Trends in algal biomass and species composition during the spring
bloom in Budd Inlet, Washington State and their possible
relationship to changes in density stratification and nutrient
(N and Si) composition and concentration.
Brooke Young, Julie Marano, Lindsey Belcher, Elissa Emde, Gerardo Chin-Leo
South Puget Sound has experienced HAB events, and there is evidence that the frequency
and severity of these events are increasing. Understanding the factors controlling
phytoplankton species composition and biomass can provide useful information to
understand when harmful species will become dominant. A study was conducted from
April to May 2009 in Budd Inlet, the southernmost inlet in Puget Sound (Washington
State) to determine environmental factors associated with changes in biomass and species
composition during the spring phytoplankton bloom. Sampling was conducted once a
week at two locations in the inlet representing areas with different degrees of density
stratification and nutrient input. One station was near the Deschutes River and the city of
Olympia and the other was near the mouth of the inlet. Surveys determined species
composition (vertical tows), abundance of dominant species (surface samples),
phytoplankton biomass (chl a) and nutrient concentrations (N and Si). Phytoplankton
biomass was consistently greater at the stratified site, but species composition was similar
for both study sites. The majority of the species detected were diatoms, and diatoms were
also the most abundant phytoplankton. A bloom of the diatom Chaetocerous sp. in early
May was the largest concentration of phytoplankton observed with 4.9 x 105 cells l-1 at
the stratified site and 2.9 x 105 cells l-1 at the mixed site. Towards the end of spring, the
number of dinoflagellate species and their abundance increased, but diatoms continued to
dominate the phytoplankton. The presence of the harmful dinoflagellate Alexandrium sp.
was observed. Diatoms of the genus Pseudo-nitzschia were also detected, but it was not
possible to determine if these cells belonged to toxic species. Dinoflagellate species
observed in vertical tows sometimes were not detected in surface samples. This is
probably due to dinoflagellate vertical migrations and underscores the importance of
sampling throughout the water column to determine the presence of dinoflagellates. Our
study also observed significant variations in phytoplankton parameters over short
transects indicating patchiness in the concentration of plankton. Trends in phytoplankton
parameters and their relation to changes in nutrient concentration and relative
composition (N:Si) will also be presented.