Scholarly article on topic 'The distribution and vertical flux of fecal pellets from large zooplankton in Monterey bay and coastal California'

The distribution and vertical flux of fecal pellets from large zooplankton in Monterey bay and coastal California Academic research paper on "Biological sciences"

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Abstract of research paper on Biological sciences, author of scientific article — Michael J. Dagg, George A. Jackson, David M. Checkley

Abstract We sampled zooplankton and fecal pellets in the upper 200m of Monterey Bay and nearby coastal regions in California, USA. On several occasions, we observed high concentrations of large pellets that appeared to be produced during night-time by dielly migrating euphausiids. High concentrations of pellets were found in near-surface waters only when euphausiids co-occurred with high concentrations of large (>10μm) phytoplankton. Peak concentrations of pellets at mid-depth (100 or 150m) during the day were consistent with the calculated sinking speeds of pellets produced near the surface at night. At these high flux locations (HI group), pellet concentrations declined below mid-depth. In contrast, at locations where the phytoplankton assemblage was dominated by small phytoplankton cells (<10μm), pellet production and flux were low (LO group) whether or not euphausiid populations were high. Protozooplankton concentrations did not affect this pattern. We concluded that the day and night differences in pellet concentration and flux in the HI profiles were mostly due to sinking of dielly-pulsed inputs in the surface layer, and that small zooplankton (Oithona, Oncaea), heterotrophic dinoflagellates, and bacterial activity probably caused some pellet degradation or consumption below 100m. We estimated that consumption of sinking pellets by large copepods was insignificant. High fluxes of pellets were episodic because they required both high concentrations of large phytoplankton and large stocks of euphausiids. Under these conditions, flux events overwhelmed retention mechanisms, resulting in large exports of organic matter from the upper 200m.

Academic research paper on topic "The distribution and vertical flux of fecal pellets from large zooplankton in Monterey bay and coastal California"

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Deep-Sea Research I

journal homepage: www.elsevier.com/locate/dsri

- DEEP-SEA RESEARCH

Saw PARTI

The distribution and vertical flux of fecal pellets from large Zooplankton in Monterey bay and coastal California

Michael J. Dagg a *, George A. Jackson b, David M. Checkley Jr.c

a Louisiana Universities Marine Consortium, 8124 Highway 56, Chauvin, LA 70344, USA b Texas A&M University, College Station, TX 77843, USA

c Scripps Institution of Oceanography, University of California, La Jolla, San Diego, CA 92093, USA

CrossMark

ARTICLE INFO

Article history:

Received 27 January 2014

Received in revised form

26 August 2014

Accepted 3 September 2014

Available online 16 September 2014

Keywords: Vertical flux Euphausiid Fecal pellets Retention efficiency

ABSTRACT

We sampled zooplankton and fecal pellets in the upper 200 m of Monterey Bay and nearby coastal regions in California, USA. On several occasions, we observed high concentrations of large pellets that appeared to be produced during night-time by dielly migrating euphausiids. High concentrations of pellets were found in near-surface waters only when euphausiids co-occurred with high concentrations of large ( > 10 pm) phytoplankton. Peak concentrations of pellets at mid-depth (100 or 150 m) during the day were consistent with the calculated sinking speeds of pellets produced near the surface at night. At these high flux locations (HI group), pellet concentrations declined below mid-depth. In contrast, at locations where the phytoplankton assemblage was dominated by small phytoplankton cells ( < 10 pm), pellet production and flux were low (LO group) whether or not euphausiid populations were high. Protozooplankton concentrations did not affect this pattern. We concluded that the day and night differences in pellet concentration and flux in the HI profiles were mostly due to sinking of dielly-pulsed inputs in the surface layer, and that small zooplankton (Oithona, Oncaea), heterotrophic dinoflagellates, and bacterial activity probably caused some pellet degradation or consumption below 100 m. We estimated that consumption of sinking pellets by large copepods was insignificant. High fluxes of pellets were episodic because they required both high concentrations of large phytoplankton and large stocks of euphausiids. Under these conditions, flux events overwhelmed retention mechanisms, resulting in large exports of organic matter from the upper 200 m.

© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND

license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

1. Introduction

Fecal pellets from large zooplankton such as copepods and euphausiids can dominate the downward flux of organic matter in the ocean (e.g., Legendre and Michaud, 1998; Thibault et al., 1999; Roy et al., 2000; Vargas et al., 2002; Wexels Riser et al., 2008; Kobari et al. 2010; González et al., 2011 ). Many factors contribute to high variability in this flux component, including biological conditions such as the concentration, type and composition of food, the size and species composition of the zooplankton, and the predator to prey size ratio. Physical conditions such as stratification and mixing also affect fecal flux (Alldredge et al., 1987). Although much of the organic matter flux is derived from pellets, most pellets typically do not sink far (Wassmann, 1998) but are modified by microbial degradation (Hansen and Bech, 1996; Hansen et al. 1996) and zooplankton activities that include pellet

* Corresponding author. Tel.: +1 206 201 3884. E-mail addresses: mdagg@lumcon.edu (M.J. Dagg), gjackson@tamu.edu (G.A. Jackson), dcheckley@ucsd.edu (D.M. Checkley Jr.).

ingestion and destruction (Noji et al., 1991; Urban-Rich 1999; Poulsen and Iversen, 2008). Consequently, the dynamics of fecal pellet degradation significantly affect the dynamics of vertical flux (Turner, 2002).

Zooplankton can modify sinking pellets by direct consumption (González and Smetacek, 1994; Huskin et al., 2004) or by breaking up the pellets into smaller, slower sinking particles (Noji et al., 1991). Breakage also enhances microbial access to pellet contents (Lampitt et al., 1990). Small ubiquitous copepods such as Oithona spp. and Oncaea spp. have been proposed as important modifiers of pellet flux, e.g. the "Oithona filter" (González and Smetacek, 1994; Svensen and Nejstgaard, 2003). Also, protozooplankton can be important consumers of pellets (Poulsen and Iversen, 2008; Poulsen et al., 2011). Euphausiids, which are strong diel migrators, have been observed to feed at depth during the day (e.g. Hamame and Antezana, 2010) where they may consume fecal pellets and thereby reduce flux. Alternatively, large zooplankton, especially swarming kinds like euphausiids and salps, have high filtration rates and produce large, rapidly sinking pellets; they can overwhelm degradation and recycling processes near the surface, resulting in high flux events.

http://dx.doi.org/10.1016/j.dsr.2014.09.001

0967-0637/© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

In addition to fecal pellets, other materials such as phytoplankton, mucous and gelatinous remains can aggregate to form major components of the vertical flux of organic matter (e.g. Silver and Gowing 1991; Silver et al. 1998; Jackson and Checkley, 20H). Flux is often in the form of aggregates of particles from a variety of sources, including fecal pellets, and all these materials are commonly observed at various levels of decomposition and degradation.

We are attempting to measure the magnitude of zooplankton consumption of these sinking materials, a process we call "gatekeeper activity", in the region immediately below the euphotic zone. Our goals are to determine the zooplankton contribution to the overall retention efficiency of pellets and other sinking organic matter, and to understand the conditions under which flux-consumption by zooplankton is an important determinant of the flux profile (Petrik et al., 2013). In this paper, we present information on the size frequency distribution of large fecal pellets, operationally defined as those retained on a 60 ^m mesh, in the upper 200 m during a 14-day study in or near Monterey Bay, California. Our specific objectives here are to show the vertical distribution of large fecal pellets and to determine some of the factors controlling the production and losses of these large pellets in the upper 200 m.

2. Material and methods

2.1. Study area and sampling

We spent July 10-23, 2010 aboard the RV New Horizon in or just offshore to the west of Monterey Bay, CA (Fig. 1). Shipboard sampling was done near a free drifting SOLOPC float set to cycle through the upper 100 m at approximately hourly intervals. SOLOPC floats are fully described in Checkley et al. (2008), Jackson and Checkley (2011) and Petrik et al. (2013).

Aboard ship, whole water samples were collected for meso-zooplankton fecal pellets using a simple method modified from that described in Wassmann et al. (1999) and Wexels Riser et al. (2001). For each collection, a CTD-rosette sampler fit with twelve 10-L Niskin bottles was used to collect water from six depths (Fig. 1, Table 1). In several cases, day and night samples at the same location were collected within a 24 h period (CTD 14/15, 25/26, 31/ 33 and 37/39). On deck, contents of Niskin bottles were gently drained into 10-L polycarbonate carboys. In the ship's laboratory, the entire content of each carboy, approximately 9.2 L, was filtered through a 60-^m mesh sieve. This mesh size was selected because smaller ones sometimes resulted in retention of too many contaminating particles for practical analysis of samples. Retained particles were backwashed into a small container and preserved in 5% Formalin-seawater solution. Final sample volume was between 150 and 250 ml. Preserved samples were kept in the dark and returned to a shore-based laboratory for analysis. Particles passing through the 60-^m sieve were not analyzed.

Table 1

Information for fecal pellet CTD casts.

CTD Date Time Latitude (oN) Longitude (oW)

14 13-Jul 01:08 36.74 122.01

15 13-Jul 12:02 36.74 122.01

25 15-Jul 02:02 37.03 123.37

26 15-Jul 15:00 37.03 123.37

31 17-Jul 14:05 36.77 122.08

33 17-Jul 23:06 36.74 122.05

37 19-Jul 14:47 36.72 122.00

39 20-Jul 01:26 36.74 122.04

42 21-Jul 14:16 36.68 122.03

46 23-Jul 02:22 36.44 123.33

48 23-Jul 15:33 36.43 122.21

so.o N I

47.5 N I

Washington

O 25 26 cast locations~\^ ^ \\

046 048 \

-123.25 -123 -122.75 -122.5 -122.25 -122 Longitude

42.5 N

40.0 N I

-121.75

35.0 N

45.0 N J I / Idaho

Oregon

32.5 N I

125.0° W 122.5° W 120.0° W 117.5°W 115.0°W

Fig. 1. The study area, showing the sampling locations for zooplankton fecal pellets. Depth contours in m. Open symbols^LO group, plus symbols=HI group.

Immediately before or after each pellet cast, additional CTD-rosette casts were made for two sets of analyses. (1) Two size fractions of chlorophyll (> 10 pm, referred to as "large" phytoplankton and < 10 pm, referred to as "small" phytoplankton) were determined on board using a cascade filtration system fit with a 10 pm polycarbonate filter and a GF/F glass fiber filter. (2) 100 ml samples from each depth were preserved in 5% acid Lugol's solution for later determination of microzooplankton concentrations.

Zooplankton were sampled using a 1-m2 MOCNESS with nine 202-pm-mesh nets, deployed 13 times between about 100 m and the surface. Eleven of these MOCNESS deployments were coordinated with our CTD-rosette sampling. Zooplankton were preserved in 5% Formalin in seawater. The last two nets of some MOCNESS deployments were used to collect animals for gut pigment analysis. Aliquots were immediately concentrated on 34 mm diameter, 20 pm-mesh NITEX discs and frozen in liquid nitrogen.

2.2. Phytoplankton, zooplankton and gut pigment analyses

On board ship, chlorophyll filters were extracted in 90% acetone for 24 h in the dark and the extract was measured with a Turner Designs fluorometer before and after acidification to determine chlorophyll a and phaeopigment concentrations (Strickland and Parsons, 1968).

In a shore based laboratory, 10 ml subsamples of microzooplankton were settled for 24 h (Utermohl, 1958) before counting. Microplankton cells including diatoms, dinoflagellates, aloricate ciliates, Mesodinium rubrum and tintinnids, were counted with an inverted microscope (Olympus-IX 51) at 200 x magnification. In most subsamples, between 100 and 500 individuals were counted but deep water at some stations contained < 100 cells in the 10 ml aliquot. Cell size of each unique cell type was measured using an eyepiece micrometer to calculate biovolume, then converted to biomass. Microplankton were assumed to be standard geometric forms (Hillebrand et al., 1999). Cell biomass was calculated using a volume-to-carbon conversion factor of 0.19 pg C pm3 for oligo-trich ciliates, M. rubrum (Putt and Stoecker, 1989) and dinoflagel-lates (Howell-Kubler et al., 1996). For tintinnids, C (pg cell_1) was determined from the equation 444.5+0.053 lorica volume (pm~3) (Verity and Langdon, 1984). Diatom biomass was calculated according to the formula of Menden-Deuer and Lessard (2000).

Ashore in the laboratory, each zooplankton sample was sub-sampled (split) by half multiple times. Entire subsamples were analyzed so that least 30 zooplankters > 1 mm equivalent spherical diameter (ESD) of two taxonomic groups, large copepods and euphausiids, were identified, enumerated and measured (width and length, of the cephalothorax for copepods and overall for euphausiids) and remaining individuals of each taxon identified and enumerated. Identification was to species for large copepods and, for euphausiids, always to family and occasionally to species. Abundance (number m~3) and volume (cm3 m_3) of these plankton types were calculated from abundance and size of individuals in a subsample, subsample fraction and the volume filtered, based on measurements using a calibrated flow meter (Checkley, unpublished).

In a shore based laboratory, individual euphausiids were sorted and analyzed for gut chlorophyll and phaeopigment contents, using well established methods (e.g. Dagg 1993; Dagg et al., 1997). Duplicate subsamples from each net were analyzed.

2.3. Fecal pellet analysis

In the laboratory, each sample was examined with a dissecting microscope. All fecal pellets were removed with a fine-bore pipette and transferred to a 34-mm diameter plastic petri dish. Contaminating particles such as small zooplankton, nauplii, eggs,

and non-pellet detritus were removed from each petri dish by pipette and the remaining particles, almost entirely fecal pellets, were photographed.

Each 34-mm petri dish containing pellets was uniformly back-lit by an LED light source (Advanced Illumination Model No BL0404-WHIIC) and photographed with a Canon EOS 40D camera fit with a EFS 60 mm macro lens. The light source was a white LED with a continuous output spectrum but heavily weighted in the blue wavelengths. The camera has a red, a green and a blue sensor, resulting in three images. We used the blue images for our analyses. During photography, the focal distance was the same for all samples and set so that the entire petri dish was included in a single image. The resulting images were saved as 3888 x 2592 pixel images in JPEG format. Each pixel was 14.7 x 14.7 pm. As an example, some pellets from CTD 31-100 m are shown in Fig. 2.

The Matlab image analysis function regionprops was used to locate and size particles in the images. The function determined the length of the major axis and the area for each object; pellet length L was estimated as the length of the major axis and the pellet diameter d was estimated by dividing the object area by L. Volume V was calculated assuming each fecal pellet was a cylinder, V = 0.25nd2L. For each particle, an equivalent spherical diameter (desd) was calculated for the volume-equivalent sphere:

desd = 6V 1=3

We also visually examined the images of the 10 largest particles in each sample because large particles contribute disproportionately to flux. In samples with high concentrations of pellets, the imaging program sometimes artificially combined several pellets that were in close proximity or overlapped slightly, and treated them as a single, extremely large, pellet. These were removed from further analysis. In addition, a few long thread-like contaminating particles present in some samples were incorrectly categorized by the software as large pellets. These were also removed from further analysis.

The normalized volume distribution, nVdesd, was determined after calculating the number distribution n as in Jackson and Checkley (2011) using volume-doubling size ranges. Units of nVdesd in this paper are parts per billion (1 ppb = 10_9cm3cm_3 = 1 mm3 m~3).

Fig. 2. Image of some fecal pellets from the 100 m sample, CTD 31.

Settling speed, vs (m d for each pellet was calculated using the relationship of Komar et al. (1981) for L and d expressed in cm

Vs = 1.21 x 103L2 (L/d)-1664

J3 0.4 o

euphausiids copepods

0.0 -Jfl""08 ooft;Ocmxoc^0^o0ai)oc^oOQoOgSg_ 0 10 20 30 40

MOCNESS net sample

Fig. 3. Biovolume of large copepods (open circles) and euphausiids (filled circles) in all MOCNESS samples examined for this study.

The carbon content of fecal pellets was calculated from pellet volume assuming a carbon to volume ratio of 55 ^g C mm derived from direct measurements made by González et al. 1994; Daly, 1997; Urban-Rich et al., 1998; and Riebesell et al., 1995. Their measurements ranged from 38 to 69 ^g C mm_3.

Fecal pellet flux (mgCmd~1) was calculated by summing the product of carbon content and settling velocity for each particle in a sample and dividing by sample volume. The 95% confidence limits of the total carbon content and the flux estimates were calculated using a bootstrap analysis performed with the Matlab bootci function with the bias corrected and accelerated method options.

There are several uncertainties associated with our determination of pellet volume concentration and flux: (a) Pellets may be broken up during collection and sieving, resulting in apparent sizes that are smaller than in situ sizes. This would not effect the volume concentrations but would lead to underestimation of settling velocity and flux. (b) Image analysis can lead to incorrect characterization of particles. We surveyed each image to remove misidentifications of several pellets as a single large pellet. This reduced the total volume concentrations by small amount. Other possible errors with image analysis, such as misinterpreting a broken or bent pellet as a larger sized pellet, were not addressed but visual examination of the images did not show many such particles. (c) The carbon:volume ratio of pellets varies with food type and density, whereas we applied a single

Fig. 4. The size frequency distribution of fecal pellets from CTD cast 42. X—length in pixels and d—diameter in pixels. A pixel is 14.7 ^m. Z — pellet abundance (number m 3). All six depths from this CTD cast are shown: 5, 25, 50,100,150, and 200 m.

— 100-■Q CX CX

■то >

c 50 -|

— 100-.o

c 50 -I

Cast 42 5 m

Cast 42 50 m

Cast 42 150 m

desd(cm)

Cast 42 25 m

Cast 42 100 m

Cast 42 200 m

desd(cm)

Fig. 5. The normalized volume distribution (nVd) spectra for fecal pellets from CTD cast 42.

literature-derived ratio to all our samples. (d) Settling speed varies with water temperature (viscosity). The equation we used, from Komar et al. (1981), was derived for a temperature of 13 °C whereas temperatures in our study ranged from about 14 °C at the surface to 8 °C at 200 m. Most of the in situ sinking velocities were therefore slightly lower than we calculated. This error is small relative to other uncertainties. Overall, most of our errors should be systematic because our sampling conditions, measurements and analytical protocols were applied consistently. As a result, the greatest uncertainties in our study should be with the specific values for volume concentration and flux rather than with the patterns we observe.

3. Results

3.1. Mesozooplankton

Large zooplankton included Calanus pacificus C5, C6F and C6M, Metridia pacifica C6F, Eucalanus californicus C5, C6F and C6M, and

several other less abundant copepods. However, these large copepods contributed little to the total zooplankton biomass whenever euphausiids were present. All samples with a high biovolume of large zooplankton were dominated by euphausiids (Fig. 3), primarily Euphausia pacifica.

3.2. Sample pellet profile

The abundance and size distribution of fecal pellets in a sample CTD cast, CTD 42, are shown in Fig. 4. Pellet abundance and size changed with depth. There were few pellets at 5 m but abundance increased with depth to 100 m and then declined somewhat at 150 and 200 m. Pellet length and diameter also peaked at 100 m. At 50 m and shallower, most pellets were 5 pixels (74 pm) in diameter or smaller; deeper they were as wide as 10 pixels (147 pm). In the 150 and 200 m samples, pellets greater than 30 pixels (441 pm) long were mostly absent, indicating large pellets decreased below 100 m in this profile.

Fecal C flux (g-C m 2 d 1)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Pellet volume concentration (ppb)

Fig. 6. The total volume (ppb) and vertical flux (mg C m"2 d"1) of pellets at each sampled depth in CTD cast 42.

The size distribution and abundance patterns from CTD 42 (Fig. 4) are more clearly seen when the abundance and size data are presented as nVd distributions (Fig. 5). Distribution patterns were fairly uniform at all depths and the peak at all depths was at an equivalent spherical diameter (desd) between 167 and 210 ^m, indicating the size distributions of pellets were similar but not identical at all depths. In this case, there was a slight maximum desd (210 ^m) at 100 m, confirming the visual impression from Fig. 4 that the abundance of larger pellets was greatest at 100 m and then decreased slightly in deeper samples. The area under each nVd curve, representing the integrated volume concentration of pellets, varied widely with depth (Fig. 6). The minimum concentration was in the 5 m sample (~ 10 ppb) and the maximum concentration was in the 100 m sample (~ 100 ppb) (Fig. 6). The vertical pattern of pellet flux closely tracked the profile of pellet concentration in Cast 42 (Fig. 6) because size composition of pellets did not vary much with depth. In summary, total pellet concentration and flux increased from the near-surface to 100 m and decreased below 100 m in this profile.

3.3. Pellet profiles

Peak diameter, total volume concentration, and total flux were used to characterize all 11 pellet profiles (Table 2). There was no consistent pattern of changing pellet size with depth among pellet profiles, as indicated by the diameter at maximum volume concentration (nVd) (Table 2). Thus, flux was mostly determined by the volume concentration, and the depth profiles of fecal pellet flux were similar to the profiles of pellet volume concentration for each cast (Fig. 7).

Although pellet volume concentration and flux varied greatly among the eleven pellet casts, profiles fell into a group of low volume/flux casts (Group LO—Casts 14, 15, 25, 26 and 46) and a group of high volume/flux casts (Group HI—Casts 31, 33, 37, 39, 42 and 48) (Fig. 7, Table 2). In the LO group, concentration was always < 20 ppb and was approximately uniform with depth (Fig. 7(a)). In contrast, in the HI group concentration was typically > 50 ppb and these profiles had substantial vertical structure (Fig. 7(b)). In both LO and HI groups, vertical distribution of volume concentration was reflected in flux of pellet material (Fig. 7(c) and (d)). The estimated carbon flux from fecal pellets ranged from ~ 0.04 to ~0.15 gCm"2 d"1 in the LO group and ~0.05 to ~5.0 gC m"2 d-1 in the HI group (Table 2).

Average sinking velocities estimated from pellet size are shown in Fig. 8. The range was about the same in both the LO and HI groups, approximately 50-250 m d-1, again indicating that flux was mostly determined by the concentration of pellets.

Some of the factors causing the pellet profiles to sort into HI and LO categories were identified by comparing pellet volume or flux with the concentration and size composition of phytoplank-ton, and the distribution and abundance of large zooplankton.

3.4. HI group

CTD casts 31 and 33 (HI group) were a day (31) and night (33) pair from inside Monterey Bay. Both had high pellet volume concentrations (Fig. 7(b)). Pellet concentration was higher in the surface at night than in the day and there was a large mid-depth (100 m) peak in pellet concentration during the day (Fig. 7(b)). Estimated sinking speeds for pellets in the upper 50 m at night were about 120-150 m d"1, equivalent to about 60-75 m in 12 h. These speeds are consistent with the observed profiles, suggesting that pellets produced in the near-surface strata at night were sinking to mid-depths by mid-day. MOCNESS deployments 7 and 8 were associated with CTD 31 and 33, and show euphausiid concentrations at night were much higher than during the day, with a nighttime maximum in the surface stratum (2-41 m) (Fig. 9(a)). Daytime euphausiid biovolume was low at all sampled depths (Fig. 9(a)). Chlorophyll concentrations, measured from CTD casts made immediately after the pellet CTD casts, were between 2 and 3 ^g l-1, and the phytoplankton assemblage was dominated by large cells, with highest concentrations at~ 20 m (Fig. 9(b)). A similar pattern of pellet concentrations was observed in another day (CTD 37) /night (CTD 39) pair of pellet profiles from the HI group (Fig. 7(b)). Here also, there was a strong diel migration of euphausiids (Fig. 9(c)), the phytoplankton assemblage was dominated by large cells (Fig. 9(d)), and the observed daytime peak in pellet concentration at mid-depth is consistent with the estimated sinking speeds of these pellets. Based on these two day-night series, we conclude that high concentrations of large phytoplank-ton cells and large stocks of euphausiids at night led to high production of pellets near the surface at night, and that pellets sinking from the surface were observed as a daytime peak in pellet concentration at mid-depth s of 100-150 m.

This day-night pattern of euphausiid and fecal pellet distributions is further but weakly supported by another pellet profile from within Monterey Bay, Cast 42. Pellet concentration in the daytime was low at the surface and peaked at mid-depths (Fig. 7(b)). Chlorophyll concentration was high at night and dominated by large cells although daytime chlorophyll concentration was lower and large cells were less abundant (Fig. 9(f)). Euphausiid biovolume during the day was low in all six sampled layers (Fig. 9(e)). We do not have an associated night profile of pellets or zooplankton. Nevertheless, this incomplete set of data is consistent with characteristics of the HI group.

3.5. LO group

The LO group did not show large peaks in the vertical profiles of pellet concentration (Fig. 7(a)). For example, CTD casts 14 and 15 were a night (14) and day (15) pair from within Monterey Bay. Both had low pellet volumes ( < 20 ppb) at all depths. Euphausiids were abundant at this location (MOCNESS 1 and 2), especially at night (Fig. 10(a)). The maximum biomass of euphausiids at night was in the 16-30 m stratum, although there was also a layer of euphausiids in the 17-30 m daytime sample. Chlorophyll concentrations were moderately high, 1-2 ^g l-1 (Fig. 10(b)), but large cells were not abundant. Similar patterns were observed in a pair of pellet profiles in the LO group from offshore, CTD 25 (night) and

Fecal pellet characteristics at each CTD cast: diameter at the maximum in volume concentration (D at max nVd), volume concentration of all pellets (ppb), and flux of all pellets at each sample depth (mg C m~2 d-1). (D) and (N) in CTD numbers signify Day and Night.

Profile type CTD Depth (m) D (^m) at max nVd Volume (ppb) 95% CI Flux(mgCm 2d 1) 95% CI

HI 31(D) 5 132 10 8-12 47 34-67

25 167 46 39-55 417 324-566

50 210 214 196-235 2330 1976-2827

100 210 305 276-340 4780 3836-6039

150 210 132 118-150 1352 1113-1763

200 265 23 18-30 161 113-242

HI 33(N) 5 167 60 52-70 460 356-673

25 167 45 38-56 412 310-661

50 265 79 67-92 894 710-1145

100 167 97 88-109 733 608-1136

150 167 125 113-138 1084 914-1314

200 167 47 41 -54 310 252-394

HI 37(D) 5 420 15 11-27 178 76-622

25 210 26 20-37 299 181-539

50 210 146 132-163 1387 1140-1791

100 132 170 156-189 1289 1070-1781

150 132 145 134-164 1043 796-1983

200 132 60 55-67 342 289-418

HI 39(N) 5 167 126 115-137 830 713-983

25 167 176 164-189 1151 1007-1347

50 167 21 16-30 233 119-580

100 167 77 69-88 646 529-811

150 132 85 76-94 600 504-760

200 167 42 38-48 224 188-305

HI 42(D) 5 167 10 8-14 61 39-95

25 167 29 25-34 162 127-230

50 167 44 38-53 333 245-492

100 210 96 87-107 731 630-867

150 167 56 49-65 385 303-590

200 167 34 30-42 238 165-476

HI 48(D) 25 132 49 43-57 320 246-431

50 132 12 10-14 49 36-73

100 132 13 11-16 55 39-86

150 167 5 4-7 18 12-29

LO 14(N) 5 265 7 4-13 67 22-191

20 132 16 12-24 143 72-296

40 167 7 6-10 31 20-51

60 132 7 6-9 31 22-45

170 132 18 15-21 74 58-97

200 132 6 5-8 21 14-40

LO 15(D) 5 265 4 3-8 30 9 -127

20 105 16 14-19 57 45-75

40 132 19 16-24 109 79-167

60 210 13 11-17 77 53-115

120 132 18 15-21 83 64-109

200 105 16 14-18 55 45-67

LO 25(N) 25 167 13 10-18 96 61-168

50 210 17 13-23 148 102-243

100 167 12 8-19 159 84-363

LO 46 (N) 5 210 3 2-4 9 5-19

25 334 5 2-12 54 19-198

50 132 4 3-6 17 11 -27

100 265 5 3-10 38 18-96

200 210 7 5-10 49 25-95

CTD 26 (day), although pellet data were limited (Figs. 7(a), 10(c) and 10(d)). Even though there were high stocks of euphausiids in the surface layers at night, pellet concentrations were low, probably because there were low concentrations of large phytoplankton cells. We conclude that these locations provided a poor feeding environment for euphausiids and, although dielly migrating, they produced few pellets, resulting in low pellet concentrations.

Low pellet concentrations and fluxes were also observed at CTD 46 (night), another offshore CTD cast (Fig. 7(a)). Euphausiid biovolume was high at night and low during the day, indicating diel vertical migration (Fig. 10(e)). Chlorophyll concentrations

were low and the phytoplankton assemblage was dominated by small cells (Fig. 10(f)). These were poor conditions for pellet production and, in spite of a large population of euphausiids, pellet concentrations and fluxes were low, consistent with the pattern seen in the other LO profiles.

The remaining pellet profile, from CTD 48 (day), did not fall clearly into either the HI or LO group but had some HI characteristics in that pellet volume and flux were high at one depth (25 m) (Fig. 7(b)). In common with other members of the HI group, chlorophyll concentrations were high and dominated by large cells (data not shown). However, in common with the LO group,

LO casts

HI casts

0 20406080100120140160180200

Pellet vol. conc. (ppb)

0 100 200

Pellet vol. conc. (ppb)

£ 100 -

о 120 -

-©-15 -□- 25

-X- 46

0.1 0.2 0.3 0.4 Fecal C flux (g-C m-2 d-1)

20406080-

120140160 180-1 200

_ 31 33 ■37 39 42 48

Fecal C flux (g-C m-2 d-1)

Fig. 7. (a) and (b) Concentration (ppb by volume) and (c)(d) flux (g C m 2 d 1) of pellets, at each depth for LO and HI casts. Note 10 x difference in scales for LO and HI casts. Photographs of the few pellets from some of the samples from CTD 25 and all of samples from CTD 26 were inadequate for processing.

average sinking rates of these pellets were low, the lowest observed during the cruise (Fig. 8), and there was no daytime peak in pellet concentration at mid-depths (Fig. 7(b)). Daytime euphausiid concentrations were low, the maximum was only 0.004 cm3 m_ 3. The potential for euphausiid pellet production at night cannot be ascertained because there are no companion nighttime zooplankton data. Thus, the supporting data are insufficient to provide a clearer categorization and this case falls between the HI and LO sites.

3.6. Gut pigments

Characteristics of the phytoplankton assemblage in the HI and LO groups are reflected in the gut pigment concentrations of euphausiids (Euphausia pacifica and Thysanoessa spinifera). For example, levels of gut phaeopigment were low in euphausiids from a LO location, ranging from near 0 to about 100 ng phaeopigment individual_ 1 in the surface layer (Fig. 11(a)), and from 0 to about 15 ng phaeopigment individual _ 1 in the intermediate depth stratum (Fig. 11(b)). In contrast, gut pigment levels in euphausiids collected a HI location

were at least 10 times higher, ranging between about 150 and 1500 ng phaeopigment individual"1 in the surface stratum (Fig. 11(c)) and about 40 and 400 ng phaeopigment individual"1 in the subsurface stratum (Fig. 11(d)). The strong relationship between gut pigment level and body size observed at night (Fig. 11(c)) may indicate that conditions at this location supported maximum feeding and pellet production rates. These observations support the idea that the high concentrations of large phytoplankton in HI locations result in high feeding and pellet production rates, compared to the much lower rates of feeding and pellet production in the LO locations, where large phytoplankton cells were rare.

4. Discussion

Fecal pellets large enough to contribute significantly to the vertical flux of particulate organic matter in the ocean are derived from several groups of zooplankton. Large, wafer-shaped pellets are generally produced by salps and doliolids, elliptical pellets by appendicularians, cylindrical pellets by copepods, and long strands

LO casts

Ave. velocity(m d 1)

Fig. 8. Calculated average sinking velocity (md_1)

by euphausiids (Caron et al., 1989; González, 1992; González et al. 1994; Yoon et al., 2001; Wexels Riser et al., 2007). Euphausiid feces are sometimes referred to as 'strands' or 'strings' (e.g. González et al., 2011) because they are not completely enclosed in membranes but we use 'pellets' here for simplicity. These broad categorizations of shape are not absolute because pellet characteristics depend on several additional factors such as food type and food concentration (Dagg and Walser 1986; Tsuda and Nemoto, 1990; Turner, 2002). Pellets in our samples were cylindrical (Fig. 2) and mostly ranged in volume from about 0.6 x 106 pm3 to 12.6 x 106 pm3. In comparison, large copepods in the size ranges we observed in our study produce pellets between about 0.3 and 1.5 x 106 pm3 (Dagg and Walser, 1986). The size and shape characteristics of pellets in our study suggest they were derived almost entirely from euphausiids. Consistent with this, we found that, in night-time samples, euphausiids were typically about as numerically abundant as large copepods and heavily dominated the zooplankton biomass whenever there was a large biomass of net-caught zooplankton.

During our study, euphausiids dielly migrated to the surface layers at night, a commonly observed behavior (e.g. Stuart and Pillar, 1990; Bollens et al. 1992; Nakagawa et al., 2003; Antezana, 2010; Liu and Sun, 2010). Comparison of our day and night MOCNESS samples indicated that almost all of the euphausiid population was below our deepest sampling depth of about 100 m during the day, and highly concentrated near the surface at night. Although diel vertical migration is primarily a predator avoidance behavior, it has been shown that the pattern can be modified somewhat by the food environment (e.g. Dagg, 1985; Dagg et al., 1997). In our study, we observed two occasions when some of the euphausiid population remained near the surface during the day and both occurred under poor feeding conditions.

A large population of euphausiids in near-surface waters at night was not sufficient to result in high pellet production. In our study, phytoplankton size structure was the most important characteristic distinguishing high-flux (HI) from low-flux (LO) locations. When the surface phytoplankton assemblage was

HI casts

Ave. velocity(m d 1)

fecal pellets from all casts and sampling depths.

dominated by small cells and had few large cells, pellet concentrations and fluxes were low, regardless of euphausiid concentrations. These euphausiids are inefficient feeders on small cells, and also the concentrations of small cells were comparatively low which would result in low ingestion rates and thus low production rates of fecal pellets. High pellet production and flux were only observed when there was a large population of dielly migrating euphausiids and a food environment with high concentrations of large phytoplankton. This was the case for D/N CTD casts 31/33, where euphausiid stocks were high at night and the phytoplank-ton assemblage was dominated by large cells. The resulting pellet flux was high, reaching almost 5000 mg C m_2d~1 at 100 m. In contrast, N/D CTD casts 14/15 had large euphausiid stocks near the surface at night but pellet flux was low, < 200 mg C m_ 2d_1, because the phytoplankton assemblage was dominated by small cells. At this location, there were even euphausiids remaining near the surface during the day but pellet production and flux remained low in that poor feeding environment. To summarize, high rates of pellet production occurred only at night when euphausiids and large phytoplankton cells co-occurred, resulting in a nighttime pulse of near-surface pellet production, measured by high concentrations of pellets in near-surface layers. These pellets, sinking at speeds of 50-250 md~1 (25-125 m half-day_ 1), resulted in mid-depth peaks (usually in the 100 m sample) in pellet concentrations in the daytime. Diel pulses of zooplankton pellet production and flux have been observed in several other studies (e.g Lorenzen and Welschmeyer, 1983; Welschmeyer et al., 1984; Bathmann et al., 1991; González et al., 1994). The episodic nature of krill pellet fluxes has also been noted in the Southern Ocean (Le Fevre et al., 1998).

We assume that almost all pellet production occurred within the depth layer of feeding. Pellet production likely began shortly after the upward migration and onset of feeding by euphausiids because the passage of food through their guts would take only approximately 20-60 min at our ambient near-surface temperatures (~ 12-14 °C) (Dam and Peterson, 1988; Hamame and

2040 6080100* 120

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20 40 60 80 100

—a— D

0 40 80 120160 <Ü

N > 10 |m N < 10 |m D > 10 |m

0.0 0.5 1.0 1.5 2.0 2.5

0 40 80 120 160

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0.00 0.15 0.30 0.45 0.60 0.75 0.90

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0.00 0.05 0.10 0.15 0.20 0.25 0.30 Euphausiid biovolume (cm3 m-3)

N > 10 |m N < 10 |im D > 10 |m D < 10 |im

0.5 1.0 1.5 2.0 2.5 3.0 Chlorophyll concentration (|ig L-1)

Fig. 9. Zooplankton (a) and phytoplankton (b) associated with HI casts 31/33: (a) euphausiid biovolume during the day (open circles) and night (filled circles) and (b) phytoplankton concentrations (solid symbols represent cells > 10 ^m and open symbols represent cells < 10 ^m); (c) and (d) are the same as (a) and (b) but for HI casts 37/39; (e) and (f) are the same as (a) and (b) but for HI cast 42. For all these casts, surface temperatures were approximately 14 °C, temperatures at the top of the thermocline (40-60 m) were about 10 °C and temperatures at 200 m were about 8 °C. Surface salinities were about 33.6 and were similar or slightly higher, 33.7, at the top of the halocline (40-60 m), and about 34 at 200 m.

Antezana, 2010). Numerous pellets would be produced by each individual euphausiid during the dark hours between sunset to sunrise, about 10 h. Upon the onset of morning descent and cessation of feeding on surface phytoplankton, pellet production by gut evacuation may continue for a short while but this would contribute little compared to the total nighttime production.

Other food sources such as protozooplankton and detritus may have contributed to fecal pellet production in our study because euphausiids in Monterey Bay and nearby offshore regions are primarily Euphausia pacifica, a species known to be omnivorous (Ohman, 1984; Nakagawa et al., 2002). Protozooplankton abundance, size and species composition were similar between HI and LO sites (Fig. 12) suggesting that protozooplankton would comprise a larger fraction of the diet under LO conditions where the phytoplankton component of the diet was small, than under HI conditions where the phytoplankton component of the diet was

large. However, these conditions, which presumably would result in a large protozoan component in the diet at LO sites, did not result in high fecal pellet concentrations. Pellet concentrations were only high when large phytoplankton dominated the parti-culate environment.

The apparent declines in pellet concentration below 100 m under HI conditions could be simply due to sinking of a pulsed input. Falling at speeds of 100-250 md_1, pellets produced near the surface over 10 h by nocturnally feeding euphausiids would form a sinking layer 40-100 m thick. Under HI conditions in our study, a pulse of pellets produced near the surface at night by dielly migrating euphausiids was observed at about 100 m after half a day, consistent with estimated sinking speeds. If sinking continued at this rate, most of these pellets would be below our deepest sampling depth after 24 h, and we would not see them. The nightly pulsed nature of pellet production combined with

20 f. 40 -60 : 80 100

0 40 80 120 160

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0.00 0.05 0.10 0.15 0.20 0.25 0.30

0 20 40 60 80 100 120

0.00 0.05 0.10 0.15 0.20 0.25 Euphausiid biovolume (cm3 m-3)

N > 10 |m N < 10 |m

0.0 0.5 1.0 1.5 2.0

Chlorophyll concentration (|ig L-1)

Fig. 10. Zooplankton (a) and phytoplankton (b) associated with LO casts 14/15: (a) euphausiid biovolume during the day (open circles) and night (closed circles) and (b) phytoplankton concentrations (solid symbols represent cells >10 цт and open symbols represent cells < 10 цт); (c) and (d) are the same as (a) and (b) but for LO casts 25/26; (e) and (f) are the same as (a) and (b) but for LO cast 46. Surface temperatures for casts shown in panel (b) were about 13 °C, temperatures at the top of the thermocline (25-30 m) were about 10 °C and at 200 m were about 9 °C. Surface salinities were about 33.5, increasing to about 34 at 200 m, with no halocline. Surface temperatures for casts in panel (d) were about 14 °C, temperatures at the top of the thermocline (40-45 m) were between 9-12 °C, and at 200 m were about 8 °C. Salinities between the surface and the halocline (about 45 m) were about 33.4, increasing to about 34.1 at 200 m. For casts in panel (f), temperatures between the surface and the top of the thermocline (about 35 m) were about 14 °C and declined to about 7.5 °C at 200 m. Salinities between the surface and the top of the halocline about 35 m) were about 33.5 and increased to about 33.9 at 200 m.

rapid sinking of pellets could explain most or all the observed vertical pattern in pellet concentration and flux. Nevertheless, we considered the possibility that some pellet consumption or degradation may have occurred during sinking.

We did not observe strong evidence for pellet degradation or loss over the depth range of our study, the upper 200 m. Generally, the proportion of pellet production that is degraded or lost during sinking is highly variable and is affected by pellet size, pellet content, sinking speed (i.e. time to pass through the upper layers), temperature, stocks and types of heterotrophs that may disrupt or consume pellets, and other factors (Legendre and Michaud, 1998; Urban-Rich et al., 1999; Viitasalo et al., 1999; Buesseler et al., 2007). One possible indicator of pellet destruction during sinking is a reduction in pellet size with depth but we did not observe any consistent decrease in pellet size in either HO or LO profiles. The volume concentration of pellets in LO profiles did not decrease with depth, indicating no degradation of pellets took

place during sinking. However, volume concentration typically decreased below mid-depths in the HI profiles. This may indicate degradation or consumption, and it is worth considering large copepods, small copepods, protozooplankton, and bacteria as possible contributors.

Large copepods did not appear to have an impact on pellet flux. Average concentrations of all large copepods below 50 m at HI locations were < 20 ind m~3 (Checkley unpublished). Assuming a generous clearance rate of 10 ml h_ 1 for each individual results in an estimated community clearance of <200mlh~1m~ 3=4.8 x 10_3 d_1 for all large copepods combined. Applying this small impact to a pulse of pellets sinking at 100-250 m d_ 1 (~ 4-10 m h_1) results in a removal of < 1 % of the sinking pellets over the estimated 12 h it would take for these pellets to sink from 100 to 200 m. From this rough estimate, it appears that large copepods were simply not abundant enough to be important consumers of sinking pellets during our study.

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Fig. 11. Gut phaeopigments (ng individual 1) in mixed Euphausia pacifica and Thysanoessa spinifera from MOCNESS hauls associated with LO CTD casts 14/15 (a is the upper layer) (b is the subsurface layer), and HI CTD casts 37/39 (c is the upper layer) (d is the subsurface layer). N=night cast, D=day cast.

Small zooplankton, Oithona and Oncaea, have sometimes been considered important consumers of sinking pellets (González and Smetacek, 1994; Svensen and Nejstgaard, 2003), although more recent studies have not supported this role for the ubiquitous Oithona species (Reigstad et al., 2005; Iversen and Poulsen, 2007). In our study, these copepods combined, analyzed from six of our MOCNESS hauls, had a maximum concentration of only 515 m- 3 in a sample near the surface. Concentrations at mid-depths were < 100 m-3. These concentrations are consistent with those from an extensive survey in Monterey Bay spanning more than two years (Hopcroft et al., 2002) that showed Oithona similis concentrations ranged between about 100 and 1000 m- 3 and Oncaea spp. concentrations between about 1 and 100 m-3. It is likely that both data sets underestimate the abundance of these small zooplankton because the mesh size of the collecting nets, 202 ^m, was too large to quantitatively capture them, especially the smaller copepodid stages, (Paffenhofer and Mazzocchi, 2003). Nevertheless, assuming a combined O. similis + Oncaea spp. concentration below 50 m of 1000 m-3, and a generous clearance rate of 2 ml cop-1 h- 1 (e.g. Svensen and Kiorboe, 2000; Lonsdale et al., 2000; Nishibe et al., 2010), results in the removal of pellets from only 24 Lm-3 over 12 h (2.4% 12 h -1 = 0.048 d-1). Although this is an order of magnitude greater than the estimated consumption by large copepods, it is a modest contribution to loss processes for pellets sinking at 50-125 m 12 h-1and it would not appear that these small copepods can contribute much to degradation and loss of these fast sinking pellets. Oithona similis however, is known to be an ambush feeder and this behavior is consistent with the concept of flux feeding (Jackson, 1993; Dagg, 1993). Assuming an individual

'scans' and removes sinking particles from an area of 1 mm by 1.5 mm (0.015 cm2) results in the continuous removal of sinking particles by O. similis from 15 cm2 over a 1 m2 area (0.15%). If this process operated over 50 or 100 m below the particle maxima, the resultant removal of sinking particles would be 7.5 or 15.0%. These rough calculations suggest it is possible for small copepods, especially Oithona, to contribute to removal of sinking particles under HI conditions in our study. Smaller copepodid stages that would not have been sampled by our nets would add an unknown amount to this consumption.

Protozooplankton (primarily dinoflagellates 20-100 ^m) have been identified as important consumers and degraders of copepod fecal pellets (Poulsen and Iversen, 2008; Poulsen et al., 2011). In our study, protozooplankton concentrations were highest near the surface but concentrations of dinoflagellates below 100 m were often several ^g C L_ 1. Application of clearance rates for dinoflagellates other than the two very large species used experimentally by Poulsen et al. (2011) result in clearance rate estimates of approximately 3.5-10% d_ 1 of deep water in our study. From these rough calculations, it seems possible that protozooplankton could contribute significantly to removal of sinking particles under HI conditions in our study but two factors argue against this. First, dinoflagellates sense copepod pellets by chemical signals that approximate the original phytoplankton consumed by the pellet-producing copepods (Poulsen et al., 2011). Highest consumption rates are on fresh pellets because the chemical signal decays rapidly (Urban-Rich, 1999; Poulsen et al., 2011). In our study, pellets below 100 m would be approximately 12 h old and chemical signals emitted would have diminished. Second, dinoflagellates

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20 40 60

Ciliate concentration (|ig C L-1)

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5 10 15 20 Dinoflagellate concentration (|ig C L-1)

Fig. 12. The vertical distribution of protozooplankton biomass (^gCL 1): (a) HI: ciliates and (b) HI: dinoflagellates from some HI casts; (c) LO: ciliates and (d) LO: dinoflagellates from some LO casts.

actively swim towards pellets releasing the chemical signal (Poulsen et al., 2011). The capture of large rapidly sinking pellets like those from euphausiids in our study is less likely compared to the smaller, slower sinking, copepod pellets used experimentally by Poulsen et al. (2011). We conclude that protozooplankton may possibly have contributed to pellet degradation and consumption below 100 m in our study but were more likely to have impacted fecal pellets near the surface because pellets were fresher and dinoflagellate concentrations were higher.

There is a potential for zooplankton that we did not sample, such as radiolarians, to alter the flux of pellets (Gowing, 1989). Beers and Stewart (1967) found radiolarian concentrations as high as 50,000 m_ 3 in coastal California waters. If radiolarians at these high concentrations were functioning as flux feeders, much of the flux could be consumed (Iversen et al., 2010). Radiolarians were not observed in our protozoan samples but were, at times, abundant in the deeper MOCNESS samples (Checkley, unpublished). However, our sampling methodologies were not designed to quantitatively assess their abundance and distribution.

Bacterial activity may have contributed to pellet degradation during sinking but we have no data on bacteria concentration or activity during our study. Bacteria have been implicated in pellet destruction in several studies (e.g. Hansen et al., 1996; Thor et al., 2003) and may also have been important in the experiments of Poulsen and Iversen (2008). Ploug et al. (2008) estimated that the bacterial respiration rate on fecal pellets was 0.15 d _ 1. If applied to our study for particles below 100 m in the HI profiles, this would suggest there is a significant consumption of pellet material by bacteria during 12 h.

It has become generally accepted that most zooplankton pellets are not exported even though pellets often comprise a large component of export. Euphausiid pellets however, are exceptionally large compared

to most other zooplankton pellets. In our study, we observed that many large pellets produced by euphausiids were exported, at least to 100 m and probably deeper, within 12 h. High export of euphausiid feces under phytoplankton bloom conditions has also been observed by others (e.g. Wexels Riser et al. (2001), (2008), Gonzales et al. (2011)). These episodic events of high flux can pass quickly to deeper, less biologically active water. We conclude that, in our study, the high concentrations of fecal pellets observed in surface waters at night were produced by dielly migrating euphausiids feeding primarily on large celled phytoplankton. Most of these pellets sank beneath our 200 m sampling zone within 24 h but small copepods (Oithona, Oncaea), heterotrophic dinoflagellates, and bacteria may have caused some pellet degradation or consumption. Consumption of sinking pellets by large copepods appeared to be insignificant.

Acknowledgments

This work was supported by the National Science Foundation (NSF) awards OCE-0927863 to GJ, OCE-0928139 to MD, and OCE-0928425 to DC. We are greatly indebted to Chih-Jung Wu and Hongbin Liu at the Hong Kong University of Science and Technology for analysis of the microzooplankton samples. We thank Rio Forrest-Baldini for her contribution at all stages of this work.

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