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2. Methods and materials
2.1. Field site The field site, West Point, is a dense subtidal Thalassia testudinum bed in northern
Biscayne Bay, Florida, USA Fig. 1. Temperature at the field site ranges annually from 18 to 318C and salinity from 30 to 43‰, with a water depth of approximately 1 m. The
sea urchin Lytechinus variegatus is the most conspicuous invertebrate member of the seagrass community. Monthly surveys were conducted at the site from September 1995
to September 1998. At each survey five parallel 50-m transects were laid out 5 m apart,
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and five 1-m quadrats were randomly selected along each transect. All urchins within these 25 quadrats were counted except for the first two surveys, in which only 15 and
ten total quadrats i.e. three and two transects, respectively, were counted.
2.2. Cage design
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Circular cages with an area of 2 m were used. Cages were constructed of 5 m lengths of stiff polyethylene netting Vexar with a mesh size of 3.2 3 3.2 cm. The sides of the
cages were 60 cm high. Each cage had four 90-cm long pieces of steel reinforcement bar rebar attached with cable ties at regular distances from each other. The rebar protruded
Fig. 1. Map of north Biscayne Bay, Florida showing the West Point field site.
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approximately 25 cm from the bottom of the netting for burial in the sediment. Cage tops were made of thin 0.5 mm diameter flexible polypropylene netting with a mesh
size of 3.2 3 3.8 cm. The thinnest possible netting was used for the cage tops in order to minimize shading effects. This netting was sewn onto the top of the cage side with
plastic weed trimmer line.
Several control treatments were employed to test for caging artifacts. Partial cages were constructed from 3.8 m lengths of netting and four pieces of rebar, as above. These
partial cage controls did not form a completely enclosed circle, having instead an opening of approximately 1.2 m 25 of the circumference. Two types of partial cage
controls were used: with a top as described above, and with no top. Top controls were also deployed; these were constructed by attaching a top to four pieces of rebar alone
i.e. no cage sides. Unmanipulated controls consisted of plots marked only by rebar. To test for cage effects on water movement, current speed immediately above the seagrass
canopy approximately 30 cm from the bottom was measured both inside and outside of the full cages. Red dye was released into the water column and the time to travel 50 cm
was measured. Dye within the cages was released just inside the cage and allowed to travel towards the opposite side. There was no significant difference between current
speed inside and outside of full cages ln-transformed data: t 5 1.75; df 5 10; p 5 0.11.
2.3. Experimental design Caging experiments were performed twice, in winter and summer. Experimental
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treatments were 0 urchins per m , 10 urchins m
20 cage, and 20 urchins m 40 cage. During the winter and fall months, the field site experiences a bloom of drift
algae primarily Laurencia spp. and Dictyota spp.. These algae disappear in the warmer months Irlandi, unpublished data. To account for the effects of these drift algae, the
winter experiment included algae and no-algae treatments 2algae for each of the experimental urchin densities. Thus, the total number of treatments were: for the
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summer, seven 0, 10, and 20 urchins per m , unmanipulated control, partial cage 1
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top, partial cage 2 top, top only; for the winter, ten 0 per m 1 algae, 0 2 algae,
10 1 algae, 10 2 algae, 20 1 algae, 20 2 algae, unmanipulated control, partial cage 1 top, partial cage 2 top, top only.
There were six replicates for each treatment. The cages were allocated, using a random number generator, to positions on a rectangular grid winter: 10 3 6 points;
summer: 7 3 6 with 1-m intersection intervals. After the cages and control treatments were set out, preliminary surveys were conducted within each replicate. Epifaunal
invertebrates visible to the naked eye were visually identified and counted over a period
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of approximately 15 min. Shoot counts were conducted in a haphazardly located 0.04-m quadrat within each experimental plot. The quadrat was permanently marked with
colored flags so that it could be followed throughout the duration of the experiment. After the initial survey, the appropriate number of urchins was added or removed from
each cage. The size range of the added urchins, which were collected from a nearby site in Biscayne Bay, was similar to that of the population at West Point. The number of
urchins in cage control plots was not altered. In the winter experiment, algae were removed from those cages designated as no-algae. Tops were sewn on immediately after
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addition or removal of urchins and algae. Summer cages were checked every 2–4 days during the experiment. Winter cages could not be visited as often because of weather
conditions, but were checked approximately once a week. Most cages had fewer than five escaped urchins at any given inspection, but at six inspections there was one cage
with more than 15 urchins missing. All missing urchins were replaced as necessary. Invading algae in the no-algae treatments were also removed as necessary approximate-
ly once every 10 days. Replacement of algae in 1 algae treatments was required in only five instances.
Each experiment ran for 6 weeks: August–September 1997 summer and January– February 1998 winter. Cages were established in the same general area on both
occasions. At the end of the 6 weeks, each plot was resurveyed. Visual counts of epifaunal invertebrates were repeated, as well as shoot counts within the permanently
marked quadrat. Within each plot a core 15 cm diameter was taken to a depth of 25 cm. Prior to penetrating the sediment the shoots to be included in the sample were
manipulated into the corer to ensure effective collection of all aboveground biomass. Core samples were sieved through a 0.5-mm mesh sieve. Seagrass samples were
separated into live below- and above-ground biomass including epiphytic algae, dried to constant weight at 708C, and weighed. All infaunal macroinvertebrates collected in
the cores were identified and counted.
2.4. Data analysis The methods employed for surveying invertebrates were effective only for infauna
and for large, slow-moving epifauna e.g. gastropods, sponges, echinoderms; highly mobile species, such as crustaceans, were not included in the study. The four most
common species were chosen as representative of the invertebrate population. Because of the low population densities found for most species see Results, the non-parametric
Kruskal–Wallis test was used to compare abundances among treatments at the end of the experiment. Given the lack of mobility of sponges, their abundance was counted at the
beginning and end of the experiment and compared for each treatment using a paired t-test.
Assumptions of normality and homoscedasticity were tested with the Shapiro–Wilk test and the Bartlett test, respectively
a 5 0.05, prior to all parametric statistical analyses. Because shoot counts were conducted within the same permanently marked
quadrat at the beginning and end of the experiments, these data were compared individually for each treatment with a paired t-test. To test for cage effects, above- and
below-ground seagrass biomass of the unmanipulated control and the three cage control treatments were compared with a one-way ANOVA. Because lack of a significant
difference among the cage controls indicated that there was no cage effect on seagrass biomass see Results, the cage control treatments were not included in further analyses.
For the summer experiment, a one-way ANOVA comparing biomass in unmanipulated control plots and the three urchin density treatments was performed. For the winter
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experiment, a two-way ANOVA was used, with urchin density 0, 10, or 20 per m and algae presence or absence as fixed factors. Control plots could not be included in
this analysis because algae were not manipulated within them, thus they lacked
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appropriate replication for the algae factor. Because no algal effects were detected see Results, the 1 and 2 algae treatments were pooled into their respective urchin
