Cross-shore exchange processes, and their effect on zooplankton biomass and community composition patterns in the Northeast Pacific

Transcription

1 Cross-shore exchange processes, and their effect on zooplankton biomass and community composition patterns in the Northeast Pacific D.L. Mackas Fisheries & Oceans Canada Institute of Ocean Sciences K.O. Coyle Univ. Alaska, Fairbanks Institute of Marine Science

2 Outline: Shelf vs deep ocean: Environments & zooplankton communities Important cross-shore exchange processes and their: Latitudinal gradients Seasonality of direction & intensity Interactions with zooplankton vertical distribution and life history

3 Environments: Shelf vs Oceanic Small & narrow High primary productivity, large phytoplankton Large Small phytoplankton, often nutrient limited Shallow water column, interacts with seabed Deep water column, interacts with mesopelagic Stratified to surface Strong alongshore currents across strong gradients Deep mixed layer Gyre: Slower & recirculation

4 Zooplankton Communities: Shelf vs. Oceanic Vancouver Island continental shelf 48 40N W z= 65m (grid = 1 mm) Alaska Gyre (Station "P") 50N 145W z= 4200m

5 Zooplankton Communities: Shelf 1-4 generations/year Smaller body sizes Less diverse Many herbivores Vertical migration less consistent Vertical migration also aids retention Often dormant eggs, on or in sediments

6 Zooplankton Communities: Oceanic 1-2 generations/year Larger body sizes More diverse Most omnivores or carnivores Vertical migration (diel or seasonal or both) Migration also aids seasonal survival Dormant as C4-adult

7 Strong spatial gradient between shelf and oceanic zooplankton communities. (shelf break) Average location : - Over or near the shelf break - Coincides with a 'front' and a strong along-front current Intensity of gradient and species mix on each side vary seasonally (Coyle & Pinchuk in press)

8 BUT the shelf-break front is: "Leaky" Mobile Seasonally transient Movements of the front and mixing across the front cause exchange between shelf and offshore zooplankton communities Geostrophic current vectors overlaid on surface temperature Strub and James 1995

9 Important physical processes causing shelf-offshore exchange Wind-driven Ekman transport Boundary current filaments and meanders Seaward propagating eddies Estuarine circulation/coastal runoff Canyons & other topography Subduction at frontal boundaries

10 Wind-driven surface Ekman transport (and compensating subsurface flow) : Coastal Offshore (curl of wind stress) (Bakun 1996)

11 Direction, intensity & timing of Ekman transport vary with season and latitude coastal convergence & downwelling coastal divergence & upwelling PFEL Index Sites (Schwing et al. 1996)

12 Above ~50N, transport during the growing season is shoreward, carrying oceanic zooplankton onto the shelf (plotted from Cooney 1988) 5 4 Zooplankton Biomas &Transport Ekman Transpor -0.5 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec -1-1 Month Zooplankton standing stock (g/m3) Zooplankton transport (10^6 tonnes/month) Onshore Ekman flow (10^12m3/month) -2

13 Isotopic & elemental tracers allow source discrimination: oceanic (blue) vs coastal (green) Off Alaska, 'oceanic' individuals are abundant on the shelf and in coastal embayments such as Prince William Sound (Kline 1999)

14 Below ~50N, surface Ekman transport is seaward during the growing season, exporting zooplankton from shelf to offshore (Mackas 1992, 1995)

15 Nearshore retention is an ecological challenge for 'upwelling zone' residents. Ontogenetic changes in vertical distribution aid life cycle completion (Peterson et al. 1979)

16 Depth distribution & transport direction vary with age AND With timing of reproduction vs. upwelling (before) egg nauplii protozoea zoea eggs and larvae mm 13mm 14mm 15mm 16mm 17mm 18mm 19mm 20mm female E. pacifica male E. pacifica C Line 19 April Longitude ( o W) Abundance (# m -3 ) Abundance (# m -3 ) Depth (m) Ts pin_juvs Epac juvs la rv ae eggs Ts pin_m Ts pin_f Epac_m Epac_f (after) Eggs, larvae and early juveniles Adults La titude Latitude La t it ud e (Lu et al. 2003)

17 Continental margin meanders, Instability of energetic boundary currents produces large seaward deflections of the shelf-ocean interface eddies, and filaments 'NASA Newsroom': SeaWiFS image - June 2002

18 California Current "cold filaments", eddies & meanders 'Coastal Transition Zone' program Seaward meanders are cyclonic (upwelling) Size ~ 400 km offshore Persistence ~ 1-2 months Transit time ~ days Strongest seaward flow = the front between cold upwelled water and warm offshore water (between yellow dots) (Huyer et al. 1991)

19 California Current "cold filaments", eddies & meanders mgc m Euphausia pacifica Euphausiid larvae Eucalanus other copepods Juv Chaetognaths Sagitta decipiens Sagitta scrippsae Heteropod larvae Doliolids & Salps Muggiaea others 0 > <8 SE>8 Geopotential Anomaly (m 2 s -2 ) (Mackas, Washburn & Smith 1991) Zooplankton biomass and community composition contours stretch along current streamlines Highest biomass is inshore/south of the peak seaward flow BUT Strong seaward transport within the band of strong current (Φ =8-9)

20 California Current "cold filaments", eddies & meanders "Export of Offspring" High Eucalanus egg production High abundance of euphausiid larvae (Smith & Lane 1991)

21 Vancouver Island filaments & meanders (Mackas & Yelland 1999) Smaller & less persistent than off California Both cyclonic (summer) & anticyclonic (winter) Highest biomass is inshore of strongest current BUT Higher biomass in seaward than shoreward flow

22 Alaska continental margin eddies & meanders J. Gower, IOS Large (~200 km) Anticyclonic Seasonal? (winter-spring max) Produce substantial crossshore transport (Okkonen et al 2003; Coyle & Pinchuk in press)

23 Alaska continental margin eddies & meanders After they form, the large eddies/meanders propagate westward North of ~ 60 N, westward movement is along the continental margin (blue arrows) But south of 60 N (red arrow), they often move offshore into the Alaska Gyre

24 'Haida' and 'Sitka' eddies Large (~200 km dia) & anticyclonic Core of low density coastalorigin water Form in late winter, move offshore in early spring Persist 2-3 years, gradually exchange water & momentum with surrounding ocean Can be mapped and tracked with satellite altimeter (Crawford et al 2002)

25 Zooplankton in 'Haida' eddies As they move seaward, eddies are colonized by continental margin (shelf, slope) and oceanic (Alaska Gyre) zooplankton species. Abundance of shelf-origin species is much higher in eddies than in surrounding Alaska Gyre but declines with eddy age. Eddies are very 'leaky'. Taxa best retained are those that avoid the surface layer (and Ekman transport) (Mackas & Galbraith 2002)

26 Behavioral and life cycle interactions Wind-driven currents have strong vertical shear, especially near the base of the mixed layer Zooplankton vertical distribution strongly affects rate and direction of transport. Surface avoidance favors retention in nearshore estuarine & upwelling environments, and in eddies

27 Mysteries How plastic or 'local' are behavioral and life cycle adaptations? Individual species are often successful in very different environments: open ocean, shelf break/slope, and deep coastal embayments (e.g. Euphausia pacifica, Neocalanus plumchrus) 'upwelling' and 'downwelling' continental shelf environments (e.g. Calanus marshallae, Pseudocalanus mimus)

28 Summary Often a strong gradient in zooplankton community at or near the shelf break But this boundary is mobile and leaky 'Upwelling' coasts appear in general to export zooplankton from shelf to offshore 'Downwelling' coasts both import from and export to the open ocean (wind vs. eddies)

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30 Usually a sharp gradient between shelf and oceanic communities (often centered over the outer shelf or near the shelf break) (Cooney & Coyle 1982)