Effects Of Mariculture On The Phytoplankton Community And Water Quality

Shen'ao Bay is one of the most important mariculture areas in Shantou,Guangdong Province and the main culture region for Gracilaria lemaneiformis in China.Three comprehensive investigations on phytoplankton community structure and water quality in Shen'ao Bay were made in May 2007,and March and December 2008.Four sampling stations representing the fish cage culture area,shellfish culture area,G lemaneiformis culture area and control area were set.Environmental factors measured included water depth,transparency,water temperature,salinity,pH,DO,TN and TP.Transparency and DO were lower in the culture area than the control site.Highest nutrient concentrations were recorded at the control area in winter,but the lowest at the control area in spring.Salinity,pH,TN and TP Changed through the seasons(P≤0.05).These results indicate that mariculture has important effects on environmental conditions of the bay.153 species,representing 82 genera and 5 phyla,were identified during the survey.Among these species,Bacillariophyta(107 species representing 52 genera) accounted for 69.9%of all species,followed by Dinophyta(30 species representing 16 genera),Cyanophyta(4 species representing 3 genera),Chlorophyta(10 species representing 9 genera) and Chromophyta Christensen(2 species representing 2 genera).There were 93 red-tide species,comprising 60.8% of all species.The most abundant species of phytoplankton were recorded in summer,followed by winter and spring.Species were spatially homogenous,and the richnest phytoplankton was recorded in G.lemaneiformis breeding areas.The dominant species in Shen'ao Bay are Pseudonitzschia,Chaetoceros,Asterionellopsis glacialis,Nitzschia closterium,Leptocylindrus, Thalassiosira,Skeletonema costatum,Melosira sulcata,Oscillatoria,Grammatophora marina and Scrippsiella trochoidea.In general,phytoplankton densities were higher in the control areas than in the mariculture locations.Phytoplankton densities in winter were significantly lower than the abundances noted in the other two seasons,with similar distributions of chlorophyllα(P＜0.01).The diversity index(d_Ma,H',D,J) ofphytoplankton indicated better water quality and a more stable phytoplankton structure in winter,followed by summer.G lemaneiformis culture therefore result in species richness,maintaining a more diverse phytoplankton assemblage.In order to further evaluate the effect of the seaweed G lemaneiformis on the phytoplankton community,an indoor enclosure experiment was conducted in March 2008.9 pools (2m×0.7m×1 m) were used in this experiment,each filled with 1000L seawater(filtered by sand). The experimental systems were divided into 3 groups(3 replicates per treatment) --- control group(without seaweed),3 kg G lemaneiformis and 6 kg G.lemaneiformis,respectively.The experimental period lasted for 10 days.During the experiment,a Skeletonema costatum bloom occurred in the control treatment with densities reaching 3.88×10⁷ cells·L^-1 on the 8^th day,higher than noted in the seaweed treatments(P＜0.01).The highest chlorophyll-a concentration was 43.9μg·L^-1 in the control group.Low densities of phytoplankton and low chlorophyll-αconcentration were observed in the pools with G.lemaneiformis.The highest densities in the 3 and 6 kg groups were 3.78×10⁶ cells·L^-1 and 1.33×10⁶ cells·L^-1,respectively.Highest chlorophyll-a concentrations of these two seaweed groups were 15.2 and 6.7μg·L^-1,respectively.The results demonstrate that G lemaneiformis culture may be an important bioremediation practice to improve water quality by reducing phytoplankton growth potential and thereby limiting red tide occurrences.