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Ed Street's recent question about potassium levels in streams and ponds,
and Charley Bay's question about fresh water reefs started me thinking
about natural water conditions and their comparison to aquarium
conditions.  Out of curiousity, I dug into the USGS data base available on
line from the National Water-Quality Assessment (NAWQA) Program.  After
studying some of the data, I built the table that follows.

The details may be a little arcane and technical for a lot of folks.  To
summarize, it looks to me like the popular approach to aquatic gardening
is to pollute the water with high nutrient levels, then offer only rather
low light levels to keep conditions under control.

NAWQA compiles data on the quality of water in a large number of
watersheds all over the US.  The data are gathered on a fixed frequency
and at special times, such as when a record was needed that traced a storm
flow event, or when information was needed for extreme high flow or low
flow conditions.  The samples were analysed by standard methods for a
large number of different analytes.  The sample points in the study were
described on the basis of the predominant land use in the watershed above
the sample point and categorized as Undeveloped, Agricultural, Urban
and Mixed Use.  Data were available for general analyses and nutrients for

From that mass of data I studied the fixed frequency water samples (not
storm hydrographs or low flow events) and decided to characterize water
from two different types of sites -- undeveloped watersheds and mixed-use

The data base provided 829 analyses from undeveloped watersheds.  
Undeveloped watersheds are generally small streams and swamps. These are
headwater streams, springs and swamps from all over the contiguous US.  I
treat these as representing unpolluted waters.  From my experience I would
characterize similar streams as being usually low in suspended sediments
and plankton, sometimes colored by organic materials, not usually capable
of supporting algae growth and supporting aquatic plant growth mostly
where the substrate is silty and (probably) fertile.

Streams and ponds in undeveloped watersheds probably contain conditions
that most closest approach what most of us would like in our aquariums:  
clear water, low algae growth, healthy growth of rooted plants.

The data base provided 2,920 analyses from mixed-use watersheds. Mixed-use
watersheds are generally larger rivers and the water quality at those
sites have been altered by urban and agricultural runoff.  Some of the
sites are highly polluted but the median water quality at these sites is
probably similar to the raw water supply used in many towns and urban
areas in the US.  From my experience, water at the mixed-use sites
probably is turbid with suspended sediments and plankton.  The rivers
support substantial algae growth, growth of floating plants and in shallow
water can support dense stands of aquatic plants.

I characterized each type of site using the median value for several
important water quality measures and nutrients.  Those median values are
tabled below.  I also added to the table some calculated values based on
the medians and added an estimate of incident light.  The right-most
column in the table contains some advice garnered mostly from this mailing
list for chemical quality and nutrient levels in planted aquariums.

                        Watershed land use      Aquarium
                      Undeveloped  Mixed Use    Advice

	General analysis
   units                     7.7          8         <7
   dissolved, mg/l            11         36
   dissolved, mg/l           2.4        8.4
   dissolved, mg/l             3         14
   dissolved, mg/l           0.9        2.9       5-20
   total, mg/l as CaCO3       43        106      50-75 (3-4 dGH)
   dissolved, mg/l           2.6         13
   dissolved, mg/l           4.7         27
   dissolved, mg/l as SiO2   9.4        9.6
   dissolved, mg/l         0.046      0.026       0.10
   dissolved, mg/l         0.008      0.010

	Phosphorus Species
   dissolved, mg/l as P    <0.01       0.04      <0.13
   total, mg/l  as P        0.01       0.08
   dissolved, mg/l as P    <0.01       0.04

	Nitrogen Species
   dissolved, mg/l as N     0.07       0.49       1. (4.4 mg/l as NO3)
   dissolved, mg/l as N     0.02       0.03       0.
   dissolved, mg/l as N    <0.20       0.30
   total, mg/l as N        <0.20       0.40

	Other Measures
Total Dissolved Solids
   residue at 180C, mg/l      89        202
   calc. mg/l as CaCO3      37.5      125.1      50-75 (3-4 dGH)
   calculated, mg/l          1.5        1.8         16
   molar ratio              19.9       32.9        >17

	Incident light
   uE/m2/s             1000-2000  1000-2000	  256 (3 watts/gal)

I believe that in most regards the medians on the table give a good
characterization of natural and mildly polluted conditions.  However, the
NAWQA data didn't included values for dissolved organic carbon (DOC) and
it is clear that the amount of DOC in the water creates a very important
division in the data set.  Black water swamps and streams from the
southeastern US (probably quite similar in chemistry to tropical black
water streams) are strongly affected by DOC.  These typically have very
fresh water with little or no alkalinity, pH frequently below 5 and iron
often in excess of 1000 mg/l.

Nutrient levels that we often see advised for planted aquariums are mostly
(excepting only alkalinity and hardness) high compared to natural streams,
and even high compared to the mixed-use streams.  I guess its no wonder
that algae is a major problem for many of us.  Incident light, on the
other hand is very low compared to natural light.

A few of the entries in the right column deserve some comments:

  The "optimum" CO2 level (and I'm sorry if this offends anyone) is
  laughably high compared to normal natural conditions.

  The incident PAR of 256 is based on 4 40-watt NO fluorescent (30
  uE/s each) tubes over a 55 gallon tank, in a hood and cover setup 
  that delivers 80% of the light to the water surface.  This is a 
  pretty bright tank.  Typically, plants seem to saturate with light 
  at about 500.  So while natural incident light is 2 to 4 times the
  saturation level, the estimated aquarium light is only about 1/2 the
  saturation level.  
  The maximum phosphorus level of 0.13 mg/l is about 0.4 mg/l as
  phosphate.  The phosphorus level is calculated to make phosphorus
  more growth-limiting than nitrogen at 4.4 mg/l as NO3.  It's based 
  on an N:P molar ratio of 17. If someone has other data about where 
  that ratio should be then this value could be recalculated.  I was a
  little surprised at how high this level was.

There are a lot of things that might be said about the comparison in the
table.  I hope others will add some points because it's certainly more
than I want to delve into right now.

Roger Miller