Zooplankton Community Structure
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UPDATED: 1 January 2009

Large zooplankton and micronekton play important roles in the export of organic material from surface waters in the open ocean. Global Ocean Flux planning models suggest that the relationship between primary production and passive particulate export flux is strongly influenced by size structure of the zooplankton community (e.g., Paffenhöffer & Knowles, 1979; Small et al., 1987; Frost, 1984). Active vertical migrations also have important implications for the transport and transformation of surface-derived organic particulates to dissolved inorganic constituents at depth (Longhurst & Harrison, 1988; Longhurst et al., 1990; Al-Mutairi & Landry, 2001; Hannides et al., 2008). The zooplankton component of the time-series sampling effort allows such processes to be considered in the interpretation of seasonal and interannual variations in measured flux and the elemental mass balance (e.g., carbon and nitrogen sources and sinks) of the euphotic zone.

At Station ALOHA, 6 net tows are scheduled per cruise. Three midnight (2200 - 0200) and 3 mid-day (1000 - 1400) oblique tows are done using a 1-m² net (3-m length) with 202-µm mesh Nitex netting. The net is towed obliquely at approximately 1 knot, from the surface to approximately 175 m and then back to the surface. Towing time is approximately 20-30 minutes. The tows are subsequently size-fractioned and analyzed for mesozooplankton wet and dry weight and C and N biomass.

• 2.1 Hardware
  

  Two net systems have been used for routine time-series collections of zooplankton at Station ALOHA. From 1994 to 2005 (Cruises 50-175), we used a 1-m² single-net frame with wire attachments and weighting similar to a MOCNESS (Landry et al., 2001; Sheridan & Landry, 2004). A flow meter with a low-speed rotor (Model 2030R, General Oceanics, Miami, FL) was attached across the net opening to measure distance towed, and a temperature-pressure data logger (Model XL-200, Richard Brancker Research, Ottowa, Canada) was fastened to the net frame to measure depth of tow. From cruise 175 to present, the collection procedure was simplified by switching to a 1-m² diameter ring net, with GO 2030R flow meter and Vemco minilog Time-Depth Recorder. Both frames are fitted with 202-µm filter mesh nets with similar aspect ratios, and they have roughly comparable mouth areas under tow. They are lowered to depth and returned to the surface similarly (by capstan). The main difference is a preceding bridle on the ring net, which may be easier to avoid by larger animals with fast escape responses compared to the side bridles of the original rectangular net. For this reason, caution is urged in comparing net collection in the largest (> 5 mm) size fraction before and after cruise 175 (November 2005). Since even very large, fast-towed nets (7.3 m² Isaacs-Kidd mid-water trawl and 96 m² Cobb nets; 2-4 kts) are unlikely to sample micronekton quantitatively (Kuba, 1970), neither of the small HOT nets is assumed to capture this fraction well.

• 2.2 Post-recovery processing
  
  • 2.2.1
    

    At the end of the tow, the outer side of the net is sprayed down with surface seawater to concentrate the animals in the collecting bucket.

  • 2.2.2
    

    As soon as possible after collection, the sample is split using a Folsom plankton splitter. Subsamples are taken for preservation and size-fractionationed biomass. Half of the tow is preserved in borate-buffered formaldehyde (0.5% final concentration), with strontium chloride (0.27 mM final concentration) added to aid in preservation of acantharians. The samples are stored in borosilicate-glass jars.

  • 2.2.3
    

    Generally ¹/₄ of the tow is size-fractioned through nested filters of the following mesh sizes: 5-mm, 2-mm, 1-mm, 500-µm, and 200-µm. Each fraction is concentrated onto a 47-mm 200-µm pre-weighed Nitex filter, rinsed with isotonic ammonium formate, placed in a labeled cryotube, and then frozen (liquid nitrogen or -85°C freezer).

• 3.1
  

  Frozen samples are stored at -85°C until processed. Then, they are defrosted at room temperature in the dark on a paper towel to blot excess moisture. Each sample (which represents a single size-fraction of the tow) is weighed wet on an analytical balance before (total fraction wet weight) and after subsamples of the zooplankton mass are set aside for gut pigment analysis and carbon/nitrogen biomass. The remaining sample is dried at 60°C, and then reweighed for determination of the fraction's mass (total sample mass is the sum of all fraction masses). The mass of the sample is normalized to the ocean surface area using the volume of seawater filtered through the net as recorded by the flow meter (= volume filtered) and the depth to which the net fished as recorded by the data logger (= depth).

• 3.2 Calculation of fraction dry weight:
  

       (1)   dwt1 = (wwt1 - fwt) - [(wwt1 - fwt) * %water]
  
       (2)   %water = [(wwt2 - fwt) - (dwt2 - fwt)] / (wwt2 - fwt)
      
       where:    
            dwt1 = fraction dry weight (mg)
  
            dwt2 = fraction dry weight (including filter weight)
                   after all subsamples removed (mg)
  
            wwt1 = fraction wet weight including filter weight (mg)
  
            wwt2 = fraction wet weight including filter weight after 
                   all subsamples removed (mg)
  
            fwt  = 47-mm 200-µm filter weight (mg)
  
            %water = water content of fraction (assume water content 
                     is the same for wwt1 and wwt2)
    

• 3.3 Calculation of fraction mass:
  

        (3)   mg  (dry wt.) m-2 = dwt1 * depth * (volume filtered)-1 * (fraction of tow)-1
  
        where:
             depth  =  towing depth from data logger pressure trace (m)
  
             volume filtered  = volume of seawater filtered through 
                                net from flow meter reading (m3)
  
             fraction of tow = fraction of tow concentrated in
                               each size-fraction (e.g., 1/2 or 1/4)
    

• 4.1
  

  Carbon and nitrogen biomass are determined using a CHN Elemental Analyzer (Perkin Elmer Model 2400) on subsamples which have been dried at 60 °C in pre-weighed combusted aluminum foil boats and then weighed on an analytical balance (to 5-places) (see Chapter 10, sections 4 - 8). The dry weight of the sample is the difference between the final balance weight (sample + boat weight) and the pre-weighed boat weight.

• 4.2 Calculation of carbon and nitrogen content of fraction:
  

       (4)  C (mg) m-2 = C * dwt1 * depth * (volume filtered) - 1 * (fraction of tow)-1
  
       (5)  N (mg) m-2 = N * dwt1 * depth * (volume filtered) - 1 * (fraction of tow)-1
  
       where:     
  
            C   = concentration of carbon (mg g-1)
  
            N   = concentration of nitrogen (mg g-1)
  
            dwt1   = fraction dry weight (g) (equation 1)
  
  	  depth  = towing depth from data logger pressure trace (m)
  
            volume filtered  =  volume of seawater filtered through
                                net from flow meter reading (m3)
  
            fraction of tow  =  fraction of tow concentrated in
                                each size-fraction
    

• net assembly (net frame, net, flow meter, data logger, bridle, towing line)
• seawater hose
• Folsom plankton splitter
• Nested filter set (200-, 500-, 1000-, 2000-, 5000-µm Nitex, each glued to the bottom of a plastic bowl)
• forceps (Millipore)
• filtered seawater (0.45 µm) in squirt bottles
• 47-mm filter apparatus (tower and base)
• 47-mm 200-µm pre-weighed Nitex filters
• Cryotubes and caps
• Isotonic ammonium formate (1 M)
• Liquid nitrogen
• Borate-buffered formaldehyde (10% Formalin, 79 mM Sodium tetraborate, pH 8.0)
• Strontium chloride (13.5 mM)
• Analytical balance (4- and 5-place)
• Spatula
• Ethanol (for rinsing)
• Filter pieces (small)
• Pre-weighed, combusted aluminum foil boats
• Petri dishes
• Oven (60°C)
• Filtration apparatus (25-mm base and 15-ml tower, tubes, vacuum)
• CHN Elemental Analyzer (see Chapter XX)

Temporal variation in mesozooplankton biomass during HOT year 16 (2004) is presented in Figure 64. Both zooplankton dry weight biomass (upper panel) and wet weight biomass (lower panel) are plotted. On average, zooplankton dry weight biomass was 12% of zooplankton wet weight biomass during the day and 13% during the night.

Nighttime dry weight zooplankton biomass during 2004 (1.36 g DW m^-2 ± 0.327 g DW m^-2) was approximately 1.63 times that of zooplankton collected during the day (0.887 g DW m^-2 ± 0.268 g DW m^-2). Wet weight biomass of zooplankton differed similarly during the night (10.4 g WW m^-2 ± 2.48 g WW m^-2) and day (7.43 g WW m^-2 ± 2.31 g WW m^-2), and night wet weights were 1.51 times day wet weights. The difference in biomass between zooplankton collected during the night and zooplankton collected during the day at Station ALOHA was significant for both dry and wet weights (Student's T-test, n=11, p<0.01), and was caused by the upward migration of deep-living zooplankton and micronekton after sunset.

Mesozooplankton dry weight biomass averages during HOT year 16 are slightly higher than averages for all eleven years of the zooplankton program (1994 - 2004: night = 1.10 g DW m^-2 ± 0.341 g DW m^-2; day = 0.705 g DW m^-2 ± 0.277 g DW m^-2). Nighttime dry weight biomass was significantly greater during year 16 (2004) than during 1994, 1995, 1996, 1997, 1998 and 1999 (Mann-Whitney U test, n≤12, p≤0.05). Nighttime biomass during 2000, 2001, 2002 and 2003 did not differ significantly from year 15 (2003). This pattern was similar for daytime zooplankton biomasses. Daytime dry weight biomass was significantly greater during year 16 (2004) than during 1994, 1995, 1996, 1997 and 1999 (Mann-Whitney U test, n≤12, p≤0.05), whereas daytime biomass during 1998, 2000, 2001, 2002 and 2003 did not differ significantly from year 16 (2004).

Al-Mutairi, H. and M.R. Landry. 2001. Active export of carbon and nitrogen at Station ALOHA by diel migrant zooplankton. Deep-Sea Res. II. 48: 2083- 2104.

Frost, B.W. 1984. Utilization of phytoplankton production in the surface layer. Pages 125-134 in Global Ocean Flux Study: Proceedings of a Workshop, September 10-14, 1984.

Hannides, C.C.S., M.R. Landry, C.R. Benitez-Nelson, R.M. Styles, J.P. Montoya and D.M. Karl. 2008. Export stoichiometry and migrant-mediated flux of phosphorus in the North Pacific Subtropical Gyre. Deep-Sea Res. I, 56: 73-88.

Kuba, D.M. 1970. Sampling midwater fish using the ten-foot Isaacs-Kidd midwater trawl and the Cobb pelagic trawl. MS Thesis, Univ. Hawaii, 35 pp.

Landry, M.R., H. Al-Mutairi, K.E. Selph, S. Christensen and S, Nunnery. 2001. Seasonal patterns of mesozooplankton abundance and biomass at Station ALOHA. Deep-Sea Res. II. 48: 2037-2062.

Longhurst, A.R. and W.G. Harrison. 1988. Vertical nitrogen flux from the oceanic photic zone by diel migrant zooplankton and nekton. Deep-Sea Res. 35: 881-890.

Longhurst, A.R., A. Bedo, W.G. Harrison, E.J.H. Head and D.D. Sameoto. 1990. Vertical flux of respiratory carbon by oceanic diel migrant biota. Deep- Sea Res. 37: 685-694.

Ortner, P.B., S.R. Cummings, R.P. Aftring and H.E. Edgerton. 1979. Silhouette photography of oceanic zooplankton. Nature 277: 50-51.

Paffenhfer, G.-A. and S.C. Knowles. 1979. Ecological implications of fecal pellet size, production and consumption. J. Mar. Res. 37: 35-49.

Sheridan, C.C. and M.R. Landry. 2004. A nine-year increasing trend in mesozooplankton biomass at the Hawaii Ocean Time-series Station ALOHA. ICES J. Mar. Sci. 61: 457-463.

Small, L.F., G.A. Knauer and M.D. Tuel. 1987. The role of sinking fecal pellets in stratified euphotic zones. Deep- Sea Res. 34: 1705-1712.

Strickland, J.D.H. and T.R. Parsons. 1972. A Practical Handbood of Seawater Analysis, Fisheries Research Board of Canada, 167 pp.