GIS analysis

Follow this link to:

Saltmarsh retreat zone (front page of Mangrove Cove Monitoring web site)

Water quality

Mangrove expansion photopoints

Bird sightings

 

 

Time changes everything

Aerials from 1956 show wrecks, filling, the log pool and numberous small moored boats in the area of Mutton Cove, with a handful of mangrove trees in less disturbed areas. The resolution of early black and white aerial photography was not sufficient for thorough ananlysis of the vegetation changes but it the expansion of mangroves could be noted. When the Management Plan for Mangrove Cove was written in 2005, analysis of the extent of mangroves and samphires on the site was undertaken, using a colour aerial photograph from 2002. This was classified into mangrove, mud flat, and samphire. That imagery has been compared since the writing of the management plan with high resolution imagery obtained in 2011, 2014 and 2017. The high resolution imagery obtained from 2011 onwards was sufficiently resolved that a "transitional" vegetation association was detectable. Therefore the 2002 imagery was reprocessed to allow this transient association to be included in all the following analyses.

 

Definitions

Mangrove means a cover of mangrove that no other vegetation or mud can be seen through, from the air - it is a measure of the canopy area.

Samphire means a mid dense or denser cover of chenopods including Sarcocornia quinqueflora and Tecticornia spp and can include some Suaeda australis (sea-blite) amongst the samphires.

Mudflats means unvegetated mud.

Supratidal means the area above normal high water, and this area can include some samphire species. It is located landward of the mangroves, against the embankment.

Transition means areas where the habitat is mixed - transition areas could be:

 

Maps of the vegetation

2002 habitats
2011 habitats
2014 habitats
2017 habitats
 
 
 

Table showing areas of each habitat in 2002, 2011, 2014 & 2017

 
2002
2011
2014
2017
Habitat comparison, 2002-2017
Habitat
m 2
%
m 2
%
m 2
%
m 2
%
Area change
Relative % change
Mangrove
2784
10%
4726
17%
5027
18%
5243
19%
+2459 m2
188% increase
Mudflats
19828
71%
20849
74%
18288
65%
18523
66%
-1305 m2
7% reduction
Supratidal
3255
12%
2215
8%
2007
7%
1479
5%
-1776 m2
55% reduction
Samphire
1460
5%
30
0%
0
0%
0
0%
-1460 m2
100% reduction
Transition
753
3%
291
1%
2832
10%
2963
11%
+2210 m3
394% increase
 

 

28080

 

100%

 

28111

 

100%

28154

100%
28209

100%

 

 

 
0.46% total area change due to georeferencing errors, canopy expansion out of measured area etc.

 

 

 

 

 

Discussion

The different habitats and vegetation associations have changed at different rates.

 

Mangroves have steadily increased their cover. All the mangrove area that existed in 2002 is intact. No area that supported mangroves in 2002 has lost any cover. There is a large new area of mangroves that has appeared in the intervening 15 years, however. That new area has been sourced variously. About 1107 m2 of what was mudflat in 2002 now supports mangroves, and 432 m2 of samphire has been completely over-run by mangroves. Of the areas that were transitional in 2002, nearly two-thirds have converted to mangrove. A large area that was considered to be supratidal now supports mangroves. Overall, by 2017 mangroves covered an extra 2459 m2 or 188% of the area they covered in 2002.

 

Samphires are the most vulnerable vegetation associationin the site. Samphires have been completely lost in the intertidal area and samphire species (Sarcocornia and Tecticornia) now only occur in the supratidal habitat. While some of the samphire areas have been over-run by mangroves (432 m2), a much larger area (947 m2) of samphire is transitioning, and will finally either stabilise as mudflats or mangroves.

 

In the 15 year period of observation, parts of this transition area have altered form several times, swapping between transition (covered with a thin cover of sea-blite) or bare mudflat. Some areas that were transitional in 2002 have converted fully to mudflat (92 m2) or to mangrove (488 m2). No transitional area has converted to samphire. New transitional areas have been sourced mostly from areas that were samphire (947 m2) or mudflat (804 m2), with smaller areas being sourced from supratidal areas that are now being flooded more frequently (as a result of sea level rise and/or embankment erosion). This accounts for 763 m2. A further small source of new transition area is mangrove trees that have suffered disease or misadventures that have left openings in the canopy (152 m2). Transitional habitat is very labile as the changes reflect both the multiple source habitats and also the process of habitat change, which may proceed down several paths. For example, common change paths include:

 

1) in changing from samphire to mangroves the habitat may pass from samphire through transitional (sick samphires) to mudflat (dead samphires eroded away), back to transitional (when the sea-blite Suaeda colonises and baby mangroves appear), ultimately becoming mangrove, or

 

2) samphires may move through transitional habitat directly to mudflats in areas that are currently too deep to support mangroves.

 

The transitional area is regularly disturbed by bait diggers (despite the signage prohibiting this activity) who leave a pitted uneven surface. The uneven, changing microtopography only supports the fast growing sea-blite, and the freshly disturbed surface may be more prone to erosion. A relatively small area has transitioned completely to mudflats (93 m2).

 

While a large area of mudflat (1107 m2) has been colonised by mangroves, and a smaller area has become transitional (804 m2) there are no areas of mudflat that have been colonised by samphire or that have accreted sufficient sediment to become supratidal.

 

Single snapshots in time do not allow any real certainty in estimating the fate of transitional and mudflat areas, however as time passes and the number of analyses increases, a better understanding of the habitat change process is developing.

 

The overal quantum of transitional cover is now 394% larger than the area this type of habitat covered in 2002.

 

The area that was classed as supratidal has shrunk, because some of it has converted to mangroves, transitional area or mudflats. Details of the changes within and between each of these habitats are presented in the following table:

 

 

Change details (2002 - 2017)
m2
Mudflat to Mudflat (unchanged) 17915.5
Mudflat to Mangrove 1107.0
Mudflat to Samphire 0.5
Mudflat to Supratidal 0.4
Mudflat to Transition 804.9
Mangrove to Mangrove (unchanged) 2612.6
Mangroves to Transition 151.8
Mangroves to Supratidal 19.4
Samphire to Mudflat 80.2
Samphire to Mangrove 432.4
Samphire to Samphire (unchanged) 0
Samphire to Transitional 947.3
Transitional to Transitional (unchanged) 172.0
Transitional to Mudflat 92.5
Transitional to Mangrove 488.2
Supratidal to Mangrove 377.9
Supratidal to Supratidal (unchanged) 1414.7
Supratidal to Mudflat 509.6
Supratidal to Samphire 0.3
Supratidal to Transitional 762.6

 

 

The reasons for habitat change can be quite complex, however the prime reason in this case is likely to be relative sea level change. The relative sea level at Port Adelaide has been changing rapidly. Reasons may include being in the graben zone of the horst and graben formation that results from the Para Fault and other local fault lines, oxidation and compresssion of underlying quaternary coastal alluvial deposits, and actual sea level rise due to warming oceans. Actual sea level rise, measured for Gulf St Vincent at Pt Stanvac has been an average of 5.2mm per annum since the mid 1990s (DEWNR, 2013). This translates to an actial sea level rise of more than 10 cm in the last 20 years, or 7.8 cm since 2002 when this monitoring program started. When this is combined with subsidence, the rise may be considerably underestimated.

 

Subsidiary causes for the observed changes include shadowing and competetion (via alellopathy) once mangroves move into an area, and possibly changes to the supply and texture of silt to the area.

 

 

More mangroves in Jervois Basin

Jervois Basin is the old name for the reach of the Port River that lies between the Jervois Bridge and the Railway Bridge. While the entirety of the river would have been lined with mangroves when Europeans settled, they were quickly removed. In the last few years they have begun to establish themselves at several locations in the Port River and the Jervois Basin is no exception. Besides Mangrove Cove itself, other seedling mangroves can be observed growing along the foreshore of NewPort Quays between Mangrove Cove and the new marina, and across the river at "Rotten Row" there is a small patch of mangroves growing in the quiet corner formed on the southern side of the eastern landfall of the Jervois Bridge. Several of these small mangroves have been large enough to detect with aerial photography since about 2011 and so they can be observed as they attempt to reclaim the shores of the river.

 

 

Reference

Department of Environment, Water and Natural Resources (DEWNR) (2013) Coastal Planning Information Package: A guide to coastal development assessment and planning policy. Revised 2013. Department of Environment, Water and Natural Resources. Adelaide.

 

 

Follow this link to:

Saltmarsh retreat zone (front page of Mangrove Cove Monitoring web site)

Water quality

Mangrove expansion photopoints

Bird sightings