Seasonal changes in the abundance and composition of picophytoplankton in relation to the occurrence of ‘Kyucho’ and bottom intrusion in Uchiumi Bay, Japan

Abstract

Uchiumi Bay experiences intermittent physical events of 'Kyucho ' and bottom intrusion.A Kyucho is an intrusion of warm surface water from the Kuroshio in the Pacific Ocean. Bottom intrusion, which contains a large amount of nitrates, phosphates, and silicates, slips through just above the continental shelf. We investigated seasonal changes in the abundance of Prochlorococcus, Synechococcus, and eukaryotic picophytoplankton while monitoring Kyucho and bottom intrusion from March to October 2002. Kyucho and bottom intrusion frequently occurred from June to September. Relatively high concentrations of nitrate + nitrite (> 0.8 µmol N l -1 ) and phosphate (> 0.1 µmol P l -1 ) were found when bottom intrusion occurred. The cell densities of Prochlorococcus were relatively high (>1 × 10 4 cells ml -1 ) when Kyucho occurred. Those of Synechococcus were high (2 to 30 × 10 4 cells ml -1 ) during the period of thermal stratification except in July, when bottom intrusion occurred. The cell densities of eukaryotic picophytoplankton were high (2 to 8 × 10 4 cells ml -1 ) in May and July. To examine the effects on picophytoplankton growth of the nutrients supplied by bottom intrusion, we conducted nutrient-enrichment experiments. The growth rates of Prochlorococcus and Synechococcus were not stimulated by the addition of any kinds of nutrients. The growth rates of Prochlorococcus were negative in most cases. In July, the growth rate of eukaryotic picophytoplankton was stimulated by nitrate and phosphate additions. Thus, Prochlorococcus detected in Uchiumi Bay might have been transported by Kyucho from the Pacific Ocean and could therefore not grow vigorously. Synechococcus may have been flushed out by bottom intrusion, and its growth was not limited by the nutrient concentrations. Eukaryotic picophytoplankton was abundant in spring, and its growth might have been limited by the nutrient concentrations in some cases. These results suggest that Kyucho and bottom intrusion have different effects on the abundance and growth rate of the 3 picophytoplankton groups.

Seasonal variations in abundance and biomass of picoplankton in an oyster-farming area of northern Japan

“…Several studies of Japanese coastal waters have reported that the maximum abundance of cyanobacteria (PEC or Synechococcus spp.) is similarly within the range of about 1-3ϫ10 5 cells mL Ϫ1 (Odate 1989, Miyazono et al 1992, Shimada et al 1995, Koike et al 2001, Katano et al 2005; this agrees with our maximum values (1.1ϫ10 5 cells mL…”

Distribution of prokaryotic picophytoplankton from Seto Inland Sea to the Kuroshio region, with special reference to ‘Kyucho’ events

“…Recently, we reported on the abundance of Prochlorococcus in a bay on the Bungo Channel (Katano et al 2005). We were also able to determine that Synechococcus in the bay consisted of a different type of PUB:PEB cells (Katano et al 2004, Katano & Nakano 2006.…”

“…In the East China Sea, no Prochlorococcus has been detected in coastal waters (Pan et al 2005). However, in Suruga Bay (Shimada et al 1995) and in Uchiumi Bay, located in the Bungo Channel (Katano et al 2005), Prochlorococcus has been detected. Since the Kyucho also occurs in Suruga Bay (Inaba et al 2003), the detected Prochlorococcus may have been transported from the open ocean.…”

“…The method for counting the Prochlorococcus and Synechococcus cells has been described previously (Katano et al 2005, Katano & Nakano 2006. The cell densities of Prochlorococcus and Synechococcus according to each type of pigment were counted with a flow cytometer (FACSvantage SE, Becton Dickinson) equipped with 2 water-cooled Argon lasers (INOVA305 and 306, Coherent): one (INOVA305) gives 488 nm of blue light, while the other (INOVA306) gives 514 nm of green light.…”

Feeding behavior of the copepod Temora turbinata: clearance rate and prey preference on the diatom and microbial food web components in coastal area

“…Since marine calanoid copepods often show species-specific feeding behavior and feed not only phytoplankton but also protozoan including ciliates that graze the microbial food web components, the feeding of calanoid copepods can affect the microorganisms through the food web cascade as well as phytoplankton community through direct grazing (Zöllner et al 2009). As eutrophication proceeds in estuaries and coastal areas, microbial food web components (e.g., bacteria, nanoflagellates, and ciliates) become key organisms in matter cycling (Katano et al 2005, Hirose et al 2008, and their interactions with herbivorous and omnivorous copepods have been considered as important linkage between microbial and herbivorous pathways in coastal marine ecosystems (Doi et al 2008, Chang et al 2009.…”