4""""" • the Surface Waters of Hiroshima Bay, a Larg-E Coastal Embayment Subject to Marked Eut ••• Rophication
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Journal of Coastal }{pspal'ch Charlottesville Effects of Surrounding Physical and Chemical Environment on the Spatial Heterogeneity in Phytoplankton Communifies of Hiroshima Bay, Japan Tetsuo Mukai Institute of Environmental Chemistry Faculty of Engineering Hiroshima University Shitami, Saijo-cho, Highashihtroshtrna-shi, 724 Japan AHRTRACT _ M l r1-\ AI. T., 1~)~ 7 E ff'f.'ct." of surrounding physical a nd chemical environment 011 1hE' spat ial hplprogt>l1pitv in phytoplankton communities of Hiros.h ima Hav, .lapan. -lournal oIC()a ..... tal Research, :q:n, ~(;!I.:n9. Charloti osvillo. ISSN 07 4!l-()~()H. ~ . •••4""""" • The surface waters of Hiroshima Bay, a larg-e coastal embayment subject to marked eut ••• rophication. were sampled over a three month period. Important aspects ofin situ environ mental het.erogeneitv include (1) the phytoplankton community, which is sensitive to variations in environmental conditions, and (2) the surrounding physical and chemical environment as habitat for the phytoplankton community. Two multivariate analyses were used to elucidate spatial relationships between these two environmental heterogeneities. Cluster analyses based on phytoplankton composition indicated three different water masses that approximately correspond to the northern, western, and central-to-southern parts of the hay. Mult.iple discriminant analysis in water quality indicated that spatial dif ferences in phytoplankton composition throughout the bay were a primary response ofthe phytoplankton community to in situ spatial variations in the physical and chemical environ ment as represented by water quality. ADDITIONAL INDEX WORDS: Aquatic ecosystem, cnrironmcntal hctcroecncit», phvtoplanhton communitv..«emi-cnclo ..... cd cuuslu] bov, uatcr qualitv, multicuriatc onalvsis. INTRODUCTION relationships exist between the distributional pat terns in the aquatic organism community present Due mainly to tidal mixing, river inputs and the and the in situ heterogeneity of the surrounding morphometric features of a water body, environ environment. mentally heterogenous areas with a variety of physi In general, aquatic ecosystems may be described cal, chemical, and biological characteristics are in terms of (1) aquatic organism communities and frequently formed spatially and temporally in es (2) the surrounding abiotic (physical and chemical) tuarine and coastal water bodies. This sit uation is habitat. Consequently, heterogeneity of aquatic particularly marked in semi-enclosed coastal em ecosystems is approachable from these two view bayments such as Hiroshima Hay, where there is points. Of the various types of aquatic organism limited water exchange and increasing exposure to communities, the phytoplankton community was environmental stresses induced by human activi selected as primary indicators of changes in the sur ties. Thus, for the development of programs for rounding environments, especially in view of its water resource management and available utiliza place in the food web and becuase of its high adap tion of such aquatic ecosystems, it is becoming tability or high sensitivity to varying environmental increasingly necessary to make allowance for in situ conditions among organism communities. Para spatial heterogeneity of the environment. In addi meters of water quality, e.g., watertemperature and tion, it is of interest to elucidate whether ecological salinity, were studied because they characterize 86() 1() rccoiccd 12 February 19H1;; accepted in rcri..... ion 2:i June conditions of the physaical environment. Dissolved I.9H6 inorganic nitrogen (DIN) and phosphorus (DIP), 270 Mukai primarily important anthropogenic su bstances in among various factors. Therefore, of the various eutrophicated coastal water bodies, are also con mu Itivariate analyses widely used in ecological sidered in this study. These water qualities are also work, the procedure proposed by (;REEN and major factors that control algal growth and hence VASCOTTO (1978) was applied as a means: in situ affect the distribution of the phytoplankton com spatial heterogeneity in the phytoplankton com munity in estuaries and other coastal waters. munity was identified by cluster analysis based on The major purpose of this study is to thus clarify the phytoplankton composition. Further, multiple the degree to which the phytoplankton community discriminant analysis (canonical analysis) of the responds to the surrounding environment, par surrounding environment, with respect t.o water ticularly from the viewpoint of spatial hetero quality as a phytoplankton habitat, was conducted geneity of aquatic ecosystems on a macro scale. In over a wide area in Hiroshima Hay, .Iapan. Results complex environments such as estuarine and coast of these statistical procedures provided useful al ecosystems, spatially unidirectional gradients of information for understanding and describing eut various environmental factors are not always evi rophication processes in Hiroshima Bay (Ml"KAI ct dent, presumably due to complex interactions aL, 1985). N 021 41 0 017 olB 10 ! Figure 1. Hiroshima Ray, -Iapan. Numbers indicate t he number of sampling stations. .Iournal of Coastal Research, Vo!. :L No. :L 19H7 Spatial Heterogeneity in Phytoplankton 271 STUDY AREA depth) were taken with Van Dorn bottles using a boat of the Hiroshima Prefecture in October 1977, Hiroshima Bay (Figure 1), situated on the west .Iune 1978, and March 1979. Salinity and water ern part of the Seto Inland Sea, is a typical example temperature were determined with a Salinity and of the semi- enclosed coastal embayment which has Temperature Measuring Bridge (Electronic a surface area of about 946 km", a volume of about Switchgear (London) Limited, Type MC-5L cali 24.2 x 109 m", and an average depth of25.6 m. The brated periodically with standard seawater and a mean freshwater discharge into the bay is about mercury thermometer. 149 x 10f) m'd-1. The northern part of Hiroshima Water samples for nutrients and chlorophyll a Bay, the most enclosed region of the bay, receives analyses were prefiltered immediately after collec large amounts of municipal-industrial effluents tion on board through a precombusted 0.8 /-lm glass generated in Hiroshima City (population is about fiber filter (Toyo GA 200) and stored in an ice chest one million) either directly or through the Ohta during the return to the laboratory. Nutrient con River. This area is thus the most polluted and eu centrations were immediately determined for dis trophicated compared to other parts of the bay. Red solved inorganic nitrogen (DIN) as ammonium-N tides also frequently occur in this area. In the west (GRASSHOFF and ~JOHANNSEN, 1972), nitrite-N ern part of Hiroshima Bay, treated sewage waters (BENDSCHNEIDER and ROHINSON, t952) and are discharged mainly from oil refineries, petro nitrate-N (WOOD ei al., 1967), and dissolved in chemical, textile, pulp and paper industries in the organic phosphorus (DIP) as phosphate- P Iwakuni- Ohtake industrialized regions located on (MURPHY and RILEY, 1962). The filter residue was the west coast of the bay. The central to southern used to determine the concentration of chlorophyll parts of the bay contrastwith these areas because it a by colorimetry after acetone extraction is largely free from land- based sources of pollution. (STHICKLAND and PARSONS, 1968). This southern area is only subject to secondary Non- prefiltered water samples used for phyto pollution associated with currents within the hay. plankton analyses were preserved with neutralized Although data relating to the loading of major nu :3l)1,) formalin and LugoI' s solution immediately after trients that stimulate algal growth are very scarce, collection for estimation of phytoplankton com the equivalent of about :31.91 tons of total nitrogen position in the laboratory. Because regional dif per day and 3.56 tons of total phosphorus per day ferences in the phytoplankton community over the entered the bay (whole- bay basis) in 1977. These entire bay were detectable at the genus level in the figures were provided by the Hiroshima pre preliminary survey, phytoplankton were identified fectural authorities. to genus and enumerated using a binocular micro Hiroshima Bay has low flushing characteristics scope (Nikon Model S-Ke) with phase-contrast due to limited water exchange capacity with neigh optics. Counts to at least1 O:~ cells were recorded for boring seas through the channels of the extreme each sample, because the increase in the value of southern part of the bay. An average period in the Shannon-Weaver diversity index, a measure of which a parcel of water remains within the bay was the structure ofthe phytoplankton community, was estimated to he of the order of 151 tidal cycles, or no longer observed at 1 O:~ cells or more in the pre approximately 76 days. This observation is based liminary experiment. on tracking miniature floating balls and tracing the Cluster analysis (based on phytoplankton com path of dyes using the large-scale hydraulic model position) and multiple discriminant analysis (based of the Seto Inland Sea containing Hiroshima Hay on water quality) were performed on a HITAC (lJESHIMA et al., 1984a, 1984b). While there are M-200H computer at Hiroshima University using other studies of Hiroshima Ray (e.g MUKAI ct al., programs contained in Biomedical Computer Pro 1980,1982,1984,1985), there is little information grams (BMI)P)(1975) and a Statistical Package for concerning in situ spatial heterogeneity of