Dynamics of a subtidal seagrass landscape: Seasonal and annual change in relation to water depth

The spatial heterogeneity of a subtidal marine landscape and the areal extent of both monospecific and mixed patches of seagrass species were studied in Tamps Bay, Florida, USA. Specifically, we examined the temporal dynamics of seagrass distribution and its relationship to water depth and the serial replacement of one species by another. The similar to 5-ha landscape was mapped at l-m intervals in the spring and fall of 1994 and 1995. The landscape consisted of monospecific and mixed patches of seagrass (47%) and bare sediment (53%). Halodule wrightii was the most abundant seagrass (similar to 74%), while Thalassia testudinum was second most common (20%), and mixed patches of H. wrightii and T. testudinum composed the remaining 6%. There was an overall increase in seagrass of 14% from spring 1994 to fall 1995. The majority of change occurred along the margins of existing seagrass patches (i.e., H. wrightii invading bare sediment). Typically, "new" patches were the result of the transition of one seagrass species to another (e.g., H. wrightii replacing T. testudinum). Water depth over the landscape ranged from 30 to 100 cm, and although seasonal differences in the distribution of seagrass species were detected, water depth in this landscape was not an adequate predictor of the observed distributional patterns of either seagrass species at any season. Temporal transitions of one seagrass species to another, or to/from bare sediment, did nor fit previously published models of seagrass succession. Instead, H. wrightii, the competitive "inferior," often replaced T. testudinum as the dominant seagrass, suggesting that factors other than competition may have a controlling influence in this landscape. We suspect that hydrodynamics (e.g., seasonal storm events) may be an organizing force in maintaining the landscape's heterogeneity.