Tropical corals and temperate barnacles - a common declining theme?

Debbie MacKenzie debimack at
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:  ).

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: ). 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
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