Biogeographical regions of the plankton of the North-Western Pacific ocean and their influence on the deep sea

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<ul><li><p>Biogeographical regions of the plankton o f the North-Western Pacif ic </p><p>Ocean and their influence on the deep sea </p><p>B. G. BOGOROV </p><p>(Received 21 October, 1957) </p><p>Abstract--Biogeographical regions in the pelagial of the ocean can and should be characterized not only by their typical species but also by their production of plankton. The biomass of plankton (weight in mg per 1 m 8) in the boreal region of the North Pacific Ocean is about ten times greater than in its tropical region. Since plankton is the basis of all food-relationships through the whole thickness of the ocean water mass, this will tell also on the abundance of benthos and the character of bottom sediments. The boreal region is richer in benthic life and its sediments contain a greater amount of silica. </p><p>INTRODUCTION </p><p>WlTIa the rapid development of fisheries- which depend either directly or indirectly on plankton-feeders - a knowledge of plankton is especially important. It can lead to an evaluation of. marine resources and even to the possibility of forecasting their utilization by man. Consequently, it is important to ascertain the distribution (horizontal, vertical and seasonal) of various plankton organisms and to determine other features of the biogeographical areas affecting the distribution of many fish*, whales* and squid. In the Pacific Ocean, the distribution of the surface, calanoids has been delimited by BRODSKY (1955-1956) and corroborated by others. There is nevertheless a need for more detailed studies of the plankton, its composition and amount (biomass), the number of generations, primary production, etc., typifying different areas, as related to the water masses. An attempt at such an evaluation has been made possible by the data collected in the North-Western Pacific by expeditions of the Institute of Oceanology of the U.S.S.R. Academy of Sciences on the Vitiaz. </p><p>The present paper is based on observations made by the author and on the following published papers: ANRAKU (1953; 1954), BEKLEMISHEV and SEMINA (1956), BOGOROV and BEKLEMISHEV (1955), BRODSKY (1950-1956), FUKUOKA (1955), HIRANO (1957; 1957a), KAWAI (1955; 1956a; 1957b), LUBNY-GERZYK (1956), MOTODA and ANRAKU (1954; 1955), NAKAI (1952), PONOrCrAREVA and BEKLEMISrmV ('1956), SEMINA (1956), SMmNOVA (1956), TANAKA (1953), VINOGRADOV (1954-55) and WmSONt (1942-1950). </p><p>PHYSICAL AND BIOLOGICAL CHARACTERISTICS OF THE SURFACE WATERS </p><p>It is more convenient to accept three biogeographical regions in the part of the Pacific Ocean: arctic, boreal and tropical, without any further subdivision into subtropical, subarctic, etc. The southern limit of the arctic region extends to the southern Chukchi Sea, where arctic and boreal forms intermingle. Over 50 per cent of the plankton fauna consists of Bering Sea or arctic species (BRODSKY, 1955a) and the remainder are representatives of Pacific boreal communities, including Eucalanus </p><p>*See p. 159. tSome of Wilson's identifications are not reliable, so that his papers arc only partly used. </p><p>149 </p></li><li><p>150 B.G. BoGoRov </p><p>bungii, Calanus cristatus, Calanus plumchrus, etc. The boreal waters extend northward from the line 40-42N. Here the Oyashio (or Kurile-Kamchatka) Current flows from the Bering and Okhotsk Seas. In winter and spring the well-cooled waters of the Okhotsk Sea enter the Pacific through the passage between the southern and central Kurile Islands. Consequently, the southern waters of the Kuri le-Kamchatka region are colder in spring than its northern waters. </p><p>The surface waters in the Pacific between the line 40-42N and the Kurile Islands usually have a temperature in winter of less than 3C, increasing in summer to 14-15C, where the cold Oyashio waters meet the warm waters of the north-eastern branch of the Kuroshio, forming a subarctic convergence, termed also as the primary polar front or the Oyashio front (MASUZAWA, 1955). To the south of the front, the waters of the Oyashio and Kuroshio intermingle and are characterized by a surface temperature of 18-20C in summer. This " zone of mixing," the " zone of transi- tion " of BOGOROV and VINOGRADOV (1955a)*, varies in width with the season and the prevailing winds. Probably a zone of mixing of boreal and tropical faunas also occurs in the Southern Sea of Japan and the Yellow Sea. The tropical waters of the Kuroshio with surface temperatures of 26-28C in summer and 18-20C in winter are dominant south of 35N. </p><p>Fig. 1. Distribution of zooplankton communities in the surface waters in the North-Western Pacific. August-October 1954. (I) Boreal complex; (2) Zone of mixing; (3) Tropical complex. Broken line is an isotherm of 18 on the surface water (from B. G. BoGoaov and M. E. Vtr~OORADOV, </p><p>1955). </p><p>The composition of the plankton in these waters varies greatly (Fig. 1 ; Tables 1-3)-]', but contrary to BgoosKv's (1955) opinion that a distinct subtropical plankton fauna occurs between the boreal and tropical faunas, no such group was found. Just as the </p><p>*The term "" zone of mixing" is preferable. "]'South of 40N, and especially south of 35N, coccoliths, Cestus amphitrites, Velella, Physalia, </p><p>heteropods, pteropods, salps, Halobates, etc., should also be included as typical. </p></li><li><p>Biogeographical .regions of the plankton of the North-Western Pacific Ocean 151 </p><p>Table 1.* Copepods found in the "zone of mixing" (surface layers down to 200 m): their distribution in boreal and tropical waters based on various published data BRODSKY, 1950; ANRAKU, 1954; NAKAt, 1952; TANAKA, 1955; WILSON, 1942; 1950t and the </p><p>author's own collections </p><p>Species </p><p>Calanus cristatus Calnus pacificus Calanus plumchrus ( tonsus) Candacia columbiae Eucalanu$ bungii Labidocera japonica Metridia ochotensis ( longa) Metridia pacifica (lucens) Microcalanua pygmaeus Oncaea borealis Pareuchaeta japonica Pseudocalanus elongatus (minutus) Scolecithricella minor </p><p>Calanus tenuicornis Candacia bipinnata Clausocalanus arcuicornis Eucalanus attenuatus Euchirella brevis Gaidius tenuispinus Microsetella rosea Oithona plumifera Oithona similis Oncaea conifera Paracalanus parvus </p><p>Temperature Om. </p><p>from 3 to 15 from 15 to 18 from 18 to 27 </p><p>Boreal waters Zone of mixing Tropical waters </p><p>+ + + + + + + + + + + + + </p><p>+ + +- + + + + + + + + </p><p>+ + + + + + + + + + + + + </p><p>+ + + + + + + + + + + </p><p>+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + </p><p>m </p><p>m </p><p>m </p><p>m </p><p>m </p><p>m </p><p>n </p><p>+ + + + + + + + + + </p><p>Acartia danae Acartia negligens Acrocalanus gibber Acrocalanus gracilL, Aetideus armatus Calocalanus pavo Caiocalanus plumulosus Candacia simplex Candacia varicans Clausocalanus furcatus Clausocalanus pergens Clytemnestra rostrata Copilia mirabilis Copilia quadrata Corycaeus anglicus Corycaeus crassiusculus Corycaeus pumilus Corycaeus speciosus Corycaeu$ typicus Eucalanus crassus Eucalanus mucronatus Eucalanus subcrassus Eucalanus subtenius Euchaeta acuta Euchaeta marina Farranula gibbula Farranula rostrata Gaetanus minor Haloptilus angusticeps </p><p>*Prpared with the assistance of A. K. H~NmCH. tSee Footnote t on page 149. </p><p>+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + </p><p>+ </p></li><li><p>152 B.G. BOGOROV </p><p>Table 1- -cont inued. </p><p>Temperature Om. </p><p>Species </p><p>Heterorhabdus papilliger Labidocera acuta Labidocera detruncata Lucicutia clausi Lucicutia flavicornis Macrosetella gracilis Mecynocera clausi Megacalanus princeps Nannocalanus minor Neocalanus gracilis Oithona attenuata Oithona nana Oithona setigera Oithona spinirostris Oncaea media Oncaea mediterranea Oncaea minuta Oncaea venusta Paracalanus aculeatus Paracalanus pygmaeus Phaenna spinifera Pleuromamma gracilis Rhinealanus nasutus Sapphirina auronitens Sapphirina nigromaculata Sapphirina stellata Temora discaudata Temora stylifera Temora turbinata Undinula darwini Undinula vulgaris Xanthocalanus pauper </p><p>from 3 to 15 from 15 to 18 from 18 to 27 </p><p>Boreal waters Zone of mixing Tropical waters </p><p>t </p><p>+ </p><p>1- </p><p>i i i T </p><p>i L i ! i </p><p>L </p><p> : i </p><p>+ + + + </p><p>q </p><p>q i- </p><p>t {- </p><p>i- t </p><p>water in the area of convergence is the product of a mixing of the boreal and tropical waters, so too the plankton of the surface layers is the product of an intermingling of species living in both water masses (Table 1). By inference, the " zone of mixing " is inhabited by the most eurythermal species from the boreal and tropical waters. </p><p>The phytoplankton contains 58 per cent of diatoms, 38 per cent of peridinians and 4 per cent of cyanophycean and silicoflagellates (SMmNOVA, 1956). There is a marked preponderance of diatoms in the boreal reg ion- three to four times the number of peridinians - both in the number of species and especially in biomass. Inversely, in tropical waters there are twice as many species as in boreal waters (Table 2). The zone of mixing contains almost equal percentages of phytoplankton species from warm (tropical), temperate-warm (southern boreal) and temperate-cold (northern boreal) waters (cf., Tables 2 and 3). </p><p>Comparison of Figs. 1 and 2 shows that the boundaries of the biogeographic regions and zone of mixing are the same. As already mentioned, the zone of mixing shifts considerably according to the season. In winter, the boreal fauna extends southwards, and in spring the zone of mixing lies south of 40N. In the summer the distribution of tropical fauna extends northwards across the zone of mixing and in the autumn the zone of mixing stretches up to 42N. The farther northwards from the main flow of the Kuroshio, the thinner becomes the layer of mixed water. Near </p></li><li><p>Biogeographical regions of the plankton of the North-Western Pacific Ocean 153 </p><p>42N the tropical representatives live in the uppermost water layers down to 50 m, less frequently to 100 m. Below this depth occur mainly representatives of the boreal fauna. Some of them are found also farther south but at ever increasing depths; </p><p>Table 2. Phytoplankton species in the several types of water in the North-Western Pacific </p><p>Type of Water </p><p>Cold water (i.e., chiefly arctic region) </p><p>Temperate cold water (i.e. chiefly northern part of boreal region) </p><p>Temperate warm water (i.e., chiefly southern part of boreal region) </p><p>Warm water (i.e., tropical region) </p><p>No. of Species recorded </p><p>10 </p><p>27 </p><p>19 </p><p>55 </p><p>Representative species </p><p>Ceratium arcticum Rhizosolenia alata f, curvirostris Rhizosolenia hebetata f. hiemalis (all have wider distribution) </p><p>Ceratium fusus Ceratium longipes Chaotoceros altanticus Chaetoceros concavicornis Thalassiosira nordenski61dii </p><p>Ceratium tripos Ceratium macrosceros Chaetoceros atfmis Chaetoceros convolutus Thalassiosira subtilis </p><p>Ceratium extensum Ceratium massiliense Ceratocorys armata Ceratocorys horrida Chaetoceros coarctatus Chaetoceros lorenzianus Chaetoceros peruvianus Chaetoceros messanensis Climacodium biconcavum Ethmodiscus rex Pyrocystis pseudonotilina Pyrocystis fusiformis Pyrocystis hamulus Rhizosolenia bergonii Trichodesmium thiebauti </p><p>Table 3. Percentages of the total number of species of phytoplankton population in the North-Western Pacific Ocean arranged according to their ecological affinities </p><p>(SMIRNOVA, 1956) </p><p>Boreal waters </p><p>Zone of mixing </p><p>Noah-eastern branch of the Kuroshio </p><p>Warm-water species 11-25C with preference </p><p>for temperatures above 18C </p><p>1% </p><p>35% </p><p>93-5% </p><p>Temperate warm-water </p><p>species 5.100C </p><p>13% </p><p>30% </p><p>6% </p><p>Temperate cold water species Cold-water 3-14C with species preference of 2-5C </p><p>5.10C </p><p>79% 7% </p><p>35% </p><p>0.5% </p><p>in the northern part of the Kuroshio they live at depths of 1000 m and more. At these depths the character of the plankton fauna tends to be much the same in all areas, </p></li><li><p>154 B.G. BOGOROV </p><p>The biogeographical regions differ from one another not only in the species occurring there, but also in the abundance of certain species, In the arctic region only two species are abundant: Calanusfinmarchicus arid Pseudocalanus elongatus (minutus). In the surface zone (0-200 m) of the boreal region, there are several abundant species : Calanus plumchrus, Calanus cristatus, Eucalanus bungii, Metridia pacifica and Oithona similis. In the southern boreal region, this plankton is superceded or complemented by a number of warm water species: Calanuspacificus, Eucalanus hyalinum and others. Farther south off the Kurile Islands Penilia avirostris occurs. </p><p>1 I 21 3 i </p><p>~.N J .t~ ~ </p><p>.) </p><p>L L </p><p>I "4 % </p><p>J </p><p>, </p><p>t </p><p>55" </p><p>50* </p><p>45* </p><p>~45" 15o* ~5s* 16o* les* ~7o* Fig. 2. Characteristic of phytoplankton (in percentage from all numbers of species) in Boreal wators, Zone of mixing and northern waters of Kuroshio. (1) Cold water species; (2) Tmpemte-eold </p><p>water species; (3) Temperate-warm species; (4) Warm-water species. August-October 1954. </p><p>The passage to the tropical region can be recognized by the appearance of flying fish, sharks, moonfish, etc. No abundant species (i.e., with respect to biomass) are found in the plankton in the tropical Pacific Ocean, but there is usually a great variety of species. In the surface waters the calanoids alone are represented by more than 100 species. </p><p>The biomass (weight of plankton per m 3) is distributed in the north-western part of the Pacific (Fig. 3) as follows: near the Kurile Islands the quantity of plankton usually averages 200 mg per m 3, with a sharp increase further south and also further offshore, in a band 30-50 miles wide. Here the biomass usually exceeds 500 mg, and in some places even 2000 and 3000 mg per m 3 (BoGoRov and VINOGRADOV, 1955). Comparison of plankton in spring and late summer indicates that this area of high zooplankton biomass is fairly stable. Patches with large quantities of plankton are also encountered elsewhere in the boreal region. Occasionally larger amounts may also occur near shore. The nearer to the tropics the less plankton there is. The warm </p></li><li><p>Biogeographical regions of the plankton of the North-Western Pacific Ocean 155 </p><p>140 145 150 155 160 ~65 170 </p><p>Fig. 3. Distribution of Zooplankton biomass in 0-100 m in the North-Western Pacific August- October 1954. (1 )&gt; 500mg]m3; (2) 250-500 rag/m3; (3) 100-200 rag/m3; (4) &lt; 100rag/m3 (from </p><p>B. G. BOGOROV, M. E. VXNOORADOV, 1955). </p><p>/ / ,.ex '~ / / </p><p>,/ </p><p>Fig. 4. Average diurnal production of carbon in mg per litre in a different part of the North-Western Pacific. August-October 1954 (determined by Oxygen method). After B. G. BoGogov and </p><p>C. W. BEKLEWaSHEV, 1955). </p></li><li><p>156 B.G. BOGOROV </p><p>waters of the Kuroshio are characterized by a low plankton biomass, usually less than 100 mg/m 3, and often below 50, or even 20 mg. In the regions between 40 and 50N the quantity of plankton is many times greater than south of 30N (NAKAI, 1952). </p><p>The low primary production of tropical waters results in a low biomass of phyto- plankton. Carbon production as determined by the oxygen method on the Vitiaz in 1954 (BoGoROV and BEKLEMISHEV, 1955) was ten times lower in the tropical region than in the boreal (Fig. 4). In areas where the phytoplankton at that time was " in bloom," i.e., at its maximum, along the shores of Kamchatka, primary production was 20 times greater than in the tropical region. </p><p>The low biomass of plankton in tropical waters seems inconsist...</p></li></ul>