In situ detrimental impacts of Prorocentrum donghaiense blooms on zooplankton in the East China Sea

Marine Pollution Bulletin 88 (2014) 302–310

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In situ detrimental impacts of Prorocentrum donghaiense blooms on zooplankton in the East China Sea Jia-Ning Lin a,b, Tian Yan a,⇑, Qing-Chun Zhang a, Yun-Feng Wang a, Qing Liu a,b, Ming-Jiang Zhou a a b

Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e

i n f o

Article history: Available online 18 September 2014 Keywords: Prorocentrum donghaiense bloom The East China Sea Zooplankton Abundance Reproduction

a b s t r a c t Large-scale algal blooms of the dinoflagellate Prorocentrum donghaiense have occurred frequently in the East China Sea (ECS) in recent decades. However, its impacts on the zooplankton in situ are still under not well understood. During a spring P. donghaiense bloom (April–May 2013) along the northern coast of Fujian Province (120°–121°3000 E, 26°3000 –28°N), we found that the bloom decreased the abundance of copepods and had no significant effect on chaetognaths and small jellyfish. However, the abundance of small jellyfish increased over the course of the study. The zooplankton community changed from being copepod and small jellyfish- to small jellyfish-dominated during the bloom. In the bloom areas, the copepod Calanus sinicus showed higher mortality and lower egg production rates (EPR) than those in the non-bloom areas. The results suggested that P. donghaiense blooms had detrimental effects on the structure of zooplankton community and the recruitments of C. sinicus. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction In the recent years, large-scale harmful algal blooms (HABs) have occurred frequently in the coastal areas of China. Zooplankton species are responsible for the exchange of materials and energy in marine ecosystems. When exposed to harmful algae, zooplankton exhibit undesirable responses, including decreased survival and feeding rates, inhibition of growth and reproduction, changes in behavior, and abnormalities in larval development (Huntley et al., 1987; Hansen, 1989; Poulet et al., 1995; Yan et al., 2009; Ianora and Miralto, 2010). The dinoflagellate Prorocentrum donghaiense is the dominant species in spring algal blooms in the East China Sea (ECS). P. donghaiense blooms have affected large areas (>1000–10,000 km2) for long periods of time (>30 d) in the ECS almost every spring since the 1990s (Zhou et al., 2003; Lu et al., 2005). These blooms are massive, with high biomass of 107 cell L1 and dominance of 90% in the phytoplankton community (Lu et al., 2005). The affected areas covered the largest estuary in China (Changjiang River estuary), the well-known upwelling systems (i.e. Zhoushan fishing ground), and systems strongly influenced by eutrophication. Such typical and complex environments made the large-scale bloom more representative and worth researching. ⇑ Corresponding author. Postal address: Institute of Oceanology, Chinese Academy of Sciences, 7, Nanhai Road, Qingdao 266071, China. Tel.: +86 532 82898589. E-mail addresses: [email protected], [email protected] (T. Yan). http://dx.doi.org/10.1016/j.marpolbul.2014.08.026 0025-326X/Ó 2014 Elsevier Ltd. All rights reserved.

P. donghaiense is a non-toxic dinoflagellate and does not release known phytotoxins (Glibert et al., 2012). However, in laboratory studies, some zooplankton species exhibited significantly decreased viability or fecundity when fed with high concentrations of P. donghaiense (108 cells L1). For example, the brine shrimp Artemia salina and the cladoceran Moina mongolica showed a decline in survival rates (Chen et al., 2007a,b), and the rotifer Brachionus plicatilis displayed inhibited swimming activity, decreased egg production rates and population growth (Wang et al., 2003; Yan et al., 2009). Several laboratory studies demonstrated that exposure to P. donghaiense caused significant inhibition of egg production, and hatching success of the copepod C. sinicus, which might be related to the low contents of some essential fatty acid in P. donghaiense (Jing–Jing Song, not published). However, these views are based solely on laboratory experiments, and relatively little is known about in situ effects of P. donghaiense blooms on zooplankton. Although studies showed that C. sinicus exhibited low survival rates in bloom areas compared to non-bloom areas during the spring bloom in 2005, the bloom was co-dominated by the toxic dinoflagellate Karenia mikimotoi and the non-toxic P. donghaiense. Therefore, field evidence of the impact of P. donghaiense blooms on zooplanktons is still scarce. In spring, coastal areas in the ECS are important spawning grounds for many commercially important fish species, including large head hairtail Trichiurus japonicus, small yellow croaker Larimichthys polyactis, and chub mackerel Scomber japonicas (Hu, 2004). Copepods, jellyfish, and chaetognaths are the main

J.-N. Lin et al. / Marine Pollution Bulletin 88 (2014) 302–310

zooplankton groups in these coastal areas. Copepods are the main food source of larvae of many fish, such as anchovy and mackerel (Uye et al., 1999; Meng, 2000). In contrast, jellyfish adversely affect fishery resources through competition and predation (Lynam et al., 2006). Changes in zooplankton community structure can have both direct and indirect effects on fish recruitments and resources. Decomposition and respiration of massive algal blooms always lead to depletion of oxygen and production of ammonia and/or toxic sulfides (Landsberg, 2002; Branch et al., 2013), which are also harmful to aquatic fauna. Therefore, clarification of the influence of P. donghaiense bloom (i.e. the peak and decay phases) on zooplankton groups (copepods, jellyfish, and chaetognaths) is key to understand its possible effects on the fishery resources in the ECS. C. sinicus is an ecologically important copepod species in the ECS (Chen, 1964) and the dominant species in bloom areas (i.e., 68.09% of zooplankton abundance) (Xu et al., 2003). The survival and recruitment of this species is crucial for the structure of zooplankton community. Therefore, the impact of in situ P. donghaiense bloom on the recruitments of C. sinicus is still in need of concern. In this study, several cruises were conducted during P. donghaiense blooms in the ECS in 2013. The objectives of this study were twofold: (1) to illustrate the effects of different phases of the P. donghaiense bloom on zooplankton community structure; (2) to measure the adverse effects of P. donghaiense blooms on the copepod C. sinicus and discuss the causes of the observed detrimental effects. The results will help us identify the potential threats to the marine ecosystem posed by large-scale P. donghaiense blooms.

2. Materials and methods 2.1. Study location and field sampling During the spring P. donghaiense bloom outbreaks in the ECS, four cruises were carried out along the northern coast of Fujian Province from April to May in 2013 (Fig. 1). Large-scale and high biomass blooms were found in two sections, ZE and FA (120°– 121°3000 E, 26°3000 –28°N), where sampling was conducted at 25 stations during the four cruises. The changes in abundances of zooplankton, and the mortality of the copepod C. sinicus, during the blooms were studied at these stations. Therein, egg production rates and essential fatty acid measurements of C. sinicus were studied in some representative stations. Zooplankton samples were collected by towing a net (mouth area, 0.5 m2; mesh size, 500 lm; length, 2 m) vertically from the bottom to the sea surface. At each sampling station, at least two zooplankton samples were collected; one was preserved in a 5% formalin-seawater solution, and the other sample was kept alive for onboard experiments. 2.2. Abundance of key zooplankton groups All samples preserved in 5% formalin-seawater solution were analyzed in the laboratory. Copepods, small jellyfish, and chaetognaths were picked out and counted under a stereomicroscope. The abundance (ind m3) of each zooplankton group was obtained using the zooplankton abundance per net divided by the volume of the filtered seawater (measured by multiplying the area of net mouth by the vertical distance through which the net was towed). All determinations were performed by strictly following the Specifications of Oceanographic Surveys (State Oceanic Administration, 1991). 2.3. Mortality of C. sinicus At each sampling station, the living sample was used to analyze the proportion of dead C. sinicus in the total count. After capture,

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the numbers of total individuals and dead individuals in the sample were measured immediately on board the research vessel. 2.4. Egg production rates of C. sinicus 2.4.1. Egg production experiments At some non-bloom stations (ZE5 on 25 April, FA 7 on 26 April, FA1 on13 May, ZE7 on 14 May, and ZE3 on 22 May) and representative peak bloom stations (FA5 on 26 April and 7 May, ZE4 on 14 May), one living sample was collected for on board egg production experiments. Plankton samples were diluted into a 20 L bucket filled with surface water and delivered immediately to the laboratory, where healthy adult females were picked out as soon as possible. Ninety fresh adult females were transferred into plastic bottles (with false bottoms of 220 lm mesh size to avoid cannibalism; 10 females per bottle). Next, 500 mL of in situ surface seawater was added to each bottle for incubation. Prior to the experiments, the surface seawater were filtered through a 100 lm mesh and then checked carefully under a stereomicroscope to avoid the unwanted introduction of nauplii and eggs. The adult females in the bottles were incubated for 1 d in an incubator set on a 12:12 h light: dark cycle. Temperature in the incubator was set at the surface seawater temperature (17–19 °C). After 24 h, eggs in each bottle were counted under a stereomicroscope. Egg production rate (EPR) was expressed as eggs female1 day1. 2.4.2. Shipboard incubation experiments A shipboard incubation experiment was conducted to further clarify the harmful effects of P. donghaiense on EPR. We conducted additional vertical hauls with the same net in a non-bloom area (station: ZE7, date: 14 May). One hundred and eighty fresh adult females were transferred into plastic bottles (18 bottles, 10 individuals per 500 mL bottle). Two series of experiments were conducted at the same time. The surface seawater collected at station ZE7 (with no P. donghaiense cells) were added to nine of the bottles as controls. The water collected in the bloom area (station: ZE2, date: 14 May), which contained high density of P. donghaiense cells at 107 cell L1, were added to the other replicate bottles as experimental groups. The surface water were also filtered through a 100 lm mesh and checked to avoid the unwanted introduction of nauplii and eggs before the experiment. Other conditions for incubation were the same as those used for the egg incubation experiment. After 24 h, eggs in each bottle were counted and EPR in each treatment was measured. 2.5. Essential fatty acid measurements of C. sinicus Additional vertical hauls were conducted and fresh, healthy C. sinicus females were collected from non-bloom stations (ZE5 on 25 April, FA 7 on 26 April, ZE7 on 14 May, FA1 on13 May, and ZE3 on 22 May) and stations with blooms in the peak phase (FA5 on 26 April and 7 May, ZE4 on 14 May). For each sample, 100 individuals were picked on the GF/F filter membrane and then preserved in liquid nitrogen. All samples were processed according to Folch et al. (1957) and Parrish (1999). Analysis of fatty acid composition was carried out using an Agilent 7890 AGC instrument. 2.6. Data analysis Data were analyzed using the Excel 2003, Origin 8.5, Surfer 8.0, and SPSS 16.0 software packages. No transformation did not meet the assumptions for the analysis of variance, so nonparametric analogs were used for all the data. For the mortality, percentages were transformed to arcsine square root, and then analyzed by nonparametric analogs (Kruskal–Wallis test). For the others, the original data were directly used. Pair-wise comparisons were

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28

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27

26 119.5

121.5

28

28

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121.5

5.22-5.23

26 122 119.5

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Fig. 1. Map of research area and locations of experimental stations in the East China Sea during four cruises from April to May in 2013. (‘‘d’’ stations: non-bloom areas; ‘‘4’’ stations: peak-bloom areas where density of P. donghaiense bloom reached more than 106 cell/L; ‘‘N’’ stations: decay-bloom areas; ‘‘’’ stations: not sampling stations).

3. Results 3.1. The outbreak of P. donghaiense bloom During the four cruises conducted from April to May 2013, the P. donghaiense bloom occurred mainly in sections ZE and FA. Based on the phase of the bloom outbreak, we divided the research area into three parts: non-bloom, bloom at peak, and bloom in decay. Areas in which the density of P. donghaiense bloom reached more than 106 cell L1 were defined as peak-bloom areas. When the bloom began to decompose, most algal cells sank to 3 m below the surface, where the densities were still high (104 cells L1). At these stations, an unpleasant smell was present at the sea surface due to the algal decay. Therefore, an unpleasant smell, and high algal densities characterized the decay-bloom areas. Other stations were defined as non-bloom stations where the bloom occurred rarely and the algal densities were lower than 104 cells L1 (Fig. 1). 3.2. Abundance of key zooplankton groups and changes in community structure

at non-bloom stations (average, 359 ind m3), lower at the peak-bloom stations (average, 121 ind m3), and lowest at the decay-bloom stations (average, 34 ind m3). To evaluate the temporal changes in copepod abundance during different bloom phases, the abundance at a representative bloom station (FA3) was measured on 26 April and 7, 13, and 21 May (Fig. 3). The bloom was peaking during the first two sample dates, with algal densities of 106 cells L1, and copepod abundance was lower than 40 ind m3. On 13 May, the algal density decreased to 104 cells L1, and copepod abundance increased to 188 ind m3. On 21 May, when the bloom was decaying, copepod abundance

2000

non-bloom areas peak-bloom areas decay-bloom areas

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Abundance (ind*m-3)

conducted by Tukey’s Tests or Dunn’s method (non-parametric). Mean differences were considered significant at the 0.05 level.

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0

In this study, copepod, small jellyfish, and chaetognaths constitute the main zooplankton groups (95% of zooplankton abundance in total), in the non-bloom, peak-bloom, and decay-bloom areas. Fig. 2 shows the abundance of copepods in the non-bloom, bloom at peak, and bloom in decay areas. The abundance was high

Time-Station Fig. 2. The abundance of copepods in non-bloom areas, peak-bloom areas and decay-bloom areas. (The line represents average value).

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200

6.5

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was only 16 ind m3. Therefore, the P. donghaiense bloom at peak had significant negative effects on copepods, and the bloom in decay was even more deleterious. We compared the abundance of small jellyfish and chaetognaths at the non-bloom, peak-bloom, and decay-bloom areas. There was no significant variation in abundance of these two zooplankton groups in these areas (Table 1). Thus, the P. donghaiense bloom had no significant impact on the abundances of these zooplankton groups. Figs. 4 and 5 shows the abundance of small jellyfish and chaetognaths over time, respectively. The temporal changes appeared to show a significant increase in small jellyfish abundance but no trend for chaetognaths. The proportions of the three key zooplankton groups (copepods, small jellyfish, and chaetognaths) in the three areas were analyzed (Fig. 6). The proportion of copepods in peak and decay bloom areas were lower than that in non-bloom areas, and the proportion of gelatinous zooplankton (small jellyfish and chaetognaths) was clearly higher. Thus, the zooplankton community structure changed from being copepod- and jellyfish-dominated to small jellyfish-dominated during the P. donghaiense bloom.

Fig. 4. The abundance of small jellyfish over the course of the study.

Abundance (ind*m-3)

Fig. 3. The abundance of copepods at typical bloom station (FA3) during different bloom phases (At 26 April and 7, May, bloom was at peak; at 21 May, bloom was in decay).

Time-Station Fig. 5. The abundance of chaetognaths over the course of the study.

3.3. Impacts of the P. donghaiense bloom on the copepod C. sinicus 3.3.1. Mortality of C. sinicus The impact of the P. donghaiense bloom on the mortality of C. sinicus was studied at 25 stations in sections ZE and FA during four cruises. The proportion of dead C. sinicus at the non-bloom stations was lower (average value, 1.88%) than that at the bloom stations. The proportion of dead individuals was highest (average value, 69.86%) in the decay-bloom areas, followed by the peak bloom area (average value, 20.44%; Fig. 7). Thus, the bloom appeared to significantly increase the mortality of C. sinicus (p < 0.05). 3.3.2. Egg production rates of C. sinicus The impact of the P. donghaiense bloom on the EPR of C. sinicus was studied using samples collected from representative Table 1 Results of analysis of variance (Kruskal Wallis Test) performed for abundance of three key zooplankton groups (copepod, small jellyfish, and chaetognaths) in non-bloom areas, peak-bloom areas and decay-bloom areas.

⁄

Variable

df

Chi-square

P

Copepod Small-jellyfish Chaetognaths

2 2 2

11.416 0.976 0.340

0.003⁄ 0.614 0.844

Correlation is significant at the 0.05 level (2-tailed).

peak-bloom stations and some non-bloom stations. The EPR of C. sinicus was significantly lower at the peak-bloom stations (0.79 eggs female1 day1), relative to 3.92 eggs female1 day1 at the non-bloom stations (p < 0.05; Fig. 8). Results of the shipboard incubation experiments were the same (Fig. 9). Females cultured in the non-bloom seawater had an EPR of 4.60 eggs female1 day1. When exposed to the water collected from peak bloom station (ZE2 on 14 May, with algal densities of 107 cells L1), the EPR declined to 0.73 eggs female1 day1. These results show that the P. donghaiense bloom had a detrimental effect on the EPR of C. sinicus. 3.3.3. Essential fatty acids in C. sinicus Fatty acid concentrations of C. sinicus collected from representative peak bloom and non-bloom stations were measured (Fig. 10). The concentrations of the following essential fatty acids in C. sinicus from peak bloom stations were significantly lower compared to those at non-bloom stations (Table 2): arachidonic acid [20:4(n6),ARA], eicosapentaenoic fatty acid [20:5(n-3),EPA], 22:5(n-3)/ 20:6(n-3) (EPA/DHA), polyunsaturated fatty acids (PUFA), monounsaturated fatty acids (MUFA), and total saturated fatty acids (TFA). However, the concentrations of docosahexaenoic fatty acid [22:6(n-3), DHA] and saturated fatty acids (SFA) did not differ between bloom and non-bloom areas.

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A

B

8.18%

15.3% 31.16%

45.6%

46.22% 53.53%

C

9.26% 19.47%

copepod small-jellyfish chaetognaths

71.27% Fig. 6. The ratios of three key zooplankton groups (copepod, small jellyfish, and chaetognaths) in non-bloom areas, peak-bloom areas and decay-bloom areas (A: non-bloom areas; B: peak-bloom areas; C: decay-bloom areas).

non-bloom areas peak-bloom areas decay-bloom areas

80 60 40 20

10

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Proportions of dead C.sinicus (%)

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6 5 4 3 2 1 0 5.7-FA5 5.14-ZE4 4.26-FA5 4.26-FA7 5.22-ZE3 4.25-ZE5 5.14-ZE7 5.13-FA1

Time-Station

Time-Station Fig. 7. The impacts of P. donghaiense bloom on the mortality of C. sinicus.

Fig. 8. The impacts of P. donghaiense bloom in situ on the egg production rates of C. sinicus.

4. Discussion

Some deleterious effects of planktonic Prorocentrum species, especially P. minimum, on copepods or some early life stages of bivalves have been reported. For example, Glibert et al. (2007) reported that the oyster Crassostrea. ariakenisis larvae had a severe reduction in survival and motility with P. minimum, although no negative effects were observed on the early stages of C. virginica in 2-day exposures (Stoecker et al., 2008). The copepod Acartia tonsa showed low egg production efficiency when feeding on exponentially growing P. minimum cells (Dam and Colin, 2005). Similar harmful effects were observed with P. donghaiense. In this study, the abundance of copepods was clearly lower at peak bloom areas, which may be due to the high mortality and low egg production rates there. Many previous studies have indicated that HAB species can negatively affect the survival, growth, and reproduction of

Egg production rates (ind*female-1 *d-1)

4.1. Possible causes of the effect of P. donghaiense bloom at peak on copepods

6

peak-bloom areas non-bloom areas

5

4

3

2

1

0 Station Fig. 9. The impacts of P. donghaiense bloom on the egg production rates (EPR) of C. sinicus – Shipboard incubation.

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1.6 1.4 0.6

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Contents (ug. mg-1)

Contents (ug. mg-1)

non-bloom areas peak-bloom areas

Time-Station

Fig. 10. The concentrations of some essential acids in C. sinicus in peak-bloom areas and non-bloom areas.

Table 2 Results of analysis of variance (Kruskal Wallis Test) performed for concentrations of some essential acids in C. sinicus in peak-bloom areas and non-bloom areas.

⁄

Variable

df

Chi-square

P

ARA EPA DHA EPA/DHA PUFA MUFA SFA TFA

1 1 1 1 1 1 1 1

5.445 5.400 1.667 5.400 5.400 5.400 3.267 4.267

0.02⁄ 0.02⁄ 0.197 0.02⁄ 0.02⁄ 0.02⁄ 0.071 0.039⁄

Correlation is significant at the 0.05 level (2-tailed).

zooplankton, likely via mechanisms related to algal toxins or other substances (Kim et al., 2000; Deeds et al., 2002; Wang et al., 2005; Zhou et al., 2011), and nutrient deficiencies (Dam and Colin, 2005; Zheng et al., 2011). P. donghaiense is a non-toxic dinoflagellate and does not release known phytotoxins (Glibert et al., 2012), which rules out the possibility that toxins from P. donghaiense led to the negative effects on zooplankton observed in this study. Studies of P. donghaiense in our laboratory demonstrated that the ratio of essential fatty acids to total fatty acids is lower compared to Alexandrium catenella and Skeletonema costatum, with low contents of ARA and EPA (Jing–Jing Song, unpublished data). In addition, P. donghaiense has also been reported to contain low amounts of phenylalanine, lysine, and histidine (Chen et al.,

2007a). In general, during bloom outbreaks, P. donghaiense constitutes 90% of the phytoplankton community, which limits the amount of nutritional compounds available for zooplankton. Copepods cannot synthesize many of the essential components needed for growth, and they rely on dietary sources for these compounds (Pond et al., 1996). In the long term, the continuous nutrient deficiency may affect the basic life activities and result in the decreased survival of C. sinicus. In our study, C. sinicus collected from bloom stations contained low quantities of EPA, ARA, PUFA, MUFA, and EPA/DHA, and the EPR was also significantly lower than that at non-bloom stations. Specific lipid accumulation, especially some PUFAs, is important for reproductive success. These results suggested that the EPA and ARA deficiency in P. donghaiense and the copepod C. sinicus might have affected the EPR, which is in accordance with data from many studies of copepods. For example, EPRs were found to depend on the quantities of EPA ingested from the diets and from the female reserves (Arendt et al., 2005; Jónasdóttir et al., 2009). EPA and ARA are the ultimate precursors for prostaglandin hormone synthesis (eicosanoids) (Sargent et al., 1999; Bransden et al., 2005), which are involved in a variety of physiological functions including reproduction (Bransden et al., 2005). Although DHA was not deficient in P. donghaiense or C. sinicus collected from bloom stations, the ratio of EPA to DHA was significantly lower in samples from bloom stations. An unbalanced ratio between different fatty acids reportedly can affect reproductive success

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(Jónasdóttir, 1994; Jónasdóttir and Kiørboe, 1996). Therefore, we propose that when P. donghaiense blooms occur and dominate over long time periods and large spatial scales, the lack of essential fatty acids (including EPA, EPA/DHA, and ARA) can inhibit the reproductive success of C. sinicus, which in turn negatively impacts the recruitments of copepods. Dietary amino acids reportedly promote high fecundity in marine copepods (Kleppel et al., 1998; Guisande et al., 1999, 2000; Helland et al., 2003). Arginine and histidine are important nutritional compounds for the metabolism of crustaceans (Koch et al., 2011), and Kleppel et al. (1998) reported that egg production in the marine copepod is significantly related to the arginine and histidine contents of diets. The low quantities of histidine in P. donghaiense might be related with the low egg production of C. sinicus, but detailed experiments are still in need to test this hypothesis. Many studies have reported that marine dinoflagellate Alexandrium species could produce some exotoxin or extracellular allelochemicals, other than paralytic shellfish poison (PSP) toxin (Emura et al., 2004; Yamasaki et al., 2008). Although P. donghaiense does not release known phytotoxins, we cannot rule the possibility that it might produce some other substances. However, whether the detrimental effect is related to the unknown substances requires further investigation. 4.2. Poor water quality during the decay of P. donghaiense During the last two cruises, an unpleasant smell was present throughout the study area at the decay-bloom areas. The mortality of C. sinicus was higher and the abundance of copepods was lower at these areas compared to non-bloom and peak-bloom areas. High biomass blooms are always associated with poor water quality due to algal metabolic activities. Decomposition and respiration of algal cells always leads to the depletion of oxygen, production of ammonia and other nitrogenous by-products, and/or in the formation of toxic sulfides (Trainer et al., 2010; Wang et al., 2012; Branch et al., 2013), which are related to the mass mortalities of marine organisms caused by algal blooms (Morrison et al., 1991; Forbes and Borstad, 1990; Matthews and Pitcher, 1996; Cockcroft, 2001; Pitcher and Weeks, 2006; Pitcher et al., 2008). In this research, the levels of oxygen at decay-bloom areas did not differ significantly from those at the other stations (Xiao-yong Shi, unpublished data). In this research, the average concentration of ammonia in the surface seawater at decay-bloom areas was only 1.00 lM (Xiaoyong Shi, unpublished data), which is in accordance with data from many studies of nutrients in the ECS. For example, Chen et al. (2006) reported the ammonia concentration ranging from 0.61 to 1.23 lM during the 2004 P. donghaiense blooms in the ECS. Li et al. (2010) reported the value ranging from 0.8 to 2.5 lM during the 2005 P. donghaiense blooms in the ECS. Mi et al. (2012) reported the average value of 0.97 lM during April in 2011 in the Southern Yellow Sea and ECS. According to the high values (more than 10 lM) reported for the ammonia toxicity in copepods (Sullivan and Ritacco, 1985; Buttino, 1994), ammonia in this research may be not the major factor responsible for the P. donghaiense effects on zooplankton. Hydrogen sulfide poisoning events often followed the decay of phytoplankton blooms dominated by dinoflagellates (Matthews and Pitcher, 1996; Emeis et al., 2002). Some studies described the detrimental effects of sulfides on copepods. For example, Tiselius et al. (2008) reported that rich sulfide levels in deep water decreased the survivorship of A. tonsa. Exposure to sulfide also affected the survival rates of male Cletocamptus confluens (Vopel et al., 1998) and the hatching success of A. tonsa (Nielsen et al., 2006). The decreased abundance of copepods caused by the P. donghaiense bloom in our study may have been related to the production of some toxic sulfides during algal decay. However, the decay

products and their effects on zooplankton still need to be identified. 4.3. The potential threat of P. donghaiense bloom to the marine ecosystem Purcell et al. (2007) proposed that eutrophication could cause the phytoplankton community to shift from a diatom- to a flagellate-based food web, which favored size reduction of the zooplankton community and could lead to jellyfish blooms. There is a growing consensus in the literature that eutrophication may lead to a decreased proportion of mesozooplankton in the community (Zervoudaki et al., 2009; Riisgård et al., 2012). In recent decades, jellyfish populations have exhibited an increasing trend in coastal areas worldwide (Purcell et al., 2007; Uye, 2008; Richardson et al., 2009; Brotz et al., 2012). Moreover, Sun et al. (2012) reported that jellyfish abundance was positively correlated with the dinoflagellate abundance. During the P. donghaiense bloom studied herein, we also found that copepod abundance decreased and small jellyfish abundance increased. Copepods constitute the most abundant components of zooplankton assemblages in the world. Herbivorous copepods are the main pathway of primary production transfer to higher trophic levels, such as fish larvae and fish. Thus, changes in the abundance of copepods have important impacts on the dynamics of fishery resources and the marine food web. For example, changes in the abundance of C. finmarchicus and C. helgolandicus contributed to the depletion of gadoid resources (Beaugrand et al., 2003). In addition, the increase of jellyfish abundance poses major threats to fishery resources. Jellyfish blooms have been associated with high mortality of fish in farms (Doyle et al., 2008), the depletion of commercial fishery resources through competition and predation (Lynam et al., 2006), and as probable intermediate vectors of various fish parasites (Richardson et al., 2009). It has been reported that frequent jellyfish blooms can apparently be a threat to the fisheries sustainability of the East Asian Marginal Seas, one of the world’s most productive fisheries grounds (Uye, 2008). The decrease in copepod abundance that accompanies the increased small jellyfish abundance may pose risks to fish recruitment and ultimately result in the depletion of fishery resources. Spring algal blooms are experiencing succession from being diatom to dinoflagellate dominated, and this has led to a shift in the classic food chain characterized by diatom–copepod-fish to those dominated by dinoflagellate-protozoan and small copepod–jellyfish (Uye, 2011; Purcell, 2012). Similarly, in this research, the zooplankton community structure changed from being copepod and jellyfish-dominated to small jellyfish-dominated during the P. donghaiense bloom. In this scenario, the structure of pelagic ecosystems can change rapidly from one that is dominated by fish to a less desirable gelatinous state. In recent decades, large-scale P. donghaiense blooms have taken place in the ECS every year, affecting areas as large as 104 km2. In the long term, the bloom with such large spatial and temporal scales, may lead to the accumulation of the observed detrimental effects. In this situation, the marine ecosystem in the ECS will hardly restore, ultimately, which may result in the marine ecosystem degradation. 5. Conclusions The P. donghaiense bloom led to significant changes in the zooplankton community, which changed from being copepod and small jellyfish dominated to being small jellyfish dominated during the bloom. P. donghaiense blooms also had detrimental consequences for the recruitments of C. sinicus. In recent decades, large-scale P. donghaiense blooms have taken place in the ECS every

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