Ecosystem Structure and Function on the Gulf of Alaska Shelf

Transcription

1 Ecosystem Structure and Function on the Gulf of Alaska Shelf

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3 GLOBEC COASTAL GULF OF ALASKA (GAK) STUDY AREA GAK or Seward Line: approx 2 km coast to oceanic GAK 1-34 years Line GLOBEC NPRB 2km

4 Setting: Complex Bathymetry

5 Slope eddies/meanders Potential shelf/slope exchange mechanisms and associated vertical motions.

6 Aleutian Low is the dominant atmospheric influence with lows entering the Gulf year-round, but less in summer The currents and water column properties respond to changes in the Aleutian Low

7 Annual Cycle of Alongshore Winds and Freshwater Runoff - year-round downwelling and - a large coastal freshwater discharge with low NO 3 - ~24, m 3 -s -1 (annual average), - Salinity exerts important control on ocean dynamics. YET A PRODUCTIVE SHELF!!

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9 The Schematic Plot of the Fresh Water Budget Estimate for Northeast Pacific Ocean

10 Salinity upper 1 m seasonal cycle A band of < 32 psu extending about 5-6 km offshore seasonally, max extent and lowest salinity in fall (coastal runoff) Interannual variability apparent highest offshore salinities in 2 and 21 Temperature upper 1 m seasonal warming with no significant temporal delay across the shelf Maximum temperatures in late summer Interannual variability apparent warmest years were 1998 (ENSO related) and 23, coolest winter in 22

11 GAK1 salinity anomalies for a) the upper 1 m and b) the lower 15 m. The respective linear slopes are -.4 year -1 and +.25 year -1.

12 GAK1 temperature anomalies for a) the upper 1 m and b) the lower 15 m. The respective linear slopes are +.31 o C year -1 and +.24 o C year -1.

13 Nutrients GAK 1 : Inner-shelf

14 GAK 4 : Middle-shelf

15 GAK 9 : Shelf-break

16 GAK 13 : Oceanic Regime

17 Surface iron concentrations decrease offshore Deep (>5m) iron concentrations >.5 nm Is outer shelf production controlled by mixing and/or the offshore spread of lowsalinity waters? Increasing salinity J. Wu (preliminary data)

18 Observations: Springtime Chlorophyll SeaWiFS [chl a]

19 April 1, 23 May 16, 23 Primary production starts on the inner shelf earlier (.5 1 month) than the mid- and outer shelf due to different stratifying mechanisms.

20 % chlor > 2 um IS MS OS Fraction of total chl >2 µm Total Total chlorophyll (mg (µg/liter) m -3 ) Bimodal distribution of CGoA phytoplankton size composition regions are either dominated by large cells (green oval) or small cells (red oval) but intermediates are not common High chlorophyll community dominated by large cells, > 2 um Low chlorophyll community dominated by small cells, < 5 um (and the biomass fraction of large cells is small)

21 Integrated Chlorophyll by Size Fraction: 21 - GAK 1 Integrated Chlorophyll by Size Fraction: 21 - GAK 9 m g C h l/m Sample Month >2 <2 & >5 <5 m g C h l/m Sample Month >2 <2 & >5 <5 Integrated Chlorophyll by Size Fraction: 21 - GAK 4 Integrated Chlorophyll by Size Fraction: 21 - GAK 13 m g C h l/m Sample Month >2 <2 & >5 <5 m g Chl/m Sample Month >2 <2 & >5 <5

22 summer 23 summer 23 Photosynthesis (µgc µgchl -1 hr -1 ) >2 <2 (large) (small) Light-saturated >2 <2 unsaturated Irradiance (moles photons m -2 6hr -1 ) Photosynthesis is saturated on sunny days but light-limited on cloudy days: light limitation may occur up to 5% of the time. (S. Strom)

23 Microzooplankton heterotrophic dinoflagellates 6 5 Biomass (μgc l -1 ) 4 Biomass (µg/l) x x1 3 1x1 3 Gym/Gyr 1 (>15 µ m ) Gym/Gyr 2 (7-15µ m ) Gym/Gyr 3 (2-6µm ) Gym/Gyr 4 (1-4µm ) Gym/Gyr 5 (1-2µm ) Gyrodinium cf. lachrym a Abundance (cells/l) 8x1 3 6x1 3 4x1 3 2x GAK Station

24 Microzooplankton - ciliates T o ta l C ilia te s - J u n e /J u ly L T O P Abundance Biomass Abundance (Cells/L) Biovolume (μgc/l) GAK Station

25 Microzooplankton grazing Phyto <5 µm : grazing : growth = 1.22 Phyto >2 µm: grazing : growth =.48 Microzoo grazing rate (d -1 ) : Phyto growth rate (d -1 ) ~5% of large phytoplankton production is consumed by microzooplankton the shelf supports a complex food web. (S. Strom)

26 Mesozooplankton Cross-shelf meso-zooplankton abundance (MOCNESS) May 1998 Cross-shelf meso-zooplankton abundance (MOCNESS) July 1998

27 Taxa Neocalanus plumchrus Neocalanus flemingeri Neocalanus cristatus Metridia pacifica Eucalanus bungii Calanus marshallae Metridia okhotensis Cross-shelf distribution - inter-annual variation ) Biomass (gm Distance (km) 1 2 Distance (km) 1 2 Distance (km) May 1998 May 1999 May 2 from Coyle et al. - LTOP data

28 Neocalanus feeding on phytoplankton small cell system Low phytoplankton concentration 15 Chlorophyll (ug/l) Ingestion (ng chl/cop/d) Ingestion < 5 um 5-2 um > 2 um Chlorophyll size category N. flemingeri 5/18/1, Total chlorophyll =.77 ug/l, Total ingestion = 24.2 ng chl/d

29 Neocalanus feeding on phytoplankton large cell system High phytoplankton concentration Chlorophyll (ug/l) Ingestion (ng chl/cop/d) Ingestion < 5 um 5-2 um > 2 um Chlorophyll size category N. flemingeri 5/27/1, Total chlorophyll = 3.73 ug/l, Total ingestion = ng chl/d

30 Neocalanus spp.don t consume a large fraction of the total daily phytoplankton production, but their preference for large cells (both phytoplankton and microzooplankton) has important implications for structuring the food web because of top down, cascade, effects - see my poster: Neocalanus spp. and the Structure of the Pelagic Ecosystem in the Subarctic Pacific Ocean M. Dagg, Louisiana Universities Marine Consortium H. Liu, Hong Kong University of Science and Technology S. Strom, Western Washington University

31 Mesozooplankton - larvaceans and pteropods Oikopleura labradoriensis famous photo by Per Flood

32 2 Appendicularian grazing on < 2 um phytoplankton (August 23) % daily production Larvacean grazing: 1-18 % of daily phytoplankton production Copepod community grazing: ~ 1 % of daily phytoplankton production 4 PWS IS IS IS MS MS MS MS OS OS OS OS

33 A. Spring large-cell dominated food web B. Summer small-cell dominated food web (dashed line = episodic event) large (>2 µm small (<2 µm microzoo 13 larvaceans? 2.6 Neocalanus 2.6 other mesozoo microzoo sinking? advection?? other mesozoo? pink salmon Units: µgc liter -1 d -1

34 Pink salmon comprise % of the total commercial catch of salmon in Alaskan waters (last 32 years) Juvenile pink salmon occupy surface waters and feed primarily during daylight. Fish move out of PWS into the inner half of the shelf (GAK 1-6) during Jul/Aug.

35 Mean Weights of Juvenile Pinks: August 21 and 22 WEIGHT (G) WEIGHT (G) AUGUST 21 PWS ACC SHELF AUGUST 22 * * Significant (*) yearto-year size differences suggest differences in food quality or quantity on inner- and midshelf between years. PWS ACC SHELF (L. Haldorson)

36 Juvenile Pink Salmon Diets - August 21, AUGUST 21 OTHER INSECT FISH APENDIC LIMACINA SHRIMP CRAB August 21: Shelf diet: Diverse (low availability of preferred foods);.2 PWS ACC Shelf AM PH EUPH LG COPE SM COPE 22 Adult return: 2.5% - LOW AUGUST 22 PWS ACC Shelf OTHER INSECT FISH APENDIC LIMACINA SHRIMP CRAB AM PH EUPH LG COPE SM COPE August 22 Diets: Limacina 23 Adult return: 8.5% - HIGH Mesozoop. Important? Causes?? (L. Haldorson)

37 plankton < 2 um - bacterioplankton -phytoplankton - microzooplankton plankton > 2 um -phytoplankton - microzooplankton Appendicularia/pteropod -growth Appendicularia -houses - detritus Copepods, Euphausiids juvenile pink salmon (July/August = post Neocalanus)

38 Climate change connections are apparent but mostly unquantified: freshwater inputs stratification coastal flow Mixed Layer Depth, nutrients (incl Fe), magnitude, type and timing of PP (ecosystem state), transport of oceanic mesozooplankton, types of zooplankton production, and pink salmon and other higher trophic levels

39 Future Directions Seward Line is currently being run 2 x yr -1, supported by NPRB GLOBEC synthesis phase (no field work) has been initiated currently physics to fish without lower trophic levels, Models mainly physical but some hope for biological NPRB Integrated Ecosystem Research Program: AO under development

40 The END