Seasonality of phytoplankton distributions in the

1
Seasonality of phytoplankton distributions in the
Galapagos Marine Reserve
A.M. McCulloch1, W.V. Sweet1, B.A. Schaeffer1,
J.M. Morrison2, D. Kamykowski1, and S. Banks3
2Contact morrisonj_at_uncw.edu, 910-962-2333 Univers
ity of North Carolina Wilmington Department of
Physics Physical Oceanography Wilmington, NC
1North Carolina State University Marine, Earth
and Atmospheric Sciences Raleigh, NC.
3Charles Darwin research Station Galápagos
Islands, Ecuador
Results
Introduction
Results
The Galápagos Archipelago is unique due to the
oceanographic conditions (Fig. 1) that come from
multiple directions creating diversity among
marine niches. Shifts in ocean
currents create spatial variability characterized
by the physical and chemical properties of the
mixing water. Temporal variations such as El
Nino and La Nina also effect the distribution
of biological communities. The interactions of
the cold and warm currents make discrete
biogeographic zones based on macroinvertebrate
and pelagic fish distributions (Fig. 2).
Objective One goal of the project is to
determine the seasonality of phytoplankton
distribution throughout the Galápagos Marine
Reserve.
BIOLOGICAL
PHYSICAL
During the March 2005 cruise, the highest
concentrations of centrate and pennate diatoms
were located from NW to SE within the Archipelago
(Fig. 7). Low concentrations were found in the
upwelling area to the west of Isabela. The
highest concentrations of dinoflagellates were
found in the upwelling area to the west of
Isabela with moderate concentrations throughout
the remaining Archipelago. During the June 2006
cruise, the highest concentrations of centrate
and pennate diatoms were found in the upwelling
area to the west of Isabela (Fig. 7). Moderate
concentrations of pennate diatoms and
dinoflagellates were found throughout the
remaining Archipelago.
Methods
Physical and biological data were collected
onboard the M/V Sierra Negra at approximately 70
stations throughout the Archipelago. We focused
on a 12-day cruise in the warm season (March) and
during the transition from the warm season to the
Garua (June). At each station, vertical profiles
were conducted with a CTD (SeaBird 19) and
SeaPoint Fluorometer to an approximate depth of
75 m. Vertical phytoplankton net tows (20 mm)
were conducted to 20 m. Phytoplankton (Fig. 3)
were preserved in 3 buffered formalin and
enumerated using a Dikon inverted microscope.
During the March 2005 cruise, the surface waters
indicate localized upwelling west of Isabela,
with warmest waters within the northwest (NW) and
southwest (SW) (Fig. 4). A band of slightly
cooler surface water (24o C) occurred NW to
southeast (SE) within the Archipelago, also seen
at the 20-m contour (18o C), which may indicate
upward mixing (Fig. 5). Higher chlorophyll a
concentrations are associated with this band of
cooler water (Fig. 6). During the June 2006
cruise, overall surface temperatures were cooler
and the thermal structure was uniform, with
warmer waters towards the northeast (NE) that
gradually cool toward the SW of the Archipelago
(Fig. 4). Noticeable upwelling was west of
Isabela. At the 20-m contour, the thermal
structure is similar to the surface, indicating a
vertically homogeneous water-column structure,
with a deep-warm, surficial water mass over most
of the Archipelago (Fig. 5). This thermal
structure is absent within the upwelling zone.
Maximum chlorophyll a concentrations are
associated with the upwelling zone to west of
Isabela (Fig. 6).
Discussion
During the March 2005 cruise, the 20-m
temperature contours suggests cooler upwelled
waters correspond to the area of high chlorophyll
a detected by MODIS. The highest concentrations
of centrate and pennate diatoms were associated
with the locations of highest chlorophyll a
concentrations. During the June 2006 cruise,
the homogenous waters to the east of Isabela
corresponded with low concentrations of centrate
and pennate diatom populations. High
concentrations of centrate and pennate diatoms
were found to the west of Isabela in the
upwelling area. Phytoplankton populations vary
both seasonally and spatially with diatom
populations typically found in areas of cold
nutrient rich waters, while dinoflagellate
populations show no indication of this pattern.