Spatial variation of Zostera tasmanica morphology and structure across an environmental gradient

Abstract

Spatial variation in morphological and structural characteristics of the seagrass Zostera tasmanica was examined in Westernport, an estuarine embayment in south-eastern Australia. An environmental gradient exists within the embayment, with moderate sediment loads and resuspension in the north correlating with high turbidity, chlorophyll a and reduced photon flux density (PFD). This gradient generally decreases towards the south, where oceanic influences and lower catchment inputs result in less turbid waters, with increased PFD and reduced water column chlorophyll a. We investigated variation in Z. tasmanica characteristics across this environmental gradient. A hierarchy of scales was examined ranging from cm within sites (meadows), km between sites, and 10s of km between regions. Structural variables examined included shoot density, seagrass cover and biomass, while morphological variables were number of leaves, leaf and total shoot weight, number of nodes, internode length and shoot length. Plants from northern regions of Westernport were characterised by low leaf numbers, low shoot weight and few internodes, suggesting either a loss of condition in these plants or younger plants with high shoot turnover. In contrast, plants from the mid to southern regions had more and longer leaves, more nodes and greater internode lengths, and structurally these meadows exhibited greater densities, cover and biomass. Partitioning of the variation showed that region (>20 km apart) contributed the greatest percentage of variation for a number of variables (e.g. shoot numbers, numbers of nodes, internode length and seagrass cover), suggesting the importance of the environmental gradient in shaping seagrass form and meadow structure. These variables are potentially useful indicators of anthropogenic influences driving seagrass morphology and structure at ecosystem scales. © Inter-Research 2005.