Trépanier,
C., Parent, S., Comeau, Y. and J. Bouvrette. 2002. Phosphorous budget
as a water quality management tool for closed aquatic mesocosms. Water
Research 36:1007-1017.
The
Montreal Biodome, was built inside of the old Olympic Velodrome back
in the early 1990’s and contains a number of biomes from the
Americas. One of these is a 1600 m2 reproduction of the
Gulf of St. Lawrence. This exhibit contains large numbers of fish,
invertebrates and seabirds, which, as you can imagine, create quite a
bit of waste in a closed system. Over the course of six years
(1992-1998) measurements were taken of dissolved reactive phosphate (DRP,
which is mostly orthophosphate), organic phosphate, ammonia, nitrite
and nitrate. These measurements allowed the staff to develop not only
a nitrogen budget (covered in a 2000 paper in Water Research
34(6):1846-1856) but also a phosphorous budget (the topic of this
paper). What this basically means is that they were able to establish
what the inputs and outputs were for both nitrogen and phosphorous in
the system and determine how these nutrients were being maintained
over the course of six years. During this time the phosphorous level
peaked at about 18 mg/L and then stabilized. The reasons for this are
many, but mainly regular cleaning of the filtration system (rapid sand
filters) and gravel washing removes particulate phosphate (detritus)
before it can be mineralized into DRP by bacteria. Also, water
losses/changes were instrumental in preventing DRP levels from rapidly
climbing. Prior to 1995 only 7-9% of the water was changed per year
compared to 16-18% per year from 1995-1998. Nitrogen levels increased
rapidly due the use of trickle filters and rapid sand filters that
promoted rapid nitrification to nitrate, which was not being removed.
In response to high nitrate levels (180 mg/L NO3-N) water
changes were increased and denitrification filters were added.
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The
uptake ratio of C, N and P by marine algae in the natural
environment is normally 106C:16N:1P by atom or 42C:7N:1P by
weight, also known as the Redfield Ratio. When the N:P ratio is
7.0 or more, phosphorous is considered to be the limiting nutrient
for algae growth in natural marine systems. Other studies
referenced in this paper noted that blue-green algae and other
potentially toxic algae become predominant when the ratio falls to
3 (by weight) or lower in freshwater lakes. Since they were
operating at a ratio of 12 (180mg/L NO3-N:15 mg/L PO4-P)
the fear was that lowering the nitrate-nitrogen to their target
value of 20 mg/L without similarly lowering the PO4-P
would result in a ratio of 1.2, a value they thought might result
in an outbreak of nitrogen fixing blue-greens or some other toxic
algae. Keep in mind that a value of 15 mg/L of PO4-P is
extremely high and would hardly if ever be encountered in a closed
reef system. Obviously the inputs and biomass in this system are
extreme.
By
far the greatest input of P was feeding. Over 76% of the P in the
exhibit came from feeding, while 26% came from bird guano (i.e.
poop). Fully 85% of these inputs were removed by cleaning filters
and detrital removal, leaving 15% to accumulate over the year.
Also 22% of the water was lost in 1998 through water changes and
spills, with a direct link between DRP levels and water loss: the
higher the DRP the greater the roll of water loss in lowering DRP.
For example, the net increase in P was 7.4 mg/L PO4-P
when water changes were at 9% between August 1992 and August 1993,
and only 1.3 mg/L PO4-P in 1998 when water changes were
at 18% per year. Both particulate organic phosphate (POP) and
dissolved organic phosphate (DOP) remained low; this was primarily
due to removal by the filtration system and cleaning, or by
mineralization to DRP.
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In
comparing both nitrogen and phosphorous budgets
several conclusions can be drawn. Phosphorous outputs
are much greater than nitrogen outputs. This is
primarily due to the fact that most phosphorous occurs
initially as particles that can be easily removed by
filters, whereas most nitrogen is present in dissolved
form (ammonia) and is rapidly converted by bacteria
into nitrate. The food also contained much more
nitrogen that phosphorous (5-10X more N than P). As a
result, much less phosphorous than nitrogen is
mineralized (32.5 g/yr vs. 71 g/yr in this system).
Finally, water changes removed 68% of the mineralized
phosphorous (orthophosphate) but only 15% of the
mineralized nitrogen (nitrate).
This
study reinforces what most home aquarists should
already know. Filtration is very important in
controlling the levels of phosphate but even more
important is regular cleaning. This is why it is
recommended to regularly clean any mechanical filters
and not to let detritus accumulate in them. The use of
protein skimmers also helps to reduce not only
orthophosphate but also helps in the removal of
particulate phosphate and particulate nitrogen.
Finally, the notion that an unfavorable ratio of N:P
(3 or less) can lead to cyanobacteria problems may
help to explain why some experience outbreaks of slime
algae in new setups or after several years of
trouble-free operation. In today’s reef aquaria, the
control of nitrogen, especially nitrate, has been
greatly simplified through the use of live rock and
live sand, which offer areas of denitrification. It is
not unusual to have levels of nitrate close to natural
seawater conditions. Therefore total nitrogen levels
in reef aquaria are rarely a problem. The question may
then be more one of phosphorous. How much do you add
and where does it go? Certainly, in a healthy system,
protein skimming, sequestering in sand beds and
regular cleaning of mechanical filters goes a long way
towards dealing with phosphorous inputs. However, the
increasing use of large amounts of fish food, liquid
invertebrate foods and live phytoplankton cultures can
lead to significant inputs of phosphorous. Without an
adequate means of removal, levels may accumulate and
create an unfavorable N:P ratio that could result in a
cyanobacterial outbreak. Again this underscores the
delicate balances that exists in our systems and if
one pushes the envelope too much, without providing
for adequate safeguards (e.g. increased skimming,
water changes, mechanical filter cleaning, a healthy
sand bed, etc. etc.), things can quickly go wrong,
something worth considering when trouble-shooting both
fish-only and reef systems.
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sponsor of this column
Some
references of interest from the scientific literature over the last few
months.
Corals
Brown,
B.E., Clarke, K.R. and R.M. Warwick. 2002. Serial patterns of
biodiversity change in corals across shallow reef flats in Ko Phuket,
Thailand, due to the effects of local (sedimentation) and regional
(climatic) perturbations. Marine Biology 141(1): 21-30.
Fax,
T.Y., Li, J.J., le S.X. and L.S. Feng. 2002. Lunar periodicity of larval
release by pocilloporid corals in southern Taiwan. Zoological Studies
41(3):288-294.
Harii,
S., Kayanne, H. Takigawa, H. Hayashibara, T. and M. Yamamoto. 2002.
Larval survivorship, competency periods and settlement of two brooding
corals, Heliopora coerulea and Pocillopora damicornis. Marine
Biology 141(1):39-46.
Hayashibara,
T. and K. Shimoike. 2002. Cryptic species of Acropora digitifera.
Coral Reefs 21(2):224-234.
Reddy,
N.S., Goud, T.V. and Y. Venkateswarlu. 2002. Seco-sethukarailin, a
novel, diterpenoid from the soft coral Sinularia dissecta. Journal
of Natural Products 65(1):1059-1060.
Wang,
G.H., Ahmed, A.F., Kuo, Y.H. and J.H. Sheu. 2002. Two new subergane-based
sesquiterpenes from a Taiwanese gorgonian coral Subergorgia suberosa.
Journal of Natural Products 65(1):1033-1036.
Warner,
M.E., Chilcoat, G.C., McFarland, F.K. and W.K. Fitt. 2002. Seasonal
fluctuations in the photosynthetic capacity of photosystem II in
symbiotic dinoflagellates in the Caribbean reef-building coral Montastraea.
Marine Biology 141(1):31-38.
Crustaceans
Tsuchiya,
M. and S. Najima. 2002. Occurrence of Trapezia associated with Acropora:
on the “wrong” host coral? Coral Reefs 21(2):160-190.
Fishes
Gordon,
A.K. and T. Hecht. 2002. Histological studies on the development of the
digestive system of the clownfish, Amphiprion percula and the
time of weaning. Journal of Applied Ichthyology 18(2):113-117.
Larval
clownfish do not have the necessary digestive structures or enzymes to
be able to digest artificial foods until 9 days after hatching
Proceedings
of the Third Annual William R. and Lenore Mote International Symposium
in Fisheries Ecology, Oct. 31-Nov.2; 2000; Sarasota, FL. Targets,
Thresholds, and the Burden of Proof. Bulletin
of Marine Science March 2002.
This
special issue of the Bulletin of Marine Science deals with fisheries,
their impacts and ways to develop sustainable use. There are several
papers of interest to anyone who has ever wondered how difficult it will
be to determine what exactly is “sustainable”. There is even a paper
dealing with the collection of marine ornamentals.
Randall,
J.E. and D.G. Fautin. 2002. Fishes other than anemonefishes that
associate with sea anemones. Coral Reefs 21(2):188-192.
Sea Anemones
Weis,
V.M., Verde, E.A., Pribyl, A. and J.A. Schwarz. 2002. Aspects of the
larval biology of the sea anemones Anthopleura elegantissima and A.
artemisia. Invertebrate Biology 121(3):190-201.
Seagrasses
Major,
K.M. and K.H. Dunton. 2002. Variations in light-harvesting
characteristics of the seagrass, Thallassia testudinum: evidence
for photoacclimation. Journal of Experimental
Marine Biology and Ecology 30(2):173-179.
