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Using Seagrasses to Understand the Condition of the Estuary

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Using Seagrasses to Understand the Condition of the Estuary Department of Water 21 Science supporting estuary management IssueIssue 1, February 21, April 2000 2013 Using seagrasses to understand the condition of the estuary Macrophytes are aquatic plants that may have fully effectively as an indicator of ecological health. These Contents submerged, emergent or floating growth forms. This include understanding: article will deal primarily with one species of fully Seagrasses within the • the natural variability of the characteristic/s estuary...............................1 submerged macrophyte, the seagrass Halophila measured ovalis (common name ‘paddleweed’), which is Seagrass distribution ........2 found within the shallows of the Swan-Canning • the expected response of the organism to changes Seagrass depth range ......2 estuary, Western Australia. For information on the in environmental condition Seagrass growth biology and historical distribution of this species • the sensitivity of the species (a successful indicator requirements .....................4 in the Swan-Canning estuary see River Science 20. has the response to environmental changes greater Measures of ecophysiology: a pilot study .......................4 Estuarine environments are particularly vulnerable than the natural variability for the measured to human impacts. Eutrophication has increased characteristic). Halophila ovalis physiological responses ....5 nutrient and organic matter deposition within This article reports on work (undertaken between Sediment conditions affect estuaries worldwide. Associated changes in the 2006 to 2008) to understand the distribution and seagrass growth ................6 condition of the sediment and decreased light physiological behaviour of H. ovalis and examines Future directions ...............6 availability results in additional stresses for benthic the potential of using this seagrass as an indicator organisms. of estuarine health in the Swan-Canning estuary. Glossary ............................7 H. ovalis is one of the most widely distributed References ........................8 seagrasses in the world. Our interest in this seagrass Seagrasses within the estuary Acknowledgments .............8 is its potential to be used as a biological indicator For more information .........8 (bioindicator) of estuarine condition. Bioindicators A good understanding of the extent and abundance of are organisms which are used to monitor the health of seagrasses is required to show evidence of changes an environment or ecosystem. The primary concept here is that if the organisms within the estuary are healthy, their environment is also in good health. By measuring appropriate characteristics of the bioindicator, environmental managers can gain information about a particular aspect of the environment, e.g. contaminant exposure, water quality or changes in the ecosystem. An advantage of using bioindicator species to assess environmental quality is that a species living in situ provides a time- integrated measure of the environmental condition. River Science There are many factors to understand before Halophila ovalis in Lucky Bay, Swan-Canning Estuary April 2013 H. ovalis (or any bioindicator species) can be used (Kilminster 2011) Page 1 in seagrass distribution. Light availability is one of ‘backscatter’. Surfaces that are hard or rough will the main controlling factors influencing seagrass produce high backscatter values, whereas smooth distribution. Light is strongly absorbed by the water surfaces produce low backscatter values. It is this and the growth of seagrass at the deepest edge of a distinguishing property which can be used to create seagrass meadow is limited by light. seagrass distribution maps, as seagrasses give a higher backscatter response than the surrounding Two long-term measures of seagrass status were proposed. The first measure was to record the bare sediment. Whilst this method is quite well presence and absence of seagrass and produce maps established for seagrasses in marine systems, the of seagrass distribution. The second measure was Swan-Canning system posed a scientific challenge to record the depth range of the seagrass, i.e. the as the shallow water limit for the technique was difference between the shallowest and deepest point poorly understood at the time. at which seagrass occurred. These measures – if Ten foreshore areas within the Swan-Canning estuary repeatable over several years – could show loss or were surveyed, equivalent to 4.4 km2. Sidescan gain of seagrass habitat as conditions improved or sonar was able to identify seagrass areas from bare declined. sediment. Backscatter values were categorised into groups describing habitat type: 1) mud/sandy mud, 2) sand, 3) seagrass low-medium density, 4) seagrass Seagrass distribution medium-high density and 5) rock/coastline. These A number of methods have been used to measure categories were groundtruthed with diver transects seagrass distribution within estuaries. Aerial identifying areas of < 5%, 5–50% and > 50% photography, sidescan sonar and video surveys, seagrass coverage (see backscatter maps). as well as diver-intensive techniques have all Producing seagrass distribution maps by sidescan been used to create seagrass distribution maps in sonar was successful and would be repeatable without Western Australia and the world. However, there operator bias. However, the equipment to carry out are limitations associated with the reproducibility, these surveys is expensive and requires experts to bias, cost, frequency, and accuracy of these operate and interpret the acquired data. Effort is methods. For example, seagrass mapping by aerial also required to groundtruth the data collected by imagery introduces errors associated with sun the sidescan sonar. Increasingly, the Department of glitter, turbidity, differences in depth and floating Water has been looking towards video surveys for macroalgae. Diver-intensive techniques are labour- a repeatable, low-tech and relatively inexpensive intensive (therefore expensive and time-consuming), method for producing seagrass distribution maps. and difficult to reproduce with accuracy since Video surveys have been tried successfully in several meadow density classification has a bias associated south-west estuaries to produce seagrass distribution with the individual diver’s interpretation. maps. Some advantages of the video surveys are the 0.60 Classifying criteria ability to identify species, estimate plant density and 450 note the presence of epiphytes or macroalgal species. Coastline Rock/Coast (> 0.65) 400 0.55 Seagrass dense (0.575–0.65) (m) Seagrass sparse (0.5–0.575) 350 Sand (0.5–0.45) Seagrass depth range 300 0.50 Mud/sandy mud (< 0.45) 250 The depth over which seagrasses are able to live 0.45 200 may provide a robust measure of habitat quality. The Distance depth range is the distance between the shallowest 150 0.40 and deepest points where seagrass is present. A larger -300 -200 -100 0 100 Distance (m) depth range would indicate better environmental Backscatter maps from sidescan sonar of Lucky Bay, also showing groundtruthed data conditions (improved light penetration). Seagrass by diver transects, where seagrass coverage: Ο ≤ 5%, Ο = 5–50%, Ο ≥ 50% depth range may provide an indicator of estuarine health by integrating the amount of light available In 2008, Curtin University (Centre for Marine for photosynthesis over an extended period. This Science and Technology) undertook a survey of surveying approach (dumpy-level method) has been the seagrass within the Swan-Canning estuary used previously in south-east Queensland (EHMP using acoustic remote sensing with sidescan 2007). sonar. Sidescan sonar uses sound waves which are transmitted in pulses. The sound waves travel Researchers from Edith Cowan University River Science through the water until they hit the sea floor which investigated the suitability of determining depth April 2013 bounces part of the wave back to the sidescan sonar range of seagrass at sites in the Swan-Canning instrument. The reflected energy is referred to as estuary using the dumpy-level method. Three sites Page 2 52A KNOT 44C MBSC (14) KNOT BEACON were studied: Como Beach, Matilda BayCRAWLEY and Waylen MBSC START Bay. Other sites (Chidley Point, Freshwater Bay and Royal Perth Yacht Club PELICAN 44B Point Resolution) were disregarded during the initial PELICAN Mounts Bay Sailing Club site selection as seagrasses were observed deeper POINT than 5 m, which was impracticable for the method. A rc The method involved three people: one person in SWAN ESTUARY MARINE PARK of (Pelican Point) V the water holding the staff gauge at the edge of the Refer to Dept of Environment seagrass meadow, a second person marking the & Conservation Publications (16) INNER (8 Knots) position of the transect and a third person reading (27) GARDEN the theodolite (a precision surveying instrument BANK used for measuring angles). Measurements were taken at the shallowest (inner) and deepest (outer) 41C NEDLANDS BATHS edge of the seagrass meadow and the depth range 41B NEDLANDS was calculated from these. (26) NEDLANDS 41A HALLMARK The shallowest seagrass meadows were located in (28) DALKEITH 42B DOLPHIN EAST 0.8–1 m of water and the deepest seagrasses were found around 2.8 m for Como Beach, Matilda SurveyM location E L atV Pelican I L Point,L E where dark blue continuous line shows pathway of sidecan sonar Bay and Waylen Bay. The largest depth range 6 × 10 (and smallest meadow extent) was seen at Como 6.4604 Beach, reflecting the steepness of the near-shore environment
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