Size–frequency patterns in morphometric characteristics of the seagrass Thalassia testudinum reflect environmental variability

Patterns of variability in structural characteristics of the seagrass Thalassia testudinum were investigated in 10 basins across Florida Bay, Florida, USA. T. testudinum is the dominant seagrass in this perturbed subtropical estuary and changes in its ecological condition are being used as a one of the response metrics for the Comprehensive Everglades Restoration Program (CERP). Patterns in the size–frequency distributions of a suite of morphometric and biomass characteristics were compared at two spatial scales (Bay and basin) and between the two sample years (1998 and 1999). Size–frequency distributions of shoot-specific and area-specific characteristics were more sensitive to both inter-annual and among-basin variation than measures of central tendency because of the high degree of variability at both spatial scales. Distributions of several parameters consistently grouped together into three or four pattern types. These pattern groups were spatially very similar to regions previously defined based upon biological, physical, chemical, and geological characteristics. The results of this study confirm that spatial heterogeneity in the size–frequency distributions of morphometric and biomass characteristics of T. testudinum support the concept of ecologically distinct regions within Florida Bay. The results also demonstrate the sensitivity of size–frequency distributions to differences in environmental factors and the significant control that the physical and chemical environment of Florida Bay exerts on T. testudinum morphology.