Consequences of an Invasive Clam on the San Francisco Estuarine Nitrogen Cycle Kleckner, A. E.

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Consequences of an Invasive Clam on the San
Francisco Estuarine Nitrogen CycleKleckner, A.
E. Wilkerson, F.Romberg Tiburon Center, San
Francisco State University, Tiburon, CA 94920
Abstract There is an ongoing threat to the
ecology of the San Francisco Estuary brought on
by invasive species. One particular invasive
species that has major impact on the estuary is
the Asian clam Corbula amurensis. It was first
discovered in Suisun Bay in 1986 and has been
generally accepted as the cause of a significant
decline in phytoplankton biomass and related fish
abundances. Invasive species are thought to
impact ecosystems because of their effects on
biological diversity, through competition with
native species of similar ecological roles for
resources and space yet much less in known about
the changes that they may cause to ecosystem
nutrient dynamics. In parts of northern San
Francisco Estuary, observed high densities of
Corbula may influence the nutrient regime as a
result of ammonium excretion. We have been
exploring the role of Corbula in the
remineralization of nitrogen through ammonium
excretion. This novel impact of an invasive clam
on the nutrient ecosystem may have management
implications.
Experimental Design This masters research
project was done in Suisun Bay (northern San
Francisco Estuary) in collaboration with the U.S.
Geological Survey. Question 1 Ten USGS sites
were sampled monthly for primary nutrients,
ammonium, chl-a, and salinity from September
2006 thru October 2007. Clam densities were
determined using a 0.05 m2 benthic grab. Question
2 Ammonium excretion rates by Corbula were
determined (see experimental setup below).
Question 1 What is Corbula habitat like?
Shallow water sampling stations

• Chl-a concentrations ranged from 13.1 µg/L to0.2
  µg/L.
• Ammonium concentrations ranged from 14.8 µM to
  2.4 µM
• Salinity ranged from 0.1-21.7ppt
• Nitrate concentrations ranged from 14.2 µM to
  42.4 µM
• No major differences in water chemistry patterns
  between deep and shallow stations

As a consequence of their high grazing rate and
potential uptake of dissolved organic nitrogen,
Corbula may have a strong influence on the
estuarine nitrogen cycle. An estimation of
ammonium excretion rate to the water column can
be made using an estimated biomass value of 16.79
gm/m2 from station 6.1 in November of 2006 (Jan
Thompson pers. com.). Assuming an ammonium
excretion rate of 5 µmol NH4/hr/gm we show that
the rate equals 0.084 mmol NH4/m2/hr. This rate
can be compared to the phytoplankton uptake rate
for Fall in Suisun Bay which is 0.025 mmol/m2/hr
(Wilkerson et al. 2006, assuming a euphotic zone
depth of 1 meter). These numbers are in
agreement and of the same order of magnitude. ?
Question 2 At what rate do Corbula excrete
ammonium?
Question 3 How does Corbula influence the
nitrogen regime of Suisun Bay?
Experimental Setup Clams were incubated in 500
mL of artificial seawater at ambient temperature
and salinity. Discrete water samples (25mL) were
collected for ammonium analysis at specific time
intervals.
Future Directions Further investigation is
needed to assess the ability of Corbula to
assimilate dissolved organic matter (DOM) into
its cellular metabolism and how much DOM Corbula
can remove from the surrounding environment. This
information will help determine a total nitrogen
budget for the clam as well as provide additional
information on the total nitrogen budget for
Suisun Bay.
?

• Rate of ammonium excretion by Corbula ranges from
  1.4-11.3 µmol N/hr/gm.

Acknowledgements The authors would like to thank
Jan Thompson of the US Geological Survey as well
as the crew of the R/V Polaris. We would also
like to acknowledge the Dugdale-Wilkerson
laboratory at the Romberg Tiburon Center, San
Francisco Sate University.