Tomoko Yoshiki

Temperature and zooplankton size structure: climate control and basin-scale comparison in the North Pacific.

The global distribution of zooplankton community structure is known to follow latitudinal temperature gradients: larger species in cooler, higher latitudinal regions. However, interspecific relationships between temperature and size in zooplankton communities have not been fully examined in terms of temporal variation. To re-examine the relationship on a temporal scale and the effects of climate control thereon, we investigated the variation in copepod size structure in the eastern and western subarctic North Pacific in 2000–2011. This report presents the first basin-scale comparison of zooplankton community changes in the North Pacific based on a fully standardized data set obtained from the Continuous Plankton Recorder (CPR) survey. We found an increase in copepod community size (CCS) after 2006–2007 in the both regions because of the increased dominance of large cold-water species. Sea surface temperature varied in an east–west dipole manner, showing the typical Pacific Decadal Oscillation pattern: cooling in the east and warming in the west after 2006–2007. The observed positive correlation between CCS and sea surface temperature in the western North Pacific was inconsistent with the conventional interspecific temperature–size relationship. We explained this discrepancy by the geographical shift of the upper boundary of the thermal niche, the 9°C isotherm, of large cold-water species. In the eastern North Pacific, the boundary stretched northeast, to cover a large part of the sampling area after 2006–2007. In contrast, in the western North Pacific, the isotherm location hardly changed and the sampling area remained within its thermal niche throughout the study period, despite the warming that occurred. Our study suggests that while a climate-induced basin-scale cool–warm cycle can alter copepod community size and might subsequently impact the functions of the marine ecosystem in the North Pacific, the interspecific temperature–size relationship is not invariant and that understanding region-specific processes linking climate and ecosystem is indispensable.

Pressurizing System for Observation of Marine Zooplankton

Planktonic animals occur from the surface to the sea bottom and they are exposed to hydrostatic pressure throughout their lifetime. A hydrostatic pressure apparatus integrating ideas of past apparatuses was developed to investigate effects of hydrostatic pressure on marine zooplankton. The pressure apparatus system is composed of a peristaltic pump, a pressure chamber, a specimen holder which is placed in the pressure chamber, a temperature-controlled bath, a CCD camera, a monitor and a video-recorder. This hydrostatic pressure apparatus made it possible (1) to incubate at a constant temperature by connecting it with a temperature-controlled bath, (2) to observe live zooplankton through a window, (3) to increase pressure gradually from 1 to 150 atm and (4) to continually record the behavior of specimens by using a CCD camera and a video-recorder. Effects of hydrostatic pressure on the egg of the marine zooplankton, were examined using the newly designed hydrostatic pressure apparatus. The apparatus is practical for experiments using a wide size range of specimens, from eggs to adult size of mesozooplankton (100-10000 mum size range). In present study, egg development time of planktonic copepod was examined; C. sinicus is the dominant zooplankton community in the coastal area of Japan. The egg development time at different experimental pressure conditions (1, 10, 50 and 100 atm) were similar to that at 1 atm. The hatching success of C. sinicus eggs at 1 atm was generally higher than 80 % within the temperature range from 5 to 25degC. At higher pressure conditions of 10, 50 and 100 atm, highly significant decreases in egg hatching success of C. sinicus occurred as pressure increased. Thus, hydrostatic pressure does not inhibit egg development but egg hatching of C. sinicus.