Potential use of marine bivalves as coastal ecosystem restoration tools under different salinity regimes

Abstract

Bivalve mollusks play an important role in coastal ecosystems and provide valuable ecosystem services to the marine environment by filtering water, serving food source and providing habitat for other marine species. Due to the filtration processes of the bivalves, they have been used worldwide as coastal ecosystem restoration tools to control eutrophication by consuming suspended organic nitrogen and microalgae. Such restoration strategy can be considered and applied to Hong Kong coastal waters, which cover a wide range of salinity regimes and a wide distribution of microalgae species. Before deploying bivalves as bio-filtration devices, it is necessary to investigate the interacting effect of salinity and algae species on the filtration capacity of the bivalve species in Hong Kong waters, in order to optimize the filtration process of different bivalve species and hence lead to improvement of water quality. However, over the past, only very few studies reported the interacting effects of these two parameters on the filtration capacity of local bivalves. This study aimed to investigate the combine effect of salinity and algal species on filtration capacity of four common bivalve species, and provide empirical information for selection of suitable culture site of different bivalve species at various salinity regimes in Hong Kong waters. To achieve this purpose, the Pacific oyster Crassostrea gigas, the Hong Kong oyster Crassostrea hongkongensis, the green lipped mussel Perna viridis and the pearl oyster Pinctada martensii were chosen as model species, which are commonly found in local coastal areas. Following standardized experiments and standard mesurement methods, their clearance rates were determined so as to evaluate their filtration capacity under five different salinities (i.e., 15, 20, 25, 30 and 35 ppt). At each test salinity, bivalves were fed with marine algae Isochrysis galbana (small in size) and Chaetoceros gracilis (large in size), respectively. The results showed that when fed with C. gracilis, C. hongkongensis had significantly higher clearance rates at all test salinities. When fed with I. galbana, P. martensii showed significantly higher clearance rates at salinities from 15 to 30 ppt. C. gigas showed higher clearance rate at 35 ppt. Based on these results, both C. hongkongensis and P. martensii are recommended to be used as restoration tools in Hong Kong waters. Particularly, C. hongkongensis is better for waters that contain larger microalgae species (e.g., C. gracilis); P. martensii is better to be cultured where small-size microalgae (e.g., I. galbana) are rich in water column. The significance and implication of the current results were finally discussed with reference to relevant studies, and the development of eco-shoreline in Hong Kong and beyond.