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