Distribution of moon jellyfish Aurelia aurita in relation to summer hypoxia in Hiroshima Bay, Seto Inland Sea

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    Article history:Received 14 January 2009Accepted 1 March 2009Available online 10 March 2009

    Keywords:moon jellyshJapanese anchovydissolved oxygen

    (Purcell and Arai, 2001; Brodeur et al., 2002), potentially causinga signicant impact on smaller zooplankton and their predators inpelagic ecosystems (Uye and Ueta, 2004; Haslob et al., 2007). InJapan, the moon jellysh is the most common jellysh species incoastal waters and its biomass has increased over recent decades

    in the literature is of qualitative evaluations of moon jellyshdistribution and its dynamics in nature.

    The Seto Inland Sea is a semi-enclosed basin, which is sur-rounded by heavily urbanized areas with extensive industrial andagricultural development, and receives signicant freshwater fromthe surrounding watersheds (Ochi et al., 1978; Okaichi et al., 1996).Eutrophication of coastal waters has been common in HiroshimaBay, the innermost part of the Seto Inland Sea (Okaichi et al., 1996).Nutritional input through the Ohta River, the northernmost part of

    * Corresponding author.

    Contents lists availab

    Estuarine, Coastal a

    .e

    Estuarine, Coastal and Shelf Science 86 (2010) 485490E-mail address: jshoji@hiroshima-u.ac.jp (J. Shoji).1. Introduction

    The moon jellysh Aurelia aurita is widely distributedthroughout the coastal waters of the world and has been consid-ered as an important predator of zooplankton because of its highconsumption rates (Moller, 1984; Costello and Colin,1994). Increasein abundance of the moon jellysh, the same as other large gelat-inous zooplankton such as cnidarians and ctenophores, hasoccurred in estuarine and ocean ecosystems all over the world

    in Tokyo Bay (Toyokawa et al., 2000; Ishii, 2001) and the Seto InlandSea (Uye and Ueta, 2004). Uye et al. (2003) observed moon jellyshaggregations of up to 250 individualsm2, whichwere estimated toconsume nearly 100% of the mesozooplankton biomass duringsummer months in the coastal waters of the western part of theSeto Inland Sea. Clarication of the mechanism of moon jellyshblooms and effects of environmental conditions on moon jellyshabundance are indispensable for forecasting and regulation ofmoon jellysh blooms. However, much of the information availableHiroshima BaySeto Inland Sea0272-7714/$ see front matter 2009 Elsevier Ltd.doi:10.1016/j.ecss.2009.03.001Biological and physical surveys were conducted in order to investigate the relationship between envi-ronmental conditions and the distribution of moon jellysh Aurelia aurita in Hiroshima Bay, western SetoInland Sea, Japan. Moon jellysh and ichthyoplankton were collected at 13 stations in Hiroshima Bayduring monthly surveys from July to September in 2006 and 2007. Surface temperature in 2006 wassignicantly lower during the August and September cruises and surface salinity was lower during allcruises than in 2007. Moon jellysh was the most dominant gelatinous plankton collected, accounting for89.7% in wet weight. Mean moon jellysh abundance in 2006 was higher than that in 2007 from Julythrough September, with signicant inter-year differences for July and September. Variability inprecipitation and nutritional input from the Ohta River, northernmost part of Hiroshima Bay, weresuggested as possible factors affecting the inter-annual variability in moon jellysh abundance in thecoastal areas of northern Hiroshima Bay. Moon jellysh were more abundant in the coastal areas ofnorthern Hiroshima Bay, where the dissolved oxygen (DO) concentration was lower, while low in thecentral part of the bay. Japanese anchovy Engraulis japonicus eggs were most dominant (58.1% in number)among the ichthyoplankton and were abundant in the central area of Hiroshima Bay. Explanatoryanalysis was conducted to detect possible effects of environmental conditions on the abundance of moonjellysh and Japanese anchovy eggs during the summer months in Hiroshima Bay. Of the environmentalconditions tested (temperature, salinity and DO of surface and bottom layers at each sampling station),bottom DO had the most signicant effect on the moon jellysh abundance: there was a negativecorrelation between the bottom DO and the moon jellysh abundance in Hiroshima Bay during summer.

    2009 Elsevier Ltd. All rights reserved.a r t i c l e i n f o a b s t r a c tDistribution of moon jellysh Aurelia auHiroshima Bay, Seto Inland Sea

    Jun Shoji a,*, Takaya Kudoh b, Hideyuki Takatsuji b, Oa Takehara Fisheries Research Station, Hiroshima University, Takehara, Hiroshima 725-0b Fisheries and Marine Technology Center, Hiroshima Prefectural Technology Research Ic Laboratory of Fisheries and Environmental Oceanography, Graduate School of Agricult

    journal homepage: wwwAll rights reserved.ta in relation to summer hypoxia in

    mu Kawaguchi b, Akihide Kasai c

    , Japanute, Ondo, Kure, Hiroshima 737-1207, JapanKyoto University, Sakyo, Kyoto 606-0068, Japan

    le at ScienceDirect

    nd Shelf Science

    lsevier .com/locate/ecss

  • analysis due to the small sample size.Exploratory analysis was conducted in order to detect possible

    effects of the environmental factors on variability in the moonjellysh and Japanese anchovy egg abundance. Temperature,salinity and DO of surface and bottom layers at each samplingstationwere used as explanatory variables and abundance of moonjellysh and Japanese anchovy eggs as dependent variables. Theenvironmental conditions (temperature, salinity and DO) andmoon jellysh and Japanese anchovy egg abundance werecompared between the two years by the use of Wilcoxon test.

    3. Results

    3.1. Environmental conditions

    Mean sea surface temperature ranged between 24.0 C (July)and 26.8 C (August) in 2006 and between 23.0 C (July) and 28.9 C(August) in 2007 (Fig. 2). There was a signicant inter-annualdifference in the surface temperature in all months (Wilcoxon test,d.f.1, July: P 0.004; August: P 0.0006; September:P< 0.0001). The surface temperature was lower in northern Hir-oshima Bay during all cruises (Figs. 3 and 4).

    Mean surface salinity increased from 20.8 in July to 28.1 inSeptember in 2006 and from 27.8 in July to 31.8 in September in2007 (Fig. 2). In all months, surface salinity in 2007 was signi-cantly higher than that in 2006 (Wilcoxon test, d.f.1, July andSeptember: P< 0.0001; August: P 0.005). The surface salinity waslower in the northern part of Hiroshima Bay.

    The mean bottom DO for all 13 stations ranged between3.3 mg L1 (September) and 5.0 mg L1 (August) in 2006 and

    andHiroshima Bay, and estuarine circulation enhance both primary andsecondary production in Hiroshima Bay (Hashimoto et al., 2006).Chlorophyll-a levels and therefore primary production of Hir-oshima Bay are among the highest of the nine bay areas of the SetoInland Sea (from east to west: Kii Channel, Osaka Bay, Sea of Har-ima, Bisan Archipelago, Sea of Hiuchi, Bingo-Geiyo Archipelago, Seaof Aki including Hiroshima Bay, Sea of Iyo, Sea of Suo: Okaichi et al.,1996). During summer hypoxia prevails in the northern coastalareas of Hiroshima Bay, with dissolved oxygen (DO) concentrationsof

  • Fig. 2. Seasonal changes in physical and biological conditions in Hiroshima Bay fromJuly to September in 2006 (open circles) and 2007 (closed circles). Mean sea surfacetemperature (WT), salinity (SA), bottom dissolved oxygen concentration (DO), abun-dance of moon jellysh (MJ) and Japanese anchovy egg (AE) are shown with verticalbars as standard deviation. Asterisks indicate signicant difference between the years(Wilcoxon test, *P< 0.05; **P< 0.01; ***P< 0.001; ****P< 0.0001).

    J. Shoji et al. / Estuarine, Coastal andthere was a signicant inter-annual difference in the DO (Wilcoxontest, d.f.1, P 0.003). The bottom DO was lower in the northernand/or eastern parts of Hiroshima Bay throughout the cruises. InSeptember 2006 and August and September 2007, some stationsindicated a bottom DO of

  • Fig. 3. Horizontal distribution of sea surface temperature (WT), bottom dissolved oxygen concentration (DO), and abundance of moon jellysh (MJ) and Japanese anchovy eggs (AE)from July to September in 2006 in Hiroshima Bay.

    Fig. 4. Horizontal distribution of sea surface temperature (WT), bottom dissolved oxygen concentration (DO), and abundance of moon jellysh (MJ) and Japanese anchovy eggs (AE)from July to September in 2007 in Hiroshima Bay.

    J. Shoji et al. / Estuarine, Coastal and Shelf Science 86 (2010) 485490488

  • andJ. Shoji et al. / Estuarine, CoastalMnemiopsis leidyi in low DO concentrations in Chesapeake Bay(Keister et al., 2000).

    Contrastingly, ichthyoplankton were less abundant in thenorthern part of Hiroshima Bay during the summer months.Swimming and/or feeding performance of major sh predatorshave been observed to signicantly decrease at DO lev-els< 2.0 mg L1 in coastal waters (juvenile striped bass Moronesaxatilis, adult naked goby Gobiosoma bosc and juvenile Spanishmackerel Scomberomorus niphonius: Breitburg et al., 1994; Shojiet al., 2005). Therefore, competition for prey and space betweenmoon jellysh and sh weakens as DO decreases in coastalhabitats. As a result, the relative importance of trophic ow fromplankton to jellysh is considered to increase in coastal waterswhere summer hypoxia prevails (Breitburg et al., 1994; Shoji et al.,2005). We suggest that low DO conditions in the coastal waters ofHiroshima Bay during summer were favorable for moon jellyshfeeding, growth and survival and resulted in the high abundancein the northern area.

    4.2. Distribution of moon jellysh in Hiroshima Bay

    In addition to the strong tolerance to low DO level, supply ofyoung stages of moon jellysh and retention of them within thecoastal areas of northern Hiroshima Bay should be included inthe possible factors that explain the high moon jellysh

    Fig. 5. Plots of moon jellysh abundance to bottom dissolved oxygen concentration (DO) inSeptember in 2006.Shelf Science 86 (2010) 485490 489abundance in this area. Hiroshima Bay is located at the mostinner part and is the most enclosed area in the Seto Inland Sea(Okaichi et al., 1996). Moon jellysh polyps use articialsubstrate such as concrete blocks, plastic oats and oyster beds(Yasuda, 2003). In Hiroshima Bay, the natural shoreline hasdecreased to less than 40% of its original existence (Okaichiet al., 1996), with much of it replaced by concrete seawallswhich are favorable for jellysh polyps. Intensive oyster culturein Hiroshima Bay (ca. 50% of the total landings of oystercultured in Japan) can provide suitable substrate for the moonjellysh polyps. We conclude that a combination of environ-mental factors such as available substrate for the polyps andhigh retention within the northern bay, in addition to theirbiological features (strong tolerance to low DO conditions),enhances the moon jellysh population in the northern part ofHiroshima Bay.

    During most of the cruises, on the contrary, highest abun-dances of Japanese anchovy eggs were observed in the centralpart of Hiroshima Bay, where bottom DO was higher. The spatialdifference in the Japanese anchovy abundance between thenorthern and central parts of the bay indicates that: (1) Japaneseanchovy spawning was less intensive in and around the northernpart of Hiroshima Bay, and/or (2) egg mortality was higher innorthern part of Hiroshima Bay. It is likely that Japanese anchovyeggs were more vulnerable to predation by moon jellysh in the

    each month. The effect of DO on moon jellysh abundance was signicant in July and

  • northern part of Hiroshima Bay where moon jellysh wereabundant during summer 2006 and 2007. Survival rates of bayanchovy Anchoa mitchilli eggs have been reported to markedlydecrease at DO< 3.0 mg L1 (Chesney and Houde, 1989) althoughthere is no information on how low DO affects survival of Japa-nese anchovy eggs. In Chesapeake Bay, distribution of bayanchovy spawners was conned to the southern (near bay mouth)region when summer hypoxia prevailed in the northern (inner

    Acknowledgements

    We are grateful for the crews of RV Aki, HPTRI, for their helpduring the eld sampling.

    References

    J. Shoji et al. / Estuarine, Coastal and Shelf Science 86 (2010) 485490490bay) region (Jung and Houde, 2004). We conclude that thenorthern part of Hiroshima Bay was less favorable for Japaneseanchovy spawning and/or egg survival due to low DO duringsummer 2006 and 2007.

    4.3. Inter-annual difference in moon jellysh abundance

    Global warming and increase in temperature of shallowwaters have been suggested to potentially enhance moon jelly-sh blooms in coastal waters of Japan (Uye and Ueta, 2004). Inthe present study, however, moon jellysh abundance washigher in 2006, when the temperature was lower during latesummer in Hiroshima Bay. Therefore, the inter-annual differencein moon jellysh abundance cannot be explained by tempera-ture condition alone, and other composite factors may betterexplain the higher moon jellysh abundance in 2006 in Hir-oshima Bay.

    Among the environmental factors tested, difference in salinitywas most signicant between the two years from July throughSeptember. The lower salinity in 2006 reects higher precipita-tion during summer, which might be favorable for growth andsurvival of moon jellysh. The total precipitation in HiroshimaCity from January through July (1120.0 mm) and from Julythrough September (689.0 mm) in 2006 was double the values in2007 (January through July: 550.0 mm; July through September:345.5 mm, Japan Meteorological Agency: http://www.jma.go.jp/jma/). Higher freshwater ow through the Ohta River, thenorthernmost part of Hiroshima Bay, increases primary andsecondary production within the bay by enhancing the nutri-tional input and estuarine circulation (Hashimoto et al., 2006). In2006, chlorophyll-a levels in the northern part of Hiroshima Baywere signicantly higher than in 2007 (Kawaguchi et al.,unpublished data). Previous stomach contents analysis (Uyeet al., 2003) and recent stable isotope analysis (Shoji et al.,submitted for publication) showed copepods are the major preysource of moon jellysh in the Seto Inland Sea. Increase infreshwater ow through the Ohta River should enhance growthand survival of moon jellysh in the northern part of HiroshimaBay by increasing copepod production. We still lack ecologicalinformation on younger pelagic (ephyra) and benthic stages(polyp and strobila) of moon jellysh in nature. Further inves-tigation on the effect of variability in freshwater ow throughthe Ohta River on primary and secondary production in Hir-oshima Bay in relation with growth and survival of moon jelly-sh ephyra, polyp and strobila stages would help to clarify themechanism of moon jellysh bloom, distribution and inter-yeaructuation in Hiroshima Bay.Breitburg, D.L., Steinberg, N., DuBeau, S., Cooksey, C., Houde, E.D., 1994. Effects oflow dissolved oxygen on predation on estuarine sh larvae. Marine EcologyProgress Series 104, 235246.

    Brodeur, R.D., Sugisaki, H., Hunt Jr., G.L., 2002. Increases in jellysh biomass in theBering Sea: implications for the ecosystem. Marine Ecology Progress Series 233,89103.

    Chesney Jr., E.J., Houde, E.D., 1989. Laboratory s...

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