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Copepod Neocalanus Cristatus in the Subarctic North Pacific

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Copepod Neocalanus Cristatus in the Subarctic North Pacific MARINE ECOLOGY PROGRESS SERIES Vol. 97: 39-46,1993 Published July 8 Mar. Ecol. Prog. Ser. White-noise-like distribution of the oceanic copepod Neocalanus cristatus in the subarctic North Pacific Atsushi ~suda',Hiroya Sugisakil,*,Takashi 1shimaru2,Toshiro Sainol,Tosiya Sato3 'Ocean Research Institute, University of Tokyo, 15-1 Minamidai 1, Nakano, Tokyo 164, Japan 'Department of Aquatic Bioscience, Tokyo University of Fisheries, 5-7 Kounan 4, Shinagawa, Tokyo 108, Japan 31nstitute of Statistical Mathematics, Minami-Azabu 4-6-7, Minato, Tokyo 106, Japan ABSTRACT. Continuous estimates of Neocalanus cristatus abundance were obtained with an electric particle counter along a cruise track in the subarctic western North Pacific in spring. In meso- to mega- scale observations, with sampling interval 510 m and coverage 2519 km, the number of patches de- creased exponentially both with increasing patch size and with density of the copepods in the patch. The maximum patch size and density were 6.6 km and 1230 ind. m-3, respectively. In micro-scale ob- servations, with sampling interval 10 to 30 m, the number of patches also decreased exponentially with patch size and density. Spectral analysis of temperature, salinity, in v~vochlorophyll fluorescense and copepod abundance showed that the copepod did not have the characteristic patch-length or wave- number dependencies observed in the other parameters These white-noise-like spectra, together with the general observations on micro- to mega-scales, suggested that N crjstatus had patches in all length scales from at least 10 m to 50 km, and that patches of a given order were constituted by the gathering of smaller-scale patches. INTRODUCTION classified as a cluster of animals having a density sev- eral times higher than average or background density. Heterogeneity of zooplankton distribution such as An extremely high density (10' ind. m-3) was reported patchiness or schooling is a long-recognized phenom- for diapausing Calanus pacificus (Alldredge et al. enon (e.g. Hardy 1936). Development of continuous 1984). For relatively large copepods such as Neo- plankton samplers has substantially contributed to calanus spp., 100 to 1000 ind. m-3 were reported as studies of patchiness characteristics such as size, den- patches (Barraclough et al. 1969, Kawamura & Hirano sity of organisms and community structure (Longhurst 1985). Size of patches also varies among zooplankton et al. 1966, Wiebe 1970, Fasharn et al. 1974, Mullin & species and among sampling tools, e.g. from several Williarns 1983). Patchiness of plankton is important for tens of cm for visually oriented copepods in coral reefs survival of pelagic fishes (LeBrasseur et al. 1969, Las- (Hamner & Carleton 1979) to several tens of km for ker 1975) and planktivorous manne mammals (Kawa- oceanic copepods (Cushing & Tungate 1963, Paffen- mura 1982),and for studies on zooplankton population hofer et al. 1981). Mackas & Boyd (1979) used spectral dynamics and community structures (Wiebe 1970).Re- analysis for continuous estimates of copepod abun- cently, the importance of patchiness has been stressed dance in the horizontal plane to investigate patch in relation to zooplankton behavior (Hamner 1988). scale. They showed that copepods did not have a char- Size and animal density of patches have been the acteristic patch length, and have a weaker wave- subjects of many studies. Usually patches have been number dependence than phytoplankton. In this study, we carried out continuous estimates of ' Present address: Tohoku National Fisheries Research an abundant and large boreal copepod, Neocalanus Institute, 21-5 Shinhama 3, Shiogama, Miyagi 985, Japan cristatus (Kr~yer),using a system like that of Mackas & O Inter-Research 1993 4 0 Mar. Ecol. Prog. Ser. 97: 39-46, 1993 P-03 electronic plankton- counting and sizing sys- tem, a Danfoss MAG- 1100/1000 electromagnetic flow meter and 5 Bran Lubbe AA11 Auto Analyzer channels. We did not ana- lyze the data obtained by the DO meter or the Auto 140'~ 150' 160' Analyzer in this study. Sea- Fig. l. Cruise track and location of micro-scale sampling station in the North Pacific. Thick water was pumped up to lines indicate area used for analysis in this study. Arrows indicate ship direction. Sampling the system the times: Line 1, 07:52 h on May 12 to 00:07 h on May 13; Line 2, 14:42 to 22:15 h on May 16; bottom (about 5 m depth). Line 3,02:21 h on May 31 to 13:36 h on June 2; Line 4,13:43 h on June 4 to 04:35 h on June 5; Flow rate of seawater Stn A, 00:08 h on May 13 to 14:41 h on May 15 and 13:36 h on June 2 to 13:43 h on June 4 through the system was maintained at 17.0 + 0.3 1 min-'. Data from each sen- Boyd (1979) to deduce patch characteristics and the re- sor were recorded together with navigation data on a lationships between copepod distribution and environ- f!oppy disk. For the plankton counting and sizing mental parameters. We also examined the spatial system, integrated data in 1 rnin were recorded. The scale and method of sampling suitable for quantitative navigation data were obtained from a Magnavox Type estimation of the copepod. 5000 hybrid navigation system through a local area network. Particles larger than 0.35 mm estimated spherical MATERIALS AND METHODS diameter which passed through the particle counter were enumerated in 10 size categories. The largest Observations were done at Stn A and along 4 tran- size category, ranging from 0.78 mm to the orifice di- sect lines during the RV 'Hakuho-Maru' cruise KH-91- ameter (3 mm), was used for the analysis. Neocalanus 3, from 10 May to 11 June 1991 (Fig. 1).A thermal front cristatus copepodite stage V and Eucalanus bungii was located around 43"N in all the transect lines. adult females were the only dominant organisms in Homogeneous subarctic water was selected for the this size category, which was revealed by microscopic analysis of Neocalanus cristatus, although they were counts and sizings of zooplankton collected by vertical also distributed abundantly in the frontal area. tows of the plankton net at Stn A (Tsuda & Sugisaki un- Therefore, the area studied was located between 43" publ.). Larger animals, such as euphausiids, were and 45" N with water temperature of <5.4"C. The area mostly trapped by a prefilter (3 mm mesh opening). was characterized by low chlorophyll a (0.7 pg I-') and Because E. bungii was distributed deeper than 40 m high nitrate concentration (16 PM) in the surface layer at Stn A (Koike in press). We carried out 2 series of observations. One con- sisted of meso/mega-scale observations performed while cruising at a ship speed of ca 30.6 km h-' along the tracks indicated by thick lines in Fig. 1. The second consisted of micro-scale observations to examine the - fine structure of the patches. Periods with low ship 4 05 speed (~1.85km h-') were selected from the data at -o Stn A. 3m Continuous recording of environmental parameters >3 2 and zooplankton abundance was done with an AMEMBO (Water Strider- AutoMated Environmental Monitor for Biological Oceanography). The AMEMBO 0 was of a design similar to the system of Boyd (1973) 0 2 4 6 with some modifications. The system consists of a bub- Particle counter count ble trap' Seabird thermosalinograph' a a Fig. 2. Relation between counts from the AMEMBO particle Turner Design 10-R in "jvo fluororneter, a DanfOss counter and visual counts of Neocalanus cristatus. *:A frag- TDO/EMCO dissolved oxygen (DO)meter, a Honchigo rnent of euphausiid was counted by the particle counter Tsuda et al.: Copepod patchiness 4 1 throughout the day (Tsuda & Sugisaki unpubl.), the patches (mean density <l73 ind. m-3 along 510 m of particles in this size range were considered to be N. track) were neglected in the meso/mega-scale obser- cristatus copepodite V. Moreover, direct visual counts vations. of copepods taken from the system outlet coincided Spectral analysis by FFT (Fast Fourier Trans- with counts from the particle counter (Fig. 2). formation) was made for the estimation of scale depen- The high-density areas (23 counts min-l) among the dencies of fluctuation in sigma-t, chlorophyll fluores- 2800 data along the 4 transect lines were regarded as cence and copepod abundance. copepod patches. Three counts min-' are equivalent ca 173 ind. m-3. and present 5 times the average den- sity at night or the average + 1.5 SD. This threshold is RESULTS comparable to those of relatively large copepod spe- cies in other studies (Barraclough et al. 1969, Mesolmega-scale observations Kawarnura & Hirano 1985). Furthermore, the median value was 0 counts min-' in the 2 transects and 1 in the The density of the copepod was frequently high from other transect; accordingly, our threshold was higher before dusk to midnight (Fig. 3). Abundance de- than that of Wiebe (1970).Patch lengths were defined creased from midnight until dawn and stayed low dur- as the number of consecutive sampling intervals (1min ing daylight. Discrete-depth net sampling showed that or 510 m) with continuous high density. We did not cal- Neocalanus cristatus was distributed in the 0 to 40 m culate patch diameter because there was no informa- layer throughout a day at Stn A (Tsuda & Sugisaki un- tion on patch shape. In this procedure, small-scale publ.). Therefore, circadian occurrence of the copepod indicated small-scale he1 vertical migration. Firstly, Morishita's Index I8 (Morishita 1959) was calculated to test the departure from randomness in the distribution. The formula is 16 = n xi(xi-l)/N(N-l), where n is the number of samples, X, is the number of copepods in the i th sample, and N is the total number of copepods.
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