Gap dynamics in a seagrass landscape

We investigated gap dynamics within a shallow subtidal landscape characteri zed by seagrass vegetation and examined the relationship between gap format ion and selected physical factors. The study was conducted over 2 y by usin g a biannual mapping of seagrass and water depth across an 48,800-m(2) area in Tampa Bay, Florida. In addition, monthly sediment deposition or erosion was recorded at 96 locations within the landscape. Gaps represented from 2 .4% to 5.7% of the seagrass landscape, and all were within monospecific sta nds of Halodule wrightii. Gaps ranged in size from 10 to 305 m(2) and most frequently decreased in size over time. Most gaps were small and short live d (less than 6-mo duration), but the second age group most frequently recor ded was at least 1.5 y old. No new species of seagrass invaded the gaps wit h Halodule replacing itself 100% of the time. Gaps were recorded over the e ntire range of water depths within the landscape. Neither gap area nor pers istence of gaps was related to water depth. However gap area was associated positively with the number of extreme sedimentation events. Gaps originate d not only from removal of interior vegetation (similar to classic gaps) bu t also from differential growth of the seagrass margin (similar to edaphic gaps). Distinct seasonal components to the mode of formation were detected with interior-produced gaps originating primarily in the winter and margin gaps most commonly during summer. These results combine to illustrate the i mportance of large-scale studies with fine-scale resolution for deciphering unique features of seagrass landscape dynamics. Our historical information suggests that a static enumeration of gaps may not provide an accurate ass essment of disturbance intensity in this system, and the seagrass mosaic pr obably is explained best by a combination of disturbance regimes and edaphi c factors, such as sediment stability. Moreover, we suggest that even in ar eas characterized by monospecific stands of vegetation and over short or mo derate time periods, gaps indirectly may influence community structure and ecosystem function via modification of habitat arrangement.