Burdekin

Burdekin

The Burdekin region, includes an aggregation of the Burdekin, Don, Haughton and Ross River catchments and several smaller coastal catchments, all of which empty into the Great Barrier Reef lagoon1. Rainfall is lower than other regions within tropical Queensland with an annual average of approximately 1,150 mm from on average 91 rain days. There is, however, considerable year-to-year variation due to the sporadic nature of tropical lows and storms. Approximately 75 per cent of the average annual rainfall is received during December to March2.

Approximately 18 per cent of the seagrass area mapped in the shallow waters (<15m) of the GBR occurs in the Burdekin NRM region 3. Intertidal seagrasses and shallow subtidal seagrasses dominate in this region, the majority of which are within coastal habitats4. Extensive seagrass meadows occur in Upstart, Cleveland, and Bowling Green Bays and off Magnetic Island. Twelve species have been found within this region5,6. Deep water (>15m) seagrasses occur in this region but are not as common or dense as occurs in regions further north7. Most fringing reefs associated with continental islands support moderately dense mixed species meadows (especially Cymodocea serrulata), which are not restricted to the confines of fringing reefs, but are also found in sheltered bays at continental islands or coastal localities4.

Major threats to seagrass meadows in the region include: coastal development (reclamation); changes to hydrology; water quality declines (particularly nutrient enrichment or increased turbidity); downstream effects from agricultural (including sugarcane, horticultural, beef), industrial (including refineries) and urban centres2,8. All four generalised seagrass habitats are present within the Burdekin region, and MMP monitoring occurs at coastal and reef seagrass habitat locations.

The coastal monitoring sites are located on naturally dynamic shallow sand banks and are subject to sand waves and erosion blowouts moving through the meadows. The Townsville (Bushland Beach and Shelley Beach) area is a sediment deposition zone, so the meadow must also cope with incursions of sediment carried by long shore drift. The Bowling Green Bay (Jerona) location is adjacent to the mouth of Barratta Creek. Sediments within this habitat are mud and sand that have been delivered to the coast during the episodic peak flows of the creeks and rivers (notably the Burdekin) in this area. 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 coastal 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. Another significant feature in this region is the influence of ground water9. The meadows are also frequented by dugongs and turtles as witnessed by abundant grazing trails and patches of cropping .

Conceptual diagrams of seagrass habitat in the Burdekin

 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%. 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.

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.

The 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 shallow banks. 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, Thalassia hemprichii and Halodule uninervis (wide leaf) often occupy the lower littoral/subtidal area, blending with Halodule uninervis (narrow leaved) and Halophila ovalis in the upper intertidal zone. Phosphate is often the nutrient most limiting to reefal seagrasses 10,11. 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 biomass12 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.

 

Marine Monitoring Program

2019 -2020 Summary

Seagrass meadows across the Burdekin NRM region increased slightly in overall condition in 2019–20 but remained POOR. Condition indicators contributing to this were:
  • abundance score was poor
  • reproductive effort score was poor
  • tissue nutrient score was moderate.
Report card of seagrass status indicators and index for the Burdekin NRM region (averages across habitats and sites). Values are indexed scores scaled from 0–100 (± SE) and graded: = very good (81-100), = good (61 – 80), = moderate (41 – 60), = poor (21 – 40), = very poor (0 – 20). NB: Scores are unitless.

Within seagrass canopy water temperatures

Intertidal and subtidal within-canopy temperatures were similar to the previous period and the long-term average. Maximum intertidal within-canopy temperatures exceeded 35°C for a total of 47 days during 2019–20, with the highest temperature recorded at 39.8°C (JR1, 3pm 07Mar20).

Maximum subtidal temperature during 2019–20 was 33.8°C (5pm, 15Feb20). Daily tide exposure was similar to the long‐term average but below the long-term median at all sites, which may have provided some respite from the elevated temperatures.

Long-term monitoring Results:

Climate and Environmental pressures 2019-2020: 20

  • Inshore seagrass sites in the region have a very high frequency of exposure to turbid waters during the wet season and they are the highest among all regions.
  • In 2019–20, exposure to turbid water (classes 1–5) remained at 100% of wet season weeks at all sites i.e. all sites monitored throughout the region were exposed to ‘brown’ or ‘green’ turbid water for the entire wet season.
  • Coastal sites (BB, SB and JR) experienced the highest exposure to ‘brown’ turbid, sediment laden, waters (100 per cent of wet season weeks categories 1–4), which is slightly higher than long-term average (94–99).
  • By contrast, reef sites at Magnetic Island were exposed predominately to ‘green’, phytoplankton rich waters for most of the wet season weeks, and there was less exposure to ‘brown’ water at reef sites compared to average.
  • Daily light levels in the Burdekin region were 10.9 mol m-2 d-1 on average in 2019–20, and therefore above the threshold thought to support optimal growth of 10 mol m-2 d-1.
  • The largest increase in daily incident light occurred at Shelley Beach, where light was 1.7 mol m-2 d-1 higher than average, owing to a large rise in light levels in the late dry season.
  • The only site with lower than average light levels, was Jerona.
  • Intertidal and subtidal within-canopy temperatures were similar to the previous period and the long-term average (Figure 52). Maximum intertidal within-canopy temperatures exceeded 35°C for a total of 47 days during 2019–20, with the highest temperature recorded at 39.8°C (JR1, 3pm 07Mar20).
  • Maximum subtidal temperature during 2019–20 was 33.8°C (5pm, 15Feb20).
Location

Townsville

Monitoring: ongoing
Principal watchers: Seagrass-Watch HQ
Location: on the large inter tidal sand bank in front of Cape Pallarenda and Bushland Beach township, between the Bohle and Black Rivers
Site code: BB1, SB1, SB2
Issues: Coastal development, land runoff
Comments:
The Bushland Beach area is a sediment deposition zone, so the meadow must also cope with incursions of sediment carried by long shore drift. The meadows are frequented by dugongs and turtles as witnessed by feeding trails. These meadows are also visited regularly by recreational fishers. Sediments within this habitat are mud and sand that have been delivered to the coast during the episodic peak flows of the creeks and rivers (notably the Burdekin) in this area.

The Shelly Beach area is a sediment deposition zone, so the meadow must also cope with incursions of sediment carried by long shore drift. The meadows are frequented by dugongs and turtles as witnessed by feeding trials and scars. Sediments within this habitat are mud and sand that have been delivered to the coast during the episodic peak flows of the creeks and rivers (notably the Burdekin) in this area.

Seagrass cover, species composition and canopy height

Macroalgae and Epiphyte cover

Location

Magnetic Island

Monitoring: ongoing
Principal watchers: Seagrass-Watch HQ
Location:
MI1:intertidal fringing reef flat in the north of Picnic Bay, adjacent to the wreck (MI1).
MI2: Fringing reef flat in the eastern corner of Cockle Bay, adjacent to the excavated boat harbour
Site code: MI1, MI2, MI3 (Subtidal)
Issues: Coastal development, ground-water seepage, land runoff, boat and pedestrian traffic.
Comments: Picnic Bay is fringed by coral reefs on its seaward edge and has seagrass growing on its intertidal flat and subtidally, beyond the reef crest. The seagrass meadow at this location is multispecific with stands of Cymodocea serrulata with Thalassia hemprichii, Halophila ovalis (lower intertidal, subtidal), Halodule uninervis (wide) (middle intertidal) and Halophila ovalis/Halodule uninervis (narrow) occupying the upper intertidal region. Patches of Syringodium isoetifolium (shallow subtidal) and Nanozostera muelleri (intertidal) have also been observed within this meadow. Cymodocea rotundata has also been previously recorded from this meadow. Within the Seagrass-Watch site Halodule uninervis(narrow) /Halophila ovalis with Nanozostera muelleri almost encroaching on Transect 1.

Hermit crabs, sesarmid crabs, Astropecten starfish are quite abundant within the site with the occasional dugong feeding trail.

Sea turtle feeding area. Large number of stingrays present. The seagrasses at Cockle bay form an extensive, multi-specific, fringing reef flat meadow. Species found within this meadow include Halophila ovalis, Halodule uninervis, (narrow and wide leaved morphologies) Cymodocea serrulata, Thalassia hemprichii and, recently, a patch of Syringodium isoetifolium has been observed.

Seagrass cover, species composition and canopy height

Macroalgae and Epiphyte cover

Location

Bowling Green Bay

Monitoring: ongoing
Principal watchers: Seagrass-Watch HQ
Location: Adjacent to the mouth of Barratta Creek
Site code: JR1, JR2
Issues: Coastal development, runoff
Comments: The meadows are also frequented by dugongs and turtles as witnessed by abundant grazing trails and patches of cropping

Seagrass cover, species composition and canopy height

Macroalgae and Epiphyte cover

Location

Bowen

Monitoring: ongoing (MMP)
Principal watchers: Jacquie Shiels, Heather Marshall, Shenna Barrett, Elmer Ten-Haken
Location: intertidal banks off Front Beach
Site code: BW1, BW2, BW3
Issues: increasing vessel use, anchor impacts
Comments: important foraging habitat for green sea turtles and dugongs.

Seagrass cover, species composition and canopy height

Macroalgae and Epiphyte cover

Location

Edgecumbe Bay

Monitoring: ongoing ad hoc
Principal watchers: Gudjuda Land & Sea Rangers, Seagrass-Watch HQ
Location: intertidal banks Edgecumbe Bay
Site code: EB1
Issues: increasing land impacts
Comments: important foraging habitat for green sea turtles and dugongs.

Location

Sandfly Creek

Monitoring: suspended
Location: Southern shore of Cleveland Bay
Site code: SC1, SC2
Issues: Sewage treatment outfall, land runoff, coastal development
Comments: Fishing grounds, dugong and turtle feeding grounds. Nursery area for mud and sand crabs.

  • The sites have not been examined since 2004.
  • Insufficient data to describe long-term trends, but early data indicated that abundances showed typical seasonal pattern (higher in late spring-summer than winter).
  • Seagrass abundance at SC2 significantly decreased since mid-2002. Isolated patches of Nanozostera muelleri were in the vicinity.
  • SC1 has always had low seagrass abundance and appears to have remained similar.
  • Species composition appears unchanged over sampling period
  • Canopy height slightly lower although not significant as highly variable
  • Algae and epiphyte currently lower than expected, but not significant as highly variable. The seagrass and mangrove aerial roots were covered in filamentous algae in July 2004, but this seems to be widespread phenomena at this time of year.
  • Sediment appears similar, although at present possibly less muddy

Seagrass cover, species composition and canopy height

Macroalgae and Epiphyte cover

Location

Rowes Bay

Monitoring: training/demonstration site
Location: Rowes Bay, Townsville
Site code: RB1, RB2
Issues: Downstream from estuarine creek, storm water and urban runoff, beach replenishment works.
Comments: The intertidal area of Rowes Bay includes several different marine habitats such as a mangrove forest, a rocky shoreline, a small muddy, coarser sandy and several rubbly reefal areas, one of which includes a tropical sponge garden on the seaward edge. Interspersed between and within all these habitats are seagrasses. Over the years this seagrass meadow has come and gone, clearly demonstrating the ephemeral nature of intertidal seagrass meadows. The Seagrass-Watch site is south-east of the estuarine creek and predominantly Halodule uninervis, interspersed with small amounts of Halophila ovalis. Macroalgae is also quite common within this site. This meadow is a highly disturbed site with regular pedestrian traffic, inputs of freshwater and associated sediment loads. Invertebrate diversity is high.

Seagrass cover, species composition and canopy height

Macroalgae and Epiphyte cover

Location

PALM ISLAND

Monitoring: archived
Location: Palm Island, Townsville
Principal watchers: Seagrass-Watch HQ
Location: on the inter tidal bank in front of Fred Clay Freedom Park (Palm Island)
Site code: PM1

Current status unknown

Seagrass cover, species composition and canopy height

Macroalgae and Epiphyte cover

Location

LUCINDA

Monitoring: archived
Location: Lucinda, Halifax
Principal watchers: Seagrass-Watch HQ
Location: on the inter tidal Sand bank in front Lucinda
Site code: HX1

Current status unknown

Seagrass cover, species composition and canopy height

Macroalgae and Epiphyte cover