Studies On Phytoplankton, Copepod Egg Production Rate And Euphausia Pacifica Grazing In The Oregon Upwelling Zone

Seasonal upwelling in the northern California Current upwelling ecosystemdrives seasonal variability in hydrographical, chemical and biological characteristics.When upwelling-favorable northerly winds starts in March or April, offshore Ekmantransport brings surface water offshore which are in turn replaced by deep cold andnutrient-rich water which upwells near the coast. Biological productivity is high in allcoastal upwelling systems because the upwelling process continuously providesnutrients by vertical transport into the surface layers which with the combination ofthe optimal light condition results in the maintenance of massive phytoplanktonbiomass and the subsequent consumption by zooplankton and planktivorous fishes. Tothe best of our knowledge, however, there are no published studies on the seasonalityof community composition of phytoplankton on a year-round basis in continentalshelf waters of the northern California Current. The egg production activities of twotypical copepod species within this ecosystem, Calanus pacificus and Calanusmarshallae, showed obscure relationships with phytoplankton species compositionand biomass. No studies of Euphausia pacifica feeding on natural planktonassemblages have been seen. In addition, upwelling process to what extent influencinginteractions among low trophic levels has still been the focus of studies in this region.This study relied on the time-series datasets of plankton field samplings and insitu experiments in the coastal upwelling zone off central Oregon. Topics included asfollows: phytoplankton community compositon; in situ phytoplankton speciescomposition and biomass impact on copepod egg production; adult Euphausiapacifica feeding on natural plankton assembalges. The main results are summarizedbelow:（1） Multivariate statistical analysis shows four clusters of phytoplankton in the Oregon upwelling zone in2009were determined: cold winter, strong coldupwelling, late upwelling season and warm autumn. Diatoms dominated thestrong cold upwelling cluster during the upwelling season （from April to July） aswell as an upwelling resumption in October in contrast to the dominance ofdinoflagellates in the warm autumn cluster during the autumn downwelling period.Both the cold winter cluster and the late upwelling season cluster werecharacterized of the diatom-dinoflagellate mixed assemblage during the beforeupwelling season as well as upwelling relaxations. The combining factor oftemperature, salinity and ammonium influenced comparatively more on thecommunity structure of phytoplankton. The none-significant relationship betweenphytoplankton biomass and any of the environmental factors indicatescomplicated interactions between physic-chemical and biological processes.（2） During the simultaneous investigations of phytoplankton in2009, C.marshallae were present from April to July during the upwelling season and fromJanuary to March during the downwelling season. C. pacificus appeared in thewinter downwelling season （from January to March, December） and in the laterupwelling season （September and October）. None-significant relationships werefound between the egg production rates （EPRs） of C. marshallae and either typeof its corresponding food availability represented by total Chl a, Chl a>5μmconcentrations, carbon biomass of total phytoplankton, diatom, dinoflagellate anddominant species or genera. However, the maximum EPRs of both species wereobserved during the late winter algal blooms, and the high and constant EPRs of C.marshallae were found during the upwelling bloom season, which togetherindicates positive effects from food availability. The variations of C. marshallaeEPR is likely impacted by the alternations of upwelling/downwelling and theresultant food changes. C. marshallae ceased egg producing or showed low EPRwhen diatom-type phytoplankton assemblages were substituted bydinoflagellate-type. The ultimate conclusions of the timing and extent that foodconditions impact egg production need support of long term data collections and the combination analysis at both the annual and decadal time scales.（3） From February to August in2010, the experiments of E. pacifica feedingon natural microplankton assemblages were firstly conducted in the northernCalifornia Current system. Food conditions represented by microplankton carbonbiomass and concentration of total Chl a varied greatly among the9experiments.Before the upwelling season, food concentrations were low in microplanktonbiomass ranging from16.2to49.8μg C L^-1and in total Chl a concentrationranging from0.41to0.87μg L^-1. During the upwelling season, due to thealternatives between upwelling and relaxation at the event time scale, the highestfood concentration appeared during a strong upwelling period in late August withmicroplankton biomass of698.5μg C L^-1and total Chl a of21.75μg L^-1. Thelower food concentrations corresponded with upwelling relaxations in late Juneand early August with microplankton biomass of69.4and93.3μg C L^-1and totalChl a of0.79and2.91μg L^-1, respectively.Filtration rate did not show the same variation trend between the valuesbased on cell counts biomass (6.9²02.8ml euphausiid^-1h^-1) and Chl aconcentration (8.0¹67.9ml euphausiid^-1h^-1), and the discrepancies suggestspecies-specific filtration rate and the likely associations with feeding modes.Ingestion rates were highly variable throughout the experiments with valuesranging from0.1to46.3μg C euphausiid^-1h^-1. The higher ingestion rates wererelated to high food concentrations during the summer upwelling season; and thenegative or lower ingestion rates were coincident with low food concentrations(below50μg C L^-1) and upwelling relaxations. Daily ration indicated highervalues （2.4%²3.3%） during the consistent upwelling period and lower values（0.04%¹.8%） in before the upwelling period. Evaluated by the basic metabolicrequirement of2⁴%of daily ingested carbon, feeding met the needs during theupwelling blooms （>5%） as well as upwelling relaxations （>2%）, but failed duringthe before upwelling season （<2%） indicating other food sources orphysiologically reducing activities. The fitted functional models in the present study were not completelycoherent with previous reports due to the prey differences between naturalmicroplankton assemblages and limited types of cultured foods. Ivlev curvesprovided the better fits for filtration rate against initial microplankton biomass,total Chl a concentration and ciliate biomass. Ingestion rate based onmicroplankton biomass fit linear or sigmoidal model better, but fit Holling type IIand sigmoidal model based on total Chl a and ciliate biomass, repectively, andagain all imply species-specific, opportunistic and adaptative feeding.Based on the electivity indices （Ei*）, E. pacifica showed strong preference fordiatoms （Pseudo-nitzschia spp., Leptocylindrus spp., Chaetoceros spp.,Astererionellopsis glacialis and Thalassiosira spp.） during the early strongupwelling season in June even when other abundant dinoflagellates （large or small）and ciliates （naked ciliates and tintinnids） were available. Strong selectivityoccurred among dominant and less dominant diatoms during strong upwellingseason in July and August, but E. pacifica did not consistently show a strongpreference for the dominant diatoms such as Chaetoceros spp. in August, insteadless dominant diatoms like Eucampia zodiacus and Lauderia annulata werepreferred. Ciliates which contributed most to total biomass were obviouslyimportant food type when other prey such as diatoms lacking, and they were alsopositively selected when diatoms were dominant which might relate with theirspecific nutrient values. Dinoflagellates seemed to be suboptimal prey because oftheir low cell amounts. Our results support the hypothesis that E. pacifica showedflexible feeding behavior with changes of natural assemblage composition,reflecting feeding adjustments to prey amounts, morphology, nutrients and theirown adaptability. E. pacifica switched feeding preference between predominantlyherbivorous and ominivorous as changes of ambient food types as well as thetiming and intensity of upwelling process.