Impacts of human disturbances on seagrass communities in the Padre Island National Seashore

Human trampling effects, such as those from wade fishermen, on seagrasses and surrounding sediments were examined using experimental trampling lanes in the Padre Island National Seashore (PAIS) in South Texas. Six sites were established throughout three regions in PAIS (Bird Island, Yarborough Pass, and Nine-Mile Hole). At each site control, low intensity, and high intensity treatments were randomly assigned to one of three 2.5-m by 5-m trampling lanes. The plots were trampled monthly from April to June 2003 and then repeated at six different sites from September to December 2003. Water column, sediment, and seagrass characteristics were measured prior to initial trampling, and two, four, and 10 months after trampling initiation. Sites trampled in spring were also sampled 16 months after initial sampling event to assess long-term seagrass recovery. After four months of spring and fall trampling, reductions in Halodule wrightii percent cover were evident in all low and high intensity lanes except for one low intensity lane during the fall trampling. By the end of the spring and fall experiments, two low intensity and six high intensity lanes remained lower than the control lanes. Resistance (condition immediately after trampling), tolerance (condition 6-12 months after trampling) and resilience (recovery over a defined period after trampling) indices were calculated to compare trampling responses in the three regions. Seagrasses in Bird Island, where higher shoot densities were consistently measured, were the most resistant to trampling in the spring (59.0%) and fall (77.5%) experiments. Yarborough Pass seagrass beds were more resilient to trampling by the end of the spring (76.3%, month 16) and fall (98.8%, month 10). The lower resilience index in the spring combined with extremely high root:shoot ratios (10.0 to 79.2) at the start of the spring experiment suggest a seasonal response to trampling. However, no long-term effects were detected as the low and high intensity lanes in this region all returned to 100% H. wrightii cover by the end of the experiment. Seagrass response in Nine-Mile Hole was extremely variable and a complete seagrass die-off in all lanes (control, low intensity, high intensity) at one site in that region occurred during the spring experiment. This die-off could possibly be a result of ammonium toxicity. Excessive porewater ammonium concentrations were measured at this site throughout the entire experiment (352 to 1000 μM) and appear to contribute to the instability of this region. Despite the general trends detected in each region, variability in the data makes it difficult to conclude with certainty whether H. wrightii was or was not affected by trampling in PAIS. Natural variability inherent to the system can be more important in determining seagrass distribution than localized disturbances