Determination of chlorophylla levels in benthic sediments: Can chla in sediments serve as a food ban

1
Determination of chlorophyll-a levels in benthic
sediments Can chl-a in sediments serve as a
food bank for Arctic benthic invertebrates? Rebe
cca Pirtle-Levy, Jacqueline M. Grebmeier, Lee W.
Cooper Dept. of Ecology and Evolutionary Biology,
University of Tennessee, Knoxville, TN 37932
Introduction
The continental shelf of the Chukchi Sea supports
an area of high benthic biomass, with decreasing
benthic biomass values as one moves offshore into
the Arctic Basin (Grebmeier and Dunton 2000). In
this current investigation the focus was on the
amount of chlorophyll-a found in surface
sediments and in depth profiles of sediment
cores. Chlorophyll-a (chl-a) was used as a
tracer for viable food sources that supply
organic matter to benthic invertebrates. Organic
matter could be buried in the sediment to be
accessed as a food source when organic carbon
flux from the water column is absent. Here we
present a comparison of surface sediment chl-a
from the spring and summer of 2004. We also look
at downcore profiles to assess seasonal changes
in chl-a levels. Samples were taken from 16
stations (Figure 1) in the spring and 27 stations
(Figure 2) in the summer.
BC4
BC2
HV1
Figure 2. HLY0403 stations
Figure 3. Examples of subcores taken from the
multi-HAPS corer.

• Methods
• A multi-HAPS benthic corer (Figure 4) was used
  to collect sediment cores (each subcore area
  0.0133 m2) .
• The cores were sectioned on the ship as
  follows in 1 cm intervals down to 4 cm, 2 cm
  intervals down to
• 20 cm, and in 4 cm intervals to the base of
  the core (Cooper et al. 1998).
• Replicate 1 cm³ plugs of sediment were
  collected from each depth interval of the core.
• Ten ml of 90 acetone was added to each
  sediment sample. The samples were stored in the
  dark at 2C for 12
• hours to extract all chl-a present in the
  sediment.
• Chl-a concentrations in the acetone were
  measured with a Turner Designs 10-AU fluorometer
  using the non-
• acidification assay described by Welschmeyer
  (1994).

Figure 6. Depth profiles of sediment chl-a
content in spring and summer of 2004. Top row
depicts Harold Valley line (HV1) and Barrow
Canyon (BC). Bottom row depicts East Hanna Shoal
line (EHS)..

• Results
• A significant negative correlation was found
  between surface sediment chl-a and water depth in
  both spring (r -0.0403, p
• 0.022, N 32) and summer (r -0.559, p
  0.000, N 54). This indicates that surface
  sediment chl-a decreases as
• water depth increases (Figure 5).
• Based upon a paired t-test, there was a
  significant difference between compared downcores
  at all stations except EHS5, p
• 0.769 (Figure 6). Most of the stations had
  higher sediment chl-a levels in the summer. An
  exception is EHS4, which had higher
• chl-a concentrations in the spring (Figure
  6).
• Station EHS6 had a bioturbation layer at the 4
  cm depth. A paired t-test indicated a
  significant difference between spring and
• summer in the upper 4 cm of the core (p
  0.009, N 8) and no significant difference below
  4 cm (p 0.910, N 10). This can
• be seen in Figure 6.

• Future Work
• A comparison between dominant benthic
  invertebrates at each station and chl-a levels
  within sediment cores will be
• used to determine if feeding modes of these
  animals influence the amount of chl-a found at
  depth in the cores.
• Sediment grain size analysis will be done to
  investigate if chl-a is retained better in
  similar phi size sediment.
• Comparison of beryllium-7 and cesium-137 with
  chl-a levels in sediment cores will give an
  indication of the freshness
• of the plant material in the sediments and
  how long they have been sustained at depth.

References Cooper, L.W., J.M. Grebmeier, I.L.
Larsen, S.S. Dolvin, A.J. Reed (1998).
Inventories and distribution of radiocaesium in
Arctic marine sediments Influence of
biological and physical processes. Chem. Ecol.
15 27-46 Grebmeier, J.M. and Dunton, K.H.
(2000). Benthic processes in the northern
Bering/Chukchi seas status and global change,
pp. 61-71. Impacts of Changes in Sea Ice and
other Environmental parameters in the Arctic.
Report of the Marine Mammal Commission
Workshop,15-17 February 2000, Girdwood, Alaska.
Available from the Marine Mammal Commission,
Bethesda, Maryland. Welschmeyer, N.A. (1994).
Fluorometric analysis of chlorophyll a in the
presence of chlorophyll b and phaeopigments.
Limnol. Oceanogr. 39 1985-1992
Figure 4. Multi-HAPS corer.
Figure 5. Comparison of surface sediment chl-a
levels vs. water depth in spring and summer.
Acknowledgements We would like to thank the
Captain and crew of the USCGC Healy for all their
support. This project was funded by NSF
OPP0125082.