Ecological study on the zooplankton community in the Oyashio region during the spring phytoplankton bloom

In marine ecosystems, zooplankton play an important role in the transfer production of both the grazing food chain and microbial food web for higher trophic revels. In addition to a food mediator role, zooplankton accelerate the vertical material flux, termed the "Biological pump". In the Oyashio region, western subarctic Pacific, nearly half of the annual primary production occurs from April to May. During this same period, zooplankton grow faster. However, it is difficult to generate an accurate evaluation of zooplankton growth rates using the ordinary sampling interval (once per month) from previous studies. For an accurate evaluation of the growth rates of zooplankton, high frequency time-series samplings during the spring phytoplankton bloom are needed. The OECOS is an international research programme for the evaluation of zooplankton responses to the spring phytoplankton bloom using high-frequency timeseries samplings. During the OECOS period, high frequency oceanographic observations, including CTD casts (approximately every day), water samplings and various net samplings, were conducted at St. A-5 in the Oyashio region from March 8 to May 1, 2007. In the present study, short-term changes in phytoplankton, protozooplankton and meso- and macrozooplankton abundance, biomass, population structure, vertical distribution, growth rates and feeding ecology were studied during the OECOS period. Based on these phenological descriptions, the present study aimed to evaluate the lower trophic levels during the spring phytoplankton bloom in the Oyashio region. For comparison, copepod data collected from other high-frequency time-series samplings during the phytoplankton bloom (SEEDS I, SEEDS II, SERIES and St. M) were gathered and compared with those of the OECOS. A thorough comparison of the five time-series data and characteristics of zooplankton responses to the phytoplankton bloom were conducted. Throughout the OECOS period, three dominant water masses (COW, MKW and OYW) occurred at the surface layer (0-50 m) over a short period of time. Because the COW contains sufficient nutrients originating from the Sea of Okhotsk, phytoplankton peaks were observed at the COW on April 7-8 and 23. The composition of diatoms was more than 74% of the chl a content during April, and centric diatoms predominated throughout the study period. The dominant species changed from Thalassiosira spp. to Chaetoceros spp. after April 20. Peaks of microzooplankton biomass were observed on April 7 and 25, and both thecate and athecate dinoflagellates dominated in the microzooplankton community. The mesozooplankton wet mass at 0-150 m ranged from 7.6-147.7 g WM /square m. The wet zooplankton biomass was low in March, but increased after April 8. For the zooplankton population structure, the copepod N. cristatus developed from C1 to C4. Significant growth of two euphausiids (E. pacifica and T inspinata) and two chaetognaths (E. hamata and P. elegans) was observed. A comparison of the mass-specific growth rate (g) showed no significant differences between species. These findings suggest that there was no food limitation for all feeding modes of zooplankton during the spring phytoplankton bloom in the Oyashio region. For the reproduction of zooplankton, reproduction of the epipelagic copepod E. bungii was initiated in response to the phytoplankton bloom peak on April 7-8, and newly recruited early copepodid stages were observed on April 12. For mesopelagic copepods, increases in the composition of spermatophore-attached C6F in April suggested the initiation of reproduction. Throughout the study period, most adult females of T. inspinata had spermatophores, and the high proportion of attached-spermatophore females to the total population (> 40%) suggested that spawning had occurred. For amphipods, the reproduction of C. challengeri and T. pacifica was initiated in April. In response to water masses, various species showed high abundance and biomass under COW-dominated conditions and low abundance and biomass under OYW-dominated conditions. Few species showed any correlations with MKW. Zooplanktonic responses to high abundance under COW and low abundance under OYW might reflect the different characteristics of each water mass. The results of FRA-ROMS analyses revealed that the temperature of OYW was lowest and induced low zooplankton growth rates. However, the geographical origin of COW is the Sea of Okhotsk, a marginal sea with high primary productivity, providing high zooplankton survival and growth rates. Temporal changes in the vertical distribution were observed for E. bungii, M. pacifica and P. scutullata. For E. bungii, the arousal from diapause and upward migration to the surface layer was initiated on April 5. DVM ceased for M. pacifica and P. scutullata after 23 and 11 April, respectively. In P. scutullata, this cessation was observed for all copepodid stages. However, in M. pacifica, the cessation was observed in all stages except C6F, which performed DVM throughout the study period. The continuous DVM behaviour of M. pacifica C6F might reflect reproduction at the surface layer. The ceased DVM of suspension feeding copepods might reflect increasing POC flux, which enables the acquisition of sufficient food at greater depths, without any DVM in April. For the mesopelagic carnivorous copepod P. elongata, continuous DVM was observed throughout the study period. Gut content analyses were conducted on mesopelagic copepods and macrozooplanktonic euphausiids and chaetognaths. Mesopelagic suspension-feeding copepods primarily fed on protozooplankton prior to the onset of the phytoplankton bloom, while resting spores of diatoms were observed in the gut contents after the phytoplankton bloom. Within the gut contents, the cell conditions were mostly intact for shallower dwelling species, and the composition of broken cells increased for deeper living species. These results might reflect the coprophagy and repacking of mesopelagic suspension-feeding copepods. For mesopelagic carnivorous copepods, the species-specific preferences of euphausiids and chaetognaths in the feeding ecology were observed throughout the study period. Within the biomass and production of the total zooplankton community, the small-sized copepod M. pacifica was dominant in March. Both the biomass and production values in April were similar to those in March, while the species composition significantly varied, that is, the composition of M. pacifica decreased and the macrozooplanktonic euphausiid E. pacifica was the most dominant species in April. A comparison of the mass-specific growth rate (g) of epipelagic copepods at various locations showed that g varied from 0.002 to 0.210. For C. finmarchicus in the North Atlantic and E. bungii in the North Pacific, overwintered adult generations reproduced at the surface layer. Common for these species, g of the newly recruited generation cohort was approximately 5 times higher than that of the adult cohort. The highest g of N. cristatus was observed for SEEDS I, characterized by the highest chl a, suggesting that the g values increased with increasing chl a within the observed chl a range (～18 mg /cubic m). As a feature of the present study, the response to the spring phytoplankton bloom and water masses on macrozooplankton, with limited ecological information taxa, was evaluated. Particularly, the euphausiid E. pacifica was the most dominant zooplankton species both in biomass and production after the spring phytoplankton bloom. Previous zooplankton studies in the Oyashio region primarily focused on mesozooplankton, such as copepods. However, because of their importance, studies on macrozooplankton and micronekton are needed in the future. From high-frequency time-series sampling, such as the OECOS project, the time-resolution analysis was significantly improved, but the problems of low analytical levels of spatial- and vertical-resolution remain. Thus, the application of visual imaging instruments, such as VPR (Video Plankton Recorder) and UVP (Underwater Video Profiler), which collect zooplankton data continuously, has been suggested as a solution for increasing the spatial- and vertical resolution of zooplankton data analysis in future studies.