JR.

sea urchins were reducing seagrass biomass. These comparisons also found significantly more aboveground seagrass biomass in treatments containing 10 and 20 sea urchins than in those containing 40 and 80 urchins on Days 6 and 12, indicating these two sets of treatments had different impacts on aboveground seagrass biomass. Although seagrass reductions were significant in treatments enclosing 10 and 20 sea urchins/m 2 (> 50%), they had not overgrazed (i.e., completely defoliated)enclosed seagrass.

In contrast, sea urchins enclosures of 40 and 80 individuals/m 2 removed all visible seagrass within 6 days. By Day 18, aboveground seagrass biomass in the treatment containing 20 sea urchins was not significantly different from the treatment containing 40 sea urchins. Since all visible seagrass was removed at a sea urchin density of 40 individuals/m 2 and seagrass biomass in treatments enclosing 20 and 40 individuals/m" were not significantly different, we estimate that sea urchin densities between 20 and 40 individuals/m 2 were sufficient to overgraze seagrasses in fall 1988.

Winter grazing experiments showed that lower sea urchin densities than required in fall 1988, were capable of overgrazing seagrass habitats. In winter 1989, no significant differences in total, seagrass and algal, aboveground biomass were detected among treatments on Day 0 (Fig. 2a; seagrass F -- 0.06, p > 0.99; algae F - 0.42, p > 0.42; total biomass F = 0.10, p > 0.98). By Day 6 all sea urchin treatments completely defoliated enclosed seagrass and pairwise comparisons found total aboveground biomass in the control treatment to be significantly higher than treatments enclosing sea urchins (X 2 = 23.71, p < 0.0001).

Algal biomass increased in all treatments except those enclosing 40 and 80 urchins after Day 6 (Fig. 2b), but aboveground seagrass biomass continued to decrease in all urchin enclosure treatments (Fig. 2c). As a result, pairwise comparisons of total aboveground biomass (seagrass and algae) on Day 18 found no significant differences between the 10 sea urchins/m 2 treatment and the control treatment. Consequently, we conservatively estimate that overgrazing densities are near 20 sea urchins/m 2 for winter 1989, but in fact the minimum overgrazing density may be much less since the regenera- tive ability of seagrasses is low during winter.

Experiments conducted in summer 1989 found that higher sea urchin densities than any of the previous seasonal experiments were required to effectively overgraze seagrass habitat. Pairwise comparisons on Day 6 found signific~xit differences between all sea urchin enclosure treatments except those enclosing 40 and 80 urchins (Fig. lb). As with previous experiments, all enclosure treatments were also significantly different from the control treatment. Only sea urchin densities ~40 individuals/m 2 were observed to completely overgraze enclosed seagrass. Similar results were found on Days 12 and 18 indicating that overgrazing densities were > 40 sea urchins/m 2 during this season.

Again in fall 1989, aboveground seagrass biomass did not vary significantly between treatment.s on Day 0 (Fig. lc; F - 0.91,p > 0.46) but differences were detected on Days 6, 12and 18 (Fig. lc; for Day6 ;~2 = 30.04,p < 0.0001; Day 12 X 2 = 24.02,p < 0.0001; Day 18 X 2 = 41.20, p < 0.0001). On eacl', of these dates aboveground seagrass biomass in treatments enclosing urchins was significantly less than in the control treatment.


4.0

3.5

3.0

2.5

2.0

1.5

!.0

0.5

0.0

Total Vegetation (Algae+Seagrass) (a)

6 18

O u O Control A ~ A 10/m 2

I - - I 20/m 2 V ~ V 40/m 2 0 - - 0 80/m 2

• , ~ ° 0 '

E o

E

Algae (b) 2.5

2.0

1.5

e-

0 6 18

3.0 Seagrass only

2.5 20 : !.5

1.0

0.5 ~ _~

0.ol ~' ~ t

(c)

0 6 18

Days

Fig. 2. Sea urchin grazing experiment conducted in St. Joseph Bay, Florida, during winter 1989; (a) total aboveground biomass (seagrass + algal aboveground biomass), (b)algae and (c)seagrass biomass per

0.01 m 2 vs. sample date (mean _+ 1 standard deviation).

Pairwise comparisons showed the I 0 and 20 urchin treatments and the 40 and 80 urchin treatments on Days 12 and 18 had different impacts on aboveground seagrass biomass. Only urchin densities of 40 and 80 individuais/m: were observed to completely defoliate enclosed seagrass. Therefore, the overgrazing density was conservatively estimated to be > 40 individuals/m 2, similar to that reported in fall 1988.


Generally speaking, the smallest urchins were used in fall 1988 and largest in summer and fall 1989 (Table II). However, no significant differences in horizontal diameter were found between treatments for any single grazing experiment (Table II).

TABLE II

Mean urchin horizontal diameters in enclosures cages on Day 0. F scores from ANOVAs on Day 0 from urchin herbivory experiments.

Season Urchin density Mean horizontal F P (> ) (m 2) diameter (mm)

Fall 1988 10 30.99 2.50 0.06 20 32.01 40 30.96 80 30.61

Winter 1989 10 38.41 1.29 0.28 20 37.20 40 38.09 80 37.99

Summer 1989 10 45.87 0.08 0.97 20 45.89 40 46.02 80 46.29

Fall 1989 10 45.83 0.70 0.55 20 45.28 40 46.28 80 46.41

All of the experimentally created overgrazings recovered to levels of aboveground biomass that were statistically indistinguishable from those of unmanipulated seagrass habitat in relatively short periods of time. For example, overgrazing plots created in November 1988 recovered within 5 months (Fig. 3a), while those created in March 1989 recovered in 3 months (Fig. 3b). A short recovery also followed overgrazing in summer 1989 when regrowth was observed within 1 month (Fig. 3c), while those experiments conducted in October 1989 returned to natural biomass levels within 4 months (Fig. 3d).

L. VARIEGATUS POPULATION DENSITIES IN ST. JOSEPH BAY

The population densities of L. variegatus in St. Joseph Bay, Florida, were quite stable between the December 1988 and May 1989 period. Mean densities during this time ranged from 12.5 individuals/m 2 in March to 14.2 individuals/m 2 in May (Fig. 4). The upper range of densities was also fairly stable, ranging from 42 individuals/m 2 to 49 individuals/m 2. Population densities increased between August and September 1989, when mean densities increased from 13.3 to 24.6 individuals/m 2 (Fig. 4). The upper range of densities also increased during this period from 36 to 72 individuals/m 2, but the highest density, 140 individuals/m 2, was observed in Januarv 1990 (Fig. 4). Over- grazing densities were commonly observed during each monthly sampling.


Fall 19ee (a) ..I.o

It : 6.32 /P ( 0.001 2.0

1.0. ( 0.766

; • ~ 0.0 1~ o Dec. 1988 Jan.i989 MQy.i9B9 d Ot

~, .t.5, ° T Summer 1989 (c)

3.0, / 2.5, O

2.0- 1 ~ le~t =I.098 •

1.0- J 0.5 ~

0.0 I August, 1989 Sept.,' 1989

2.5

2.0

1.5-

1.0-

0 , 5 '

0.0 March

Winter 1989 (b)

. i t 7 0.777 ~ OP 0.448

1989 May,1999

H Overgrazed plots Natural

3.0

2 . 0

1 .0 •

0.0 Nov.

rQ, ,gee (d)

l e.~ I =

1989 Feb., 1989

Sample Date

Fig. 3. Recovery of experimentally overgrazed seagrass habitat during (a)fall 1988, (b)winter 1989, (c) summer 1989, and (d) fall 1989 in St. Joseph Bay, Florida. Aboveground biomass per 0.01 m 2 vs. sample

date (mean + 1 standard deviation).

In addition, in January 1989, a sea urchin aggregation measuring 42 m long by 0.5 m wide was observed. Urchin densities in this aggregation ranged from 62 to 132 individuals/m 2 (n = 5). Collections of aboveground biomass from 0.01 m 2 quadrats either side of the aggregation, designated as grazed and ungrazed, found differences of approximately 90% to be significant (Fig. 5; t = 7.91, p < 0.0001).

DISCUSSION

Results indicate that L. variegatus densities sufficient to defoliate seagrass habitats are common in St. Joseph Bay, Florida, as well as other locations (Table I). Although the productivity of T. testudinum in St. Joseph Bay is unknown, our experiments suggest that the observed sea urchin densities can reduce seagrass biomass by more than 50%, and can potentially influence the distribution of subtropical seagrass habitat.


¢N

E

e - , , I

c

80 ~-140

6O , e

40 ]

T ~b qb

, / , / / ' , , / \ ; o 1 1 ! ] Dec Jon Mar Apt May Jun ~lul AugSeptOct Nov Dec Jan Feb Mot

1988 1989 1990

Fig. 4. Monthly densitxes ofL. variegatus in St, Joseph Bay, Florida. Mean density per I m 2 vs. sample date. Error bars rep, rese, nt the range of densities encountered while conducting field censuses of sea urchin

densities.

¢'4 E

0 c~

E

J¢

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

t = 7.91 p ( 0.0001

Grozed Ungrozed

Fig. 5. Aboveground seagrass biomass associated with a sea urchin feedi~g front during January 1989 in St. Joseph Bay, Florida (mean + 1 standard deviation).

A review of pertinent literature further supports the role of sea urchin herbivory in controlling seagrass habitat structure. In addition to the overgrazing event reported by Camp et al. (1973), Moore & McPherson (1965)estimated that in areas of Biscayne Bay, Florida, annual consumption ofL. variegatus was roughly equal to annual seagrass productivity (9 kg/m2). L. variegatus has also been associated with several areas of low seagrass biomass in offshore grassbeds along the western coast of Florida (Zieman &


Zieman, 1989) and the Florida Keys (Bach, 1979). In Kingston Harbor, Jamaica, Greenway (1974) estimated that this urchin consumed slightly under 50 ~o of the weekly Thalassia production.

It has been hypothesized that energy stored within seagrass rhizomes allows seagrasses to regrow leaves following defoliation (Phillips, 1960; Buesa, 1974; Greenway, 1974, 1976; Dawes & Lawrence, 1979; Tomasko & Dawes, 1989). Seagrasses can also translocate stored nutrients along rhizomes towards areas of increased metabolic activity such as new leaf growth (Libes & Bourdresque, 1987; Tomasko & Dawes, 1990). Based on this information and on our results, one "overgrazing" event should not have a long-lasting influence in seagrass-dominated systems since regrowth ofleaves occurs rapidly. However, repetitive harvesting of leaves leads to decreased regrowth of leaves (Phillips, 1960; Buesa, 1974; Greenway, 1974, 1976), ~fignificant reductions in belowground root and rhizome biomass (Dawes & Lawrence, 1979; Williams, 1987) and potentially unvegetated substrates (Phillips, 1960). Specifically, Buesa (1974)found that T. testudinum was adversely affected (i.e., slower leaf regrowth) by as few as three repeated croppings of the same area in as little as 30-50 days. Greenway (1974) found significant declines in leaf regrowth following five successive clippings of the same plot. Phillips (1960) found that repeated harvesting of T. testudinum was sufficient to cause the almost total disappearance of aboveground biomass after 4 months. Based upon these studies, three to six repeated grazings of the same plot per year by sea urchins should be sufficient to cause permanent declines in seagrass in the eastern Gulf of Mexico, probably due to nutrient depletion of seagrass rhizomes.

We also hypothesize that "drift algae" abundance may play an important role in limiting the long-term effects of sea urchin grazing on seagrass habitats, particularly in winter months when seagrass production is lowest, in the northern Gulf of Mexico (Iverson & Bittaker, 1986). L. variegatus is a generalist herbivore which eats plants in proportion to their relative abundance (Lowe, 1974; Lawrence, 1975a; Ogden, 1980). In the northern Gulf of Mexico, "drift algae" are present in large accumulations during the cooler months of the year (Zimmermaa & Livingston, 1976b, 1979). We suggest that if "drift algae" become scarce, perhaps due to poor algal recruitment or intense herbivory by large numbers of "mesograzers" such as snails, amphipods or caridean shrimp, the potential for long-lasting overgrazings of seagrasses by sea urchins will increase. Extraordinarily high numbers of sea urchin recruits could also deplete "drift algae" and detrital material and then consume live seagrasses.

Finally the question must be asked, if sea urchins densities are commonly sufficient to overgraze seagrass habitats why are there so few accounts of seagrass overgrazings? We suggest two possibilities. First, most sea urchins are stenohalic (Moore, 1966; Lawrence, 1975b) and are unable to tolerate depressed salinities in nearshore grass beds where most work has been accomplished, owing to freshwater runoff during intense storms (Goodbody, 1961; Lawrence, 1975b). Second, few studies have been conducted in the offshore seagrass habitats of the eastern Gulf of Mexico, where sea urchins are


common (Camp et al., 1973; Hooks et al., 1976; Zimmerman & Livingston, 1976a). These seagrass meadows are substantial, extending intermittently along Florida's west coast, covering more than an estimated 3000 km 2 (Iverson & Bittaker, 1986).

We believe that the importance of these results is twofold. First, seagrasses provide complexity to otherwise featureless habitats (Heck & Wetstone, 1977). In turn, this complexity provides refuge for smaller prey (Stoner, 1980a,b,c; Coen etal., 1981; Nelson, 1981a,b; Peterson, 1982; Stoner, 1982; Heck & Thoman, 1984; Summerson & Peterson, 1984), as well as nursery during the early life of many species and foraging areas for many adult fishes and birds (Thayer et al., 1975; Robertson, 1977; Bell et al., 1978a,b; Weinstein & H e e l 1979). As a result animal biomass can be several orders of magnitude higher in seagrass than nearby unvegetated habitats (Heck & Thoman, 1984; Orth et al., 1984). Thus, losses in seagrass habitat due to sea urchin overgrazings can have consequences not only in terms of lost productivity by plants but also lost productivity of macrofauna which occur in these habitats.

Second, this work attempts to understand the specific factors that establish the boundaries, sizes and alternate states of soft-bottom, seagrass-dominated habitats. Determining these factors should lead to an increased understanding of processes that create and maintain alternative stable states (vegetated vs. unvegetated habitats) in soft-bottom benthic environments and allow for comparison with those processes already identified in hard bottom kelp forest habitats.

ACKNOWLEDGMENTS

The authors thank L. Coen, M. Judge, J. Lawrence, S. Morgan, D. Tomasko, and R. Zimmer-Faust for their constructive criticisms of this manuscript. In addition, we thank the following students: M. Brownlie, M.A. Foster, P. Harper, J. Lavelle, N. Mitchell, D. Nadeau, J. Nelson, S. Riemer, S. Rikard, P. Steven, D. Webb, and C. Welch for their assistance with tasks associated with this study. Special thanks are extended to R. Modlin and the Alabama Marine Environmental Sciences Consortium for logistical support. This work was supported in part by the Biological Oceanography Program of the National Science Foundation (Grant OCE-8810234) and the National Science Foundation Experimental Program to Stimulate Competitive Research (Grant R11-8996152).

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