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    MAPPING SHALLOW WATER SEAGRASS WITH

    LANDSAT TM SATELLITE DATA IN TORRES STRAIT

    Mervyn Thomas

    Brian Long

    Thomas Taranto

    June 1997

    REPORTMR-GIS 97/6

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    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 northwesternTorres Strait, using Landsat TM satellite data. The study area was 4,545 km2. It was

    sampled by divers in November / December 1993 at 251 sites. Percentage cover ofseagrass, water depth and substrate type were recorded at each site. A spatial statisticalmodel was developed, relating seagrass cover with the pixel values of blue, green and redlight recorded by the satellite. The usefulness of this model for prediction of seagrassdensity from satellite imagery was assessed. Landsat TM satellite data did not provide anacceptable basis for spatial prediction of seagrass density outside the area sampled.Nevertheless, Landsat TM data may be useful in improving the interpolative mapping ofseagrass density withinthe sampled area. In this study it improved the predicted residualsums of squares statistic by 2.3%.

    IntroductionSeagrass is critical habitat for dugongs, turtles, and some commercially important prawnsand fish. Torres Strait has one of the largest areas of seagrass in Australia; seagrass bedsin Torres Strait are larger than the Gulf of Carpentaria by a factor of 10 and areequivalent to the total estimated area of seagrass along the Queensland coast (Long andPoiner, 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 useof 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 ofcoastal 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 mayprove useful for mapping the shallow water sub tidal habitats in much of the TorresStrait region. The purpose of this study was to test the utility of Landsat TM satellite datato map seagrass beds in northwestern Torres Strait where water depths are < 15 m formost 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 ofPapua 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 physicaloceanography and sedimentary geology of the Torres Strait. The Straits are shallow (< 15m) with strong tidal currents due to large pressure gradients between the Arafura andCoral Sea (Bode and Mason, 1994). Water speeds exceeding 2.5 m.s-1occur in the narrowchannels between some islands and reefs (Admiralty, 1973).

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    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 km500

    Cape York

    Moa Is.Badu Is.

    Orman Reefs

    Dauan Is.

    Boigu Is.

    Buru Is.

    Aldai Reef

    Mabuiag Is.

    AUSTRALIA

    Torres Strait

    PNGMai River

    142 143 E

    10 S

    N

    Figure 1. Map of Torres Strait, showing the boundaries of the study area sampled for seagrass inNovember 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 TorresStrait: a dry season and a wet season. The dry season runs for seven months from May toNovember with an average rainfall of 21.4 mm month-1. The wet monsoon season lastsfor 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 alsodistinct. During the dry season, south-east trade winds blow from E and SE 90% of thetime. 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 dryseason, 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 themonsoon than dry season (6 and < 1 days.month-1respectively). There is little net flowof water through Torres Strait although there are seasonal differences in the direction ofnet flow. The dry season has a net westerly flow with the south-east trade winds There isa net eastwardly flow over the wet monsoon season, when westerlies and north westerliesprevail (Wolanski et al., 1988). The winds and currents stir up the bottom sediments inshallow water areas of central Torres Strait which results in a turbidity maximum zone incentral Torres Strait (Harris, 1988).

    The southern limits of the study area were Badu and Moa Island, situated mid-way acrossthe Straits. Boigu and Dauan Island near the S coast of Papua New Guinea formed thenorthern boundary. The eastern limit of the study area was formed by a line NE from

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    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 4

    Moa Island through the Orman reefs and N to Dauan Island The 142 nd meridian oflongitude 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 unitarea, 18.9 km2, was chosen on the basis of three factors:

    estimated time taken to sample a site (15 min),

    time to travel between sites,

    total time (three weeks) available for field sampling.

    All primary sampling units were sampled, giving complete sampling coverage of thestudy area. It was impractical to sample the whole primary sampling unit (18.9 km2), and100 m2 sites were sampled in each unit. The position of each site within each primarysampling 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 ofapproximately 100 m2and estimated the percentage cover of seagrass and algae as well asrecording descriptions of the substratum. Seagrass samples were also collected, sortedand enumerated to species level, but for this study we only used the presence/absencedata. Water visibility (m) and water depth (m) were also recorded at each site and asediment sample was taken for grain-size analysis.

    Figure 2. Map of Torres Strait study area showing sites sampled for seagrass.

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    0 25 50 75 Kilometers

    N

    # Recentsamplesites

    Torres100k Basemapforeshoreislandmainlandreef

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    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 5

    Data analysis

    The data analysis occurred in two stages. In the first stage, and exploratory analysis wasused, to identify the appropriate functional form of predictive relationships, and tocharacterise large scale spatial trends and small scale spatial dependencies. The second

    stage involved estimating the parameters of a predictive relationship, using generalisedleast squares, based on a spatially structured error covariance matrix. The predictiverelationship was then assessed using cross validation.

    The exploratory analysis was based on a generalised additive model (GAM) (Hastie andTibshirani, 1990). Square root of percentage cover of seagrass was analysed using anormal error structur