Species and functional traits composition
During the four seasons surveys in 2006 - 2007, a total of 118 fish species were captured in the Zhoushan Fishing Ground. These species differed in their seasonal preference with 61, 58, 65, and 73 species in spring, summer, autumn, and winter, respectively (Supplementary information Table 1). The species composition varied across seasons, either by number or by weight (Fig. 2). The top 10 fish species of IRI in different seasons accounted for 89.8%, 71.4%, 91.7%, and 71.3% by number in spring, summer, autumn, and winter, respectively. Among these species, only Amblychaeturichthys hexanema and Apogon lineatus were captured in all seasons, of which the former was the most important species in spring, summer, and winter, while the later was the most important one in autumn. The second-most-important species was Larimichthys polyactis, Champsodon snyderi,Acropoma japonicum in spring, summer, autumn (and winter), respectively (Table 2).
In terms of functional traits composition of fish communities, five out of seven traits were significant different between at least two seasons (Table 3). Specifically, CWMK was significantly lower in spring than that in summer and winter; CWMLFM was significantly lower in autumn than that in spring and winter; CWMQ/B was significantly lower in spring than that in autumn and winter; CWMMTP was significantly higher in autumn than that in other three seasons; CWMGT was significantly lower in summer than that in other three seasons.
Table 2 The top 10 fish species for Index of Relative Importance (IRI) in different seasons in the Zhoushan fishing ground are indicated in bold. (/ = no data recorded.) .
Species
|
IRI in Spring
|
IRI in Summer
|
IRI in Autumn
|
IRI in Winter
|
Amblychaeturichthys hexanema
|
4843.16
|
1730.40
|
5519.48
|
2546.47
|
Larimichthys polyactis
|
1449.38
|
21.50
|
2761.21
|
96.32
|
Lophiomus setigerus
|
1077.35
|
2.60
|
/
|
648.83
|
Chelidonichthys kumu
|
787.62
|
/
|
2397.19
|
954.86
|
Sillago japonica
|
300.91
|
/
|
543.68
|
4.22
|
Pleuronichthys cornutus
|
246.10
|
206.42
|
1679.72
|
70.91
|
Cynoglossus abbreviatus
|
206.16
|
118.53
|
6024.67
|
190.94
|
Cynoglossus trigrammus
|
183.91
|
/
|
4075.53
|
301.82
|
Apogon lineatus
|
155.93
|
279.26
|
18871.26
|
571.12
|
Cynoglossus sinicus
|
107.27
|
/
|
/
|
1.04
|
Champsodon snyderi
|
28.44
|
733.59
|
531.34
|
79.83
|
Dysomma anguillare
|
47.72
|
705.06
|
2834.53
|
54.77
|
Acropoma japonicum
|
9.25
|
549.64
|
6370.72
|
996.34
|
Lepidotrigla japonica
|
/
|
263.42
|
/
|
/
|
Psenopsis anomala
|
/
|
158.16
|
2302.66
|
/
|
Muraenesox cinereus
|
53.06
|
143.89
|
2683.18
|
58.34
|
Argyrosomus macrocephalus
|
71.88
|
138.18
|
/
|
1.46
|
Minous monodactylus
|
25.35
|
80.17
|
4346.50
|
0.38
|
Harpadon nehereus
|
13.31
|
24.63
|
4196.09
|
583.08
|
Pseudorhombus arsius
|
74.55
|
22.26
|
4061.06
|
2.63
|
Ophichthus apicalis
|
29.34
|
12.29
|
3353.45
|
/
|
Pennahia argentata
|
/
|
21.83
|
3328.98
|
/
|
Conger myriaster
|
41.04
|
/
|
1758.78
|
354.52
|
Collichthys lucidus
|
2.33
|
73.83
|
2327.20
|
106.73
|
Table 3 Functional traits composition based on CWM in spring, summer, autumn, and winter in the Zhoushan fishing ground, respectively. Same letters upper right corner indicate significant differences in functional traits between seasons (Kruskal-wallis test, p < 0.05).
Functional traits
|
Spring
|
Summer
|
Autumn
|
Winter
|
CWMK
|
0.41±0.06ab
|
0.54±0.15a
|
0.58±0.31
|
0.55±0.23b
|
CWMLFM
|
16.99±2.59a
|
15.28±3.77
|
13.54±5.91ab
|
17.72±6.64b
|
CWMQ/B
|
7.54±1.92ab
|
8.48±2.12
|
9.24±2.76a
|
9.18±2.79b
|
CWMTL
|
3.58±0.11
|
3.62±0.15
|
3.68±0.17
|
3.62±0.19
|
CWMMD
|
273.22±249.49
|
191.51±99.52
|
167.75±60.69
|
249.63±138.70
|
CWMMTP
|
19.76±1.86a
|
21.14±2.68bc
|
23.37±1.90abd
|
19.34±2.52cd
|
CWMGT
|
2.67±0.40a
|
2.32±0.43abc
|
2.54±0.69b
|
2.56±0.59c
|
Community structure and biodiversity
The results of One-way ANOSIM testing showed fish communities substantially overlapped but still significantly vary among four seasons (taxonomic structure: Global R = 0.206, p < 0.05; functional structure: Global R = 0.107, p < 0.05). In NMS diagrams, both taxonomic structure and functional structure were scattered in all seasons (Fig. 3).
The results of kruskal-wallis test revealed that inconsistent changes of TD and FD with seasons (Fig. 4). Three TD indices (i.e., species richness, Simpson’s evenness, and variance of abundance) and 2 FD indices (i.e., FRic and FDiv) were significant difference between at least two seasons. Specifically, species richness in summer was significantly lower than that in spring and winter, while FRic in summer was significantly lower than that in other 3 seasons (Fig. 4a,d). Simpson’s evenness was significantly higher in winter than that in autumn, while FEve was non-significantly different between any 2 seasons (Fig. 4b,e). Variance of abundance was significantly higher in summer than that in winter, while FDiv was significantly lower in summer than that in spring and autumn, and significantly higher in autumn than that in winter (Fig. 4c,f).
The relationship between biodiversity to environmental variables
In this study, both TD and FD indices were observed significantly related (Pearson correlation test) to several environmental variables at least in one season (Fig. 5). In spring, species richness was not significantly related to any environmental variables, while FRic was significantly related to COD; Simpson’s evenness was significantly related to three environmental variables including COD, PO43-, and DP, while FEve was significantly related to 10 environmental variables including salinity, COD, AN, NN, IN, DN, TN, PO43-, DP, and silicate; variance of abundance was significantly related to depth and DP, while FDiv was not significantly related to any environmental variables. In summer, species richness was significantly related to depth, IN, DN, and TN, and variance of abundance was significantly related to depth. In autumn, only Simpson’s evenness was found to significantly related to nitrite; In winter, species richness was significantly related to three environmental variables including PO43-, Dp, and TP, while FRic was significantly related to only one environmental variables (i.e., PO43-).
Fish community structure and related to environmental variables
When analyzing the effects of environmental variables on fish community structure, results showed that eight of 15 variables including salinity, DP, IN, TN, silicate, PO43-, nitrite, and temperature were significantly related to taxonomic structure (p < 0.05), and these variables totally explained 51.4% of variance of taxonomic structure, while three variables including AN, TN, and COD were significantly related to functional structure (p < 0.05), and these variables totally explained 97.4% of variance of functional structure (Fig. 6A, D).
Different species and functional traits of fish differently responded to environmental variables. As water temperature or nitrite increased, abundance of Lepidotrigla japonica, Hoplobrotula armata, Chaeturichthys stigmatias, and Aseraggodes kobensis increased while Hippocampus kelloggi, Liparis tanakae, Oxyurichthys macrolepis, and Sebastiscus marmoratus decreased. When salinity increased, Abantennarius nummifer and Bregmaceros mcclellandi became more abundant while Ctenotrypauchen chinensis, Coilia nasus, Arius sinensis, and Eleutheronema tetradactylum became less abundant. The influence of DP, IN, TN, silicate, and PO43- on species abundance were opposite with the influence by salinity (Figure 6A). In terms of the response of functional traits of fish to environmental variables, as AN increased, the value of CWMLFM increased, while the value of CWMMTP declined. When TN increased, the value of CWMMD declined. When COD increased, the value of CWMK increased, and the values of CWMMD and CWMGT declined (Figure 6B).