Zooplankton Grazing and Secondary Production in the Indian Ocean

Transcription

1 Zooplankton Grazing and Secondary Production in the Indian Ocean Michael Landry Integrative Oceanography Division Scripps Institution of Oceanography University of California, San Diego SIBER Workshop Goa, India, October 2006

2 Zooplankton Diversity Omnivorous Suspension Feeders Euphausia pacifica Carnivores MesoZoo MicroZoo 200 µm

3 Overview Concepts, Methods & Relationships Microzooplankton Rate Overview Mesozooplankton Rate Overview Food-Web Flux Synthesis Summary & Implications

4 Net Growth Rate (d -1 ) MicroZoo Grazing - Dilution Method Coastal Station Station N2, 8 m Y = X (r 2 = 0.84) Grazer Density (Relative) Oceanic Station Nutrient Addition Treatments Station N7, 8 m Y = X (r 2 = 0.96) No Nutrient Controls Grazer Density (Relative) Coast Oceanic Grazing, m (d -1 ) Growth, µ n (d -1 ) Net Growth (d -1 ) Growth, µ o (d -1 ) m + net µ % PP - m/µ o Nutr Limited µ µ o /µ n Data Available: # Experiments Reckman & Veldhuis (1997) Arabian Sea 8 Landry et al. (1998) Arabian Sea 61 Caron & Dennett (1999) Arabian Sea 29 Edwards et al. (1999) Arabian Sea 27 Brown et al. (2002) Arabian Sea (58) taxon-specific Patterson et al. (in prep) SW Australia

5 MicroZoo Grazing & 2 Production CO 2 50% Fenchel & Findlay (1983) Verity (1985) PP m:μ BP PP Anderson & Ducklow (2001) Production GGE = 30% Straile (1997) egesta

6 MesoZoo Grazing Methods Gut Fluorescence: Uses gut phaeopigments (Ph) -- i.e., Chl a breakdown by-products -- as tracer of feeding on phytoplankton. Measured on individual spp. or mixed assemblages. Isotope Uptake: Measures uptake of 14 C and 3 H-labeled particles into the >200-µm size-fraction in short-term (1 h) incubations. Involves in situ incubations in special containers with mesh concentrated MesoZoo from 10- m depth range. Assumed to measure grazing on both phytoplankton ( 14 C) and detritus/heterotrophs ( 3 H). Data Available: Grazing = [Ph] gut * Gut Turnover Rate No MesoZoo rate estimates in the US JGOFS database. Extensive studies of net-collected abundance & biomass: JGOFS, IO Roman et al. (2000) DSR II. Grazing was experimentally measured by the isotope technique at 6 stations on 2 cruises (NE Monsoon - January 95; Spring Intermonsoon - March 95) and extrapolated to grazing & production at all stations for all US JGOFS cruises/seasons (4).

7 MesoZoo Grazing & Production Phyto Phyto, MicroZoo Detritus, & Carnivory Total Ingestion CO 2 (40%) Biomass Size Str Growth or P/B Model (Hirst & Sheader 1997) Estimated Production f (B, T C, size) GGE = 30% Egesta (30%)

8 Grazing & Production by MicroZooplankton

9 MicroZoo as Primary Consumers CO 2 40% PP PP 71% PP BP ~10% PP 2 Production 24% PP

10 Spatial & Seasonal Variability Rate (mg C m -3 d -1 ) Aug 95, SW Monsoon Nov-Dec 95, NE Monsoon Phyto Production MicroZoo Grazing Brown et al. (2002)

11 Taxon-Specific Differences Production Grazing Rate (mg C m -3 d -1 ) Prochlorococcus Synechococcus Brown et al. (2002)

12 All Phytoplankton Taxa are Significantly Grazed by MicroZoo Diatoms Production Grazing Rate (mg C m -3 d -1 ) Brown et al. (2002)

13 MicroZoo as Primary Consumers CO 2 40% PP PP 71% PP BP ~10% PP 2 Production 24% PP

14 Microbial Food Web

15 Carbon Flows through MicroZoo Cumulative Trophic n Resp Prod Transfer n = mean # trophic steps before MesoZoo All estimates as % of PrimProd

16 Grazing & Production by MesoZooplankton

17 Temporal & Spatial Variability in Biomass and Ingestion January Late NE March Spr Inter Aug-Sept Late SW December Early NE Roman et al. (2000)

18 MesoZoo Ingestion vs. Primary Production GRAZING RATIO = Ingestion/Prim Prod Grazing Ratio January Late NE March Spr Inter Aug-Sept Late SW December Early NE Roman et al. (2000)

19 Ingestion Biomass (mm C m -2 d -1 ) (mm C m -2 ) Grazing Production Mean Estimates of MesoZoo Parameters Ratio (mm C m -2 d -1 ) Jan Mar Aug Dec LNE SI SW ENE Jan Mar Aug Dec LNE SI SW ENE Jan Mar Aug Dec LNE SI SW ENE Jan Mar Aug Dec LNE SI SW ENE Roman et al. (2000)

20 A Food-Web Flux Analysis

21 Sum of the Parts % PrimProd Feeding by MicroZoo 81 Feeding by MesoZoo 40 Bacterial Carbon Demand ~ 100 > 200%

22 Phyto Trophic Fluxes in the Arabian Sea Major S-line Stns Means of 4 cruises Rates - mmol C m -2 d -1 Phyto Resp etc. GPP C-PP 85 Higher Trophic Levels G meso 19 G micro 72 Meso Micro Est I meso = * 14 C-PP n=1 n=2 2 P Transfer 24 7 BCD BGE = 15-20% Virus 9 9 HBact BP 18 DOC

23 Variable Partitioning of Grazing on Phytoplankton Grazing Ratio (G meso :G micro ) late SW Aug-Sept early NE Dec late NE Jan Spr Inter March S2 S4 S7 S11 S15

24 Summary Measured rates make sense in a food-web context Closed grazing balance Constrained fluxes Dynamics: We know little about: Interannual & long-term trends Short-term community responses & transitions Modeling: Explicit trophic connection between diatoms & dominant grazers (MicroZoo) - regulate Si:N utilization? Both Micro & MicroZoo are multi-level boxes Regulatory (top-down) role of MesoZoo?