The relative importance of light and hydrodynamics in the decay and transport of vegetative seagrass fragments

Abstract

The exchange of genetic material between seagrass meadows is fundamental to maintaining their genetic diversity and, consequently, resilience against stressors and disturbances. Increasingly, biophysical models are being used to predict the movement and dispersal of seagrass propagules (i.e. fruits, seeds, vegetative fragments) to better understand connectivity among populations. Describing how propagules behave under different hydrodynamic and environmental conditions is critical to ensuring the accuracy and reliability of such models. However, this information is lacking for vegetative fragments which have been relatively understudied compared to sexual propagules as a vector for long-distance dispersal. To address this knowledge gap, we investigated: (i) the effects of shading on the decay rates, buoyancy and morphometrics of fragments in two tropical seagrass species (Halophila ovalis and Thalassia hemprichii) over time; and (ii) the transport speeds of fragments of four species (T. hemprichii, Cymodocea rotundata, H. ovalis and Halodule uninervis) under varying wind, wave and current conditions in a flume. Our study revealed that higher shading resulted in a faster rate of fragment decay and buoyancy loss over time. Various morphometric measurements also decreased with lower light levels after the month-long experiment. Through our flume experiments, we found that winds can contribute to the transport of seagrass fragments, although the average wind drag of 0.915% was lower than the 4% previously documented for reproductive seagrass shoots. Additionally, small waves (those with smaller wave height and lower frequency) led to faster transport speeds than large waves. The results of this study provide important experimentally-obtained parameters that are crucial for the development of accurate predictive models for seagrass fragment dispersal.