Introduction to the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 12

Figure 1.5 Two examples of towed, undulating vehicles that we have used in shelf sea research.
Left: Seasoar, looking like a short, stubby-winged aeroplane. Right: Scanfish, a relatively
simple wing-shaped vehicle that incorporates a downward-looking echosounder used for
automated seabed avoidance - a very useful feature when working in shallow seas.
(Photos by J. Sharples.)

1.5.2 Sensors for biogeochemistry and beyond

The basic physical measurements of pressure, temperature and electrical conductivity
are, in principle, straightforward; the challenge lies largely with achieving the accur-
acy needed to quantify the small changes in density that drive mean flows in the
ocean. When we want to make measurements of biogeochemical or ecological
parameters, the challenge becomes far more extreme. There are still many measure-
ments that require water to be collected and analysed using laboratory equipment
on the ship, or even back ashore. We might need to transport samples from remote
locations back to our institutes in such a way as to minimise the possibility of
degradation, which could involve freezing to very low temperatures or chemical
preservation. In addition to all of these practical issues, there is a key intellectual
challenge in all interdisciplinary work: how can we make biogeochemical and eco-
logical measurements that are compatible with the spatial and temporal patchiness
that our physical measurements routinely show? This is an exciting and rapidly
evolving field of ocean-observing technology.
The first, and by now most common, biogeochemical instrument that became
available for routine use alongside temperature and conductivity was the chloro-
phyll fluorometer. The idea of measuring phytoplankton pigment as a proxy for
phytoplankton biomass was pioneered by Gordon Riley in the 1940s, using
methods developed in Plymouth by H.W. Harvey. The original colorimetric
analyses of filtered seawater samples was replaced by a fluorometric technique;
the plant pigment chlorophyll a fluoresces at a known, quite specific wavelength of
light as a mechanism for dumping energy from photons that it cannot use in
photosynthesis. We will describe fluorescence and photosynthesis in more detail in