Plankton abundance and size structure in the northern North Pacific Ocean in early summer

Phyto- and zooplankton abundance and size structure were investigated in the northern North Pacific Ocean (37.0–49.5^oN along 180^o longitude) during June of 1987. The area between 41.5^oN and 47.5^oN was expected to be abundant in phytoplankton since both water column stability and nutrient availability were favourable for phytoplankton growth. Actual phytoplankton abundance was, however, low between 41.5^oN and 45.5^oN. In particular, abundance of net phytoplankton (>10μm) was low although picophytoplankton (<2 μm) was abundant between 41.5^oN and 43.5^oN. The zooplankton community between 41.5^oN and 45.5^oN was characterized by a large amount of net zooplankton and a small amount of microzooplankton. This situation may be best explained by top-down control; that is, net zooplankton grazing suppressed net phytoplankton and microzooplankton abundance. The diminished microzooplankton promoted the increase of picophytoplankton. Because planktivorous Pacific saury were abundant between 40.0^oN and 41.0^oN, it is further speculated that the low biomass of net zooplankton south of 41.0^oN resulted from feeding by Pacific saury. The reduced grazing pressure of net zooplankton may have enhanced the growth of net phytoplankton and microzooplankton in this region.     

Interannual variation in Neocalanus biomass in the Oyashio waters of the western North Pacific

We examined the interannual variation in Neocalanus copepod biomass in the Oyashio waters in spring and summer from 1972 to 1999. In the mid‐1970s, mesozooplankton biomass in spring was high; however, it decreased significantly in the late 1970s. The timing of the decrease in mesozooplankton biomass corresponded to the 1976/77 climatic regime shift. The biomass of N. flemingeri, which dominated the Neocalanus community, was roughly constant from 1980 to 1999. Although species‐level estimates of Neocalanus biomass were not available for the 1970s, a previous study reported that Neocalanus copepods were the predominant mesozooplankton in the Oyashio waters in spring during the 1970s. Neocalanus copepods dominated the mesozooplankton community throughout the 1970s, and their biomass decreased in the late 1970s. Springtime net community production, an index of new production, also decreased in the late 1970s. We suggest that the reduction in new production negatively affected Neocalanus food availability, resulting decreased copepod biomass. New production may have been limited by a combination of subsurface iron supplies, increased vertical density gradient, and reduced vertical water mixing in winter, which resulted in diminished iron entrainment in winter. In summer, mesozooplankton biomass significantly decreased and increased synchronously with the 1976/77 and 1988/89 climatic regime shifts. The biomass of N. plumchrus, which dominated the Neocalanus community, was low in the 1980s and increased in the early 1990s. The biomass of the second‐most dominant copepod, N. cristatus, also increased in the early 1990s. Neocalanus copepods were reported to be a dominant component of the mesozooplankton community in the 1970s; Neocalanus biomass was high in the mid‐1970s and decreased in the late 1970s. Japanese sardine (Sardinops melanostictus), an important predator of Neocalanus copepods, exhibited interannual variation in standing stock that was inversely related to mesozooplankton biomass. At their peak in 1984, sardines consumed 32–138% of the daily Neocalanus production during summer. Therefore, predation pressure on Neocalanus by Japanese sardine is likely to affect interannual variation in mesozooplankton biomass during the summer.     

Seasonal dynamics in a coastal Vibrio community examinedby a rapid clustering method based on 16S rDNA

ABSTRACT:   A rapid clustering scheme for Vibrio specieswas developed based on the analysis of 16S rDNA, which compriseda group-specific polymerase chain reaction (PCR) and restriction fragmentlength polymorphisms (RFLP) of the PCR-amplified 16S rDNA. The group-specificPCR, using a specific primer Vib1F, clustered the Vibrio speciesinto two large groups: Vib1F⁺ group and Vib1F^– group.The PCR-RFLP, using Sca I and Bln I, further dividedthese groups into smaller groups. Finally, 46 Vibrio speciesand eight related species could be clustered into 14 groups usingthis rapid clustering method. Seasonal dynamics of the Vibrio communityin Yoshimi Bay, Hibiki-nada Sea, Japan, were examined based on therapid clustering method. In both seawater and sediments, the groupcomprised Vibrio parahaemolyticus , Vibrio alginolyticus , Vibriocampbellii , Vibrio carchariae , Vibrio harveyi and Vibrionatriegens and was predominant when the seawater temperaturewas above 20°C, whereas the group of Vibrio splendidus biotypeI and Vibrio lentus was abundant when the temperature wasaround or below 20°C.     

Effects of elevated pCO2 on the early development of the commercially important gastropod,Ezo abalone Haliotis discus hannai

We used an up‐to‐date, a high accuracy CO₂ manipulation system to investigate the sensitivity of organisms to CO₂ acidification, rearing marine calcifiers under elevated CO₂ in running water. We evaluated the effects of elevated partial pressures of carbon dioxide (pCO₂) in seawater on larvae of the commercially important marine gastropod Ezo abalone Haliotis discus hannai. In larval Ezo abalone, no effect of exposure to <1100 μatm pCO₂ seawater was observed in fertilization, malformation, or mortality rates until 15 h after fertilization. However, compared to control larvae in seawater (450 or 500 μatm pCO₂), the fertilization rate and the hatching rate (15 h after fertilization) decreased with increased pCO₂ exposure (1650 and 2150 μatm pCO₂) and the malformation rate increased significantly, with the larval shell length being smaller 75 h after hatching. These results suggest that ocean acidification will potentially impact the marine population of Ezo abalone as a human food source in the future.