The “BOUSSOLE” Buoy—A New Transparent-to-Swell Taut Mooring Dedicated to Marine Optics: Design, Tests, and Performance at Sea

A new concept of oceanographic data buoy is described, which couples a taut mooring and a “transparent-to-swell” superstructure, and is specifically designed for the collection of radiometric quantities in offshore environments. The design of the thin superstructure addresses two major requirements: stabilizing the instruments in the water column and avoiding shading them. The development of the buoy is described, starting with the theoretical assessment and then describing the various stages of development leading to the latest version of the mooring and buoy. Its performance at sea is also analyzed. This new platform has been deployed in the deep waters (>2400 m) of the northwestern Mediterranean Sea for about 4 yr (since September 2003) and provides a quasi-continuous record of optical properties at this site. The data are used for bio-optics research and for calibration and validation operations of several European and U.S. ocean color satellite missions. The plan is to continue the deployment to build a decadal time series of optical properties. The instrument suite that is installed on this buoy is also briefly described, and sample results are shown to demonstrate the ability of this new system to collect the data at the desired frequency and quality.

A Note on Seabed Irradiance in Shallow Tidal Seas

(1) The tidal cycle and that of solar elevation interact to produce patterns in seabed irradiance during the day which change over the springs-neaps cycle. In locations where high water springs is at midday, there is a single peak in seabed irradiance in the middle of the day at neap tides and two peaks in seabed irradiance (one in the morning, the other in the evening) at spring tides.(2) These cycles also interact to produce a springs–neaps cycle of daily mean irradiance, which is relevant to the growth of benthic algae. The pattern of this cycle will also vary from place to place. In locations where low water springs is in the middle of the day the pattern is straightforward: maximum daily irradiance always occurs at spring tides. However, where high water springs is in the middle of the day, the cycle can reverse as the days get longer. The timing of the switch in the cycle depends upon the water clarity.(3) Variations in attenuation over the springs–neaps cycle will also be important. In the Menai Strait, a springs–neaps cycle in K is often (but not always) observed, higher attenuations occurring at spring tides when seabed sediments are stirred up by the stronger tidal currents. This will reduce seabed irradiance at spring tides and tend to favour greater irradiance at neaps. In our model runs, K was kept constant because at present it is difficult to predict these changes. However, in our observations there is evidence of greater seabed irradiance at neap tides associated with greater water clarity. Variations in cloud cover during the day can also affect the regular cycles predicted by the model.The authors are grateful to Professor Ernest Naylor for encouragement and for information on animal behaviour and light. Members of the Marine Optics Group at Menai Bridge helped with the sampling during the experimental work in July 1994.

A History of Early Optical Oceanographic Instrument Design in Scandinavia

Interest in the optical characteristics and variability of the sea has grown for nearly two centuries. Most of the early work in this area was performed by European investigators. Perhaps the earliest reference to an optical oceanographic research cruise can be found in the book by Otto Krümmel (1886), in which the author refers to the Rurik circumnavigational cruise of 1817 made by Otto von Kotzebue. In these studies von Kotzebue made measurements using optical instrumentation comprised of a piece of red cloth tethered to a line and lowered into the sea. With this technique, von Kotzebue was able to crudely measure the depth of penetration of light. This technique was refined by using a white plate, and the first measurements in the Pacific (at 10°N 152°W) yielded measurements of 49 meters. It is worth noting that this work was done several decades before the famous efforts of Secchi (1866). Efforts to incorporate photographic techniques to characterize the underwater light field were also developing in the late 1800’s. In March, 1885 some experiments were made in the waters off Nice, France, in which a photographic plate was submerged to depths of several hundred meters. Additional historical information can be found in the classical textbook by Sauberer and Ruttner (1941). Theoretical treatments of optical oceanography developed somewhat later. Ludvig Valentin Lorenz published the first works on the theoretical aspects of marine light scattering. This work, originally published in Danish (Lorenz, 1890), was subsequently translated into French in 1915. Martin Knudsen (founder of International Council for the Exploration of the Sea, and developer of some of the fundamental concepts for making hydrographic calculations) also had concerns about marine optics as reflected in correspondence he sent to Professor Otto Pettersson (father of Hans Pettersson) in Sweden: . . . In studying those provinces of water and particularly of sea water, which are of importance to the organisms living therein, the study of the light contents of the water must occupy the prominent place. Light contents play in many respects a similar part to that of oxygen content but have not been so strongly investigated as the latter. . . .