Temporal and spatial variations in seagrass standing-stock at a meadow in Port Curtis, Queensland

Studies to assess the health of seagrass beds in Port Curtis have been undertaken since 1994. Much of this work has been directed towards a relatively small bed at Wiggins Island near the mouth of the Calliope River, and has primarily involved intensive quantitative sampling of seagrass standing stock and assessments of its temporal relationships with sediment grain size, algal cover and macrobenthic community parameters. Two species of seagrass Zostera capricorni and Halophila ovalis occur throughout the intertidal zone at Wiggins Island. The former is most prolific, however both seagrass species are temporally subordinate to mat-forming, filamentous, green algae. Both algae and seagrass communities at Wiggins Island undergo annual cycles of growth and die-back that are broadly six months out of phase. The key period of growth for seagrass occurs between August and November and coincides with increasing seawater temperatures and daylight hours; it dies-back through February and June when seawater temperatures and daylight hours are declining. Algae, in contrast, flourish at the end of summer and die-back during spring. These seasonal cycles are almost certainly the result of natural cyclic phenomena, however a range of other environmental processes (including unpredictable climatic events and biological interactions) are understood to be contributing to large inter-annual differences in standing-stock at this location. In particular, flood events in the Calliope River are known to have an immediate impact on the benthic composition at Wiggins Island. Transportation and the reworking of fine sediments during major flow events (October 1998, February 1999 and October 2001) have resulted in significant declines in the biomass of seagrass, and the abundance and richness of infaunal organisms. Whether rapid algal growth at Wiggins Island following major flood events simply represents an opportunistic response to habitat availability, or possibly a response to elevated nutrient runoff derived from the Calliope River catchment, remains to be determined.