Monitoring seagrass changes in Indian River Lagoon, Florida using fixed transects

Long-term monitoring of fixed transects is an important management tool for determining the health of seagrass ecosystems. In-the Indian River Lagoon, Florida, semiannual monitoring (summer-winter) of 76 fixed transects was established in 1994 to quantify changes in the seagrass. The transects are perpendicular to shore along the 250-km-long axis of the lagoon. Transects are sampled using non-destructive techniques to measure percent cover and canopy height, of each seagrass species every 10 m along the line until the deep edge of the bed is reached. Shoot counts are also made at the center and deep edge of the beds at each transect location. Halodule wrightii was found to be the dominant species on a lagoon-wide scale. It was present in every transect but one and comprised, on average, 36% cover. Other species found included: Syringodium filiforme (9.5% cover), Ruppia maritima (5.2% cover), Thalassia testudinum (2% cover), Halophila johnsonii (2% cover), Halophila decipiens (0.6% cover), and Halophila engel mannii (0.5% cover). Percent cover was significantly lower in the winter compared to the summer for Halodule wrightii, Halophila decipiens, and Halophila engelmannii. Canopy height was significantly lower in the winter for Halodule wrightii, S. filiforme, T. testudinum; and Halophila decipiens. T. testudinum and Halophila johnsonii reached their Atlantic coastal northern limit in the mid-lagoon region. During the period 1994 to 1998, seagrass beds in the lagoon increased in abundance in most areas; at 65 of the 76 transect sites; seagrass beds increased in width. On average for all sites, the distance from shore to the deep edge of the seagrass beds increased 26% during this period. At some specific locations the increase in transect length was quite dramatic. In the Cocoa Beach segment of the lagoon, for example, average transect length increased 207%. Year-to-year, lagoon wide variability in seagrass abundance was high, especially that of the less abundant species such as R. maritima, Halophila engelmannii; Halophila decipiens, and Halophila johnsonii where increases in cover exceeded 500% in some cases. Intermittent droughts and hurricanes also had significant impacts on annual seagrass abundance. This study illustrates how long-term, repeated survey measurements are necessary to develop an understanding of the natural variability of seagrass systems, and ultimately, how to separate this natural variability from that induced by anthropogenic causes.