Sensitivity of transects across a depth gradient for measuring changes in aerial coverage and abundance of Ruppia megacarpa Mason
Introduction
Managers often require an assessment of whether a dominant aquatic macrophyte is stable within a system, or undergoing long term increases or declines in aerial coverage and abundance. Such information is sought to determine whether a system is being impacted and, subsequently, to assess the effectiveness of management decisions taken to ameliorate an impacted system. Where an impact has occurred, however, measurement of these long term directional changes in aerial coverage and abundance of aquatic macrophytes is difficult and often inconclusive due to large variation in estimates. Many methods have been used, but the usefulness of the different methods depends on the macrophyte species and system characteristics (Kirkman, 1996). Wilson Inlet is an estuary with a predominantly rural catchment (Hodgkin and Clark, 1988), showing signs of eutrophication, suggesting that both short and long term management strategies are required (Atkins et al., 1993). The aim of this study was to assess the use transects across a depth gradient for determining long term changes in aerial coverage of Ruppia megacarpa within Wilson Inlet.
Wilson Inlet is a seasonally closed lagoonal estuary located on the southwest coast of Australia. It is a ‘bar-built’ estuary (Lane and McComb, 1988), approximately 14 km long by 6 km wide and closed to the ocean for approximately 7 months of the year (Lukatelich et al., 1987). A channel to the ocean is cut through the sand bar annually, usually in August when the water level within the Inlet reaches 1.015 m above low water mark (Lukatelich et al., 1987). This increase in water depth is caused by runoff from winter rains being trapped in the closed estuary.
Wilson Inlet has two deeper basins (3+ m) in the eastern and western regions of the Inlet (Fig. 1). Shallow flats (1–1.5 m) occur around the edges of the system. These are particularly wide along Eden Bank, west of the Hay River-mouth and near to Poddy Point (Fig. 1). Surveys conducted in 1982, 1983 and 1994 found these shallow flats to have the highest biomass of the dominant aquatic macrophyte R. megacarpa. Maximum biomass varied from 400 g m−2 (Bastyan et al., 1995) to 1200 g m−2 (Lukatelich et al., 1987).
Long et al., 1996 studying tropical seagrass meadows, concluded that inter-annual changes in aerial extent of seagrass of 50% or more and changes in above ground biomass of 70% or more were cause for concern. These authors suggest that once baseline mapping has occurred, statistically valid measures should be taken on a subset of the meadows (Long et al., 1996). This is particularly important in systems dominated by Ruppia, which has high annual variation relative to other seagrass species (Thorne-Miller et al., 1983). Previous studies within Wilson Inlet have produced baseline maps of R. megacarpa meadows. This was based on a visual assessment of a series of transects and depth contours using manta tows (Bastyan et al., 1995).
Light is often considered the main determining factor controlling the maximum depth limit, growth and photosynthesis of submerged aquatic plants (Verhoeven, 1980, Bulthuis, 1983, Dennison and Alberte, 1986, Dennison, 1987, Duarte, 1991). This relationship has been used to predict growth and survival of submerged aquatic vegetation from simply measured water quality parameters such as Secchi depth (Dennison et al., 1993). Use of a Secchi disc is a simple method for measuring attenuation of light through water and is often used to approximate the attenuation coefficient (Kirk, 1994). Secchi depth is equivalent to between 10% (Strickland 1958 in Dennison, 1987) and 18% (Bulthuis, 1983) of surface irradience and when averaged over the year has been found to approximate the light compensation depth for the seagrass Zostera marina (Dennison, 1987), and other species (Adair et al., 1994). Seagrass distribution data indicate that submerged angiosperms require at least 5–15% of surface irradience to survive (Bulthuis, 1983). In situ reductions in light intensity have been found to cause total death of seagrass, where light drops to less than 16 and 10% surface irradiance for Thalassia testudinum and Halodule wrightii, respectively, (Czerny and Dunton, 1996). When incident light to a Ruppia meadow in the Blackwood River Estuary was reduced to 20% of incident irradiance for 100 days, there was a 50% reduction in biomass (Congdon and McComb, 1979).
Permanent and temporary transects have been frequently used for comparing depth limits of aquatic macrophytes with their light requirements (Middelboe and Markager, 1997), and for assessing variation in the distribution of macrophytes (Orth and Moore, 1988, Comin et al., 1993, Skubinna et al., 1995). Such an approach can potentially provide both specific and general information about the growth and survival of submersed aquatic macrophytes.
The questions addressed in this paper were: (1) can Secchi depth measurements be used to predict when R. megacarpa meadows are declining or expanding? and (2) can depth transects be used to detect long term changes in aerial cover and abundance of R. megacarpa?
Section snippets
Methods
Permanent transects were established perpendicular to depth contours from inside of R. megacarpa meadows and beyond the edge of the meadows. These were compared to Secchi depth data collected weekly within the Inlet.
Depth contours of transects
All transects had a similar mean depth range, from 1.0/1.5 m depth (0 m) down to 2.5/3.0 m (50 m) (Fig. 2). There were two different patterns of depth contour in the three transects. South Gutter and Poddy Point were almost level from 0 to 30 m along the transect, then showed a gradual increase in depth until 40 m along the transect where a very steep increase occurred to the end of the transect. In contrast, Springdale Beach showed a gradual increase in depth along the entire length of the transect (
Discussion
Different annual cycles in percent cover of R. megacarpa meadows at different locations in Wilson Inlet, suggests that monitoring transects in individual meadows will be useful in determining long term changes in aerial coverage and abundance of seagrass in the system.
Variation in light availability did not explain all patterns in percent cover along depth transects in the Inlet. R. megacarpa growing along the South Gutter and Poddy Point transects may be highly influenced by other factors such
Acknowledgements
Funding for this work was provided by Water and Rivers Commission, WA, Australia. This study constitutes work towards a Ph.D by TJBC, who was co-supervised by AJ McComb. Secchi data was supplied by Geoff Bastyan, who also provided valuable input in the initial stages of this study. Thanks to Jane Wilshaw, Simon Montgomery, Bernie Reagler, Mike Howell, Grey Coupland and Tom Davis for their assistance in the field.
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