Population structure of a newly recorded (Halodule uninervis) and native seagrass (Halophila ovalis) species from an intertidal creek ecosystem

The presence of the seagrass Halodule uninervis was documented for the first time outside the Chilika lagoon at Haripur creek, along with previously documented Halophila ovalis from the same region. The population structure (density; biomass; leaf growth rate; and morphometrics) and environmental parameters was assessed for both seagrass species. The pH levels of the water column above both seagrass meadows were lower than the standard oceanic pH of 8.2, likely attributable to upstream freshwater influences. Halophila ovalis grew on sandy and H. uninervis on silty substrates. High nutrient (nitrate and phosphate) concentrations in the water column of the creek influenced the growth of macroalgae (Ceramium sp. and Gracilaria verrucosa) on the leaves of H. uninervis. Leaf reddening was only observed in H. ovalis leaves. Under similar environmental conditions, the shoot density and biomass (above- and below-ground) of H. ovalis were 2-fold lower that for H. uninervis. The leaves of H. uninervis were 9-fold longer than H. ovalis, whereas H. ovalis had 5-fold wider leaves than H. uninervis. The leaf plastochrone interval of H. ovalis (2.3 days) and H. uninervis (9.6 days) was similar for the Indian Ocean region. Consequently, the H. ovalis leaf growth rate was 2-fold lower than that for H. uninervis. H. ovalis shoot internodes were 2.6-fold longer than for H. uninervis, whereas the root lengths of H. uninervis were longer than for H. ovalis. The shorter root length of H. ovalis resulted in a higher branching frequency than for H. uninervis. The total C and N content were higher in the H. ovalis leaves than those of H. uninervis. This study suggest seagrass responses to similar environmental conditions are species-specific. Regular assessment of the seagrass population structure and water quality monitoring of Haripur creek are essential for better management and conservation of these important seascapes.