Phytoplankton Response to Nutrient Drawdown in Growout Experiments Stations AD

1
Influence of San Francisco Estuary Outflow on the
Near-Shore Gulf of the Farallones Phytoplankton
Community A. Marchi, R. Dugdale, A. Parker, V.
Hogue, K. Lew, J. Fuller and F. Wilkerson Romberg
Tiburon Center-San Francisco State University,
3152 Paradise Drive, Tiburon, CA 94920
Phytoplankton Response to Nutrient Drawdown in
Grow-out Experiments (Stations A-D)
Physical, Chemical and Biological Parameters in
July 2007
Abstract Coastal zones worldwide are heavily
impacted by nutrient inputs from rivers and
estuaries, especially nitrogen due to increasing
population size in the coastal zones and the use
of nitrogen fertilizers. The impact of estuarine
eutrophication on adjacent coastal ecosystems has
been a focus of recent coastal science. This link
has been unstudied for San Francisco Bay (SFB),
which supplies eutrophic water to the Gulf of the
Farallones. Our study sought to understand the
influence of anthropogenically modified SFE water
on phytoplankton productivity and natural
phytoplankton assemblages in the near-shore Gulf
of the Farallones Marine Sanctuary. Three
cruises were completed this year (2007) aboard
the National Marine Sanctuary research vessel
(R/V Fulmar) during the months of February, May,
and July. Temperature, salinity, inorganic
nutrients, DIC-DIN pools, flow cytometry, 15NO3,
and 15NH4 productivity rate measurements were
collected, and shipboard grow-out enclosure
experiments were conducted on water collected
from the most near-shore stations.
NO3
C
Estuarine Near-shore Ecosystem Model
NH4
This Studys Near-shore Enclosure Phytoplankton
Drawdown
Typical San Francisco Bay Phytoplankton Drawdown
Enclosure Results When San Francisco Bay water
is enclosed and exposed to 50 surface irradiance
and sampled for 4 days, NH4 first declines to low
non-inhibiting concentrations, then NO3 uptake
proceeds rapidly with the production of
chlorophyll. The same pattern is seen in the
enclosures from the buoyancy plume. The drawdown
of NO3 is delayed more in the water sampled
closest to the Golden Gate. Nitrate
concentrations did not decrease until ammonium
concentrations lowered enough to allow the uptake
of nitrate (about day 2) (see graphs above). 
Nitrate depletion was greatest in the presence of
lower ammonium concentrations and this was
coupled with a greater increase in phytoplankton
(as measured by chlorophyll-a).  The near-shore
phytoplankton uptake response is similar to what
is generally observed inside San Francisco Bay
(see above graph on right). Additionally, rapid
silicate drawdown (see graph below) suggests that
near-shore phytoplankton communities are
generally diatom dominated and is supported by
our flow cytometry data (not shown here).
http//farallones.noaa.gov
Rationale Our scientific objectives are to
understand the role of two different sources of
nutrients for the lower trophic levels that feed
the food web of the Gulf of the Farallones 1)
coastal upwelling and 2) outflow from San
Francisco Bay. Each of these sources is likely
to be more important seasonlly and under
different climate conditions (see conceptual
model above). Upwelling occurs primarily in
spring and summer, while major freshwater flow
and outflow from San Francisco Bay occurs in
winter and early spring during the rainy and
snowmelt periods. The additional nutrient source
from San Francisco Bay will supplement the more
traditionally considered upwelling sources and
ensure plentiful nutrients to fuel the Farallones
food web. The dual nutrient sources may be one
reason for the high productivity condition of the
Gulf as evident from the large resident marine
bird and mammal populations. The form of
nitrogen dominating the dissolved inorganic
nitrogen pool is also important. In San
Francisco Bay ammonium (NH4, an anthropogenically
produced nutrient) has been shown to delay and/or
reduce primary productivity by its inhibitory
action on phytoplankton nitrate uptake. The
ratio of NH4NO3 and the outflow concentration of
NH4 (if it is above a threshold of 4 µM) may
modulate primary production in the Gulf of the
Farallones. To evaluate whether NH4 may be
impacting phytoplankton physiology directly we
used grow-out enclosure experiments which were
tracked for 4 days, isolated from grazing
pressure and light limitation.
Results The buoyancy plume along the Marin Coast
originating from San Francisco Bay is
characterized by low salinity, warm temperatures
and elevated NH4 concentrations. A band of
relatively low chlorophyll also occurs within
this plume. However, there are actually two
water masses along the Marin coast, separated by
a persistent oceanic front at Bolinas, seen most
clearly in the temperature contours (see above
plots). The colder water in the Bolinas to Point
Reyes region is from upwelling. Throughout the
plume, NH4 concentrations are inhibitory to NO3
uptake. As the surface water is advected away
from the coast, and NH4 is drawdown below 4 µM,
then surface chlorophyll increased. The effect
can be seen by comparing the NH4 and chlorophyll
contours. The effect of the NH4 in preventing
access of the phytoplankton to NO3 is
demonstrated in the enclosure experiments (right
side of poster).
Acknowledgements We wish to thank the crew of
the Gulf of the Farallones Sancuatry ship R/V
Fulmar, Jan Roletto of the Gulf of the
Farallones National Marine Sanctuary, and the
many helpful interns and students during the
research cruises.