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What’s that white stuff in the seaweed?

by Debbie MacKenzie, March 2003

(click on photos below for larger views)

Late August, 2002, on the wave-swept rocky coastline of Nova Scotia, appeared what resembled snowdrifts in the seaweed. The generally dark vista of seaweeds exposed at low tide, a sight long familiar but not particularly interesting to most people, was now unmistakably interrupted by a stark white band of Irish moss (normally a dark red algae). And the area in general was affected, not just a few rocks. Older coastal residents, people who have spent lifetimes on the water fishing here, admitted that this was definitely a new sight. Nobody could recall seeing live seaweed look quite this way before. Why not? What had happened? Weather patterns and water temperatures during the summer of 2002 were not unusual. A year earlier I had been struck by the color change in Irish moss, which had shifted, over decades, from red to greenish yellow on familiar rocks in this area, but even I was unprepared for the sight of such extreme whiteness in the seaweed.

Over a century ago, scientists described the tendency of this essentially red (when healthy) algae to fade to green and yellow when it grew in particularly stressful, "unfavourable" locations, such as “tide pools near the high water mark.” (Harvey, 1849) So in that sense, the loss of color in this algae is not a new development. But for the most part, this plant has been seen and described simply as a dark red seaweed, and some early writers of detailed descriptions of the species failed to make any mention of the existence of non-red specimens (best example: Darbishire, 1902). But the summertime tendency to lose color has ever so gradually become part of what is considered ‘normal’ for Irish moss. The ‘snow banks’ in my pictures simply represent today's ‘normal’ taken to a new extreme. But ‘normal’ is one of the things that has been subtly changing. It seems now that a long and gradual change has occurred, such that 'unfavourable habitat' leading to color loss in this plant now extends to virtually everywhere that the Irish moss grows. And that the trend is still intensifying. The areas shown in the photographs above offer what should be ideal conditions for the growth of healthy Irish moss. These rocks are in fairly deep water with constant wave action; a far cry from the impoverished conditions in the still, shallow water of a tide pool…yet, the seaweed really is colorless.

The following article represents an attempt to alert scientists and the public to unmistakable signs of stress in the intertidal zone, indicators that might go a long way toward explaining exactly what is underlying today’s generalized failure to thrive of marine fish, shellfish, etc. The essence of the problem, as I see it, is one of simple starvation, in which the photosynthesis on which all marine life ultimately depends for food is gradually declining. That, I am convinced, is the key message in the white Irish moss.


See article published in the Halifax Herald about dying seaweed, 2001.

See gallery of Irish moss photos (in the intertidal zone or the subtidal zone, in varying depths of water and wave action - practically everywhere that Irish moss grows along this (Nova Scotian) coastline, it is now prone to losing its red color during the summer months.)

See color changes in rockweed which point to the same conclusion, and argue against the alternative hypothesis that increasing UV radiation is the major cause of the stress on seaweeds.

See my original seaweed article discussing the consistency of today's broad changing trends across marine plant life in general with the hypothesis of a steady decline in nitrogen availability in seawater.

See the contracting range of barnacles, which suggests declining food availability for this common intertidal animal.

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