To understand the distribution and seasonal changes of macrobenthos in Zhoushan sea area, two cruise surveys were carried out in the summer and winter. A total of 219 species were recorded in the sampling sediment during the two seasons. Two-way ANOVA results showed that the differences were not significant among sections in all the community parameters (species number, biomass and density) and species diversity (Shannon-Weiner index and Pielou's evenness index), but significant between seasons in species number, density and Shannon-Weiner index. This might be influenced by two factors as follows: (1) water currents' differences between seasons; (2) considerable differences in temperature between seasons.
Macrobenthic communities are key components in the functioning of coastal systems. Benthic organisms produce considerable changes in physical and chemical conditions of the sediment, especially in the water-sediment interface. They also promote the decomposition of organic matter, nutrient recycling and energy transfer to other links within the food web (Gaudéncio and Cabral, 2007). They modify sediment chemical components (Norling et al., 2007), alter sediment stability and near-bed hydrodynamics (Norkko et al., 2001), release and draw down oxygen, nutrients and particulates from the water column (Kamp and Witte, 2005), supply and consume pelagic organisms (Snelgrove et al., 2001) as well as break down and recycle detritus and primary production (Duchêne and Rosenberg, 2001). They can alter the sediment in the depth of > 20 cm (Dauwe et al., 1998). Macrobenthos mostly live in the surface of sediments with abundant oxygen and organic substance. Their food resources mainly include microbial (bacteria, microalgae, protist and fungi), meiobial and organic substance. Bottom-layer fish and some mammals prey on benthos as their food (Herman et al., 2000). Thus, the secondary productivity of benthos is the important part of material and energy flow in the marine ecological system and benthos is crucial to the near-bed water ecological system.
Zhoushan sea area (ZSSA) contains the Zhoushan fishing ground, one of four world famous fisheries, which is located at the east of Zhejiang province and outside of the Yangtze River. The fishing ground is influenced by the mutual action of Yangtze River diluted fresh water, Zhejiang seashore currents, Taiwan warm currents and the Yellow Sea cold water mass. Every year 1 × 1012 m3 of fresh water runoff flows into the sea. Research has already been conducted in the China Sea area including Yangtze River estuary (Li et al., 2007), Jiaozhou Bay (Yuan et al., 2007), and Xiangshan Bay (Gao et al., 2003), but there have been no reported studies on macrobenthic fauna in ZSSA thus far. In spite of obvious interest in that area and the important role of macrobenthos in aquatic ecosystems, the seasonal change in distribution and community structure of macrobenthos in ZSSA is poorly understood, especially in relation to the dominant factor influencing that change. This study aimed to analyse: (1) the distribution and seasonal change of macrobenthos in ZSSA; (2) the diversity of macrobenthos in ZSSA; (3) the dominant influencing factor on community parameters of macrobenthos in ZSSA.
Materials and methods
Study area, sampling and study method
This study is part of a project on comprehensive investigation and evaluation of coastal areas in China. To understand the distribution and seasonal change of macrobenthos, two cruises survey were carried out in ZSSA. Based on latitude, a total of 29 stations were divided into 3 sections in ZSSA, which were labeled 05, 06 and 07, respectively. The ranges of investigation were 122.375°E ∼ 124.000°E, 30.000°N ∼ 30.780°N, respectively (Fig. 1). Benthic samples were collected with a weighted, manually deployed 0.01 m2HNM grab from August 15th to August 17th at 2006 in the summer, and from January 14th to January 26th at 2007 in the winter. The study involved sampling the benthic specimens twice at each station and only the samples with a penetration of more than 10 cm were kept for faunal analysis. The material was processed through a sieve with a mesh size of 0.5 mm and the retained fraction was fixed in 4% neutral formalin. Temperature, salinity and dissolved oxygen were measured at the bottom using a Valeport CTD meter, and the depth was recorded. In the laboratory, samples were firstly sorted under a binocular dissecting microscope, and then identified to the lowest possible taxonomic level, counted and weighed.
Species diversity of each sample was calculated by the Shannon-Weiner index (H′) (Shannon and Weaver, 1963). Evenness of each sample was calculated by Pielou's index (J) (Pielou, 1975). A two-way ANOVA (Analysis of variance) was used to test differences in benthic parameters and community parameters among the sections and between the seasons. Simple linear regression was used to test the correlativity between the community parameters and temperature. All data were tested for normality (Kolmogorov–Smirnov test) and homogeneity of variances (Bartlett test) before the two-way ANOVA.
In total, 219 species, including 82 species of Annelida, 60 species of Mollusca, 37 species of Arthropoda, 20 species of Echinodermata, 2 species of Bryozoa, 2 species of Sipuncula, 1 species of Nemertea, 9 species of Coelenterata and 6 species of Chordata, were recorded in the sampling sediment during the two-season study period in ZSSA.
Species number, biomass and density
The two-way ANOVA (i.e. the factors of seasons and sections) shows the following: the number of species is significantly different between seasons (F1, 110 = 29.0279, P < 0.01) but not among sections (F2, 110 = 0.7437, P = 0.48); the biomass has no significant differences either in sections (F2, 110 = 0.7104, P = 0.49) or seasons (F1, 110 = 0.0030, P = 0.96), and the density is significantly different between seasons (F1, 110 = 10.1732, P < 0.01) but not among sections (F2, 110 = 0.0287, P = 0.97) (Fig. 2∼ 4). It was interesting that the maximum value appeared at 123° E. The values at the stations O7-6, O6-7, O5-6 are the maximum of their own sections.
The two-way ANOVA shows: the Shannon-Weiner index of macrobenthos has no significant differences among sections (F2, 110 = 0.8965, P = 0.41) but between seasons (F1, 110 = 13.9028, P < 0.01); and the Pielou's evenness index has no significant differences either in sections (F2,110 = 0.260, P = 0.77) or seasons (F1,110 = 1.224, P = 0.27) (Fig. 5, Fig. 6).
Temperature, Salinity and Dissolved oxygen
The two-way ANOVA shows: the temperature is significantly different between seasons (F1, 110 = 93.83, P < 0.01) but not among sections (F2, 110 = 0.120, P = 0.89), the salinity has no significant differences either in sections (F2, 110 = 1.26, P = 0.29) or seasons (F1, 110 = 1.79, P = 0.18), and the dissolved oxygen has no significant differences either in sections (F2, 110 = 1.32, P = 0.28) or seasons (F1, 110 = 0.00, P = 1.00).
The relationship between the community parameters and temperature
Simple linear regression was used to test the relationship between the community parameters and temperature. The results are shown in Table 1. The species number and density were significantly related to the temperature in the summer.
The seasonal change of distribution in macrobenthos
One of the key objectives of coastal biology is to determine whether coastal fauna show any seasonal patterns in their activities. Recent studies have indicated that seasonal input of surface-derived food materials can drive seasonality of reproduction in some coastal animals. These studies have shown that the greatest abundance value and species number are found in the summer; while the lowest values are found in the winter (Lu and Wu, 2007; Zajac and Whitlatch, 1982; Bonsdorff and Österman, 1985). Our results, as expected, showed the greatest differences between summer and winter with temperature and macrobenthos distribution; whereas no significant difference occurred with salinity and dissolved oxygen. Therefore we could conclude that temperature might be one of the main factors affecting the macrobenthos distribution.
However, the relationship between abundance and temperature is more complicated. It was analyzed whether or not bottom temperature was related to the species number and density was also analyzed. Results showed that bottom temperature was negatively related to the species number and density in the summer; while it was positively related in the winter but with less significance (Table 1). Living organisms have an optimal temperature range for growth, at which the species number will rapidly reach a peak value. The community population and density vary with the species number. The relationship between the temperature and species distribution has already been examined in other researches (Zhang et al., 2007; Mfilinge and Tsuchiya, 2008).
The effect of spatial scale on distribution of macrobenthos
Another factor influencing species distribution is their location. Recently, much attention has been paid to the effect of spatial scale on distribution patterns in ecological studies (Schneider, 1994). Spatial heterogeneity is regarded as a central factor in ecological systems (Pickett and Cadenasso, 1995). The application of multi-scale analysis in benthic studies is worthwhile, not only when considering the influence of spatial patterns on sampling methodology (Morrisey et al., 1992), but also when attempting to relate the scale of abiotic and biotic processes to structural patterns observed in benthic communities (Kendall and Widdicombe, 1999). Many studies show that the spatial distribution of living organisms is dependant on latitude differences, which our results did not confirm. Species number and abundance reach a peak value at 123° E. Ning et al. (2004) reported that the middle ZSSA, where the runoff waters of Yangtze River are diluted, 100 km away from the estuary of Yangtze River and Hangzhou Bay, appeared to contain the maximum of Cyanophyta abundance, phytoplankton current quantity, primary productivity and zooplankton. The sea area mentioned above corresponds to our sampled region, so it was concluded that the diluted water of the Yangtze River had greater effect on the macrobenthos community than the latitude in ZSSA.
A total of 219 species were recorded in ZSSA during two cruise surveys. Two-way ANOVA results showed that the differences were not significant among sections in all the community parameters (species number, biomass and density) and species diversity (Shannon-Weiner index and Pielou's evenness index), but were significant between seasons in species number, density and Shannon-Weiner index. It can be concluded that considerable differences in temperature between summer and winter might influence the distribution of macrobenthos in ZSSA. It can also be concluded that diluted water of the Yangtze River has a greater effect on the macrobenthos community than the latitude in ZSSA. However, more research needs to be conducted in order to study the function of macrobenthic ecology, due to few reports in this area.
We thank Hu Yue-Mei and Dong Yong-Ting for their assistance in the laboratory. We are also grateful to two anonymous reviewers for valuable comments on the manuscript. This work was supported by the Grants from Scientific Research Fund of the Second Institute of Oceanography, SOA (JG200815; JT0806) and Grants from Project on Comprehensive Investigation and Evaluation of Coastal Areas in China (908-01-04; 908-ZC-II-04; 908-02-04-02; ZJ908-01-01-2).