Analysis Of Ecological Effects Of Southern Dalajia Island Artificial Reef Area In Daya Bay, Guangdong, China

In order to rehabilitate the ecological environment and increase fisheries resources of Day Bay, Guangdong Ocean and Fisheries Administration has deployed 1829 reinforced steel-concrete hollow reefs in Daya Bay during the period of December 2002-August 2004. All modular reefs were cast in two batches within an area of 6.8 km² in the southern part of Dalajia Island, with a total volume of 60178m³.Four comprehensive surveys in the reef area were made including one cruise prior to casting reef (November 2002) and other three cruises post-cast in July 2004, August 2005 and November 2006. The ecological effects of artificial reefs were analyzed ecosystem-based using ecosystem health evaluation and ecosystem services valuation, through characterization of shifts in nutrients and the main biota of the area. The results showed that:1) Following deployment of the artificial reefs, concentration of COD, DIN, andPO₄^3- -P in the reef area were higher than those pre-deployment of reef, COD, DIN andactive phosphate increased by 73-149%, 172-224% and 14-129% respectively. Suspended solids concentrations were reduced. These indicators for all sites reach the first category water quality standards of China. Hence, artificial reefs appear to improve water quality. The N:P ratio before reef placement approximated Redfield (16.29) indicating no P limitation, while N/P ratio significantly increased and substantially greater than the Redfield ratio after reef placement (ranged from 39.63 to 116.17) suggesting P-limitation. Consequently, growth of phytoplankton was limited prominently by phosphorous. With these increases, potential eutrophic conditions still prevailed.2) Surface sediments of reef areas were principally clay-silt and sandy-silt. Following reef deployment, pH and Hg content in surface sediments of the reef area decreased, while contents of organic matter, Cu, Pb, Zn and Cd increased to some degree, withmeans appearing higher than those of control site. In July 2004, lead content of a station was a little more than the first category sediment quality standards of China (levels exceeded standards by 78 percent). In November 2006, organic matter content of three stations and Lead content of one station (exceeded standards by 0.52-fold) were slightly more than China's first category sediment quality standards. Cu, Pb, Zn, Cd and Hg in surface sediment were so minimal that potential ecological risks of these elements were low.3) The species and density of phytoplankton in the reef area were mainly diatoms as all dominant species belonged to Bacillariophyta. Seasonal changes in dominant species were obvious. Additionally, an inter-annual variation in phytoplankton community structure was evident. After reef deployment, densities of phytoplankton slightly increased year after year (ranged from 441.05×10⁴ cell/m³ to 711.1×10⁴cell/m³). There was little difference in phytoplankton densities between reef areas and control site during the survey period. The maximum concentration of chlorophyll a in the reef area was noted prior to artificial reef casts. However, maximum primary productivity after reef deployment occurred in July 2004, with slightly higher in the surface layer than that of bottom layer.4) The post-deployment period was also typified by an increased in species and biomass of zooplankton in the reef area year by year. Densities of zooplankton were significantly higher than before the reefs were deployed, and densities and biomass of zooplankton were higher in the reef area than noted at the control site. The dominant species of zooplankton were quite variable, with no consistent community structure pattern for the zooplankton. In August 2005, the density of zooplankton in the reef area (1260.2 ind/m³) exceeded a relatively high historical level (1013.4 ind/m³ in 2004), but biomass (154.2 mg/m³) was lower than historical levels (472.8 mg/m³ in 2004). This shift reflected a switch to dominance of small-sized zooplankton.5) Following reef deployment, species and quantities of ichthyoplankton in the reef area exceeded those noted prior to deployment. Further, there were more species and numbers of ichthyoplankton in the reef area than in the control site during the same period. Fish eggs of family Sparidae, the main taxon accounted for 60.9% of the total fish eggs found in November 2006. We concluded that artificial reefs have an apparent protective performance on fish eggs and larval and juvenile fish.6) The main groups of benthic invertebrates in the reef area were molluscs, crustaceans and annelids, most of which belong to tropical and subtropical warm water species and inshore widely distributed species. 2-3 years after reef deployment, the biomass and densities of macrobenthos in the reef area had substantially increased compared to pre-deployment levels, and levels at the reef areas were higher than observed at the control site. Hence, proliferation performances of artificial reefs on zoobenthos are obvious.7) Nekton species increased by 64-91% and densities increased by 12.13-21.22 fold in the reef area surveyed by trawl net. Nekton species increased by 13.33-46.67% and catch rates increased by 36.96-70.80 fold in the reef area investigated by gill-net. Species, resource densities and catch rates within reef area were higher than those of the control site. The densities and catch rates of the dominant species markedly increased. Hence, artificial reefs played an important role in resource conservation.8) The southern Dalajia Island artificial reef ecosystem remained healthy throughout the survey period. With increasing time, it is anticipated that ecosystem health, through the parameters described above, will be further improved.9) The southern Dalajia Island artificial reef ecosystem services are valued at 203 million yuan RMB, the services value per unit area was estimated at 996,900 yuan RMB km^-2.a^-1. The initial investment of 13 million yuan RMB should be recovered in 2 years. Net benefit for a 30-year life span is 190 million yuan RMB, a ratio of input to output is 1:15.6.