Underwater Area Research Articles

Cartographie des biocénoses marines de Guadeloupe à partir de données SPOT (récifs coralliens, phanérogames marines, mangroves)

In the framework of a comprehensive study of the coastal marine ecosystems of Guadeloupe Island (French West Indies), a remote sensing survey of marine communities was conducted in the Bay of the Grand Cul-de-Sac Marin. Pictures of SPOT satellite were used. At first sight, XS data from SPOT did not seem to be well adapted to extract information from deepest underwater areas. In the present study, a method for analysis of XS data was developed, which increases tenuous spectral differences associated to very low intensity signals. This technique led to a better distinction of objects, even the deepest ones. Reference surfaces were then identified in the field and a reliable supervised classification was conducted, which reflected the high diversity of ecological themes (up to 32) of the area to 20 m depth. A total of 23 700 ha of biota was clearly identified. Mangrove covered up to 2 700 ha. Submerged areas were constituted of 15 700 ha of soft bottoms and 3 800 ha of hard substrate bottoms. On the coral reefs, flourishing coral communities accounted only for 113 ha (11.2%). This result enhances the alarming situation of these communities in Guadeloupe. Finally, seagrass beds took up to 8 200 ha (i.e. 42% of the total sublittoral area, 58% of soft bottom substrates). This ecosystem revealed to be of primary importance within the studied area and particular attention was paid to seagrass beds of Thalassia testudinum. Twelve themes were retained to describe them, based on the coverage rate of the substratum by the plants and the type of the sediment colonised. The final document expresses well the high complexity of the studied area. This map constitutes a reference of the present status of the coastal benthic communities of the ‘Grand Cul-de-Sac Marin’ and can be useful for scientists and the administrative managers of that area.

Underwater acoustic animal guidance system

A system and method for developing acoustic underwater stimuli for guiding aquatic animals such as fish, to or from an underwater area. Fish captured from the underwater area are placed in a sound recording chamber and their sounds recorded. The spectrum of the recorded sound is analyzed and is used to vary a primary sound spectrum is accordance with predominate frequencies contained therein to create a sound believed to have the most efficient guidance effect on the fish since the fish are believed to be most responsive to sounds produced by the fish themselves. The captured fish are then placed in a behavior chamber preferably located in the underwater area. The created sound or stimuli is applied to behavior chamber to determine its effect on the fish contained therein and may be modified to arrive at the most effective sound. This sound then is applied to the underwater area and measured to ensure that is has been properly applied.

Icebergs in the Southern Ocean (Abstract)

Relatively little data on the distribution of Antarctic icebergs were available prior to 1980. The published literature included size data of about 5000 icebergs, and position data of 12 000 icebergs. There were indications that the size data were biased in favour of larger icebergs.A programme of systematic iceberg observations was therefore initiated by Norsk Polarinstitutt in 198! through the SCAR Working Group on Glaciology. This programme is based on standard “blue” forms distributed to all ships going to Antarctica. The icebergs are recorded every 6 h and in Five length groups: 10–50, 50–200, 200–500, and 500–1000 m, and those over 1000 m are described individually.The amount of data has increased greatly from the start in 1981–82. The position of 70 000 icebergs, including 50 000 that had been size classified, were on file at Norsk Polarinstitutt by December 1985, and the data set is growing rapidly. Most ships travelling to and from Antarctica now participate in collection of the data. (Fig.1 shows the locations of the icebergs sighted.) Fig. 1.Location of iceberg observations under the programme initiated in 1981. Main ship tracks are clearly reflected. The average observation represents 14 icebergs.The size distribution of the classified icebergs observed under this programme up to December 1985 is given in Table I: Table IThe “standard size” (length, width, and thickness) is based on our observations from three Antarctic expeditions which carried out dedicated iceberg studies. Many icebergs are of course not right-angled parallelepipedal in shape, but this is a good approximation for most of the larger icebergs.The data are based both on visual sightings and on radar observations. Duplicate observations from a ship moving at slow or zero speed are as far as possible eliminated, both during observation, and by critical appraisal before the data are filed. The data editing also includes evaluation of data quality, especially in connection with radar observations, and comparison of positions and dimensions of the large icebergs in order to reduce to a minimum repeated observations from different vessels of icebergs >1000 m. These account for most of the iceberg mass (see Table I).Consideration of iceberg-distribution patterns and the observed area of the Southern Ocean, and of duplicate observations, indicates more than 300 000 icebergs south of the Antarctic Convergence, with a total ice mass of about 1016 kg. Consideration of mean residence times indicates an annual iceberg production from the continent of 23–1015 kg, which is considerably higher than most other recent estimates. This also suggests that the Antarctic ice sheet is in balance.The data indicate large regional differences in iceberg sizes, the most noticeable being between the two sides of the Antarctic Peninsula, and between the Amery Ice Shelf/ Prydz Bay area and the remainder of East Antarctica. These differences are probably mainly related to different calving sites.About one-third of the observed icebergs are over the continental shelf of Antarctica. The total under-water area of these icebergs is two orders of magnitude less than the under-water area of the Antarctic ice shelves. The annual total iceberg melting and its effect on the water masses over the continental shelf has been calculated from ocean-water temperature variations at 200 m depth and estimated melt rates. This turns out to be an order of magnitude less than the annual effect of melting sea ice. The iceberg data considered here are probably under-represented with respect to the smallest sizes, and they do not include icebergs that have become <10 m. Inclusion of these ice bodies would increase the total melt.

Icebergs in the Southern Ocean (Abstract)

Relatively little data on the distribution of Antarctic icebergs were available prior to 1980. The published literature included size data of about 5000 icebergs, and position data of 12 000 icebergs. There were indications that the size data were biased in favour of larger icebergs. A programme of systematic iceberg observations was therefore initiated by Norsk Polarinstitutt in 198! through the SCAR Working Group on Glaciology. This programme is based on standard “blue” forms distributed to all ships going to Antarctica. The icebergs are recorded every 6 h and in Five length groups: 10–50, 50–200, 200–500, and 500–1000 m, and those over 1000 m are described individually. The amount of data has increased greatly from the start in 1981–82. The position of 70 000 icebergs, including 50 000 that had been size classified, were on file at Norsk Polarinstitutt by December 1985, and the data set is growing rapidly. Most ships travelling to and from Antarctica now participate in collection of the data. (Fig.1 shows the locations of the icebergs sighted.) Fig. 1. Location of iceberg observations under the programme initiated in 1981. Main ship tracks are clearly reflected. The average observation represents 14 icebergs. The size distribution of the classified icebergs observed under this programme up to December 1985 is given in Table I: Table I The “standard size” (length, width, and thickness) is based on our observations from three Antarctic expeditions which carried out dedicated iceberg studies. Many icebergs are of course not right-angled parallelepipedal in shape, but this is a good approximation for most of the larger icebergs. The data are based both on visual sightings and on radar observations. Duplicate observations from a ship moving at slow or zero speed are as far as possible eliminated, both during observation, and by critical appraisal before the data are filed. The data editing also includes evaluation of data quality, especially in connection with radar observations, and comparison of positions and dimensions of the large icebergs in order to reduce to a minimum repeated observations from different vessels of icebergs >1000 m. These account for most of the iceberg mass (see Table I). Consideration of iceberg-distribution patterns and the observed area of the Southern Ocean, and of duplicate observations, indicates more than 300 000 icebergs south of the Antarctic Convergence, with a total ice mass of about 1016 kg. Consideration of mean residence times indicates an annual iceberg production from the continent of 23–1015 kg, which is considerably higher than most other recent estimates. This also suggests that the Antarctic ice sheet is in balance. The data indicate large regional differences in iceberg sizes, the most noticeable being between the two sides of the Antarctic Peninsula, and between the Amery Ice Shelf/ Prydz Bay area and the remainder of East Antarctica. These differences are probably mainly related to different calving sites. About one-third of the observed icebergs are over the continental shelf of Antarctica. The total under-water area of these icebergs is two orders of magnitude less than the under-water area of the Antarctic ice shelves. The annual total iceberg melting and its effect on the water masses over the continental shelf has been calculated from ocean-water temperature variations at 200 m depth and estimated melt rates. This turns out to be an order of magnitude less than the annual effect of melting sea ice. The iceberg data considered here are probably under-represented with respect to the smallest sizes, and they do not include icebergs that have become <10 m. Inclusion of these ice bodies would increase the total melt.

Restoration of Seagrasses: With Economic Analysis

The technique of restoration of populations of land plants is an old one that is well understood by scientists, agronomists, forestry experts, and the general public. However, only recently has attention been turned to restoring underwater areas with socalled seagrasses. From 1947 onwards, attempts have been made to restore suitable marine areas by using important species; however, only very recently (from August 1973) have large-scale restoration projects been undertaken.The efforts that have been made to replant various seagrass species by different techniques are reviewed, but only two seagrasses, Zostera noltii and Thalassia testudinum, have been restored on a large scale. Both of these can be restored successfully; however, the seeding method of Thalassia appears to be the most practical operation of all, and the most potentially valuable for future research into large-scale restoration, as it allows flexibility of location and of depth of transplantation, and involves the minimum of damage to the donor site. It also ensures the most rapid regrowth, besides being in the long run the most economical method yet devised of restoring a seagrass community. An economic model (see Technical Appendix to this paper) and analysis for restoration costs are given and finally a wide range of uses of restoration of such beds of seagrasses are enumerated.

Port Royal, Jamaica: The archaeological problems and potential

In September 1968, one of the authors (P. Mayes), having been appointed Head of Mission by the British Ministry of Overseas Development to act as advisor to the Jamaica National Trust Commission, made recommendations which formed the basis for the establishment of the Port Royal Project. The Project is concerned with the excavation, conservation and presentation of 17th century Port Royal, which was founded in 1655 and destroyed by an earthquake in 1692 (see Fig. 1). For the purposes of archaeology the area was divided into three: the Archaeological-Historical Reserve, encompassing the protected underwater area and lands currently occupied by government facilities at the west end of Port Royal; the area in the centre of Port Royal which is protected archaeologically by modern housing and the area at the east end of Port Royal in which it was proposed that commercial expansion should be allowed on land held on lease from the Jamaican government. Work began early in 1969 in a section of this last area, which was threatened, at that time, by the construction of an hotel. This area was part of the shoal water section of sunken Port Royal (see below and Fig. 2). Since that date excavation headquarters and conservation laboratories have been established in the old naval hospital and the construction of a major museum, the first phase of the presentation programme, has begun.

Survey techniques in underwater archaeology

The land archaeologist is, in most instances, helped by the topography of the ground; there are generally good reasons for believing a certain area to be of archaeological interest and the irregularities of the ground will give him leads as to where he should dig. The underwater archaeologist may have no such indications on which to base his search, especially if he is interested in finding a well-preserved ancient wreck. Shallow wrecks can often be found near dangerous rocks but such wrecks are liable to be broken up by succeeding storms with the result that they have little or no interest to the marine archaeologist. There are three ways in which ancient wrecks may be discovered. Amateur or professional divers find sunken ships in the course of other diving activities. For instance in the eastern Mediterranean hundreds of sponge divers have been active during many years and have covered large underwater areas visually. This has resulted in the accurate positioning of many wrecks. The divers involved have in most instances not precisely pin-pointed the wreck position and although the general area is known they have to be refound. Secondly, the possible existence of wrecks have been suggested by trawler operators either working for fish or sponges. Amphora and in exceptional instances bronzes have been found in their nets. These will give approximate positions and again precise location must be accomplished by other methods. The third approach to finding ancient sunken ships is by searching large areas in the blind hope that something of interest will be found. Unfortunately the sea is very large! Even when using methods having a wide sweep, such as sonar, the time and effort involved make this approach almost impossible except in confined areas where there are reasons to believe that wrecks would have occurred.

U.S. TANKERS FREED FROM CORROSION

Following a number of years of trial and experiment, two American chemical tankers, the S.S. Marine Chemist and S.S. Marine Dow‐Chem, chartered by the Dow Chemical Co., of Midland, Michigan, U.S.A., for carrying chemicals from the Dow Texas Division to the markets of the Atlantic seaboard, have been rendered virtually corrosion proof, both in respect of their hulls and tanks. The hulls and ballast tank interiors have been equipped with modernised cathodic devices while the chemical tanks and pipes have been specially lined. Both tankers are owned and operated by Marine Transport Lines Inc. So effective has been the protection afforded by the magnesium anodes with which the hulls have been equipped that cleaning and maintenance painting of the underwater areas have been practically eliminated.

Cathodic Protection of an Active Ship in Sea Water

H.M.C.S. “New Liskeard,” an Algerine Class minesweeper (length 225 ft., beam 351/2 ft., underwater area 7500 sq. ft., 960 displacement tons) on active duty with the Royal Canadian Navy, has been fi...