07 February 2011

Rare seagrass at Kranji Nature Trail

Wei Ling spotted large meadows of the rare Beccari's seagrass on Kranji Nature Trail at a recent Mega Marine Survey there. And yesterday, Jerome and I saw more of them.
I head back to Kranji this evening to find out just how extensive these meadows are at Kranji.

Why is this seagrass so special?

Beccari's seagrass (Halophila beccarii) is rare globally! The IUCN lists it as Vulnerable worldwide. It says the seagrass is estimated to occupy less than 2,000 km² in our part of the world. This is because it has a patchy and fragmented distribution in the Indo-Pacific. It also has a narrow, restricted depth range. It is mainly found in the intertidal zone, an area where there is a lot of human disturbance. Although it is fast-growing and may recover quickly from disturbance, global population trends indicate this species is declining.

In Singapore, this seagrass has so far been seen only at Sungei Buloh Wetland Reserve proper (near the Mangrove Boardwalk) and at Chek Jawa, Pulau Ubin.

Beccari's seagrass is easily overlooked as it is tiny and resembles scum or algae when out of water.
But a closer look reveals how pretty they are. Those seen at Kranji were mostly green, while those we see at Chek Jawa tend to be brown or reddish. They have banded leaves, so Siti also calls them Tiger seagrass.
This seagrass has tiny narrow leaves on long skinny stems that emerge as a rosette from the underground rhizomes.
The area covered by this rare seagrass at Kranji is quite awesome. I was busy marking the locations with a GPS, when I realised, it would have been easier to mark the areas where they were NOT found. The seagrass meadows were growing under almost every large mangrove tree along Kranji, particularly among trees with pneumatophores like the Api-api putih (Avicennia alba) which are plentiful on this shore.
The Beccari's seagrass meadows form an almost continuous strip parallel to the Kranji Nature Trail shore, under the mangrove trees, extending out to the mudflats to about where the mangrove tree pneumatophores end. The meadows range in width from 1m to about 10m.
The seagrasses were also lushly growing on the sand bars in front on some parts of the shore. In Jan 09, I did go to Kranji specifically to look for Beccari's seagrass. But I didn't come across such vast meadows at that time. Did the seagrasses grow explosively recently? Due to the rainy weather? We noticed that during the mass death at Chek Jawa in 2007, Beccari's seagrasses there did very well. There's still so much more to learn about our seagrasses!
Seagrasses have underground stems and with their many roots, form a mat that traps and holds sediments. Here's a patch of Beccari's seagrass obviously forming mounds on the mudflats.
I noticed that mangrove saplings are often found in the middle of the seagrass meadows. Most of the saplings that I noticed were Perepat (Sonneratia alba). I wonder if the meadows helps young mangrove trees to grow, and visa versa? So much more for me to learn!
A closer look at the many young Perepat saplings growing in the middle of the seagrass meadows.
A study also found that juvenile horseshoe crabs were found in meadows of Beccari's seagrass. So these meadows may be important for all kinds of babies, plants and animals! Here's more about the horseshoe crabs at Kranji.

The only large areas without seagrasses were those heavily impacted by trash. In this photo below, the seagrasses are found in the red circle, while the trashy area in the foreground had no seagrasses.
As the IUCN noted, Beccari's seagrass is vulnerable to human impact. Such as pollution and trampling. Yesterday, we encountered a group of three men seemingly removing a driftnet that was placed earlier at high tide. The net was full of leaves and fishes, we noticed them removing large Scats.
They appear to have arrived by bicycle.
And were well prepared with buckets and bags.
Besides the seagrasses, as usual, I am distracted by other mangrove stuff. Growing on a mangrove tree, I noticed this thorny climber. Joseph Lai earlier kindly identified a similar plant we saw at Pulau Ubin as Caesalpinia crista. This plant is also listed for Sungei Buloh in the Guide to the Mangroves of Singapore. Here's more about the plant on The Total Vascular Flora of Singapore Online.
I also got distracted by some pretty snails typical of our mangroves: The large lovely lemon yellow Black-mouthed mangrove periwinkle (Littoraria melanostoma), and the handsomely striped Lined nerite (Nerita articulata).
I also came across a bunch of Mangrove drills (Chicoreus capucinus) near what seems to be their egg capsules laid on a dead tree trunk.
Oh dear, this large Api-api bulu (Avicennia rumphiana) has fallen right over. But it seems to be valiantly struggling to survive, with new roots growing downwards toward the ground, and new shoots sprouting at right angles to the trunk. Erosion is a serious problem at Sungei Buloh, and one that is being seriously tackled in the Sungei Buloh Masterplan.
Erosion has also affected this magnificent Nyireh bunga (Xylocarpus granatum). While this is sad, it does give a glimpse of what is usually going on underground. Here, we can see all the little roots growing down from the buttress roots of the tree.
As I walked in to Kranji today, I noticed men laying out a very long absorbent boom on the grass next to the Kranji dam sluice gates. The man in charge told me they were about to open the sluice gates at the Kranji dam and told me not to go near the water at the gates. This is good advice.
As I was leaving, I noticed the boom laid in front of the sluice gates. Perhaps to trap any floating chemicals or trash when they release the water? That would be nice!
More about Kranji mangroves in this older post.

More about Kranji Nature Trail on the wildsingapore website and Sungei Buloh Wetland Reserve website, with a downloadable brochure (pdf).

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