1. 46054 11 m/s Water mass subduction & eddy effects on phytoplankton distributions in the Santa Barbara Channel, California Libe Washburn 1, Mark Brzezinski 2, Nick Dellaripa 3, and Chris Gotschalk 3 1 Marine Science Institute & Dept. of Geography, UC Santa Barbara, washburn@eri.ucsb.edu, 2 Dept. of Ecology Evolution & Marine Biology, mark.brzezinski@lifesci.ucsb.edu,washburn@eri.ucsb.edumark.brzezinski@lifesci.ucsb.edu 3 nick@physics.ucsb.edu, Marine Science Institute, UC Santa Barbara, 4 Marine Science Institute, UC Santa Barbara, gots@lifesci.ucsb.edu nick@physics.ucsb.edugots@lifesci.ucsb.edu Abstract: Observations using towed, undulating vehicles show that phytoplankton layers occur well below euphotic zone depths in the Santa Barbara Channel (SBC). The deep layers are typically found where density surfaces slope steeply suggesting isopycnal mixing and advection cause downward transport of the phytoplankton. Significant correlation between salinity and chlorophyll along isopycnals supports this interpretation. The observations also indicate that eddy processes cause downward transport of phytoplankton. Data were obtained during 16 cruises conducted three times per year from 2001 to 2006 as part of the Santa Barbara Coastal Long Term Ecological Research project (SBC-LTER). Other sampling during the cruises reveals that wind-driven upwelling and cyclonic eddies largely control the spatial patterns of phytoplankton primary productivity. Cyclonic eddies influence productivity by uplifting isopycnal surfaces and by occasionally entraining phytoplankton and nutrients from water upwelled near Point Conception. Eddy-enhanced productivity changes were superimposed on variable levels of channel-wide productivity caused by wind-driven upwelling. The combined effects of upwelling and cyclonic circulation enhance phytoplankton biomass and productivity in the SBC compared to elsewhere in the Southern California Bight. 1. Wind-driven downwelling of phytoplankton - Current velocity contours on Line H show high chlorophyll, low salinity water mass advecting eastward & low chlorophyll, higher salinity water mass advecting westward. 10 km Santa Rosa I. 1% wind-driven downwelling of phytoplankton Study site and methods - 16 cruises (spring, summer & fall, winter), 2002-2006 on RV Pt Sur - 25 CTD/rosette stations in grid & 7 stations on PnB line from mainland to Santa Rosa Island - phytoplankton primary productivity at 5 m on grid & PAR sensor for euphotic zone (1% light) - towed profiler surveys (Scanfish & Triaxus); this poster focuses on lines C and H (red lines) Approach: Use fluorescence-derived chlorophyll as dye to infer downward transport from near-surface. High spring productivity: upwelling conditions Strongly upwelling-favorable wind before & during cruise 7: May 2003 3 May 200310 May17 May 24 May CTD Grid line Cline H Cyclonic eddy & domoic acid in a spring bloom: May 2003 Anderson et al. (2006) mg m -3 19 May 2003 2003 1600 – 2000 -40 -30 0 40 30 20 2. Eddy-driven downwelling of phytoplankton eastward current speed (cm s -1 ) 10 km - Deep phytoplankton in core of eddy indicate downward transport out of euphotic zone. - Highest chlorophyll concentration above eddy is found above center. eddy-driven downwelling of phytoplankton 3. Isopycnal subduction & mixing of phytoplankton -30 -20 030 20 25.6 25.5 25.7 -Phytoplankton primary productivity in the SB Channel driven by upwelling winds and cyclonic eddies. - Large fractions of chlorophyll biomass (cruises: 8 - 42%) are found below 1% light depth. -Three vertical transport mechanisms identified: 1. wind-driven downwelling 2. eddy-driven downwelling 3. subduction & mixing along sloping isopycnals. -Speculations for future research: 1. Vertical transport of phytoplankton out of the euphotic zone important contributor to the demise of blooms. 2. Mechanisms are significant transport pathways for organic carbon to depth. Acknowledgements: Thanks to many graduate & undergraduate students, technicians, and RV Pt. Sur crew members who participated in the cruises. Funding: - Santa Barbara Coastal Long Term Ecological Research Project - NOAA/IOOS & the Southern California Coastal Ocean Observing System -State of California’s Coastal Ocean Current Monitoring Program Summary and future directions Santa Barbara Coastal Long Term Ecological Research - Chlorophyll distribution along cruise Line C during downwelling-favorable winds - Chlorophyll below 1% light depth (yellow dashed line) along Santa Rosa Island is consistent with wind-driven downwelling of phytoplankton - Cyclonic rotation uplifts isopycnals by at least 40 m - Elevated chlorophyll concentrations occur above region of isopycnal uplift suggest nutrient injection into euphotic zone Santa Cruz I. Line C - Red arrow shows average wind speed over 48 hrs preceding Line C at buoy 46054. - Winds are downwelling favorable winds along north side of Channel Islands. profiler track Line C Santa Rosa I. Santa Cruz I. 1% Line C Line H Three mechanisms for downward transport of phytoplankton Surface currents show cyclonic (counter- clockwise) eddy. Core of eddy is in solid body rotation that extends below 300 m depth (vorticity =  ≈ 0.5f ). eastward currents (cm s -1 ) Isopycnal distribution of salinity & chlorophyll - Chlorophyll below euphotic zone (red dots) is linearly correlated with salinity. - Linear correlation indicates chlorophyll concentration behaves as conservative tracer. - Consistent with downward mixing of phytoplankton by isopycal mixing and advection   = 25.5 kg m -3   = 25.6 kg m -3   = 25.7 kg m -3 seafloor near Anacapa Island 1% Line C Line H Line C - Upper left: primary productivity distribution during LTER cruise 7. Strongly upwelling favorable spring winds upwelled nutrient rich waters before & during the cruise. - Upper right: surface currents and surface relative vorticity. Circular current pattern with high vorticity was associated with high high productivity & high chlorophyll concentration. - Lower left: Satellite chlorophyll distribution show large phytoplankton bloom (red shades). - Lower right: Distribution of domoic acid produced by Pseudo-nitzchia australis. - Upper right: Distribution of wind stress during LTER cruise 7 (solid arrows) and averaged the 2 weeks preceding the cruise. - Upper left: Primary productivity distribution during LTER cruise Sampling during SBC-LTER cruises: