Pulsating patches: a model for the spatial and temporal dynamics of the threatened seagrass Halophila johnsonii

Abstract

The Indian River Lagoon (IRL), FL, USA, is arguably the most. biologically diverse estuarine system in the continental USA. An important. part of any marine ecosystem, seagrass beds provide food, habitat, and nursery for both vertebrate and invertebrate inhabitants. Their high productivity and sensitivity to water quality changes make seagrasses primary indicators of overall coastal ecosystem health, Halophila johnsonii (Johnson's seagrass), currently federally listed as a threatened species, is one of the rarest seagrass species, and is found within the IRL usually only approximately between Sebastian and Jupiter Inlets, continuing south to northern Biscayne Bay. Quantitative data for the present study is a Subset of the monitoring of all 7 seagrass species in the IRL starting in 1994 with 1 m(2) quadrats placed every 10 in along each transect from shore to the deep edge of the seagrass bed. The present study reports long-term data for H. johnsonii from the 35 transects within its range in the IRL sampled continuously (summer and winter) from 1994 through 2007. Our objective was to provide insight and to test. hypotheses about the dynamic nature of H. johnsonii. In addition, we developed an index of retrospective effect. size to assess the importance of ecological factors and their interactions as well as to develop a comparative basis for future seagrass studies, This erratic, sparse, but persistent species increased slightly in coverage over time but. with accompanying high variability so that regional stability over time is maintained by local unpredictability. Summer abundance on average appears to follow a 2 to 3 yr increase, then a single year's precipitous decrease over the 14 yr of our summer observations. Based upon the results from this long-term and thorough data set, we propose a new model of asynchronous, 'pulsating patches' in both space and time for describing the long-term Survival strategy of H. johnsonii.