Joji Ishizaka

Global verification of critical depth theory for phytoplankton bloom with climatological in situ temperature and satellite ocean color data

An investigation is made of the global relationship between seasonal variations of the surface mixed-layer depths derived from monthly climatological hydrographic data and seasonal variations of the surface pigments from monthly satellite ocean color data. At middle and high latitudes of the western North Pacific and the North Atlantic, shallowing of the mixed-layer depth from winter to spring largely explains basin-scale features of the spring bloom of phytoplankton in terms of Sverdrups critical depth theory. In these areas the spring bloom occurs from middle to high latitudes along with the increase of insolation from winter to spring. In the eastern North Pacific and the Southern Ocean the absence of a spring bloom is difficult to explain using the critical depth theory because Sverdrups parameters are treated as constants, which in nature vary with physiological and ecological conditions. At northern latitudes the termination of fall bloom corresponds to a deepening in the mixed layer beyond the critical depth. Sverdrups critical depth theory is found useful as a first step in examining the general pattern of phytoplankton seasonality.

Response of the equatorial Pacific to chlorophyll pigment in a mixed layer isopycnal ocean general circulation model

The influence of phytoplankton on the upper ocean dynamics and thermodynamics in the equatorial Pacific is investigated using an isopycnal ocean general circulation model (OPYC) coupled with a mixed layer model and remotely sensed chlorophyll pigment concentration from the Coastal Zone Color Scanner (CZCS). In the equatorial Pacific heat accumulation due to a higher abundance of chlorophyll pigments in the equatorial Pacific leads to a decrease of the mixed layer thickness. This generates anomalous westward geostrophic currents north and south of the equator. In the western equatorial Pacific, these anomalous geostrophic currents merge into and strengthen the equatorial undercurrent (EUC), supplying water mass from the 200 m depth to the eastern equatorial Pacific. This chlorophyll-induced response of the undercurrent enhances upwelling around 110W, resulting in a lower sea surface temperature (SST) than without chlorophyll. Thus, thermal gradients due to absorption of solar radiation by phytoplankton may contribute remotely to equatorial upwelling in the eastern Pacific.

Basin scale estimates of sea surface nitrate and new production from remotely sensed sea surface temperature and chlorophyll

The highly variable nature of T-N relationships in oceanic waters has restricted nitrate (N) measurements from remotely sensed sea surface temperature (SST) to small time and space domains. Here we show that if changes in T-N relationships resulting from phytoplankton (chlorophyll a) are taken into account, remote sensing can be exploited to provide high resolution maps of sea surface nitrate (SSN) that are valid over much larger scales than has been previously possible. We illustrate the potential of the method for monitoring basin scale, interannual variations in SSN in the north Pacific Ocean using co-registered imagery of SST and chl a and demonstrate the usefulness of such data for estimating basin scale annual new production.

Temporal and Spatial Variability of Phytoplankton Pigment Concentrations in the Japan Sea Derived from CZCS Images

Temporal and spatial variability of phytoplankton pigment concentrations in the Japan Sea are described, using monthly mean composite images of the Coastal Zone Color Scanner (CZCS). In order to describe the seasonal changes of pigment concentration from the results of the empirical orthogonal function (EOF) analysis, we selected four areas in the south Japan Sea. The pigment concentrations in these areas show remarkable seasonal variations. Two annual blooms appear in spring and fall. The spring bloom starts in the Japan Sea in February and March, when critical depth (CRD) becomes equal to mixed layer depth (MLD). The spring bloom in the southern areas (April) occurs one month in advance of that in the northern areas (May). This indicates that the pigment concentrations in the southern areas may increase rapidly in comparison with the northern areas since the water temperature increases faster in spring in the southern than in the northern areas. The fall bloom appears first in the southwest region, then in the southeast and northeast regions, finally appearing in the northwest region. Fall bloom appears in November and December when MLD becomes equal to CRD. The fall bloom can be explained by deepening of MLD in the Japan Sea. The pigment concentrations in winter are higher than those in summer. The low pigment concentrations dominate in summer.

Meridional distribution and carbon biomass of autotrophic picoplankton in the Central North Pacific Ocean during late Northern Summer 1990

The meridional distribution of autotrophic picoplankton groups in the central north Pacific was studied during the late northern summer of 1990. Sampling was along a section at 175°N which extended from 45°N to 8°S. The section is far from coastal regions and included subarctic, central gyre, and equatorial areas. Five autotrophic picoplankton groups, autotrophic microflagellate, red-fluorescing picoplankton,Synechococcus, prochlorophyte, and orange-fluorescing picoplankton, were identified from samples taken at stations distributed along this section. These five groups showed distinctive differences in their meridional and vertical distributions. The autotrophic microflagellates and red-fluorescing picoplankton showed distributions that were similar to that of chlorophyll a, which was dominated by the <3 μm size fraction. However, the vertical distribution of these groups was different.Synechococcus was found mostly in surface waters (PAR<10%) and was particularly abundant in the Kuroshio Extension and south of the equatorial region where the nitracline was shallow (50–75 m). Prochlorophytes were abundant in the deep euphotic layer (PAR 1-0.1%) from the south of the Kuroshio Extension to the south of the equatorial area. Orange-fluorescing picoplankton, which may be one kind of cyanobacteria but is larger than typical Synechococcus, were mostly distributed in the oligotrophic surface waters of the central gyre. The carbon biomass estimates for these organisms showed that these five groups dominated in different areas. The vertical distribution of carbon biomass did not correspond to that of chlorophyll a in the central gyre and south of the equator because of the larger carbon/ chlorophyll a ratio of Synechococcus and orange-fluorescing picoplankton relative to that of the other picoplankton.

Size and taxonomic plankton community structure and carbon flow at the equator, 175‡E during 1990–1994

Abstract Size and taxonomic structure of plankton community carbon biomass for the 0.2–2000 μm equivalent spherical diameter range were determined at the equator at 175°E in September 1990–1993 and April 1994. Total biomass of the plankton community ranged from 1944 to 3448 mg C m −2 . Phytoplankton, zooplankton and bacteria carbon biomasses were 604–1669 mg C m -2 , 300–797 mg C m 2 , and 968–1200 mg C m -2 , and the percentages were 31–54%, 15–26%, and 29–54%, respectively. Biomass of heterotrophic bacteria was always the largest fraction and Prochlorococcus biomass was second. Heterotrophic and autotrophic flagellates and dinoflagellates in the nanoplankton size range and copepods (adults and copepodites) in the mesoplankton range were also high. Relatively small biomass was observed in the microplankton size range. The differences in integrated biomass of plankton community for El Nin˜o type oligotrophic conditions of September 1990–1993 and non-El Nifio type mesotrophic conditions of April 1994 were generally small compared with the interannual difference during 1990–1993. However, the percentage of Prochlorococcus in phytoplankton carbon biomass was larger in non-El Nin˜o year. Biomasses of cyanobacteria, diatom, dinoflagellates, nauplii of copepods, and crustaceans other than copepods were larger in the non-El Nin˜o year. Primary production increased significantly from El Nin˜o to non-El Nin˜o years. Carbon flow through the plankton food chain was estimated using the plankton carbon biomass data, primary production measurements, and published empirical relationships.

Evaluation of coastal upwelling effects on phytoplankton growth by simulated culture experiments

Upwelling effects of subsurface water on phytoplankton growth were evaluated by 9 simulated culture experiments of coastal upwelling. Particular attention was paid to the effects of nutrient enrichment on the surface phytoplankton by the upwelling of nutrient rich subsurface water and of the exposure of the subsurface phytoplankters to surface radiation. The following are the results obtained: the lag period of phytoplankton growth was inversely related to water temperature; the maximum yield of phytoplankton was proportional to the amounts of available initial nutrients; the specific growth rates of phytoplankton were a function of both the initial nutrient concentrations and water temperature; and the maximum specific growth rate was simply proportional to water temperature. According to the relations found, a simple equation is presented for the estimation of phytoplankton growth in a given upwelling. Succession of species in the phytoplankton assemblage in upwelled water mass was also taken into consideration.

Calibration and Validation of the Ocean Color Version-3 Product from ADEOS OCTS

We present calibration and validation results of the OCTS’s ocean color version-3 product, which mainly consists of the chlorophyll-a concentration (Chl-a) and the normalized water-leaving radiance (nLw). First, OCTS was calibrated for the inter-detector sensitivity difference, offset, and absolute sensitivity using external calibration source. It was also vicariously calibrated using in-situ measurements for water (Chl-a andnLw) and atmosphere (optical thickness), which were acquired synchronously with OCTS under cloud-free conditions. Second, the product was validated using selected 17 in-situ Chl-a and 11 in-situnLw measurements. We confirmed that Chl-a was estimated with an accuracy of 68% for Chl-a less than 2 mg/m3, andnLw from 94% (band 2) to 128% (band 4). Geometric accuracy was improved to 1.3 km. Stripes were significantly reduced by modifying the detector normalization factor as a function of input radiance.

Towards a General Description of Phytoplankton Growth for Biogeochemical Models

The growth of phytoplankton is fundamentally important to biogeochemical cycling in the sea, and models of this process are essential to describing the fluxes of carbon, nitrogen, and many other elements in the ocean. We address here nitrogen-based models that predict the photosynthesis and growth of phytoplankton for use in basin-scale simulations of marine biogeochemical processes. These models describe light absorption by photosynthetic pigments and biological transformations of carbon and nitrogen, so they must specify, implicitly or explicitly, the cellular chemical composition of phytoplankton (i.e., chlorophyll a, C and N) and photosynthesis per unit chlorophyll a as a function of irradiance (P B vs. E).

Phytoplankton Pigment Distributions in Regional Upwelling around the Izu Peninsula Detected by Coastal Zone Color Scanner on May 1982

Phytoplankton pigment (chlorophylla+pheopigments) distributions in a regional upwelling around the Izu Peninsula obtained by the Coastal Zone Color Scanner (CZCS) on May 23, 1982, were compared with ship-observed pigment and satellite sea-surface-temperature distributions. Pigment concentrations detected by the CZCS were positively correlated with the ship-observed pigment concentrations. However, they were about factor of 5 smaller when atmospheric correction parameters known for typical oceanic and land aerosol were used and when the parameters were estimated with the “clear water algorithm”. When the atmospheric correction parameters were adjusted so that a pigment concentration derived by CZCS was equivalent to a concentration obtained by the ship at a coincide location, the pattern and magnitudes of the CZCS-derived pigment distributions showed remarkable agreement with ship-observed pigment distributions. Thus, the normal atmospheric correction algorithm may not be suitable for waters around Japan, and the development of better atmospheric correction methods combined with more verification programs is required. The pigment distributions showed patterns that were similar to those observed in sea-surface-temperature distributions. Cold water showed higher pigment concentrations, and warm water showed lower pigment concentrations. The Kuroshio, which can be identified by generally warm, low pigment water, showed a large meander which flowed offshore at Shiono-misaki, looped back onshore from Hachijo Island to Omaezaki and then flowed northeast along the Izu and Boso Peninsulas. Locally upwelled water along the Izu Peninsula was seen clearly in the sea-surface-temperature and CZCS pigment distributions as a region of cold water and high pigment concentrations. Cold upwelled waters were also found at the eastern side of the Izu Islands, but pigment concentrations in these waters was not always high. This difference in the two upwelling regions may be caused by different physical and biological interactions.