Monitoring a sea urchin overgrazing event in Outer Florida Bay
William C. Sharp and John H. Hunt
Florida Fish & Wildlife Conservation Commission
Florida Marine Research Institute, Marathon, FL
In August 1997, an extraordinarily dense aggregation of the variegated urchin, Lytechinus
variegatus Lamark, was discovered within the extensive manatee grass (Syringodium filiforme
Kützig) meadow in Outer Florida Bay, approximately 19 km north of Marathon, FL. By the time
of its discovery, the aggregation had either completely defoliated or severely damaged
approximately 9 km2 of the meadow. When the damaged portion of the seagrass meadow was
discovered in Florida Bay, the population dynamics of the urchin aggregation and the short-term
impacts of their grazing on the seagrass and sediment were assessed. This work revealed that
this urchin aggregation was composed of a single cohort that had consumed all of the aboveground seagrass biomass and damaged much of the below-ground seagrass biomass in an area
estimated to be approximately 9 km2, causing the depletion of the associated molluscan
communities and the resuspension of fine-grained sediments. Because the event potentially
posed the threat of additional larger-scale ecosystem perturbations, we continued periodic
monitoring of urchins and the long-term impacts on the seagrass meadow.
We observed that the urchin cohort continued to damage the meadow through much of 1998, but
by late that year, the formerly well-formed aggregation had clearly becoming more diffuse. The
remnants of the urchin aggregation could still be detected in 1999, though the mean urchin
density was approximately an order of magnitude lower than 1997. Yet, even at these lower
densities, visual assessment by divers revealed continued decreases in seagrass biomass in the
area of this aggregation, suggesting urchin abundance remained sufficient to damage the seagrass
bed. Moreover, by late 1999 the size-frequency of the urchin population indicated that it was
now composed not only of urchins from the cohort responsible for the overgrazing event, but
also of smaller, presumably younger urchins. However, we could not assess if this younger
cohort was present in numbers sufficient to cause prolonged damage to the meadow.
Additionally, commercial spiny lobster fishermen reported unusually large numbers of L.
variegatus within their traps located in nearby areas of the meadow.
Consequently, in 2000, we established additional monitoring sites and continued to evaluate
urchin population dynamics and their effect on the seagrass bed. We also monitored the seagrass
community within areas of the meadow that had previously undergone intense defoliation. We
established sites in a spatially stratified manner according to the relative damage to the seagrass
bed caused by the overgrazing event during 1997-98 and established four sites in the portion of
the meadow that had been completely (or nearly) defoliated during September 1997 (hereto
referred to as “Defoliated 97"), two in an area that urchin overgrazing had visibly reduced the
seagrass biomass, but where sparse seagrass remained (“Overgrazed 98"), and two in an area that
were undamaged by urchins (“Undamaged”). Each site was sampled during the winter (Dec Feb) and summer (August - October) during both 2001 and 2002.
Our initial sampling conducted during December 2000 detected no unusually dense aggregations
of urchins and no signs of continued overgrazing. Given the life span of L. variegatus, which
has been estimated to be approximately four years, we concluded that a large portion of the
urchin aggregation responsible
for the overgrazing event of
Figure 1 Boxplots comparing seagrass biomass at
1997-98 had perished through
Defolatied 97, Overgrazed 98, and Undamaged sites from
the Winter 2001 sampling period (Dec – Feb 2001) through
senescence by 2000, and that
the Summer 2002 sampling period (Aug 2002).
the abundance of urchins
originating from subsequent
cohorts were not present in
densities sufficient to cause
additional damage to the
meadow. Seagrass biomass
had increased at both
Defoliated 97 and Overgrazed
98 sites from early 2001 to the
summer of 2002, but remained
greatly reduced compared to
sites that had been not been
impacted by urchin grazing
(Figure 1).
800
Defoliated 97
Biomass (dry wt. g/m 2)
700
Overgrazed 98
600
Undamaged
500
400
300
200
100
0
N=
12
12
12
Winter 2001
12
12
12
Summer 2001
12
12
12
Winter 2002
12
12
12
Summer 2002
Because the effects of urchin
overgrazing on S. filiforme
remains poorly understood we
will continue to evaluate our monitoring sites annually and will be conducting manipulative field
experiments to further evaluate the effect of urchin herbivory on this seagrass.
William Sharp, FWC, FMRI, 2796 Overseas Hwy, Suite 119, Marathon, FL 33050, Phone: 305289-2330, Fax: 305-289-2334, bill.sharp@fwc.state.fl.us, Question 5 - Higher Trophic Levels