The link between upper ocean biogeochemistry and the deep ocean:
An example from a ten year time series in the Northeast Atlantic
Lampitt, R. S
Southampton Oceanography Centre, UK

Much of the geological record in the deep ocean was created as a result ofbiogeochemical processes in the upper part of the water column. The factors that determine
the rate and characteristics of primary production affect to a very large extent the rates atwhich material is lost from the upper ocean due to gravitational settling of particles. It istherefore of considerable importance for us to develop an understanding of the processes
responsible for the time varying downward flux of particulate matter.
Since 1989 studies have been conducted at a single location on the Porcupine Abyssal
Plain (PAP) in the Northeast Atlantic (4800m water depth)(Figure 1). The objective has been
to examine the seasonal and interannual trends in downward particle flux, to elucidate thephysical and biological factors that control this and to determine the influence of flux
variations on the benthic community and the biogeochemistry of the upper few centimetres ofthe sediment.
90

Bank60
allBankH
atton45-4504590135

180all
Ro
ck0m
200
300

200m

-900m0m

4000m-135North-90-18010055-45

TroughRoc
k0Ab Poys rcusa pinlP elain50Time series site45-20-15-10-50West

Figure 1. Location of the ten year study site in the Northeast Atlantic. The water depth is 4800m.
Daily meteorological data (Figure 2) have been used to drive a model of upper ocean
biogeochemistry providing a derivation of particle export from the upper mixed layer andhence the flux in the deeper parts of the water column. These model results are compared todownward particle flux measured using time series sediment traps at 3000m depth (1800m
above bottom). The depth of winter convection varies by more than a factor of two,interannual variation in the measured timing of the major particle flux peaks varies by one
month (Figure 3) and integrated annual flux by a factor of 1.5 (Figure 4). There is close
agreement between the model output of flux at 3000m depth and that measured by sediment
trap although neither show a consistent trend of increasing or decreasing flux over this decade(Figure 4). The model output is also compared to semi-quantitative estimates of the timevarying benthic load of phytodetrital material and midwater concentrations of marine snow
both derived using time lapse photography.

25

89

90

91

92

93

94

95

96

97

98

99

Wind Speed(m/s)

2015
1050

Air temp.
o( C)

2015

Mixed Layer Depth (m)100-100
-200
-300
-400
-500-600

UML (model)
UML (Measured)-700-800
89

90

91

92

93

94

95

96

97

98

99

YearecovNDctOugSep

A

Jul

Jun

ayM

A

arM

pr1989
1990
1992
1994
1995
1996
1997
1998
1999

Dry Mass Flux
at 3000m

200

Flux (mg/m2/d)

Feb

JanFigure 2. Variation in meteorological forcing over the Porcupine Abyssal Plain and the effect on thedepth of the upper mixed layer. Deeper mixing introduces more nutrients into the upper sunlit zone
leading to enhanced primary production and hence greater downward particle flux. Air temperature
and wind speed were taken from NCEP re-analysis provided by NOAA-CIRES, Climate Diagnostics
Center, Boulder, Colorado, USA from their web site (http:// www.cdc.noaa.gov/).

15010050
1995

0050100150

200250300350

Day Number(Mid-sampling point)

Figure 3. Downward flux of the dry mass of particulate material at 3000m over the Porcupine
Abyssal Plain. Each data point represents the average flux over a predetermined period of time which
may be as short as a week but during periods of constant flux such as in the winter, longer samplingintervals were selected.

The biological community of the underlying sediment has changed markedly over thistime period with dramatic increases in the population size of some large benthic species
which were previously rare. The expectation was that explanation for this would be found in
the primary supply of organic material to the seabed. Similarly the phytodetrital layer that has
previously been a prominent feature on the seabed in early summer has not been observed
since 1996. To date the causes of this change in community structure have not been elucidated
and may be related to subtle qualitative changes in the food supply rather than to changes in
the integrated annual organic carbon flux.

322Flux (g/m /y)ModelMeasured

10
1988

1990

1992

1994

Year

1996

1998

2000

Figure 4. Annual integrated downward flux of organic carbon at 3000m depth as determined by themodel driven by upper ocean biogeochemistry and as measured by sediment traps at that depth.