Spring bloom in a hypereutrophic lake, Lake Kasumigaura, Japan—IV : Inductive factors for phytoplankton bloom

The limiting factor for inducing the spring bloom of phytoplankters in 1980 at Tsuchiura Harbor of Lake Kasumigaura was shown to be temperature. The bloom started at a water temperature higher than 10°C and lasted until the temperature was up to approx. 20°C. The concentration of inorganic nitrogen in the lake was found to have a profound effect on the induction of the spring bloom as a nonlimiting factor. Other inorganic nutrients contributed mostly to the species succession of phytoplankters comprising the spring bloom.     

Spring bloom in a hypereutrophic lake, lake Kasumigaura, Japan—I Succession of phytoplankters with different accessory pigments

Phytoplankters appearing during the spring bloom of 1980 in Lake Kasumigaura were classified into three groups characterized by different accessory pigments. Phytoplankters of the first group possess phycocyanin, those of the second group possess chlorophyll b, and those of the third group possess chlorophyll c. Only eucaryotic phytoplankters, belonging to the second or third group, were predominantly present during the bloom. Predominance of the second and third groups alternated during the bloom as individual phytoplankton pulses. The relationship between the population density of all phytoplankters and the abundance of phytoplankters possessing chlorophyll c could be expressed as a highly significant regression curve (r = 0.7997) throughout the spring bloom in Tsuchiura Harbor: Y = 102.9 e^−0.03138X + 29.04, where X is the concentration of chlorophyll a in all phytoplankters in lake water, and Y is the percentage of chlorophyll a possessed by phytoplankters possessing also chlorophyll c.     

Spring bloom in a hypereutrophic lake, lake Kasumigaura, Japan—II Succession of phytoplankton species

Numerous phytoplankton species in Lake Kasumigaura were shown to comprise the Spring bloom in 1980. The predominant phytoplankton species that appeared during the Spring bloom belonged to 28 genera. A few species among them increased at any time to form a phytoplankton pulse, but the few species of phytoplankters that comprised a phytoplankton pulse did not exclude completely other phytoplankters. During the Spring bloom, one pulse was replaced by another pulse composed of a new grouping of different species. Throughout these replacements by natural selection in the bloom were numerous other species by phytoplankters at levels of abundance somewhat less than those which formed phytoplankton pulse.     

Spring bloom in a hypereutrophic lake, lake Kasumigaura, Japan—III Phytoplankton abundance in standing stock of organic matter in lake water

The rapid increase of phytoplankton biomass was the major process for the increase of particulate organic matter in Lake Kasumigaura in 1980. During the first phytoplankton pulse in the bloom relative abundance of each component of organic matter in the lake water was: dissolved organic matter 100, phytoplankton 28, bacteria 27, and detrital particle 27. This relative abundance was maintained as a steady-state oscillation throughout the spring bloom similar to that usually observed in moderately eutrophic waters. Thus phytoplankton abundance in the standing stock of organic matter of the hypereutrophic water during the spring bloom does not show any marked difference from that of moderately eutrophic waters.