Transcript Amanda ASLO 2004 poster - Institute of Marine Science

Abundance, biomass and production of Oithona similis in the Gulf of Alaska.
Amanda Byrd, R.R. Hopcroft, K.O. Coyle – University of Alaska Fairbanks
Introduction
Results
Egg production in copepod species may be the largest component of copepod production and is a
parameter routinely monitored in ecosystem studies. Production is often focused on the female of the
species as it is often the largest, longest lived and easiest to stage. Enumeration of copepods with
length-weight ratios and egg mass can be easily converted to production (mass per unit time). The
number of eggs produced daily per female can be seen as a direct effect of the environment and has
often been related to resources such as temperature and resource quality and concentration.
The egg carrier Oithona similis is found in many ecosystems from the poles to the equator and is the
most abundant copepod in the Gulf of Alaska. O. similis is one of the smallest common copepod
species, but one of the most important due to its year round abundance and reproduction. Egg
carrying species generally produce fewer clutches of eggs than egg dispersing species, and often carry
the clutch for many days. In the past, most studies were focused on the larger copepod species, which
are typically considered the most important secondary pelagic producers. Smaller species may be
more important in energy flow due to faster growth rates than the larger species (Hopcroft et al.
1998). This work will determine if such an hypothesis holds for sub-polar environments.
Female Abundance (# m-3)
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Pws 1
Pws 2
Kip 2
Gak 1
Gak 2
Gak 4
Gak 9
Gak 12
Gak 13
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The average population abundances in 2001 and 2002 were 1530 and 1524 individuals m-3, and
biomass (figure 1B) was 2.67 and 2.73 mg m-3, respectively. Females were present year round
with an average of 250 and 208 m-3 in 2001 and 2002 , respectively. In 2001, mid-shelf Gak4 and
Gak9 had consistently high female abundance, whereas in 2002 the highest abundance was found
more inshore at Gak1 to Gak4.
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Clutch sizes (Figure 1C) and average female length were consistently highest in May and
consistently low in March and December of both years. Gak9 had the highest clutch size in May
2001 with 26 eggs per clutch while Pws1 and Pws2 had the highest clutch sizes in May 2002.
Egg Clutches ranged from 8 to 26 eggs per clutch with an average of 17 eggs per clutch.
Mar Apr May Jul Aug Oct Dec Mar Apr May Jul Aug Oct Dec
2001
Specific Egg Production was relatively consistent across all nine stations with a mean of 2.3%
daily in 2001 and 2.8% daily in 2002 (Figure 1D). SEP at Gak1 and Gak9 were consistently high
in 2001 and Gak4 and Gak9 were general higher in 2002. In both years July and August had the
highest SEP, while March and December were lower in both years.
2002
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Biomass (mg m )
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Methods
Six stations along the Seward Line in the Gulf of Alaska (Gak1, 2, 4, 9, 12 and 13) and three stations
in Prince William Sound (Pws1 Pws2 and Kip2) were sampled seven times each in 2001 and 2002.
Sampling was undertaken by deploying two 25cm diameter 53μm mesh nets to a depth of 100m,
collecting plankton on the upward tow. The contents were immediately combined and preserved with
10% formalin. Flow meters were used to record the volume filtered (~10 m3 per pair).
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Discussion
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Clutch Size (eggs female )
SEP = (egg/female) HR (egg weight/female weight) (Nielson et al. 2002)
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Mar Apr May Jul Aug Oct Dec Mar Apr May Jul Aug Oct Dec
The copepods were enumerated and measured using a digitizer to estimate the biomass of O. similis.
Eggs and egg sacs were counted to determine egg sacs per female.
Specific egg production (SEP) was estimated by the egg-ratio method, which considers the egg:female
ratio of the population (including females not carrying eggs), the egg hatching rate (HR d-1) at an in
situ temperature, and mass of the egg and female:
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There is a higher overall abundance at the Prince William Sound stations (Pws1, Pws1 and
Kip2), although biomass was highest on the Seward Line. Data suggests slightly higher biomass
and abundance in 2001 at Gak4 and Gak9; Gak4 is located on an inner shelf with a semipermanent eddy, and Gak9 is located on the shelf break where a front often occurs. In 2002, the
higher biomass was found at Gak1 and Gak2 in the Alaska Coastal Current. The higher
abundances could be due to a concentration of prey and O. similis. Gak13 generally has the
lowest biomass, which could be attributed to the more oligotrohpic oceanic habitat.
In May clutch sizes were highest, consistent with observations for most other copepod species in
this region. March and December’s lowest clutch sizes could be due to the lower temperatures
and lower food concentrations in the water. The clutch sizes increased from March to May,
where a maximum was reached. The temperatures also increase as do the prey concentrations
through this time period.
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Specific egg production is affected by in situ temperature and hatching rate. Nielson et al. (2002)
notes that temperature is the main factor in determining hatching rate. Specific egg production
has a similar pattern to biomass following the annual cycles of primary productivity and
temperature. March and December had low biomass and SEP, likely due to the low temperatures
and limited primary productivity of the Gulf of Alaska during these months.
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Mar Apr May Jul Aug Oct Dec Mar Apr May Jul Aug Oct Dec
2001
2002
Conclusion
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Specific Egg Production (% d )
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There appear to be repeatable seasonal patterns, with March and December having the lowest
production and biomass, and an upward increase to the warmer months of May and July. In
future correlation to environmental and biological data will be undertaken, and stronger patterns
may be revealed with the inclusion additional locations and samples from 2003.
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Literature cited
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Hopcroft R.R and Roff J.C, Lombard D (1998) Production of Tropical Copepods in Kingston Harbour, Jamaica, the important
of small species, Marine Biology 130: 593-604
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Mar Apr May Jul Aug Oct Dec Mar Apr May Jul Aug Oct Dec
Figure 1. Map of study area and station Locations .
2001
2002
Figure 2 Female Population abundances (A), Biomass (B),
Clutch size (C), and Daily Specific Egg Production (D) at each
station over the 7 sample occasions each in 2001 and 2002.
Nielsen T.G, Møller E.F, Satapoomin S, Ringuette M, Hopcroft R.R (2002) Egg Hatching Rate of the cyclopoid copepod
Oithona similis in Arctic and Temperate Waters 236: 301-306
This research is supported buy the U.S. GLOBEC Northeast Pacific Program, Jointly funded by the
National Science Foundation and the National Oceanic and Atmospheric Administration under the NSF
Grants OCE-0105236 & OCE-0109078.