The Townsville region is on the dry tropical coast of north-eastern Queensland. Intertidal and shallow subtidal seagrasses predominate and tend to form multi-specific meadows that are arranged in mono-specific bands across a depth gradient. True reefal seagrasses are also rare in this region, but most fringing reefs associated with continental islands support moderately dense mixed species meadows. Area of seagrass in the region is estimated at 130 km2, of mostly moderate (11-49%) and light (1-10%) cover.
In the Townsville region, there are many seagrass meadows along Cape Cleveland, the Strand, Cape Pallarenda, and around Magnetic Island. The main seagrass species in shallow waters near Townsville are Halophila ovalis, Halodule uninervis, Zostera capricorni, and Cymodocea serrulata. Thalassia hemprichii can also be found on the reef flats of Magnetic Island. Halophila spinulosa that has been washed up from deeper waters can sometimes be found. Despite the variable inter-annual rainfall that occurs along this coastline, cyclones are a frequent feature(~15 cyclones/decade). In April 2000, intertidal seagrass meadows in the Townsville region were decimated by cyclone "Tessi" but are now virtually recovered.
The distribution of seagrasses along this coastline is predominately influenced by seasonal (April-November) south-easterly trade winds. Seagrass meadows generally establish in places that offer protection from these winds, such as the large north opening bays and the lee ward sides of continental islands. The combination of seasonal terrestrial run-off, frequent cyclones, strong south-easterly trade winds and large tidal runs (in the south) creates significant coastal turbidity. Consequently seagrasses that inhabit this area are subjected to low light regimes, and high influxes of freshwater and sediment. To survive this regime seagrasses need to exhibit high vegetative growth rates and prolific seed banks. This has probably led to the predominance of opportunistic species, such as Halodule and Halophila within this region.
The greatest threat to seagrass throughout this region is agricultural land clearing (both grazing and cropping) and its inherent problems of soil erosion and associated loads of nutrients and pesticides.
Monitoring occurs at three or the four generalised seagrass habitats within the Townsville region: coastal, estuarine and reef.
Below is a conceptual diagram of coastal seagrass habitat in the Townsville region. Coastal habitats are generally on naturally dynamic intertidal sand flats and are subject to sand waves and erosion blowouts moving through the meadows. While episodic riverine delivery of freshwater nutrients and sediment is a medium time scale factor in structuring these coastal seagrass meadows, it is the wind induced turbidity of the costal zone that is likely to be a major short term driver. In these shallow coastal areas waves generated by the prevailing SE trade winds are greater than the depth of water, maintaining elevated levels of suspended sediments, limiting the amount of light availability for photosynthesis during the trade season. Intertidal seagrasses can survive this by photosynthesizing during periods of exposure, but must also be able to cope with desiccation. Another significant feature in this region is the influence of ground water.
Estuarine habitats within this region are in open estuaries/inlets and tend to be continuous with the coastal habitats. Both tidal and subtidal distributions are heavily influenced by their proximity to creeks and rivers. Delivery of freshwater and suspended solids, including clays, colloids, fine organic matter, is the major impact to all meadow types in this habitat. This influence extends beyond the estuarine zone during high flow events. Of the total sediment being delivered to the GBRWHA lagoon, the long term average discharge of sediment from this region represents 20%-40% (Burdekin Dry Tropic Board 2005). The major impact of terrestrial run-off on near-shore environments occurs during cyclones or heavy monsoonal rains and is only delivered during infrequent flooding events. Relatively low sediment discharge occurred over the intervening years. During flood events these habitats are subject to scouring, a combination of fast flow and course sediments. The habitats are productive nursery areas, particularly the southern parts of Cleveland and Bowling Green Bay as these are areas targeted by commercial and recreational fishers.
Reef habitats are mainly represented by fringing reefs on the many continental islands within this area. Most fringing reefs have seagrass meadows growing on their intertidal flats. Nutrient supply to these meadows is by terrestrial inputs via riverine discharge, re-suspension of sediments and groundwater supply. The meadows are typically composed of zones of seagrasses. Cymodocea serrulata and Thalassia hemprichii often occupy the lower intertidal/subtidal area, blending with Halodule uninervis (wide leaved) in the middle intertidal region. Halophila ovalis and Halodule uninervis (narrow leaved) inhabit the upper intertidal zone. Studies from overseas have often implicated phosphate as the nutrient most limiting to reefal seagrasses. Experimental studies on reef top seagrasses in this region however, have shown seagrasses to be nitrogen limited primarily with secondary phosphate limitation, once the plants have started to increase in biomass (Mellors 2003). In these fringing reef top environments fine sediments are easily resuspended by tidal and wind generated currents making light availability a driver of meadow structure.
Within seagrass canopy water temperatures
Within canopy temperature was monitored at coastal and reef-platform locations and generally follow a similar pattern. Mean temperatures were mostly within the 20 – 30°C range, with highest mean temperatures in the January to February period. No extreme temperatures (>40°C) were recorded over the last 12 months. Maximum temperatures peaked several times throughout the year at all locations, generally during the time of low spring tide of the Dry season.
Townsville
