Nutrient Pollution in Coral Reef Waters
with data from Curaçao waters
By Dr. Gert Jan Gast
Syllabus for the Reef Care Curaçao Workshop on Nutrient Pollution
with Dr. Brian Lapointe, Curaçao, 23 Oct 1998.
Reef Care Curaçao Contribution no. 5, 1998
Additional information
Reef Care Curaçao
P.O. Box 676
Curaçao, Netherlands Antilles
E-mail: info@reefcare.org
Web Site: http://www.reefcare.org
Permissions and Inquiries
Information contained in this publication may be freely quoted, provided the author and this publication are properly
acknowledged.
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Contents
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General introduction
The coral reef ecosystem
Eutrophication effects in coral reefs
Eutrophication on Curaçao
State of the coral reefs along the southern shore of Curaçao
Afterword
Map of Curaçao
General introduction
Living nature can be divided in different areas with specific plants and animals. The group of organisms in a certain
area and the way in which these organisms interact with each other is called an ecosystem. Examples of ecosystems
are coral reefs, deserts, taigas, rain forests and savannas. The outward boundaries of ecosystems are set by physical
factors such as temperature, rain and geological shape of the land. Polar regions are different from the tropics in
temperature, but within the tropical zone there are wet and arid areas. The geological shape can mean plains or
mountains, small islands or large continents, shallow or deep sea. Organisms are adapted to live under specific
circumstances and the occurrence of such circumstances determines which animals and plants can be found where.
These influences are called abiotic factors. Relations between the organisms determine the detailed composition of
that ecosystem (e. g. predators eating prey, trees create a place for birds to build a nest, corals and sponges compete
for space on a reef flat, etc.). Such influences of organisms on each other are called biotic factors. The availability of
nutrients depends on a combination of abiotic and biotic factors.
The term 'nutrient' in a broad and general sense means food. Organisms need nutrition or food to obtain the necessary
energy and building materials to grow, maintain and reproduce. However, more commonly the term nutrients is used
for the chemical elements nitrogen and phosphorus. With nutrient pollution or eutrophication we mean an increase in
nitrogen (usually as ammonium or nitrate) and phosphorus (as phosphate) in a natural environment. Before I go into
the details of eutrophication, let me first explain the role of the elements in an ecosystem.
Plants fix energy from sunlight into organic material in a process called photosynthesis. Plant eating animals
(herbivores) obtain the necessary energy to live by eating plants. Animal eating animals (carnivores) eat herbivores
or other carnivores. This way energy is transferred through the food chain from plants to herbivores to carnivores. It
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is important to realize that there is only one input in the system: plants fixing sunlight. All other organisms depend on
the presence of plants for their energy. Energy is transferred through the ecosystem until it is lost.
Aside from fixing energy into organic material, a plant needs building materials to make itself: stem, leaves, roots,
flowers; the whole thing. These materials are usually expressed as their chemical elements, e.g. carbon (C), nitrogen
(N), phosphorus (P), hydrogen (H), oxygen (O), etc.. In reality these elements are bound in organic molecules. C, O
and H form the largest part of living or organic matter. Nitrogen is a necessary element in for example protein
molecules and phosphate occurs in cell membranes. Also, both elements N and P are necessary parts of DNA. Other
elements are needed in small amounts to form a body, such as iron and copper. In a whole living body these materials
are needed in certain amounts. Plants in the sea consist of C and N and P in a ratio of approximately 106:45:5. Plants
need to obtain these different elements in different amounts from the environment. C is present in CO 2, H is in water
(H2O), N in NH4 (ammonium) or NO3 (nitrate) and P in PO4 (phosphate). O is present in almost all these molecules.
The ratio in which these building materials are available is mostly not the same as the ratio in which they are needed.
Of one of these elements there will be less available relative to the others, which means that this element becomes
limiting for growth. In the sea there is of course water enough and H and O are never a problem. CO 2 dissolves into
the water from the atmosphere and is usually sufficiently present as well. The limiting nutrient is most commonly N
or P (although there are areas where neither N or P, but iron is limiting). Hence the common use of the term nutrient
pollution for excess inputs of ammonium (NH4), nitrate (NO3) and phosphate (PO4).
Like energy, nutrients are transferred through the ecosystem as one organism eats another. There is, however, an
important difference: nutrients are not used up, but become released again. Animals that eat plants burn 80 - 90% of
their food for energy and use only the rest for growth of their body and reproduction. This means that they eat far
more N and P than they need and the surplus has to be excreted. Also every organism dies at some time and when
bacteria break down the remains, nutrients become available again. The essential difference with energy is that
nutrients are cycled through an ecosystem. Plants take up inorganic nutrients from their environment and fix them in
organic material, animals eat plants and excrete organic nutrients, and bacteria convert these back to inorganic
nutrients, which can be used by plants again. As long as none are lost, nutrients could in theory be recycled forever
through an ecosystem. In reality, ecosystems are not closed and nutrients are imported and exported: animals move in
or away, water currents bring or take away organisms and molecules, dead organisms disappear into deep water, etc..
In long living ecosystems the import and export of nutrients are usually balanced: as much comes as goes out.
One of the major effects of humans on their environment is that we change the nutrient balance by increasing the
nutrients concentrations. We use fertilizers in agriculture, which is nothing else than nutrients for those plants we
wish to grow. These plants cannot use all the nutrients we supply and much of the loading is lost to the environment.
Sewage consists of nutrients in organic and, if treated in a sewage plant, inorganic forms. These nutrients are
generally discharged into our environment. At the same time we often reduce the capacity of the nature around us to
use these nutrients by removing the natural vegetation for agriculture or urban development. Humans eutrophy their
environment and the larger and denser the population is, the stronger the nutrient pollution.
So, why is this a problem? We are basically giving plants and thereby all the animals in the ecosystem materials that
they need, don’t we? The answer is that reality is not that simple. Yes, plants need nutrients, but only a limited
amount. The problem is that increases in nutrients lead to changes in the ecosystem. Some plants are specialized to
survive in an environment with low nutrient concentrations, while other plants dominate with high nutrient
concentrations. When nutrient levels are increased the ecosystem shifts from low nutrient specialists to high nutrient
specialists. Ultimately this leads to completely different ecosystems under long term eutrophication. Generally this
leads to a reduction of the diversity within ecosystems and variation between ecosystems.
The coral reef ecosystem.
Coral reefs consist of many different organisms: macro-algae, stony corals, soft corals, sponges, ascidians, snails,
mussels, crabs, lobsters, fish, etc. Macro-algae are plants such as seaweeds, coralline algae, small turf algae, etc.. The
other groups are basically animals, but there are a few important strangers: animals that have unicellular plants living
in their skin. These algae are called zooxanthellae or in short zoox. These combinations (or symbionts) behave partly
as plant and partly as animal. Most well known are stony corals, but other examples are some soft corals, some
sponges, giant clams and the upside-down jellyfish (Cassiopeia). The zooxanthellae fix the energy of sunlight by
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photosynthesis and the animal catches food from the water column (small bacteria, algae and animals). Because of
the dependence on sunlight corals can only live in clear and shallow waters. The zoox give energy to the coral (or
another host) and the coral gives nutrients in return. By living together in a symbiosis both organisms do better than
if they were living apart. The other animals without zooxanthellae make a living by catching food particles from the
water column or by eating other reef organisms.
Stony corals make and shape the coral reef. They contribute most to reef building by the limestone skeletons they
make to grow. The collection of stony corals in a reef creates the three dimensional structure of the reef. There are
many gaps and crevices between and under coral colonies, which serve as hiding places for many other organisms to
survive in a coral reef. Where corals disappear many other creatures such as lobsters and the colorful fish are lost as
well. Besides corals crustose coralline algae also contribute to reef growth by calcification. At the same time that a
coral reef is built by corals, it is also broken down again by bio-erosion. Parrotfish eat small algae growing on dead
coral, but in the process of eating grind the old coral skeleton to dust or sand. Boring sponges and mussels drill holes
in coral skeletons. The balance of reef growth and destruction determines whether a reef as a whole increases or
decreases. This balance depends on how many of which organisms are present in a reef ecosystem.
Coral reefs occur typically in waters with low nutrient concentrations. Reef organisms are adapted to survive under
these low nutrient concentrations. Corals and macro-algae can take up inorganic nutrients directly from the
surrounding water although these occur in very low concentrations. Organic nutrients are gained in food collected
from the water column. As the surrounding oceanic water flows over the reef many nutrients are subtracted from this
water. Another, and possibly very important, input of nitrogen is fixation of atmospheric N2 into amino acids.
Nutrients are also lost to the overlying water column and taken away with the current. Nutrients are not contained
cycle after cycle in the reef ecosystem, but rather taken up by the plants and animal-plant symbionts and lost again
when excreted by higher trophic levels. Nutrients flow through the food chain and they are converted from one form
to another in the process. Aside from the classic food chain described above, bacteria are responsible for many
transformations of nutrients. Bacteria can for example convert ammonium to nitrite to nitrate. All in all the reef
ecosystem is a extensive, complex network of compartments that do different things with different nutrients.
Eutrophication effects in coral reefs.
Increases of nutrient concentrations have various effects on the coral reef ecosystem. The first set of problems occurs
on the level of individual organisms. In corals the zoox—coral symbiosis becomes disturbed with high nutrient
concentrations. When corals are kept in aquariums with high ammonium concentrations for a few weeks, the zoox
multiply strongly and coral growth stops. It is not known yet what exactly happens, but it is quite clear that the zoox
use the energy from photosynthesis to grow themselves instead of giving it to the coral. Elevated nutrient
concentrations are bad news for corals. On the level of primary producers (plants) the competition between corals
and macro-algae is influenced by nutrient concentrations. Since the bottom surface area on a reef is limited (1 m2 is 1
m2) reef organisms have to compete for space. They all need a hard bottom to attach themselves to and cannot grow
on top of each other. With low nutrient concentrations corals are able to keep algae away and overgrow them. With
high nutrient levels algae get a competitive advantage and start to overgrow corals. As algae do not calcify, reef
growth is reduced. An important factor influencing the coral vs. macro-algae competition is fisheries. Many fish,
such as parrotfish, eat macro-algae. When these fish are removed, the control on macro-algal growth is removed,
which is again bad for corals. On a more complex ecosystem level, eutrophication can lead to an increase in bacteria,
phytoplankton and their consumers in the water column. More particles become available in the water column and
bio-eroders, such as boring sponges and mussels can use this extra food. This leads to more drilling of holes in the
coral skeleton, which weakens the corals. If their skeleton is not strong, they easily break off during storms. Again
bad news for corals.
There are various well known examples of effects of eutrophication on coral reefs in the scientific literature. On
Barbados reduced growth, reduced reproduction, reduction of successful settlement and changes in the coral
composition have been recorded. On Jamaica the coral reef has been replaced by a macro-algae or seaweed reef
helped strongly by heavy overfishing and a disease that wiped out the sea urchins. In Kaneohe Bay, Hawaii, heavy
sewage discharge led to enormous amounts of macro-algae (up to 2.5 m high!) and an almost complete loss of corals.
After the sewage discharge was diverted to deep water away from the island, the macro-algae disappeared and corals
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have come back to a reasonable extent. Although the precise causative mechanisms are often still unclear, there is no
doubt that eutrophication has serious negative effects on the health of coral reefs. —
Figure 1. Inorganic nitrogen concentrations at 2 m depth in coral reef and the adjacent oceanic water in Curaçao.
A: Oceanic water compared to non-eutrophied reef water. B: Eutrophied compared to non- eutrophied reef water. C:
Harbor water. DIN = Dissolved Inorganic Nitrogen (= NH4+ + NO2- + NO3-). Mean ± sd (n=4).
Eutrophication in Curaçao.
On Curaçao there are three main sources of nutrient pollution. The first and most obvious is sewage discharge along
Willemstad. At Marie Pompoen, beside the Avila Beach Hotel and near the old library in Punda there are 3 sewage
pipes that discharge about 1000 m3 of untreated sewage per day. A lot of sewage is also discharged into the Waaigat,
Schottegat and Anna Bay (there is a report that indicates the amounts by Letitia Buth and Tico Ras). Also industrial
waste from the refinery, the slaughter house, etc. is discharged into the Schottegat. With each outgoing tide and after
heavy rain the water flows out of the Anna Bay and over the reef east or west of it. A second, more erratic, source of
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mainly inorganic nutrients is runoff with rain. Rain water runs off the streets and through drainage channels to sea
and takes rubbish, sediment, etc. from the land to the water. The third and least visible source of nutrients is
groundwater seepage. Only 38% of the households on Curaçao are connected to the sewage system and the rest uses
septic tanks. In septic tanks organic matter is broken down by bacteria, which changes nutrients to inorganic forms,
but does not remove them. Water from the septic tanks sinks down into the bottom. Most of the bottom under
Willemstad consists of fossil reefs, which are very porous and the water easily seeps through. On average, the total of
septic tank water is equal to the total of rain water that seeps through to the groundwater. This results in very high
nitrate concentrations in the groundwater. At the same time some groundwater is pumped up and used for irrigation,
but this happens mainly further away from the town. When more and more septic tank and rain water is added to the
groundwater, this is eventually pushed sideways out of the ground and onto the reef. It will depend strongly on the
amount of rain in a year or season how much comes out, but come out it does.
Which of these influences can be recognised in patterns of nutrient concentrations at Curaçao? I sampled water at
different stations along the coast of Curaçao and measured inorganic nutrients from February 1994 until March 1995
with 2 or 3 week intervals (see map on page 4)
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Station Ocean is 3 km away from the island in deep oceanic water.
Station Upcurrent is just east of Fuik, where clean oceanic water comes in and flows over the reef. There is no
urban development or agriculture at Eastpoint and hence no influence of humans.
Station 2 is in front of Jan Thiel Lagun.
Town Reef is just west of the Avila Beach Hotel in front of Punda.
Harbour is in the mouth of the Anna Bay under the floating bridge.
Station 5 is between Sonesta and Caribbean hotels.
Station 7 is Slangenbaai
Downcurrent is Pestbaai (a.k.a Vrijgezellenbaai).
In Fig. 1A a comparison of nitrate concentrations is made between non-eutrophied reef water and the adjacent ocean.
The nitrate concentration was always higher in the water above the reef. The ammonium concentration was the same
in reef and oceanic water (the same concentrations as nitrate). Only nitrate was enhanced. Since there are no
influences of humans at this reef such as sewage and groundwater seepage, this nitrate must have been excreted by
reef organisms or micro-organisms in the sediment. This is a natural phenomenon in reef waters and has previously
been found on other reefs, e.g. in the Great Barrier Reef. There was no difference in phosphate concentrations at Fuik
and Ocean (Fig. 2).
Figure 2. Phosphate concentration at 2 m depth in eutrophied and non-eutrophied reef waters and the adjacent
oceanic water. Mean ± sd (n=4).
Fig. 1B shows a comparison of eutrophied and non-eutrophied reefs in DIN (Dissolved Inorganic Nitrogen, which is
ammonium + nitrite + nitrate; nitrite is less than 1 % of DIN). On most of the days I sampled, DIN levels were
strongly elevated in front of town. There is large variation through time because sewage discharge is not continuous
and the water current over the reef flat is highly variable. Note that the scale of Fig 1 B is ten times larger than that of
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1A. DIN at Town Reef consists roughly of 50% ammonium and 50% nitrate. Phosphate concentrations were also
usually higher at Avila than at Fuik or in the adjacent ocean (Fig. 2). From July to October 1994 weekly samples
were collected and more reef stations were included. The results are shown in figure 3, where the open circles
indicate Ocean, the filled diamonds Avila and the bars various reef sites. Different patterns become clear in this
figure. In week 6 (22 Aug.) both ammonium and phosphate peaked. Smaller peaks of these two nutrients occurred in
week 2. This is a general phenomenon that can also be seen in figures 1B and 2: ammonium and phosphate are both
high or both low. The reason is that sewage is collected in large underground reservoirs which are emptied when full.
This results in an erratic discharge pattern. At some days I sample in a sewage cloud, but at other days I caught
normal reef water. The latter does not mean that there was no sewage discharge, but merely that I missed it. Another
pattern that emerges from figure 3 is that nitrate behaves very differently from ammonium. Nitrate was not elevated
in week 6, but clearly so in week 9, 10 and 11. This shows that the source of nitrate cannot be sewage (the reason no
nitrate comes out with sewage is that bacterial activity in the underground reservoirs depletes oxygen, which prevents
oxidation of NH4 to NO2 to NO3). This nitrate came into the reef by groundwater seepage. There was rain at the end
of August and some groundwater with high nitrate concentrations gradually seeped out during the following weeks.
We can conclude that there is nutrient pollution in the reef water column in front of Punda caused by sewage
discharge and groundwater seepage.
Figure 3. Nutrient concentrations at 2 m depth in fringing coral reef waters and adjacent ocean in Curaçao. Stations
along the southern shore. Town reef is eutrophied, bars indicate sites away from eutrophication. Mean ± sd (n=4).
Accuracy is accuracy of the measurement methods.
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DIN levels in the Anna Bay are shown in figure 1C. Note that the scale is again enlarged by almost a factor 10. This
DIN consists for about 80% of ammonium. Large fluctuation is caused by the tide. With in going tide I sampled reef
water flowing in and with ebbing tide I sampled increasingly more polluted water from further down the bay. These
values are extremely high. The reason is that severe eutrophication is combined with a very long residence time of
the water. Sources of eutrophication are sewage, runoff and groundwater seepage (indicated in this case by silicate
levels; silicate originates from the volcanic core of the island). The water exchange time of the Anna Bay and
Schottegat is probably in the order of magnitude of 100 days (it must be noted that this figure depends strongly on
the amount of rain). It takes a very long time before nutrients are washed out of the bay. Phosphate concentrations in
the Harbour are comparable to those at Avila. This shows that groundwater seepage is relatively more important,
because phosphate is bound to limestone in the old reefs. An important question is how far the pollution from the
Harbour reaches along the shore. As shown in Fig. 4 elevated ammonium, nitrite and nitrate can be measured up to 4
km down current of the harbour with outgoing tide. This number should be used very carefully, as I did this long
transect on one day only. When there are high waves and a strong current, the pollution will be diluted very rapidly,
but on calm days the harbour water may reach much further along the coast.
Figure 4. Dissolved inorganic forms of nitrogen in coral reef waters and adjacent oceanic water along the southern
coast of Curaçao on 22 February 1994, with outgoing tide. Numbers on the x-axis indicate distance in km to the
harbour in the middle of town. Town ranges from 4.5 km east to 4 km west of Harbour. Mean ± sd (n=4).
State of the coral reefs along the southern shore of Curaçao.
Healthy, well developed reefs with many species and large colonies can be seen near Eastpoint where clean oceanic
water arrives at the island and no eutrophication occurs. Aside from a few spots with local problems (Fuik, Caracas
Bay) reefs are still in rather decent shape up to Seaquarium where the construction of the Seaquarium breakwaters
and beach have completely exterminated the coral reefs. Between Seaquarium beach and Princess Beach there is a
drainage channel which is usually dry, but with heavy rain much sediment and organic nutrients come out causing
very turbid waters. Between this channel and the Anna Bay there are 3 sewage pipes and some artificial beaches. The
reefs are strongly degraded over this stretch. I have measured in 1991 that both total coral cover and the number of
species are reduced by 50% at Avila compared to the reef east of Seaquarium. Acropora palmata and A. cervicornis
and Porites porites are completely gone, the Agaricia’s have mostly disappeared and only head corals survive
(Diploria’s, Montastrea’s, Colphophyllia natans, Siderastrea siderea, Porites asteroides) and Madracis mirabilis
manage to hold. These findings have more recently been repeated by students of Prof. Rolf Bak at Marie Pompoen
and Avila. They also showed that especially the baby corals are missing. This is very worrisome, because new
recruitment is needed to get restoration of the reefs. Another important finding has been by Erik Meesters and
students who have shown that more injuries occur on corals in front of the town and that these heal slower. West of
the Anna Bay the reef terrace is bare coral rock for the first kilometer or so. There is nothing that survives the
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mixture of high nutrients, metals, oil and other toxic chemicals that come out of the harbor bay. In front of Holiday
Beach there are some poorly developed corals again that try to make a living. The coral reef basically isn’t much
until a few kilometers past the Piscadera Bay. In general, the coral reefs in front of Willemstad are strongly degraded
which is related to the presence of that town.
It is impossible to determine exactly how much of this degradation has been caused by nutrient pollution. Other
important direct or indirect destructive factors are or could be: sedimentation (both from runoff and artificial
beaches), overfishing, toxic chemicals in sewage (what are effects of chloride and detergents on corals?) and oil
pollution. Moreover such negative influences often enhance each other. However, there is no doubt that nutrient
pollution is a serious problem afflicting the health of the coral reefs along Curaçao and that counter measures should
be taken.
The simplest and cheapest “end of the pipe solution” to nutrient pollution is to literally lay the end of the pipes
further out in deeper water away from the reefs. This would certainly reduce the direct effects of sewage discharge on
the coral reef of Curaçao and should be considered as a TEMPORARY first solution. On the long term this would
not solve all the problems and lead to healthy coral reefs. First, this discharge still contributes to the general
eutrophication of the ocean and may lead to negative effects on far longer time and spatial scales than we can
currently see. Moreover, we have no idea what effects of chemicals in the sewage are and these may very well be
pretty destructive at very low concentrations. Dilution is no solution to pollution. The only way to solve the
eutrophication problem is to connect all households (no more septic tanks) and industries to a sewage system, treat
that water and remove the nutrients. Although this is very expensive and will take considerable time to develop, it is
the solution that should be worked towards. Using secondary treated sewage water for irrigation is a good alternative
to removing the nutrients in a tertiary treatment step as long as these nutrients do not sink into the groundwater and
seep out to the reef. Second, as long as the harbour stays as heavily polluted as it is at the moment, it will remain a
source of nutrients and other toxic chemicals. It is really time that the industry around Schottegat is forced to live up
to modern environmental standards and clean up their rubbish. Third, runoff and sedimentation will stay serious
problems that have to be dealt with. Basins with vegetation (the good old mangroves) should be constructed in which
the sediment in runoff water can sink out before the water reaches the sea. Artificial beaches should not be allowed
anymore. Every reef in front of an artificial beach is a desert.
Afterword
What Curaçao needs is a long term commitment and realistic plan to counter the effects of the dense
population on its reefs and these intentions need to be enforced by laws. Healthy coral reefs along the
southern coast of Curaçao are no utopian idea. If industries, dive operators, recreational users,
environmental NGO’s, underwater park management and government services are willing to commit
themselves and cooperate both among each other and with scientists and law enforcers, the coral reefs of
Curaçao can be saved and improved. Essential, however, is that protection of the reefs is not placed second
in priority to short term economical gains. It is realistic to state that if nothing is changed Curaçao will have
no more coral reefs in a few decades. But not all is bleak and hopeless, construction of a sewage system for
the Punda side of town is now finally underway while a sewage system for the Otrobanda side was
completed a few years back. Things are changing and can change further. It is up to the people of Curaçao
to choose the changes in the right direction, not only for the wellbeing of the corals and all the other reef
organisms, but for themselves as well. Curaçao without coral reefs would be a sad development indeed.
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