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This project is concerned with mapping the distribution of sea grass communities around the coast of Tasmania. In selected areas, the mapping covers three time periods spread over more than forty years. To achieve this goal, reference was made to the literature on remote sensing, the mapping of seagrasses and other vegetation, and to previously published seagrass surveys. Other workers in this field were also consulted The most appropriate techniques and technologies were adopted within the constraints of the time and resources available, and the size of the area to be covered.

'This chapter looks at the literature on the surveying and monitoring of sea grasses. It also considers the issues of vegetation boundaries applied to seagrass communities, and discusses the concept of a Geographic Information System (GIS), and its fuction in mapping and data management in this project.

3.1 Seagrass mapping - an overview

In the last two decades there have been numerous Australian and overseas studies that have mapped seagrass communities in conjunction with the measurement of other parameters (VIMS 1989). Although very small areas of seagrass can be mapped by field sampling alone, to do the same for large stretches of coastline would be prohibitively costly, and in such cases some form of remote sensing is usually employed. Shallow water environments can most efficiently be surveyed from an aerial platform from which images of the distributional patterns of the vegetation can be recorded (Kelly 1 980).

This section looks at techniques for the essential field sampling, or ground truthing of remotely sensed sea grass communities, and then outlines the major issues relating to different remote sensing technologies.

3.1.1 Field

Any means of mapping underwater vegetation using remotely aquired data must be complemented by a field sampling program to 'ground truth' or validate the images (Orth & Moore 1983). 'Ground truth' parameters can be defined as those ground features that generate the signal recorded by a camera or scanner through the atmosphere (Untz et al. 1976) .

. .

The initial aim of ground truthing is to verify that an underwater feature is

•., indeed a seagrass bed, and not objects such as macroalgae, mollusc beds or :-:\_of

Chapter 3. Seagrass Mapping

rocks. For example,� in some areas off Adelaide, South Australia, macroalgae such as Ulva spp. have been found using the root mat of dead

Posidonia beds as

a substratum,� and can be confused with healthy seagrasses in aerial photography

(V.

Neverauskas, pers. comm.). Similarly, Lo and Crowell (1992) mapping seagrasses from aerial photographs in Florida in 1990, found that their 1988 maps had included areas of algae.

While sampling, important parameters not accessible through remote sensing can also be recorded. These might include the species composition of the communities, their depth, the substratum, measurements of the biomass of the seagrasses and the extent of epiphyte growth. However, sampling strategies have often been poorly designed, leading to difficulties in linking their results to existing databases and later research (VIMS 1989).

3.1.1.1 Position fixing

An accurate means of fixing the position of sample sites is important in mapping

seagrass beds at any scale (VIMS 1989). The need to be able to monitor slow changes in decline or recovery of seagrass beds ideally requires an accuracy of 1-5 m in fixing the position of boundaries and sample sites. This level of accuracy can be achieved using current surveying techniques, but may present cost and resource problems in projects covering larger areas.

Optical means of position fixing available include the marine compass, the theodolite or sextant. Electronic methods i;nclude radar, satellite navigation or a Global Positioning System (GPS). The expense and error of earlier GPS technology is being overcome with the evolution of lower cost hand held GPS receivers capable of giving positions to accuracies of a few metres (GIS User 1992). This should make accurate, rapid position fixing accessible to projects such as this in future, although this approach was not available in this survey.

Optical technologies vary in cost and ease of deployment. A theodolite used in conjunction with an electronic distance measurer will give accuracies to ± 1 m. This combination is best suited to locating permanent positions such as transects for small project areas covering individual bays or estuaries. A sextant in the hands of a skilled operator will give a ± 5 m accuracy, and requires three accurately fixed locations in the area to achieve this (VIMS 1989).

The simplest, cheapest, but least accurate method is the marine compass. This r�quires two visible landmarks to derive a position fix, and accuracies of ± 20 m in inshore areas, declining to ± 100 m offshore can be expected. In small estuaries and bays, with clear landmarks and reference to aerial photographs and charts

·�� .. _{. _ � . '}

for bathymetry, simple line-of-sight observations can be made to similar

accuracies using two or more pairs of landmarks. Although such inaccuracies may seem excessively large, they must be compared to errors generated when mapping from aerial photography. An object of 0.5 mm on a 1:25 000 aerial photograph is equivalent to 12.5 m on the ground, and on a 1:40 000 photograph the same sized object is equivalent to 20 m on the ground. Photograph scales of these magnitudes are common in recent and archival aerial photographs in Tasmania.

Electronic methods, such as radar and satellite navigation, have a range of accuracies, and vary considerably in cost and ease of use (VIMS 1989). These were not used in this survey.

3.1.1.2 Sampling strategies

Sampling strategies must be tailored to the outcome sought. They range between a qualitative observation of the species present, with possibly a coarse assessment of density, to quantitative approaches where biomass and other parameters are measured.

Qualitative methods seeking to give a measure of seagrass abundance tend to be subjective. They require clear definitions to be of use for later research. Relative estimates of percentage cover allow some quantitative analysis of the data, although the possibilities for such analysis are limited to non-parametric or ranking tests (Orth & Moore 1983). Perc�ntage cover can be estimated in the field, or from remote imagery such as aerial photographs. Field estimates offer more scope for accuracy, such as measuring the distance beween shoots to calculate shoot density. Estimation of the percentage cover from aerial photographs allows consistency in comparing different time periods, and is less expensive for large projects, though it is less accurate than field measurements (see Section 3.3.1).

Quantitative methods include establishing transects, and various random sampling strategies. Transects give an accurate measure of vegetative change in a seagrass community along an environmetal gradient, such as salinity or depth. They can be sampled by SCUBA, by a remote vehicle, or more superficially from the surface using a periscope, grab or dredge. Underwater photography or video recording can be employed, or, if SCUBA is used, direct observations can be recorded on a tablet. Many studies have used transects (for example: May

et al.

1978; Orth

et al.

1979; Neverauskas 1987a; Kirkman

et al.

1988; Hillman

et al.

1990), and the establishment of permanent transects using markers embedded in t�e sediment is recommended for long term studies of biomass or Chapter 3. Seagrass Mapping

boundary change

(H.

Kirkman, pers. comm.).

Random sampling is useful for detailed studies of an area, and can be refined by using a stratified sampling approach where homogeneous sub-sections of the overall area are each randomly sampled (Orth & Moore 1983). These authors cite Bulthuis (1981) as an example of the use of stratified random sampling. He divided the study area of Western Port, Victoria, into 25 recognisable strata subdivided into sites that were then randomly sampled. In all random sampling approaches, the statistical issues of the size of each sample and the number of samples taken must be balanced against the cost and effort required to attain increased statistical significance.

3.1.1.3 Sampling apparatus

This section is concerned with physical means of sampling of seagrass vegetation. For some methods of measuring distribution, productivity and density, the physical removal of material from a community is not necessary. If using

SCUBA or snorkelling, quadrats can be surveyed, and many parameters can be measured directly and non-destructively. These include species identification, density, leaf blade dimensions and growth rates, estimates of epiphyte coverage and substratum data. Measurements of the biomass of seagrass or epiphytes by dry weight necessarily requires the removal of shoots and rhizome material.

If working from the surface, other means must be used, which involves apparatus designed to retrieve a sample. If samples. are required solely for qualitative assessment, the reliability and performance of the sampler is not as important as for quantitative sampling (Eleftheriou & Holme 1984). For the latter,. a corer that takes a plug of the vegetation and sediment is more reliable than a grab, which can become snared with long seagrass leaves and fail to reach the sediment and seagrass roots. A corer delivers a sample of consistent surface area,. important for statistical analysis (Orth & Moore 1983). The sample size is commonly 0.1 m2• However, the apparatus needs to be heavy to penetrate the sediment and seagrass root mat, and requires a mechanical means of release and retrieval. If only species identification is needed,. qualitative sampling from the surface can employ a small dredge. In the case of Tasmanian seagrasses,. where the small number of subtidal species can be easily identified from vegetative shoot material,. a grab or dredge is adequate. However,. it is often necessary to obtain a sample of root material to distinguish between Heterozostera tasmanica and

Zostera muelleri, and an efficient,. light-weight and relatively non-destructive apparatus is required For this study,. a modified double-sided anchor dredge was constructed (see Section 4.2).

3.1.2 Remote

Due

to

the shallow marine and estuarine habitat of seagrasses, the community

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