Variability of Winter-Spring Bloom Phaeocystis pouchetii Abundance in Massachusetts Bay

Abundance of the prymnesiophyte Phaeocystis
pouchetii was quantified via light microscopy at 2-week to
monthly intervals in Massachusetts Bay (southern Gulf of
Maine, NW Atlantic) during 1992–2012. Variability in the
abundance and seasonal cycle of Phaeocystis are described
and synoptic hydrographic, nutrient, and meteorological
data were analyzed to identify factors that may influence
Phaeocystis abundance. The maximum Phaeocystis abundance
was 14 × 106 cells L−1 (10 Apr 2008). It was frequently
(5 of 8 years) absent prior to year 2000, but not thereafter.
Seasonally, it first appeared in February to early March,
reached peak abundance in mid-April, and persisted until
May or early June for a duration of 0–112 days (mean 34 days).
A long-term alternation between Phaeocystis and centric diatom
abundance was apparent, suggesting winter-spring selection
of either Phaeocystis or centric diatoms. Phytoplankton
community analysis suggested that blooms affected the rest of the phytoplankton community. Phaeocystis blooms were
manifest as a substantial increase in particulate nutrients above
normal levels. Phaeocystis blooms were preceded in February
by a slightly elevated concentration of NO3 (9.3 vs. 6.5 μM
when absent) and PO4 (0.99 vs. 0.79 μM when absent).
Blooms were also preceded by elevated ratios of NO3/PO4,
NO3/Si, and PO4/Si, and warmer, saltier waters reflecting
reduced river discharge. The correlation with salinity and
river discharge suggests that Phaeocystis bloom variability
is partially determined by annually varying circulation processes
that determine the degree of low nutrient, low salinity coastal
water intrusion into Massachusetts Bay.