Foodweb support for the threatened Delta smelt:
Microzooplankton dynamics in the low salinity zone of San Francisco Bay
J.K.
1
York ,
B.
1
Costas ,
G.
1
McManus ,
A. M.
2
Slaughter ,
T.
2
Ignoffo ,
W.
2
Kimmerer
1 Department
of Marine Sciences, University of Connecticut, Groton, CT
2 Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, CA
Introduction
Results
San Francisco Bay and Delta
Microzooplankton Herbivory
Suisun
Bay
Carquinez
Strait
sampling area
0.5-5 psu
Mar-Aug 2006
San
Joaquin
River
2.0
July *
April**
1.5
1.0
0.5
0.0
0
0.2
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-0.5
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0.8
1
dilution
dilution factor
Research Questions:
1)How does microzooplankton
grazing impact phytoplankton
(herbivory)?
2) How does microzooplankton
grazing impact bacteria (bactivory)?
3) How does copepod grazing impact
microzooplankton, and how does
this affect lower trophic levels?
• Plot shows results from 2 (of 10 total) representative
sampling dates.
• Phytoplankton growth rates (y-intercept) ranged from
0.19 to 1.97 doublings per day, increasing from spring to
summer.
• Microzooplankton grazing rates (slope) ranged from
0.04 to 1.66 per day, with a general trend towards higher
rates in summer.
• In Spring 9-42% of phytoplankton primary production
was grazed by microzooplankton; in Summer up to 84%
was grazed.
Materials and methods
• Dilution Experiments -- Determine the effect of dilution of filtered Bay water on
phytoplankton net growth rates in bottle incubations. Increased growth in more
dilute treatments indicates release of phytoplankton from microzooplankton grazing
pressure.
• Trophic Cascade Experiments -- Determine the cascade of effects of copepod
grazing on food web by experimentally manipulating copepod concentrations (low,
medium, high). Lower microzooplankton growth rates (due to copepod grazing) are
expected to result in increases in phytoplankton biomass due to release from
grazing pressure.
Additional work
- determine microzooplankton abundance by microscopic enumeration
- assess microzooplankton bactivory by experiments with fluorescently labeled bacteria
apparent phytoplankton growth rate
(day-1)
CA
San
Pablo
Bay
microzooplankton growth rate (day-1)
Sacramen
to River
In summer microzooplankton grazing
presented a strong “top-down” control on
phytoplankton biomass.
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0.4
0.2
0
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6
12
18
24
30
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-0.8
microzooplankton
-1
Copepod grazing pressure decreased the
growth rate of microzooplankton.
The decrease in microzooplankton did not
result in the expected increase in
phytoplankton or bacterial biomass.
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This may be because of a more complex food
web, with multiple food items making up the
diet of P. forbesi.
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6
12
18
24
30
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phytoplankton
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Bacteria Growth Rate/Day
20
apparent growth rate (d-1)
0
Trophic Cascade Experiments
2.5
Kilometers
San
Francisco
Our work is part of a
collaborative research program
to characterize the foodweb of
the low salinity zone (LSZ) of the
northern San Francisco Estuary
(SFE) to assess the potential for
food limitation of Delta Smelt.
Our focus is the role of
microzooplankton (20-200µm)
which are the main consumers of
phytoplankton in most estuarine
systems. In addition, they are
part of the “microbial loop” by
which dissolved organic matter
is included in the food web.
Conclusions and implications
Future work will focus on a broader
understanding of the contribution of
microzooplankton to copepod diets, in an
effort to determine whether copepods (the
preferred food for Delta Smelt), may be food
limited.
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Acknowledgments
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bacteria
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6
12
18
24
30
The authors wish to thank Captain David Morgan and
David Bell for their assistance aboard R/V Questuary, as
well as the Captain and crew of R/V Polaris. Funding
for this project was provided by CALFED Science
Program Grant # SCI-05-C107.
copepod abundance-1
Pseudodiaptomus forbesi (L )
• Copepod grazing decreased the growth rate of
microzooplankton.
• The decrease in microzooplankton abundance had no
effect on lower trophic levels (phytoplankton, bacteria).
• This preliminary evidence suggests that P. forbesi may
also graze on phytoplankton.
Further information
For more information, email: joanna.york@uconn.edu