Seagrass Resources and Condition Within Padre Island National Seashore, Texas: A GIS Analysis

Onuf, Christopher; Ingold, Jaimie

A survey of the seagrass resources of Padre Island National Seashore was conducted in fall 2002 and 2003, with additional sampling through 2006 to resolve questions. GIS overlays on U.S. Geological Survey orthophoto quads were made of depth with respect to mean sea level and frequency of occurrence of seagrass out of five samples collected for every station sampled. These data were used to draw boundaries of area submerged at mean sea level and seagrass meadow, in relation to the boundary of the National Seashore. Frequency of seagrass occurrence, mean plant height, shoot density, plant height * shoot density, live biomass, and dead biomass on a 1’ latitude by 0.25’ longitude grid were collected and their distribution plotted in space and according to depth. Seagrasses covered almost two-thirds of the regularly flooded part of Laguna Madre within the borders of the seashore. Comparisons with earlier surveys showed that substantial areas of seagrass cover had been lost in deep water between 1988 and 1998 as a result of a persistent phytoplankton bloom, and little recovery has occurred since. Maximum depth of seagrass occurrence responded to changes in water clarity. In contrast, much of the cover at shallow to intermediate depths lost at the south end of the study area between 1988 and 1998 was replaced by 2003. The seven stations withgreatest plant height all were located in this area of recent recolonization. Continuity ofcover as measured by frequency of occurrence was high except near the edge of seagrassmeadow. Decrease in this measure may be an indicator of meadow fragmentation,3signaling deterioration of seagrass meadow before loss. The other measures of condition were so variable as to be insensitive indicators of impending change.