Geoscience Reference
In-Depth Information
include less control over the location and direction of coverage, power limitations, and
limited ability for near-real-time data. Gliders are slower than AUVs, but have longer dur-
ation capabilities.
6.6.1.2 Temporal Resolution
Fluorometers have been used on buoys in oceans, estuaries, and lakes; attached to docks
and bridges; or at cabled observatories. CDOM fluorometers, due to new low power con-
suming light sources and low power detectors, have become a common addition to many
aspects of coastal ocean observatories. With fluorescence, a measurement of NOM can be
obtained at temporal resolutions of 1 second or less allowing time-varying distributions
to be studied. Some moored platforms can even be of a profiling nature and provide both
temporal and spatial data (e.g., bottom stationing ocean profiler [BSOP]; Langebrake et al.,
2002
).
6.6.2 Importance of Scale
The availability of sensitive,
in situ
sensors and appropriate deployment platforms allows
for capturing NOM measurements over challenging scales. Thin layers of particles, plank-
ton, or bacteria have been known to concentrate on density benches (regions where sharp
changes in temperature and/or salinity occur in natural waters), especially during stratifica-
tion events (Chen and Gardner,
2004
). These distributions are difficult to study due to their
ephemeral nature and small vertical distributions (they can be centimeters to meters thick).
Conversely, river plumes can extend hundreds of kilometers (e.g., Amazon) with freshwater
sitting upon higher density seawater, thus isolating the fresher river plume waters with all
their nutrients, particles, contaminants, and organisms. In addition to these spatial meas-
urement challenges, distributions of organic matter change temporally due to tides, winds,
runoff, and episodic events such as storms or effluents. To capture these temporal varia-
tions, repeated measurements over time (e.g., via transects, buoys, or profilers) are required
over the area of interest. The following three field experiments are provided as case studies
for examining NOM at varying scales using
in situ
sensors on towed platforms.
6.6.2.1 Neponset River Estuary in Boston Harbor- Small
Temporal and Spatial Scales
The Neponset River Estuary drains the second largest river into Boston Harbor, MA, and
is a tidally dominated urban system fringed with tidal salt marshes. To examine tidal and
seasonal variations in DOM sources and distributions within the estuary, surveys were con-
ducted using a miniature towed instrument package called the Mini-Shuttle (Gardner et al.,
2005
). With all hydrographic and optical sensors mounted within 4 cm of each other, ver-
tical salinity variations of 20 PSU over just 1-2 m depths can be delineated (
Figure 6.15
).
This study demonstrated that a significant but seasonally varying input of CDOM within
the estuary originates from the degradation of salt marsh organic material. This input yields
