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seagrass; landsat
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    MAPPING SHALLOW WATER SEAGRASS WITH LANDSAT TM SATELLITE DATA IN TORRES STRAIT   Mervyn Thomas Brian Long Thomas Taranto June 1997 REPORT  MR-GIS 97/6  T O R R E S S T R A I T S E A G R A S S M A P P I N G 2 Executive Summary  The objective of this study was to map the shallow water seagrass beds of northwestern  Torres Strait, using Landsat TM satellite data. The study area was 4,545 km 2 . It was sampled by divers in November / December 1993 at 251 sites. Percentage cover of seagrass, water depth and substrate type were recorded at each site. A spatial statistical model was developed, relating seagrass cover with the pixel values of blue, green and red light recorded by the satellite. The usefulness of this model for prediction of seagrass density from satellite imagery was assessed. Landsat TM satellite data did not provide an acceptable basis for spatial prediction of seagrass density outside   the area sampled. Nevertheless, Landsat TM data may be useful in improving the interpolative mapping of seagrass density within   the sampled area. In this study it improved the predicted residual sums of squares statistic by 2.3%. Introduction Seagrass is critical habitat for dugongs, turtles, and some commercially important prawns and fish. Torres Strait has one of the largest areas of seagrass in Australia; seagrass beds in Torres Strait are larger than the Gulf of Carpentaria by a factor of 10 and are equivalent to the total estimated area of seagrass along the Queensland coast (Long and Poiner, 1997). Torres Strait supports one of the largest populations of dugongs in the  world which is a reflection of the importance of seagrass there.  The attenuation of light through the water column is a major limiting factor for the use of Landsat TM technology to map subtidal habitats. Red light penetrates to 5 m, green to 15 m and blue to 30 m in waters with moderate suspended sediments (typical of coastal waters in the Great Barrier Reef lagoon. Much of central Torres Strait, however, is shallow - with large tracts of seabed < 15 m deep. Thus Landsat satellite data may prove useful for mapping the shallow water sub tidal habitats in much of the Torres Strait region. The purpose of this study was to test the utility of Landsat TM satellite data to map seagrass beds in northwestern Torres Strait where water depths are < 15 m for most of the study area. Materials and Methods Description of the study area  Torres Strait lies between the NW coast of Cape York Peninsula and the S coast of Papua New Guinea, and connects the Coral and Arafura Sea (Fig. 1). Wolanski et al. (1988), Harris (1988) and Bode and Mason (1994) have described the physical oceanography and sedimentary geology of the Torres Strait. The Straits are shallow (< 15 m) with strong tidal currents due to large pressure gradients between the Arafura and Coral Sea (Bode and Mason, 1994). Water speeds exceeding 2.5 m.s -1  occur in the narrow channels between some islands and reefs (Admiralty, 1973).  T O R R E S S T R A I T S E A G R A S S M A P P I N G 3 100 km500Cape YorkMoa Is.Badu Is.Orman ReefsDauan Is.Boigu Is.Buru Is. Aldai Reef Mabuiag Is. AUSTRALIA Torres StraitPNG Mai River 142° 143° E10° SN   Figure 1 . Map of Torres Strait, showing the boundaries of the study area sampled for seagrass in November 1993.  The strong tidal currents have created sand waves in many areas of Torres Strait (Harris, 1988) including north western Torres Strait. There are two distinct seasons in Torres Strait: a dry season and a wet season. The dry season runs for seven months from May to November with an average rainfall of 21.4 mm month -1 . The wet monsoon season lasts for five months from December to April with an average monthly rainfall of 311 mm at  Thursday Island (Admiralty, 1973). The prevailing winds for the two seasons are also distinct. During the dry season, south-east trade winds blow from E and SE 90% of the time. Wet monsoon winds are more variable; blowing from the NE, N and NW for 30% of the time. The average wind speed is lower in the wet monsoon, 5 knots.h -1 , than dry season, 7.9 knots.h -1 , and the number of calm days is also lower in the dry season, < 1 day.month -1  than wet monsoon, 2.1 days.month -1 . There are more gales during the monsoon than dry season (6 and < 1 days.month -1  respectively). There is little net flow of water through Torres Strait although there are seasonal differences in the direction of net flow. The dry season has a net westerly flow with the south-east trade winds There is a net eastwardly flow over the wet monsoon season, when westerlies and north westerlies prevail (Wolanski et al., 1988). The winds and currents stir up the bottom sediments in shallow water areas of central Torres Strait which results in a turbidity maximum zone in central Torres Strait (Harris, 1988).  The southern limits of the study area were Badu and Moa Island, situated mid-way across the Straits. Boigu and Dauan Island near the S coast of Papua New Guinea formed the northern boundary. The eastern limit of the study area was formed by a line NE from  T O R R E S S T R A I T S E A G R A S S M A P P I N G 4Moa Island through the Orman reefs and N to Dauan Island The 142 nd  meridian of longitude formed the western boundary (Fig. 1). Field sampling: Inter-reefal areas  The seagrass at 251 subtidal sites in the study area were sampled in November / December 1993 (Fig. 2). The study area was first divided into primary sampling units  which were each 4.5 km east-west and 4.2 km north-south. The primary sampling unit area, 18.9 km 2 , was chosen on the basis of three factors: ã estimated time taken to sample a site (15 min), ã time to travel between sites, ã total time (three weeks) available for field sampling.  All primary sampling units were sampled, giving complete sampling coverage of the study area. It was impractical to sample the whole primary sampling unit (18.9 km 2  ), and 100 m 2  sites were sampled in each unit. The position of each site within each primary sampling unit was chosen randomly. Global Positioning System (GPS) satellite navigation  was used to locate the sites in the field. At all sites divers searched an area of approximately 100 m 2  and estimated the percentage cover of seagrass and algae as well as recording descriptions of the substratum. Seagrass samples were also collected, sorted and enumerated to species level, but for this study we only used the presence/absence data. Water visibility (m) and water depth (m) were also recorded at each site and a sediment sample was taken for grain-size analysis. Figure 2 . Map of Torres Strait study area showing sites sampled for seagrass. ######################################################################################################### ############################################################################################################################################################## 0 25    50 75 Kilometers N # RecentsamplesitesTorres100kBasemapforeshoreislandmainlandreef 
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