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Zooplankton Distribution and Transport in the California Current Off Oregon

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Zooplankton Distribution and Transport in the California Current Off Oregon Vol. 508: 87–103, 2014 MARINE ECOLOGY PROGRESS SERIES Published August 4 doi: 10.3354/meps10835 Mar Ecol Prog Ser Zooplankton distribution and transport in the California Current off Oregon D. Wu1, M. Zhou1,*, S. D. Pierce2, J. A. Barth2, T. Cowles2 1University of Massachusetts Boston, 100 Morrissey Blvd, Boston, MA 02125, USA 2Oregon State University, 104 Kerr Administration Building, Corvallis, OR 97331, USA ABSTRACT: Transport and retention of zooplankton biomass in the shelf and slope regions off Oregon, USA, were studied in June 2002, using high-resolution measurements of temperature, salinity, depth, fluorescence, water current and zooplankton abundance. We employed 2 different analytical methods for minimizing divergence in the geostrophic current fields based on measure- ments from a vessel using an acoustic Doppler current profiler. We detected high zooplankton biomass in coastal upwelling areas on the shelf, and exchanges between shelf waters with high zooplankton biomass and offshelf waters with low zooplankton biomass via cross-isobath currents. In the shelf area of Heceta Bank off Newport bounded by the 153 m isobath, the standing zooplankton biomass was ~4 × 104 t C. The major flux of zooplankton biomass into the area occurred at the northern boundary at a rate of 1 × 103 to 2.5 × 103 t C d−1 or a specific rate of 0.03 to 0.06 d−1 based on 2 different analytical methods. The flux at the southern boundary was 1 order of magnitude lower, and a significant flux out of the area occurred at the 153 m isobath at a rate of ~0.8 × 103 to 3.7 × 103 t C d−1 or a specific rate of 0.02 to 0.09 d−1. These rates are comparable with the zooplankton growth and mortality rates of ~0.1 d−1 previously reported for this region. Offshelf transport of zooplankton contributes significantly to biomass losses in shelf ecosystems and in turn fuels offshelf ecosystems. KEY WORDS: Shelf · Coastal retention · Physical-biological coupling · Productivity Resale or republication not permitted without written consent of the publisher INTRODUCTION periment of 1982−1986 showed that surface jets and eddies were more energetic in the summer than in The California Current system off the Oregon and the winter (Rienecker & Mooers 1989, Strub et al. California coasts is highly productive during spring 1991). The Coastal Transition Zone Experiment in the and summer upwelling seasons. The association of late 1980s concluded that the cold filaments origi- these high productivities with mesoscale physical nated from continuous, meandering jets which sepa- features has sparked a number of scientific studies on rated onshelf and offshelf waters (Strub et al. 1991), physical, chemical and biological processes and the and that deep phytoplankton layers in the offshelf sustainability of fisheries within the California Cur- water originated from the subducted coastal cold rent. The Coastal Ocean Dynamics Experiment in the water at the transition or converging zone (Brink & early 1980s revealed that the intensive offshore jets Cowles 1991). In the early 1990s, the Northern Cali- associated with cold filaments penetrated more than fornia Coastal Circulation Study discovered that the 100 m deep and transported coastal biota from the velo city variability on time scales from weeks to shelf to hundreds of kilometers off the shelf (Kosro & months was produced by mesoscale eddies moving Huyer 1986). The data from the Ocean Prediction off the shelf (Largier et al. 1993). The Eastern Bound- Through Observations, Modeling, and Analysis Ex- ary Current Project was conducted in the early 1990s, *Corresponding author: [email protected] © Inter-Research 2014 · www.int-res.com 88 Mar Ecol Prog Ser 508: 87–103, 2014 focusing on mesoscale variability of physical and bio- using towed optical and acoustic devices to resolve logical fields. Results from this project indicated that both physical and biological fields at the same loca- maxima of zooplankton abundance and biomass co- tion and time, e.g. in the California Current system incided with mesoscale eddies (Huntley et al. 1995). (Huntley et al. 1995, Zhou et al. 2001, Barth et al. Processes of zooplankton transport and population 2002), in the Georges Bank region (Wiebe et al. 2001, dynamics associated with these mesoscale eddies Benfield et al. 2003, Ullman et al. 2003) and in north- were further studied using the size-structured zoo- ern Norwegian shelf areas (Fossheim et al. 2005, plankton model and objective interpolation meth od, Zhou et al. 2009, Zhu et al. 2009). Although results revealing that the generation time scale of mesozoo- from these early studies elucidated qualitative rela- plankton and macrozooplankton varies from 30 to tionships between physical and biological fields, 100 d (Huntley & Lopez 1992, Zhou & Huntley 1997, quantitative estimates of transport and retention of Zhou et al. 2001). Assuming a current of ~10 cm s−1, zooplankton biomass and the rates of zooplankton the advective time scale over a mesoscale or topo- population dynamic processes were rarely found. To graphic feature of 50 to 100 km is ~6 to 12 d, which is elucidate these process rates, the physical processes much smaller than the generation time scale (Hunt- of advection and retention need to be resolved. ley & Lopez 1992, Zhou et al. 2001). Thus, the effects A cruise was conducted between 1 and 17 June of advection processes on zooplankton distribution 2002 as part of the US Global Ocean Ecosystems and productivity are expected to be on the first order Dynamics Program (GLOBEC) Northeast Pacific of magnitude compared to other physical, chemical Study (NEP) for surveying physical and biological and biological processes. fields using towed and shipboard physical-biological The California Current is generally southward, sensors (Barth et al. 2005). Here, we use these inte- close and parallel to the coast north of Newport, Ore- grated physical and biological data to make quantita- gon (Barth et al. 2000, 2005). The bathymetry of the tive estimates of transport, retention and process region leads to the formation of mesoscale eddies or rates, potential errors based on different analytical meanders over Heceta Bank, which entrap upwelled methods, and limits of survey and analytical meth- cold water near the coast. Frequently, mesoscale ods. Our aim was to assess local food web dynamics. eddies and filaments spin off from the coast and translate westward. The California Current sepa- rates from the coast at Cape Blanco and flows south- MATERIALS AND METHODS westward (Kosro et al. 1991, Barth et al. 2000). A subsurface northward undercurrent of 5 cm s−1 was Data observed between 35° and 50°N along the shelf break in the depth range between 125 and 325 m The study area extended from 44° 37’ N off New- (Pierce et al. 2000). In the upper 200 m, the verti- port, Oregon, to 41°44’ N off Crescent City, Califor- cally integrated volume transports of the southward nia, and from the coast to ~100 km offshore (Fig. 1). California Current and the northward undercurrent The mesoscale survey was conducted over a period are ~3 and 0.8 Sverdrup, respectively (Barth et al. of 5 d with 7 cross-shelf transects ~0.25° latitude 2000, Pierce et al. 2000). The dynamics of these apart. Two fine-scale surveys were then conducted in mesoscale features are associated with baroclinic the Heceta Bank and Cape Blanco regions, with 8 instability of the California Current (Pierce et al. cross-shelf transects at latitudinal intervals of ~0.05º 1991), topographic features (Haidvogel et al. 1991), to 0.15° (Fig. 1). A towed SeaSoar instrument pack- and wind stress (McCreary et al. 1991). Biological age was employed during the survey and included 2 processes are coupled with mesoscale physical pro- pairs of SBE 911 plus conductivity, temperature and cesses in the California Current (Huntley et al. 1995, depth sensors (CTD; Sea-Bird Electronics) for the Zhou & Huntley 1997, Barth et al. 2002). Phyto - hydrographic data, a flourometer (Wet Lab) for rela- plankton and zooplankton biomasses enhanced by tive fluorescence as a proxy of phytoplankton bio- upwelling in coastal areas will be either transported mass, and an optical plankton counter (OPC; Focal away by the California Current or retained by eddies Technologies) for zooplankton sized between 0.25 and meanders. and 2.4 cm in equivalent spherical diameter (ESD). To understand the fate of zooplankton in eddies The SeaSoar undulated from the surface to ~10 m and jets requires resolving zooplankton distributions above the bottom in coastal areas, or a maximum and processes at the scale of eddies and jets. Signifi- depth of ~200 m in offshore areas at a ship speed of 7 cant efforts have been made in the last 2 decades to 8 knots. The SeaSoar undulating cycle varied from Wu et al.: Zooplankton in the California Current 89 0.02 m s−1 using navigation and bottom track, respec- tively (Barth et al. 2005). Wind measurements were obtained from NOAA’s National Data Buoy Center for Station 46050 located at 44° 37’ N, 124° 30’W, ~37 km offshore of Newport, Oregon (www.ndbc. noaa. gov). Zooplankton taxonomic data were based on live samples collected on 31 May 2002 over Heceta Bank using a 0.5 m2 ring net with 202 µm mesh towed in the upper 100 m at 1 knot (Fig. 1) (W. Peterson unpubl. data). Data processing The OPC provides plankton counts in 3494 digital sizes corresponding to a size range between 0.25 and 24 mm in ESD (Herman 1992). To use the carbon unit, the ESD of a zooplankter was converted to its body carbon based on the equation of Rodriguez & Mullin (1986) developed specifically for the Califor- nia Current system by assuming the length to width aspect ratio of 1.61 for copepods (Huntley et al.
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