eutrophication and coral physiology

[F.Marubini]

I have recently completed my Ph.D. thesis entitled 'THE PHYSIOLOGICAL 
RESPONSE OF HERMATYPIC CORALS TO NUTRIENT ENRICHMENT'. 
With this message, I am presenting a summary of my thesis. I am very keen to receive 
any feedback/queries from other coral-list members interested in the subject. 
The project was carried while I was a student of Peter Spencer Davies 
at  Glasgow University.  All laboratory and field experiments were 
based  at the Bellairs Research Institute of McGill University 
in Barbados. 
The first paper from this work has been recently published: 
F. Marubini, P.S. Davies (1996)  Nitrate increases zooxanthellae population 
density and reduces skeletogenesis in corals.  Mar. Biol. 127: 
319-328. 

	THESIS SUMMARY

keywords: hermatypic corals, eutrophication,  nitrate, phosphate,
carbon budget, calcification, ecotoxicology.

	Nutrient enrichment of tropical waters constitutes an increasing
threat to the health and biodiversity of coral reefs. In order to
manage these ecosystems effectively, the onset of nutrient pollution
has to be closely monitored. This thesis examined the possibility of
using some physiological responses of hermatypic corals as an
early-warning bio-assay, to detect nutrient enrichment before reef
deterioration has taken place. To this aim, the physiology of the
common branching coral Porites porites and the massive coral
Montastrea annularis was studied both in the laboratory and on the
reef under different nutrient conditions. By measuring the organic and
inorganic productivity of corals and by constructing carbon budgets,
it was hoped to relate differences in the fixation, allocation and
utilisation of carbon to differences in nutrient regimes. 
	Nubbins of Porites porites and explants of Montastrea annularis were 
chosen as the experimental units. Nubbins were obtained by cutting coral tips
(approx. 20 mm.), grounding their cut surface flat, and gluing them
onto a perspex tile with cyanoacrylate glue. To obtain explants, a
coral head was cored under a drill press fitted with a hole saw. Cores
were then cut to fit, and sealed into polyethylene cups with
underwater epoxy putty. 
	A new culturing system was developed to grow
corals successfully in the laboratory under completely controlled and
repeatable conditions. This system (the ‘photostat’) consisted of
glass aquaria (30x21x18 cm) placed in a constant temperature
water-bath under metal halide lamps. The aquaria were fitted with
specially designed air lines and coral trays to maintain a strong
water motion around the corals, independent of the rate of water-flow.
A peristaltic pump ensured a daily water turn-over.
	A new improved carbon budget methodology was developed by comparing the well
established methods of Davies (1984) and Muscatine et al. (1984) on
Porites porites. These methodologies differed in the measurements of
zooxanthellae respiration rate (RZ) and zooxanthellae growth rate (m).
RZ, DAVIES  was found to be twice as small as RZ, MUSCATINE (RZ,
DAVIES = 18.1 mgC cm-2d-1  vs. RZ, MUSCATINE = 33.1 mgC cm-2d-1), but
this accounted for a difference of only 3% when RZ was expressed as a
percentage of the total daily carbon input. By comparison, a 25-fold
difference between methods occurred in the component of carbon
required for the daily growth of the zooxanthellae. Davies’ method
measured the net rate of zooxanthellae growth (uNET) from the increase
in surface area, assuming a constant zooxanthellae population density.
In this case uNET was only 1.65 mgC cm-2d-1. Muscatine’s method
measured the gross rate of zooxanthellae growth (uGROSS) from the
mitotic index of freshly isolated zooxanthellae, assuming a duration
of cell division (td) of 11h. This accounted for a daily expenditure
of 41.1 mgC cm-2d-1. The assumption of td might make this method prone
to error. However, assuming that the measurement of uGROSS is correct,
two new budget components had to be introduced to account for the
large difference between uGROSS and uNET. These were expulsion and
digestion, which had not been previously recognised. The latter had
important consequences on the shape of the carbon budget because any
carbon fixed in zooxanthellae that are digested constitutes an
intrinsic part of total carbon translocated to the host. Therefore,
Davies’ budget, using uNET, overestimated translocation by the amount
of carbon lost in expulsion, and Muscatine’s budget, using uGROSS
underestimated translocation by the amount contained in digested
zooxanthellae. The new methodology incorporated these components. The
carbon fixed by gross photosynthesis was still assumed to be the only
source of carbon to the system. Carbon expenditure by the
zooxanthellae was then divided into respiration, net growth and
expulsion. The remaining carbon made up the component of total
translocation, integrating the processes of translocation of fixed
carbon from zooxanthellae to host, and digestion of zooxanthellae.
Carbon was used by the host for respiration and growth, and any
surplus was assumed to be lost from the symbiosis. 
	The effects of elevated nitrate on the budget components were tested 
for both Porites porites and Montastrea annularis in a month-long laboratory
experiment. Corals were grown in the photostat under oligotrophic
seawater and under three concentrations of nitrate (1, 5 and 20 uM).
The response was the same in both coral species, and similar to
previous reports on the effects of elevated levels of ammonia. Under
higher nitrate concentration (5 and 20 uM), corals had a higher rate
of photosynthesis per surface area, and a higher zooxanthellae
population density. The freshly isolated zooxanthellae had a higher
nitrogen, protein and chlorophyll content per cell when corals were
grown in enriched seawater than in oligotrophic seawater. This is
further evidence that zooxanthellae in hospite in oligotrophic
seawater are nitrogen limited. The amount of carbon fixed in
photosynthesis and available for translocation to the host was found
to increase with nitrate enrichment. Hence, the overall organic
productivity of corals appeared to be enhanced by nitrate. In
contrast, the growth rate of corals measured by buoyant weighing was
significantly reduced by nitrate enrichment. The average growth rate
of Porites porites decreased from 1.24 mg cm-2d-1 (control) to 0.68 mg
cm-2d-1 (20 uM NO3), and that of Montastrea annularis decreased from
1.14 mg cm-2d-1 (control) to 0.51 mg cm-2d-1 (20 uM NO3). It was
suggested that, under elevated nitrate, the increased carbon
requirements of the higher zooxanthellae population density promoted
carbon competition between zooxanthellae and calicoblastic cells.
Since zooxanthellae are in the gastrodermal cells closer to the pool
of dissolved inorganic carbon in seawater, they had a competitive
advantage over the calicoblastic cells, and calcification was reduced.
This was defined as the ‘endogenous carbon limitation of
calcification’. 
	A similar experimental design was used to test the effects of 
phosphate enrichment on corals. Phosphate was added to
oligotrophic water to give four treatments of 0, 0.2, 1 and 5 mM PO4.
Overall, no significant change in the organic productivity of corals
was measured. Phosphate enrichment resulted in a significant reduction
of the daily calcification rate of Porites porites, but not of
Montastrea annularis. 
	In order to test if changes in water quality on the reef affected 
coral physiology, nubbins of Porites porites and explants of 
Montastrea annularis were grown for a month at three sites along a 
eutrophication gradient on the west coast of Barbados. The most 
oligotrophic site was the offshore one (OS) with low nutrient 
concentration and high light. The intermediate site (BRI) was 
characterised by higher nutrient concentration and high light. The 
most polluted site (SG) had both high nutrients and low light 
penetration. At the end of the exposure period corals of both species 
could be discriminated between sites on the basis of their physiological 
characteristics alone. Corals at OS showed some evidence
of nitrogen limitation with a significantly lower zooxanthellae
population density, lower nitrogen and chlorophyll content per
zooxanthella, and lower photosynthetic efficiency than at the other
sites. At BRI, corals attained significantly higher rates of gross
photosynthesis and calcification, and their zooxanthellae contained
significantly higher amounts of photosynthetic pigments. Corals at SG
were characterised by a high zooxanthellae population density, high
nitrogen and photosynthetic pigment content per cell, and relatively
low primary productivity and calcification. Thus corals at each site
were found to respond to both nutrient enrichment and irradiance
levels in a combined manner. 
	The use of discriminate function analysis was pivotal in identifying 
those physiological variables that are most sensitive to nutrient enrichment 
(‘primary’ characters), and those that are highly dependent on 
irradiance and only secondarily on nutrient levels (‘secondary’ 
characters). Photosynthetic pigments’ concentration constituted 
‘primary’ characters. These were found to increase with nutrient 
concentration (from OS to BRI), and remain high as environmental 
degradation brought about a decrease in irradiance (from BRI to SG).
The rates of gross photosynthesis, respiration and calcification corresponded
to ‘secondary’ characters. These were related to environmental degradation 
by a single-humped curve, increasing with nutrient enrichment and decreasing again as the
reduction in light developed. Thus corals in the most oligotrophic
site (OS) and the highly degraded site (SG) could not be separated on
the basis of ‘secondary’ characters alone. Therefore in contrast to
expectations, this study found that a reduction in the growth rate or
in the organic productivity of corals per se cannot be taken to imply
the presence of stress factors. 
	The carbon budgets and the simple ratio of dayPgross/24hRc, were 
found to be entirely dependent on the rate of photosynthesis when corals from different nutrient
environments were compared. This was the case because the budget
expenditure components that were found to differ significantly between
treatments (for example, zooxanthellae population density), were very
small when compared to photosynthesis. Therefore, in relation to
nutrient enrichment, carbon budgets and the ratio dayPgross/24hRc were
included with photosynthesis among the ‘secondary’ characters. 
	Among the physiological parameters measured in this study, the ‘primary’
characters and in particular the photosynthetic pigment content per
surface area, were identified as the parameters with the highest
potential for the development of a bio-assay to detect the onset of
nutrient enrichment on coral reefs. 

Francesca Marubini
Queen Mary & Westfield College
Mile End Rd. - London E1 4NS - UK

e-mail: f.marubini at qmw.ac.uk
tel: UK-0171-9755555 x4781
fax: UK-0181-9830973