Population ecology of Aulacoseira granulata in Xijiang River

Aulacoseira granulata,a highlighted centric diatom,belongs to Aulacoseira,Coscinodiscaceae,Coscinodiscales,Centricae,Bacillariophyta.Through the seasonal and continuous investigation for the Xijiang River at 2009,we found that Aulacoseira granulata had four variant forms in morphology: Aulacoseira granulata var.granulata,Aulacoseira granulata var.angustissima,Aulacoseira granulata var.curvata,Aulacoseira granulata var.angustissima f.spiralis. As an eutrophication indicator,it often got some attention because of its dominance in freshwater resources,such as rivers,lakes and reservoirs,and even in classical brackish estuary. Muller reported firstly that two types of cell valves existed in the filament chain in A.granulata at 1903,and this could promote to detect it from other Aulacoseira species.However,this finding did not attract other researchers′ attention until the morphology and ecology study on this species by Kilham and Kilham at 1970s.Thus,an important epoch for A.granulata study came into being.The early studies focused on morphology and structure of filament,variation characteristics and population ecology.At the beginning of this century,several studies referred to toxicity of heavy metals to A.granulata and the role extracellular polysaccharide of A.granulata. Although over 100 years have past,few studies on A.granulata could be found in China,except for author′s research work in the Pearl River Estuary. The Pearl River is the third large river of China and the biggest one of southern China.As the largest tributary of the Pearl River,Xijiang River is with length of 2074.8 km and basin area of 355000 km2.Here,the population ecology of A.granulata was studied in Xijiang River during the whole 2009.Six sampling sites(S1—S6) overlapping the main stream of Xijiang River were set to study the spatial pattern,among of them,S1—S4 sites were investigated in April and July respectively,S5 was a long-term settled site at downstream where water samples were collected every five days to study the temporal variation pattern,and S6 was set at the river estuary where water samples were collected seasonally to study the export of A.granulata from S5.Our results showed that the average proportion of A.granulata to total phytoplankton was 25.37%,and the significant positive relationship between its abundance and total phytoplankton reflected the predominant position of A.granulata in phytoplankton community.We have found the whole four morphological forms of A.granulata during the whole year,and they were var.granulata,var.angustissima,var.angustissima f.spiralis and var.curvata. Among of them,var.granulata showed absolute dominant proportion(average 91.47%),which benefited from its own special morphological characteristics.The annual variation pattern of A.granulata abundance showed apparent two peaks,which appeared in August and November respectively,was mainly due to the suitable water temperature、water column conditions and high nutrients level.Before the appearance of the first peak,there were no apparent fluctuations for A.granulata,which maybe because that the positive role of rising water temperature was covered by the negative role from the dilution impact of high discharge and intense fluctuations caused by frequent rain.The spatial distribution pattern showed that the highest values usually appeared in S1、S5 and S6,which was in accordance with the spatial distribution of high nutrients.