Viable diatoms and chlorophylla in continental slope sediments off Cape Hatteras, North Carolina

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Pergamon

Deep-SeaResearchII, Vol. 41. No. 4,6, pp. 767 782, 1994 0967-0645(94)00010-7

Copyright© 1995ElsevierScienceLtd Printed in Great Britain. All rights....... d 119670645/94$7./)1)+ 0.(1(}

V i a b l e d i a t o m s and c h l o r o p h y l l a in c o n t i n e n t a l slope s e d i m e n t s off Cape Hatteras, North Carolina LAWRENCE B . C A H O O N , * RICHARD A . L A W S t a n d CARRIE J. THOMAS +

(Received 10 August 1993; in revised form 2 February 1994; accepted 15 February 1994) Abstract--Continental slope sediments off Cape Hatteras, North Carolina, were sampled by box coring in late s u m m e r , 1992. The chlorophyll a concentrations measured in sediments from 16 sites at depths ranging from 530 to 2003 m averaged 19.9 mg chl a m -2 , a concentration much higher than observed elsewhere on the eastern U.S. continental slope, indicating high depositional rates for microalgal material. The variability in the chlorophyll a values suggests strong environmental heterogeneity at both small and large spatial scales in this slope habitat, probably a consequence of both topography and bioturbation. Viable diatoms were found in sediment samples across the range of depths sampled, and up to 14 cm deep in sediments, indicating high rates of deposition and bioturbation. Bulk sediment samples contained planktonic, tychopelagic and benthic diatoms, indicating that both phytoplankton and benthic microalgac from the continental shelf may be sources of organic matter for these slope sediments.

INTRODUCTION

THE continental slope off Cape Hatteras, North Carolina (Fig. 1), has been hypothesized to be a "depocenter" for sediment and organic carbon originating from the Mid-Atlantic Bight (WALSH, 1988). Organic carbon burial rates of approximately 70 g C m 2 year i and sediment accumulation rates of approximately 1 cm year- ~have been reported in this area (SCHAFF et al., 1992; DEMASTERet al., 1994). Both estimates are more than an order of magnitude greater than respective rates reported for other areas of the continental slope of the eastern U.S. (SCHAFFet al., 1992; EMERYand UCHUPL 1972). High deposition rates off Cape Hatteras are probably a result of several circulation features. Colder Virginia Current water flows southward along the continental shelf of the Mid-Atlantic Bight and encounters the warm Gulf Stream moving to the northeast just off Cape Hatteras. Predominantly southward bottom flow along the continental shelf of the Mid-Atlantic Bight, particularly east of the "Bumpus line" ( B u M P U S , 1973; RHOADSet al., 1994), moves suspended material toward the shelf break off Cape Hatteras. Surface waters outwelling from Chesapeake Bay can carry organic material to the shelf break (WII,LIAMS and GODSHALL, 1977; WIEBEetal., 1987). Coastal storm events and Gulf Stream meanders

* D e p a r t m e n t of Biological Sciences, U N C Wilmington, Wilmington, NC 28403, U.S.A. - D e p a r t m e n t of Earth Sciences, U N C Wilmington, Wilmington, NC 28403, U.S.A. SDepartment of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, U . S . A . 767

768

L . B . CAHOON etal.

Kitty Hawk

kManteo

Carolina Platform (:?ape

Hatteras

f"

P

I~re~ead

Cape Lookout

A TLA NTIC OCEAN

"Ca~ F,=

/

/

~

Fig. 1. Areaof investigation,ManteoLeaseBlock467, off Cape Hatteras, North Carolina.

along the northern shelf of the South Atlantic Bight, particularly Raleigh Bay (Fig. 1), can drive near-bottom current speeds sufficient to suspend fine sediments and organic material, which may then be advected seaward (RODOLFOet al., 1971; BLANTON, 1991). Organic matter deposited in the continental slope sediments off Cape Hatteras probably derives largely from phytoplankton growing in shelf and inner slope waters of the MidAtlantic Bight. Phytoplankton productivity in the Mid-Atlantic Bight has been extensively measured and described, e.g. WALSH (1988). Phytoplankton also bloom in water outweiled from Chesapeake Bay (WlEBE et al., 1987). Benthic microalgae growing in continental shelf sediments also may be a source of organic matter exported downsiope. Benthic microalgal chlorophyll a concentrations/area in sediments of the North Carolina continental shelf are as much as four times the depthintegrated phytoplankton chlorophyll a in the overlying waters (CArtOON et al., 1990). Production by a taxonomically distinct benthic microflora in sediments of the North Carolina continental shelf has been found to be approximately equal to phytoplankton production in the overlying water column at depths <40 m (CArtOON and COOKE, 1992). Pennate diatoms account for most of the taxa and biomass of these benthic microalgae (CAHOON and LAws, 1993). A productive benthic microflora may be found to approximately 100 m off Onslow Bay, North Carolina (CArtOON et al., 1990, 1992). Sediment sampling in deeper slope sediments also has revealed significant concentrations of chlorophyll a and viable centric and pennate diatoms (CAHoON and COOKE, 1989; LAWS

Viable diatoms and chlorophylla

769

and CAHOON,1992), although there was no evidence that these microalgae were growing in situ (CAI-IOON and COO~:E, 1989).

The presence of chlorophyll a and viable diatoms in slope sediments, particularly at greater depths, suggests rapid rates of accumulation of organic matter, and, through analysis of the taxonomic composition of the assemblage, also may indicate the relative importance of planktonic and benthic sources of this organic matter. We report here the results of our investigations of the distribution of viable diatoms and concentrations of chlorophyll a in slope sediments off Cape Hatteras. This study was part of a larger study funded by the Minerals Management Service to describe the biological, chemical, physical, and geological characteristics of the continental slope off Cape Hatteras, an area proposed for hydrocarbon exploration. METHODS The study area was centered on the Manteo 467 lease block, which lies approximately 72 km east-northeast of Cape Hatteras (Fig. 1). The slope is quite steep (average 30-35 °) in this area with many canyons, ridges, and gullies (DIAz and BLAKE, 1994). Bottom topography and the interaction of the Gulf Stream, which is deflected to the east off Cape Hatteras, and the southward flowing Virginia Current drive the complex oceanographic conditions in this area (CSANADYand HAMILTON,1988). Transient features, including Gulf Stream meanders and eddies, fronts, stratification, and storm events, also control oceanographic conditions in this area. Sediments are primarily hemipelagic sandy silts, with roughly even proportions of sand, silt, and clay (DIAz and BLAKE, 1994; DEMASTERet al. , 1994). Sediment samples were obtained using a 0.16 m 2 BX-640 Ocean Instruments box core deployed by R.V. E n d e a v o u r during field work from 26 August to 6 September 1992. Sixteen box core samples were taken along several onshore-offshore transects and at other selected locations at depths ranging from 530 to 2000 m (Fig. 2). Each box core was partitioned into sixteen 10 × 10 cm subcores and processed immediately after recovery. Disturbed subcores were discarded. Samples for analysis of chlorophyll a were collected from 16 box core sites using a 10 cm long by 2.5 cm (i.d.) core tube. Five replicate samples were taken from different subcores within each box core and immediately frozen. These were later extracted in 100% acetone (1 : 1 sediment to acetone by volume) and analyzed by the double extraction spectrophotometric technique of WHITNEY and DARLEY (1979). This method partitions the acetone extract with hexane, followed by measurement of absorbance of the hexane phase at 663 nm before and after acidification. The acetone phase retains chlorophyll c, chiorophyllides a and b, and pheophorbides a and b. The hexane phase contains chlorophylls a and b (if present) and pheophytins a and b. Comparison of absorbance values for the hexanc extracts before and after acidification with 50% HCI eliminates interference by the pheophytins in determination of intact chlorophylls. Chlorophyll a has a much higher absorbance coefficient at 663 nm than chlorophyll b (LORENZEN, 1967), which is also typically less concentrated in marine sediments than chlorophyll a (FuRLONC and CARPENTER, 1988; CArtOON et al., 1992), so this method permits the concentration of intact chlorophyll a to be determined. Since chlorophyll a degrades rapidly ( < hours) in dead cells, measuring its concentration gives a relative estimate of the amount of living microalgae. However, chlorophyll a can be retained for long periods of time by heterotro-

770

L . B . CAHOONet al. 35°50

'

35040 .

35030 '

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• Analysed 0 Not analysed

35°20'N

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Contours in meters

75 °O0'W

74 °50'

74 °40'

Fig. 2. Bathymetric chart of Manteo Lease Block 467 area, showinglocations of box core sample sites for R.V. Endeavour cruise, 26 August~ September 1992. phic microalgae living in the dark, e.g. HELLEBUSTand LEWIN (1977), and by resting stages or cysts of microalgae. Sediment for culture of viable diatoms was obtained from 15 box core sites (Fig. 2) with a 15 cm long by 5.5 cm (i.d.) core tube. Each core was split longitudinally, and sediment samples were collected at 2 cm intervals down from the top. These were refrigerated in the dark. For culture, a small aliquot of sediment from each sample was streaked onto a nutrient agar plate using a wire loop. Sterile technique was used throughout the culture procedure, including alcohol-flame sterilization of wire loops between samples and autoclaving media and glassware. The plates, which consisted of 1.5% Difco Bacto-Agar in sterile, filtered seawater with modified "f/2" medium (GuILLARD and RYTHER, 1962; Guillard, personal communication), were then incubated under a bank of 40 W cool white

771

Viable diatoms and chlorophyll a

Table 1.

Station # 18 41 2 33 SA 9 10 34 42 3 19 11 21 5 44 36 SA 111

Sediment chlorophyll a data from R. V. Endeavour cruise, 8/25/92-9/4/92. Stations as in Fig. 2; "n" = number of subsamples analyzed from each core Depth (m)

n

530 5911 600 610 620 630 775 785 812 812 815 1410 1501 1535 1567 2003

(5) (5) (5) (5) (5) (5) (5) (5) (5) (5) (5) (5) (5) (4) (3) (5)

mg Chla m 2 + s.d. 57.9 31.0 5.16 5.55 5.49 2.10 19.5 65.2 16.7 23.2 8.23 23.1 14.7 25.8 11.6 3.74

+_ 105 _+ 22.5 + 7.35 -+ 9.36 +_ 3.44 + 2.88 +_ 39.4 +_ 102 _+ 22.8 _+ 24.7 _+ 5.56 _+ 44.5 _+ 9.85 _+ 44.2 _+ 6.80 _+ 3.11

rug Chla (g sed) i + s.d. 1.71 0.92 0.90 0.17 0.19 0.08 0.81 2.29 0.71 0.83 0.28 0.87 0.50 1.03 0.56 /).12

_+ 3.15 _+ 0.71 _+ 1.26 _+ 0.30 _+ 11.13 _+ 0.10 + 1.65 _+ 3.44 _+ 1.09 _+ 0.86 + 0.19 _+ 1.66 + 0.35 _+ 1.78 _+ 1.65 + 0.1//

fluorescent lights at room temperature for several weeks and were regularly monitored for growth. Viable diatoms produced small brown colonies, which were examined with dissecting and compound microscopes and scanning EM. Samples from selected colonies and bulk sediment samples were treated with hydrogen peroxide and nitric acid solutions to remove organic matter, followed by acidification with hydrochloric acid to remove carbonates (CAHOONand LAWS, 1993). Cleaned colony samples and bulk sediment subsamples were then settled on 22 mm 2 coverslips according tO LAWS (1983) and mounted on microscope slides with Hyrax mounting medium (R1 = 1.71). Identifications of diatoms were done using an Olympus BH-2 compound microscope with differential interference contrast illumination at 1250 x . Identifiable valves in several cleaned bulk samples were counted in randomly chosen fields until ->500 valves were counted in order to estimate relative abundances of species/genera and life mode groups. Photomicrographs of selected slide fields were taken at 625x on the light microscope using differential interference contrast illumination or on an ISI SX40A scanning electron microscope using a 30kV accelerating potential. RESULTS

Chlorophyll a was found in all box core samples that were extracted (Table 1). Overall mean concentrations were 19.9 mg chl a m -2 or 0.75 ~tg (g dry sediment) ~. There was no discernible trend in chlorophyll a concentrations vs depth, primarily owing to high variability among samples and among replicates within samples. Nor was there any latitudinal trend, although some transects yielded somewhat higher chl a levels than others. These large-scale differences likely reflect topographic heterogeneity. Because the sediment samples used for chlorophyll a analyses integrated a 10 cm deep core, the variability among individual replicate samples must be attributed to small-scale horizontal and vertical patchiness in the distribution of chlorophyll a-containing organisms.

772

L . B . CAHOON et al.

Table 2. Viable diatom data from R. V. E n d e a v o u r cruise, 8 / 2 5 / 9 2 - 9 / 4 / 9 2 . " + " denotes one or more colonies cultured from raw sediment; " - " denotes no colonies cultured; " N D " denotes no culture attempted Sediment depth interval (cm) Station # 18 41 2 33 SA 9 10 34 42 3 19 11 21 5 44 SA10

Depth (m)

0-2

2-4

4-6

6-8

530 590 600 610 620 630 775 785 812 812 815 1410 1501 1535 2003

+ + + + + + +

+ + +

+ +

+ + + +

+ +

+

+ + +

+ + +

+ + +

+

+

ND + +

10-12

12-14

ND

ND + +

+ +

+

+ +

8-10

+ + + + +

+ + +

+ +

+

Viable diatoms were cultured from 14 of the 15 sites sampled (Table 2). Only site 44 (depth = 1535 m) failed to yield viable diatoms. Viable diatoms were distributed throughout the sediment to depths of up to 14 cm, the deepest sample depth. There was, however, a slight tendency toward fewer viable diatoms at the deeper sample depths, particularly at the deeper box core sites. Quantitative analyses of viable diatom concentrations are problematic, owing to the difficulty of quantifying sediment subsamples taken by wire loops for streaking and the likelihood that any culture technique selects for a subset of viable cells. Several cultured species, all very small forms, have been tentatively identified. These included at least one species of the pennate diatom, Nitzschia, and the small pennate diatom, Delphine& surirella, a benthic (possibly tychopelagic) species (Fig. 3). Tychopelagic diatoms typically sink to the bottom but are easily suspended by turbulence (BOLD and WYNNE, 1985). Species lists prepared from detailed examinations of two acid-cleaned bulk sediment samples from the shallowest and deepest sites (18 and SA 10, respectively; Fig. 2) include a variety of diatom taxa, which were identified by their frustule morphology, and life mode groups, as well as a silicoflagellate, Distephanus sp., found in both samples (Table 3). The diatom assemblage from site 18 included approximately equal numbers of planktonic (8/23 classifiable taxa), benthic (10/23), and tychopelagic (5/23) diatoms, as classified by CAHOONand LAWS(1993). The diatom assemblage from site SA 10 had a somewhat higher proportion of planktonic diatoms (9/17 classifiable taxa) than benthic (5/17) and tychopelagic diatoms (3/17). The tychopelagic centric diatom Paralia sulcata and the (probably) benthic pennate diatom Delphine& surirella were the most frequently observed species in these samples. Unidentified resting spores also were observed in both samples. Diatom counts show that benthic and tychopelagic diatoms account for >70% of all identifiable valves at three sites between 530 m and 2003 m (Table 4). The cleaned samples contained many diatom fragments, which is typical of sediments, because bioturbation and

Viable diatoms and chlorophyll a

Fig. 3.

Delphineissurirella grown in culture from raw sediment samples taken at site SA 10. Scale bar is 1 micrometer.

773

> © © z

Viable diatoms and chlorophyll a

775

transport processes damage frustules readily (Figs 4 and 5). However, m a n y of the benthic forms, particularly Delphineis surirella, were whole cells with both valves and girdle bands intact (Figs 4 and 5), suggesting that m a n y of these forms reach the sampling sites in viable condition. Specimens of Aulacoseira spp., a freshwater diatom, were identified from samples taken at sites 18 and SA 10, DISCUSSION Average chlorophyll a concentrations in the study area are approximately half those reported for continental shelf sediments off North Carolina (CAHOON et al., 1990; CAHOON and COOKE, 1992), and are much lower than values reported from slope sediments underlying upwelling z o n e s (BLAKE et al., 1992) or estuarine sediments (e.g. SuN et al., 1993). H o w e v e r , these average concentrations are much higher than values reported from the shallow continental slope off Onslow Bay, south of the study area (CAHOON et al., 1990, 1992). Such relatively high chlorophyll a concentrations in these sediments imply relatively high fluxes of viable cells to the bottom in the study area. Carbon flux to slope sediments attributable to viable, chlorophyll a-containing microalgae can be calculated very approximately with the equation: Chla d = Chlao

e -kd

where Chla 0 is an initial value for chlorophyll a concentration in the sediment, Chla d is the chlorophyll remaining after a time period, d, assuming no new chlorophyll a is added, k is the decay rate coefficient for chlorophyll a in the sediment, and d is the time period in days. Using an average value for chlorophyll a concentration in the sediment of 19.9 mg m -2 and a decay rate coefficient of 0.1 (for " b o u n d " chlorophyll a in oxic sediment (see discussion of bioturbation below), SUN et al., 1993) and solving for chlorophyll a concentration after one day, a chlorophyll a loading rate of 1.80 mg m -2 day -1 or 693.5 mg m -2 year - I is calculated. Assuming a C : chlorophyll a ratio of 50:1 in diatoms yields a carbon flux rate of approximately 34.7 g C m -2 year -1 , or approximately half of the total carbon flux reported by SCHAFFet al. (1992) and DEMASTER et al. (1994). Thus, viable diatoms might represent a significant portion of the total organic matter fluxing into slope sediments off Cape Hatteras. The presence of viable diatoms in the sediments must be interpreted carefully. Only a few of the many species we identified in a limited examination of bulk sediment samples were brought into culture, and all were very small forms. Any culture technique undoubtedly selects for certain species among many that are present as live cells.

Fig. 4. Selected fields of view illustrating common taxa found in prepared sediment samples from Cape Hatteras slope sites. Scale bar is 20 micrometers. Taxa are listed top to bottom and left to right. (a) Site SA 10: Paralia sulcata, Diploneis bombus; (b) Site 19: Biddulphia sp., Delphineis surirella, Thalassiosira eccentrica, Diploneis smithii, Paraliasulcata, Delphineis surirella; (c) Site 19: Diploneis papula, Delpheneis surirella, Paralia sulcata, Rhaphoneis amphiceros, Paralia sulcata; (d) Site 18: Trachyneis aspera, Nitzschia panduriformis, Delphineis surirella; (e) Site 21: Delphineis surirella, "'Navicula" sp. (girdle view), Coscinodiscus radiatus, Paralia sulcata, Cymatosira belgica? (girdle view); (f) Site 21: Delphineis surirella (girdle view), Delphineis surirella (three valve views), Diploneis sp., Rhaphoneis amphiceros; (g) Site 21: Delphineis surirella (two valve views), Amphora sp., Nitzschia sp.; (h) Site 18: "'Navicula" sp. (girdle view); (i) Site 18: Distephanus sp. (silicoflagellate), Paralia sulcata. Also note the large number of fragments of planktonic and benthic forms.

776

L.B. CAHooyetal.

Microscopic examination showed that many of the species identified in Table 3 were represented by cells that were observed to be more or less intact (Figs 4 and 5). Few diatom resting spores were observed, so viable cells probably account for most of the chlorophyll a we measured. However, it is not possible to determine how much chlorophyll a might have been contributed by each life mode group within the total assemblage. Mixotrophy among phytoplankton, particularly diatoms, is not well understood, but mixotrophic diatoms might account for a significant fraction of the viable cells cultured and of the chlorophyll a found in the sediments. However, our culture conditions (light + inorganic nutrient medium) favored photoautotrophic forms, and attempts to culture mixotrophs with organic media in the dark yielded no colonies. Thus we think that the chlorophyll a observed in slope sediments represents viable autotrophic cells and resting stages. The microalgae taxa identified in the bulk sediment samples include planktonic forms, as expected, but also include many tychopelagic and benthic forms typical of shelf sediments (CAHOON and LAWS, 1993). The species list in Table 3 is based on an examination of samples from the shallowest and deepest sites sampled in this study. A more thorough examination of more samples will certainly yield a much longer list of species. Those identified here, however, probably include the more common forms, many of which were seen repeatedly. It is significant that the species lists from the two sites are so similar. Benthic and tychopelagic diatoms dominated cell counts from three sites spanning the range of depths sampled in this study. These observations contrast with those of SCHUETTEand SCHRADER (1979, 1981), who found continental slope sediments underlying upwelling areas commonly dominated by valves of planktonic diatoms, e.g. Actinoptychus, Actinocyclus, Cyclotella, Coscinodiscus, and Thalassiosira. Our observations suggest a common source for at least some of the organic material at all these slope sites, and also indicate that benthic and tychopelagic microalgal assemblages in adjacent continental shelf habitats, which are not well quantified, may also contribute significant amounts of organic material to the continental slope habitat off Cape Hatteras. The presence of the freshwater diatoms, Aulacoseira spp., is more difficult to explain, but may indicate river output through the Chesapeake Bay or runoff as possible sources of organic materials to the slope as well. CAHOON and LAWS (1993) found the benthic microflora at continental shelf sites off North Carolina to be dominated by Cocconeis disculoides, a monoraphic epipsammic diatom that attaches very firmly to sand grains. This species was considerably less abundant than several others that do not attach as firmly, such as Paralia sulcata, Delphineis surirella and Diploneis spp., three of the most frequently observed taxa in the slope samples. This observation suggests that physical processes transport down-slope a subset of the shelf microalgae assemblage, or that microalgal assemblages in areas from which down-slope transport originates are dominated by the taxa we observed in the slope samples. We interpret the widespread distribution of chlorophyll a and viable diatoms across the slope to depths as great as 2000 m in the study area to indicate that frequent, energetic events transport organic material into this slope habitat from the adjacent shelf and overlying waters. WASHBURN et al. (1993) documented off-shelf transport of sediment by mesoscale eddies. Gulf Stream eddies and frequent storm events may provide sufficient energy frequently enough to drive significant export of shelf-derived organic material down-slope off Cape Hatteras. The variability in chlorophyll a concentrations within and

Viable diatoms and chlorophyll a

Fig. 5. S e l e c t e d f i e l d s o f v i e w i l l u s t r a t i n g c o m m o n t a x a f o u n d i n p r c p a r e d s e d i m c n t s a m p l e s f r o m Cape Hatteras slope sites. Scale bar is 20 micrometers. Taxa are listed top to bottom and left to right. (a) Site 21: Diploneis smithii, Navicula sp., Dictyoneis sp., background shows a large Coscinodiscus sp.; (b) Site 21: Nitzschia pamturifimnis , Delphineis surirella (two valve views), Thalassiosira eccentrica; (c) Site SA I0: ActinopO,chus splendens, Paralia sulcata, Diploneis papula, Actinoptychus senarius, Diploneis bombus; (d) Site 18: (~vmatosira Iorenziana (girdle view of four cells): (c) Site SA 10: Paralia sulcata, Cocconeis disculoides.

777

779

Viable diatoms and chlorophyll a

Table 3. Diatom taxa identified in bulk samples o f sediment, 0-1 cm depth, from two continental slope box core sites off Cape Hatteras, North Carolina. Designations of planktonic (P), benthic (B), or tychopelagic (P/B) life modes follow CAHOON and LAWS (1993) Taxon

Li~ mode

Site 18 (530 m) Centric

Pennate

A ctinocyclus sp. A ctinoptychus senarius Coscinodiscus sp. Cyclotella stylorum Cymatosira belgica C. lorenziana Eunotogramma laeve Hyalodiscus sp. Odontella aurita Paralia sulcata Plagiogramma sp. Rhizosolenia sp. Thalassiosira sp. Amphora sp. Cocconeis disculoides C. scutellum Delphineis surirella Diploneis bombus D. papula D. srnithii Epithemia sp. Fragilaria construens Navicula spp. Nitzschia spp. Raphoneis amphiceros Thalassionema nitzschiodes

P

P P

P P/B P/B

B P P/B P/B B P P B

B B B B B B ? P/B '~ '~

B P

Site SA (2003 m) Centric

Pennate

A ctinocyclus octanarius Actinoptychus senarius A. splendens Coscinodiscus spp. Cyclotella stylorum Cymatosira lorenziana Nitzschia sp. Odontella aurita Paralia sulcata Rhizosolenia sp. Stephanodiscus sp. Thalassiosira spp. Amphora sp. Cocconeis disculoides Delphineis surirella Navicula forcipata Nitzschia panduriJbrmis Thalassionema nitzschiodes

P P P P

P P/B ? P/B P/B p P p B

B B B B p

780

L.B. CAHOONet al. Table 4. Relative abundances o f benthic, planktonic and tychopelagic diatoms and resting stages" identified in sediment samples f r o m Sites 18 (530 m), 21 (1410 m), and SA 10 (2003 m). Species were classified as in Table 3, except that Achnanthes spp. , Cocconeis spp., Navicula spp., Nitzschia spp. , Diploneis spp. and Pleurosigma spp. were counted as benthic f o r m s and Chaetoceros spp., Coscinodiscus spp., Rhizosolenia spp. and Thalassiosira spp. were counted as planktonic f o r m s

Location

Life mode group

Dominant taxa

No. of valves

% Total

236 113 150 115 110 20

45.7

Site 18 Benthic D. surirella

Tychopelagic P. sulcata

Planktonic Resting stages

29.1 21.3 3.9

Site 21 Benthic D. surirella

Tychopelagic P. sulcata C. lorenziana

Planktonic Resting stages

275 125 110 75 35 121 14

52.9

199 97 192 175 17 91 28

39.0

21.1

23.3 2.7

Site SA 10 Benthic D. surirella

Tychopelagic P. sulcata C. lorenziana

Planktonic Resting stages

37.6

17.8 5.5

among box core samples also implies considerable heterogeneity in the small- and large-scale processes and topography controlling the distribution of organic matter in this slope area. The distribution of chlorophyll a and viable diatoms throughout the top 14 cm of sediment (at least) implies that bioturbation is an important process in these slope sediments, as others have found (SCnAFF et al., 1992; BLAKEet al., 1992; DEMASTER et al., 1994). Head-down deposit feeders are abundant in this slope habitat and commonly extend to depths of 14 cm or more in the sediment (BLAKE and HILBtG, 1994). DOBBS and WnrVLAfCn (1982) have shown that the maldanid polychaete, Clymenella torquam, extends its tail out the end of its tube and "hoes" the surface of the sediment with its anal crown. This sediment, which can contain viable diatoms, is transported down the tube to the feeding void near the anterior end of the worm. This activity is thought to be a means of stimulating the "microbial garden" associated with head-down feeders (YtNGSr and RHOADS, 1980). If the matdanid species of the Cape Hatteras slope do this, it may explain how viable diatoms are injected deep into the slope sediments. A c k n o w l e d g e m e n t s - - T h c authors thank R. J. Diaz for coordinating this research effort, which was supported by the Minerals Management Service, U.S. Dept. of Interior, Contract No. 14-35-0001-30672. This is contribution No. 096 of the Center for Marine Science Research at UNC Wilmington.

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