Dataset: HOT Zooplankton Community Structure

Principle
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. Field Operations
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 meter squared 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, Ottawa, Canada) was fastened to the net frame to measure depth of tow. From HOT cruise 175 to present, the collection procedure was simplified by switching to a 1 meter square diameter ring net, with GO 2030R flow meter and Vemco minilog Time-Depth Recorder. Both frames are fitted with 202 micron 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. As reported by Valencia et al. (2018), the two net systems were compared in a series of tows on the same cruise, revealing no significant differences in areal estimates of mesozooplankton biomass for either day or night tows (Mann-Whitney test, p > 0.05). They are therefore assumed to be equally efficient samplers in the time series. 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
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. 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.
Generally ¼ 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 -85ºC freezer).

3. Determination of Mass
3.1. Frozen samples are stored at -85ºC 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 60ºC, 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. Particulate C and N
4.1. Carbon and nitrogen biomass are determined using a CHN Elemental Analyzer (Perkin Elmer Model 2400) on subsamples which have been dried at 60ºC 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