Classical food webs and the microbial loop off Western Australia: A new method to estimate simultaneously micro- and mesozooplankton grazing impacts

Transcription

1 Classical food webs and the microbial loop off Western Australia: A new method to estimate simultaneously micro- and mesozooplankton grazing impacts J. Anthony Koslow 1,4, Joanna Strzelecki 1, Harriet Paterson 2 and Stéphane Pesant 3 1 CSIRO Marine and Atmospheric Research, Perth, Australia 2 University of Western Australia, Perth, Australia 3 Laboratoire d Océanographie de Villefranche/mer, Villefranche-sur-Mer, France 4 Scripps Institution of Oceanography, University of California, SD, USA

2 Classic and microplankton food webs Pico-Phytoplankton Tintinnid Other microzooplankton Phytoplankton 1 µm Copepod Fish

3 Trophic cascades Copepods may release small phytoplankton from grazing pressure by intermediate consumers (protists) copepod Trophic cascades may be strong in oligotrophic waters where the dominant phytoplankton are too small (<5µ) to be directly consumed by most mesozooplankton ciliate Small phytoplankton

4 How to measure simultaneously micro- & mesozooplankton grazing + interactions? How can the effects of micro- and mesozooplankton grazing + their predator-prey interactions be estimated in an experiment? Conventional solution: Two sets of experiments: 1. Mesozooplankton (Incubation method (Frost, 1972)) 2. Microzooplankton (Dilution method (Landry and Hassett, 1982)) But this is tedious & time-consuming Can we determine: micro and mesozooplankton grazing on phytoplankton and mesozooplankton predation on microzooplankton from a single experimental method?

5 Grazing experiments T o natural assemblages of plankton from surface water Mesozooplankton microzooplankton phytoplankton mesozooplankton Control (24 hrs) Mesozoo excluded (= ) microzooplankton phytoplankton Treatments (24 hrs) from T o with natural assemblages of mesozooplankton manipulated, varying concentrations in 3 treatment levels short vertical tows from 5 m depth Phytoplankton and microzooplankton bucket sampling Incubation in ambient temperature and light Measured: Phytoplankton: Chl a and HPLC Microzooplankton: abundance and biomass Mesozooplankton: abundance and biomass

6 Feeding Rates P t = P exp [(G p -I zp Z) t ] (Frost 1972) P t & P - phytoplankton concentrations at start and end of experiment t - duration of experiment (e.g. 24 hr) G p growth rate of phytoplankton I zp Ingestion or grazing rate of mesozooplankton on phytoplankton Z Mesozooplankton concentration Modified for microzoo + mesozoo grazing: P t = P exp [(G p -I m M-I zp Z) t] M t = M exp [(G m -I zm Z) t] I m Grazing of microzooplankton on phytoplankton M Microzooplankton concentration G m Growth rate of microzooplankton I zm Grazing of mesozooplankton on microzooplankton

7 How can we solve for Gm, Im, I zm & I zp from one set of incubation experiments? 1) P t = P exp [(G p -I m M-I zp Z) t] 2) M t = M exp [(G m -I zm Z) t] In control, Z =, solve for G m from Eq 2: 1) M t = M exp [(G m ) t] 2) Solve G m from intercept in regression (Eq 2): ln(m t ) = [G m t + ln(m )] - I zm Zt ln(m t ) G m t+ln(m ) -I zm t Solve for I m 1) I m ~ G m /.33 (.33: growth efficiency) Z 2) In control, P t = P exp [(G p -I m M) t] (G p estimated from C 14 incubations) I zp & I zm from slopes of linearized Eq 1) & 2)

8 Study site Study site Stations: A - 1 m C - 1 m E 1 m Quarterly sampling over 3 years

9 Composition of grazers A January Abundance and biomass C abundance A, C, E July biomass A cladocerans calanoids calanoids cladocerans cyclopoids calanoids cyclopoids calanoids cyclopoids E abundance calanoids cyclopoids cyclopoids E biomass calanoids siphonophores zoeae siphonophores C biomass E chaetognaths chaetognaths cyclopoids calanoids Siphono phores

10 Composition of prey (microzooplankton & phytoplankton) Abundance Biomass Jan July Jan July A C E A C E A C E A C E dinoflagellate diatom 5µm ciliate

11 Phytoplankton production & size structure, Microzooplankton biomass Primary production (µ g C L -1 d -1 ) Primary production (bars) & proportion > 5 um (line) Jan 4 A Jan 4 C Jan 4 E Jul 4 A Jul 4 C Jul 4 E Microzooplankton Date biomass & Station (control) (bars) and fraction of daily primary production (line) Proportion >5 µ m Biomass (µ g C L -1 ) Jan 4 A Jan 4 C Jan 4 E Jul 4 A Jul 4 C Jul 4 E Fraction (B/PP) Sampling date & station

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15 Microzooplankton growth rate & ingestion: relation to primary production Microzooplankton productivity (G) v Primary productivity Microzooplankton Ingestion v Primary Production Microzooplankton G Primary production (µg C L -3 d -1 ) Microzoop Ingestion (µ g C µ g C -1 d -1 ) Primary production (µ g C L -3 d -1 ) Apparent asymptotic relationship between microzooplankton rates of production & ingestion and primary productivity

16 Comparison of mesozooplankton grazing & dilution experiments 12 % Total Primary Production Grazed by Microzooplankton: Dilution cf Mesozooplankton Grazing Experiment % Grazed - Mesozooplankton Expt :1 line % Grazed - Dilution Expt

17 Mesozooplankton grazing Grazing rate (I) (µ g C µ g C -1 d -1 ) Mesozooplankton grazing: phytoplankton (maroon) & microzooplankton (blue) Jan 4 A Jan 4 C Jan 4 E Date & Station Jul 4 A Jul 4 C Jul 4 E % Microzooplankton Production (G m *M) grazed by Mesozooplankton Mesozooplankton grazing (I) appears variable, low But it has a significant impact on microzooplankton production at the experimental densities % Grazed Mesozooplankton biomass (µ g C L -1 )

18 Trophic cascade? Mesozooplankton biomass and % small phytoplankton production grazed by microzooplankton % Small phyto production grazed Mesozooplankton biomass (µg C L -1 )

19 Mesozooplankton grazing impacts at ambient prey densities D wt (mg m 2 ) % Consumed 1 Mesozooplankton consumption of large phytoplankton production (> 5 µm) at high & low abundance levels Feb 2 Mar 2 May 2 Aug 2 Nov 2 Dec 2 Jan 4 A Jan 4 C Jan 4 E Jul 4 A Jul 4 C Jul 4 E Date & Station 4 m 1 m 3 m 1 m day 1 m night Mesozooplankton consumption of microzooplankton production at high and low mesozooplankton abundances Natural prey densities: 2 25 µg C L -1 Grazing impacts appear modest Consistent with Calbet % production consumed Jan 4 A Jan 4 C Jan 4 E Jul 4 A Jul 4 C Jul 4 E Date & Station

20 Summary Modified Frost equations used to calculate microzooplankton grazing (I), productivity (G), and mesozooplankton grazing rates on phytoplankton and microzooplankton from one set of experiments Good agreement between microzooplankton grazing rates estimated from dilution and mesozooplankton grazing experiments Microzooplankton grazed most of phytoplankton production (phytoplankton mostly in < 5 µm fraction) Mesozooplankton grazing imposed a trophic cascade in experiments, but grazing rates were low, with little impact at natural grazer densities.

21 Acknowledgements James McLaughlin - identification of microzooplankton Funding: Strategic Research Fund for the Marine Environment (SRFME): WA govt & CSIRO