Since posting my original seaweed article on this website in August, 2001, I have continued to observe the changes in the seaweeds. Most remarkable now is the extreme deterioration that is occurring in many Fucus specimens, especially the larger ones. Also the contrasting pictures of pigmentation patterns in Fucus species as compared to Ascophyllum nodosum is becoming more prominent. A relationship is suggested between nutrient shortage and a heightened susceptibility to "sunburn" (red/dead tissue - damage induced by extreme levels of heat/light/dessication) in both species, although the pattern of colors is reversed between them.
Most recent seaweed photos from Shad Bay, Nova Scotia, taken December 28, 2001:
Still wet, this boulder is covered with 'healthy' looking Ascophyllum nodosum plants and Fucus specimens that appear to be deteriorating. All of the older tissue on the largest Fucus plant has shrivelled and changed to a dark brown-black color. Shorter Fucus plants growing higher on the same rock are also showing the dark discoloration of their older tissue, but to a lesser degree.
Another view of the same seaweeds.
Following, a few photos that show the proximity of remarkably deteriorating Fucus plants to 'healthy' looking Ascoplants (normally colored, not shrivelling or becoming necrotic, although they do appear to be becoming increasingly 'close-cropped'):
The photo above shows normally colored Asco, well established clumps, that appear to have recently lost a large number of fronds to breakage. "Sick" fucus is also visible in this view. The deterioration patterns in the two types of seaweed are noticeably different - fucus becomes brown, withered and necrotic, and also increasingly susceptible to breakage - while Asco simply becomes more susceptible to breakage.
A closer view of the smaller plants living at the upper vertical limit of their range. The fucus is showing discoloration of the "wings" of the older tissue. The area that is darkening shows a reddish hue in this picture - "red" discoloration usually being associated with "sunburn" (heat/light damage) in these types of seaweeds...this raises the question of whether or not "sunburn" could be one element of the tissue damage that is occurring here. (Strange though, since this photo was taken December 28th, 2001.)
For comparison, the following two photos were taken during August of 2001, showing the pattern of color changes, including "red"(sunburn?) in both Asco and Fucus specimens. An interesting reversal of the color patterns is apparent between the two species: In the Asco the color gradient from holdfast to tip under stressful summer conditions is from green => yellow => red, while in the fucus (that is undergoing older tissue loss) the color gradient from holdfast to tip is the reverse: red => yellow => green.
In the Asco the "sunburn" at the tips is explained as damage that has been caused by intense levels of heat, light and dessication, such as what would be expected to occur on hot summer days with low tide at midday. Fucus is normally subject to the same type of "sunburn," and a few examples were certainly seen last summer that showed fucus with reddened tips similar to what had happened in the Asco, but, as in the lower photo above, fucus plants were more commonly showing the healthiest color at the tips (green) and then the bleached effect (yellow) induced by low nutrients and high sunlight(?) and beyond that the tissue appeared red before becoming necrotic and sloughing off. The susceptibility to "sunburn" (and also to the yellow bleaching effect) might be expected to normally occur first at the tips of plants, it seems to me, since the tips would naturally receive the highest dose of sunlight (and possibly the lowest nutrient availability) due to their tendency to float in the surface layer of water.
One detail of the differences between the physiology of these two seaweed species may account for the remarkable difference in their pattern of susceptibility to "sunburn." According to the literature, Fucus species (as well as kelps) have a built in transport mechanism for nutrients with the effect that nutrients that have been stored in the older tissues are actively transported ("translocated") to the tips when new growth is stimulated at the ends of the fronds. New growth demands more nutrients than the amount available for direct absorption from seawater (especially summertime) so the tips draw nutrients out of the older tissue, which have been stored there during previous seasons when the availability of nutrients in the water was greater. If insufficient nutrient stores have been accumulated in the older tissue to support the demand by the new growth, the older tissue may become severely depleted to the point that physical breakdown occurs. (At least this seems like a possible explanation to me.) The idea that as the older tissue is drained of nutrients its susceptibility to light/heat damage increases seems like a plausible explanation for the seemingly unlikely appearance of sunburn (red) and bleaching (yellow) at positions lower than the very tips of the plant. If so, this pattern in fucus illustrates how susceptibility to sun damage is strongly related to nutrient stores, and not a simple function only of exposure to heat and light (or else the tips would be bleached/burnt - in contrast, their ability to draw on the older plant tissue for nutritional support likely accounts for their healthier green color and greater resistance to the damaging effects of light and heat). The lack of nutrient translocation in Asco probably explains why the older tissue can maintain its green color and tissue mass, while bleaching (yellowing) and "burning" (red) effects are consistently seen only at the tips of Asco specimens growing in low-nutrient seawater.
These recent observations seem possibly to also point to an increasing tendency for Asco fronds to be broken off by wave action at a faster rate than they are being replaced.
Poor physical condition in multiple marine species in this area (exposed to open NW Atlantic ocean) seems to suggest the possibility that there is now a generalized shortage of nutrients in the seawater. The weaknesses in these seaweeds probably reflects a drop in the overall nutrient availability to them year-round, not just the extreme lows of summertime. The unusual appearance of "sunburn" in the Asco in this area this past summer was not the result of unusually high temperatures - because those did not occur - more likely it reflected a lowering of the resistance of these plants to this type of damage...one of their natural defenses that depends upon being adequately nourished.
A few more illustrations (photos taken during September, 2001, at Shad Bay)...
Small Fucus plants, living high in the intertidal zone, showing "sunburn" in older tissue. Also Asco specmens showing a high proportion of broken fronds.
A larger Fucus specimen adrift, showing the changes discussed above.
Several sizes of Fucus plants showing the pattern of changes, with more severe breakdown of the larger specimens.
Another close view of the changes in Fucus living at the upper limit of its range showing broken tips with resultant "bunchy" new growth as well as "sunburn" in older sections. Only the very tips are spared from the apparent "sunburn" effect...
