Response of a mixed Philippine seagrass meadow to experimental burial

Abstract

The effect of burial due to sudden sediment loading was examined in a mixed Philippine seagrass meadow through the experimental deployment of sediment (0, 2, 4, 8, and 16 cm deposited over the experimental plots). The responses in shoot density, vertical growth, and branching of the species present were assessed 2, 4, and 10 mo following disturbance. Shoot density responses were strongly species-specific. The large Enhalus acoroides maintained shoot density at all burial treatments, and only showed evidence of decline by the end of the experiment. Thalassia hemprichii and, to a lesser extent, Cymodocea rotundata showed a sharp decline in shoot density even at moderate burial treatments, from which they failed to recover. The accompanying species (Halodule uninervis, Syringodium isoetifolium, and Cymodocea serrulata) showed an initial decline in shoot density followed by recovery. The small Halophila ovalis showed an opportunistic growth in plots receiving intermediate (buried by 4 and 8 cm sediment) disturbance, reaching shoot densities well in excess of those on control plots. The results suggest a pattern of species loss following disturbance by sediment burial corresponding to the sequence, T. hemprichii -> (C. rotundata, S. isoetifolium, H. uninervis) -> C. serrulata -> E. acoroides. Vertical growth increased significantly for all species with differentiated vertical shoots, except C. serrulata. The examination of the time course of vertical growth imprinted on the shoots of the dominant species, T. hemprichii, revealed a rapid response to burial through increased internodal length, which was maintained over 8 mo following the disturbance. The resulting cumulative vertical growth along the experiment was linearly correlated with the degree of burial imposed on the plants. Branching of vertical shoots also increased significantly (73 to 96%, depending on the species) with burial. Experimental burial induced changes in shoot age distribution of some of the species, involving rearrangements, through selective mortality or recruitment, of the contribution of young shoots to the populations. The results obtained show major differences in species response to small-scale disturbance, closely linked to predictions derived from consideration of species growth rate and size, and provide evidence of the importance of small-scale disturbance in the maintenance of multispecific seagrass meadows.