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 exte nt 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 seria l 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 landsc ape consisted of monospecific and mixed patches of seagrass (47%) and bare sediment (53%). Halodule wrightii was the most abundant seagrass (similar t o 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 1 995. The majority of change occurred along the margins of existing seagrass patches (i.e., H. wrightii invading bare sediment). Typically, "new" patch es were the result of the transition of one seagrass species to another (e. g., H. wrightii replacing T. testudinum). Water depth over the landscape ra nged from 30 to 100 cm, and although seasonal differences in the distributi on of seagrass species were detected, water depth in this landscape was not an adequate predictor of the observed distributional patterns of either se agrass species at any season. Temporal transitions of one seagrass species to another, or to/from bare se diment, did nor fit previously published models of seagrass succession. Ins tead, H. wrightii, the competitive "inferior," often replaced T. testudinum as the dominant seagrass, suggesting that factors other than competition m ay have a controlling influence in this landscape. We suspect that hydrodyn amics (e.g., seasonal storm events) may be an organizing force in maintaini ng the landscape's heterogeneity.