Tatsuo Miyazaki

Light, nutrients and primary productivity in Lake Biwa: An evaluation of the current ecosystem situation

Simple correlation and multiple regression analyses were performed to examine the relationship between primary productivity and environmental factors in the north basin of Lake Biwa. The primary production rates used in the analyses were estimated monthly or bimonthly during the growing season (April–November) in 1992, 1996 and 1997 with the 13C method. Elemental (C, N and P) contents of seston were used to assess nutrient conditions. Analyses revealed that 86% of variance in depth-integrated primary production rates (areal PP) can be explained by changes in light intensity, and sestonic C, N and P concentrations. Water temperature had no effect on areal PP. To assess relative effects of light and nutrients on PP, the P:B ratio was estimated by normalizing PP with sestonic C. The areal P:B ratio correlated most significantly with the sestonic N:P ratio, followed by light intensity. When regression analyses were made at each depth, however, the P:B ratio correlated significantly only with the sestonic N:P ratio at 0 and 1 m depths, while light intensity was also incorporated into the regressions at deeper than 2.5 m. In these regressions, the P:B ratio was negatively correlated with sestonic N:P ratio but positively with light intensity. The results suggest that the primary production rate in this lake was mainly limited by P relative to N supply rates, but was not free from light limitation in a large part of the epilimnion. In Lake Biwa, the vertical water mixing regime as well as the nutrient supply seem to be important in determining the growth and composition of primary producers, since the surface mixing layer extends into 10–15 m depths during most of the growing season.

Effects of ammonium supply rates on competition between Microcystis novacekii (Cyanobacteria) and Scenedesmus quadricauda (Chlorophyta) : simulation study

Abstract Competition between the cyanobacterium Microcystis novacekii and the green alga Scenedesmus quadricauda was studied, using a simulation model where nitrogen-limited chemostat systems were considered. The effects of dilution rate and ammonium concentration were examined. The dilution rate determined the dominant species and its final biomass. M. novacekii dominated at lower dilution rates ( −1 ), while S . quadricauda dominated at higher dilution rates (>0.65 d −1 ). Ammonium concentration in the influx culture medium determined the final biomass of the dominant species. However, it exerted no influence on the determination of the dominant species and also the displacement time. Initial cellular N:C ratio that is an index of initial nutrient status of algae and initial biomass did not affect the determination of dominant species or its final biomass. Initial biomass affected the displacement time. This result explains in part the occurrence of blooms of Microcystis in nutrient-rich and slow-flowing waters.

Spatial coexistence of phytoplankton species in ecological timescale

The species diversity of phytoplankton is usually very high in wild aquatic systems, as seen in the paradox of plankton. Coexistence of many competitive phytoplankton species is extremely common in nature. However, experiments and mathematical theories show that interspecific competition often leads to the extinction of most inferior species. Here, we present a lattice version of a multi-species Lotka–Volterra competition model to demonstrate the importance of local interaction. Its mathematical equilibrium is the exclusion of all but one superior species. However, temporal coexistence of many competitive species is possible in an ecological time scale if interactions are local instead of global. This implies that the time scale is elongated many orders when interactions are local. Extremely high species diversity of phytoplankton in aquatic systems may be maintained by spatial coexistence in an ecological time scale.

Feeding behavior and effect of prey availability on Sanderling Calidris alba distribution on Kujukuri Beach

ABSTRACT The distribution of Sanderling Calidris alba, and intertidal infaunal biomass was monitored over an annual cycle. In addition, the feeding behavior and fecal contents of Sanderling were studied in order to examine the relationship between the seasonal distribution of Sanderling and prey availability at Kujukuri Beach, Chiba Prefecture. Sanderling were observed on the beach from August through winter to May with an estimated total number of ca. 600 birds consistently being recorded from September to May. The numbers were supplemented by passage migrants with peaks in August, late January/early February and April. Direct observation, prey availability and fecal evidence indicated that Sanderling fed mainly on the surf clam Donax (Chion) semigranosus, the isopod Excirolana chiltoni and the mysid Archaeomysis vulgaris as well as insects. During winter (December to February) when the densities of E. chiltoni and A. vulgaris markedly decreased, Sanderling distribution was positively correlated (P<0.01) with the distribution of D. semigranosus, which formed its main prey during this period. Sanderling were observed to direct their attacks on D. semigranosus at the surf clams muscular foot and the shell part was subsequently discarded. We conclude that Kujukuri Beach is important both as an over-wintering site and as a temporary stopover site for migrant Sanderling and that over-wintering Sanderling respond to the seasonal variation in the abundance of their main prey.

Ammonium supply mode and the competitive interaction between the cyanobacterium Microcystis novacekii and the green alga Scenedesmus quadricauda

We performed competition culture experiments using the bloom-forming cyanobacterium Microcystis novacekii (Kom.) Comp. and the green alga Scenedesmus quadricauda, (Turpin) Brebisson to study the effects of nutrient supply modes on the competition of phytoplankton. The cells were grown in continuous cultures under nitrogen limitation. Ammonium, as the sole nitrogen source, was supplied either continuously or in pulses as one pulse per two days, and the effects of the mode of nutrient supply were examined. While both the species grew well in monoculture, the growth of one species was suppressed by the presence of the other species in the mixed cultures, indicating the occurrence of competition for the supplied ammonium. The competitive outcomes depended on the mode of ammonium supply: under continuous supply, though both the species tended to grow together, the growth of S. quadricauda was suppressed by the presence of M. novacekii, compared to that in the monoculture. Under the pulsed supply, M. novacekii tended to be suppressed by S. quadricauda. This suggests that M. novacekii is a species that grows better in low ammonium supply, while S. quadricauda is a species that grows better in high ammonium supply. The temporally high concentration of ammonium immediately after the pulsed nutrient supply provided an opportunity for S. quadricauda to outgrow or almost exclude M. novacekii.

Power Law for Extinction Process in Multiple Contact Process

We deal with a multiple contact process, that is a modification of the contact process. This system contains N kinds of species ( N = 10) on a finite-sized square lattice. Simulations are carried out using two different methods: local and global interactions. It is found that the waiting time to the extinction of the first species follows a power law. Moreover, we find that local interaction promotes the coexistence of multiple species.

Effects of gamete behavior and density on fertilization success in marine green algae: insights from three-dimensional numerical simulations

We developed a numerical simulation of mating experiment to study effects of phototactic gamete behavior and density on fertilization success, using the C++ programming language, and pseudo-parallelization methods with input parameters based on experimental data. In our experiments, we found that gametes with positive phototaxis are favored, particularly in shallow water, because they can search for potential mates on the two-dimensional (2-D) water surface rather than randomly in three dimensions. We also found evidence that sperm (male gametes) limitation might not be the dominant selective force in the evolution of isogamous or slightly anisogamous marine green algae because almost all of female gametes can be fertilized on the 2-D water surface meaning they might not be under sperm limited conditions. Gamete density also appears to affect mating success seriously. These findings were produced by some technical progress made recently to rapidly and correctly count the numbers of zygotes formed calculating the locations of huge numbers of male and female gametes in the test tank. Both gamete behavior and density might be determined by environmental conditions of habitat, particularly the depth of water.

Evolution of gamete size in primitive taxa without mating types

An ESS model to better understand the evolutionary dynamics of a primitive non-mating type gamete size was developed with reference to the PBS (Parker, Baker and Smith’s) theory, which was based on total numbers of zygotes formed and the zygote survival rates. We did not include mating types since it has been suggested that primitive mating systems did not have mating types. As input parameters, we used experimental data on gamete motility of marine green algae. Based on hard sphere collision mechanics, we detailed the fertilization kinetics of gametes that swim in water prior to fusing with their partners through a set of coupled, non-linear differential equations. These equations were integrated numerically using typical values of the constant parameters. To estimate the relative zygote survival rate, we used a function that is sigmoid in shape and examined some evolutionarily stable strategies in mating systems that depend on optimizing values of the invasion success ratio.