Measurement problems in the field

There are three main sources of error in the measurement of underwater irradiance in the field: wave action, fluctuations in surface-incident flux due to drifting clouds and perturbation of the light field by the ship. The convex part of a surface wave acts as a converging lens and will focus the incident light at some depth within the water. As the wave moves along, so this zone of intense light travels with it. This is essentially the same phenomenon as that which produces the well-known moving patterns of bright lines to be seen on the bottom of a swimming pool on a sunny day. A stationary upward-pointing irradiance meter will be subjected to a series of intense pulses of radiation of duration typically of the order of some milliseconds as the waves move past overhead. There are, in addition, rapid negative fluctuations in irradiance caused by the de-focusing effect of the concave part of the wave.

An irradiance meter traversing a series of depths will also be subject to these rapid temporal fluctuations in intensity. Dera and Gordon (1968) showed that the average fractional fluctuation in irradiance for a given surface wave field increased with depth to a maximum and then decreased, eventually to insignificant levels. For example, at a shallow coastal site, where Kd(525 nm) was 0.59 m"1, the average fractional fluctuation in Ed(525 nm) rose to a maximum value of —67% at about 0.5 m depth, and then decreased progressively with depth, falling to —5% at about 3 m. It was found that the clearer the water, the greater the depth at which significant rapid temporal fluctuation could be detected. The upwelling light field, by contrast, is hardly at all affected by this wave-focusing phenomenon. At the above site, for example, the average fractional fluctuation in Eu was only —5% over the whole depth range. On the basis of a long-term series of measurements at a deep-sea mooring in the Mediterranean Sea, Gernez and Antoine (2009) found that the conditions for maximum fluctuations in downward irradiance (510 nm) at 4 m depth under clear skies are for wave heights of —0.5 m, or wind speeds between — 1 and 5ms"1. Fluctuations under clear skies are reduced for wave heights greater than —1.5m or wind speeds in excess of —7ms"1.

To extract good values for the vertical attenuation coefficient, some kind of smoothing of the data must be carried out. At its simplest, in the case of a manual instrument with a meter readout, the operator can concentrate on taking observations at any given depth, only between the 'blips'. Alternatively, a simple electronic damping circuit can be used to smooth out the fluctuations.287,699 In the case of sophisticated rapid-profiling instruments, which continuously record irradiance, complete with all the wave-induced rapid fluctuations, as they descend,1249 a variety of mathematical smoothing procedures can subsequently be applied to the stored data.1246

Fluctuations in the light field due to clouds differ from those due to waves first in that they are much slower, and second in that their effects are manifest throughout the whole illuminated water column and affect the upwelling and downwelling light streams to the same extent. The procedure most commonly used to overcome this problem is to monitor the incident solar flux continuously with a reference irradiance meter on deck, and use the data so obtained to adjust (for purposes of determining vertical attenuation coefficients, or reflectance) the concurrently obtained underwater irradiance values as appropriate. Davies-Colley et al. (1984) found that a more satisfactory (less variable) correction for changing ambient light was achieved if the reference meter were placed within the water at some fixed depth, preferably the one at which irradiance was reduced to 10 to 20% of the surface value.

Computer modelling of the light field476 indicates that the third problem perturbation of the field - by the ship - is relatively unimportant for measurements of downward irradiance made on the sunny side of the vessel under clear skies, but that errors under overcast conditions can be significant, and measurements of upward irradiance are strongly influenced by the ship's presence under either type of illumination. To solve this problem, techniques have been developed for deploying the instrument at some distance from the ship.1443 Since the seriousness of the problem is a function of the size of the boat, it is of more significance for oceanographers than for limnologists. Calculations by Gordon and Ding (1992) show that in measurement of the upward irradiance (or radiance), self-shading by the instrument itself can cause significant error. The error is greatest in strongly absorbing waters, and under vertical Sun, and increases with the diameter of the instrument and the absorption coefficient of the water.

A measurement problem of a different nature to those discussed above is that of obtaining a representative set of measurements within a realistic time period when, as is invariably the case in the sea, the area of interest is very large. To address this problem, Aiken (1981,1985) and Aiken and Bellan (1990) have developed the undulating oceanographic recorder, an instrument platform which is towed 200 to 500 m behind the ship. It is designed to follow an undulating trajectory within the water: for example, when towed at 4 to 6ms"1 (8-12knots) it moves between near-surface depths and ^70 m with an undulation pitch length of ~1.6 km. The platform is fitted with a suite of sensors measuring a range of oceano-graphic parameters, including downwelling and upwelling light in a number of wavebands across the photosynthetic range. The distance from the towing vessel is such that ship shadow and wake problems are eliminated. The data can be logged at, for example, 10-s intervals for durations of 11 h, so that essentially continuous information over long stretches of ocean can be accumulated.

Another approach to the problem of collecting optical data over large areas of ocean is to use autonomous underwater vehicles (AUVs), which have the capability to traverse substantial distances and depth intervals, in a pre-programmed manner. An example is the Seaglider, developed by Eriksen et al. (2001) at the University of Washington. This is a slim, low drag, light-weight (52 kg), 1.8 m long vehicle with internal mechanisms and circuitry housed within a pressure hull enclosed by a fibreglass fairing to which wings, a rudder and a trailing antenna are attached. Propulsion is achieved by buoyancy control effected by variation of vehicle-displaced volume. This involves pumping hydraulic oil from an internal to an external reservoir, or letting it flow back, as required. As the vehicle sinks or rises, the wings provide hydrodynamic lift to propel the vehicle forward. It can operate down to a depth of 1000 m. Data can be collected from onboard instruments both as the Seaglider dives and as it rises again to the surface. At the surface, the Seaglider dips its nose to raise its antenna out of the water. It then determines its position using Global Positioning System (GPS), uploads its oceanographic data to the Iridium data telemetry satellite and then downloads a file with any new instructions that may be necessary. While to date this vehicle does not seem to have been fitted with instruments for measuring the underwater light field, this would presumably be possible. Perry et al. (2008) have carried out measurements with a Seaglider equipped to record chlorophyll a fluorescence and the water backscattering coefficient. The vehicle carried out V-shaped transects, approximately 450 km long, off the coast of Washington State, USA. Each transect took ~30 days with three dives per day, with 5-km spacing between dives.

Wijesekera et al. (2005) used an AUV fitted both with a multispectral irradiance meter, operating in seven wavebands, an ac-9 nine-channel optical spectrometer for measuring absorption and beam attenuation coefficients and a scattering sensor. The AUV was used to follow cross-shelf transects extending from 4.34 to 18.47 km from the Oregon coast at depth of 2, 4, 6, 8 and 10 m.

There has been in recent years a major increase in the deployment of unattended instruments in situ for prolonged periods to carry out long-term monitoring of the underwater light field or of optical properties of the water. All such instruments are subject to the problem of biofouling, the formation of layers of algae, and eventually invertebrate animals, on optical windows. Depending on the environment this can become serious in as little as a week, and should certainly be expected within about two weeks if no countermeasures are taken. We saw earlier (§3.2) that automatic periodic injection of bromine proved effective to keep a moored absorption meter clear. Wiping mechanisms have been used.1126 The HydroRad spectroradiometer (see below) is available in a version with a copper anti-fouling shutter that automatically protects the light collector from biofouling between readings.

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