Guangtao Zhang

Breeding places, population dynamics, and distribution of the giant jellyfish Nemopilema nomurai (Scyphozoa: Rhizostomeae) in the Yellow Sea and the East China Sea

In East Asian waters, concern about giant jellyfish blooms, including Nemopilemanomurai (Cnidaria: Scyphozoa: Rhizostomeae), has increased in recent decades. Based on surveys in 2012 and 2013, as a part of the Chinese Jellyfish 973 Program, we investigated the life cycle in situ of the planktonic stages of this species in the Yellow Sea and the East China Sea. We found the following results: (1) Offshore of the Changjiang River is one of its principal breeding places, and is the area where the pelagic stages of N. nomurai appear earliest in the YS and ECS. (2) The general distribution pattern of pelagic stages of N. nomurai described previously in the YS and ECS is confirmed from the 2012 and 2013 surveys: From this small area (31.50–33.00°N, 122.00–122.75°E) usually in late May and early June, it expanded into the 31.5–36°N sea area (aggregating in 32–34°N) in June, then continued to expand northward and southward to 30–37°N by August, and then shrank to 34–37°N in October. (3) The biomass and abundance of N. nomurai in 2012 both in June and August were higher than those in 2013. (4) We confirmed that almost all pelagic stages of N. nomurai in the YS and ECS were confined north of 30°N. The breeding places, population dynamics, and mechanism of formation of the distribution pattern of N. nomurai in the YS and ECS in recent years are presented.

Can tintinnids be used for discriminating water quality status in marine ecosystems

Ciliated protozoa have many advantages in bioassessment of water quality. The ability of tintinnids for assessing water quality status was studied during a 7-yearcycle in Jiaozhou Bay of the Yellow Sea, northern China. The samples were collected monthly at four sites with a spatial gradient of environmental pollution. Environmental variables, e.g., temperature, salinity, chlorophyll a (Chl a), dissolved inorganic nitrogen, soluble reactive phosphate (SRP), and soluble active silicate (SRSi), were measured synchronously for comparison with biotic parameters. Results showed that: (1) tintinnid community structures represented significant differences among the four sampling sites; (2) spatial patterns of the tintinnid communities were significantly correlated with environmental variables, especially SRSi and nutrients; and (3) the community structural parameters and the five dominant species were significantly correlated with SRSi and nutrients. We suggested that tintinnids may be used as a potential bioindicator for discriminating water quality status in marine ecosystems.

Changes in the planktonic copepod community in a landlocked bay in the subtropical coastal waters of Hong Kong during recovery from eutrophication

Tolo Harbour is a poorly flushed bay in the northeastern corner of Hong Kong. Eutrophication caused by discharge of untreated and secondarily treated sewage into the bay was first detected during the 1970s. Increased nutrient input led to a noticeable increase in algal biomass and algal bloom occurrences. Nutrient reduction measures, including the construction of a pipeline to export all sewage effluents from Tolo Harbour, were introduced during the late 1980s. Decline in nutrient levels and decrease in the number of algal blooms have been recorded since 1998 when all nutrient reduction measures became fully operational. Zooplankton samples collected during 2003–2004 revealed that Tolo Harbour still contained a higher density and lower diversity of planktonic copepods compared to Mirs Bay, a less-polluted sea area outside Tolo Harbour. A comparison between data collected in this study to those collected during 1988–1990, several years before nutrient reduction measures were to be fully implemented, showed a decrease in copepod densities and an increase in copepod diversity. Small copepods, notably species of Paracalanus and Oithona, dominated the copepod communities in both periods, but there was an increase in species evenness during 2003–2004, caused by an increase in the number of dominant species. These observations confirm that eutrophication may lead to increases in copepod densities accompanied by increased dominance of small species.

Boreal Tintinnid Assemblage in the Northwest Pacific and Its Connection with the Japan Sea in Summer 2014

Tintinnids are planktonic ciliates that play an important role in marine ecosystem. According to their distribution in the world oceans, tintinnid genera were divided into several biogeographical types such as boreal, warm water, austral and neritic. Therefore, the oceanic tintinnid assemblage could be correspondingly divided into boreal assemblage, warm water assemblage and austral assemblage. The purpose of this study was to investigate the characteristics of boreal tintinnid assemblage in the Northwest Pacific and the Arctic, and to identify the connection between boreal tintinnid assemblage and neighboring assemblages. Surface water samples were collected along a transect from the East China Sea to the Chukchi Sea in summer 2014. According to the presence of boreal genera and warm water genera, three tintinnid assemblages (the East China Sea neritic assemblage, the Japan Sea warm water assemblage, and the boreal assemblage) were identified along the transect. The boreal assemblage extended from the Chukchi Sea to the waters north of the Sōya Strait. Densities peaks occurred at stations in the two branches of the Alaska Current and decreased both northward and southward. The densities were <10 ind./dm3 at most stations in Arctic region. The dominant genera (Acanthostomella, Codonellopsis, Parafavella, and Ptychocylis) accounted for 79.07±29.67% (n = 49) of the abundance in the boreal assemblage. The densities of the dominant genera covaried with strongly significant positive correlations. Tintinnids with lorica oral diameter of 22–26 μm and 38–42 μm were dominant and contributed 67.35% and 15.13%, respectively, to the total abundance in the boreal assemblage. The distribution and densities of tintinnids in the study area suggest that the Sōya Strait might be a geographical barrier for tintinnids expansion.

Effects of temperature regime and food supply on asexual reproduction in Cyanea nozakii and Nemopilema nomurai

Cyanea nozakii and Nemopilema nomurai are two major giant jellyfish species frequently blooming in East Asian waters in recent decades. In order to inspect their asexual reproduction performance in climate regimes associated with global warming, variations of podocyst reproduction and strobilation were investigated at five different temperature regimes (approximate simulation of summer in warm years and cold years: temperature increased from 18 to 25°C and then decreased to 18 and 10°C in three months, respectively; approximate simulation of autumn in warm years and cold years: temperature decreased from 18 to 10°C in two and three months, respectively; control: constant 18°C), and three food frequencies (once per 3d, once per 6d, and unfed). Strobilation in C. nozakii occurred at warmer temperature (≥18°C) but lower temperature (≤18°C) in N. nomurai. More than 90% of ephyrae of C. nozakii and N. nomurai were released in the thermal ranges of 22–25°C and 10–13°C, respectively. Higher percentages of polyps produced podocysts, and increased podocyst production was observed at warmer temperature (>18°C) and more food supplies in both species. The numbers of ephyrae in C. nozakii and podocysts in N. nomurai were significantly increased by prolonged duration of 18–25°C in summer during warm years. Simultaneously, prolonged duration of 10–18°C significantly increased strobilation percentage, ephyra, and podocyst production in N. nomurai in the autumn. More polyps of N. nomurai strobilating in the autumn proved adverse to outbreaks of medusae in the following summer. The results of this study indicate that the response of asexual reproduction to ocean warming appears species specific. Increased ephyra production in C. nozakii but higher podocyst reproduction in N. nomurai is expected in summer and autumn during warm years.

Accelerated recruitment of copepod Calanus hyperboreus in pelagic slope waters of the western Arctic Ocean

Shelf-basin advection is essential to subsistence of the Arctic copepod Calanus hyperboreus population in high basin area. Its abundance, population structure and body size in pelagic layer were investigated with samples collected over a large range in the western Arctic Ocean during summer 2003, to evaluate the geographical variation in recruitment pattern. Calanus hyperboreus was absent from the shallow areas of the Chukchi Sea and most abundant in the slope area between the Chukchi Sea and Chukchi Abyssal Plain (CS-slope). Total abundance varied between 1 110.0 and 5 815.0 ind./m2 in the CS-slope area and ranged from 40.0 to 950.0 ind./m2 in the other areas. Early stages (CI–IV) dominated in the CS-slope area, whereas CV and adult females were frequently recorded only in deep basin areas. Geographical difference of prosome length was most evident in CIII, with average ranging from 2.48 to 2.61 mm at the CS-slope stations and 2.16–2.37 mm at the others. Abundance of early developmental stages (CI–CIV) correlated positively with Chl a concentration, but negative correlation was observed in late stages (CV–adult). Our results indicated that C. hyperboreus can benefit from primary production increase through accelerated development in the first growth season and the productive CS-slope area is a potential source for slope-basin replenishment.

Selective suppression of in situ proliferation of scyphozoan polyps by biofouling

An increase in marine artificial constructions has been proposed as a major cause of jellyfish blooms, because these constructions provide additional substrates for organisms at the benthic stage (polyps), which proliferate asexually and release a large amount of free-swimming medusae. These hard surfaces are normally covered by fouling communities, the components of which have the potential to impede the proliferation of polyps. In this study, we report an in situ experiment of polyp survival of four large scyphozoan species found in East Asian marginal seas that were exposed to biofouling, a universal phenomenon occurring on marine artificial constructions. Our results showed that the polyps of three species (Nemopilema nomurai, Cyanea nozaki, and Rhopilema esculentum) attached to the artificial surfaces were completely eliminated by biofouling within 7-8months, and only those of moon jellyfish (Aurelia sp.1) in the upper layers could multiply on both artificial materials and other organisms (e.g., ascidians and bryozoans). Fouling-associated competition and predation and suppressed asexual reproduction of podocysts were observed to contribute to the loss of polyps. This study shows that the natural distribution of polyps is defined by the biofouling community that colonizes the surfaces of artificial constructions. Consequently, the contribution of marine constructions to jellyfish bloom is limited only to the ability of the jellyfish species to reproduce asexually through budding and inhabit solid surfaces of fouling organisms in addition to inhabiting original artificial materials. We anticipate that fragile polyps will colonize and proliferate in harsh environments that are deleterious to biofouling, and we propose special attention to polyps in antifouling practices for excluding the possibility that they occupy the available ecological space.

Inter-annual variation of the summer zooplankton community in the Chukchi Sea: spatial heterogeneity during a decade of rapid ice decline

Ice decline is believed to benefit the pelagic food chain in Arctic shelf ecosystems, but the impacts of ice decline are usually difficult to detect owing to the overlap of ice decline with natural variability. To evaluate the responses of zooplankton communities to sea ice reduction in the Chukchi Sea, we combined zooplankton samples collected in the early summers of 2003, 2008, 2010, and 2012 and compared the inter-annual changes in the subregions with various physical and biological characteristics. Three geographically separate communities were identified by a hierarchical cluster analysis. The Bering Sea water influenced the central (CCS) and south (SCS) Chukchi Sea communities, which had total abundances that were approximately ten times higher than that of the north (NCS) Chukchi Sea community, and the inter-annual variability was dramatic. The SCS and CCS shared dominant taxa of Calanus glacialis, Pseudocalanus spp., barnacle larvae (nauplii and cypris), and Oikopleura vanhoffeni, while Pacific and neritic species were recognized as the dominant species in only the SCS. The inter-annual variations in the dominant species assemblages can be explained by the variability in oceanic circulation and the counteractions between copepods and barnacle larvae. Despite the numerical fluctuations, an increase in the average abundances in the Pacific-influenced region and the summer establishment of the C. glacialis population are proposed to be the most pronounced responses to ice decline. The NCS, which is governed by cold Arctic water, was characterized by low abundances and a constant dominant taxa assemblage. This area was also characterized by the presence of the high Arctic species Calanus hyperboreus and a lack of barnacle larvae. The total abundance of the NCS doubled from 2003 to 2008, while the community structure remained consistent. These results indicate that sea ice reduction has a positive effect on the zooplankton community, but heterogeneity is the main obstacle to the detection of the zooplankton community in the Western Arctic Ocean.

Zooplankton community structure, abundance and biovolume in Jiaozhou Bay and the adjacent coastal Yellow Sea during summers of 2005-2012: relationships with increasing water temperature

Zooplankton abundance, biovolume and taxonomic composition in Jiaozhou Bay and the adjacent coastal Yellow Sea were evaluated using ZooScan measurement of samples collected by net towing every August from 2005 to 2012. Zooplankton abundance and biovolume ranged from 1 938.5 to 24 800 ind./ m3 and 70.8 to 1 480.1 mm3 /m3 in Jiaozhou Bay and 73.1 to 16 814.3 ind./m3 and 19.6 to 640.7 mm3 /m3 in the coastal Yellow Sea. Copepods were the most abundant group in both regions, followed by Noctiluca scintillans and appendicularians in Jiaohzou Bay, and chaetognaths and Noctiluca scintillans in adjacent coastal Yellow Sea. Over the study period, the most conspicuous hydrographic change was an increase in water temperature. Meanwhile, a general decrease in zooplankton abundance was observed, particularly in copepod populations. Based on redundancy analysis (RDA), the warming trend was the key environmental factor influencing to decrease of copepod abundance. The proportion of small-sized copepods increased while the mean size of all copepods decreased, in significant correlation with water temperature. Our results indicate that zooplankton, particularly copepods, are highly sensitive to change in water temperature, which is consistent with predicted impacts of warming on aquatic ectotherms. Due to their dominance in the zooplankton, the decline in copepod size and abundance could lead to an unfavourable decrease in energy availability for predators, particularly planktivorous fish.

Seasonal phenology of the heterotrophic dinoflagellate Noctiluca scintillans (Macartney) in Jiaozhou Bay and adjacent coastal Yellow Sea, China

Seasonal variations in numerical abundance, cell diameter and population carbon biomass of the heterotrophic dinoflagellate Noctiluca scintillans were studied for 10 years from 2004 to 2013 in Jiaozhou Bay and adjacent coastal Yellow Sea, China, and their ecological functions were evaluated. In both areas, N. scintillans occurred throughout the year and demonstrated an essentially similar seasonality; the cell abundance increased rapidly from the winter minimum to an annual peak in late spring and early summer, and decreased gradually toward the autumn-winter minimum. The peak abundance differed by years, and there was no consistent trend in long-term numerical variations. The cell diameter also showed a seasonal fluctuation, being larger in spring and early summer than the other seasons. Estimated carbon biomass of N. scintillans population reached to a peak as high as 90.3 mg C/m3, and occasionally exceed over phytoplankton and copepod biomass. Our results demonstrate that N. scintillans in northwestern Yellow Sea displays the seasonal phenology almost identical to the populations in other temperate regions, and play important trophic roles as a heterotroph to interact with sympatric phytoplankton and copepods.