869

DISCHARGE: EFFECTS ON MORTALITY AND FEEDING

OF AN URCHIN (LYTECHINUS VARIEGATUS)

AND A GASTROPOD (LITHOPOMA TECTUM)

E. Irlandi, S. Maciá and J. Serafy

ABSTRACT

We conducted laboratory experiments to study the effects of rapid salinity fluctua

associated with freshwater canal discharge on survivorship and grazing rates of two co

mon herbivores in South Florida seagrass beds, Lytechinus variegatus (Echinoderm

and Lithopoma tectum (Gastropoda). Urchins suffered 100% mortality when subjec

to a short-term change in salinity from 36 to 2 to 36‰, a salinity change characteristi

a major canal discharge event. Gastropod survivorship, however, was unaffected by sh

term exposure to reduced salinity. Urchins and gastropods both survived exposure 

less severe oscillation in salinity (from 36 to 18 to 36‰). Grazing rates of urchins 

snails were affected by the 50% drop in salinity, but the effects were different for e

group. Urchins demonstrated decreased grazing while gastropods increased their c

sumption of algae after exposure to reduced-salinity water. Our results suggest that w

management strategies can significantly influence field distributions and feeding of 

vertebrate grazers in South Florida seagrass beds and other areas where freshwater run

is controlled by canal locks.

In South Florida and other coastal regions, man-made canals drain low-lying l

agricultural development and for flood control in urban areas (Fig. 1). The canals 

natural surface flow and periodically release freshwater into coastal bays and 

Large volumes of water that are discharged when the flood gates are opened can 

cally change salinity at the mouths of canals over very short time intervals with s

fluctuating from full seawater to full freshwater and back up again within a ma

few hours (Fig. 2) (Fatt, 1986; Wang and Cofer-Shabica, 1988; Cofer-Shabica and 

1990). The drop in salinity is less severe with increasing distance from the canal 

and reductions in ambient salinity can be experienced beyond the immediate vic

freshwater discharge. Depending on the hydrodynamics of the area, plumes of lo

ity water drift from discharge sites and potentially influence plant and animal pop

that are distant from canal mouths (Wang and Cofer-Shabica, 1988; Cofer-Shab

Wang, 1990).

Marine animals respond to reduced salinity stress in a variety of ways. Motile 

may move out of an area when salinities are reduced (Perez, 1969; Moser and 

1989; Jury et al., 1994; 1995), but sessile and slow-moving invertebrates are at th

of the changes in salinity occurring in the surrounding water column. Many m

tightly close their operculum to limit short-term exposure to sub-optimal condition

enport, 1982). Some mollusks also have the capacity for ion regulation (Kinne

providing limited protection from osmotic stressors. Other invertebrates, includi

noderms, however, have a permeable body wall and poor ion regulating abilities 

et al., 1985) making these animals more susceptible to higher osmotic stress whe

ties are altered.
The urchin, Lytechinus variegatus, and the gastropod, Lithopoma tectum, are c

invertebrate grazers in Biscayne Bay, Florida, USA (Moore et al., 1963; Olive

pers. obs.). Based on preliminary data (Irlandi and Maciá, unpubl. data), pers. o

records from the Dade County Department of Environmental Resource Managemen

County, Miami, FL), urchin and gastropod abundance tend to be greater in areas o

and more stable salinity regimes typical of the eastern side of the bay, but they do 

Figure 1. Map of Biscayne Bay, Florida, and the numerous canals that drain agricultural f

urbanized areas of its watershed.
regions where salinities are less than full-strength seawater. Given the overwh

influence of freshwater discharge from the canal system in south Florida and the 

dynamics of the region, many areas of Biscayne Bay are subject to salinity fluctua

some degree (Fatt, 1986; Wang and Cofer-Shabica, 1988; Cofer-Shabica and Wang

even locations that support healthy populations of both species of invertebrate

effects of salinity fluctuations on survivorship and feeding behavior of echinoder

gastropods may have cascading effects on seagrass-dominated ecosystems by 

grazing rates on seagrass-associated primary producers (e.g., seagrass, macroalg

phytes). In this study we conducted laboratory experiments to examine the ef

Figure 2: Reproduction of data presented in Wang and Cofer-Shabica (1988) demonstrating 

fluctuation associated with Mowry Canal in Biscayne Bay, FL.
short-term reduced salinity stress, characteristic of the influence of canal discha

survivorship and grazing rates of L. variegatus and L. tectum.

METHODS

SALINITY REDUCTION AND SURVIVAL. — We conducted two salinity stress experiments to 

the effects of freshwater pulses associated with canal discharge on survivorship of L. va

The experiments were carried out in sixteen 18.9-L buckets arranged into groups of fou

group received water from an additional 18.9-L bucket that served as a mixing chamber 

We collected 16 urchins ranging in test diameter from 40.2 to 71.3 mm in December, 19

seagrass beds adjacent to the University of Miami’s Rosenstiel School of Marine and Atm

Science. The collection site was dominated by turtle grass (Thalassia testudinum) and 

adjacent to a canal mouth. Urchins were placed individually in each bucket and held in 

seawater for 5 d prior to use in the experiments. Twice during the 5-d period we fed the u

mix of green and decayed leaves of turtle grass. In the first experiment, we exposed ha

urchins (i.e., eight individuals) to a rapid decline in salinity (the other eight served as contro

ambient levels (36‰) to near freshwater (2‰) and then back to ambient levels to mimic 

canal discharge event. This was achieved by turning off the saltwater flow into two of th

buckets and pumping in fresh water that had been aerated for 24 h to remove water t

chemicals. We measured salinity in each of the buckets every 10 min to quantify changes 

ity. After 50 min, we resumed saltwater flow to the two experimental mixing buckets and c

to record salinity levels in all of the buckets every 10 min for an additional 70 min. Twe

hours later, we examined all animals and recorded whether they were alive or dead.

We conducted a second salinity stress experiment with a new group of L. variegatus f

the same protocol as above, except the salinity was reduced by 50% (reduction from appro

Figure 3. Schematic representation of one of the four experimental set-ups testing the e

reduced salinity on urchin and gastropod survival.
36 to 18 then to 36‰) to mimic a less drastic canal discharge event and/or lesser salinity 

that might result from plumes of freshwater drifting through the bay. In this experiment, we 

one replicate from the final analysis because detritus was clogging the water line limiting the 

of freshwater delivered to the bucket.

Also in December, 1995, we used the same experimental procedure to examine the im

freshwater discharge from canals on survivorship of the gastropod L. tectum. However, 

mortality was observed with a severe drop in salinity (see results below) we did not su

snails to a 50% drop in salinity.

Figure 4. The average salinity plotted over the duration of the experiments examining the e

reduced salinity on urchin (a: 100% freshwater dilution, b: 50% freshwater dilution) and g

(c: 100% freshwater dilution) survival. The X’s represent plots for control buckets and t

circles represent treatment buckets. Error bars represent ±1 SE. Lack of an error bar ind

variation about the recorded mean salinity.
SALINITY REDUCTION AND GRAZING. — In another series of laboratory experiments we t

effect of a non-lethal 50% reduction in salinity on grazing rates of urchins and snails. In F

1996, we collected 20 urchins (50.0-68.8 mm test diameter) and placed them in individu

plastic buckets filled with seawater and supplied with an air stone. The buckets were pla

constant temperature water bath (25°C) in a greenhouse. We emptied the water in each bu

refilled it with new seawater every other day. The urchins were fed Thalassia testudinum le

miscellaneous macroalgae (mostly Laurencia poitei, Polysiphonia echinata) for approxim

d followed by 48 h with no food. We then exposed the urchins to a short-term 50% drop in 

by emptying half of the seawater out of all of the buckets. Ten of the buckets were refi

aerated seawater (control group) while the other 10 were refilled with aerated freshwater 

mental group). We measured salinity in all of the buckets and after approximately 1 h, we 

each bucket and refilled them all with fresh seawater. We repeated the same procedur

following day resulting in two consecutive days of 1 h of reduced salinity stress. After the 

were replenished with fresh seawater on the second day of stressing, we placed 25 ml of 

echinata - 25 ml measured by water displacement in a graduated cylinder) in each bucket. 

h we determined the volume of algae left in each bucket. The change in volume of a

analyzed as the dependent variable in a one-way ANOVA. Cell means for this and all sub

experiments were tested to confirm homoscedasticity of error variances prior to anal

max, α = 0.05).

We did a similar experiment using L. tectum (14.1-17.5 mm, shell height) to determin

fects of a non-lethal 50% drop in salinity on their feeding. For this experiment, eight repli

each treatment (control and stressed) were used. We used small amounts of algae (

cervicornis) to feed the snails. It was difficult to accurately measure volume of these smal

ties of algae. Instead, we measured initial and final wet weights of the algae added to each 

The change in wet weight was calculated and used as the dependent variable in a one-way A

RESULTS

SALINITY REDUCTION AND SURVIVAL. — In the first experiment with the urchins, 

dropped from 36 to ca. 2‰ over a 30-min time interval in all eight of the exper

buckets, and remained at 36 to 37‰ throughout the experiment in the controls (F

Low salinity was maintained in the treatment buckets for an additional 20 min f

by a rapid increase back to ambient levels (36-37‰). After 24 h, 100% of the 

exposed to the drop in salinity had died while all urchins in the control buckets we

(Table 1). In the second experiment, we reduced the salinity by almost 50% over 

30-min time interval (Fig. 4b); both control and experimental urchins survived th

drop in salinity (Table 1).

L. tectum were also exposed to a similar reduction in salinity from seawater 

freshwater. Salinity dropped from 36 to ca. 2‰ in approximately 30 min and re

retawhserf=

S t seicep T nd reatmen % Dea

Lytechinus variegatus 1 80 00% FW 10

c 80 ontrol

Lytechinus variegatus 5 70 0% FW

c 80 ontrol

Lithopoma tectum 1 80 00% FW

c 80 ontrol
low for an additional 20 min before the seawater flow was restored and salinity 

turned to ambient levels (Fig. 4c). All snails survived exposure to freshwater (T

SALINITY REDUCTION AND GRAZING. — For the trials with L. variegatus the averag

ity during the freshwater stressing on the first day in the experimental buckets was 

(± 0.21 SE) while it was 38.5‰ (± 0.17 SE) in the control buckets. On the secon

Figure 5. Volume of algae consumed by control and stressed (short-term, 50% reduction in 

L. variegatus (a) and wet weight of algae consumed by control and stressed L. tectum (b

bars represent ± 1 SE and unlike letters denote a statistical difference between treatments

L. tectum
‰ (± 0.36 SE) while it remained high at 37.6 ‰ (± 0.16 SE) in control bucke

water temperature in all buckets was between 24.5 and 25.0° C on both days. Twe

hours after the second freshwater dosing, the control group had consumed signi

more (F(1,18 )= 6.5; P = 0.02) algae than those in the experimental group that had 

enced two consecutive days of freshwater pulsing (Fig. 5a).

In the trials with L. tectum the salinity in the experimental buckets dropped to 

(± 0.40 SE) while it was 36.3‰ (± 0.37 SE) in controls on the first day. On the 

day of stressing, experimental buckets averaged 18.8‰ (± 0.25 SE) while contro

aged 37.1 ‰ (± 0.13 SE). The water temperature in all buckets on both days was b

27 and 28° C. The ANOVA on the change in wet weight of algae over the 24-h 

indicated that stressed Lithopoma consumed more algae than non-stressed snails 

9.9; P = 0.0072) (Fig. 5b).

DISCUSSION

The effect of reduced salinity stress differed between gastropods and urchins. 

teristic of other gastropods, adult L. tectum showed limited influence of lowered 

on survival, while urchins suffered high rates of mortality when subjected to p

near-zero salinity water. The degree of tolerance to salinity variation, however, m

with ontogeny. For example Kinne (1971) found the eggs and larvae of other sp

gastropods could not survive low salinities that are tolerated by adults. Early s

development in urchins are also adversely affected by low salinities. Fertilizati

variegatus gametes is greatly reduced, larval development is abnormal, and larva

morphosis is suppressed at salinities lower than 30‰ (Roller and Stickle, 199

narrow range of tolerance by early invertebrate life stages to lowered salinity may 

limit the distribution of L. variegatus and L. tectum in Biscayne Bay to regions 

from canal mouths. In regions where adult populations are established, the influ

freshwater plumes that drift from canal discharge sites appears to be minimal fo

tum with no effects on survival and increased grazing rates. For urchins, howe

data show that drastic and rapid decreases in salinity are capable of killing adult 

and that while less catastrophic changes in salinity do not result in mortality, 

significantly lower grazing rates of urchins on macroalgae. Reductions in grazi

may be the result of impaired use of tube feet that occurs at sub-optimal sa

(Lawrence, 1975).

Grazing is an important process that can influence community composition a

system function in many marine ecosystems. It has been suggested that intense 

pressures in tropical and subtropical waters may be responsible for limiting algal 

thus allowing reef-building corals to persist (Potts, 1977; Brock, 1979; Sammarco

Johannes et al., 1983; Steneck et al., 1988). In temperate waters, various levels of 

by urchins maintain kelp beds and barren grounds (Ebeling et al., 1985; Harr

Reed, 1985; Watanabe and Harrold, 1991). L. variegatus and other urchins are 

cant grazers in seagrass ecosystems (Camp et al., 1973; Greenway, 1976; Ogden

Zieman and Zieman, 1989; Valentine and Heck, 1991; Klumpp et al., 1993; H

Valentine, 1995). They consume both live and dead seagrass leaves (Montagu

1991; Klumpp et al., 1993), and can create barren patches when urchin densities 
in seagrass beds (Lowe, 1975; Lowe and Lawrence, 1976; Vadas et al. 1982; Klin

Lawrence, 1984). Accumulations of drift algae often harbor large numbers of fa

provide additional structural complexity to seagrass beds (Virnstein and Howard

Kulczycki et al., 1981; Holmquist, 1994). When algal abundance is high and re

times are long, drift algae can also have deleterious effects on the underlying s

(Holmquist, 1992). Differential rates of grazing on algae brought about by period

ing of freshwater can thus directly affect both the fauna and the flora of subtropical s

communities.

Grazing by gastropods like L. tectum and other micrograzers is important in r

epiphyte loads on seagrass blades (Howard, 1982; van Montfrans et al., 1982; O

van Montfrans, 1984; van Montfrans et al., 1984; Neckles et al., 1993). Epiphy

shades the surface of the leaves and has been shown to significantly decrease gr

several species of seagrasses including T. testudinum (Irlandi, unpub. data) and 

marina (Orth and van Montfrans, 1984; Williams and Ruckleshaus, 1993; Sho

1995; Wright et al., 1995). Our data suggest that grazing of epiphytes by L. tect

periodically be increased in Biscayne Bay when populations are subjected to p

low salinity water.

Most previous investigations on the effects of fluctuating salinity on subtidal 

invertebrates have involved tidally-driven salinity changes which are more cyc

gradual than those associated with canal discharge. Canal discharge can be sud

the rates of water flow are often as high as 300 cfs in Biscayne Bay (Fatt, 1986). O

show that water management practices in South Florida can potentially influence 

distributions and grazing rates of herbivores; factors that may have cascading ef

seagrass ecosystem function.

ACKNOWLEDGMENTS

Special thanks to B. Orlando, R. Orhun, C. Schmitz, and L. Kaufman for assisting us wit

collection and running the experiments. We are grateful to T. Capo who provided tanks a

for the survival trial. Gratitude is also extended to G. Diaz and C. Rivero for graphics. T

was partially supported by funding from the Army Corps of Engineers/WES, project# DA

94-K-0032, to The University of Miami’s Center for Marine and Environmental Analyses

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DATE ACCEPTED: January 16, 1997.

ADDRESS: University of Miami, Rosenstiel School of Marine and Atmospheric Science, Di

Marine Biology and Fisheries, 4600 Rickenbacker Causeway, Miami, Florida 33149-109