An assessment of seasonal abundance of Halophila ovalis (R. Brown) Hooker f. and potential for conservation in Hong Kong

Abstract

Seagrasses are a group of marine flowering plants which are species of conservation interest due to their high ecological value and their globally declining populations. Currently, the five seagrass species in Hong Kong are potentially threatened by human factors such as coastal development and water pollution. However, little research has been conducted on the biology of seagrasses in Hong Kong, focusing mainly on Zostera japonica. It is important to fill in the information gap of the other seagrass species for conservation as their environmental requirements are different. This study, therefore, aims to investigate the seasonality of Halophila ovalis at San Tau, which is the most common seagrass species in Hong Kong, relate seasonality to environmental conditions, and assess the potential for conservation.

Monthly monitoring of the H. ovalis seagrass bed at San Tau, Lantau was conducted from January 2017 to March 2018. Seagrass abundance, including the percentage cover, total seagrass bed area, leaf density and leaf size were measured using quadrats, GPS unit, core samples and ruler, respectively. Seedling production, flowering and fruiting were also observed during each visit. Associated macro-epifauna were enumerated using quadrats and transects, and the epifauna abundance on seagrass bed was compared with the adjacent bare mudflat, and related to seagrass abundance. To assess the feasibility of transplantation, cores of seagrass were transplanted in January and April 2017 and the growth of the transplanted seagrasses and the donor sites were monitored each month. Various environmental parameters, including seawater temperature, salinity, dissolved oxygen level, water nutrient content, turbidity, suspended solid, tidal regime, were recorded using a combination of in situ data loggers, portable water quality meters during each visit, weather data from Hong Kong observatory website, and water quality data of the North Western Water Control Zone from Environmental Protection Department (EPD). These environmental factors were compared with the changes in seagrass abundance using principal correlation analysis to investigate which parameters affecting the growth of H. ovalis.

H. ovalis at San Tau showed distinct seasonality in percentage coverage. The change in percentage coverage was most negatively correlated to suspended solid concentration and turbidity of seawater, and positively correlated to light intensity and temperature. It was likely that light was the most limiting factor and that high suspended solid and turbidity negatively affected the growth of the seagrass by reducing photosynthesis. The results provide baseline data for future monitoring of seagrass. The seagrass bed was completely disappeared around late July 2017, which was most likely due to tropical cyclone, and signs of recovery were absent as of March 2018. The transplanted seagrasses were able to expand after 4 months in similar habitat, while the donor site showed a very slow recovery rate, which indicated that transplantation is not a good mitigation measure to conserve seagrasses in Hong Kong due to the small population sizes and the potential damage to donor sites.

The seagrass bed was dominated by two common mud snail species, Pirenella asiatica and Batillaria zonalis, and the seagrass abundance was negatively correlated with abundance of these fauna. The epifaunal abundance within the seagrass bed was 1.4-5.6 times higher than that on bare mudflat, which demonstrated the ecological value of seagrass bed.