|
I have spent much of
the last couple of years taking an indirect look at what happens
in reef tanks with regard to various chemical constituents
(Shimek, 2001, 2002a, b, c), primarily the toxic heavy metals,
referred to in the hobby as “trace elements.”
Some of these materials are biologically necessary, but none
has been ever shown to have any benefits at concentrations
above those found in natural sea water. Indeed, most of them
have been shown to be both acutely and chronically toxic at
even slightly higher concentrations (see, for example, the
discussion in Shimek, 2002d and these references: Alutoin,
et al., 2001; Breitburg, et al. 1999; Goh, and Chou, 1992;
Heyward, 1988; Negri, and Heyward, 2001; Reichelt-Brushett
and Harrison, 1999). Those studies of documented toxicity
notwithstanding, I have found that many of these chemicals
have exceptionally high concentrations in the liquid medium
of reef aquaria, as well as within the food we add to the
systems.
There are numerous documentations of presumed
cases of metals toxicity in our systems, mostly due to copper
contamination. Additionally, there are likely many other cases
of “hidden” metals poisoning. Most of these events,
from the point of view of the aquarist, likely occur haphazardly
and sporadically enough that the actual cause of mortality
has either been overlooked or attributed to some other cause.
I think that most cases of “acclimation death”
are due to insufficient acclimation to the heavy metals concentrations
found in our tanks. Most marine organisms can detoxify these
materials to one degree or another, but it takes time for
the metabolic pathways which can do this (mostly the production
of metallothioneins, the proteins used to bind and detoxify
metals) to become active. This activation may take from several
hours to several days.
Many, if not most, coral reef organisms
are from shallow water and, as such, most of them are subject
to significant and rapid changes in salinity. Salinity changes
of several parts per thousand are not uncommon with tidal
shifts and as a result of heavy rains. For example, a few
hours of heavy tropical rains can often drop the salinity
of a coral reef lagoon from 36 ppt to 30 ppt down to a depth
of twenty feet or more. While this stresses the animals, most
of them will not die. Likewise, after several days of hot,
dry conditions, lagoonal animals often have to contend with
salinities of 39 ppt or more. If this occurs during a spring
tide period where the tidal exchange is minimal, the salinity
may remain elevated for weeks. When the tidal exchanges do
become more extreme, such hot saline water flows out over
the reef and animals that a few minutes earlier might have
had much lower salinity in waters surrounding them. Again
this stresses the animals, but it generally doesn’t
result in the immediate mortality often referred to in the
reef aquarium hobby as “salinity shock.” Frankly,
for animals in good health otherwise, transient salinity shock
simply is very unlikely to cause any long-term stress. On
the other hand, sudden exposure to high concentrations of
heavy metals, even if they might be tolerable after acclimation,
will be lethal.
I have documented the metal concentrations
from samples of aquarium tank water taken from 23 tanks across
the United States (Shimek 2002a, b) and are summarized in
Table 1, below. These data are unambiguous and conclusive,
showing that in these tanks, many of the toxic heavy metals
concentrations exceeded demonstrated lethal levels for many
corals and other marine organisms (Shimek, 2002d)
There has been much discussion of these
results with various proposals being put forth as to why the
tanks should not show the effects of toxic metals, even though
the metals concentrations were apparently high enough to cause
such effects. Some of the explanations for ameliorating effects
were:
|
•
|
The metals were simply not toxic. |
|
•
|
The metals were being chemically bound in solution by
several possible classes of organic chemicals, such as
humic acids. |
|
•
|
The metals were being bound into some sort of particulate
material in the unfiltered water, and the high readings
were a spurious result. |
All of the ameliorating effects listed
may be true to some degree. For example, the metals may not
be toxic to some adult organisms even though they are toxic
to larvae and immature organism. Toxicity is often demonstrated
with larvae or immature organisms as they are known to be
more sensitive. Perhaps in some cases, the adults kept in
reef aquaria are not as affected as are these other stages.
For example, heavy metal toxicity, particularly to adult organisms,
is often a long-term process occurring over several years
and would not likely be immediately noticeable in many cases,
except that “previously healthy” animals die of
unknown causes after four or five years in an aquarium system.
Those animals that have some method to detoxify the chemicals
typically build up a significant body burden of the metals,
which, in turn, results in long-term effects such as abnormal
behavior, neurological damage, sterility and eventually early
death. Additionally, any materials in solution, be they bound
to a humic acid, an iron hydroxide, or some other chemical,
are ingested by the organisms during their normal feeding
(Chong and Wang, 2000; Sundelin et al., 2001). Digestive processes
have the potential to release any bound metals, with net effect
of transferring them to the animals. Additionally, many invertebrate
organisms including corals, actively absorb organic materials
across their outer epithelial surfaces. Such absorption will
result in ingestion of these metals bound to the organic molecules.
Finally, particulate material in the water is actively consumed
by many coral reef animals; most notably for aquarists, many
of the corals, tridacnid clams, and other suspension feeders.
Finally, the presence of organic material may increase trace
metal uptake significantly over what occurs in water with
less organic materials in it (Guo, et al. 2001; Vasconcelos,
et al., 2001; Wang and Dei. 2001). In effect, the amelioration
of metals concentrations by organic binding or precipitation
likely simply replaces acute toxic effects with long-term
chronic ones.
Actual testing of reef aquarium water for
toxic effects has not been done. I hoped to report on the
results of some sea urchin larval bioassays in time for this
writing, by testing various aquarium waters for toxic effects.
However, I have been thwarted in those efforts by Mother Nature.
Hurricanes Lila and Isidore impacted the Gulf Coast earlier
this autumn, resulting in excessive amounts of fresh water
runoff, which in turn caused mass spawnings of the local sea
urchins around the facility of my source for sea urchins.
As I need “unspawned” sea urchins for the tests,
I now have to wait until a different species becomes gravid,
probably sometime after the first of the year
Two means exist for rendering tank waters
less toxic. The first is the precipitation or removal of the
metal from the aqueous milieu. Once the material has been
removed from the water, it is not toxic. It has to be in the
water for organisms to either absorb it or ingest it. One
way this precipitation might happen is biologically, as when
corals detoxify the mildly poisonous metal, strontium, by
incorporating it into their skeletons. Another way to precipitate
the chemicals from the water may happen by strictly inorganic
means, such as the precipitation of heavy metals by various
iron hydroxides. Additionally, particulate substrates such
as those found in deep sand beds, have in their depths anaerobic,
sulfide-rich conditions that facilitate the precipitation
of many of these poisonous materials as non-toxic sulfide
minerals. Deep sand beds significantly favor the precipitation
and retention of heavy metal sulfides, and are thus likely
a significant factor in rendering tank waters less toxic.
Precipitated materials in the deep sand beds accumulate with
time, but they are not toxic as long as they remain insoluble
in the sediments. If those materials should become soluble,
however, they would present serious and acute toxicity problems.
Such precipitates may become soluble if:
|
•
|
they are exposed to the aerobic conditions
in shallower sediments, or |
|
•
|
they are exposed to acidic conditions, such as might
happen during a calcium reactor malfunction, or |
|
•
|
they are eaten by a deposit-feeding animal, or |
|
•
|
nutrient loading of the sediments causes bacterial populations
to create a more acidic bed. |
Any of these events can cause either acute
toxicity, or accelerate chronic poisoning, and the events
can vary significantly in duration and effect. To remove or
reduce the threat of some catastrophic event happening, and
to avoid long-term or chronic poisoning, it is to the aquarist’s
advantage to reduce the accumulation of these materials either
in the water or in the sediments.
The second way to detoxify tank water is
to chemically or physically export the chemical out of the
system. Several export methods exist for aquarists, but their
effectiveness has hitherto not been evaluated in aquariums.
In this article, I will describe some of the results from
examining a few of the various ways commonly employed to export
materials from aquaria.
Materials and Methods:
In this study I was, through the kind and
generous help of several aquarists, able to examine the chemical
constituents of four types of export materials. I examined
4 samples of skimmate, the liquid resulting from the foam
that makes it through a foam fractionation device. I also
examined one sample of skimmer sludge, the greenish-gray or
black “mud” from the inside of a skimmer. One
other person was scheduled to return some sludge, but had
to drop out of the study, so unfortunately there was only
one sample of sludge. This material is largely bacterial in
composition, but as it is washed away during skimmer cleaning,
it is a reasonable material to examine. There were three samples
of Caulerpa; two samples of Caulerpa cupressoides,
one of Caulerpa racemosa. The results from these
algae were pooled for the analyses. Two samples of Xenia
of unknown species were processed. I also processed one sample
of a Sarcophyton species. However, as this latter
species is uncommonly used as an export, I will report on
it only as a comparison to the commonly exported soft coral,
Xenia.
The samples were analyzed as in the Tank
Water Study (See Shimek, 2002a, for details). Precleaned and
treated jars were sent to me from the analytical laboratory.
I repackaged the jars and sent them to the participants. All
samples were collected, frozen and returned to me. I refroze
them and then sent them to the laboratory.
Varying amounts of the materials were obtained,
and in some cases there was insufficient material present
to run all the tests. In these cases, the tests were prioritized.
The sample results were reported in “as is” basis,
based on the total wet weight of the samples. Tests for metals
concentrations and nitrogenous compounds were done on all
of the samples. Tests of the conventional nutrients and caloric
content, however, were not done on several of the samples
of skimmate and one of the Xenia samples. The samples
had to be dried for these analyses and skimmate is, apparently,
a fairly “weak tea.” Even a liter of skimmate
contained insufficient material to perform all the tests.
The two Xenia samples were of differing sizes, reflecting
the growth rates in the two different aquaria that they originated
in. One of the samples was simply too small for all the tests.
After processing and analyses, the data
were returned to me for further analyses. It is important
to realize that, as with the other tests in this series, the
statistics that are presented are simply descriptive. I ran
no statistical tests comparing the effectiveness of the various
exports because there aren’t enough samples for the
results to be meaningful. However, there are plenty of data
to examine and discuss in the remainder of this article.
All of the descriptive statistics were
calculated using spreadsheets, and are subject to operator
error. If you find that I have made an error in calculations,
please contact me and I will correct it.
Results
and Discussion:
Baseline Conditions
In the discussion that follows, the exports
will be discussed in relation to three distinctly different
types of water. The first is natural sea water (NSW), with
the various metals concentrations as reported in Pilson(1998).
It is important for our purposes to realize that these data,
as authoritative as they may be, may have little to relevancy
to the waters over natural reefs. Relatively little chemical
oceanographic sampling has been done over reef areas and,
in many cases, what has been done was performed in a manner
that makes comparisons to reef tanks dubious. For example,
samples taken three meters above a reef samples “reef
waters,” to be sure, but how those waters relate to
what is happening closer to the reef is largely unknown. Most
aquarists sample their reefs in far closer proximity than
many oceanographers would dream of doing, and the few samples
taken close to reefs show some very significant differences
from the water a few meters above them. (see, for example:
Stimson and Larned, 2000).
In all marine areas within a few inches
of the substrate, the chemical composition of the water largely
reflects what the nearby organisms have been doing to it.
This is particularly the case with biologically “interesting”
chemicals, and these, of course, are precisely the items of
interest to aquarists. Reef water data taken a few meters
above the reef might be, and has been, legitimately argued
to tell little about the reef. Unfortunately, this limits
comparisons severely, and I have taken the easy way out and
just discussed NSW in the familiar general terms.
There is also the comparison with
artificial sea water, and here two different comparisons can
be made. The first is to what I refer to as the “average
reef tank” water as determined by the Tank Water Study
(Table 1 and Shimek 2002 b). These comparisons will be made
first. The second comparison is to the average of the artificial
sea water mixes described by Atkinson and Bingman (1999) and
that comparison will be made subsequently.
|
Table
1. Average values of elemental concentrations
in natural sea water and from the tank study, showing
the “excess” amount of materials in an average tank,
assuming the average tank volume of 191.3 liters and
a specific gravity of 1.025. All values
are in parts per million (mg/kg).
Blank cells indicate that the data are not available.
Values that are “0.000000” do not indicate a value of
zero, but rather indicate the actual value is less than
1 part per trillion (the average concentration is less
than 10-12). The variance measures
in the average tank data are the sample standard deviations.
Arsenic has no variance measure in the study as it was
only found in one tank.
|
|
Element
|
Natural Sea Water
Average
|
Average Tank Values
± Variance
|
Excess Concentration
in
Average Tank (Difference
of Averages)
|
Excess Mass in
Average tank in mg
|
|
Aluminum
|
0.00027
|
0.140 ± 0.070
|
0.140
|
33.88
|
|
Antimony
|
0.000146
|
0.018 ± 0.007
|
0.018
|
3.50
|
|
Arsenic
|
0.001723
|
0.02
|
0.018
|
3.58
|
|
Barium
|
0.01374
|
0.015 ± 0.008
|
0.001
|
0.25
|
|
Beryllium
|
0.000000
|
Not Detected
|
|
|
|
Boron
|
4.6
|
3.94 ± 1.42
|
-0.665
|
-130.42
|
|
Cadmium
|
0.000079
|
Not Detected
|
|
|
|
Calcium
|
400
|
400.4 ± 85.1
|
0.400
|
78.45
|
|
Chromium
|
0.000208
|
Not Detected
|
|
|
|
Cobalt
|
0.000001
|
0.0002 ± 0.0001
|
0.000
|
0.04
|
|
Copper
|
0.000254
|
0.024 ± 0.005
|
0.024
|
4.66
|
|
Iodine
|
0.05076
|
0.447 ± 0.518
|
0.396
|
77.71
|
|
Iron
|
0.000056
|
Not Detected
|
|
|
|
Lead
|
0.000002
|
Not Detected
|
|
|
|
Lithium
|
0.1725
|
0.666 ± 1.462
|
0.494
|
96.79
|
|
Magnesium
|
1272
|
1326 ± 139
|
54.0
|
10,591
|
|
Manganese
|
0.000027
|
Not Detected
|
|
|
|
Mercury
|
0.000000
|
Not Detected
|
|
|
|
Molybdenum
|
0.00959
|
0.016 ± 0.017
|
0.006
|
1.26
|
|
Nickel
|
0.00047
|
0.024 ± 0.006
|
0.024
|
4.61
|
|
Phosphorus
|
0.0713
|
0.328 ± 0.745
|
0.257
|
50.35
|
|
Potassium
|
380
|
405.2 ± 61.1
|
25.20
|
4,942
|
|
Silicon
|
2.81
|
1.270 ± 1.30
|
-1.540
|
-302.03
|
|
Silver
|
0.000003
|
Not Detected
|
|
|
|
Sodium
|
10561
|
10850 ± 1246
|
289.0
|
56,680
|
|
Strontium
|
13
|
6.786 ± 1.69
|
-6.214
|
-1,219
|
|
Sulfur
|
884
|
789.6 ± 68.9
|
-94.400
|
-18,514
|
|
Thallium
|
0.000012
|
0.015 ± 0.005
|
0.015
|
2.94
|
|
Tin
|
0.000000
|
0.095 ± 0.01
|
0.095
|
18.63
|
|
Titanium
|
0.00001
|
0.007 ± 0.001
|
0.007
|
1.37
|
|
Vanadium
|
0.001527
|
0.00002 ± 0.0000
|
-0.002
|
-0.30
|
|
Yttrium
|
0.000022
|
Not Detected
|
|
|
|
Zinc
|
0.000392
|
0.212 ± 0.021
|
0.212
|
41.50
|
Except for a few elements, the concentrations
of elements in the water of an average reef tank, as defined
by the Tank Water Study, bear little resemblance to the concentrations
of the same elements in NSW. In fact, the relationship between
reef aquarium water and NSW outside of a couple of the major
ions, such as sodium and calcium, is so tenuous that one could
legitimately say there is NO consistent relationship. Reef
aquarium water has roughly the same amount of salt and calcium
as in NSW, but the concentrations of all other ions differ
from the concentrations found in natural sea water. Not only
that, but they differ with inconsistent magnitudes and directions
(Table 1). Nonetheless, many of the ions are substantially
more abundant in reef aquarium water than in NSW, and often
these are the ions with the highest potential for toxic effects,
such as copper, iodine, nickel, and zinc.
The water found in an average tank, however,
is not gathered from some place and added to a tank “as
is.” Rather, it is the result of several continuous
and simultaneous processes that occur to some initial water
volume. The initial water can come from either natural sea
water or artificial sea water, or a mixture of the two. Once
the tank is filled with water, no matter where or how it originates,
the initial tank water is manipulated by both the organisms
in the reef aquarium system and the aquarist. Organisms actively
alter the water in several ways.
|
•
|
They secrete materials that actively bind
to and alter the chemical properties of some of the dissolved
constituents of tank water. |
|
•
|
Environmental conditions of some of the microhabitats
in the tank, particularly in a deep sand bed and within
live rock, may facilitate the adsorption and removal of
many chemical constituents. Organisms, mostly bacteria,
living in these areas are largely responsible for the
changes from “normal” tank conditions, and
as such are responsible for the changes observed. |
|
•
|
Some organisms may actively sequester and hold in their
bodies many essential biological materials such as phosphorus,
iron, and nitrogen (see, for example, Vasconcelos and
Leal, 2001). |
Additionally, the aquarist manipulates
the levels of these chemicals by adding foods, some chemical
additives, making water changes, and by exporting various
materials.
What organisms are doing to these chemical
balances in any given tank is open to supposition; no numerical
data about any organism’s secretion or accumulation
of materials are known from aquaria, and precious few of these
data are available from natural reef areas. That organisms
are manipulating their chemical environment is a given. However,
reef aquarium concentrations of many of the dissolved trace
elements are so different from actual marine concentrations
that it is impossible to even reasonably speculate about what
is happening in a tank. Simply put, there are too few data
available to generalize.
The manipulations that aquarists make to
the chemicals found in the tanks may be estimated, given a
couple of baselines, and I have attempted to do just that
in previous articles in this series (See Shimek: 2002a, 2002b,
and especially 2002c), but prior to this article few data
have been available on the various means of exportation of
materials. The results of the analyses of these materials
are shown in Table 2.
|
Table
2. Average
values of Export Products. All
values are in parts per million (≈
mg/kg). Blank cells indicate that the data are
not available; generally because the material was not
detected in the sample. The variance measures
in the average tank data are the sample standard deviations.
Values without variance measures indicate only one datum
for the element in the particular export product or
element. N = Number of samples. Colored
values indicate either the highest single concentrations,
or the highest concentrations within the same variance
range.
|
|
Element
|
Export Product
|
|
Skimmate; N=4 Average
±
SSD
|
Sludge N=1
|
Caulerpa sp.;
N=3 Average ±
SSD
|
Xenia sp.; N=2
Average ±
SSD
|
Sarcophyton sp.
N=1
|
|
Aluminum
|
45.43±76.48
|
560
|
38.33±24.91
|
53.50±4.95
|
|
|
Arsenic
|
0.470
|
|
|
|
|
|
Barium
|
0.370±0.621
|
1.900
|
0.177±0.060
|
0.170±0.099
|
0.42
|
|
Boron
|
4.030±3.932
|
17.00
|
6.000±1.838
|
5.400±2.404
|
6.600
|
|
Cadmium
|
0.128±0.103
|
0.890
|
0.200±0.135
|
0.325±0.021
|
|
|
Calcium
|
2207
±
3734
|
37000
|
1743±1136
|
3350±1344
|
2700
|
|
Chromium
|
0.190±0.255
|
0.880
|
|
|
0.090
|
|
Cobalt
|
0.133±0.094
|
1.200
|
0.400±0.440
|
0.190±0.057
|
0.570
|
|
Copper
|
1.385±1.243
|
3.700
|
0.587±0.201
|
0.515±0.346
|
0.660
|
|
Iodine
|
18.80±28.86
|
130
|
48.67±23.29
|
135±191
|
95
|
|
Iron
|
25.09±41.49
|
180
|
4.567±1.650
|
3.950±5.586
|
2.700
|
|
Lead
|
0.477±0.168
|
|
0.463±0.194
|
0.415±0.035
|
|
|
Lithium
|
0.340±0.286
|
|
1.403±1.195
|
0.555±0.403
|
2.400
|
|
Magnesium
|
847±537
|
2000
|
797±491
|
1600±566
|
1800
|
|
Manganese
|
3.315±6.457
|
23.00
|
1.663±1.440
|
0.340±0.170
|
|
|
Molybdenum
|
1.295±1.394
|
11.000
|
1.380±0.430
|
2.300±0.990
|
1.900
|
|
Nickel
|
0.890
|
8.200
|
0.400
|
0.575±0.120
|
5.800
|
|
Phosphorus
|
37.23±61.91
|
250
|
80.33±34.06
|
370±297
|
160
|
|
Potassium
|
328±219
|
480
|
2550±3161
|
1075±177
|
1100
|
|
Silicon
|
24.75±43.53
|
3.800
|
12.73±9.92
|
8.500±12.021
|
2.200
|
|
Sodium
|
10252±7037
|
6800
|
11667±12013
|
5520±7750
|
16000
|
|
Strontium
|
35.10±50.36
|
310
|
24.33±17.10
|
28.50±7.778
|
55.00
|
|
Sulfur
|
662±416
|
880
|
800±334
|
1600±566
|
1000
|
|
Thallium
|
0.550
|
|
|
|
|
|
Tin
|
0.178±0.123
|
|
0.723±0.372
|
0.460±0.297
|
|
|
Titanium
|
0.600
|
|
0.273±0.140
|
|
|
|
Vanadium
|
0.360
|
|
0.445±0.191
|
|
0.390
|
|
Yttrium
|
0.050
|
0.320
|
|
|
|
|
Zinc
|
2.005±2.875
|
9.900
|
1.420±1.225
|
42.10±47.94
|
1.900
|
Just about everything in a tank appears
to be exported by skimming, albeit in often very low concentrations.
Skimmer sludge appears to remove much more material on a per
unit weight basis, although both Caulerpa and Xenia
also concentrate some of the exportable materials in their
tissues. Comparison with Table 1 will show that some items
are significantly concentrated when compared to tank waters.
Some aquarists have been concerned that
some of their exporting methods will alter the salinity of
the tank by preferentially removing salt. This does not appear
to be the case. The sodium concentration in skimmate and Caulerpa
is effectively the same as in the tank water, while in skimmer
sludge and Xenia, it appears substantially lower
than in tank water. Since sodium and salt concentrations are
correlated, it is unlikely that one can export excess salt
by any normal export method. However, as some tank water with
salt in it leaves the system with each export mechanism, those
that contain more water (such as skimmate) do take out some
salt and necessitate the regular replenishment of the salt
and salinity.
Possibly of more concern is the export
of calcium by all of the exports. Significant amounts of calcium
are is found in all of the exports; skimmate contains about
2200 ppm calcium, sludge about 37000 ppm, Caulerpa
about 1743 ppm and Xenia about 3350 ppm. Put another
way, the analyzed sample of skimmer sludge was 3.7 percent,
by weight calcium. Harvesting a pound of skimmer sludge removes
about 17 grams, over half an ounce, of calcium from the system.
If all of this calcium was present as calcium carbonate, and
it is doubtful that it would be, then removal of a pound of
skimmer sludge would remove about 42 grams, or about an ounce
and a third of calcium carbonate. The average tank in the
Tank Water Study had a volume of 191. 3 liters and contained
a calcium concentration of about 400 mg/kg. Assuming a specific
gravity of 1.025, this means that the average tank contained
((191.3) x (1.025)) or 196.0825 kg of artificial sea water.
This water, in turn, contained about (196.08 x 0.4) = 78.4.grams
of calcium. In other words, removal of one pound of skimmer
sludge, would remove 17/78.4 or about 21.7 percent of the
calcium in the sea water of the tank. Skimming enough to build
up skimmer sludge can be a major way to remove calcium from
a tank. In tanks with continual supplementation of calcium,
such losses are like not to be noticeable, but in tanks with
periodic supplementation, provided they also are accumulating
skimmer sludge, the sludge may noticeably be removing calcium.
The bacteria in the sludge, or inorganic precipitates, perhaps
of calcium phosphate, trapped in the sludge may be responsible
for much of the drop seen in calcium concentrations. .
All aquarists should be concerned about
the removal of excess toxic materials, particularly heavy
metals and toxins produced by organisms from their systems.
The export mechanisms can only remove materials found in water,
either as suspended, colloidal or dissolved materials, removed
by foam fractionation, or by the same kinds of materials that
get removed by organism absorption or ingestion. Materials
that have been deposited by precipitation into the system’s
sediment or by adsorption on to surfaces, or by incorporation
into non-exported organisms are not removed from the system.
Even though it appears that many of the toxic heavy metals
such as copper, zinc, and nickel would be exported rather
rapidly, this may not actually be the case. Again, examining
the data from an “average” reef tank (Shimek,
2002c), and using those as a basis for calculation, and using
the data in Table 2 as an estimate of export rate, the amount
of each export material needed to bring the elemental concentrations
in an average tank down to that of natural sea water may be
calculated (Table 3). I have converted the amount of exports
to pounds, as most of the readers in the United States will
be more familiar with that measure. Remember that when we
consider liquid measurements, the old saying that, “A
pint is a pound, the world around,” and you can convert,
for example skimmate volume roughly into pounds.
|
Table
3. Concentrations
of elements in the various exports, and the amount of
each export (in pounds) necessary to bring the concentrations
from those found in an “average” reef aquarium to the
concentration found in natural sea water (NSW). The
“average” tank data (volume = 191.3 liters with a specific
gravity of 1.025) from Shimek, 2002c. Excess Mass
= The number of milligrams of the element found in the
average reef tank above what would be found in a tank
of the same size filled with NSW. Lb = Number of
pounds of the export necessary to remove the excess. Values
with no variance measure indicate the element was detected
in only one sample. ND = Not Detected. Values
of 0.00 lb indicate less than one hundredth of a pound
would be necessary to remove the excess.
|
|
Element
|
Excess
Mass in (mg)
|
Skimmate;
N=4;
Average ± SSD
|
Lb.
|
Sludge,
N=1
|
Lb.
|
Caulerpa,
N=3
Average
± SSD
|
Lb.
|
Xenia,
N=2
Average ±
SSD
|
Lb.
|
|
Aluminum
|
33.88
|
45.43±76.48
|
1.64
|
560.00
|
0.13
|
38.33±24.91
|
1.94
|
53.50±4.95
|
1.39
|
|
Antimony
|
3.50
|
ND
|
|
ND
|
|
ND
|
|
ND
|
|
|
Arsenic
|
3.58
|
0.47
|
16.78
|
ND
|
|
ND
|
|
ND
|
|
|
Barium
|
0.25
|
0.37±0.62
|
1.47
|
1.90
|
0.29
|
0.18±0.06
|
3.08
|
0.17±0.10
|
3.20
|
|
Beryllium
|
ND
|
ND
|
|
ND
|
|
ND
|
|
ND
|
|
|
Boron
|
-130.4
|
Average Tank Concentration
Below That Of Natural Sea Water
|
|
Cadmium
|
ND
|
0.13±0.10
|
|
0.89
|
0.00
|
0.20 ±
0.13
|
0.00
|
0.33±0.02
|
0.00
|
|
Calcium
|
78.45
|
2,206±3,733
|
0.08
|
37,000
|
0.00
|
1,743±1,135
|
0.10
|
3,350±1,344
|
0.05
|
|
Chromium
|
ND
|
0.19±0.25
|
0.00
|
0.88
|
0.00
|
ND
|
|
ND
|
|
|
Cobalt
|
0.04
|
0.13±0.09
|
0.65
|
1.20
|
0.07
|
0.40±0.44
|
0.21
|
0.19±0.06
|
0.45
|
|
Copper
|
4.66
|
1.39±0.24
|
7.40
|
3.70
|
2.77
|
0.59±0.20
|
17.46
|
0.52±0.35
|
19.89
|
|
Iodine
|
77.71
|
18.80±28.86
|
9.09
|
130.00
|
1.32
|
48.67±23.29
|
3.51
|
135±191
|
1.27
|
|
Iron
|
ND
|
25.09±41.49
|
0.00
|
180.00
|
0.00
|
4.57±1.65
|
0.00
|
3.95±5.59
|
0.00
|
|
Lead
|
ND
|
0.48±0.17
|
0.00
|
ND
|
|
0.46±0.19
|
0.00
|
0.42±0.04
|
0.00
|
|
Lithium
|
96.79
|
0.34±0.29
|
626
|
ND
|
|
1.40±1.20
|
152
|
0.56±0.40
|
384
|
|
Magnesium
|
10,591
|
847±537
|
27.52
|
2,000
|
11.65
|
797±
491
|
29.25
|
1,600±566
|
14.56
|
|
Manganese
|
ND
|
3.32±6.46
|
0.00
|
23.00
|
0.00
|
1.66±1.44
|
0.00
|
0.34 ±
0.17
|
0.00
|
|
Mercury
|
ND
|
ND
|
|
ND
|
|
ND
|
|
ND
|
|
|
Molybdenum
|
1.26
|
1.30±1.39
|
2.14
|
11.00
|
0.25
|
1.38 ±
0.43
|
2.00
|
2.30 ±
0.99
|
1.20
|
|
Nickel
|
4.61
|
0.89
|
11.41
|
8.20
|
1.24
|
0.40
|
25.38
|
0.58 ±
0.12
|
17.66
|
|
Phosphorus
|
50.35
|
37.23±61.91
|
2.98
|
250.
|
0.44
|
80.33±34.06
|
1.38
|
370.±
297
|
0.30
|
|
Potassium
|
4,942
|
328±219
|
33.20
|
480
|
22.65
|
2,550±3,161
|
4.26
|
1,075 ±
177
|
10.11
|
|
Silicon
|
-302
|
24.75±43.53
|
-26.8
|
3.80
|
-175
|
12.73 ±
9.92
|
-52.1
|
8.50 ±
12.02
|
-78.1
|
|
Silver
|
ND
|
ND
|
|
ND
|
|
ND
|
|
ND
|
|
|
Sodium
|
56,680
|
10,253±7,037
|
12.16
|
6,800
|
18.34
|
11,667±12,013
|
10.69
|
5,520±7,750
|
22.59
|
|
Strontium
|
-1,219
|
Average Tank Concentration
Below That Of Natural Sea Water
|
|
Sulfur
|
-18,514
|
Average Tank Concentration
Below That Of Natural Sea Water
|
|
Thallium
|
2.94
|
0.55
|
11.76
|
ND
|
|
ND
|
|
ND
|
|
|
Tin
|
18.63
|
0.18±0.12
|
231
|
ND
|
|
0.72±0.37
|
56.67
|
0.46±0.30
|
89.11
|
|
Titanium
|
1.37
|
0.60
|
5.03
|
ND
|
|
0.27±0.19
|
11.03
|
ND
|
|
|
Vanadium
|
-0.30
|
0.36
|
-1.81
|
ND
|
|
0.45±0.19
|
-1.46
|
ND
|
|
|
Yttrium
|
ND
|
0.05
|
0.00
|
0.32
|
|
ND
|
|
ND
|
|
|
Zinc
|
41.50
|
2.01±2.87
|
45.54
|
9.90
|
9.22
|
1.42±1.23
|
64.30
|
42.10±47.94
|
2.17
|
When one considers the results in Table
3, one must also consider the time necessary to skim out enough
foam to make a pound of skimmate, or to grow a pound of skimmer
sludge on the inside of the skimmer column, or to grow a pound
of Caulerpa or Xenia. As an example for
copper, which has a total excess mass in the average tank
of 4.66 mg, it would take 7.4 pounds of skimmate or 2.77 pounds
of skimmer sludge or 17.46 pounds of Caulerpa or 19.89 pounds
of Xenia to bring the copper concentration back into line
with that in natural sea water. So, how long would it take
to accumulate a gallon of skimmate, or twenty pounds of Xenia?
The calculations in Table 3 represent a tank that is not being
fed or having the chemical concentrations altered in any way.
Such an assumption is unrealistic, of course, but it does
allow an estimation of some export rates. It likely overestimates
skimmer efficiency, however, because as the concentration
of the various materials drop, they will likely be removed
at a lower rate. Additionally, reef aquaria need to be fed,
and feeding adds these elements to a system.
The average daily feeding ration for some
reef tanks was calculated from the data given in the “Tank
Water Study.” That ration may be used along with some
estimated export rates to assess the efficiency of the export
methodology (Table 4). The water was assumed to be “average
artificial sea water” using the average of the artificial
sea water mixes measured by Atkinson and Bingman (1999), and
the “average tank water” from the “Tank
Water Study (Shimek, 2002b, c).” I used the following
values for each export:
|
•
|
One pound (= one pint) of liquid skimmate
(condensed skimmer foam) produced per day, |
|
•
|
One half pound (= one standard measuring cup) of skimmer
sludge produced each week, |
|
•
|
One pound of Caulerpa grown each week, and |
|
•
|
One-quarter pound of Xenia grown each week. |
I think they are reasonable estimates.
However if you disagree, substitute some other values and
recalculate; this table is the result of simple algebra.
|
Table
4. Tank trace element budget, assuming a tank of
net volume Of 297.8 liters with a specific gravity of
1.025. The tank data and average daily ration
are taken from Shimek, 2002c. All of the export
methods are working simultaneously.
|
|
A.
Exports
|
Amount Assumed Exported
in Pounds Per Week
|
|
Skimmate
|
7 lb/wk
|
|
Sludge
|
0.5 lb/wk
|
|
Caulerpa
|
1lb/wk
|
|
Xenia
|
0.25 lb/wk
|
|
B.
Results of calculations assuming water made from the
average of the salt water mixes tested by Atkinson and
Bingman (1999). Net Daily Reduction = Daily Ration
– All Exports. Difference = The difference between
the “Artificial Salt Mix” Tank and Natural Sea Water.
|
|
Element
|
Average
Tank Total
(mg)
|
Net
Daily Reduction
(mg)
|
After
1 Day Difference
|
Number
of days of all exports necessary to match NSW and the
number of pounds of export needed to bring the tank
to NSW levels (rounded to the nearest pound).
|
|
|
Pounds
of Export Necessary
|
|
Days
|
Skimmate
|
Sludge
|
Caulerpa
|
Xenia
|
Total
|
|
Aluminum
|
77838
|
316
|
77522
|
245
|
245
|
17
|
34
|
10
|
306
|
|
Barium
|
143
|
1.19
|
141
|
116
|
116
|
8
|
16
|
5
|
145
|
|
Cadmium
|
84.7
|
0.70
|
84.0
|
120
|
120
|
8
|
17
|
5
|
150
|
|
Chromium
|
2549
|
0.48
|
2548
|
5269
|
5269
|
369
|
738
|
211
|
6586
|
|
Cobalt
|
466
|
0.87
|
465
|
533
|
533
|
37
|
75
|
21
|
666
|
|
Copper
|
733
|
2.78
|
730
|
263
|
263
|
18
|
37
|
11
|
328
|
|
Iron
|
367
|
96.6
|
271
|
3
|
3
|
0
|
0
|
0
|
4
|
|
Lead
|
797
|
0.61
|
797
|
1301
|
1301
|
91
|
182
|
52
|
1626
|
|
Lithium
|
83829
|
1.04
|
83774
|
80379
|
80379
|
5626
|
11253
|
3215
|
100473
|
|
Manganese
|
370
|
12.79
|
358
|
28
|
28
|
2
|
4
|
1
|
35
|
|
Molybdenum
|
797
|
7.25
|
790
|
109
|
109
|
8
|
15
|
4
|
136
|
|
Nickel
|
591
|
4.57
|
587
|
128
|
128
|
9
|
18
|
5
|
161
|
|
Silver
|
1080
|
-0.002
|
1080
|
Silver is gradually
increasing
|
|
Titanium
|
248.
|
0.39
|
248
|
630
|
630
|
44
|
88
|
25
|
787
|
|
Vanadium
|
1007
|
0.36
|
1007
|
2764
|
2764
|
193
|
387
|
111
|
3455
|
|
Zinc
|
172
|
24.96
|
147
|
6
|
6
|
0
|
1
|
0
|
7
|
|
C.
Results of calculations assuming average tank water
as determined from hobbyist tanks (in Shimek, 2002c).
All other calculations as above. Empty cells indicate
elements not detected in the average tank water.
|
|
Element
|
Average
Tank Total
|
Net
Daily Reduction (mg)
|
After
1 Day Difference
|
Number
of days of all exports necessary to match NSW
and the number of pounds of export needed to bring
the tank to NSW levels
|
|
|
Pounds
of Export Necessary
|
|
Days
|
Skimmate
|
Sludge
|
Caulerpa
|
Xenia
|
Total
|
|
Aluminum
|
52.8
|
316
|
-264
|
0.17
|
0.17
|
0.01
|
0.02
|
0.01
|
0.21
|
|
Barium
|
4.6
|
1.2
|
3.4
|
3.87
|
3.87
|
0.27
|
0.54
|
0.15
|
4.83
|
|
Cadmium
|
|
|
|
|
|
|
|
|
|
|
Chromium
|
|
|
|
|
|
|
|
|
|
|
Cobalt
|
0.06
|
0.87
|
-0.81
|
0.07
|
0.07
|
0.00
|
0.01
|
0.00
|
0.09
|
|
Copper
|
7.33
|
2.78
|
4.55
|
2
|
1.61
|
0.11
|
0.23
|
0.06
|
2.01
|
|
Iron
|
|
|
|
|
|
|
|
|
|
|
Lead
|
|
|
|
|
|
|
|
|
|
|
Lithium
|
203.3
|
1.0
|
202.3
|
144
|
143.53
|
10.05
|
20.09
|
5.74
|
179.43
|
|
Manganese
|
|
|
|
|
|
|
|
|
|
|
Molybdenum
|
4.9
|
7.2
|
-2.3
|
0.67
|
0.67
|
0.05
|
0.09
|
0.03
|
0.84
|
|
Nickel
|
7.3
|
4.6
|
2.8
|
1
|
0.57
|
0.04
|
0.08
|
0.02
|
0.72
|
|
Silver
|
|
|
|
|
|
|
|
|
|
|
Titanium
|
2.1
|
0.4
|
1.7
|
4
|
4.43
|
0.31
|
0.62
|
0.18
|
5.53
|
|
Vanadium
|
0.01
|
0.36
|
-0.36
|
0.02
|
0.02
|
0.00
|
0.00
|
0.0
|
0.02
|
|
Zinc
|
64.7
|
25.0
|
39.8
|
2
|
1.59
|
0.11
|
0.22
|
0.06
|
1.99
|
In considering the values in Table 4, remember
all of the export methods are acting over the same time, so
in Table 4B to export sufficient Vanadium to reduce the “average
tank water to levels found in natural seawater, the tank would
have to be skimmed for 2764 days, removing 2764 pounds of
skimmate, 193 pounds of sludge, 387 pounds of Caulerpa, and
111 pounds of Xenia for a total of 3455 pounds…all from
a tank of less than 100 gallons. Obviously, while the data
may indicate the relative effectiveness of the filtration
methods for the various elements, those data do not reflect
reality in those situations were only relatively small amounts
of materials may be removed by the export method. Even with
this caveat, however, the data from the “average artificial
sea water” and the “average tank water”
are wildly different. These differences likely reflect the
processing of the water constituents by the organisms in the
tank. As in actual marine situations, the concentration of
the aqueous medium surrounding the organisms is maintained
by the organisms with minimal effect by purely physical or
inorganic processes. The organisms that modify the water the
most are the bacteria and cyanobacteria (and some other microalgae)
living in the tank. They may secrete materials that bind with
toxic metals and make them insoluble, or by the action of
their metabolism, they may lower the oxygen tension within
sediments or porous rock resulting in the precipitation of
some of the toxic trace metals as sulfide minerals or insoluble
iron hydroxides (Booij, et al, 2001; Pichler, et al 1999,
Pichler, et al., 2000). The sulfur necessary for such for
minerals would be the result of anaerobic metabolism of bacteria.
Instead of fearing the small amount of hydrogen sulfide gas
dissolved in a deep sand bed, aquarists should rather be thankful
it is there as an indicator that copper, zinc, and other toxic
materials are being sequestered in the sediments by the anoxic
conditions in a deep sand bed. Additionally, sediment bacteria
can also be incorporating significant amounts into their own
biomass. In fact, this statement is also operative with regard
to the free-living bacteria in the water and in water borne
particulates, as wells as the mucus, surfaces, and associated
particulates of the soft corals and algae. All of these materials
may be incorporate toxic materials.
Sediment precipitation notwithstanding,
the export mechanisms available in the average system seem
capable of removing some of the excess materials relatively
efficiently (Table 4C) – as long as the remaining excess
is detoxified by sediment dwelling bacteria and algae.
These calculations indicate a couple of
other potential problems. Freshly mixed artificial sea water
is heavily laden with a tremendous excess of potentially toxic
heavy metals. Just how toxic this material is remains open
to question, however anecdotal and other evidence from invertebrate
embryologists indicates it is significantly toxic (See Strathmann,
1987). If freshly mixed artificial sea water contains some
toxic concentrations of some trace metals, these metals will
not be detoxified immediately in a tank, and until they are,
they will be adding to the cumulative toxic chemical load
found in the tank animals. This could occur with each water
change.
Additionally, tank sediment beds and the
porosity of rocks represent a limited volume for detoxified
materials. In essence, the sediment beds and rock porosity
has a finite capacity for detoxification. Sooner or later,
these volumes will become saturated and toxic heavy metals
may begin to accumulate in tank waters, or in portions of
the sediment which are at the aerobic/anaerobic boundary.
Such boundaries are found in deep sand beds and inside of
live rock, and their positions fluctuate in nature and in
our tanks, primarily with the input of organic matter (food)
to the system. If many toxic metals such as copper have been
deposited in these marginal areas and the feeding regime in
the tank is altered so that that the depth of the boundary
changes, significant amounts of toxic materials may be released.
In natural situations, heavy metal pollution
typically results in the deposition of metals in the sulfide-rich
anoxic layers deep in the sediments. It is likely that a similar
deposition pattern occurs in deep sand beds. Consequently,
the anoxic areas of the deep sand bed would be the place where
tank waters would be detoxified. Water is slowly moved through
these areas by the cumulative motions of the animals in the
upper layers of the sand beds. This slow percolation of water
results in these areas accumulating organic materials. The
bacterial utilization of this organic material results in
the elimination of oxygen in the deeper sediments. This, in
turn, facilitates the removal of metals from solution. Durable, colorful,
and modern kitchenware from Tupperware Catalogue.However,
the accumulation of organic material in these sediments also
results in these anoxic sediment layers getting thicker with
time. If this occurs, the level where free oxygen occurs becomes
shallower. Most animals cannot tolerate anoxic conditions,
and will not penetrate those layers. This chain of events
leads to a positive feedback loop, working over extended periods.
As organic material builds up in tanks, the anoxic layers
become deeper forcing the animals into shallower sediment
layers. As this occurs, the animals can pump less water through
the deep layers. This reduction in pumping facilitates the
increase in thickness of these anoxic layers seen in highly
polluted areas – or in old reef tanks. In severely polluted
situations in natural ecosystems, the anaerobic sediments
may actually start at the sediment-water interface (Rosenberg,
1976). In aquaria, a situation such as this is very unlikely;
the system will likely crash before it occurs.
It is tempting to suggest that the solution
to this quandary would be simply to vacuum the sediments of
all organic materials. However, such vacuuming would destroy
the functionality of the sand bed as far as its beneficial
aspects of excess nutrient processing and feeding the reef
are concerned. However, as an alternative to breaking the
tank and sand bed completely down, a thorough vacuuming of
the sediment followed by re-inoculation of the sediment fauna
must be considered as a viable alternative. It is important
to note, however, that such vacuuming might not remove much
in the way of precipitated metals. Rather, it would be a way
to keep the organic loads from becoming extreme. It may also
drastically affect pH, redox, oxygen levels and release lots
of rather unpleasant anaerobes into the water column.
Alternatively, maintenance of a highly
diverse and densely populated sand bed will help utilize much
or all of this excess organic material and thus prevent the
accumulation of excess organic material. Using this method
of sand bed maintenance requires careful monitoring of the
sand bed, and period replenishment of the faunal diversity.
Likewise, changes in the overall tank pH
cycle may result in brief transient periods of acidic sediment
conditions. During these periods, many of the bound heavy
metals may become soluble. This would result in transient
exposures to these metals. As heavy metal poisoning is commonly
cumulative, the final lethal effects of such brief periods
of toxicity may be seen only after several months or years.
Organic Exports:
Of course, export of metals is only part
of the story. Aquarists need to worry about the accumulation
of excess organic nutrients. In this context, I am considering
carbon and nitrogen containing compounds as “organic.”
Generally, aquarists focus on the visible aspects of the accumulation
of these materials, such as the excessive growth of algae
and cyanobacteria or large accumulations of detritus. The
invisible aspects of organic accumulation in sediments, as
discussed above, however, are probably far more serious.
Unfortunately, the determination of organic
exports is considerably more difficult than is the determination
of metal exports. This is due to the multiplicity of organic
export pathways. Carbon and nitrogen containing compounds
enter into metabolic pathways and may be broken down to give
byproducts that are gaseous. Such gaseous metabolites can
leave the tank through any air-water interface and, as a result,
are hard to measure or even estimate.
In this study, there was another complication.
I gave the trace metal analyses higher priority than the analyses
of these organic materials. Consequently, if the samples were
too small for the complete analytical array to be done, the
tests that were not done related to the organic factors. This
resulted in several incomplete analyses (Table 5).
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Table
5. Nitrogenous
and Organic Exports.
Data for nitrogenous materials in mg/kg (=ppm).
Empty cells indicate insufficient sample for the test.
The caloric value was measured for only one Xenia
sample.
|
|
|
|
Average±SSD
|
|
Material
|
Sludge
|
Skimmate
|
Caulerpa
|
Xenia
|
|
Nitrate
+ Nitrite Nitrogen
|
2
|
2.40±2.27
|
107.00±46.68
|
0.61±0.49
|
|
Ammonia
Nitrogen
|
530
|
78.33±105.57
|
113.67±45.61
|
585.00±583.47
|
|
Total
Kjeldahl Nitrogen
|
4500
|
421.00±652.99
|
1016.67±317.54
|
8600.00±5392.59
|
|
Calories
(Cal/gm)
|
|
|
1460.00±729.93
|
2500
|
|
Fat
%
|
|
0.17±0.09
|
0.16±0.11
|
0.55
|
|
Moisture
%
|
79
|
93.67±5.69
|
94.00±1.00
|
86.00±8.49
|
|
Total
Protein %
|
2.6
|
|
0.60±0.17
|
4.95±3.61
|
Aquarists often worry about the removal
of nitrogen products from their systems, but it is clear that
several of the export methods are fairly efficient at removing
nitrogen compounds. On a per weight basis both skimmer sludge
and Xenia export significant amounts of nitrogen, probably
as protein in bacteria and tissue respectively. Comparisons
utilizing carbohydrates are incomplete, but tend to indicate
that Xenia was again the most efficient export of these
materials.
Export efficiency may be measured in a
couple of ways, however, and although on a per weight basis
Xenia appears to be the best export mechanism, Caulerpa
grows far faster in most tanks and it would accumulate a lot
more of the needed export per unit time.
Conclusion:
Prior to doing this study, I was quite
convinced that none of the export methodologies that were
available to aquarists were very good. I was surprised to
find that that is definitely not the case with regard to many
of the elements needing export. Foam fractionation, coupled
with organism export, decidedly provides ways to remove many
elements and to keep them from accumulating, given a normal
feeding regime. The problem comes with the initial levels
of heavy metals concentration found in artificial salt mixes.
Unless these excessive amounts of metals can be exported,
they will accumulate and, with the passage of time and associated
water changes, will become more potentially troublesome. Heavy
metal accumulation is organism mediated with both active and
passive processes facilitating it. The accumulation products
will likely be located in the sediments and inside the porous
aquarium rock. If there is also the accumulation of significant
organic material in the sediments over time, this may result
in periodic transient or chronic low level releases of toxic
heavy metals. Heavy metal poisoning in such situations would
typically be a cumulative process, resulting in mortality
after several months or years. Because of this, sediment cleansing
or replacement every few years coupled with the replacement
of porous rock substrates may be necessary to prevent heavy
metal poisoning of the aquarium’s inhabitants. Alternatives
to this drastic and traumatic treatment might include the
use of toxic metal sponges, polyfilters and carbon. All of
these treatments may all be more efficient and less potentially
hazardous than sand bed trauma, however the efficiency of
such processes is really unknown. In other words, more work
remains to be done before a satisfactory export methodology
is available to reef aquarists.
Acknowledgements:
I wish to acknowledge the participation
of Guy Comstock, John Link, Joe Peck, Gene Schwartz, Sandra
Shoup, James Waltman and William Wiley, who contributed both
funds and samples. Howard Pierce, contributed money to defray
the cost of the study additional funds were available from
the Tank Water Study, and thus I wish to thank to the donors
to that study as well. I thank Eric Borneman and Skip Attix
for their comments and thorough reviews.
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