[L&O Featured Article] L&O Featured Article, Vol. 52 (3) May 2007

[L&O Feature Articles Announcements]

L&O Featured Article in Issue 3 of Volume 52

Aksnes, Dag L., Mark D. Ohman, and Pascal Rivière. 2007. Optical effect on
the nitracline in a coastal upwelling area. Limnol. Oceanogr. 52(3):
1179-1187.

Introductory comments by Marcel Babin (L&O Associate Editor)

The use of the Secchi disk to measure an index of clarity in natural surface
waters was introduced in the late 19th century.  It has since become widely
used for routine monitoring.  Despite its empirical and somewhat inaccurate
character, and although sophisticated underwater optical sensors have become
commercially available during the last two decades, the use of the Secchi
disk remains very popular because of the simplicity of its use, and because
of its intrinsic robust calibration.  Consequently, millions of Secchi depth
records have been accumulated over the last century.  Many are archived and
easily accessible. The large amount of data compensates for their medium
quality.

Tyler (1970) and later Preisendorfer (1986) provided optical models that
rigorously relate Secchi depth to beam attenuation (c) and vertical diffuse
attenuation (Kd) coefficients.  In the open ocean, c is generally correlated
with Kd.  Consequently, the depth of the euphotic zone (often defined as the
depth where downward irradiance is 1% of its surface value), which depends
on Kd, can be estimated from the Secchi depth.  The Secchi depth can also be
used to estimate the concentration of phytoplankton chlorophyll because the
latter correlates with both c and Kd.
The power of the global archive of Secchi depth is well illustrated by three
examples from past studies. Lewis et al. (1988) analyzed the global NODC
Secchi depth data and found spatial and temporal associations with ocean
dynamics and new production.  Falkowski and Wilson (1992) analyzed a large
set of Secchi depth from the North Pacific and found that phytoplankton
biomass had not changed significantly in response to climate change since
1907.  Recently, Aksnes (2007) showed that there is a link between the
biomass of visually constrained fish stocks and horizontal visibility in the
Black Sea, using coincident fish stock and Secchi depth records covering the
period between the late 1960s and the late 1990s.

In this issue’s featured paper, Aksnes and collaborators use Secchi depth
data to examine the dynamics influencing the vertical distribution of
nitrate in the ocean surface layer, and the link with phytoplankton growth.
During the 1980s, a number of studies showed that the depth of the
nitracline results from a balance between the uptake by phytoplankton in the
euphotic zone, and the supply from the bottom by physical processes
(upwelling and turbulent diffusion).  The present study is based on the
analysis of combined Secchi depth and nitrate concentration measurements
made off the coast of California and shows that light penetration is a major
forcing factor affecting the nitracline. The extensive dataset, which covers
21 years of observation, reveals a good relationship between the nitracline
depth and steepness and the Secchi depth.  These relationships are excellent
when seasonal variations and a nearshore to offshore gradient area are
analyzed.  Aksnes and collaborators interpret these statistical
relationships using a simple and elegant 1-dimensional steady-state model
that includes nitrate uptake by phytoplankton, light penetration, and
nitrate supply by upwelling.  Results explicitly show how the nitracline
depth and steepness are related to the Secchi depth.

In the open ocean, low upward supply of nitrate leads to low phytoplankton
biomass, which results in low vertical attenuation of incoming solar
radiation, which in turn leads to a deep nitracline because of the high
nitrate uptake by phytoplankton a greater depth. Elevated upward supply of
nitrate generally sustains higher phytoplankton biomass, which results in
higher vertical attenuation of incoming solar radiation and leads to a
shallow nitracline.  In this simple reasoning, light attenuation may appear
to be a passive consequence of biomass and not a driving factor by itself.
In other words phytoplankton self-shading would determine the depth of the
nitracline.  But the analysis of Aksnes and collaborators suggests that
vertical attenuation is a forcing factor by itself.  To better illustrate
their argument, they cite the case of a system where allochthonous dissolved
and particulate substances decrease Kd independently of phytoplankton
biomass.  In such a case, Kd actually determines how deep light, and the
resulting nitrate uptake by phytoplankton, extends into the oceanic nutrient
pool, and thus determines the depth of the nitracline.

This paper forces us to reevaluate our conception of the interactions
between physics, optics, and biology in the ocean, suggesting that optics
may play an even larger role in the dynamics than previously thought. If
anything, it reinforces the point that to accurately model the oceanic
ecosystem, optics must be described carefully. In addition to its scientific
merit, this paper’s simple presentation provides excellent material for
stimulating reflection and debate among students and researchers.

References

Aksnes, D. L. 2007.  Evidence for visual contraints in large marine fish
stocks.  Limnol. Oceanogr.  52: 198-203.

Falkowski, P. G. and C. Wilson. 1992. Phytoplankton productivity in the
North Pacific ocean since 1900 and implications for absorption of
anthropogenic CO2.  Nature 358: 741-743.

Lewis, M. R., N. Kuring, and C. Yentsch. 1988. Global patterns of ocean
transparency: Implications for the new production of the open ocean.  J.
Geophys. Res. 93: 6847–6856.

Preisendorfer, R. W. 1986. Secchi disk science: visual optics of natural
waters. Limnol. Oceanogr. 31: 909-926.

Tyler, J. E. 1968. The Secchi disc. Limnol. Oceanogr. 13: 1-6.