The Cairns region is on the wet tropical coast of northeastern Queensland. It includes several major catchments and river systems. Agricultural (e.g. grazing, forestry and cropping) and urban developments are the primary land uses in coastal catchments and contribute high loads of nutrients and sediments to nearshore coastal waters.

Thirteen seagrass species have been recognised for this region. The most extensive areas of seagrass occur around Low Isles, Green Island and Cairns Harbour. Intertidal seagrass meadows in the region are situated along nearshore sand and mud banks and mostly consist of Halodule uninervis and Halophila dominated meadows. In Cairns Harbour however, meadows are dominated by Zostera capricorni. Shallow subtidal coastal meadows consist of Halodule uninervis and Halophila communities mostly found along the sheltered coasts and harbours. Cymodocea spp., Thalassia hemprichii, Syringodium isoetifolium and a suite of Halophila species dominate intertidal reef platform habitats of this region.

Seagrass meadows throughout the region are characterised by low nutrient concentrations and are primarily nitrogen limited. Nitrogen inputs to marine waters can therefore directly promote seagrass growth. The indirect consequences of high sediment and nutrient runoff can on the other hand detrimentally influence seagrass distribution and seasonality in the region. From December to March (wet season) low seagrass abundance often coincides with high rainfall and high loads of sediments and nutrients to nearshore marine waters. Sediments and nutrients decrease light availability for seagrass by reducing light penetration in the water column and promoting the growth of algae that can shade seagrass blades. Warm water temperatures during summer also promote growth of algae and can lower productivity of seagrass meadows. Coupled with physical disturbance from storm activity seagrass growth and abundance may therefore be severely depleted over the wet season. Seagrass meadows are generally most abundant in October-November following the dry season that has more favorable light and temperature conditions.

Monitoring of intertidal seagrass meadows occurs in three seagrass habitat types in the region: coastal, reef and estuarine.

Below is a conceptual diagram of coastal seagrass habitat in the Cairns region – major controls are pulsed terrigenous runoff from seasonal rains, salinity and temperature extremes, and sediment disturbance.

Below is a conceptual diagram of reef seagrass habitat in the Cairns region. Shallow unstable sediment, fluctuating temperature, and variable salinity in intertidal regions characterize these habitats. Physical disturbance from waves and swell and associated sediment movement primarily control seagrass growing in these habitats. Reef seagrass habitats in the region are often adjacent to areas of high tourism use and boating activity with propeller and anchor scarring impacts. Globally, nutrient concentrations are generally low in reef habitats due to the coarse nature of the sediments coral sand. In these types of carbonate sediments the primary limiting nutrient for seagrass growth is generally phosphate. This is due to the sequestering of phosphate by calcium carbonate sediments. In this region seagrass meadows inhabiting the near shore inner reefs and fringing reefs of coastal islands inhabit a mixture of terrigenous and carbonate sediments, such as Green Island. Seagrasses at this location have been shown to be nitrogen limited (Udy et al. 1999).

Below is a conceptual diagram of estuarine seagrass habitat in the Cairns region. These habitats are also closely associated with mangrove forests, characterised by fine sediments and prone to high sedimentation and anoxic conditions. The physical and biological connectivity between these habitats is of high importance to support the detrital food chains and the productive faunal communities. The dominant influence of estuary habitats is terrigenous runoff from seasonal rains. Increased river/creek flow results in higher sediment loads which combine with reduced light to create potential light limitation for seagrass (McKenzie 1994). Estuary habitats also have higher loadings of micro and macro-algal epiphytes than those in other seagrass habitats. Salinity fluctuations and scouring make estuary and inlet habitats a seasonally extreme environment for seagrass growth. Estuary habitats also include the ports and shipping activities (including dredging).

Within seagrass canopy water temperatures

Mean monthly within seagrass canopy water temperatures in the Cairns region are generally within the 22 – 31°C range, with highest mean temperatures in the late Dry and Monsoon seasons. Temperatures at the coastal and reef-platform locations generally follow a similar pattern. Mean temperatures over the 2010/11 summer were the highest recorded since monitoring was established, although no extreme temperatures (>40°C) were recorded.

Extreme within seagrass canopy water temperature (41.5°C) was recorded at YP1 on the 5th February 2008. The maximum temperatures recorded across the region over the last 12 months were in early March 2010 during low spring tides. Maximum temperatures reached 37.2°C at Green Island (8th March) and 38°C at Yule Point (9th March). Yule Point temperatures also peaked at 38°C on two other occasions (9th and 27th February).

Reef Rescue Marine Monitoring Program (2009-2010)

Seagrass meadows were monitored at Green Island (reef) and Yule Point (coastal). In 2009/10, seagrass remained in a good state and the meadows either expanded or stabilised over the monitoring period. Seed banks and reproductive effort decreased below the GBR long-term average, indicating lower recovery potential to disturbances.

Leaf tissue nutrient ratios suggest the potentially higher light environment in reef habitats than coastal, however lower C:N ratios at Green Island since 2006 indicate decreasing light availability which may be a consequence of elevated epiphyte fouling. Leaf tissue nutrient ratios at Yule Point indicate high levels of nutrients, with N:P ratios generally increasing over time indicating increased nitrogen availability. Epiphyte fouling of seagrass increased at most locations and was well above the GBR long-term average.

Overall results suggest poor water quality at Yule Point with low light availability and nutrient enrichment (elevated N). Macro-algae abundance remained negligible over the monitoring period. Climate across the region was hotter, windier and wetter in 2009/10 than the long-term average, and within canopy water temperatures were also significantly higher than previous years. Overall the status of seagrass in the region was rated as fair.

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Ellie Point

Monitoring: suspended

Principal watchers: Tom Collis & Far North TAFE
Occasional and past watchers: Seagrass-Watch HQ
Location: On the north of Cairns Harbour, adjacent to the mouth of the Barron River
Site code: EP

EP1 position: S16.87617 E145.77796
Best tides: <0.8m Cairns (Port 59060)

Issues: Land runoff & boat traffic
Comments: Popular recreational fishing area and turtle feeding grounds

Status (Feb05):

Site has not been examined since 6 April 2004.
Site appears to be showing a fairly typical seasonal pattern of seagrass abundance (higher in late spring-summer than winter).
Site is dominated by Zostera capricorni with Halodule uninervis and Halophila ovalis;
Site appears to be recovering in distribution and cover since the substantial loss of seagrass area and abundance reported in December 2001 (Campbell et al 2002).
Results of monitoring indicate that seagrasses at Ellie Point appear relatively healthy and that there had been an overall increase from values recorded in February 2002.

Green Island

Monitoring: ongoing

Principal watchers: Seagrass-Watch HQ
Location: Reef-platform on Great Barrier Reef mid shelf reef approximately 27 km north east of Cairns
Site code: GI1, GI2

GI1 position: S16.76163 E145.97290 (heading: 180 degrees)

GI2 position: S16.76140 E145.97614 (heading: 180 degrees)
Best tides: <0.8m Green Island (Port 59070)

Issues: Elevated nutrients and land runoff
Comments: Green Island is a vegetated sand cay approximately 12 ha in area with a maximum elevation of 4.5m. There are extensive seagrass meadows in the waters surrounding Green Island with at least 9 species (Halodule, Cymodocea, Halophila, Thalassia, Syringodium) identified. Abundance of seagrasses highest in the sub-tidal area in the north western lagoon. Monitoring of reef habitats occurs at Green Island, on the large intertidal reef-platform south west of the cay. The meadow is dominated by Cymodocea rotundata and Thalassia hemprichii with some Halodule uninervis and Halophila ovalis.

The distribution of seagrass around Green Island has changed substantially in the last 50 years, possibly from poor water quality, a consequence of increases in tourist visitation and increased nutrients emanating from the adjacent sewage outfall. The most dramatic change however, has been the seagrass species composition, with the species Syringodium isoetifolium now dominating most of the lagoon meadows. How these changes in the seagrass composition and abundance on Green Island will effect the sea turtle, dugong and fisheries is unknown. Further reading Udy et al (1999). The location is a green (no fishing) zone of the GBRMP, and a dugong and turtle feeding ground.

Status (March 2011):

Seagrass abundance appears to follow typical season pattern (higher in late spring-summer than winter).
Seagrass abundance relative to the seagrass guidelines indicates that the seagrass meadows at Green Island are in a FAIR condition.
Figure below shows the seagrass status for each quarterly monitoring event since November 2001.

Seagrass abundance has decreased slightly over the past 12 months at both sites.

Sites are dominated by Cymodocea rotundata, Thalassia hemprichii, Halodule uninervis and Halophila ovalis.
Seagrass species composition appears to fluctuate throughout the year, however no long-term trend is apparent.

Seagrass canopy height is generally higher at GI1 than GI2.

Macro-algae at Green Island is predominately composed of Halimeda spp. and abundance generally increases in spring/early summer. Mean covers is predominately less than 10% and within levels of natural change.
Epiphyte cover on seagrass leaf blades at Green Island sites was variable and although not significant, appear to be increasing. Abundances at Green Island are above the GBR long-term average for reef habitats.

No seeds have ever been found at Green Island meadows.

Seagrass-Watch monitoring on Green Island is proudly supported by Great Adventures Cruises and Green Island Resort.

Green Island

Yule Point

Monitoring: ongoing

Principal watchers: Seagrass-Watch HQ

Occasional and past watchers: Tom Collis & Far North TAFE
Location: Coastal intertidal sand banks, protected by an extensive fringing reef
Site codes: YP1 YP2

YP1 position: S16.56932 E145.51240 (heading: 50 degrees)
YP2 position: S16.56387 E145.50925 (heading: 50 degrees)
Best tides: <0.8m Port Douglas (Port 59040)
Issues: Storm water & land runoff
Comments: The seagrass meadows at Yule Point are located on naturally dynamic intertidal sand banks. These meadows are dominated by Halodule uninervis with some Halophila ovalis. A small patch of Zostera capricorni has appeared closer inshore in recent years, however it is outside the monitoring sites. The meadows at Yule Point are often exposed to regular periods of disturbance from wave action and consequent sediment movement. The sediments are relatively unstable restricting seagrass growth and distribution. The meadows are also popular dugong feeding grounds, and grazing trails are abundant.

Status (March 2011):

Seagrass abundance appears to follow typical season pattern (higher in late spring-summer than winter). Sites are dominated by Halodule uninervis and Halophila ovalis.
Seagrass abundance relative to the seagrass guidelines indicates that the seagrass meadows at Yule Point are in a GOOD condition.
Seagrass abundance has decreased over the past 12 months at both sites from the highest abundances recorded at this location in 2009.
Figure below shows the seagrass status for each quarterly monitoring event since August 2000.

Although the seagrass abundance and meadow extent has fluctuated over the last decade, sites appear similar to 1967, when den Hartog (1970) photographed the area and described the species present and sediment condition.

Sites are dominated by Halodule uninervis and Halophila ovalis.
In 2010, the proportion of Halophila ovalis at the YP1 meadow increased above it’s long-term average (22.5%) and has continued to increase; indicating higher levels of physical disturbance (e.g., drainage channels) across the site.

Seagrass canopy height shows a clear season pattern - correlating with abundance of Halodule uninervis.
Canopy height is slightly higher at YP2 than YP1.

Epiphyte cover has recorded peaks which generally correlates with seagrass abundance.
Epiphyte cover on seagrass leaves has continued to remain high and above the GBR long-term average at Yule Point over the past 12 months.
Macroalgae generally increases in spring/early summer, but does not appear of concern.
Percentage cover of macro-algae at coastal sites was consistently lower than the GBR long-term average and at Yule Point abundance has declined over the last four to five years

A persistent and abundant seed bank was present at Yule Point. Seed abundance has significantly declined at both sites over last 12 months.
The proportion of germinated seeds was high at this location, possibly a consequence of the high level of sediment movement and disturbance to the sites.