Tropical corals and temperate barnacles - a common declining theme?
Debbie MacKenzie
debimack at auracom.com
Tue Nov 6 14:18:52 EST 2001
Dear Coral-list,
A large amount of scientific attention in recent years has been directed
toward studying the fate of corals, tropical sessile plankton-feeding
invertebrates. And disturbing negative developments affecting these
populations have been well documented (...this is clearly not news to this
group, but bear with me, I still think that I have something useful to
offer.) Human nature being what it is, the commercial value that we
perceive to be associated with healthy corals has probably been the main
driver of this area of scientific research.
In the temperate zones populations of sessile plankton-feeding
invertebrates also exist, but they have been, and continue to be, generally
considered to hold no 'value' to humans. Therefore scant attention has been
paid to their fate as changes evolve in the marine ecosystem. A case in
point is the common barnacle that inhabits rocky intertidal habitat in
temperate zones worldwide. Detailed descriptive studies of barnacle
physiology have been done, but it has proven impossible (for me, at least)
to find population assessment work on barnacles that documents changes
occurring over time. A bit of research confirmed my suspicions, however,
that populations of barnacles exposed to the clean open ocean have been
undergoing a decline (this is true, at least in Nova Scotia, Atlantic
Canada, as described in the barnacle article recently posted on my website:
http://www.fisherycrisis.com/barnacles.html ).
It appears to me that there may be a common theme underlying the decline of
temperate barnacles and the decline of tropical corals. It struck me
particularly when looking at the changes that have occurred, over the last
50 years, on a previously barnacle-dominated reef at Hall's Harbour, Nova
Scotia (see pictures and description of Hall's Harbour in the "findings"
section of my barnacle article:
http://www.fisherycrisis.com/barnacles.html#D ). The decrease in barnacle
cover and increase in macroalgal cover that has occurred at this location
seemed to me to echo the accounts of similar transformations that have
occurred in tropical coral reefs over recent decades. Could changes in
plankton, specifically a decline in the abundance of zooplankton in oceanic
waters, be what has triggered and is being reflected in these changing
pictures of stationary plankton feeding populations? Declines in oceanic
zooplankton abundance have been well documented in temperate zones, but
I've yet to see reports of similar studies in the tropics. My previous
inquiries to this list regarding trends in tropical zooplankton abundance
didn't turn up any information on the topic - does it exist?
The macroalgal overgrowth is interesting, especially when it occurs in
clean water (where nutrient-enhancement, if it has indeed occured, has been
at below detectable levels - this is true in some deteriorating tropical
locations - no?) I think that it bears pointing out that macroalgae can
thrive in nutrient-poor waters more successfully than can plankton (or
plankton feeders).
From the book "Seaweed Ecology and Physiology" by Lobban and Harrison
(1994), p 203:
"Extensive analysis of the chemical composition of marine plankton has
revealed that the ratio relating carbon, nitrogen, and phosphorus is 106 :
16 : 1 (by atoms) (i.e. C : N = 7 : 1 and N : P = 16 : 1). This is commonly
referred to as the Redfield ratio. Decomposition of this organic matter
occurs according to the same ratio. However, Atkinson and Smith (1983) have
recently shown that benthic marine macroalgae and seagrasses are much more
depleted in phosphorus and less depleted in nitrogen, relative to carbon,
than are phytoplankton. The median ratio C : N : P for seaweeds is about
550 : 30 : 1 (I.e., C : N = 18 : 1 and N : P = 30 : 1). An important
ramification of these observations is that the amounts of nutrients
required to support a particular level of net production are much lower for
macroalgae than for phytoplankton. In addition, seaweeds, on average,
should be less prone to phosphorus limitation with their N : P ratio of 30
: 1 than are phytoplankton, with an N : P ratio of 16 : 1. The high C : N :
P ratios in seaweeds are thought to be due to their large amounts of
structural and storage carbon, which vary taxonomically. Niell (1976) found
higher C : N ratios in the Phaeophyceae than in either the Chlorophyceae or
Rhodophyceae. The average carbohydrate and protein contents of seaweeds
have been estimated at about 80% and 15%, respectively, of the ash-free dry
weight (Atkinson and Smith 1983). In contrast, the average carbohydrate and
protein contents of phytoplankton are 35% and 50%, respectively (Parsons et
al. 1977)."
Therefore a marine ecosystem that was experiencing a steady decline in
total 'nutrient inventory' might be expected to display a decline in
plankton and plankton-feeders first, with an advantage then being dealt to
macroalgae in areas where they previously were in competition for space
with those plankton feeders.
If the decline in plankton-feeders was caused by increasing levels of
malnutrition, what signs and symptoms would be expected? It seems to me
that the array of problems that have appeared in corals in recent decades
pretty well sums up the picture that would be predicted: loss of the
heavier feeders first (e.g. Acropora), weakened immune systems and
increased susceptibility to infectious diseases and possibly malignancies,
decreased ability to withstand environmental stressors such as intense
light and higher temperatures, increased likelihood of dying during the
annual season of least food availability. The increasing presence of
cyanobacteria in different forms (overgrowing corals, free floating blooms)
would also be a predictable systemic reaction to a decreasing
nitrogen-content in the system. (Regarding the disappearing barnacles, I
obviously have no information on what pathologies preceded their demise.
All I have is the evidence today of their absence from areas that they
previously dominated...and some pretty strong clues that relative increased
food availability is the major factor determining which areas continue to
support barnacle growth.)
I do realize that some polluted coastal areas do have elevated levels of
nitrogen and other nutrients, and all sorts of consequences of that. But
the major bulk of oceanic water has not been measurably affected by
nutrient enhancement - and it's critical to also figure out exactly what is
changing in the 'clean' seawater. I also realize that there are very
significant differences between corals and barnacles. But the main features
that they do have in common - being small sessile plankton-feeding animals
that are gradually disappearing from (clean) marine habitat - may provide a
useful clue as to "what is wrong with this picture?"
Just thought I'd run this one by you...
All opinions greatly appreciated,
Debbie MacKenzie
http://www.fisherycrisis.com/barnacles.html
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