Sub-tidal seagrasses of the Torres Strait: spatial patterns and nutritional value for marine herbivores

Carter, Alex B., Sheppard, James K., Coles, Robert G., and McKenzie, Len J. (2007) Sub-tidal seagrasses of the Torres Strait: spatial patterns and nutritional value for marine herbivores. Report. Department of Primary Industries and Fisheries (DPI&F), Cairns, QLD, Australia.

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The distribution and nutritional attributes of sub-tidal seagrasses from the Torres Strait, a narrow stretch of water between Cape York (Queensland, Australia) and Papua New Guinea, are described in this report. This project, Understanding Ecosystem Processes, was a joint tender to CRC Torres Strait by the Queensland Department of Primary Industries and Fisheries (Marine Ecology Group), CSIRO, Geoscience Australia, and the Queensland Museum.

Seagrasses analysed in this study were collected using a 1.5m epibenthic sled with net backing (10 mm square mesh) and using a stratified representative pattern during a broad scale survey conducted March - April 2005. Nutritional components of seagrass were analysed using near infrared reflectance spectroscopy.

We examined the distribution and nutritional value of seagrasses in relation to depth and other physical characteristics to increase understanding of factors influencing seagrass distribution and spatial variation in the nutritional value of seagrasses to marine herbivores. Torres Strait seagrasses provide critical habitat for commercial and traditional fishery species including penaeid prawns, tropical rock lobster, dugong and green sea turtles.

Seagrass was recorded at 33.3% of the 168 sub-tidal sites surveyed. The region was characterised by high species diversity with eleven species recorded: Halophila spinulosa, H. ovalis, H. decipiens, H. capricorni, Syringodium isoetifolium, Cymodocea serrulata, C. rotundata, Enhalus acoroides, Halodule uninervis (wide), Thalassia hemprichii, and Thalassodendron ciliatum. Sub-tidal seagrasses typically occurred in large multi-species meadows and displayed a clear east-west partitioning, being abundant in central and western Torres Strait and uncommon in the eastern Torres Strait. Seagrass was most likely to grow in sites where sediments were comprised primarily of muddy sand to gravelly sand. T. hemprichii, C. rotundata, H. uninervis and S. isoetifolium were recorded at depths greater than 25 m, considerably deeper than the 5 - 15 m range these species have previously been found.

The nutritional value of the four most common sub-tidal seagrass species: H. spinulosa, H. ovalis, C. serrulata and S. isoetifolium, was examined. Composition of digestible cell contents (nitrogen, water soluble carbohydrates, starch) and indigestible cell wall components (neutral detergent fibre, acid detergent fibre, lignin) were evaluated as well as in vitro dry matter digestibility and organic matter. There were no significant effects of individual sample site or sample site latitude on the nutrient concentrations of individual seagrass species. When starch was mapped there were two distinct regions containing seagrasses with high nutritional value (north-central and south-western regions). With the exception of a negative linear relationship between starch concentration and water depth for S. isoetifolium, depth had no significant effect on any of the nutrient levels sampled across any seagrass species.

The nutritional profiles of the four tropical seagrasses we sampled were consistent with previous studies of the same species. Significant variation for total starch, water-soluble carbohydrates, in vitro dry matter digestibility, organic matter, acid detergent fibre, neutral detergent fibre and lignin was recorded among the four species. Seagrass nitrogen and starch concentrations, the most important nutrients for marine herbivores, were significantly correlated with species and with plant component. For all seagrass species, the above-ground component (shoots and leaves) possessed greater nitrogen concentration than the below-ground component (roots and rhizomes), which possessed greater starch concentration. S. isoetifolium had the highest nitrogen concentration (1.40 ± 0.05% DW). However, it also had higher fibre concentration (38.2 ± 0.68% DW) relative to the other four species. H. ovalis possessed the highest starch concentration (2.76 ± 0.12% DW) and highest digestibility (83.24± 0.66 % DW) as well as the lowest fibre (27.2 ± 0.66% DW). The combination of broad geographic and depth distributions and nutritional superiority suggests H. ovalis and S. isoetifolium provide a potentially important food source for marine herbivores that forage sub-tidally in the Torres Strait.

The area surveyed represents almost the entire Torres Strait region and provides a valuable snapshot of the distribution and nutritional value of sub-tidal seagrasses in this region. This study contributes to the maintenance of information on seagrass distribution in the Torres Strait, and provides the first baseline study of regional variation in nutritional composition of the four most common sub-tidal seagrasses in Torres Strait. This information can be used in developing Torres Strait environment and fisheries management plans.

Item ID: 39874
Item Type: Report (Report)
ISSN: 0727-6273
Keywords: recovery, resilience, management, environmental monitoring, marine monitoring, marine water quality, seagrass, reproduction, seagrass habitats, Great Barrier Reef, port development, dredging
Funders: CRC Torres Strait
Projects and Grants: DPI&F Marine Ecology Group (MEG)
Date Deposited: 25 Aug 2015 06:48
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050206 Environmental Monitoring @ 50%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050205 Environmental Management @ 50%
SEO Codes: 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 50%
96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960503 Ecosystem Assessment and Management of Coastal and Estuarine Environments @ 50%
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