Interacting physical, chemical and biological forcing of phytoplankton thin-layer variability in Monterey Bay, California

Abstract During the 2005 Layered Organization in the Coastal Ocean (LOCO) field program in Monterey Bay, California, we integrated intensive water column surveys by an autonomous underwater vehicle (AUV) with satellite and mooring data to examine the spatiotemporal scales and processes of phytoplankton thin-layer development. Surveying inner to outer shelf waters repeatedly between August 18 and September 6, the AUV acquired 6841 profiles. By the criteria: [(1) thickness ≤3 m at the full-width half-maximum, (2) peak chlorophyll at least twice the local background concentrations, and (3) a corresponding peak in optical backscattering], thin layers were detected in 3978 (58%) of the profiles. Average layer thickness was 1.4 m, and average intensity was 13.5 μg l−1 above (3.2x) background. Thin layers were observed at depths between 2.6 and 17.6 m, and their depths showed diurnal vertical migration of the layer phytoplankton populations. Horizontal scales of thin-layer patches ranged from 10,000 m. A thin-layer index (TLI), computed from layer frequency, intensity and thinness, was highest in mid-shelf waters, coincident with a frontal zone between bay waters and an intrusion of low-salinity offshore waters. Satellite observations showed locally enhanced chlorophyll concentrations along the front, and in situ observations indicated that phytoplankton may have been affected by locally enhanced nutrient supply in the front and concentration of motile populations in a convergence zone. Minimum TLI was furthest offshore, in the area most affected by the intrusion of offshore, low-chlorophyll waters. Average thin-layer intensity doubled during August 25–29, in parallel with warming at the surface and cooling within and below the thermocline. During this apparent bloom of thin-layer populations, density oscillations in the diurnal frequency band increased by an order of magnitude at the shelfbreak and in near-bottom waters of the inner shelf, indicating the role of internal tidal pumping from Monterey Canyon onto the shelf. This nutrient transport process was mapped by the AUV. Peak TLI was observed on August 29 during a nighttime survey, when phytoplankton were concentrated in the nutricline. Empirical orthogonal function decomposition of the thin-layer particle size distribution data from this survey showed that throughout the inner to outer shelf survey domain, the layers were dominated by phytoplankton having a cross-section of ∼50 μm. This is consistent with the size of abundant Akashiwo sanguinea cells observed microscopically in water samples. During a subsequent and stronger intrusion of low-salinity offshore waters, spatially-averaged vertical density stratification decreased by > 50%, and phytoplankton thin layers disappeared almost completely from the AUV survey domain.

[1]  James M. Sullivan,et al.  Coastal thin layer dynamics: Consequences to biology and optics , 2010 .

[2]  Andrew M. Fischer,et al.  A coastal ocean extreme bloom incubator , 2008 .

[3]  Harry Leach,et al.  Vertical Microscale Distribution of Phytoplankton in Relation to the Thermocline , 1979 .

[4]  F. G. Figueiras,et al.  Stoichiometric variations of N P, C and O during a Gymnodinium catenation rod tide and their Interpretation , 1992 .

[5]  P. Franks.  Thin layers of phytoplankton: a model of formation by near-inertial wave shear , 1995 .

[6]  T. Smayda.  Adaptive Ecology, Growth Strategies and the Global Bloom Expansion of Dinoflagellates , 2002 .

[7]  J. Largier,et al.  Upwelling shadows as nearshore retention sites: the example of northern Monterey Bay , 1997 .

[8]  R. Marin,et al.  Mapping phytoplankton in situ using a laser‐scattering sensor , 2008 .

[9]  R. Eppley,et al.  Chlorophyll maximum layers of the southern-california bight and possible mechanisms of their formation and maintenance , 1981 .

[10]  Franklin B. Schwing,et al.  Bifurcated flow from an upwelling center: a cold water source for Monterey Bay , 1994 .

[11]  R. L. Smith,et al.  Coastal upwelling ecosystems , 1981 .

[12]  C. Storlazzi,et al.  Long-term, high-frequency current and temperature measurements along central California: Insights into upwelling/relaxation and internal waves on the inner shelf , 2003 .

[13]  John P. Ryan,et al.  Northern Monterey Bay upwelling shadow front : observations of a coastally and surface-trapped buoyant plume , 2009 .

[14]  John J. Cullen,et al.  Effects of nitrate on the diurnal vertical migration, carbon to nitrogen ratio, and the photosynthetic capacity of the dinoflagellate Gymnodinium splendens , 1981 .

[15]  Andrew M. Fischer,et al.  Influences of upwelling and downwelling winds on red tide bloom dynamics in Monterey Bay, California , 2009 .

[16]  John P. Ryan,et al.  Mass Stranding of Marine Birds Caused by a Surfactant-Producing Red Tide , 2009, PloS one.

[17]  James H. Churnside,et al.  Thin scattering layers observed by airborne lidar , 2009 .

[18]  A. Longhurst.  Analysis of Marine Ecosystems , 1981 .

[19]  John P. Ryan,et al.  Phytoplankton thin layers caused by shear in frontal zones of a coastal upwelling system , 2008 .

[20]  Francisco P. Chavez,et al.  Observations of upwelling and relaxation events in the northern Monterey Bay during August 2000 , 2005 .

[21]  P. Donaghay,et al.  Thin layers and species-specific characterization of the phytoplankton community in Monterey Bay, California, USA. , 2010 .

[22]  Michael S. Twardowski,et al.  Temporal and spatial occurrence of thin phytoplankton layers in relation to physical processes , 2001 .

[23]  L. C. Breaker,et al.  The circulation of Monterey Bay and related processes , 1989 .

[24]  M. Silver,et al.  The emergence of Cochlodinium along the California Coast (USA) , 2008 .

[25]  John P. Ryan,et al.  Linking the physiology and ecology of Cochlodinium to better understand harmful algal bloom events: A comparative approach , 2008 .

[26]  C. Torrence,et al.  A Practical Guide to Wavelet Analysis. , 1998 .

[27]  Francisco P. Chavez,et al.  Seasonal fluctuations of temperature, salinity, nitrate, chlorophyll and primary production at station H3/M1 over 1989-1996 in Monterey Bay, California , 2000 .

[28]  Kenneth S. Johnson,et al.  In situ ultraviolet spectrophotometry for high resolution and long-term monitoring of nitrate, bromide and bisulfide in the ocean , 2002 .

[29]  Percy L. Donaghay,et al.  Toward a theory of biological‐physical control of harmful algal bloom dynamics and impacts , 1997 .

[30]  W. Broenkow,et al.  The Role of Internal Tides in the Nutrient Enrichment of Monterey Bay, California" , 1982 .

[31]  O. Holm‐Hansen,et al.  Reliability of estimating chlorophyll a concentrations in Antarctic waters by measurement of in situ chlorophyll a fluorescence , 2000 .

[32]  R. Kudela,et al.  Cryptic Blooms: Are Thin Layers the Missing Connection? , 2008 .

[33]  W. Broenkow,et al.  Surface circulation and replacement of water in Monterey Bay , 1978 .

[34]  Andrew M. Fischer,et al.  Coastal Ocean Physics and Red Tides: An Example from Monterey Bay, California , 2005 .