Geoscience Reference
In-Depth Information
6.2 Historical Perspective of
In Situ
Sensors
The advent of field fluorometers was necessitated by a pressing need, particularly in ocean
environments, to overcome difficulties in obtaining measurements from remote locales and
over great depths. Limitations of discrete Niskin bottle sample collection prevented the
capture of fine-scale measurements over sharp temporal and spatial gradients. This section
is dedicated to the timeline of field sensor development beginning with the earliest instru-
ment designs through the common designs of the present day. It offers an appreciation of
previous efforts within the community to move past sampling limitations and describes
how this work has improved the collective understanding of aquatic DOM.
6.2.1 Chlorophyll Field Sensors: Precursors to
In Situ
DOM Fluorometers
The first fluorescence field sensor was customized for chlorophyll measurements in open
ocean environments (Lorenzen,
1966
; Scripps Institute of Oceanography). A Turner III
benchtop fluorometer was modified to extend the response of a standard photomultiplier
tube (PMT) from 650 to 685 nm, thereby increasing sensitivity for chlorophyll by an order
of magnitude. A blue fluorescent lamp was used to excite the sample, where the primary
filter with a maximum transmission at 430 nm isolated the blue light close to the maxi-
mum excitation wavelength of chlorophyll (440 nm). A submersible pump was used to
pump unfiltered seawater via a through-hull fitting (~2 m deep) into a flow-through cuvette.
Results showed a strong linearity between discrete chlorophyll extractions on filter pads
and
in vivo
chlorophyll fluorescence data; therefore discrete data were used to calibrate the
sensor. Over a continuous 21-day collection (Lorenzen,
1966
), it was demonstrated that
flow-through fluorescence instrumentation was a viable means of collecting rapid meas-
urements with improved temporal and spatial resolution. This work was also the first to
address two critical environmental concerns with field instruments: (1) the interference of
bubbles on flow-through measurements, where they implemented a bubble trap and subse-
quently gravity-fed the water to the instrument; and (2) biofouling, where daily cleaning
with alcoholic KOH was used to prevent fouling.
A decade later, Herman and Denman (
1976
) developed a truly
in situ
method with the
argument that the pumping of water is not “
in situ
” because it introduces some mixing of
chlorophyll samples. To avoid this phenomenon, a Variosens Fluorometer (Impulsphysik
GmbH, Hamburg, Germany) was mounted onto a Batfish towed instrument package
(
Figure 6.1
) and data from this first
in situ
instrument was compared to a Turner Designs
fluorometer that was fed by a towed submersible pump. Both were calibrated with dis-
crete samples. Although the Variosens Fluorometer provided fluorescence values that were
biased high, had slow response times for capturing signal change through sharp vertical
gradients, and exhibited data interpretation challenges due to time lag with pumped water
samples, the
in situ
profiler remained the optimal choice over the pumped system. Shortly
after, Geiskes et al. (
1978
) mounted the same model Variosens Fluorometer on a rosette
