Download Document

"The Gulf of Alaska Seward Line - 2005 & 2006
Russell R. Hopcroft, Kenneth O. Coyle, Tomas J. Weigngartner, Terry E. Whitledge
[email protected]
Institute of Marine Science, University of Alaska Fairbanks
COOL
WARM
COOL?
18
0
-10
1990
2000
C
100
80
60
40
20
0
12
10
8
6
4
2
0
2001
2002
2003
2004
2005
2006
140
1999
2000
2001
2002
2003
2004
2005
80
60
40
20
30
25
20
15
10
0
1998
1999
2000
2001
2002
2003
2004
2005
2006
1998
60
40
20
1999
2000
2001
2002
2003
2004
2005
2000
1500
1000
500
0
300
4000
3000
2000
1000
0
1999
2000
2001
2002
2003
2004
2005
2006
1999
2000
2001
2002
2003
2004
2005
1999
2000
2001
2002
2004
2005
2006
1998
1999
Stg3
2000
2001
2002
2003
2004
2005
2003
2004
2005
2006
During the late summer, water temperatures are at their highest and we
continue to see significant variability in abundance of the small copepods that
dominate the zooplankton (i.e. Oithona, Acartia, Pseudocalanus – Fig 6). More
interestingly, during warm years “southern” species often appear within the
zooplankton communities. During the 1997/98 El Niño, the copepod
Mesocalanus tenuicornis became common in nearshore waters, while during
2005 the small copepod Paracalanus parva was spread completely across the
Seward Line (Fig 7) . The copepod Calanus pacificus was more consistent in
occurring during warm years in offshore waters, but remained notably common
even during 2006. When common these “warm” water species have the
potential to change the size-spectra of the zooplankton and may alter the
foraging efficiency of visual predators such as fish.
3500
3500
Pseudocalanus spp. (Summer vert)
3000
2500
2000
1500
1000
500
Fig.6. Abundance of the
copepods Pseudocalanus
and Oithona that dominate
the Seward Line during late
summer.
3000
2500
2000
1500
1000
500
Oithona similis (Summer vert)
0
0
1998
1999
2000
2001
2002
2003
2004
2005
1998
2006
1999
2000
2006
2001
2002
2003
2004
Acartia spp. (May vert)
8000
2006
500
Paracalanus parva (Summer vert)
6000
4000
2000
400
300
200
100
0
1998
1999
2000
2001
2002
2003
2004
2005
2006
2004
2005
2006
Year
0.20
OTHERS
Chaetognatha
Cnidaria
Pteropods
Oikopleura spp.
Calanus pacificus
Eucalanus bungii
Metridia spp.
Paracalanus parva
Pseudocalanus spp.
Oithona similis
0.15
0.10
50
100
30
20
16
12
8
0
1998
0.05
0
Calanus pacificus (Summer vert)
4
0.00
200
2005
Year
Year
2006
150
200
1999
2000
2001
2002
2003
Fig.7. Abundance and biomass of
dominant zooplankters along the Seward
Line during early September 2005, noting
two prominent southern species.
Distance from shore (km)
100
75
50
25
0
1998
2003
400
Oithona similis (May vert)
Mean Abundance (No m -3)
Mean Abundance (No m-3)
2500
2002
2006
5000
Pseudocalanus spp. (May vert)
2001
0
Fig.3. Abundance of the
dominant copepod species
along the Seward Line during
May. 95% confidence errors
are indicated for the long term
mean (red symbol, green bar)
and each year (black).
1998
2000
40
80
2006
3000
1998
1999
5
0
0
4
0
Biomass (g
100
6
2
Calanus marshallae (May MOC)
120
8
2
100
Mean Abundance (No m-3)
Mean Abundance (No m -3)
4
0
1998
Metridia spp. (May MOC)
Mean Abundance (No m-3)
Eucalanus bungii (May MOC)
16
Mean Abundance (No m -3)
120
35
14
6
10
Results (continued):
Neocalanus cristatus (May MOC)
Mean Abundance (No m-3)
Mean Abundance (No m -3)
140
8
1998
18
N. plumchrus & N. flemingeri (May MOC)
10
12
Mean Abundance (No m-3)
180
12
Fig.5. Abundance of the
larvacean Oikopleura and the
pteropod Limacina along the
Seward Line during May
Mean Abundance (No m -3)
1980
Limacina helicina (May MOC)
14
Mean Abundance (No m-3)
1970
Mean Abundance (No m-3)
1960
16
14
Abundance (No m3)
1950
Like all biological communities we can see changes in abundances of species
between years. Of the large copepods that dominate the spring, the largest,
Neocalanus cristatus, shows no significant pattern across years, while the slightly
smaller N. plumchrus/flemingeri show significantly higher abundances in 3 years
and lower abundances in 3 years (Fig 3). Similarly, Eucalanus bungii, and Metridia
pacifica, show significant variation between years, while Calanus marshallae shows
large increases in abundance during 2005& 2006. Smaller species (i.e. Oithona,
Pseudocalanus, Acartia) are also variable, but there appears to be little consistency
in pattern between species. Although warm years may not affect abundance, they
do effect growth rates & passage of stages through the ecosystem (Fig 4). Like the
copepods, the mucus-net feedering Oikopleura and Limacina are variabile, but for
Limacina, higher abundances occur only during “warmer” springs (Fig 5).
2000
Oikopleura spp. (May MOC)
Mean Abundance (No m -3)
-5
Results (biological):
1999
18
16
Methods:
1998
Stage5
5
D
160
1998 1999 2000 2001 2002 2003 2004 2005
B
10
Fig.2. A) The El Niño Southern Oscillation (ENSO)
Index, B) the Pacific Decadal Oscillation (PDO) and
warm/cold regimes, C) May temperatures along the
Seward Line, D) Temperatures and salinity profiles at
Gak1 for 2005 & 2006 cruises compared to the 30 year
mean and 95% confidence intervals at that location.
Results (physical):
20
1998 1999 2000 2001 2002 2003 2004 2005
WARM
1940
Long-term observations began the fall of 1997, with 6 or 7 cruises conducted
annually until 2004 under the NSF/NOAA GLOBEC program. During 2005 &
2006, NPRB-funding allowed continuance of cruises in early May and early
September. Sampling during cruises consisted of 13 stations along the
Seward Line stretching from the coast to well beyond the shelf break, and 3-5
stations in western Prince William Sound (Figure 1). Sampling during all
cruises consisted of: A) profiles of temperature, salinity, nutrients and
chlorophyll, B) stratified sampling of larger zooplankton and integrated
sampling of smaller zooplankton, C) estimation of the community rates of
primary production and the rates of secondary production for the dominant
zooplankton species.
40
0
m3)
We have long appreciated that the ocean experiences variation from year to
year, but only recently appreciate that long-term trends and even pronounced
shifts may also be occurring. Short-term events such as El Niño and La Niña
can result in changes in ecosystem productivity that result in increased or
decreased survival of commercial and non-commercial species. Longer term
changes can result in fundamental shifts in ecosystem structure and function,
such as the 1976 regime shift in the Gulf of Alaska that resulted in a change
from a shrimp dominated fishery to one dominated by pollock, salmon and
halibut. One of the greatest challenges to detecting and understanding such
changes is the lack of appropriate oceanographic time-series that couple
these physical events to their biological manifestations. The multidisciplinary
time-series of the northern Gulf of Alaska’s Seward Line allows such
observation of short and long-term changes in the oceanography of a region
that is critical to Alaska’s fisheries, subsistence and tourist economies.
60
Stg4
Stage4
20
80
Percentage Stage
Stage5
40
Mean Abundance (No m -3)
summer PDO
Introduction:
Fig.1. Sampling area. Experimental sites indicated in larger red dots.
60
0
15
To date the study period has encompassed the strong 1997/98 El Niño, the
strong 1999 La Niña, the moderate 2002/03 El Niño, and the anomalously
warm non-El Niño years of 2000 & 2005 (Fig 2a). A strong El Niño is
expected for 2007. Each of these events is apparent in the physical time
series (Fig 2c). It is not apparent if a 1998/99 regime shift as suggested by a
change in PDO sign (Fig. 2b) has occurred within the time-series. With both
the 2005 and 2006 profiles consistent with the long-term deep warming trend
observed at the inshore station Gak1 for which a 30-year physical time series
is available (Fig. 2d,e).
80
Percentage Stage
A
The Seward Line in the Northern Gulf of Alaska has been the focus of
multidisciplinary sampling for 9 years. Here we report on the observations of
physical oceanography, nutrients, phytoplankton, and zooplankton over that
period. In particular, we compare the patterns during 2005 and 2006 to the
prior period, comment on inter-annual variability and consider how events
such as El Niños and regime shifts may be impacting this region.
Fig.4. Stage distribution of the
Neocalanus copepods during
May. During warm years, later
stages are more abundant,
suggesting they will complete
the growth phase of their life
cycle and descend to depth
earlier. Once at depth, they are
no longer available to their
commercial predators (i.e. fish).
Neocalanus cristatus
100
Stg3
Abstract:
Neocalanus plumchrus/flemingeri
100
1998
1999
2000
2001
2002
2003
2004
2005
2006
Acknowledgements:
This is a contribution to the North Pacific Research Board’s Ocean monitoring program, and
the US GLOBEC program (jointly funded by the NSF and NOAA).