Download High concentrations of marine snow and diatom algal mats in the

High concentrations of marine snow and diatom algal mats in the North Pacific
Subtropical Gyre: implications for carbon and nitrogen cycles in the oligotrophic
ocean
C. H. Pilskalna*, C. H., T. A.Villarealb, M. Dennettc, C. Darkangelo-Woodd, and G.
Meadowse
aBigelow
Laboratory for Ocean Sciences, 180 McKown Point Road, West Boothbay
Harbor, ME 04575, USA
bMarine
Science Institute, University of Texas at Austin, 750 Channel View Drive,
Port Aransas, Texas 78373, USA
cDepartment
of Biology, Woods Hole Oceanographic Inst., Woods Hole, MA 02543,
USA
dUniversity
e School
of Maine, School of Marine Sciences, Orono, ME 04473 USA
of Naval Architecture and Marine Engineering, College of Engineering,
University of Michigan, Ann Arbor, MI 48109, USA
*Corresponding author. Tel: 207-633-9668; fax: 207-633-9641
E-mail address: [email protected] (C. Pilskaln)
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Abstract
A Video Plankton Recorder (VPR) and remotely operated vehicle (ROV) were
utilized on three cruises in the oligotrophic North Pacific Subtropical Gyre (NPSG)
between 1995 and 2002 to quantify the size and abundance of marine snow and
Rhizosolenia diatom mats within the upper 305 m of the water column. Quantitative
image analysis of video collected by the VPR and an ROV-mounted particle imaging
system provides the first transect of marine snow size and abundance across the central
North Pacific gyre extending from 920 km NW of Oahu to 555 km off Southern
California. Snow abundance in the upper 55 m was surprisingly high for this
oligotrophic region, with peak values of 6.0-13.0 x 103 aggregates m-3 at the western and
eastern-most stations. At stations located in the middle of the transect (farthest from HI
and CA), upper water column snow abundance displayed values of ~0.5-1.0 x 103
aggregates m-3. VPR and ROV imagery also provided in-situ documentation of the
presence of nitrogen-transporting, vertically migrating Rhizosolenia mats from the
surface to >300 m with mat abundances ranging from 0-10 mats m-3. There was clear
evidence that Rhizosolenia mats commonly reach sub-nutricline depths. The mats were
noted to be a common feature in the North Pacific gyre, with the lower salinity edge of
the California Current appearing to be the easternmost extent of their oceanic
distribution. Based on ROV observations at depth, flux by large (>1.5 cm) mats is
revised upward 4.5 fold, yielding an average value of 40 µmol N m-2 d-1, a value equaling
previous estimates that included much smaller mats visible only to towed optical systems.
Our results suggest that the occurrence across a broad region of the NPSG of particulate
organic matter (POM) production events represented by high concentrations of
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Rhizosolenia mats, associated mesozooplankton, and abundant detrital marine aggregates
may represent significant stochastic components in the overall carbon, nitrogen and silica
budgets of the oligotrophic subtropical gyre. Likewise, their presence has important
implications for the proposed climate-driven, ecosystem reorganization or domain shift
occurring in the NPSG.
Subject keywords and regional index terms: Biogeochemical cycles, carbon cycle,
nitrogen cycle, particle flux, North Pacific Subtropical Gyre, 24-32o N; 168-123o W
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1. Background
Fast-settling marine aggregates are believed to be responsible for the export of
particulate organic carbon and nitrogen from the surface mixed layer to the deep ocean
interior, with aggregate size, density, and numerical abundance being the parameters
most important to determining their flux through the water column (Shanks and Trent,
1980; Honjo et al., 1984; McCave, 1984; Asper, 1987; Alldredge and Gotschalk, 1988).
Their abundance and size frequency are indicative of the trophic structure of the
planktonic consumer community and thus the pathway of exported and remineralized
carbon through the marine system (Michaels and Silver, 1988; Karl, 1999). Additionally,
marine aggregates are mid-water environments where elevated rates of microbial and
macro-zooplankton-mediated organic matter regeneration may occur (Alldredge et al.,
1986; Caron et al., 1986; Alldredge and Silver, 1988; Simon et al., 1990; Smith et al.,
1992; Steinberg et al., 1994). However, data on the size distribution and abundance of
marine aggregated material from the largest marine biome on earth—the expansive,
open-ocean subtropical gyres of the world ocean--are severely lacking. To date, the
focus of ‘marine snow’ studies has been primarily concentrated on continental margin
settings or relatively productive pelagic systems such as the Panama Basin, the
mesotrophic North Atlantic, the Ross Sea, the Equatorial Pacific, or the Black Sea
(Gardner and Walsh, 1990; Asper et al., 1992; Lampitt et al., 1993; MacIntyre et al.,
1995; Pilskaln et al., 1998; Diercks and Asper, 1997; Asper and Smith, 1999).
Subtropical gyres such as the North Pacific are crucial regions for understanding the
role of the ocean in controlling atmospheric CO2 levels because of their global
dominance and high gross production and export of biogenic carbon (Emerson et al.,
1997; 2001; Karl, 1999; Karl et al., 2001; 2002). Both US JGOFS time-series programs
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(Hawaii Ocean Time-series Study, HOTS; Bermuda Atlantic Time-series Study, BATS)
have revealed the importance of the subtropics in regulating global CO2 via the solubility
and biological carbon pumps and documented their high sensitivity to climate change
(Karl et al., 1997; 2001; Michaels et al., 2001).
François et al. (2002a, b) recently demonstrated that the fraction of organic carbon
exported from the surface that reaches the bathypelagic depths (defined as the transfer
efficiency, Teff) is controlled by a complex interplay of particle properties and
biochemical processes occurring in the 100-1000 m mesopelagic “remineralization” zone
(Buesseler, 1998; Armstrong et al., 2002; François et al., 2002a, b). It is within this
region of the ocean interior that a marked attenuation of 80-97% of the particulate
organic matter flux occurs (Suess, 1980; Martin et al., 1987; Pace et al., 1987; Lampitt
and Antia, 1997). Particle size distribution, packaging morphology (e.g., fecal pellets,
algal aggregates, or marine snow), sinking rate and mineral ballasting, coupled with the
activity of mesopelagic microbial and zooplankton communities, ultimately determine the
latitudinal variation observed in particle remineralization length scales and thus the
efficiency of carbon transfer to the bathypelagic zone (Boyd and Newton, 1999). The
subtropical gyres display an intermediate carbon transfer efficiency between low-latitude,
productive regions (highTeff) such as the Arabian Sea or the equatorial upwelling zones,
and high latitude, diatom-dominated regions (low Teff; François et al., 2002a, b).
Therefore, it is of particular interest to examine the characteristics and abundance of large
(~300 m->1 mm) particle aggregates to assess their contribution to the ecosystem
structure, nutrient cycling and nutrient delivery within the vast subtropical gyre regions.
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In addition to marine snow, another class of large particle aggregates that play an
important role in nutrient transfer in the warm, subtropical gyres are macroscopic, multispecies associations of Rhizosolenia spp. diatoms, which may reach up to 10’s of cm in
size (Carpenter et al., 1977; Alldredge and Silver, 1982; Villareal and Carpenter, 1989).
Termed mats, these Rhizosolenia diatom associations exhibit buoyancy-regulated
migration to depth for nutrient acquisition (Villareal et al., 1993; 1999). Rhizosolenia
mats are particularly abundant in the NPSG, where they migrate between deep nutrient
pools (below 80-100 m) for nitrate uptake and the surface for photosynthesis (Villareal et
al., 1993; 1999). The migration corresponds to a biological transport of nitrate (new
nitrogen) across the deep chlorophyll maximum and provides a source of new production
in the surface mixed layer. Although large mats can be seen on the surface, their fragility
and relative scarcity (in #’s m-3) have required SCUBA divers for collection and
enumeration (Alldredge and Silver, 1982). As a consequence, investigators have rarely
included them in studies of open ocean biogeochemistry. Models suggest that most mats
should occur below diver-accessible depths (Richardson et al., 1998), but the requirement
for SCUBA collections creates inherent depth and time restrictions on observations.
Villareal et al. (1999) provided the first quantitative observations of mats at subnutricline depths in the central North Pacific, resulting in a revised upward nitrate
transport term representing over 50% of the export production (Villareal et al., 1999).
However, a significant unanswered question remains: how patchy or spatially extensive is
the occurrence of vertically migrating Rhizosolenia mats in the NPGS?
2. Methods
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Particle aggregates such as marine snow and algal mats are porous and fragile and
thus extremely difficult to study quantitatively with water bottles, filtration systems, or
nets. The additional challenge of sampling sufficient water to collect these large, rare
aggregates effectively in oligotrophic systems has biased most studies towards high
particle concentration, non-open ocean environments. The use of non-destructive, in-situ
water column imaging techniques as we have employed in the present study allows for
the collection of marine aggregate size and abundance data over spatially extensive
oceanic regions within the limited time period of a research cruise.
To quantify marine snow and Rhizosolenia mat distributions and abundance in the
subtropical central North Pacific, we conducted three 1-month cruises from Hawaii to
California or Hawaii to Hawaii during the summer months (June, July, August) of 1995,
1996 and 2002 (Fig. 1). In 1995, ROV malfunctions allowed us to complete only 2
stations at ~25o N, 166o W, with an ROV-mounted, in-situ particle imaging system; in
1996, we completed 10 in-situ particle imaging stations along ~30-32o N between ~158
and 123o W using a video plankton recorder or VPR (Fig. 2; Davis et al., 1992a, b;
Darkangelo, unpubl.). In 2002, a Benthos, Inc. Open-Frame ROV (operated by the
University of Michigan’s School of Marine Engineering) equipped with a Pulnix color
video camera was deployed at 10 stations to collect and document Rhizosolenia mat
occurrence well below diver-accessible depths within the 28-32o N latitudinal zone from
159o W to 124o W (Fig. 1 and 2). No quantitative marine snow imagery was obtained in
2002.
2.1. Acquisition of marine snow imagery and post-cruise analysis
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2.1.1. 1995 data set
A structured light and camera system identical to that used by Pilskaln et al. (1998) in
Monterey Bay was mounted to a small ROV (Hydrobot 2000, Hydrobotics Engineering
Canada, Inc.) enabling a high resolution monochrome video camera to image a 3.2 liter
portion (13.9 x 12.4 x 18.5 cm) of the structured light beam located 1 m from the camera
dome and at a 90o angle to the camera view (Fig. 1a). The real-time video is transferred
through the ROV tether to the ship, where it was recorded in S-VHS format. The ROV
was deployed in 1995 to pre-specified survey depths at 20 m intervals between 10 and
180 m. At each survey depth, particle video was collected as the ROV traveled
horizontally for 10 minutes at a speed of approximately 1 knot. Post-cruise, the
structured light video was sub-sampled every 2 seconds, digitized, and quantitatively
analyzed for snow size and abundance within the particle size range of >0.5 mm to >5
mm (Davis and Pilskaln, 1992; Pilskaln et al., 1991; 1998). Total water volume analyzed
for each survey depth was approximately 960 liters.
2.1.2. 1996 data set
We employed a VPR system consisting of a video camera and synchronized red strobe
light source, a video recorder and a CTD (Fig. 2b). The CCD (charged coupled device)
camera on the VPR is synchronized at 60 fields per second (fps) to a xenon strobe (600
nm), and the camera is positioned to face the strobe flash at an oblique angle (Davis et al.,
1992a, b). The intersection of strobe light volume and the camera’s field of view
represents an individual image volume of 4.8 cm x 4.5 cm x 3.6 cm (approximately 0.08
liters). Video plankton recorders have been previously used to collect video imagery for
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the quantitative analysis of plankton and marine particulates (Davis et al., 1992a, b; Davis
et al., 1996; Villareal et al., 1999; Dennett et al., 2002). All components are mounted on
a frame that was tow-yo’d up and down from the ship’s hydrowire at 12 m min-1 for four
round-trips between the surface and 150 m with a vessel speed of approximately 1-2
knots. During the tow-yo’s, VPR imagery is recorded internally on Hi-8 video format
and converted to digital format in the lab following the cruise. The 1996 continuous VPR
imagery was sub-sampled post-cruise on a 2-second interval, digitized, and quantitatively
analyzed for snow size and abundance by the protocols of Pilskaln et al. (1998). The
aggregate size and abundance data was averaged and binned into 10 m depth intervals
between 0 and 150. A mean volume of 16 liters per 10 m depth interval was analyzed for
marine snow size and abundance data from the VPR video.
Images selected for quantitative analysis were reviewed for the presence of micro- and
macro-zooplankton to ensure that organisms were not included in the marine snow
aggregate counts.
The quantitative image analysis procedure is detailed in Pilskaln et al. (1998). The
system consists of a high-end VCR with dynamic frame-by-frame tracking ability, an
Overlay Frame Grabber (OFG) from Imaging Technology Inc., PC, and a Windowsbased, quantitative image analysis software package called OPTIMAS. The OFG,
interfaced to the PC, exports digitized images into the image processing program.
2.2. Rhizosolenia mat imaging and post-cruise analysis
2.2.1. 1996 data set
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For each station, the complete VPR tow-yo series videotape was reviewed post-cruise
to document Rhizosolenia mat occurrence. Based on the tow-yo speed of the VPR (equal
to ~12 m/minute wire out and in) and the dimensions of the individual image volume,
new water volume was be imaged approximately every 0.25 seconds. From this
information, we estimate that the total VPR-imaged water volume that we reviewed for
mat occurrence over each 10 m depth interval per complete tow-yo series at a station was
127 liters. The water volume imaged by the VPR at each station was 4-5.3 m3 (Villareal
et al., 1999). Mats, appearing as large, well-defined bright objects within a dark field,
were identified by their distinctive morphology consisting of intertwining diatom chains
forming macroscopic aggregations up to several centimeters in diameter (Villareal and
Carpenter, 1989; Villareal et al., 1996; 1999). Size and depth was noted for each
Rhizosolenia mat sighting. The OPTIMAS-based image processing system described
above was used to determine mat sizes. Rhizosolenia mats within the near-surface waters
were also enumerated by SCUBA divers at the stations using a 1 m2 frame equipped with
a General Oceanics flowmeter bearing a slow speed rotor (Trent et al., 1978; Villareal, et
al., 1996; 1999). The frame was swum in a circle (9 m diameter) at 6 depths between the
surface and 18.3 m (Villareal, et al., 1996; 1999).
2.2.2. 2002 data set
To document Rhizosolenia mats throughout the upper 305 m of the water column in
the central North Pacific, we deployed at eleven stations a mid-sized ROV (Benthos, Inc.,
Open Frame vehicle equipped with a Pulnix low-light color camera) operated through a
1000 m tether. For each ROV dive, we deployed the vehicle to a maximum depth
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between 100 and 305 m and then slowly ascended back to the surface, recording the
hours of observation per dive. When a mat was seen, the depth and approximate size
were noted by scale bars placed within the field of view of the camera. Diffuse lighting
coupled with the lack of in-water field–of-view calibration prevented the determination
of the total water volume surveyed by the ROV.
2.3. TEP analyses
In 1996 we examined 23 individual diver-collected Rhizosolenia mats for the presence
of transparent exopolymer polysaccharide particles (TEP) following the Alcian Blue
staining and colorometric analysis methods detailed in Passow and Alldredge (1995) and
in Engel and Passow (2001). Mats were gently filtered onto 0.4 m pore size
polycarbonate filters, stained with pre-filtered, 0.02% aqueous Alcian Blue solution in
0.06% acetic acid, rinsed with distilled water, and kept refrigerated until their return to
the laboratory for analysis (Passow and Alldredge, 1995). Filters stained with the Alcian
Blue solution were used as stained filter blanks and replicate analyses were completed on
the majority of the samples. Stain adsorption is linearly related to the amount of
polysaccharide present in the samples and is measured with a spectrophotometer (Passow
and Alldredge, 1995).
2.4. Hydrographic data
Hydrographic and transmissometer casts were conducted within 1-2 hrs directly
before or after all marine aggregate imaging surveys (Brzezinski et al., 1998; Villareal et
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al., 1999) on all cruises. Data from the 1996 VPR-mounted CTD was time-synchronized
to the video imagery post-cruise.
3. Results
3.1. Marine snow
Total marine snow abundance and size distributions at the two 1995 and ten 1996
stations represent an eastward transect along across the central North Pacific from Hawaii
to California as shown in Figure 3. High snow abundance values (up to 13 x 103
aggregates m-3) within the upper 75 m were observed at the western-most stations near
the Hawaiian Islands, and at the most eastern station within the California Current (Fig.
3). Below 75 m, these values decrease by a factor of 2-3 (Fig. 3). The dominant
aggregate size class was 0.5 – 1.0 mm at all stations (Fig. 3). The 1.0 – 2.0 mm class was
of secondary importance with the presence of 2.0 – 3.0 mm size snow particles being
relatively scarce and primarily limited to the 1995 data set (Fig. 3). Marine snow of
greater than 3 mm in size was extremely rare at all sites. Plots of marine snow particle
size spectra (Fig. 4) defined as the number of particles within a specific size range
divided by the extent of the size range and the volume sampled (Jackson et al., 1997)
provide additional detail about the marine snow size spectrum. The size spectra for the
1995 stations located within the Hawaiian Island chain (Fig. 4a) reveal a wider range in
particle diameter than the 1996 stations. This may be a reflection of the difference in the
1995 and 1996 particle imaging systems wherein a larger water volume was imaged in
1995 as compared with 1996. McCave (1984) and Jackson et al. (1997) have shown that
one of the most important constraints on the maximum particle size for which a particle
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size spectrum can be determined is sample volume. This becomes particularly important
when particle size spectra are obtained and compared from simultaneous deployments of
multiple instruments (Jackson et al., 1997). Spatial and temporal variability between our
1995 and 1996 stations where the marine snow size data were collected is also a
significant factor to be considered in the observed differences in the particle size spectra.
Particle size spectra for the 1996 stations are presented as three groups in Fig. 4b-d
based on the clustering and slope of the spectral plots for all depths at the stations. Fig.
4b shows the particle spectra for the 1996 stations 3-5 that were located immediately
north of the Hawaiian Island chain, and Fig. 4c displays the spectral plots for the 1996
stations 6-11 located farther west within the subtropical gyre (Fig. 1). The major
differences between the latter two spectral plot groupings are that stations 3-5 (Fig. 4b)
show higher particle abundance at all depths (e.g., spectral lines shifted higher) and the
presence of larger diameter particles (e.g., occurrence of particles within larger particle
size classes) as compared to stations 6-11 (Fig. 4c). Our interpretation of this variability
is that particle size and abundance properties at stations 3-5 was likely affected by the
stations’ closer proximity to the Hawaiian Islands, where biological particle production
and detritus abundance would be expected to be greater than in the mid-gyre regions.
The final size spectra group is represented by 1996 station 12 (Fig. 4d) located within
the California Current off the coast of Southern California (Fig. 1) and displays a
separation of the data into two spectral slope groups. The upper three spectral slopes are
from data collected between 45 and 65 m, whereas the spectral slopes for all other depths
are clustered below (Fig. 4d). At this station, the aggregate maximum between 45 and 65
m was associated with large subsurface peaks in fluorescence and cp (particle beam
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attenuation), suggesting that algal-rich, marine snow particles were forming in the
subsurface fluorescence maximum (Fig. 5). Such particles are represented by the upper
three spectral slopes in 4d. Above and below the 45-65 m aggregate/fluorescence peaks,
the slopes of the particle size spectra (Fig. 5) and the plot of abundance and size (Fig. 3)
indicate fewer and smaller particles. Either the large, algal-rich particles associated with
the fluorescence peak physically disaggregated as they sank below 65 m, or (more likely)
they were rapidly consumed by grazers. Interestingly, Dennett et al. (2002) showed an
abundance peak in colonial radiolaria also centered between 45 and 65 m at station 12
based on the analysis of extracted 1996 VPR images.
In contrast to the California Current station 12, several of the 1996 western stations
(3 and 5) displayed marine snow abundance maxima centered within or at the base of a
defined pycnocline where no fluorescence peak was observed (Fig. 5). The association
of the pycnocline with an accumulation of porous particulates has been documented in
previous studies and is believed to result from shear-induced particle collision and
aggregation or changes in aggregate sinking rate (Jackson, 1990; MacIntyre et al., 1995;
Pilskaln et al., 1998). Large aggregate abundance peaks of 5–12 x 103 aggregates m-3
observed in the upper water column at stations 3 and 5 in the absence of an associated
chlorophyll fluorescence peak suggests that the aggregates were not rich in fresh algal
material. Secondary aggregate abundance maxima at stations 3 and 5 were seen between
90–110 m and were coincident with deep fluorescence peaks (Fig. 5). It is noteworthy
that the aggregate size spectra from the depths centered on the deep fluorescence peaks at
stations 3, 5 and 12 indicate the presence of particles in the larger and less common snow
size classes between 2.0 and 4.0 mm (Fig. 3 and 5).
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3.2. Rhizosolenia Mats
Mats were noted over the entire depth range imaged by the 1996 VPR (0-150 m) and
in 2002 by the ROV (0-305 m). The video imagery clearly displays the characteristic
structural and textural qualities of the mats including the typical occurrence of the 4-5
species of larger Rhizosolenia imbedded as chains in a matrix consisting of the smaller
diameter R. fallax (Fig. 6). Fig 6a captures the speckled appearance commonly found in
mats dominated by R. acuminata. This species has short chains (2-4 cells) of only a few
mm in length whereas the other mats have rigid chains of cm-scale length that appear as
elongate rods in Fig. 6b-d. In the 1996 VPR imagery, mats were observed at only the
three western-most stations (stations 3-5. Fig. 1; Villareal et al., 1999); in 2002 we
observed mats with the ROV throughout the water column at ten out of 11 stations. A
frequency distribution with depth of the total number of mats counted from the VPR and
ROV imagery documents a trend of elevated mat presence in the upper 60-70 m and a
general decrease with depth (Fig. 7a).
The cumulative count of mats from the bottom survey depths of the 1996 VPR tows
and 2002 ROV dives demonstrates that 84% of the total mats observed were below the
diver-accessible depth of approximately 20 m (Fig. 7b). Over 50% of the mats
documented on the 1996 VPR video were <1.5 cm in size (longest dimension), which is
approximately the minimal mat size observed by SCUBA divers when completing counts
and collections between 0 and 20 m (Villareal et al., 1999). VPR-viewed mats displayed
average dimensions of 1.7 cm (length) x 1.0 cm (width), yielding a volume calculation of
2.3 ml assuming a cylindrical shape. These mats were much smaller than the reported
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average mat lengths of 3.5 cm of diver-surveyed mats (Table 1; Villareal and Carpenter,
1989; Villareal et al., 1996; 1999). Rhizosolenia mats counted from the 2002 ROV video
were also larger, averaging 2.9 cm in the longest dimension (Table 1).
In both 1996 and 2002, Rhizosolenia mats were observed at sub-nutricline depths (Fig.
8a). In 2002, mats were noted on all ROV dives at depths greater than the 1 m nitrate
depth (Fig. 8b). No mats were observed by the ROV at Sta. 16 where a small number of
mats were seen and collected by divers in the upper 20 m.
We measured a wide range of Rhizosolenia mat-associated TEP concentrations from
124 to 35500 g Xanthan Equivalent l-1 (g Xeq l-1) with a mean of 4000 g Xeq (+1600
g Xeq l-1 standard error). The unit relates the amount of adsorbed Alcian Blue to the
equivalent weight of polysaccharide Gum Xanthan used as a standard (Passow and
Alldredge, 1995; Engel and Passow, 2001). Our TEP values fell within the range of
previously reported TEP values obtained in mesocosm experiments using phytoplankton
cultures and from natural diatom assemblages collected in coastal, inland sea, and openocean Atlantic regions (Engel and Passow, 2001; Passow, 2002; Engel, 2004). Our
calculated median TEP value of 1500 g Xeq l-1 indicates that a significant portion of the
samples displayed very high TEP concentrations similar to values obtained from more
productive vs. oligotrophic systems (Engel and Passow, 2001; Engel, 2004).
4. Discussion
4.1. Significance of marine snow and Rhizosolenia mats to biogeochemical cycling in
the NPSG
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The relatively high abundance of > 5 mm-sized marine snow particles at various
stations within this oligotrophic region was not expected. Abundances of 5-10 x 103
aggregates m-3 at the two 1995 stations and the three western-most 1996 stations were
similar to those reported for productive pelagic systems such as the Equatorial Pacific
(Diercks and Asper, 1997) or the Northeast Atlantic during the spring/summer bloom
period (Lampitt et al. 1993). The values reported here are also surprisingly comparable
to marine snow abundances reported from a deep coastal upwelling system (Pilskaln et
al., 1998). Our observations from the NPSG leave us with several provocative questions:
What are the primary sources of detrital and algal particles contributing to the formation
of a substantial volume of marine snow in the oligotrophic, sub-tropical gyre which is
assumed to be dominated largely by a picoplankton-supported microbial food web?
Additionally and more importantly, what are the implications of the observed high
abundances of marine snow and diatom mats for particulate carbon and silica
remineralization and transfer efficiency in this particular region?
The size distribution of primary producers and the consumer trophic structure
determine the composition and the magnitude of carbon export, and thus the observed
biological community associations are important to modeling the fate of carbon (and
silica) in different systems (Michaels and Silver, 1988; Karl, 1999). Marine snow
aggregates are mid-water microenvironments or patches of enriched concentrations of
organic and inorganic nutrients (Alldredge and Silver, 1988). Our observations of a
sizeable abundance of >0.5 mm-sized marine snow particles with a distribution showing
a general loss of larger particle classes with depth (i.e., Fig. 3) suggests the presence of a
grazer/flux feeding/particle-mining food web (Karl, 1999; Stemmann et al., 2004). A
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scenario of large particle production would in turn suggest enhanced POM export
compared to that resulting from the microbial food chain (Michaels and Silver, 1988;
Peinert et al., 1989; Karl, 1999).
Although we have only qualitative observations, we noted in 1996 and 2002 that the
occurrence of high numbers of Rhizosolenia mats was coincident with diver and
VPR/ROV observations of abundant macrozooplankton such as salps and ctenophores as
compared to stations where the mats were rare. We also noted a fairly consistent
association of 1-2 harpactacoid copepods per mat in the 0-20 m diver-collected mats
(Pilskaln and Villareal, unpublished). Similar associations of mesozooplankton with
Rhizosolenia mats have been documented in a previous study by Carpenter et al. (1977).
Dennett et al. (2002) reported a high sub-surface abundance of colonial radiolarians at the
three western-most 1996 stations, as well as the easternmost station, where maximum mat
and aggregate abundance values were likewise obtained. Thus, there appears to be
regions of enhanced protozoan and zooplankton activity associated with areas of elevated
Rhizosolenia mat abundance in the NPSG, providing a source of particulate organic
material for the formation of marine snow aggregates. An additional source of POM in
the NPSG that could lead to an increase in the formation of detrital aggregates is the
occurrence of summer blooms of the cyanobacterium Trichodesmium and the symbiontcontaining Rhizosolenia spp. and Hemialus spp. diatoms (Wilson, 2003; Letelier et al.,
2004; Montoya et al., 2004). The N source for these blooms may be nitrogen fixation by
either Trichodesmium or the nitrogen-fixing symbiont Richelia found associated with
Rhizosolenia and Hemiaulus diatoms, or an influx of deep N via vertical migrations of
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Rhizosolenia mats between the deep nutricline and the surface (Venrick, 1974; Karl et al.,
1997; Karl, 1999; Wilson, 2003; Letelier et al., 2004; Montoya et al., 2004).
An abundance of large detrital particles suggests that higher particle flocculation rates
exist in the NPSG than what would be predicted from classic coagulation theory applied
to a low, suspended-particle density environment (Peinert et al., 1989; Jackson, 1990;
Hill, 1992; Karl et al., 2001). Our TEP analyses of diver-collected mats indicates that
TEP production can be significant during blooms of Rhizosolenia as has been observed in
blooms of other large diatoms (Passow, 2002; Engel, 2004). Interestingly, our
Rhizosolenia TEP concentrations were very similar to values reported for coastal and
inland sea regions such as the Baltic Sea as compared to that reported for open-ocean
regions such as the northeast Atlantic (Engel and Passow, 2001; Passow, 2002; Engel,
2004). Substantial TEP production in areas of abundant Rhizosolenia mats would
increase the total volume of particles, thus enhancing the probability that larger sinking
particle aggregates could form and transport POC and biogenic silica to the deep ocean
interior. We suggest that the sufficient potential for large seasonal pulses in the NPSG of
carbon- and silica-rich Rhizosolenia-rich particle aggregates, either through TEP-initiated
flocculation of mats following summer bloom periods or through the consumption of
mats by vertically large migrating predators, needs to be considered in the overall carbon
and silica budgets of the region. Ship et al. (1999) reported a revised estimate of NPSG
silica production by Rhizosolenia mats within the upper 150 m of 317 mol Si m-2 d-1
based on VPR-collected abundance data. This daily production value represents almost
one-third the rate of the entire non-mat diatom assemblages in the central North Pacific
and is indicative of the substantial but previously overlooked impact that mats may have
19
on the global silica budget (Shipe et al. 1999). The export of mats may also play an
important role in the accumulation of biogenic silica in abyssal sediments as evidenced
by Smith et al. (1996) who reported Rhizosolenia spp. as components of extensive
phytodetritus deposits in the Pacific.
Annual estimates of the sub-euphotic export of particulate organic and inorganic
material in the NPSG are expectedly low as represented by the data sets from the Hawaii
Ocean Time Time-series (HOT) station ALOHA located at 22o 45’ N, 158o W (Karl et
al., 1996). Sub-euphotic particulate carbon export measured at 150 m is 6.7% (5 year
mean) of contemporaneous primary production, although strong interannual variability is
observed in production and export (Karl et al., 1996). To date, there have been no
observations of abundant Rhizosolenia mats or marine snow noted at the HOT site as
have been observed farther north at ~30o N (Villareal et al., 1996; Villareal et al., 1999).
Considering the significant impact that Rhizosolenia mats and aggregates may have on
the export and cycling of N, C, and Si in the NPSG, it would be of interest to quantify
their occurrence at ALOHA and thus reassess how representative the ALOHA site is of
the open Pacific Ocean.
4.2. Deep nitricline mat observations: New implications for upward nitrogen transport
The surface distribution of Rhizosolenia mats agrees reasonably well with the model
prediction of 22% of the particulate N in mats being in the upper 10 m (Richardson et al.,
1998), but the prediction of subsurface secondary maxima is not supported. This is
probably due to the boundary conditions of the model which do not allow mats to sink
farther than 20 m (no mat counts existed below this depth prior to 1996). In 1996, our
20
VPR data from stations east of Hawaii revealed measurable mat abundance at and below
the 80-140 m nitricline (Fig. 8; Villareal et al., 1999). Comparison of the 1996 VPRdetermined mat abundance with SCUBA diver counts between 0 and 20 m on the same
cruise revealed that the relatively small (<1.5-2.0 cm) mats documented from the surface
to 150 m are 5-10 times more abundant than the larger (3-4 cm) mats observed with
SCUBA (Villareal et al., 1999). The explanation for the differences in the data obtained
by the two techniques is that the SCUBA method was biased towards observing and
counting the obvious, large mats. The smaller mats are simply not seen because of their
small size and poor contrast with the surrounding water (Villareal et al., 1999). The
difference we noted between the measured size ranges of the mats observed by SCUBA
(all >2.5 cm in longest dimension) vs. by the VPR (60% <1.5 cm) supports this
explanation. Divers rarely report mats <1 cm in size (Villareal and Carpenter, 1989;
Villareal et al., 1999), and thus VPR-documented mats represent an additional population
of smaller mats not quantified in previous SCUBA-based studies.
The significance of “new” observations and quantification of smaller mats at depth is
that the data allowed a recalculation of the upward flux of nitrate provided by
Rhizosolenia mats. Using the ratio of VPR small mat/diver large mat abundance and
nitrogen transport integrated over 150 m and averaging over seven cruises, Villareal et al.
(1999) provided an average upward transport of new nitrogen due to migrating
Rhizosolenia mats of 40 + 28 mol N m-2 day-1. Previously, Villareal et al. (1996)
reported a calculated range of mat-mediated N transport in the oligotrophic ocean of 3.940 mol N m-2 day-1 based solely on diver-based mat counts between 0-20 m. However,
the higher value was derived using maximum surface mat accumulation data and lacked
21
validation from mat abundance estimates through the water column (Villareal et al.,
1996). Deployment of the VPR to nutricline depths provided the validation for the higher
upward N fluxes with mat counts from the surface to 150 m and the documentation of a
small size class of mats that had been previously missed by divers.
In this paper our ROV and VPR observations confirm the widespread distribution of
mats at depths of greater than 20 m thus confirming a key element to the migration model
and providing the means to verify some assumptions in previous N flux calculations.
Villareal et al. (1999) were necessarily limited to diver observations in the upper 20 m for
the large size class of Rhizosolenia mats. If we use the ROV counts of large mats to scale
the 0-20 m diver estimates, we note that 4.5 times as many large mats were observed
below 20 m as above. Applying this correction to the estimates of nitrogen transport via
mats presented in Villareal et al. (1999) yields the net effect of large mat transport
increasing to equal the previously reported total large + small mat transport of 40 µmol N
m-2 d-1. The calculated N transport by small mats alone of 10 µmol N m-2 d-1 (Villareal et
al., 1999) may be superimposed on the large mat value to yield a higher total matmediated nitrogen transport value of 50 mol N m-2 day-1, taking into account the
uncertainties in the overlap between the mat observations made by the two optical
systems. For comparison, the representative turbulent nitrate flux is estimated at 200
mol N m-2 day-1 (Richardson et al., 1998) indicating that the mat-mediated new nitrogen
input of 40 mol N m-2 day-1 is on average 20% of the turbulent input but may be as high
as 75% at some locations (Villareal et al., 1999). It should be noted that mat N transport
directly imports new nitrogen into the mixed layer and thus the comparison to turbulent
fluxes is not straightforward as the mat-transported nitrate is captured deep in the
22
euphotic zone and cannot diffuse into the mixed layer. As a result of our mat
observations completed over several years with different optical systems and SCUBA, we
can say with greater certainty that the occurrence of smaller mats is patchier than that of
the larger sized mats. Based on our current observations, we conclude that large mats are
abundant at depth, and that our estimates of upward N transport are in the 40 mol N m-2
day-1 range just due to large, readily visible mats.
It is noteworthy that the small mat class was observed in 1996 with the VPR in the
same zone where large, late summer chlorophyll blooms persisting over several months
have been recently documented by SeaWiFS imagery (Wilson, 2003), but were absent
from observations further to the east where such blooms are less frequent. If small mats
are as patchy as our VPR observations suggest, then their flux, superimposed on a
background of large mat flux, could provide the episodic nitrogen input required by
Wilson’s model to sustain the satellite-observed blooms.
Much like the migration of Trichodesmium to depth (Villareal and Carpenter, 2003),
the depth of nutrient acquisition is an important physiological constraint on the migration
model. An overlay of our 2002 ROV observations on the nitrate data clearly shows mats
present at the top of the nutricline at ~80-100 m and at sub-nitricline depths along a
station transect similar to that of 1996 (Fig. 1 and 9). In six instances (Sta. 6, 8, 10, 13,
14, 16 and 17) mats were not observed at the maximum ROV survey depth. These data
suggest that mats do not migrate far into the nutricline and, at some stations, may not
have a significant presence in the nitricline at all. Observations must be extended deeper
into the nutricline to resolve this question.
23
The cruise transects highlight the widespread occurrence of Rhizosolenia mats in the
NPSG. To date, mats have been observed at every station occupied between 25° and 32°
N and as far west as 175.4° E. The ROV observations indicate that the eastern boundary
of mat distribution is linked to the lower salinity characteristic of the seaward influence
of the California Current system (Fig. 9). This general longitudinal boundary has been
noted in other cruises as well (Villareal, unpublished observations). Rhizosolenia mats
are not an unusual feature of the NPSG, merely one that requires specialized sampling to
reliably detect their presence.
4.3. NPSG ecosystem domain shift and the role of rare particle aggregations
Historically, the subtropical gyres have been considered to be homogenous,
oligotrophic deserts with biomass production being severely limited by available
inorganic nitrogen or nitrogen and phosphorus combined (Eppley et al., 1973; 1977; Karl,
1999; Karl et al., 2001). However, it has been shown relatively recently that they
actually represent dynamic ecosystems characterized by strong seasonal, interannual, and
even decadal variability that is highly sensitive to climate change (Karl, 1999; Dore et al.,
2002; Karl et al. 2002; Neuer et al., 2002; Letelier et al., 2004). A climate-driven,
ecosystem reorganization or domain shift in the NPSG has been proposed with important
implications for biogeochemical cycling in all subtropical gyres (Karl, 1999; Karl et al.,
2001). Observed increases in water column stratification over several decades and
decreased inorganic nutrient availability in the NPSG are proposed as the driving
mechanisms behind the hypothesized shift in the phytoplankton community structure
toward an ecosystem dominated by prokaryotes vs. eukaryotic photoautotrophic
populations. Karl et al. (2001) clearly describe the cascading effect on oceanic
24
ecosystem structure and trophic interactions predicted to result from continued warming,
enhanced stratification of the upper water column, and low nutrient availability. Under
this scenario, selective pressure would favor organisms such as picoplankton capable of
growing at reduced nutrient concentrations and bacteria with the ability to fix nitrogen.
The observed abundance in the stratified NPSG water column of nutrient-rich particle
aggregations such as Rhizosolenia mats and marine snow indicates that the hypothesized
ecosystem domain shift may not be moving unilaterally from one dominant system to
another. Alternatively, it may consist of parallel or co-existing communities of
organisms that bring nitrogen into the system (such as nitrogen fixers and vertically
migrating Rhizosolenia mats) as well as the pervasive organisms of oligotrophic food
webs that benefit from the additional nitrogen directly. Climate forcing will likely cause
the relative proportions of the two communities to shift back and forth (Karl, 1999; Karl
et al., 2001). Thus we have the opportunity to examine ecosystem change in one of the
largest domains on earth as a function of measurable, greenhouse gas warming trends,
and make predictions of how carbon export will fluctuate as a result.
Conclusions
The contemporary view of the NPSG is that the occurrence of the large eukaryote
phytoplankton-herbivore grazer food chain is primarily episodic and that the
photoautotrophic picoplankton-supported microbial food web is always present (Karl,
1999). We suggest that our observations across a broad region of the NPSG of
significant POM production events represented by high concentrations of Rhizosolenia
mats, associated mesozooplankton, and abundant detrital marine aggregates may lead to
25
export events that represent important stochastic components in the overall
biogeochemical budgets of this subtropical gyre system. Our results are necessarily
limited to the summer period of low wind activity because of the operational constraints
of diving operations. However, the spatial extent of the particle-rich waters suggests a
process important at basin scales. In-situ sampling combined with long-term time-series
data collection in this vast oceanic region is the only means by which we will be able to
understand the cause and timing of such events, including the periodic intense summer
chlorophyll blooms documented by Wilson (2003), and provide the proper time and
space integration to assess their contribution to the Pacific biological carbon pump.
Our data provide visual confirmation of Rhizosolenia algal mats at sub-euphotic
depths across a broad expanse of the gyre, and highlight the value of in-situ imagery via
towed instrumentation or remotely operated vehicles for detailing the distribution of
macroscopic phytoplankton and detrital aggregates in the open sea. In addition, the
ability of towed imaging systems to resolve even smaller colonial forms such as
Trichodesmium (Davis, et al., 1992b; Pilskaln, unpublished) indicates it may be a useful
tool for resolving questions about vertical migration in this nitrogen-fixing genus as well
(Letelier and Karl, 1998). In-situ imaging provided the first data set of marine snow size
and abundance variability across a major ocean gyre, making it a powerful method for
quantifying various factors that impact particulate biogeochemical cycling and export in
the ocean.
26
Acknowledgements
We are extremely grateful to the our fellow RoMP colleagues M. Brzezinski, F.
Lipschultz, and M. Altabet and all the project graduate students for their support,
assistance, and humor at sea, and are similarly grateful to the captains and crews of the
R/V Moana Wave, New Horizon, and Melville. We thank D. Caron and S. Gallager for
use of/assistance with the VPR in 1996, F. Lipschultz for nitrate profile data, E. Perry for
completing the 2002 mat counts, M. Cadwallader and D. Thompson for ROV and dive
operations assistance in 2002, and B. Tupper for production of graphics. This project
was primarily supported by NSF Biological Oceanography Program grant OCE-9423471
to C. Pilskaln, OCE-9415923 and OCE-9414372/OCE-0094591 to T. Villareal, and
assisted by OCE-9314533 to D. Caron.
27
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