Environmental correlates of Thalassia hemprichii status descriptors: an evaluation of tools for diagnostic monitoring

Accurate evaluation of target organisms is fundamental to quantify responses to disturbances, elucidate thresholds and strengthen effective management. Threats to seagrasses in Singapore are excessive nutrients and sedimentation, leading to an evaluation of descriptor utility to detect plant response to nutrient and light availability using Thalassia hemprichii as the model species. A combination of popularly used seagrass physio-morphological descriptors and epibiont biomass were measured to assess plant response to environmental quality. Results showed convincing support for the use of leaf pigments as indicators of light availability and salinity changes. Water column nitrification and phosphorus availability had strong effects on leaf nutrient content, leaf morphology and epibiont biomass. Relationships between leaf length and light contradicted expected patterns, where shorter leaves were associated with lowered light. This may be attributable to hydrodynamic or sedimentation regimes and hence, is not a reliable indicator of light limitation. There was strong support for inclusion of epibiont biomass as part of nutrient response evaluation since this descriptor correlated strongly with water column nutrients and appeared to limit light availability to the seagrass. This study demonstrated the importance of integrating various levels of plant descriptors to reliably assess seagrass status, employing different combination of descriptors to monitor specific stressors.