Masayuki Takahashi

Measurement of photosynthetic production of a marine phytoplankton population using a stable 13C isotope

The use of stable isotope of carbon, 13C, for the determination of the photosynthetic rate of a marine phytoplankton population was examined. Particular concern was paid to the effects of non-phytoplanktonic organic carbon and the enrichment of inorganic carbon on the estimation of the photosynthetic rate. Photosynthetic rates determined by the 13C method showed a remarkable agreement with those determined by the 14C method. Insitu determinations of photosynthetic rate were made in three different water types: open ocean, coastal and neritic waters, which included oligo- and mesotrophic waters, by using the 13C method established.

Size structure of phytoplankton biomass and photosynthesis in subtropical Hawaiian waters

In a subtropical Hawaiian ecosystem, phytoplanton size structure analyses (November–December, 1980) showed that ultraplankton (>3μm), nanoplankton (>20μm) and netplankton (>20μm) accounted for ca. 80, 98, and 2% of total chlorophyll standing stock, respectively, on the basis of chlorophyll. Similar trends were evident, for other biomass indices (e.g. cell numbers, total cell volume, ATP, particulate organic carbon, particulate organic nitrogen). The ultraplankton fraction consisted primarily of small flagellates (1 to 3 μm diam) and coccoid cells (≊1 μm diam); the 3 to 20 μm fraction was represented by dinoflagellates, coccolithophores, diatoms, and chrysophytes; and the netplankton fraction consisted principally of dinoflagellates and centric diatoms. Community photosynthesis had a size distribution similar to that of biomass. Sinking rates for the 3 μm, 3 to 20 μm, and >20 μm fractions averaged 0.0, 0.09, and 0.29m d−1, respectively. The absence of measurable sinking rates for the ultraplankton, together with the relative abundance of biomass in this fraction, result in very small phytoplankton losses due to sinking in such subtropical surface waters.

Abundance of picophytoplankton in the subsurface chlorophyll maximum layer in subtropical and tropical waters

A distinctive chlorophyll maximum was detected around 60-m depth in the western North Pacific Ocean and the South China Sea, and almost 55% of the total chlorophyll in the entire water column was found within 50 m around the subsurface chlorophyll maximum (SCM) layer. More than 70% of the chlorophyll was contained in picoplankton which passed through a 3-μm Nuclepore but retained on 0.22-μm Millipore filters at the SCM as well as the surface layers. By transmission electron microscopic observations, the picoplankton were identified as aChlorella-like coccoid green alga having a section size of 1.2 to 1.5 μm and cyanobacteria of 0.5 to 2 μm. No obvious difference in these two dominant groups was observed in the SCM and the surface samples except in numerous and heavily stacked thylakoids in the former samples.

Importance of picocyanobacteria biomass (unicellular, blue-green algae in the phytoplankton population of the coastal waters off Japan

Picocyanobacteria, which specifically excited the phycobilin pigments by green light, were numerically counted under an epifluorescence microscope in the Kuroshio and the Oyashio between 1 and 9 July 1983, and in coastal waters between 10 and 12 August 1983 off Japan. The fluorescence, and the various morphological and chemical characters of the picocyanobacteria were evaluated by using monospecific strains isolated from the study area. Plasma volume determined on the epifluorescence microscopic photographs was converted into cellular organic carbon using a relation obtained from the isolates. Percentages of the picocyanobacteria biomass in terms of cellular carbon ranged between 8.3 to 79.4% of the total picophytoplankton (<3 μm), and between 4.7 to 46.4% of the total phytoplankton. The larger percentages of the picocyanobacteria occurred consistently in low chlorophyll waters.

Ultraplankton growth rates in a subtropical ecosystem

The ultraplankton (cell diameters >3 μm), which compromises about 70% of the biomass of phytoplankton in subtropical surface waters near Oahu, Hawaii, was isolated for growth rate studies. The specific growth rate (μ) was estimated from the rate of increase of the chlorophyll biomass during incubations in the absence of grazers. This growth rate of the ultraplankton ranged from 0.037 to 0.071 h−1 (=1.3 to 2.5 doublings d−1) during a period when P:B ratios of 5 to 14.5 μg C μg−1 chl a h−1 prevailed. The co-occurrence of atypically high P:B ratios and nonlimiting ambient nutrient concentrations suggests that the calculated values are higher than those characteristic of such subtropical ecosystems in general. Rates of ammonium uptake and photosynthesis by the >3 μm fraction were also compared to those of larger fractions. Organisms in the >3 μm fraction assimilated NH4+at a rate which was about 75% greater than that of the 3 to 20 μm size fraction. Comparison of μ and P:B data collected over a 2 mo period (November–December, 1980) shows that the correlation between these two rate indices is nonlinear. The predominance of small-celled phytoplankton in oligotrophic waters is explained, in part, by its higher μ, its higher nutrient assimilation rates, and the absence of its loss through sedimentation.

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.

Upwelling plumes in sagami bay and adjacent water around the Izu Islands, Japan

Water plumes, 20 km long or less, identified by low temperature, high salinity and high nutrient concentrations, were observed on the eastern side of Izu Islands where the Kuroshio Current or its branch flowed eastward. The T-S diagrams and the vertical profiles of oceanographic variables indicated that the water plumes resulted from the upwelling of subsurface water. A newly formed plume, characterized by a sharp temperature front and high nutrient concentrations, contained less chlorophyll than did old plumes. It is suggested that the upwelling plumes are maintained for a period long enough to allow luxuriant growth of phytoplankton.

Phytoplankton Species' Responses to Nutrient Changes in Experimental Enclosures and Coastal Waters

Phytoplankton growth is controlled by factors that influence intracellular processes, such as radiation, nutrients, and temperature; and factors that influence the entire cell, such as grazing, sinking, mixing, and advection. To evaluate the effects of certain of these factors on growth, small-scale containers are acceptable for short-term experiments, but continuous culture systems are recommended for long-term observations. Examining the growth of mixed phytoplankton assemblages in a multitrophic system requires large-scale facilities and very large containers. The present study examines the effects of macronutrients (nitrate, phosphate, and silicate) additions on phytoplankton species grown in large enclosures (1335 m3 ) containing multitrophic communities, during the CEPEX (Controlled Ecosystem Populations Experiment) Foodweb I experiment (Grice et al. 1980), at Saanich Inlet, British Columbia, Canada.

Abundance and life history ofNeomysis intermedia Czerniawsky in Lake Kasumigaura

AbstractWeekly observations ofNeomysis intermedia in Lake Kasumigaura showed two major peaks in abundance during spring and autumn (more than 104 individuals m−2) and minimum levels in summer and winter (less than 103 individuals m−2). Their increase in abundance followed a high egg ratio, suggesting that the increase in abundance was caused by a high reproductive rate. Major contributors to mysid population decreases include fish predation and commercial fisheries, and possible horizontal migration of the mysids.nN. intermedia showed two types of life history in the lake. One type (overwintering generation) has a life span of about 6–7 months and produces about 27 eggs per brood. Another, appearing from spring to autumn, matures in 3–6 weeks at a smaller size, and produces 12 eggs per brood. The reproductive season ofN. intermedia was continuous from March through November.

Dynamics of dissolved oxygen during algal bloom in Lake Kasumigaura, Japan

Abstract Dynamics of dissolved oxygen during an algal bloom were studied in Lake Kasumigaura. Great amounts of oxygen arose from photosynthesis, and the concentration of dissolved oxygen reached 190% of saturation at 12 h. The majority of the dissolved oxygen produced was liberated into the atmosphere or consumed by microorganisms. Only minor fractions were transported into the dysphotic zone due to the low eddy diffusion coefficient in deeper waters of the euphotic zone.