Seasonal changes in plankton biomass, production and community structure in southern Japan*

CCCC REX Workshop on PICES 13th Annual Meeting October 14th, 2004 Hawaii Convention Center

TORU KOBARI Faculty of Fisheries, Kagoshima University

* Partly supported by grants from the Japan Society for the Promotion of Science and the Fisheries Agency of Japan Contents

1. Background

2. Sampling and experimental methods

3. Results Seasonal patterns Vertical distribution Biomass Production Community structure Growth rate Mortality rate Carbon budget 4. Summary EpipelagicEpipelagic MarineMarine EcosystemEcosystem

Mammals Fishes

>200µ Meso Grazing Copepod, Euphausiid Microbial Food Web Food Web 20-200µ Micro Diatom Ciliate

2-20µ Nano Heterotrophic Nano-Flagellate

0.2-2µ Pico Bacteria, Cyanobacteria EarlyEarly studiesstudies

Station PaPa

Site H Little knowledge of plankton food web in the oligotrophic waters ALOHA

NOPACCS EQPAC

Zooplankton grazing activity Standing stock of phyto- and zooplankton Seasonal variation

Satellite image of sea surface chlorophyll a ObjectiveObjective

Seasonal changes in biomass, production and community structure of phyto- and zooplankton in the Kuroshio

Carbon budget within phyto- and zooplankton

Structure of plankton food web SamplingSampling

Sampling station 30˚N, 131˚E (Kuroshio)

Sampling period June 2002 - October 2003 200m (Monthly to Bimonthly)

Oceanographic observations

1000m Station A Temperature, Salinity (CTD) Pico- to Micro-plankton (Niskin bottles) Kuroshio 0,10,20,30,50,75,100 m Meso-plankton (Plankton net) 0-100 m MicroscopicMicroscopic AnalysisAnalysis Pico-plankton (<2µm) Bacteria Cyanobacteria Other pico-phytoplankton Nano-plankton (2-20µm) Autotrophic Nano-Flagellate (ANF) Heterotrophic Nano-Flagellate (HNF) Micro-plankton (20-200µm) Centric or Pannae Diatom Autotrophic Micro-flagellate (AMF) Heterotrophic Micro-flagellate (HMF) Naked Ciliate Tintinnid Ciliate Nauplii Meso-plankton (>200µm) Copepod Gelatinous zooplankton Other zooplankton ConversionConversion FactorsFactors

Picoplankton: cell numbers were directly converted to carbon Bacteria 12.8 fgC/cell Cyanobacteria 250 fgC/cell Other picophytoplankton 220 fgC/cell

Nano- to Meso-plankton: carbon contents were estimated from biovolume

Autotrophic Nano-Flagellate (ANF) Log10C=0.863Log10V×-0.363 Heterotrophic Nano-Flagellate (HNF) 0.12 pgC/um3

Centric or Pannae Diatom Log10C=0.758Log10V×-0.422 Autotrophyic Micro-Flagellate (AMF) C=0.216V0.939 Heterotrophic Micro-Flagellate (HMF) C=0.216V0.939 Naked Ciliate 0.19 pgC/um3 Tintinnid Ciliate C=444.5+0.053LV Nauplii 0.05 pgC/um3 Gelatinous mesoplankton 0.003 pgC/um3 Other mesoplankton 0.06 pgC/um3

C: Carbon (µgC), V: Volume (µm3), LV: Lorica volume (µm3) DilutionDilution ExperimentsExperiments

Following by Landry & Hassett (1982), Landry et al. (1995)

Experiments February to October 2003 (monthly)

Incubating sea waters Mesoplankton-Free SW ＋ Particle-Free SW

Diluted series Enriched bottles: 30, 50, 75, 100% Unenriched bottles: 100% (to examine nutrient-limited growth)

Enriched nutrients

NO3 : NH4: PO4 : Fe = 7.5µM : 0.5µM : 0.5µM : 1nM VerticalVertical DistributionDistribution ofof PhytoPhyto-- andand ZooplanktonZooplankton BiomassBiomass

0 10 * 20 * 30 * 50 * 75 * 100 Jun. Aug. Oct. * Nov. Feb. Mar. Apr. May Jul. Aug. Sep. Oct. 02040 0 10 * 20 * 30 * 50 * 75 * 100 * 02040 Biomass (mgC m-3) Pico Nano Micro

* No data PhytoplanktonPhytoplankton BiomassBiomass andand CommunityCommunity StructureStructure )

-2 1500

1000 Pico Nano (ANF) 500 Micro

Biomass (mgC m Biomass (mgC * 0

100

75 Cyano 50 OtherPico 25

Pico-composition (%) 0

100 Naked MF 75 Thecate MF 50 Centric 25 Pannae 0 Micro-composition (%) JJASONDJFMAMJJASO Month *sea surface only ZooplanktonZooplankton BiomassBiomass andand CommunityCommunity StructureStructure 2106.7 )

-2 2000 Pico 1500 Nano (HNF) 1000 Micro 500 * Meso Biomass (mgC m Biomass (mgC 0

100 Naked MF 75 Thecate MF 50 Naked Ciliate 25 Tintinnid 0 Micro-composition (%) Micro-composition Nauplii 100

75 Gelatinous 50 Copepoda 25 Others 0 Meso-composition (%) Meso-composition JJASONDJFMAMJJASO Month *sea surface only GrowthGrowth raterate

2 Bacteria ) -1 1.5 Cyanobacteria Other pico-phyto

h rate (day Nano-phyto 1 Micro-phyto

0.5 Maximum growt

0 Feb. Mar. Apr. May Jul. Aug. Sep. Oct.

Month NutrientNutrient effectseffects onon growthgrowth raterate

Pico Nano & Micro 2 ) -1 1

0 wth rate (day

-1

-2 Bacteria Other Pico Nano-phyto Unenriched gro Cyano Micro-phyto -3 -3 -2 -1 0 1 2 -3 -2 -1 0 1 2

Enriched growth rate (day -1) Enriched growth rate (day -1) VerticalVertical ProfilesProfiles ofof TemperatureTemperature andand SalinitySalinity

0 28 >29 20 24 28 40 26 22 60 26

Depth (m) 24 22 80 20 20 18 Temperature (ÞC) 100

0 <34.0 <34.0 34.2 20 34.2 34.4 40

34.4 60 34.6 Depth (m) 34.6 34.8 80 34.8 Salinity (PSU) 100 J J ASONDJFMAMASO J J

Month sampling date RelationshipRelationship betweenbetween GrowthGrowth andand MortalityMortality

Pico Nano & Micro

3 ) -1 2

1 Mortality rate (day Bacteria Other pico Nano-phyto Cyano Micro-phyto 0 01230123

Maximum growth rate (day -1) Maximum growth rate (day -1) CarbonCarbon BudgetBudget inin PlanktonPlankton FoodFood WebWeb Production: mgC m-2 Respiratory requirement : mgC m-2

Meso-zoo Meso-zoo 46.7 93.9

321.8 167.2

Micro-phyto Micro-zoo Micro-phyto Micro-zoo 140.5 38.8 27.4 74.0

110.8 211.3

ANF HNF ANF HNF 212.6 299.9 0.0 1091.5

749.8 2728.7

Pico-phyto Bac Pico-phyto Bac 125.6 172.6 470.0 1483.9

Mixing period (February-May) Stratified period (July-October) SummerySummery

Seasonal pattern of phytoplankton • Autotrophic nano-flagellates and cyanobacteria contributed to phytoplankton biomass. • No seasonal pattern was observed for phytoplankton biomass. • Seasonal phytoplankton dynamics might be affected by nutrients from the neighboring waters. Seasonal pattern of zooplankton • Bacteria and copepods dominated zooplankton biomass. • Zooplankton biomass increased during the stratified period. • Seasonal patterns of biomass might be resulted from temperature-dependent growth. Plankton food web • Grazing food web was functional along microbial food web during the mixing period. • During the stratified period, microbial food web was predominated and carbon flow seems to be complicated.