Abstract
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 × 10
6
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 NO
3
(9.3 vs. 6.5 μM when absent) and PO
4
(0.99 vs. 0.79 μM when absent). Blooms were also preceded by elevated ratios of NO
3
/PO
4
, NO
3
/Si, and PO
4
/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.