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Apparent phytoplankton bloom due to island mass effect
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Journal of Marine Systems 69 (2008) 238 – 246 www.elsevier.com/locate/jmarsys
Apparent phytoplankton bloom due to island mass effect Daisuke Hasegawa a,⁎, Hidekatsu Yamazaki b , Takashi Ishimaru b , Hideki Nagashima b , Yoshio Koike b a
b
Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J1 Faculty of Marine Science, Tokyo University of Marine Science and Technology, 5-7, Konan 4, Minato-ku, Tokyo 108-8477, Japan Received 12 August 2005; received in revised form 20 April 2006; accepted 29 April 2006 Available online 3 March 2007
Abstract A continuous monitoring of temperature and chlorophyll-a (Chl-a) concentration from a surface water monitoring system and a towed free fall instrument (MVP) around a small island in the Kuroshio showed low sea surface temperature (SST) and high surface Chl-a concentration (SCC) distribution in the lee of the island that indicates typical “island mass effect” phenomena. When the observed Chl-a profiles (0 to 250 m) were integrated, the total amounts in the lee side data were slightly smaller than those of the upstream side of the island. The difference was statistically significant at the 95% confidence level. The cross section diagram of Chl-a indicated the diffusion of subsurface Chl-a maximum (SCM) from the upstream to the downstream flanks of the island. The diffusivity of SCM and the change of potential energy require the same level of strong turbulent dissipation rate at the flanks of the island. That is consistent with our previous direct measurement in a similar hydrodynamic condition. Therefore, the observed high SCC is due to turbulent diffusion of SCM, and clearly showed that high SCC does not require any new production. Although a high fluorescence field behind an isolated island in a strong flow is often visible from satellite images, the images do not necessarily indicate an enhanced primary production at that moment. © 2007 Elsevier B.V. All rights reserved. Keywords: Kuroshio; Subsurface Chl-a maximum; Island wakes; Turbulent mixing
1. Introduction The Kuroshio is oligotrophic (Takahashi et al., 1985; Kaneko et al., 1998) due to strong stratification; however, it is known that this low-nutrient flow path is chosen as major spawning grounds by numerous small pelagic fish species (Watanabe et al., 1997). Flow perturbation by an island or a seamount has been suspected as one of the important processes for the ecosystem of the Kuroshio,
⁎ Corresponding author. E-mail address: [email protected] (D. Hasegawa). 0924-7963/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2006.04.019
because the Kuroshio always passes the Nansei Islands and the Izu–Ogasawara Ridge (Fig. 1A). The enhanced biological productivity in the vicinity of an island is known as the “island mass effect” (Doty and Oguri, 1956). Numerous studies have shown enhanced biological productivity and sometimes with evidence of upwelling, such as a doming of isopycnals (e.g. Heywood et al., 1990; Coutis and Middleton, 1999), a cold water formation, and increased levels of the nutrient content in the lee of islands in the Kuroshio (e.g. Takahashi et al., 1980; Kimura et al., 1994). Previous studies conjectured that the nutrification of the euphotic zone leads to an enhanced primary productivity (Takahashi et al., 1985;
D. Hasegawa et al. / Journal of Marine Systems 69 (2008) 238–246
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Fig. 1. (A) Absolute Dynamic Topography (color scale in meters) around Southeast Asia on 27 September 2003. White lines indicate bathymetry. (B) The enlarged bathymetry around Nakano-shima. Dotted line indicates the observational ship track with ADCP, thermo-salinograph and MVP around the island. Black circles show MVP stations.
Furuya et al., 1986; Simpson and Tett, 1986; Heywood et al., 1990; Odate and Furuya, 1998; Signorini et al., 1999) and increased levels of zooplankton (copepods)
concentrations (Toda, 1989). However, these physical events associated with the “island mass effect” are understood only qualitatively, due to the complexities of
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turbulent flows around islands, as well as observational difficulties. Consequently, the rate of contributions to the biological enhancement of the ecosystem is not clearly quantified. Satellite data are useful for showing the elevated level of production around an isolated island for an extensive area. However, the information is limited to the surface features. In this study, we conducted a survey around an island in the Kuroshio with a free fall instrument to investigate the distribution of phytoplankton in subsurface layers. 2. Survey and instrumentations We made a field survey in the vicinity of Nakanoshima from the Research and Training Vessel R.T.V. Umitaka-maru. The island is located to the south of Kyusyu Island, Japan (Fig. 1). The size of Nakano-shima is approximately 5.5 × 11 km. During the experiment, the Kuroshio was flowing directly at Nakano-shima so that the “island mass effect” could be expected. CTD (FSI, Micro-CTD2) and fluorometer (WET Labs, FLF300) measured fine structure hydrography. These sensors were mounted on a free fall fish of MVP (BOT, MVP300 Multi-sensor). The deployment system allows the fish to continuously deploy from a vessel without stopping. Current data were obtained from a shipboard 38.4 kHz Acoustic Doppler Current Profiler (ADCP, RDI) and surface water was monitored by a thermo-salinograph system that consisted of a temperature–conductivity sensor (Ocean Systems, OS-200CT), and a fluorometer (WET Labs, WETStar). 3. Results and discussions 3.1. Current During the experiment, the Kuroshio was flowing northeast across a relatively shallow region between the edge of the continental shelf and the Nansei Islands. The current turned eastwards at the northern edge of the Nansei Islands then hit several islands (e.g. Kuchinoshima and Nakano-shima, see Fig. 1). The incident current was blocked by the islands and accelerated at the flanks of the islands (Fig. 2A). Current distribution from the ADCP data shows that the flow was accelerated at the flanks of Nakano-shima, the surface flow velocity reached 1.8 m s− 1 around Stations 5 and 23 (Fig. 2A and B). Since the shape of the island is conic, the spread angle of the deeper flow at the front side of the island is wider (∼130° at 176 m depth) than the flow at the surface layer (∼ 50° at 32 m depth).
The vertical section of horizontal velocities along the mean flow direction and the right-angle component (Fig. 2C and D) show the flow accelerations at the flanks of the island (at Stations 4–6, 12–17 and 21–27). These flow accelerations were caused by a flow blockage due to the island. The blocked water has to converge to the side flow. The incident flow is usually in geostrophic balance that shows a baroclinic flow (strong flow at the surface). Normally, even with the baroclinic condition (i.e. vertically sheared), background shear in the Kuroshio is not sufficient to generate turbulence, due to strong stratification; however, when the shear 2 level i exceeds the critical level (i.e., RiuN 2 = Au Az b1=4, where Ri is the gradient Richardson number, N is the buoyancy frequency and ui is horizontal velocity), turbulent mixing can take place. Theoretically, the velocity distribution for twodimensional irrotational flow past a circular cylinder would double the far-field incident flow, 2U, at the flanks (Fig. 3A). Assuming that a three-dimensional flow structure past the cylinder as the superposition of two-dimensional layers (Fig. 3B), the maximum shear intensity would reach 4 times by virtue of the acceleration of each of the layers (Fig. 3C). Thus, the Ri can be reduced by a factor of one-fourth from the original value before the acceleration. Consequently, the strong shear at the flanks of the island may create instability, resulting in a vertical mixing of the water column. 3.2. Temperature Sea surface temperature (SST) was monitored, using a thermo-salinograph that took water samples at a depth of 12 m (Fig. 2E). SST was almost constant at 28.8 °C at the front side and along the flanks of the island (Stations 16 to 27). However, the SST gradually dropped towards the center of the lee, with the lowest value (28.0 °C) observed near Station 7. Downstream of the channel between Nakano-shima and Kuchino-shima (between Stations 2 and 5), the temperature was constant but relatively lower about 0.2 °C compared to the value at the front side of the island. Assuming that the incident vertical profile of the temperature for this region is steady and uniform, then the lower temperature at the lee side of the island and the downstream of the accelerated flow is due to upwelling and vertical mixing caused by the “island mass effect”. The vertical cross section of the temperature, measured by the MVP, shows shoaling of the 28.0 °C isotherm as evidence of the upwelling (Fig. 2F). The 28.0 °C isotherm corresponded to the bottom of the
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Fig. 2. Current vectors at (A) 32 m and (B) 176 m depth. The vertical section of horizontal velocities along the main current axis; (C) SE (135°) and its orthogonal component; (D) NE (45°). (E) Surface temperature is in red and Chl-a is in green. White circles represent the depth-integrated Chl-a. Vertical sections of (F) temperature and (G) Chl-a with density contour lines (white dashed lines).
surface mixed layer (SML), that existed around the island, and the doming of the isotherm pushed the SML upward and with turbulent mixing resulted in lowering the SST that was observed from the thermo-salinograph data.
3.3. Chlorophyll-a As opposed to the SST distribution, the surface Chl-a concentration (SCC) was high at the lowest SST region (green line plotted in Fig. 2E). The SCC at the front side
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Fig. 3. Flow past a circular cylinder (R, radius) for an idealistic (potential flow) case (A). The vertical profile of the horizontal velocity (B) and profiles of shear intensity (C). Dashed lines indicate the far field incident flow profiles, and solid lines indicate the accelerated profiles at the flanks of the cylinder θ = ±π/2.
and to the flanks of the island (16 to 26) was relatively low and the data range was between 0.13 and 0.15 mg m− 3. The SCC increased towards the lee of the island and reach a maximum value of 0.4 mg m− 3 near Station 7 where the minimum SST was observed. In the downstream area of Kuchino-shima (Stations 1 to 2), the SCC is higher than at the front of the island and indicates another “island mass effect”. The SCC is generally regarded as a measure of productivity in that region. Satellite images and time series readings of fluorescence measured from a shipboard thermo-salinograph are often used to identify areas of high production. Do the values of SCC indicate actual new production at the region? Generally, in the oligotrophic region, when upwelling or/and turbulent mixing take place, extra nutrient is supplied to the euphotic zone from the lower water column where light limits the growth of phytoplankton cells. Then, diatoms with high growth rate would increase rapidly (Furuya et al., 1986), in comparison with originally dominated species in SCM. However, since the original percentage of diatoms in the region is usually low, at most a few percent, and there is a time lag to adapt phytoplankton to a new environment (Ishizaka et al., 1983). Thus, it takes at least a day to get a visible increment and takes a few days to see multiple increment as a clear bloom, and which would take place at hundreds kilometers off from the island (Thomas et al., 1980; Ishizaka et al., 1983). When a sizable island, i.e. projected diameter N10 km, in a relatively weak flow traps an isolated mixed water mass in the immediately down stream, so
that phytoplankton bloom can take place, as a consequence, the depth integrated biomass increases. The flow passage from the upstream to the downstream (Stations 19 to 11) is about 14 km, thus the mean current speed (0.9 m s− 1) provides an advective time scale of about 4.3 h. Therefore, that is not long enough to match the typical time scale required for phytoplankton blooms induced from enriched euphotic layers. Actually, the depth-integrated Chl-a value decreased along the transect from the upstream to the downstream (Stations 16 to 11 in Fig. 2E). In addition, the averaged depth-integrated Chl-a value (Table 1) in the downstream (31.2 mg m− 2, for Stations 6 to 15 and 25 to 29) is lower than the averaged value of the upstream (36.6 mg m− 2, for Stations 16 to 24). The difference is statistically significant (at the 95% confidence level). The phytoplankton content actually decreased in the wake, that might be associated with the advection related with surface divergence (upwelling) or biomass loss due to predation.
Table 1 Regionally averaged depth-integrated Chl-a of the upstream (Stations 16 to 24) and the downstream (Stations 6 to 15 and 25 to 29) Depth-integrated Chl-a [mg m− 2]
Upstream Downstream
Mean
S.D.
Number of profiles
36.6 31.2
4.6 5.1
9 15
The difference between these two means is statistically significant according to t-test (at the 95% confidence level).
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Consequently, since we confirmed no evidence of a phytoplankton bloom, the observed increment of SCC may be explained by redistribution of phytoplankton in the region. Although the nutrient level at the surface is usually depleted, the nutrient level in the subsurface layer may be sufficient to support production, and this condition usually results in a subsurface Chl-a maximum (SCM) at the bottom of the euphotic zone. SCM is undetectable from surface information, and the surface value does not reflect the total biological productivity in that region. The Kuroshio region is typically oligotrophic, but SCM has been identified (Takahashi et al., 1985; Odate and Furuya, 1998). SCM commonly occurs at a depth between 30 and 100 m (Takahashi et al., 1985). According to our data, the SCM appears at the pycnocline (σθ = 22.25 kg m− 3) on the front side and on the flanks of the island (Stations 16–26). The maximum value of the SCM is roughly 0.5 mg m− 3 at Stations 19–21 (Fig. 2G). In comparison with those profiles with a clear peak in the upstream (Chl-a N 0.4 mg m− 3 , at Stations 18–22), the SCM gradually faded from the upstream to the downstream and the prominent peak finally disappeared altogether around Station 12 (Figs. 2G and 4A). Assuming that the vertical structure of Chl-a is steady in the incident flow, then the fading of the SCM along the streamline from the upstream to the lee of the island may be explained by vertical diffusion due to turbulent mixing. This assumption should be reasonable for a snapshot observation because the horizontal distribution of phytoplankton is relatively uniform where no episodic input of nutrients exists, such as storm induced mixing (Hayward, 1987). Prior to our experiment, no significant storm was reported in the area. In the vicinity of the island, flow disturbance took place due to flow blockage by the island. In order to evaluate the stability of the water column in the area, Ri was computed using an observed flow and density field of 16 m data resolution for the upper 255 m. About 19.4% of computational cells were Rib 1/4 and the 52.5% was Ri b 1.0. Thus significant sections of the water column were unstable to cause turbulent mixing. Hasegawa et al. (2004) reported one of the largest rates of turbulent kinetic energy dissipation O (× 10− 4 W kg− 1) for open ocean at the flanks of Aoga-shima at those times when the Kuroshio hit the island directly. In order to estimate the rate of turbulent kinetic energy dissipation ε, for our observations, we estimate the vertical eddy diffusivity Kz, from the distribution of Chl-a and infer ε from Kz.
243
The diffusion equation for Chl-a concentration is AC AC A2 C þ uj ¼ Kj ; At Axj Axj Axj
ð1Þ
where t is time, xj = (x, y, z) are spatial coordinates, C (x, y, z, t) is Chl-a concentration, uj = (u, v, w) are velocity components and Kj = (Kx, Ky, Kz) are diffusion coefficients. We take the coordinate system that aligns with the main stream direction on the x axis, and uj = (U, 0, 0). We also assume the lateral diffusion is negligible in comparison with the vertical diffusion. Then the diffusion equation for the observed SCM diffusion process may be expressed as follows: AC AC A2 C þU ¼ Kz 2 : At Ax Az
ð2Þ
The solution for a point source case is given as follows: Cðt; x; zÞ ¼
z2 exp − ; 1 4Kz t ð4pKz tÞ2 M dðnÞ
ð3Þ
where M is the total amount of Chl-a for the water column and δ(ξ) is the delta function with ξ = x − Ut. Thus the problem can be analyzed as a simple one-dimensional diffusion case. A similar approach has been used to infer the vertical eddy diffusivity from a dye experiment (Ledwell et al., 2000). Since this solution is same as the pffiffiffiffiffiffiffiffiffiffi 1=ffiffiffiffiffiffiffiffiffi 2pr2 expð−z2 =2r2 Þ with a Gaussian distribution, p standard deviation, r ¼ 2Kz t , the relationship between the standard deviation and the vertical eddy diffusivity can be written as follows: r2 ¼ 2Kz tYKz ¼
1 Ar2 : 2 At
ð4Þ
We estimate Kz from the rate change of the variance (σ2). For our data, the vertical profiles of Chl-a are asymmetric across the peak. Thus, we have defined the standard deviation σ for the upper layer (σup) and the lower layer (σlow) separately. Z r Dpeak
Z CðzÞdz ¼ 68% of
Dbase
CðzÞdz;
ð5Þ
Dpeak
where Dpeak is the depth of Chl-a peak and Dbase is the upper/lower limit of the integration. We assume the
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Fig. 4. (A) Profiles of buoyancy frequency at Stations 11 to 18 (region a in Fig. 2). Colored circles indicate Chl-a concentration. (B) Chl-a profiles. The y-axis indicates squared vertical distance from a peak (SCMD). Black diamonds and white circles show one standard deviation from SCMD. (C) Potential energy anomaly for the upper and the lower 50-m layer from SCMD.
Chl-a field is uniform and steady, thus we converted the horizontal distance, x, to the time scale,
Kz ¼
1 Ar2 1 Dr2 f 2 At 2 Dx=jU¯ j:
ð6Þ
Since strong stratification at the lower layer (Fig. 4A) suppresses the vertical mixing, the rate change of σ at the lower layer is smaller than that of the upper layer (Fig. 4B). The estimated Kz (Table 2) was threefold larger in the upper layer (0.23 m2 s− 1) than the lower layer (0.07 m2 s− 1).
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The turbulent kinetic energy (TKE) is supplied by large-scale shear and the TKE is dissipated by viscous friction at a rate (ε), while roughly 100 × γmix % of the kinetic energy is converted to potential energy (Gregg, 1987). The relationship between the diffusivity and the rate of kinetic energy dissipation is ec ¼
N 2 d Kz ; gmix
ð7Þ
where γmix is the mixing efficiency. We take the maximum efficiency (0.2) to estimate a conservative vale of ε (Osborn, 1980). The estimated kinetic energy dissipation rate reaches εc = 1.1 × 10− 4 W kg− 1 for the lower layer and εc = 0.3 × 10− 4 W kg− 1 for the upper layer (εc in Table 2). The upper layer value is smaller than the lower layer value, but both in the same order, and these values are consistent with the values observed in the vicinity of Aoga-shima while the hydrographic condition was similar. At the flanks of Nakano-shima, we observed a decrease in SST and the uplift of the pycnocline as evidence of the upwelling and the turbulent mixing due to flow perturbation by the island. Since SCMD was consistent with the depth of the pycnocline, we took SCMD as the reference depth to calculate the potential energy of the water column to estimate the intensity of turbulence. For simplicity, we took the potential energy anomaly, ϕ, as follows: Z 1 h /¼ ðq − q ð8Þ ¯ Þd g d fdf; h 0 where ρ is water density ( ρ¯ = 1024 kg m− 3) and ζ is a vertical distance referenced to SCMD, ζ = |z − SCMD| and ζ = |z − SCMD + h|, for the upper and the lower layers. We took the layer thickness h as 50 m. The relationship between the rate of kinetic energy dissipation and the potential energy anomaly is
245
Table 2 Estimated vertical diffusivity and turbulence energy dissipation rate from the vertical diffusion of the Chl-a profiles (εc) and the rate change of the potential energy due to turbulent mixing (εϕ) at the flank of the island (Stations 16 to 11 of Fig. 4B and C) N Kz (×10− 2 s− 1) (m2 s− 1) Upper 0.54 Lower 1.75
0.23 0.07
εc εϕ (× 10− 4 W kg− 1) (× 10− 4 W kg− 1) 0.3 1.1
0.2 0.8
shallow, thus bottom friction may generate additional turbulent mixing in the deeper layer. In short, adjacent to the island, strong turbulence is taking place against the strong stratification, mixing the water column vertically. 4. Conclusions At the lee of Nakano-shima island in the Kuroshio, we observed cold water formation with high Chl-a concentration that suggests a conventional “island mass effect” phenomena. However, observed features revealed “apparent phytoplankton bloom” due to the island mass effect in the region. The cooling of the surface waters and the elevated level of the SCC at the lee of the island can be explained by the mixing of the water column and the diffusion of SCM, due to strong turbulence caused by the flow perturbation around the island. Therefore, SCC features require special attention in order to differentiate elevated new production from “apparent bloom” due to the diffusion of SCM. Although we have only focused on the Chl-a feature adjacent to an island, topographically induced turbulence significantly modifies the physical and biological conditions that result in a true phytoplankton bloom downstream. Thus, considerable efforts are required to quantify the effect of the mixing caused by small islands in the Kuroshio. Acknowledgments
A/ 1 D/ f e/ ¼ ¯ j: gmix d q gmix d q ¯ At ¯ Dx=j U 1
ð9Þ
The estimated kinetic energy dissipation rate reaches εϕ = 0.8 × 10− 4 W kg − 1 for the lower layer and εϕ = 0.2 × 10− 4 W kg− 1 for the upper layer (Table 2). These values are consistent with εc based on Chl-a distribution. The difference in the turbulent intensity might be explained by the nature of velocity profile associated with the Kuroshio. Usually, strong shear exists at the pycnocline and causes strong turbulence in the deeper layer (Fig. 3B and C). Another possibility is bottom friction. At the vicinity of the island, bottom depth is
We thank the crew of the R.T.V. Umitaka-maru for support during the experiment. N. Horimoto and Y. Matsumoto provided technical support for calibration of the fluorometers. J. Yoshida, Y. Tanaka, Y. Kitade and T. Masuda provided valuable discussion on this manuscript. Also, we thank two reviewers for their constructive suggestions. The bathymetric data were obtained from NOAA's National Geophysical Data Center (http://www. ngdc.noaa.gov/ngdc.html) and the Japan Oceanographic Data Center (http://www.jodc.go.jp/). The Absolute Dynamic Topography data were provided by AVISO (http:// www.aviso.oceanobs.com/). The project was partially
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funded by a Grant-in-Aid for Science Research (B) 16310005 from the Japan Society for the Promotion of Science. References Coutis, P.F., Middleton, J.H., 1999. Flow–topography interaction in the vicinity of an isolated, deep ocean island. Deep-Sea Res. I 46, 1633–1652. Doty, M.S., Oguri, M., 1956. The island mass effect. J. Cons. - Cons. Int. Explor. Mer 22, 33–37. Furuya, K., Takahashi, M., Nemoto, T., 1986. Summer phytoplankton community structure and growth in a regional upwelling area off Hachijo Island, Japan. J. Exp. Mar. Biol. Ecol. 96, 43–55. Gregg, M.C., 1987. Diapycnal mixing in the thermocline: a review. J. Geophys. Res. 92, 5249–5286. Hasegawa, D., Yamazaki, H., Lueck, R.G., Seuront, L., 2004. How islands stir and fertilize the upper ocean. Geophys. Res. Lett. 31, L16303. doi:10.1029/2004GL020143. Hayward, T.L., 1987. The nutrient distribution and primary production in the Central North Pacific. Deep-Sea Res. I 34, 1593–1627. Heywood, K.J., Barton, E.D., Simpson, J.H., 1990. The effects of flow disturbance by an oceanic island. J. Mar. Res. 48, 55–73. Ishizaka, J., Takahashi, M., Ichimura, S., 1983. Evaluation of coastal up-welling effects on phytoplankton growth by simulated culture experiments. Mar. Biol. 76 (3), 271–278. Kaneko, I., Takatsuki, Y., Kamiya, H., Kawae, S., 1998. Water property and current distributions along the WHP-P9 section (137°–142°E) in the Western North Pacific. J. Geophys. Res. 103 (C6), 12,959–12,984. Kimura, S., Choo, S.H., Sugimoto, T., 1994. Characteristics of the eddy caused by Izu-Oshima Island and the Kuroshio Branch Current in Sagami Bay. J. Oceanogr. Soc. Jpn. 50, 373–389.
Ledwell, J.R., Montgomery, E.T., Polzin, K.L., Laurent, L.C.S., Schmitt, R.W., Toole, J.M., 2000. Evidence for enhanced mixing over rough topography in the abyssal ocean. Nature 403, 179–182. Odate, T., Furuya, K., 1998. Well-developed subsurface chlorophyll maximum near Komahashi No. 2 Seamount in the summer of 1991. Deep-Sea Res. I 45, 1595–1607. Osborn, T.R., 1980. Estimates of the local rate of vertical diffusion from dissipation measurements. J. Phys. Oceanogr. 10, 83–89. Signorini, S.R., McClain, C.R., Dandonneau, Y., 1999. Mixing and phytoplankton bloom in the wake of the Marquesas Islands. Geophys. Res. Lett. 26, 3121–3124. Simpson, J.H., Tett, P.B., 1986. An induced mixing experiment. In: Bowman, J., Yentsch, M., Peterson, W.T. (Eds.), Tidal Mixing and Phytoplankton Dynamics, vol. 17. Springer-Verlag, Berlin, pp. 483–495. Takahashi, M., Koike, I., Ishimaru, T., Saino, T., Furuya, K., Fujita, Y., Hat-tori, A., Ichimura, S., 1980. Upwelling plumes in Sagami Bay and adjacent water around Izu Islands. J. Oceanogr. Soc. Jpn. 36, 209–216. Takahashi, M., Nakai, T., Ishimaru, T., Hasumoto, H., Fujita, Y., 1985. Distribution of the subsurface chlorophyll maximum and its nutrient–light environment in and around the Kuroshio off Japan. J. Oceanogr. Soc. Jpn. 41, 73–80. Thomas, W.H., Pollock, M., Seibert, D.L.R., 1980. Effects of simulated upwelling and oligotrophy on chemostat-grown natural marine phytoplankton assemblages. J. Exp. Mar. Biol. Ecol. 45, 25–36. Toda, H., 1989. Surface distributions of copepods in relation to regional up-wellings around the Izu Islands in summer of 1988. J. Oceanogr. Soc. Jpn. 45 (4), 251–257. Watanabe, Y., Zenitani, H., Kimura, S., 1997. Variations in spawning ground area and egg density of the Japanese Sardine in the Pacific coastal and oceanic waters. Fish. Oceanogr. 6, 35–40.
Journal of Marine Systems 69 (2008) 238 – 246 www.elsevier.com/locate/jmarsys
Apparent phytoplankton bloom due to island mass effect Daisuke Hasegawa a,⁎, Hidekatsu Yamazaki b , Takashi Ishimaru b , Hideki Nagashima b , Yoshio Koike b a
b
Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J1 Faculty of Marine Science, Tokyo University of Marine Science and Technology, 5-7, Konan 4, Minato-ku, Tokyo 108-8477, Japan Received 12 August 2005; received in revised form 20 April 2006; accepted 29 April 2006 Available online 3 March 2007
Abstract A continuous monitoring of temperature and chlorophyll-a (Chl-a) concentration from a surface water monitoring system and a towed free fall instrument (MVP) around a small island in the Kuroshio showed low sea surface temperature (SST) and high surface Chl-a concentration (SCC) distribution in the lee of the island that indicates typical “island mass effect” phenomena. When the observed Chl-a profiles (0 to 250 m) were integrated, the total amounts in the lee side data were slightly smaller than those of the upstream side of the island. The difference was statistically significant at the 95% confidence level. The cross section diagram of Chl-a indicated the diffusion of subsurface Chl-a maximum (SCM) from the upstream to the downstream flanks of the island. The diffusivity of SCM and the change of potential energy require the same level of strong turbulent dissipation rate at the flanks of the island. That is consistent with our previous direct measurement in a similar hydrodynamic condition. Therefore, the observed high SCC is due to turbulent diffusion of SCM, and clearly showed that high SCC does not require any new production. Although a high fluorescence field behind an isolated island in a strong flow is often visible from satellite images, the images do not necessarily indicate an enhanced primary production at that moment. © 2007 Elsevier B.V. All rights reserved. Keywords: Kuroshio; Subsurface Chl-a maximum; Island wakes; Turbulent mixing
1. Introduction The Kuroshio is oligotrophic (Takahashi et al., 1985; Kaneko et al., 1998) due to strong stratification; however, it is known that this low-nutrient flow path is chosen as major spawning grounds by numerous small pelagic fish species (Watanabe et al., 1997). Flow perturbation by an island or a seamount has been suspected as one of the important processes for the ecosystem of the Kuroshio,
⁎ Corresponding author. E-mail address: [email protected] (D. Hasegawa). 0924-7963/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2006.04.019
because the Kuroshio always passes the Nansei Islands and the Izu–Ogasawara Ridge (Fig. 1A). The enhanced biological productivity in the vicinity of an island is known as the “island mass effect” (Doty and Oguri, 1956). Numerous studies have shown enhanced biological productivity and sometimes with evidence of upwelling, such as a doming of isopycnals (e.g. Heywood et al., 1990; Coutis and Middleton, 1999), a cold water formation, and increased levels of the nutrient content in the lee of islands in the Kuroshio (e.g. Takahashi et al., 1980; Kimura et al., 1994). Previous studies conjectured that the nutrification of the euphotic zone leads to an enhanced primary productivity (Takahashi et al., 1985;
D. Hasegawa et al. / Journal of Marine Systems 69 (2008) 238–246
239
Fig. 1. (A) Absolute Dynamic Topography (color scale in meters) around Southeast Asia on 27 September 2003. White lines indicate bathymetry. (B) The enlarged bathymetry around Nakano-shima. Dotted line indicates the observational ship track with ADCP, thermo-salinograph and MVP around the island. Black circles show MVP stations.
Furuya et al., 1986; Simpson and Tett, 1986; Heywood et al., 1990; Odate and Furuya, 1998; Signorini et al., 1999) and increased levels of zooplankton (copepods)
concentrations (Toda, 1989). However, these physical events associated with the “island mass effect” are understood only qualitatively, due to the complexities of
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turbulent flows around islands, as well as observational difficulties. Consequently, the rate of contributions to the biological enhancement of the ecosystem is not clearly quantified. Satellite data are useful for showing the elevated level of production around an isolated island for an extensive area. However, the information is limited to the surface features. In this study, we conducted a survey around an island in the Kuroshio with a free fall instrument to investigate the distribution of phytoplankton in subsurface layers. 2. Survey and instrumentations We made a field survey in the vicinity of Nakanoshima from the Research and Training Vessel R.T.V. Umitaka-maru. The island is located to the south of Kyusyu Island, Japan (Fig. 1). The size of Nakano-shima is approximately 5.5 × 11 km. During the experiment, the Kuroshio was flowing directly at Nakano-shima so that the “island mass effect” could be expected. CTD (FSI, Micro-CTD2) and fluorometer (WET Labs, FLF300) measured fine structure hydrography. These sensors were mounted on a free fall fish of MVP (BOT, MVP300 Multi-sensor). The deployment system allows the fish to continuously deploy from a vessel without stopping. Current data were obtained from a shipboard 38.4 kHz Acoustic Doppler Current Profiler (ADCP, RDI) and surface water was monitored by a thermo-salinograph system that consisted of a temperature–conductivity sensor (Ocean Systems, OS-200CT), and a fluorometer (WET Labs, WETStar). 3. Results and discussions 3.1. Current During the experiment, the Kuroshio was flowing northeast across a relatively shallow region between the edge of the continental shelf and the Nansei Islands. The current turned eastwards at the northern edge of the Nansei Islands then hit several islands (e.g. Kuchinoshima and Nakano-shima, see Fig. 1). The incident current was blocked by the islands and accelerated at the flanks of the islands (Fig. 2A). Current distribution from the ADCP data shows that the flow was accelerated at the flanks of Nakano-shima, the surface flow velocity reached 1.8 m s− 1 around Stations 5 and 23 (Fig. 2A and B). Since the shape of the island is conic, the spread angle of the deeper flow at the front side of the island is wider (∼130° at 176 m depth) than the flow at the surface layer (∼ 50° at 32 m depth).
The vertical section of horizontal velocities along the mean flow direction and the right-angle component (Fig. 2C and D) show the flow accelerations at the flanks of the island (at Stations 4–6, 12–17 and 21–27). These flow accelerations were caused by a flow blockage due to the island. The blocked water has to converge to the side flow. The incident flow is usually in geostrophic balance that shows a baroclinic flow (strong flow at the surface). Normally, even with the baroclinic condition (i.e. vertically sheared), background shear in the Kuroshio is not sufficient to generate turbulence, due to strong stratification; however, when the shear 2 level i exceeds the critical level (i.e., RiuN 2 = Au Az b1=4, where Ri is the gradient Richardson number, N is the buoyancy frequency and ui is horizontal velocity), turbulent mixing can take place. Theoretically, the velocity distribution for twodimensional irrotational flow past a circular cylinder would double the far-field incident flow, 2U, at the flanks (Fig. 3A). Assuming that a three-dimensional flow structure past the cylinder as the superposition of two-dimensional layers (Fig. 3B), the maximum shear intensity would reach 4 times by virtue of the acceleration of each of the layers (Fig. 3C). Thus, the Ri can be reduced by a factor of one-fourth from the original value before the acceleration. Consequently, the strong shear at the flanks of the island may create instability, resulting in a vertical mixing of the water column. 3.2. Temperature Sea surface temperature (SST) was monitored, using a thermo-salinograph that took water samples at a depth of 12 m (Fig. 2E). SST was almost constant at 28.8 °C at the front side and along the flanks of the island (Stations 16 to 27). However, the SST gradually dropped towards the center of the lee, with the lowest value (28.0 °C) observed near Station 7. Downstream of the channel between Nakano-shima and Kuchino-shima (between Stations 2 and 5), the temperature was constant but relatively lower about 0.2 °C compared to the value at the front side of the island. Assuming that the incident vertical profile of the temperature for this region is steady and uniform, then the lower temperature at the lee side of the island and the downstream of the accelerated flow is due to upwelling and vertical mixing caused by the “island mass effect”. The vertical cross section of the temperature, measured by the MVP, shows shoaling of the 28.0 °C isotherm as evidence of the upwelling (Fig. 2F). The 28.0 °C isotherm corresponded to the bottom of the
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Fig. 2. Current vectors at (A) 32 m and (B) 176 m depth. The vertical section of horizontal velocities along the main current axis; (C) SE (135°) and its orthogonal component; (D) NE (45°). (E) Surface temperature is in red and Chl-a is in green. White circles represent the depth-integrated Chl-a. Vertical sections of (F) temperature and (G) Chl-a with density contour lines (white dashed lines).
surface mixed layer (SML), that existed around the island, and the doming of the isotherm pushed the SML upward and with turbulent mixing resulted in lowering the SST that was observed from the thermo-salinograph data.
3.3. Chlorophyll-a As opposed to the SST distribution, the surface Chl-a concentration (SCC) was high at the lowest SST region (green line plotted in Fig. 2E). The SCC at the front side
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Fig. 3. Flow past a circular cylinder (R, radius) for an idealistic (potential flow) case (A). The vertical profile of the horizontal velocity (B) and profiles of shear intensity (C). Dashed lines indicate the far field incident flow profiles, and solid lines indicate the accelerated profiles at the flanks of the cylinder θ = ±π/2.
and to the flanks of the island (16 to 26) was relatively low and the data range was between 0.13 and 0.15 mg m− 3. The SCC increased towards the lee of the island and reach a maximum value of 0.4 mg m− 3 near Station 7 where the minimum SST was observed. In the downstream area of Kuchino-shima (Stations 1 to 2), the SCC is higher than at the front of the island and indicates another “island mass effect”. The SCC is generally regarded as a measure of productivity in that region. Satellite images and time series readings of fluorescence measured from a shipboard thermo-salinograph are often used to identify areas of high production. Do the values of SCC indicate actual new production at the region? Generally, in the oligotrophic region, when upwelling or/and turbulent mixing take place, extra nutrient is supplied to the euphotic zone from the lower water column where light limits the growth of phytoplankton cells. Then, diatoms with high growth rate would increase rapidly (Furuya et al., 1986), in comparison with originally dominated species in SCM. However, since the original percentage of diatoms in the region is usually low, at most a few percent, and there is a time lag to adapt phytoplankton to a new environment (Ishizaka et al., 1983). Thus, it takes at least a day to get a visible increment and takes a few days to see multiple increment as a clear bloom, and which would take place at hundreds kilometers off from the island (Thomas et al., 1980; Ishizaka et al., 1983). When a sizable island, i.e. projected diameter N10 km, in a relatively weak flow traps an isolated mixed water mass in the immediately down stream, so
that phytoplankton bloom can take place, as a consequence, the depth integrated biomass increases. The flow passage from the upstream to the downstream (Stations 19 to 11) is about 14 km, thus the mean current speed (0.9 m s− 1) provides an advective time scale of about 4.3 h. Therefore, that is not long enough to match the typical time scale required for phytoplankton blooms induced from enriched euphotic layers. Actually, the depth-integrated Chl-a value decreased along the transect from the upstream to the downstream (Stations 16 to 11 in Fig. 2E). In addition, the averaged depth-integrated Chl-a value (Table 1) in the downstream (31.2 mg m− 2, for Stations 6 to 15 and 25 to 29) is lower than the averaged value of the upstream (36.6 mg m− 2, for Stations 16 to 24). The difference is statistically significant (at the 95% confidence level). The phytoplankton content actually decreased in the wake, that might be associated with the advection related with surface divergence (upwelling) or biomass loss due to predation.
Table 1 Regionally averaged depth-integrated Chl-a of the upstream (Stations 16 to 24) and the downstream (Stations 6 to 15 and 25 to 29) Depth-integrated Chl-a [mg m− 2]
Upstream Downstream
Mean
S.D.
Number of profiles
36.6 31.2
4.6 5.1
9 15
The difference between these two means is statistically significant according to t-test (at the 95% confidence level).
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Consequently, since we confirmed no evidence of a phytoplankton bloom, the observed increment of SCC may be explained by redistribution of phytoplankton in the region. Although the nutrient level at the surface is usually depleted, the nutrient level in the subsurface layer may be sufficient to support production, and this condition usually results in a subsurface Chl-a maximum (SCM) at the bottom of the euphotic zone. SCM is undetectable from surface information, and the surface value does not reflect the total biological productivity in that region. The Kuroshio region is typically oligotrophic, but SCM has been identified (Takahashi et al., 1985; Odate and Furuya, 1998). SCM commonly occurs at a depth between 30 and 100 m (Takahashi et al., 1985). According to our data, the SCM appears at the pycnocline (σθ = 22.25 kg m− 3) on the front side and on the flanks of the island (Stations 16–26). The maximum value of the SCM is roughly 0.5 mg m− 3 at Stations 19–21 (Fig. 2G). In comparison with those profiles with a clear peak in the upstream (Chl-a N 0.4 mg m− 3 , at Stations 18–22), the SCM gradually faded from the upstream to the downstream and the prominent peak finally disappeared altogether around Station 12 (Figs. 2G and 4A). Assuming that the vertical structure of Chl-a is steady in the incident flow, then the fading of the SCM along the streamline from the upstream to the lee of the island may be explained by vertical diffusion due to turbulent mixing. This assumption should be reasonable for a snapshot observation because the horizontal distribution of phytoplankton is relatively uniform where no episodic input of nutrients exists, such as storm induced mixing (Hayward, 1987). Prior to our experiment, no significant storm was reported in the area. In the vicinity of the island, flow disturbance took place due to flow blockage by the island. In order to evaluate the stability of the water column in the area, Ri was computed using an observed flow and density field of 16 m data resolution for the upper 255 m. About 19.4% of computational cells were Rib 1/4 and the 52.5% was Ri b 1.0. Thus significant sections of the water column were unstable to cause turbulent mixing. Hasegawa et al. (2004) reported one of the largest rates of turbulent kinetic energy dissipation O (× 10− 4 W kg− 1) for open ocean at the flanks of Aoga-shima at those times when the Kuroshio hit the island directly. In order to estimate the rate of turbulent kinetic energy dissipation ε, for our observations, we estimate the vertical eddy diffusivity Kz, from the distribution of Chl-a and infer ε from Kz.
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The diffusion equation for Chl-a concentration is AC AC A2 C þ uj ¼ Kj ; At Axj Axj Axj
ð1Þ
where t is time, xj = (x, y, z) are spatial coordinates, C (x, y, z, t) is Chl-a concentration, uj = (u, v, w) are velocity components and Kj = (Kx, Ky, Kz) are diffusion coefficients. We take the coordinate system that aligns with the main stream direction on the x axis, and uj = (U, 0, 0). We also assume the lateral diffusion is negligible in comparison with the vertical diffusion. Then the diffusion equation for the observed SCM diffusion process may be expressed as follows: AC AC A2 C þU ¼ Kz 2 : At Ax Az
ð2Þ
The solution for a point source case is given as follows: Cðt; x; zÞ ¼
z2 exp − ; 1 4Kz t ð4pKz tÞ2 M dðnÞ
ð3Þ
where M is the total amount of Chl-a for the water column and δ(ξ) is the delta function with ξ = x − Ut. Thus the problem can be analyzed as a simple one-dimensional diffusion case. A similar approach has been used to infer the vertical eddy diffusivity from a dye experiment (Ledwell et al., 2000). Since this solution is same as the pffiffiffiffiffiffiffiffiffiffi 1=ffiffiffiffiffiffiffiffiffi 2pr2 expð−z2 =2r2 Þ with a Gaussian distribution, p standard deviation, r ¼ 2Kz t , the relationship between the standard deviation and the vertical eddy diffusivity can be written as follows: r2 ¼ 2Kz tYKz ¼
1 Ar2 : 2 At
ð4Þ
We estimate Kz from the rate change of the variance (σ2). For our data, the vertical profiles of Chl-a are asymmetric across the peak. Thus, we have defined the standard deviation σ for the upper layer (σup) and the lower layer (σlow) separately. Z r Dpeak
Z CðzÞdz ¼ 68% of
Dbase
CðzÞdz;
ð5Þ
Dpeak
where Dpeak is the depth of Chl-a peak and Dbase is the upper/lower limit of the integration. We assume the
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Fig. 4. (A) Profiles of buoyancy frequency at Stations 11 to 18 (region a in Fig. 2). Colored circles indicate Chl-a concentration. (B) Chl-a profiles. The y-axis indicates squared vertical distance from a peak (SCMD). Black diamonds and white circles show one standard deviation from SCMD. (C) Potential energy anomaly for the upper and the lower 50-m layer from SCMD.
Chl-a field is uniform and steady, thus we converted the horizontal distance, x, to the time scale,
Kz ¼
1 Ar2 1 Dr2 f 2 At 2 Dx=jU¯ j:
ð6Þ
Since strong stratification at the lower layer (Fig. 4A) suppresses the vertical mixing, the rate change of σ at the lower layer is smaller than that of the upper layer (Fig. 4B). The estimated Kz (Table 2) was threefold larger in the upper layer (0.23 m2 s− 1) than the lower layer (0.07 m2 s− 1).
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The turbulent kinetic energy (TKE) is supplied by large-scale shear and the TKE is dissipated by viscous friction at a rate (ε), while roughly 100 × γmix % of the kinetic energy is converted to potential energy (Gregg, 1987). The relationship between the diffusivity and the rate of kinetic energy dissipation is ec ¼
N 2 d Kz ; gmix
ð7Þ
where γmix is the mixing efficiency. We take the maximum efficiency (0.2) to estimate a conservative vale of ε (Osborn, 1980). The estimated kinetic energy dissipation rate reaches εc = 1.1 × 10− 4 W kg− 1 for the lower layer and εc = 0.3 × 10− 4 W kg− 1 for the upper layer (εc in Table 2). The upper layer value is smaller than the lower layer value, but both in the same order, and these values are consistent with the values observed in the vicinity of Aoga-shima while the hydrographic condition was similar. At the flanks of Nakano-shima, we observed a decrease in SST and the uplift of the pycnocline as evidence of the upwelling and the turbulent mixing due to flow perturbation by the island. Since SCMD was consistent with the depth of the pycnocline, we took SCMD as the reference depth to calculate the potential energy of the water column to estimate the intensity of turbulence. For simplicity, we took the potential energy anomaly, ϕ, as follows: Z 1 h /¼ ðq − q ð8Þ ¯ Þd g d fdf; h 0 where ρ is water density ( ρ¯ = 1024 kg m− 3) and ζ is a vertical distance referenced to SCMD, ζ = |z − SCMD| and ζ = |z − SCMD + h|, for the upper and the lower layers. We took the layer thickness h as 50 m. The relationship between the rate of kinetic energy dissipation and the potential energy anomaly is
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Table 2 Estimated vertical diffusivity and turbulence energy dissipation rate from the vertical diffusion of the Chl-a profiles (εc) and the rate change of the potential energy due to turbulent mixing (εϕ) at the flank of the island (Stations 16 to 11 of Fig. 4B and C) N Kz (×10− 2 s− 1) (m2 s− 1) Upper 0.54 Lower 1.75
0.23 0.07
εc εϕ (× 10− 4 W kg− 1) (× 10− 4 W kg− 1) 0.3 1.1
0.2 0.8
shallow, thus bottom friction may generate additional turbulent mixing in the deeper layer. In short, adjacent to the island, strong turbulence is taking place against the strong stratification, mixing the water column vertically. 4. Conclusions At the lee of Nakano-shima island in the Kuroshio, we observed cold water formation with high Chl-a concentration that suggests a conventional “island mass effect” phenomena. However, observed features revealed “apparent phytoplankton bloom” due to the island mass effect in the region. The cooling of the surface waters and the elevated level of the SCC at the lee of the island can be explained by the mixing of the water column and the diffusion of SCM, due to strong turbulence caused by the flow perturbation around the island. Therefore, SCC features require special attention in order to differentiate elevated new production from “apparent bloom” due to the diffusion of SCM. Although we have only focused on the Chl-a feature adjacent to an island, topographically induced turbulence significantly modifies the physical and biological conditions that result in a true phytoplankton bloom downstream. Thus, considerable efforts are required to quantify the effect of the mixing caused by small islands in the Kuroshio. Acknowledgments
A/ 1 D/ f e/ ¼ ¯ j: gmix d q gmix d q ¯ At ¯ Dx=j U 1
ð9Þ
The estimated kinetic energy dissipation rate reaches εϕ = 0.8 × 10− 4 W kg − 1 for the lower layer and εϕ = 0.2 × 10− 4 W kg− 1 for the upper layer (Table 2). These values are consistent with εc based on Chl-a distribution. The difference in the turbulent intensity might be explained by the nature of velocity profile associated with the Kuroshio. Usually, strong shear exists at the pycnocline and causes strong turbulence in the deeper layer (Fig. 3B and C). Another possibility is bottom friction. At the vicinity of the island, bottom depth is
We thank the crew of the R.T.V. Umitaka-maru for support during the experiment. N. Horimoto and Y. Matsumoto provided technical support for calibration of the fluorometers. J. Yoshida, Y. Tanaka, Y. Kitade and T. Masuda provided valuable discussion on this manuscript. Also, we thank two reviewers for their constructive suggestions. The bathymetric data were obtained from NOAA's National Geophysical Data Center (http://www. ngdc.noaa.gov/ngdc.html) and the Japan Oceanographic Data Center (http://www.jodc.go.jp/). The Absolute Dynamic Topography data were provided by AVISO (http:// www.aviso.oceanobs.com/). The project was partially
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