"Sand beaches are very harsh environments, ..."So starts an article by NOAA, an overview of the animals living on and in the sand. It doesn't seem quite right, does it?
|Boundary Bay, retreating tide. Harsh? Yes!|
Flat sand, warm, shallow water, fresh air with a hint of salt in it, the cry of gulls, distant laughter, blue sky overhead. Perfect! Just being there gives us a new lease on life. But ... harsh,they say?
It depends on your situation; if we had to live there 24/7/52, we might see things in a different light.
"... encompassing most of the rigors of the rocky intertidal (high wave action, wide temperature range, periodic tidal exposure) with the addition of high abrasion levels and lack of firm substrate for attachment. Beach fauna exhibit the characteristics of communities in harsh environments, namely low species diversity but large numbers of individuals of each species."
I would add the lack of hiding places, refuges from predation. And predators there are many; around here, they're mostly of the two-legged variety, some with wings, others with shovels and buckets.
When we parked at Centennial Beach Monday afternoon, the tide was starting down; just the first few metres of sandy beach were exposed. We stopped at the edge to watch hordes of mud snails, exposed on the sand, and all streaming as fast as they could drag their stripy shells downstream, towards the retreating water. A doomed enterprise; the tide goes out faster than they slide, and they would end up having to wait out the dry spell all afternoon.
We waded in, meandering back and forth from one newly-risen sandbar to the next. The water was bathtub warm in spots already, where the current was slower.
Once we'd passed the snail zone, there wasn't much to see. Occasional patches of seaweed or eelgrass provided shelter to small critters. I captured a tiny greenmark hermit, watched it scramble around my hand; so tiny, it had a long walk to the edge. When it got there, I replaced it in the weeds.
|Green ribbon seaweed harbours an abandoned clam shell, and probably a few multi-legged beasties.|
|Bleached seaweed remnants float out to sea.|
|A tangle of rockweed and eelgrass stranded on a sandbar. It reminds me of the tossing bouquet that a bride would carry.|
|Rockweed turns orange and yellow as it dries in the sun.|
Apart from the bits of floating or stranded weed, the beach looks empty, because everything that can flee or dig or find a convenient patch of eelgrass has gone into hiding. Sometimes they leave signs of their passing, holes or humps in the sand. A smallish lump with a round hole in the centre may hide a clam, but most have dug themselves in deeper. Careless clams get offered free flying lessons by hungry gulls. A few hard landings, and they're lunch.
|Assorted worms make larger hills, usually topped with a coiled poop decoration.|
|I don't know what makes these.|
Boundary Bay sand is on the muddy side. Further inland from our walk, the mud gets deeper and mushier; even going straight out from Centennial Beach, we cross spots where our feet sink deep into the sand long after the water has gone down. The finer grains of silt and clay retain water better than coarse sand and shell remnants.
In this area, the sand may contain up to 89% fine sediments (97% further inland, where it is almost impossible to walk). This means that, along with the water, the sand is rich in organic remnants, food for the residents. However, it also means that the space between the sand grains is packed with mud, reducing the space for small animal life. (Meiofauna; I haven't forgotten that good word!)
A few inches under the sand surface, there is a layer of blue-black sediment. This is an area starved of oxygen, which has been used by the animals above it, and is not replenished due to the density of the compacted silt. There, contaminants settle, giving the layer its black colour.
When a burrower digs past this layer, it shovels the anoxic mud up to the surface, leaving a black patch. I dug down into a few of these, looking for mud shrimp.
|Mud shrimp burrow opening?|
I couldn't dig fast enough. I was using my bare hand; I wouldn't want to risk damaging one with a digging tool. The sand was loose and easy to remove. A few inches down, I found the burrow; a hole about big enough for two fingers before the sand caved in, gushing water. I followed it down and down, around a corner and down again, tossing out handfuls of lumpy sand as I went. About eight inches down, the sand from above filled the hole as fast as I shovelled. I gave up.
A second burrow gave me the same result. But I ran my fingers through the excavated sand, to see what made the lumps.
|One pink macoma clam, and many purple and brown clams, species undetermined.|
I put the clams back in the hole, and filled it in again. No sense exposing the clams to the ever-hungry gulls!