Time Series of Vertical Flux of Zooplankton Fecal Pellets on the Continental Shelf of the Western Antarctic Peninsula" (2010)

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5-2010

Time series of vertical flux of ooplanktz on fecal pellets on the continental shelf of the Western Antarctic Peninsula

Miram Rayzel Gleiber College of William and Mary

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Recommended Citation Gleiber, Miram Rayzel, "Time series of vertical flux of zooplankton fecal pellets on the continental shelf of the Western Antarctic Peninsula" (2010). Undergraduate Honors Theses. Paper 694. https://scholarworks.wm.edu/honorstheses/694

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Time series of vertical flux of zooplankton fecal pellets on the continental shelf of the Western Antarctic Peninsula

An honors thesis submitted in partial fulfillment of the requirement for the degree of Bachelors of Science in Biology from The College of William and Mary

Miram Rayzel Gleiber

Accepted for ______(Honors)

______Dr. Lizabeth Allison, Director

______Dr. Deborah Steinberg

______Dr. Randy Chambers

______Dr. Jonathan Allen

Williamsburg, VA April 27, 2010

I Table of Contents:

List of Tables…………………………………………………………………………… III

List of Figures…………………………………………………………………………... IV

Abstract………………………………………………………………………………….. 1

Introduction……………………………………………………………………………... 2

The biological pump and carbon flux Importance of zooplankton fecal pellets in carbon export The West Antarctic Peninsula: zooplankton and carbon flux in a changing climate

Methods…………………………….……………………………………………………..7

Sediment Trap deployment and retrieval Sample processing Fecal Pellet Analysis Carbon Calculations Data analysis

Results…………………………………………………………………………………... 11

Fecal pellet carbon flux Fecal pellet shape Fecal pellet size Fecal pellet color

Discussion……………………………………………………………………………….. 15

Seasonality of flux in the Antarctic Fecal pellet contribution to total POC Contribution of different taxa to fecal pellet flux Possible contribution of different feeding modes to fecal flux Implications of the changing Antarctic climate for fecal pellet carbon flux

Conclusion…………………………………………………………………………….… 23

Acknowledgements ……………………………………………………………………...24

Tables ………………………………………………………………………….………... 25

Figures ………………………………………………………………………………….. 27 . Literature Cited………………………………………………………………...…….… 43

II List of Tables:

Table 1. Sample carbon flux and dates of collection………………………………… 25

Table 2. Phytodetritus in samples……………………………………………………. 26

Table 3. Fecal pellets size data …………………………………………………….... 26

III List of Figures:

Figure 1. The Biological Pump (Ducklow et al., 2008) ……………………………… 27

Figure 2. PalLTER sediment trap locations ………………………………………….. 28

Figure 3. Schematic and picture of sediment trap ……………………………………. 29

Figure 4. Fecal pellet shape and color ………………………………………………... 31

Figure 5. Fecal pellet flux from Jan. 2005 – Jan. 2009 ………………………………. 32

Figure 6. Monthly fecal pellet flux (Jan. 2005 – Jan. 2009) ………………………… .33

Figure 7. Seasonal fecal pellet flux (Jan. 2005 – Jan. 2009) ……………………….... 34

Figure 8. Proportion of fecal pellets in total POC flux ……………………………… .35

Figure 9. Phytodetritus obvious in samples …………………..……………………… 36

Figure 10. Visual observation of samples in each season …………………………… 37

Figure 11. Monthly cylindrical and non-cylindrical pellets in POC flux …………… .38

Figure 12. Monthly ovoid and tabular pellets in POC flux ………………………….. 39

Figure 13. Correlation between cylindrical and ovoid pellets ……………………….. 40

Figure 14. Size class frequency distribution of fecal pellets …………………………. 41

Figure 15. Fecal pellet color …………………………………………………………. 42

Figure 16. Correlation between pellet volume and total POC flux ……………….….. 43

Figure 17. Regression of non-phytodetritus pellet volume and total POC ………...… 43

IV Abstract:

The contribution of zooplankton fecal pellets to particulate organic carbon (POC) flux in the Western Antarctic Peninsula (WAP) was investigated to predict the effect of changes in the zooplankton community due to rapid climate change on flux of fecal pellets to the deep sea.

Fecal pellets were collected in a moored sediment trap from 2005 to 2009, along the continental shelf of the WAP. Fecal pellet shape, size, and color were quantified to assess flux of pellets from different zooplankton taxa, and compared between seasons and years. Fecal pellet POC flux constituted a significant proportion of total POC flux, with summer pellet C flux (68% of total) significantly higher than in winter (36%) (n = 11,270). Cylindrical pellets, produced by euphausiids, contributed to a monthly mean of 79% of fecal pellet flux, with copepod and salp pellets contributing significantly less (15% and 6%, respectively, of total fecal pellet flux).

Cylindrical and ovoid pellets were significantly higher in summer samples, while 90% of tabular pellet flux occurred in the winter. Tabular pellets were the largest (median size = 0.8 µgC/fecal pellet), followed by cylindrical pellets (0.2 µgC/fecal pellet), with ovoid pellets the smallest

(0.02 µgC/fecal pellet). The largest tabular pellets contained up to 3-orders of magnitude more carbon than cylindrical pellets. Most fecal pellets were dark or light brown in color (suggesting an herbivorous or omnivorous diet), and <1% of pellets were white or red (detritivorous/ carnivorous diet). Zooplankton fecal pellets, especially from krill, are a significant component of carbon flux in the WAP. Loss of sea-ice due to climate change is making this region less suitable for krill, and more suitable for salp populations. Loss of krill and increase in salps could significantly change export of POC to the deep sea in this region, resulting in enhanced, but highly variable flux, with a possible increase in phytodetritus contribution to flux.

1 Introduction

The oceans play a crucial role in the global carbon cycle by taking up natural and anthropogenic CO2 from the atmosphere, thus how climate warming may be changing the ocean’s carbon cycle is an important question. Zooplankton play a key role in the marine carbon cycle by grazing on primary producers (phytoplankton) in surface waters and then egesting dense, fast-sinking fecal pellets, thereby transferring carbon from the surface to the deep ocean where it can be sequestered (Bishop et al., 1977; Turner and Ferrante, 1979; Dunbar and Berger,

1981; Urrere and Knauer, 1981; Steinberg et al., 2000; Ducklow et al., 2001). In this thesis I investigate how changes in zooplankton community structure in the Western Antarctic Peninsula

(WAP), a region of rapid climate change, affect flux of zooplankton fecal pellets to depth.

The biological pump and carbon flux

Through processes collectively termed the “biological pump”, dissolved inorganic carbon

(DIC) is converted by phytoplankton via photosynthesis into organic forms, and exported to the deep ocean (Fig. 1; Ducklow et al., 2001). This vertical export occurs via: passive sinking of particulate organic carbon (POC), ‘active transport’ by zooplankton diel vertical migration, and the physical mixing of dissolved organic carbon (DOC; Volk and Hoffert, 1985; Ducklow et al.,

2001). The major export by the biological pump is thought to be via passive sinking of POC, which is comprised of phytoplankton aggregates, detritus, living and dead cells, molts, and zooplankton fecal products (Fowler and Knauer, 1986).

Carbon dioxide (CO2), the inorganic form of carbon present in the atmosphere, along with other nutrients, is fixed during photosynthesis by phytoplankton in the ocean’s euphotic layer. Phytoplankton can release dissolved organic carbon (DOC) into the surface layer, or may

2 be consumed via zooplankton grazing. The DOC that builds up in the ocean’s surface layer (i.e., the portion that is not used by bacteria for their metabolism) can be vertically mixed to depth by physical processes (Ducklow et al., 2001).

Zooplankton play a crucial role in the biological pump through herbivorous grazing of phytoplankton and subsequent egestion of dense, rapidly sinking fecal matter that delivers POC to depth. The transport of POC is accomplished through passive sinking of fecal pellets and aggregates, and also by active transport by mesopelagic zooplankton that vertically migrate to the surface at night to feed, then egest fecal pellets in the deeper ocean during the day (Atkinson et al., 1996; Morales, 1999; Steinberg et al., 2000; Kobari et al., 2008). Most of the sinking POC flux, however, does not reach the deep sea due to flux attenuation in the mesopelagic zone by bacteria, zooplankton, and nekton that degrade, consume, and respire the POC back to its inorganic form (Buesseler et al., 2007; Buesseler et al, 2008b; Steinberg et al, 2008b). Microbes, both ‘free-living’ in the water column and ‘attached’ to particulate matter metabolize the undigested or partially digested material in the fecal pellets or other types of sinking particles, thus decomposing them (Karl et al., 1988). Mesopelagic zooplankton play a major role in changing the sinking rate of POC flux by repackaging (i.e., reingesting) suspended and sinking particles, or by fragmenting larger particles into smaller, non-sinking particles through swimming activity or ‘sloppy feeding’ (Lampitt et al., 1990; Noji 1991; Lampitt et al., 1993;

Dilling and Alldredge, 2000; Wilson et al., 2008).

Importance of zooplankton fecal pellets in carbon export

Once at depth, carbon is stored for millions of years, sequestering inorganic atmospheric carbon (CO2) to the deep ocean (Ducklow et al., 2001). The sinking rate of POC is governed by

3 Stokes Law, stating that the sinking of particles is proportional to their size and density, and inversely proportional to viscosity. Thus, macrozooplankton are integral in carbon flux as they produce rapidly sinking fecal pellets, bypassing sources of attenuation and resulting in carbon export to the seabed with a high transfer efficiency (Volk and Hoffert, 1985; Buesseler et al,

2007). Previous studies found zooplankton fecal pellets effective transporters of carbon, with sinking rates of pellets 10 to 100-fold that of phytoplankton cells (Turner, 2002). Zooplankton fecal pellets quickly and efficiently transport carbon to the deep sea, and have been used as important environmental indicators as contaminants show up in sinking pellets on a short timescale. In a study of Mediterranean sediment traps after the Chernobyl disaster, radioactivity was observed in zooplankton fecal pellets collected at 200m, seven days after peak radioactivity was recorded at the surface (Fowler et al, 1987).

Fecal pellet production studies provide insight into the types of pellets different species produce. Euphausiids produce long, cylindrical pellets often easily broken as they sink in the water column (Marshall and Orr, 1955; Fowler and Small, 1972). Copepods produce ellipsoid or ovoid pellets with round or pointed ends, and are encased by a peritrophic membrane that makes pellets more resistant to bacterial degradation and breakage via zooplankton swimming and feeding behavior (Gauld, 1957; Martens, 1978; Yoon et al., 2001). Salps (gelatinous pelagic tunicates) produce very large, fast-sinking ‘fecal flakes’ that have been observed to contribute to major flux of POC when blooms are present (Wiebe et al., 1979; Urrere and Kanuar, 1980; La

Fevre et al., 1988; Anderson, 1998).

Sinking rates greatly vary with fecal pellet size, shape, and nutritional content, thus POC flux is affected by zooplankton community structure and mode of nutrition (Noji 1991; Carroll et al., 1998; Wilson et al., 2008). Copepod pellets exhibit the slowest sinking rate, 5-220 m d-1

4 (Smayda, 1971; Small et al., 1979; Yoon et al., 2001), cylindrical krill pellets have sinking rates ranging from 16-862 m d-1 (Fowler and Small, 1972; Yoon et al., 2001), and salp pellets have the fastest sinking rates from 42-2700 m d-1 (Yoon et al., 2001; Phillips et al., 2009).

Zooplankton have a variety of feeding modes which can result in variations in particle packaging (Urrere and Knauer, 1981; Urban-Rich et al., 1998). Salps, for example, are indiscriminate feeders that ‘repackage’ particles suspended in the water column into dense, rapidly sinking fecal pellets (Anderson, 1998; Madin and Diebel, 1998), while some pellets such as those from carnivorous chaetognaths, can contain buoyant lipid from prey, resulting in slower sinking pellets (Dilling and Alldredge, 1993). Variation in zooplankton community structure can be reflected in fecal pellet abundance and composition, with the contribution of zooplankton fecal pellets to total sinking POC in different ocean environments ranging from insignificant to nearly 100% (Bishop et al., 1977; Asper, 1987; Turner, 2002; Wilson et al., 2008).

The Western Antarctic Peninsula: zooplankton and carbon flux in a changing climate

Zooplankton fecal pellet production plays an integral role in the biological pump and is particularly relevant in regions experiencing the effects of climate change. The Western

Antarctic Peninsula (WAP) is experiencing one of the most rapid rates of warming on Earth; since 1950 the annual mean temperature has increased by 2ºC, with average midwinter temperatures rising by 6ºC (Vaughan et al., 2003, Ducklow el al., 2007). As a result, the marine ecosystem in the WAP has been undergoing rapid physical climate change in the past few decades from a cold-dry, polar-type climate to a warm-humid, sub-Antarctic-type climate

(Ducklow et al, 2007; Ducklow et al., in press). These changes have had a major impact on seasonal sea ice coverage, with sea ice in this region decreasing by 40% in the last thirty years

5 (Stammerjohn and Smith, 1997; Stammerjohn et al, 2008), posing serious concern for primary production and sea-ice dependent biota in the WAP.

Phytoplankton communities contribute to primary production at the base of the WAP marine food web, as a main food source for grazing zooplankton. Since 1979, the amount of

Chlorophyll a (an indicator of phytoplankton abundance) along the WAP shelf has declined significantly by about 12% (Montes-Hugo et al., 2009; Montes-Hugo et al., 2010). This decline reflects the shift in seasonal sea ice, which many Antarctic phytoplankton communities need to establish blooms, as well as changes in physical water column mixing from wind (Stammerjohn and Smith, 1997; Montes-Hugo et al., 2009; Montes-Hugo et al., 2010).

Zooplankton populations in the WAP also reflect the changes occurring throughout this marine ecosystem due to climate change, through the relative abundance of two major grazers: krill (Euphausia superba) and salps (Salpa thompsoni) (Quetin and Ross, 2001; Atkinson et al.,

2004; Ross et al., 2008). Antarctic krill abundance is dependent on both summer food availability and winter sea ice necessary for larval recruitment and survival, while salps can survive in regions of lower-productivity and are not dependent on sea ice (Quetin and Ross,

2001; Atkinson et al., 2004). Data from net tows in the Southern Ocean from 1926-2003 show a significant decrease in krill since 1976 in the WAP region, while salp densities have increased throughout the entire Southern Ocean (Atkinson et al., 2004). Thus, changes in sea ice cover and primary production are resulting in visible shifts in the WAP zooplankton community. Since the zooplankton community structure directly affects carbon flux through fecal pellet production and vertical migration, changes in the zooplankton community could potentially impact biogeochemical cycling in the Antarctic ecosystem.

6 This thesis examines the fecal pellet composition of a time series of sediment trap samples collected from Jan. 2005 – Jan. 2009, located offshore of the WAP along the continental shelf (Ducklow et al., 2007; Ducklow, 2008a). The WAP sediment trap has been deployed annually since 1992 as part of the Palmer Long Term Ecological Research (Pal LTER) program funded by the National Science Foundation (NSF). The Antarctic shelf experiences high vertical particle flux that draws CO2 to the deep sea (Walsh et al., 1981; Karl et al., 1996) and thus is an important mechanism of CO2 storage in the ocean carbon system (Ducklow et al., 2008b).

Past studies have examined total POC in the Pal-LTER time series of sediment trap samples (Ducklow et al., 2008b), but they have not analyzed fecal pellet composition. My study aims to gain insight into the importance of zooplankton fecal pellets to the particle flux of the

WAP, and to determine if changes in zooplankton fecal pellet flux reflect changes in the WAP zooplankton community composition. Ultimately this information can be used to assess if changes in zoooplankton community composition in the WAP may lead to changes in the efficiency of the biological pump.

Methods

Sediment Trap deployment and retrieval

Samples were collected in a moored sediment trap (PARAFLUX Mark 78H 21-sample trap; McLane Research Labs, Falmouth, MA) deployed annually on the continental shelf located about 130km off the Western Antarctic Peninsula (64º30’S, 66º00’W) (Fig. 2, 3). The bottom depth at the deployment site is 350m, with the trap suspended at a depth of 170m. The trap contains 21 sample bottles that autonomously collect samples throughout the year at intervals varying from 7-30 days, corresponding with anticipated seasonal flux (Ducklow et al., 2008b).

7 During peak flux in the Austral summer (Nov. – Jan.) the sample carousel rotates bottles weekly, and in the Austral winter (Feb. – Oct.) intervals were monthly. There were gaps of 5 days between mooring recovery and deployment. The 21 plastic sample collection bottles on the trap were prepared before deployment with a Milli-Q deionzed water rinse and filled with a 7.5g

NaCl L-1 solution and 2% borate-buffered formalin in filtered seawater (Ducklow et al., 2008b).

While this thesis examines sediment trap samples deployed annually in the Austral summer from

Jan. 2005 to Jan. 2009, this sediment trap has been collecting samples since 1992 as part of the

Palmer LTER time-series (Ducklow et al., 2008b). Sediment trap deployment occurred during annual research cruises to this area of study; the 2005-2009 trap deployments and recoveries were done aboard the R/V Laurence M. Gould with the most recent recovery Jan. 10, 2009.

Sample processing

After retrieval, trap samples were gently rinsed through a 1000µm Nitex® mesh to remove large swimmers (zooplankton that swam into the sediment trap and died), non-swimmer material replaced in sample, sealed, and stored at 5ºC until processed in the laboratory. Starting with the 2008 series, samples were pre-screened and all swimmers and molts were individually removed by hand. Zooplankton remains and molts were preserved in 2% formaldehyde.

Swimmer-free trap samples were sent to the Marine Biological Laboratory in Woods Hole for

CHN analysis. There samples were split using a plankton splitter; half (except for cups with insufficient sediment) were treated with dilute HCl to remove inorganic carbon, then analyzed for total organic Carbon and Nitrogen on a CHN Analyzer (Karl et al., 1991b, Ducklow et al.,

2008b), and the other half preserved with formalin for pellet analysis.

8 Fecal Pellet Analysis

The fecal pellet analysis protocol was adapted from Wilson et al. (2008). Preserved sediment trap samples were analyzed using an Olympus SZX12 dissecting scope with Olympus DP71 digital camera under bright-field and dark-field illumination. Samples too dense to analyze were split using a plankton splitter until the sample was split to a manageable amount. The range of splits analyzed was 1/1 -1/256 of the total trap sample. This split subsample was poured into a petri dish with a grid containing pre-measured squares. Squares were randomly chosen and pellets within were photographed until at least 100 pellets of each type were photographed

(although some samples contained too few pellets to reach a count of 100). Larger, rare pellets were photographed from the entire sample prior to the subsample split. Pellet volumes in splits were later extrapolated to the whole sample to account for all pellets by taking into account the area of the dish and number of squares counted.

Digital images were analyzed with Image-Pro Plus© software measuring recognizable pellets for size and color (average RGB values) as in Wilson et al. (2008). Light field images were used to quantify size and dark field images used to quantify color; constant camera settings and light conditions were used. Pellets were visually categorized by shape (ovoid, cylindrical, amorphous, and tabular) and color (dark brown, light brown, and red/white) (Fig. 4). Fecal pellet production experiments in the field gave a general idea of the types of zooplankton producing these pellet shapes: copepods (ovoid), krill (cylindrical), salps (tabular) (Steinberg et al., 2009). Fecal pellet color has been used as a general indicator of zooplankton feeding mode, with dark brown pellets produced by herbivores (feeding on phytoplankton), light brown pellets produced by omnivores

(feeding on diatoms, protozoan, and detritus), and red, transparent, or white pellets produced by carnivores feeding on crustaceans (red) or gelatinous plankton (transparent/white) (Urrere and

9 Knauer, 1981; Noji et al., 1991, Dilling and Alldredge, 1993; Urban-Rich et al., 1998; Wilson et al., 2008). Pellets may leech color when preserved in formaldehyde over time (Hansen et al.,

1996), thus pellet color as an indicator of food source must be used cautiously.

Pellets were recorded as broken, lacking a peritrophic membrane, fragmented or otherwise degraded. The presence of particles not recognizable as pellets was recorded, including phytodetritus (phytoplankton detritus), fecal “fluff”, and foraminiferans.

Carbon Calculations

The volume of measured pellets was converted to carbon to calculate fecal pellet carbon flux for analysis. Pellet volume was calculated using pellet length and width measurements and volume formulas for an ovoid, cylinder, and tabular shape. Pellet volumes were converted to carbon using a conversion factor of 0.018mgC mm-3. This value was calculated from the regression of total pellet volume, from samples observed to be almost entirely composed of pellets, with total POC flux values obtained from CHN analysis. Our conversion factor is on the low end of those from the literature, which range from 0.01-0.15 mgCmm-3 (Carroll et al., 1998;

Urban-Rich et al., 1998; Wassmann et al., 2000; Turner, 2002; Wilson, 2008). However, our initial calculations using a midrange conversion factor of 0.08 mgCmm-3 (Wilson et al., 2008) resulted in erroneously high pellet flux values (i.e., higher than total POC flux), thus we used the total POC flux to constrain our pellet volume-C conversion. Total pellet C in a sample was converted to carbon flux (mgCm-2d-1) by dividing by the known area of the trap (0.5m2) and days of samples collection for each bottle (ranging from 7-31 d).

10 Data analysis

The total carbon flux from pellets for each sample was compared to total POC flux, with values also combined and averaged for months and seasons (Summer = Oct. – April; Winter =

May – Sept.). Sample bottles from June and July 2005, June and Sept. – Dec. 2007 were not analyzed for pellets as samples were either not preserved properly or did not contain enough fecal material.

The monthly flux by pellet shape, size, and color was also compared. For size analysis, pellets for different shapes were binned in ten size classes and samples of each pellet shape normalized to 1000 pellets: ([number of pellets in a size class/total number of pellets of that shape] x 1000). Size classes were based on pellet carbon values for a variety of size and shaped pellets photographed for analysis. Size classes ranged from <0.001 – 0.05 µgC/pellet for small pellets, 0.5-1.0 µgC/pellet for medium pellets, and 1.0- >5.0 µgC/pellet for large pellets. T-tests and Univariate ANOVA were used to calculate differences between pellet carbon flux for seasons, pellet shapes, and years.

Results

Fecal pellet carbon flux:

The total pellet carbon flux in sediment trap samples Jan. 2005 – Jan. 2009 shows a seasonal difference between the austral summer (Oct-April) and winter (May-Sept.), with peak pellet fluxes in summer (Table 1). Peak fecal carbon flux for each year appears in samples taken between Jan. 20-27th in 2005 (24.0 mgCm-2d-1), Feb. 12-March 1 in 2006 (168.4 mgCm-2d-1),

Jan. 15-22 in 2007 (20.9 mgCm-2d-1), and very early in the summer season, Nov. 22-29, in 2008

(65.7 mgCm-2d-1) (Fig. 5, Table 1). For the years 2005-2007 fecal pellet carbon flux was lowest

11 in the winter months, and remained low (<0.1 mgCm-2d-1) until mid-summer (Jan.) when flux increased by100-fold or greater. In 2008 however, flux was low from June-Nov., and in mid-

Nov. increased by over 200-fold (Fig. 5, Table 1).

This pattern of seasonal flux is also apparent when individual samples are averaged by month (Fig. 6). Monthly pellet carbon flux was significantly higher in summer months (mean ± standard error: 10.6 ± 4.1 mgCm-2d-1, n = 26) than winter months (0.2 ± 0.1 mgCm-2d-1, n = 16)

(Independent-samples t-test; p = 0.022). Seasonal mean values were not significantly different between years for either summer or winter fluxes (Fig.7; Univariate ANOVA: summer & year: F

= 1.036; df = 4; p = 0.41; winter & year: F = 1.051; df = 3; p = 0.41).

Fecal pellet POC is a large proportion of the total POC flux, with recognizable pellets ranging from 8 - 180% in the summer months (Nov. 2006 and Jan. 2009, respectively) and 1 -

125% in the winter months (June 2008 and July/Aug. 2008, respectively) of the total POC flux calculated from CHN analysis (Fig. 8). Outliers occurring in periods of flow flux were removed as they were up to seven times the total POC Flux (Fig. 8). Mean pellet proportion of total POC for all months was 58 ± 8%. The mean proportion of total POC comprised of recognizable fecal pellets was significantly higher in the summer (68 ± 9%, n = 25) than in the winter (36 ± 14%, n

= 14) (Independent-samples t-test; p = 0.05).

In addition to fecal pellets, total POC flux includes other particulate matter such as phytodetritus and fecal “fluff”– decomposed phytoplankton and degraded pellets, respectively

(Fig. 9). From qualitative visual observations, phytodetritus was obvious in 33.3% of summer samples analyzed (n=57), and 53.3% of winter samples analyzed (n=15; Table 2). Samples with obvious phytodetritus occurred in months with a lower mean proportion of pellets in POC flux

12 (mean% ± S.E.: phytodetritus obvious = 30.8 ± 9.0, n=12, outlier removed; not obvious = 84.6 ±

16.2%, n=27).

Photographs taken from samples illustrate the above seasonal trends (Fig. 10). High numbers of pellets and little phytodetritus or fecal “fluff” comprise the late-Summer samples.

Fall samples have decreasing numbers of pellets, some contain tabular pellets, and some with increasing non-pellet material. Winter samples generally had more non-pellet material, including phytodetritus and fecal ‘fluff’, with some samples containing tabular pellets. Spring to early-

Summer samples were similar to fall samples.

Fecal pellet shape:

Monthly mean carbon flux varied significantly between the three fecal pellet shapes, with cylindrical pellets having considerably higher flux (mean flux ± S.E. mgCm-2d-1: cylindrical=6.5

± 2.7; ovoid=0.2 ± 0.05; tabular =0.03 ± 0.01; n=42; Univariate ANOVA: cylindrical & non- cylindrical: p=0.012, df = 41). Ovoid and tabular mean monthly flux were not significantly different from each other (mean flux difference: ovoid * tabular = 0.17, p=0.997). Mean carbon flux for any of the three pellet types did not vary among years (p = 0.50).

Flux of cylindrical pellets ranged from <0.1 mgCm-2d-1 in July and Aug. 2007 to 98.1 ±

69.0 mgCm-2d-1 in Feb. 2006. Mean flux of cylindrical pellets was highest in summer months, contributing up to 10 - 100 times more carbon flux in these months than both ovoid and tabular combined (Fig. 11). Flux of ovoid and tabular pellets (combined) was higher than cylindrical pellets in some winter months (e.g., April 2005, July and Aug. 2006, and May-Aug. 2008). The non-cylindrical pellets constituted 92.8% of total pellet flux in April 2005, 12.8 times higher flux than the cylindrical pellets in this month (Fig. 11). Peak heights in cylindrical pellet flux

13 correspond to the total pellet POC flux (Fig. 6; 8), and cylindrical pellets constituted on average

80.7 ± 4.0% of the total pellet flux, with flux in some months comprised almost entirely of cylindrical pellets (up to 99.9%).

Ovoid pellet flux was >0.1 mgCm-2d-1 from mid to late summer each year (Jan. – April), and was <0.05 mgCm-2d-1 in the winter and early summer months, for all years (Fig. 12). Ovoid pellet flux from 2005-2008 was 0.2 ± 0.05 mgCm-2d-1 and was highest in April 2007 with a flux of 1.4 mgCm-2d-1. Tabular pellets contributed the least to pellet carbon flux, with a mean flux of

0.03 ± 0.01 mgCm-2d-1, and maximum monthly flux of 0.2 mgCm-2d-1 in Jan. 2005. Although tabular pellets did not significantly differ between years, this pellet type occurs with the highest frequency throughout the year in 2008, occurring in Feb., March, July, and Aug. (Fig. 12).

Flux of both cylindrical and ovoid pellets were higher in Jan.-April of each year, and there was a significant positive correlation between flux of these two pellet types (r2=0.255; n=42; p=0.001; Fig. 13).

Fecal pellet size:

Fecal pellets were categorized into 10 different size classes as shown by the frequency distribution in Fig. 14. Tabular pellets occur in the highest frequency in the larger size classes with 93% in classes >0.1 µgC/pellet, with the largest being 121.4 µgC/pellet (Table 3).

Cylindrical pellets were normally distributed, occurring most frequently among the medium size classes but with pellets falling into all ten size-classes (Table 3). Over 80% of cylindrical pellets fall in the 0.01-1.0 µgC/pellet size classes, with the highest frequency of pellets (39%) in the 0.1-

0.5 µgC/pellet size class. Ovoid pellets were the smallest, with 99.7% of pellets below 0.1

14 µgC/pellet in size, and the highest frequency of pellets (43%) between 0.01-0.05 µgC/pellet

(Table 3).

Fecal pellet color:

Average seasonal flux for different color categories of pellets indicates most flux is comprised of dark brown or light brown pellets indicating mostly herbivorous or omnivorous feeding (Fig. 15). Dark brown pellets (herbivores) constitute 60.2% of the total fecal pellet flux in both seasons, and light brown pellets (omnivores) constitute 39.2% of total fecal pellet flux in both seasons. Carnivory of either crustaceans (indicated by red pellets) or gelatinous zooplankton

(indicated by white or transparent pellets) is not prevalent in any season, with total carnivory contributing to <1% of all fecal pellet carbon flux.

There were no significant differences in pellet color between winter and summer seasons.

For summer months all three pellet color categories are present in the same proportions for all pellets (see above). While in the winter proportions vary slightly, dark brown = 55.6%; light brown = 44.7%; and carnivory = <0.1%. This difference is likely due to the low flux in the winter, and high flux in summer.

Discussion

Seasonality of flux in the Antarctic

The polar regions of the oceans are characterized by high intra-annual variability in flux due to seasonal sea ice coverage and retreat (Lampitt and Antia, 1997; Ducklow et al., 2008b). Many sites in the Antarctic marginal ice zones have a significant annual peak in sedimentation during the ice-free season (Austral summer; Fischer et al., 1988; Honjo et al., 2000; Ducklow et al.,

15 2008b). This seasonal variability has been previously observed throughout the PalLTER sediment trap time series (1993-2008) in total POC flux (Ducklow et al., 2008b). My analysis of sediment trap samples from 2005-2008 for fecal pellet flux shows this pattern to also be significant in flux of fecal pellet POC. Mean summer pellet flux of 10.6 mgCm-2d-1, was significantly higher than winter pellet flux of 0.2 mgCm-2d-1. Pellet flux ranged from insignificant amounts (in both summer and winter months) to 99 mgCm-2d-1 as the highest summer peak, and 1 mgCm-2d-1 as the highest winter peak.

Fecal pellet contribution to total POC

Fecal pellet contribution to total POC flux is highly variable, with studies throughout the world’s oceans finding this proportion to range from ‘insignificant’ to nearly 100% of total flux

(Turner, 2002). Fecal pellet proportion of total flux was variable throughout the time-series, but there was a significant seasonal difference (major outliers removed). Fecal pellets constituted a higher proportion of total POC flux in the summer (68%) than in the winter (36%) with values ranging from 8-180% in the summer (outlier of 725% removed), and 1-125% in the winter

(outlier of 328% removed). The occurrence of values greater than 100% is discussed below. In contrast, fecal pellet composition in mid-latitude oligotrophic and mesotropic regions of the

Pacific exhibited proportions ranging from 14% to 35%, and 3% to 29%, respectively (Wilson et al., 2008). As this study used similar methods to Wilson et al., 2008, it allows for a unique comparison of these two regions. Since the Pacific regions studied in Wilson et al. (2008), are not seasonally light and ice-limited, phytoplankton and zooplankton populations do not experience seasonal population changes with the same magnitude as the WAP. Other studies in mid-latitude regions where production can occur year round resulted in similar proportion of

16 fecal pellets in POC flux. In the oligotrophic regions of the North Atlantic subtropical gyre, pellet contribution to POC flux averaged 30% (Huskin et al., 2004), and in the Mediterranean fluxes remained <25% (Carroll et al., 1998). In the polar regions of this study, light and sea-ice cover limits primary production and subsequent zooplankton populations in the winter, while in the summer zooplankton fecal pellets become the main source of sinking carbon (Ducklow et al.,

2008b; Montes-Hugo et al., 2008).

While pellets are major contributors to POC flux in the summer months, as hypothesized, they cannot constitute more than 100% of the total POC flux, as calculations yielded in some samples. This error is likely due to accuracy of the fecal pellet carbon-to-volume conversion factor. While studies similar to this use carbon conversion factors ranging from 0.01 to 0.15 mgCmm-3 (Urrere and Knauer, 1981; Wassmann et al., 2000; Wilson et al., 2008), preliminary calculations using a mid-range conversion of 0.08 mgCmm-3, yielded results that were clearly an overestimation, with fecal pellet POC flux 100% or more of total POC flux in nearly all samples.

Thus, we used the slope of the regression of fecal pellet volume, from samples visually observed to be composed almost entirely of pellets, with measured total POC, yielding a conversion factor of 0.018 mgCmm-3 (Fig. 16, 17). This study provided a unique opportunity to do so as over

10,000 pellets were analyzed, with significant correlation between pellet volume per samples and total POC. Overestimation might also be due to calculations of loose and transparent pellets with the sample formula as whole pellets, or varying carbon content depending on species its diet. For better accuracy of carbon content of pellets, further studies would need to measure the carbon content of pellets with a known volume.

In addition to fecal pellets, total POC flux includes other particulate matter such as phytodetritus and fecal “fluff”. Phytodetritus is composed of decomposed phytoplankton, also

17 contributing to carbon flux, although taking longer than pellets to reach the deep-sea with sinking rates of 100-150 m d-1 (Lampitt, 1985). Phytodetritus was obvious in 33% of summer samples and 52% of winter samples, with the mean percentage of pellets contributing to POC in samples with obvious phytodetritus 31%. These data seem consistent with hypothesis that when less fecal pellet matter is present, less zooplankton are present to graze on phytoplankton, and more sinks as phytodetritus. Thus phytodetritus may account for much of the non-pellet compositions of total POC in samples with less proportion of pellets. Samples from summer months that had obvious phytodetritus present, along with high proportion of fecal pellets, might be due to phytoplankton blooms present in the region. Patchiness of plankton is an inevitable source of error with sediment trap samples as it is difficult to monitor plankton composition in the whole region year-round (Turner, 2002). Finally, fecal “fluff”, partially degraded fecal pellets, is difficult to quantify, thus our estimate of the importance of fecal pellet C to total flux is likely underestimated (Wilson et al. 2008).

Contribution of different taxa to fecal pellet flux

Cylindrical pellets from krill contributed most significantly to flux, constituting a monthly mean of 80% of total fecal pellet carbon flux. Flux of ovoid pellets from copepods and tabular pellets from salps was significantly lower than for cylindrical pellets, although tabular pellets were common in winter months when they contributed to 90% of all tabular pellet flux.

These results reflect previous observations and hypotheses about zooplankton contribution to flux in the Antarctic environment. Euphausiids and copepods both graze on phytoplankton, thus populations are concentrated following sea ice retreat in the spring and summer months when phytoplankton bloom (Quentin and Ross, 2001). In contrast to fecal pellet studies in non-polar

18 regions which indicate euphausiid pellets contribute in similar or lower amounts to pellet flux as ovoid copepod pellets (Carroll et al., 1998; Wilson et al., 2008), the Antarctic marginal ice zone is dominated by euphausiids (Le Fevre et al., 1998; Ross et al., 2008). Tabular pellets are produced by salps which are indiscriminate filter feeders, acting like a vacuum, ingesting all particles in the surrounding area (Mandin and Diebel, 1998; Phillips et al, 2009). Thus they do not rely as heavily on seasonal sea ice and can thrive in winter months when populations of copepods and krill are lower.

Much of the variability in zooplankton fecal pellet flux and thus the carbon exported to the deep-sea results from variability in size (carbon per pellet) of pellets produced by different taxa.

Size class frequency distribution for pellet size of different pellet types revealed ovoid pellets to fall in the smallest size classes (median = 0.02 µgC/fecal pellet), cylindrical in the medium classes but with the largest range throughout all size classes (0.2 µgC/fecal pellet), and tabular pellets to dominate the larger size classes (0.8 µgC/fecal pellet). Tabular pellets included many very large pellets (>5.0 µgC/fecal pellet), with some even up to 3 orders of magnitude higher in size than cylindrical pellets. Salps make up a smaller amount of the biomass in the Southern

Ocean compared to euphasiids (Ross et al., 2008), but due to the large size of pellets they produce, and common presence in blooms, they are major contributors to carbon flux when abundant (Phillips et al., 2009).

The median pellet size of all pellets measured in this study was 0.07 µg C/fecal pellet. In comparison, a similar study of sediment trap samples collected from 150m in depth in the oligrotrophic subtropical gyre and the mesotrophic subarctic North Pacific Ocean, yielded median pellet sizes of 0.036 µg C/fecal pellet and 0.170 µg C/fecal pellet, respectively (Wilson

19 et al., 2008). Median pellet size of pellets in the WAP fell in between these two environments indicating differences in zooplankton populations.

Possible contribution of different feeding modes to fecal flux

Fecal pellet color can vary as a result of zooplankton diet. Dark brown pellets may indicate herbivory on phytoplankton, lighter brown to white pellets resulting from omnivorous diets of detritus, protists, fecal pellets, and suspended particles, while pellets of carnivores may be red if feeding on crustaceans or transparent or white if feeding on gelatinous zooplankton (Urrere and

Knauer, 1981; Noji et al., 1991; Dilling and Alldredge, 1993; Hansen et al., 1996; Urban-Rich et al, 1998; Wilson et al., 2008). Using these general guidelines, herbivores and omnivores contributed the most to fecal pellet carbon flux in all samples, with carnivores contributing <1% of all pellets. The main carnivore in the Southern Ocean is chaetognaths (Ross et al., 2008), but their pellets tend to float in the water column as they contain buoyant lipids, and for this reason are possibly not present at depth in sediment trap samples (Dilling and Alldredge, 1993). There is little difference between summer and winter in mean flux of brown and light brown pellets.

There is a difference in dominant feeding mode between years throughout the times series, but the large amount of error in samples makes this data too inconclusive to draw any significant inferences to zooplankton nutrition. While fecal pellets can leech color over time when preserved in formaldehyde (Hansen et al., 1996), this does not seem to be a major source of error in this study, as samples from more recent years (2008) had a higher mean of lighter pellets than earlier years (2005-2007). A more likely source of error is the sampler bias, with qualitative color category assigned to pellets changing over time as the analyst is exposed to more variation in pellets. While measured RGB (red, green, blue) values were a quantitative form of color

20 analysis, variations in light pollution in the dark field images used for analysis introduces too much error into the data. Future studies might limit color analysis to quantitative RGB values in a controlled environment to better assess color. Also scanning electron microscopy (SEM) could be used to confirm or refute fecal pellet contents as assumed by color analysis.

Implications of the changing Antarctic climate for fecal pellet carbon flux

Seasonal differences in the Antarctic in pellet flux is clear from this four-year time series, and although evidence for long-term changes in pellet flux would require a much longer time series, inferences can be made as to possible effects of long-term changes in the zooplankton community on pellet flux. Seasonal pulses of POC flux reflect sea ice cover and retreat in the

Western Antarctic Peninsula in both timing and magnitude. The 2005/2006 winter season had both the highest total POC flux and fecal pellet POC flux of years studied. When compared to the recorded sea-ice cover over the PalLTER sediment trap mooring (Stammerjohn et al., 2008), this peak corresponds to a year with prolonged sea ice, while the lowest summer flux in Sumer

2007/2008 corresponds to a year with a shorter season of sea ice. Thus, the length of seasonal sea-ice cover impacts the magnitude and timing of zooplankton fecal pellets contributing to flux in the region. Data from the entire PalLTER time series indicates peak POC flux occurs much later in the season. This is hypothezised to be due to loss of duration and magnitude of winter sea ice due to atmospheric warming (Ducklow et al., 2008; Stammerjohn et al., 2008, Ducklow et al., in press).

As zooplankton are significant contributors to carbon flux in the WAP, changes in the composition of zooplankton with climate change will likely lead to changes in the biological pump in this region. My results indicate that cylindrical pellets, produced by krill contribute

21 significantly to fecal pellet flux. While tabular pellets produced by salps were rare in samples, salp pellets contain up to 3 order of magnitude more carbon per pellet than krill, and sink at rates up to double that of euphausiid pellets. There is currently a shift in relative populations of euphausiids and salps occurring in the Western Antarctic Peninsula due to decline in krill as a result of loss in winter sea-ice extent (Atkinson et al., 2004). The krill populations that characterize the major zooplankton composition of the Antarctic rely heavily on winter sea-ice for both summer phytoplankton blooms and winter larval recruitment (Quetin and Ross, 2001;

Atkinson et al., 2004; Montes-Hugo et al., 2009; Montes-Hugo et al, 2010). Salps are indiscriminate filter feeders that can thrive in regions of much lower-productivity, and warmer water with a complete absence of sea-ice (La Fevre et al., 1998; Madin and Diebel, 1998). A decrease in krill populations with a subsequent increase in salps has major implications both upward through the food chain, and down through nutrient cycling. Krill transfer carbon and other nutrients to apex predators in the Antarctic, while salps contain only a fraction of the nutrients; thus the observed shift in zooplankton has major implications for the survival of top

Antarctic predators (Le Fevre et al., 1998; Quentin and Ross, 2001; Atkinson et al., 2004;

Ducklow et al., 2007; McClintock, et al., 2008; Steinberg, et al., 2009; Ducklow et al., in press).

Krill fecal pellets are the significant contributor in POC flux, while salp pellets contribute only a fraction of this amount. But, with salp pellets containing thousands of times more carbon than krill and sinking at rapid rates to the deep-sea (Turner, 2002; Phillips et al., 2009) it is likely this shift in the zooplankton due to climate change will result in a significant change in the carbon export system. With a greater contribution of salps, carbon export may both be enhanced and more sporadic, due to the tendency of salps to form blooms. An alternative hypothesis is that the mucus net, with which salps feed can easily become clogged by dense phytoplankton (Harbison

22 et al., 1986). Thus, when phytoplankton blooms are present the salps will be unable to efficiently repackage carbon to pellets, as krill do, resulting in increasing phytodetritus and microbes (Le

Fevre, et al., 1998).

Conclusion

This study indicates the importance of zooplankton as a major contributor to the export of carbon to the deep-sea in the Western Antarctic Peninsula. Euphausiids (krill) contribute most to this flux, especially during summer phytoplankton blooms. With the loss of sea-ice extent, and subsequent reduction in summer phytoplankton blooms due to climate change (Stammerjohn et al., 2008; Montes-Hugo et al., 2010; Ducklow et al., in press), this region is becoming less suitable for euphausiids, resulting in a shift in zooplankton population to gelatinous salps

(Quentin and Ross, 2001; Atkinson et al., 2004). Salps produce large, dense pellets that can export thousands of times more carbon than euphasiids pellets. If changes in this region persist, carbon flux may be enhanced, but more variable, with a possible increase in phytodetritus contribution to flux.

23 Acknowledgements

I am grateful to so many people who have helped, supported and believed in me through both this thesis and all of my academic pursuits and passions. First, I am extraordinarily grateful to Dr. Debbie Steinberg, for advising me in this honors thesis and as a mentor in furthering my passions in marine biology, and inspiring my love of zooplankton through her own ceaseless excitement for these critters. The incredible research opportunities and experiences she has given me are incomparable, and I am grateful every day for finding my way to her lab, as these experiences have truly shaped my future. I am also incredibly grateful to Dr. Grace Saba for bringing me into the VIMS zooplankton lab over two years ago to help with research for her PhD dissertation, starting me on this phenomenal path that has shaped my undergraduate career. I would also like to thank everyone I have worked with in the zooplankton lab; Kate Ruck, Lori Price, Dr. Stephanie Wilson, and especially Joe Cope who helped me learn many of the techniques for pellet analysis and so much more! I am also grateful to all of the PIs and support staff for the Palmer LTER, especially Dr. Hugh Ducklow and technician Matthew Erickson at Marine Biology Laboratory (MBL) in Woods Hole for the sediment trap samples and CHN data for total POC flux. I would like to thank the captain and crew of the R/V Laurence M. Gould that made the deployment and retrieval of samples possible. I would also like to thank Adrian G. “Casey” Duplantier Jr. for the private gift, matched by 1st Advantage Federal Credit Union of Newport News, making it possible for me to go on the PalLTER cruise. The Palmer LTER is made possible by a NSF grant. I would like to thank my committee members, Dr. Randy Chambers and Dr. Jonathan Allen. I would especially like to thank Dr. Chambers as he brought me into my first research experience at William and Mary, and has advised me in my undergraduate major. I would also like to thank Dr. Lizabeth Allison and Dr. Paul Heideman for supporting my decision to do a biology honors thesis and assist with any questions along the way. I would like to thank my family, especially my parents who have supported everything I have done to pursue my passion for marine science and truly letting me design my own future. I also am grateful to my wonderful friends for always being supportive, for everything from putting up with me excitedly showing them pictures of fecal pellets during lunch, to always encouraging and believing in me.

24 Tables

Table 1. Fecal pellet POC flux (mgC m-2d-1) for each collection bottle, including

opening and closing dates. Samples not analyzed for pellets indicated by an asterisk.

Table 2. Visual observation counts of obvious phytodetritus in samples (n = 72).

Table 3. Median and mean carbon/pellet values (µg/pellet) for different pellet shapes.

26 Figures

Figure 1. Schematic of the ‘biological pump’, described in Ducklow et al., 2001.

Figure 2. Left: Palmer Station Long Term Ecological Research (LTER) study region on the West Antarctic Peninsula (WAP) with sampling grid (black box) and labeled bathymetry. Right: Main sampling grid sampled each Jan. since 1993. Yellow cone denotes location of long-term sediment trap mooring (64º30’S, 66º00’W). Red circle indicates location of Palmer Station (USAP). Vertical dashed black line denotes the continental shelf. (Ducklow et al., 2008)

(a)

(b)

Figure 3. (a) Schematic diagram of the moored sediment trap (PARALUFX Mark 78H 21- sample trap). Trap is moored to a bottom weight with float at top; it is remotely released for collection and redeployment. Honeycomb baffle keeps out larger object that might clog the trap. Step motor is used to rotate bottles at set time intervals. (McLane Research Labs). (b) Photograph of the sediment trap being deployed. (www.vims.edu)

(a) Euphausiid (krill) b d

a 2 cm

(b) Copepod - Calanus

a d

b 5 mm

e 2 mm

c b

5 cm e

4 cm

Figure 4. Different zooplankton taxa produce different types of fecal pellets. Euphausiids produce long, cylindrical pellets (a), copepods produce small ovoid pellets (b), and salps produce big, loose, tabular-shaped pellets (c). Pellets colors indicate diet: dark brown are herbivorous (d), light brown are omnivores (e), and red/white/transparent are carnivores (f).

Figure 5. Fecal pellet POC flux (mgC m-2d-1) for each sample collected in the PalLTER sediment trap, located on the west Antarctic Peninsula shelf, from Jan. 2005 to Jan. 2009 indicated by black bars. The total POC flux for each sample is plotted with a line graph. For 2005, samples 7 and 8 were not analyzed for pellets as they were partially degraded. Also, for 2007, samples 9, and 11-21 were not analyzed for pellet because not enough material was available. Stars indicate samples not analyzed for pellets.

Figure 6. Fecal pellet POC flux for each month from Jan. 2005 – Jan. 2009. Pellet flux in summer months consist of average combined samples thus bar indicate mean flux ± stand error. Total POC in for each month is represented by the line graph. Stars indicate samples not analyzed for pellets.

Figure 7. Average fecal pellet POC flux for each season is indicated with bars for mean flux ± standard error. Mean total POC flux for seasons is indicated with a line graph. Summer values are means of samples collected during summer months October – April. Winter values are means of samples collected during winter months May – September. Due to samples unable to be analyzed for pellets, mean for Winter 2005 only includes May, August and September; Winter 2006 includes only May, July and August; Summer 2007/08 includes only Jan.-April. Independent t-test indicated significant difference in seasonal mean flux. A Univariate ANOVA found no significant difference between seasons for each year.

Figure 8. Monthly proportion of fecal pellets in total POC flux. Samples for July and Aug. 2008, and Jan. 2009 are outliers and removed in calculations. Stars indicate samples not analyzed for pellets.

(a) (b)

01/05/06-01/06/06 01/03/06-01/04/06

Figure 9. (a) Photograph of sample with obvious phytodetritus (white material). (b) Photograph of sample without phytodetritus. Both photos are taken on 10x magnification of initial sample.

Figure 11. Photograph from samples to show differences in pellet type, density, degradation and amount of phytodetritus throughout the year. All photos taken on 10x magnification of entire sample, with the following exceptions: “Late Summer”, 2006=1/256 split; “Summer”, 2007=1/32 split; “Winter”, 2007=1/16 split. Black arrows in graph indicate sample show in photographs below. Star in top left corner indicates samples with phytodetritus present. White arrows point to tabular shape pellets probably produce by salps.

(a) (b)

2005 2006

Figure 11. Fecal pellet flux for each pellet shape for (a) 2005, (b) 2006, (c) 2007, (d) 2008 sediment trap samples. Pellet flux in summer months consist of average combined samples thus bar indicate mean flux ± stand error. UNINOVA indicates significant difference in mean flux between pellet types with Tukey Post-Hoc indicating significance is from difference between mean flux for cylindrical and other pellet types. Test also found no significance between flux from different pellet types between years. Stars indicate samples not analyzed for pellets.

(a) (b)

2005 1.06 2006

Figure 12. Fecal pellet flux for ovoid and tabular/amorphous pellets for (a) 2005, (b) 2006, (c) 2007, (d) 2008 sediment trap samples. Pellet flux in summer months consist of average combined samples thus bar indicate mean flux ± stand error. UNINOVA indicates significant difference in mean flux between pellet types with Tukey Post-Hoc indicating no significance between ovoid and tabular/amorphous flux, but between cylindrical with each of these shapes. Test also found no significance between flux from different pellet types between years. Stars indicate samples not analyzed for pellets.

Figure 13. Correlation between ovoid pellet POC flux and cylindrical pellet POC flux.

Correlation was linear and significant. One outlier was removed before analysis. R2 =

0.255; n = 42; df = 40; p = 0.001.

Figure 14. Carbon/pellet size class frequency distribution for different pellet shapes in all samples. Pellet counts were normalized to 1000 pellets for each pellet shape ([number of pellets in a size class/total number of pellets] x 1000).

Figure 15. Fecal pellet POC flux categorized by zooplankton feeding method for each season. Observed pellet color indicated feeding method (dk. Brown = herbivore; lt. brown = omnivore; red/white/transparent = carnivore). Carbon flux is displayed as mean ± S.E.

Figure 16. Significant positive correlation between total pellet volume (mgm-2d-1) and total POC flux (measured from CHNs) for all samples (r2 = 0.891; n=69; p<0.001).

Figure 17. Significant positive correlation between total pellet volume (mgm-2d-1) for samples obvious without phytodetritus (mostly composed of pellets) and total POC flux (measured from CHNs) for all samples (r2 = 0.735; n=38; p<0.001). Pellet volume to carbon conversion factor = 0.018 calculated from slope = 54.958C-1.

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