Thursday, November 27, 2014

The rotten egg zone

Boundary Bay is about 13 kilometres (8 miles) across at the base, and encompasses about 100 square kilometres. In the centre, there is deep water, but most of the northern end is a flat intertidal plain, filled with silt from two rivers and many creeks.

2009 Google map. The intertidal zone is shaped somewhat like an amphipod, looking towards the east.

The head and the spine of the amphipod are mud. Deep, maybe waist-deep, sticky, squelchy mud, continually replenished with run-off from the farms and bogs of the Delta brought in by the Serpentine and Nicomekl Rivers at the "head" end. It's bird and worm country; people can't walk there, don't even take their boats and paddle boards in that direction.

As the silt merges with the sand along the western shore, the mud develops a variety of disagreeable odours. Sometimes there are dead zones, where clams and crabs lie dead on the surface; the air smells of rotting flesh, choking and impregnating our clothes. At other times, everything smells fishy or of untamed compost heaps. These are often the result of human pollution; fertilizer and industrial contaminants, construction debris, oils and paints.

The more common rotten egg smell of hydrogen sulfide is almost pleasant in comparison. It is a sign of abundant organic matter, teeming with busy, happy life.

It all depends on the size of the mud particles. Where the sand is coarse, water flows through, carrying oxygen to the tiny critters living there. Mud worms in their burrows dance a slow, sinuous dance, pushing the water along; clams squirt water and sediment as they move about. We see it as clean sand. It's soft underfoot, but not sticky; we dig in it, walk on it, sunbathe on it. If we notice any odour, it is the fresh tang of salt water.

Where fine silt clogs the water passageways, the water becomes stagnant, and free oxygen is depleted. Life goes on, even in these conditions; after all, there's plenty of food. It just has to be metabolised differently. Some of the animals living here use hydrogen sulfide as an energy source. The sand, to us, looks mucky and off-colour. And smells of rotten eggs. We look for better places to play.

Sticky sand/mud. Lugworm heaven. 2008

Slightly wetter muck, with worm poop.

Where the oxygen levels are lowest (anaerobic or anoxic zones), the muddy sand forms a black layer, sometimes just below the surface of the tide flat, or in other spots some 6 inches down, depending on the ratio of sand to silt.

Further south along the shore of the Bay, although the silt doesn't reach quite this far, and we have acres of clean sand, there are still pockets of black sand underneath. Around the worm "mountains", there is often a circle of black sand that the worm has eaten and ejected to the surface.

If we grab a handful out of the black layer, it feels firmer than the sand around it, and, again, it smells of rotten eggs.

And this brings me to the mysterious network stuff I was wondering about a couple of months ago. Continued tomorrow.

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