Seasonality of phytoplankton distributions in the Galapagos Marine Reserve A.M. McCulloch 1, W.V. Sweet 1, B.A. Schaeffer 1, J.M. Morrison 2, D. Kamykowski 1, and S. Banks 3 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 (~24 o C) occurred NW to southeast (SE) within the Archipelago, also seen at the 20-m contour (~18 o 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). Introduction Methods Results Discussion 1 North Carolina State University Marine, Earth and Atmospheric Sciences Raleigh, NC. 2 Contact:University of North Carolina Wilmington Department of Physics & Physical Oceanography Wilmington, NC 3 Charles Darwin research Station Galpagos Islands, Ecuador The Galpagos 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 Galpagos Marine Reserve. 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 m) were conducted to 20 m. Phytoplankton (Fig. 3) were preserved in 3 % buffered formalin and enumerated using a Dikon inverted microscope. Figure 3. Phytoplankton of the Galapagos Marine Reserve including Centrate Diatoms, Pennate Diatoms, and Dinoflagellates. MARCH 2005JUNE Figure 6. Chlorophyll concentration from Modis satellite images during March 2005 and June Figure 2. The Galapagos Marine Reserve separated into five biogeographical zones based on macroinvertebrates. Blue dots indicate CTD profiling and phytoplankton net tow stations. Figure 7. Phytoplankton concentration (#s/L) of the Galapagos Marine Reserve for both the warm season (March 2005) and the transition from the warm season to the Garua season (June 2006) Figure 4. Surface temperature contours for March 2005 and June Figure meter temperature contours for March 2005 and June PHYSICAL: Results BIOLOGICAL: 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. 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. Figure 1. Current flow surrounding the Galapagos Marine Reserve.