Gap Dynamics in a Seagrass Landscape Susan S. Bell,* Bradley D. Robbins, and Susan L. Jensen Department of Biology, University of South Florida, Tampa, Florida 33620-5150, USA ABSTRACT We investigated gap dynamics within a shallow subtidal landscape characterized by seagrass vegeta- tion and examined the relationship between gap formation and selected physical factors. The study was conducted over 2 y by using a biannual map- ping 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 stands 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 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 land- scape. 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 originat- ing 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 histori- cal information suggests that a static enumeration of gaps may not provide an accurate assessment of disturbance intensity in this system, and the sea- grass mosaic probably is explained best by a combi- nation 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 commu- nity structure and ecosystem function via modifica- tion of habitat arrangement. Key words: gaps; landscape; patch dynamics; sea- grasses; sedimentation; Florida; Tampa Bay; Halod- ule wrightii. INTRODUCTION Gaps, commonly defined as openings created in structured environments by disturbance events (for example, Whitmore 1989), have received consider- able attention in terrestrial settings (Brokaw 1987; Lawton and Putz 1988; Canham 1989; Kubo and others 1996) but less so in aquatic ecosystems (Kay and Keough 1981; Karlson 1983; Sousa 1984). Whereas most information on gaps comes from investigations on vegetation, some data on gaps in benthic faunal assemblages are also available (for example, Paine and Levin 1981). Studies of gap dynamics have provided insight into comparative life history features of both the organisms that colonize gaps early in succession and those more typical of later stages (for example, Dayton 1971; Shumway and Bertness 1994). The contribution of gaps to the maintenance of species diversity (see Petraitis and others 1989) is a common theme among the investiga- tions on this topic. Received 17 September 1998; accepted 26 April 1999. *Corresponding author: e-mail: sbell@chumal.cas.usf.edu Ecosystems (1999) 2: 493–504 ECOSYSTEMS  1999 Springer-Verlag 493