Gap Dynamics in a Seagrass Landscape

Abstract

We investigated gap dynamics within a shallow subtidal landscape characterized by seagrass vegetation and examined the relationship between gap formation and selected physical factors. The study was conducted over 2 y by using a biannual mapping of seagrass and water depth across an 48,800-m2 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 stands of Halodule wrightii. Gaps ranged in size from 10 to 305 m2 and most frequently decreased in size over time. Most gaps were small and short lived (less than 6-mo duration), but the second age group most frequently recorded was at least 1.5 y old. No new species of seagrass invaded the gaps with Halodule replacing itself 100% of the time. Gaps were recorded over the entire range of water depths within the landscape. Neither gap area nor persistence of gaps was related to water depth. However gap area was associated positively with the number of extreme sedimentation events. Gaps originated not only from removal of interior vegetation (similar to classic gaps) but 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 importance 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 assessment of disturbance intensity in this system, and the seagrass mosaic probably is explained best by a combination of disturbance regimes and edaphic factors, such as sediment stability. Moreover, we suggest that even in areas characterized by monospecific stands of vegetation and over short or moderate time periods, gaps indirectly may influence community structure and ecosystem function via modification of habitat arrangement.