Moreton Bay

Between August and December 1987 the first detailed survey documented the broad extent of seagrass distribution in Moreton Bay and adjacent estuaries⁷. Hyland et al., 1989⁷ identified seven species of seagrass, and estimated approximately 14,170 ha of seagrass meadows (light to dense) and 12,500 ha of sparse or patchy seagrass areas between Coolangatta and Noosa. A recovery of seagrass in Deception Bay and Pumicestone Passage was evident between 1981 and 1987 following the large scale declines of seagrass in these areas during the early seventies.

In August 1998, the distribution of seagrass between Caloundra and Logan River mouth (southern Moreton Bay) was mapped as part of the initiation of the Healthy Waterways Initiative⁸. Approximately 19,109 ±700 ha was mapped within Moreton Bay. The greatest coverage was found in the eastern Bay, Waterloo Bay and northern Deception Bay. The Bay was dominated by Zostera capricorni and the greatest diversity can be found in the eastern Bay⁸.

In 2001 a number of the sites examined in 1998 were re-visited (476 in total), however the meadow extent was not interpreted⁹. The majority of sites ground truthed were within Moreton Bay (south of Bribie Island bridge). Only two meadows (intertidal banks) were examined in southern Pumicestone Passage.

From March to July 2002, seagrass was mapped in Pumicestone Passage between Caloundra and the Bribie Island bridge, including several of the major creeks¹⁰. Approximately 750 ground truth points were examined and non-rectified aerial photography (Beach Protection Authority, July 1999 at 1830m) was used to assist with boundary determination.

The most detailed survey of the bay (between the Bribie Island bridge and Kangaroo Island, from July to August 2004), mapped 183 km2 of seagrass meadows (revised from original 189 km2 using GIS and excluding macroalgae)¹¹. Approximately 10% of Moreton Bay was covered by seagrass, most of which was found on the shallow banks (<5m)¹¹. An additional 10km2 of deep water seagrass (5 12m depth) meadows was mapped in the Middle Banks areas south and west of Tangalooma Point in 2002 and 2005^12,13. Overall, this equates to just under 5% of the total area of seagrass shallower than 15m along eastern Queensland coast. Roelfsema et al., 2009¹¹ estimated that a little over half (56%) of the seagrass meadows in Moreton Bay were of low cover (between 1-25%). Although the % cover is low, these meadows are of high importance to the dugong of the bay as they are grazed on more heavily than denser meadows¹⁴. As adult dugongs have been reported to eat about 28 to 40 kilograms a day¹⁵, it is no coincidence, that one the highest population of dugongs adjacent to a major population centre occurs in the region.

The area of highest dugong population in the bay is the Eastern Banks¹⁶, where seagrass cover has been mapped annually from 1988-2010 using course scale resolution (30 m pixel size) archived satellite imagery (Landsat Thematic Mapper)¹⁷.

The most recent mapping of seagrass communities in Moreton bay has been in response to the impacts from flooding. From mid December 2010 to mid January 2011, heavy rainfall across southern Queensland river catchments, due to TC Tasha combining with a trough during the strongest La Niña weather pattern since 1973, resulted in some of the highest floods in over 30 years. On 11 January 2011 at around 2:30 pm EST, the Brisbane River broke its banks and on 13 January it peaked at 4.46 metres in Brisbane. The floods were the sixth highest in the city's history. Significant flood plumes were reported along the western shores of the bay, immediately adjacent to the Brisbane River mouth. Using the 2004 approach^11,18 remapped the Bay between June and September 2011, and reported 182.3 km2 of seagrass meadows. The survey, however, did not include 23 km2 areas that were mapped in 2004 as seagrass, due to limited resources. If the same survey areas are compared between the 2004 and 2011 surveys, the seagrass extent increased from 160 km2 to 182.3 km2 (i.e. 14%) respectively. The increase in seagrass extent was driven by an increase in area on the Eastern Banks, which was greater than the net loss predominantly occurring on the Western and Southern Bay areas¹⁹. No significant conclusion can be drawn as to whether the loss in seagrass cover on the Western side of the Bay was due to the 2011 floods, as no sufficient data was available immediately prior to the flood.

Following on from the post-flood surveys of 2011, ongoing research on the Eastern Banks area of the Moreton Bay, has mapped seagrass percentage cover, species and biomass in June 2011, February 2012, and June 2012 (C. Roelfsema, UQ, Pers. Comm.). The project is currently finalising the February/March 2013, and June 2013 mapping. This project will compare the results with previous surveys in September 2004, August 2007, September 2008, and December 2009.

During the last week of January 2013, ex-TC Oswald moved south and produced extreme rainfall and flooding in the Brisbane catchments and the ranges along the NSW/QLD border (the area-averaged rainfall for the Brisbane River catchment was very similar to that in the 2011 event) (www.bom.gov.au, accessed 04 March 2013). This resulted in Brisbane River flood waters discharging into Moreton Bay, the second time in 24 months. However, coupled with the floodwaters in 2013 were strong-gale force winds, high tides and associated wave action, resulted in damage to littoral systems and resuspension of sediments throughout the bay.

Concerns have been raised of widespread sediment discharges throughout Moreton Bay and Pumicestone having a more extensive effect on seagrass and marine systems than occurred in January 2011. The extent of the January 2013 sediment pollution plume has been described as heavy and extensive, which was possibly in part exacerbated by the high levels of sediment pollution ‘left in the system’ following the January 2011 floods²⁰. The general perception is that "damage to seagrass and dependent iconic dugong and turtle species is likely to be significant"²⁰. There are also concerns that the ‘easily mobilised sediment and nutrients’ in the upper catchments have again been ‘recharged’ and vulnerable to further flushing during future rains.

Findings from all the assessments conducted to date, indicate that seagrass presence throughout the region is most likely influenced by shelter, sediment characteristics, water turbidity and tidal exposure.