Sediment and nutrient dynamics in coastal intertidal seagrass of north eastern tropical Australia

Mellors, Jane Elizabeth (2003) Sediment and nutrient dynamics in coastal intertidal seagrass of north eastern tropical Australia. PhD thesis, James Cook University.

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Abstract

In this thesis, I aimed to assess the interactions between the sediment structures and the nutrient environments of tropical seagrasses inhabiting intertidal meadows within the central region of the Great Barrier Reef World Heritage Area. I first surveyed 11 intertidal seagrass meadows and created an inventory of their sediment, nutrient, and seagrass parameters. This investigation revealed that the porewater nutrient environments of these meadows were low in a global context and below the critical values (60–100 μM NH4 + and 10 μM PO4 3-) that have been suggested as being limiting to seagrass growth. The biomass of these meadows was also low, with below ground biomass (roots and rhizomes) constituting the largest fraction (c. 60%) of the total biomass. In contrast, the concentration of plant tissue nutrients (%N and %P) recorded for Halodule uninervis and Halophila ovalis were higher than those (1.8%N and 0.2%P) calculated by Duarte (1990) as the thresholds distinguishing between seagrass meadows that would respond to nutrient enhancement from those meadows that would not. I then examined the following four aspects of the generally accepted views concerning seagrass ecosystem function for a variety of locations spanning >500 km of coast: i) that the presence of seagrass regulates sediment structure and nutrient environment; ii) that the abundance of seagrass modifies sediment structure; ii) that seagrass abundance is affected by its sedimentary nutrient environment; and iv) that plant tissue nutrients reflect their sedimentary nutrient environment. I found that the presence of the structurally small seagrasses I examined did not regulate sediment structure and nutrients. Additionally at this scale, seagrass abundance did not appear to moderate sediment structure, or be influenced by sediment nutrient levels, which in turn were not reflected in the concentrations of plant tissue nutrients. The most salient outcome from these investigations was the significance of geographical location on the properties I studied, indicating the important influences of the local environment (physical, chemical and nutrient) on local populations of seagrass. The relationships between the abundance of the seagrass and the plant tissue nutrients to the sediment nutrients may have reflected the time of year (July, Southern Hemisphere winter) in which these investigations were undertaken. Consequently I examined intraannual variation within two locations to establish the effects of season on seagrass abundance, tissue nutrients, growth and sediment nutrients. I found that the ambient nutrient levels, seagrass abundance, plant tissue nutrients and plant growth parameters within the two Halophila ovalis meadows sampled, differed between the months selected to represent a seagrass senescent season and a growing season. In general, sediment nutrients were greater during the month representing the senescent season, whilst seagrass abundance and growth were greater during the month representing the growing season. With this knowledge, I designed an experiment that investigated the response of sediment nutrients and seagrass of the two Halophila ovalis meadows to additions of Osmocote® fertilizer. These experiments highlighted the complexity of field experimentation because of the inability to control environmental parameters. Despite the positive and relatively consistent response of sediment nutrients to the additions of fertilizer, the response of Halophila ovalis varied according to location, season and experimental manipulation. Halophila ovalis at Bolger Bay (characterized by clay mineralogy) showed P limitations during the senescent season (August), with no significant response during the growing season (November). In contrast, Halophila ovalis at Picnic Bay (carbonate sediments), displayed no significant response to additional nutrients during the senescent season, but displayed primary N limitation with secondary P limitation during the growing season. The significant responses were relative to the experimental control and not the ambient field control, indicating the need to examine past experimental evidence in relation to how the fertilizer was added and the type of control used to compare the response. Overall, intertidal seagrass meadows that are characterized by structurally small species in this region do not conform to the generally accepted views on seagrass, sediment, and nutrient interactions. These tropical meadows are substantially influenced by seasonal cues. The predominant effect of location also indicates the importance of the local environmental/geographical history to which the seagrass meadow has been subjected. This result was highlighted by the nutrient enrichment, where the two different meadows displayed opposing patterns of nutrient limitation despite their proximity Overall my results suggest that a broad-brush management approach to the research and management of seagrass habitats in this dynamic and environmentally variable region is inappropriate.

Item ID: 1148
Item Type: Thesis (PhD)
Keywords: Sediment structures, Nutrient environments, Tropical seagrasses, Central region of the Great Barrier Reef World Heritage Area, Porewater nutrient environments, Biomass, Plant tissue nutrients, Halodule uninervis, Halophila ovalis, Sedimentary nutrient environments, Local environment, Effects of season, Fertilizer, Intertidal seagrass meadows
Date Deposited: 25 Oct 2006
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 0%
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