Water Analyses
Salinity - is a measure of the total
concentration of all ions found in water. Seven ions make up the bulk of
a salinity reading. These are sodium, potassium, calcium, magnesium, chloride,
sulfate, and bicarbonate. Salinity is typically measured in parts per thousand
(ppt). Full-strength seawater is typically around 35 ppt. Estuarine water
is classified as having 3 ppt while freshwater has less than 0.3 ppt. Any
water having more than 1 ppt salinity can be expected to have a salty taste.
Penaeid shrimp generally exhibit ideal growth at salinity values between
15-25 ppt, but it is a highly adaptable species and has been grown in commercial
settings at salinities ranging from 2-38 ppt. Tilapias show varying degrees
of salinity tolerance depending on species but all can tolerate salinities
lower than 5 ppt.
Potassium - concentrations in water
tend to be highly variable and tend to vary with salinity. The concentration
of potassium in seawater is normally around 380 ppm.
pH - values between 7.0 and 8.0 are
generally considered ideal for aquaculture.
Temperature - the ideal water temperature
for the culture of tropical fishes and shrimps is between 82F to 86F.
Total Hardness, Calcium & Magnesium
- total hardness is the concentration of calcium and magnesium in water
expressed in milligrams per liter (mg/l) of equivalent calcium carbonate
(CaCO3). Water having total hardness values of 0-75 mg/l is generally
classified as soft water, while values above 150 mg/l is categorized as
hard water. Full strength seawater has a total hardness of 6,600 mg/l.
Hardness is important to crustaceans because they have heavily mineralized
exoskeletons and it is often thought that low hardness levels may limit
their growth (Greenway, 1974). The lower limit for total hardness for
the proper development of crustacean's exoskeletons is believed to be
50 mg/l (Boyd, 1990).
Phosphorous - is perhaps the most
important nutrient influencing the natural productivity of aquatic systems.
Phosphorous is critical for proper development of phytoplankton and phytoplankton
is one of the basic building blocks of aquatic productivity. Estuarine
areas typically have much higher concentrations of phosphorous in the
water and this is why they are so productive. Open ocean environments
typically have very low concentrations of phosphorous and productivity
is generally low. Concentration of phosphorous in pure seawater is usually
around 0.07 mg/l.
Total Ammonia Nitrogen (TAN) -
TAN consists of two fractions, un-ionized ammonia (NH3) and ionized ammonia
(NH4+) and is the by-product of protein metabolism. TAN is excreted from
the gills of fish as they assimilate feed and is produced when bacteria
decompose organic waste solids within the culture system. The un-ionized
form of ammonia-nitrogen is extremely toxic to fish. The fraction of TAN
in the un-ionized form is dependent upon the pH and temperature of the
water. At a pH of 7.0, most of the TAN is in the ionized form, while at
a pH of 8.0 the majority is in the un-ionized form, While the lethal concentration
of ammonia-nitrogen for many species has been established, the sublethal
effects of ammonia-nitrogen have not been well defined. Reduction in growth
rates may be the most important sublethal effect. In general, the concentration
of unionized ammonia-nitrogen should not exceed 0.05 mg/1.
Nitrite-nitrogen (N02-) - is a product
of the oxidation of ammonia-nitrogen. Vitrifying bacteria (Nitosomonas)
in the production system utilize ammonia-nitrogen as an energy source
for growth and produce nitrite-nitrogen as a by-product. These bacteria
are the basis for biological filtration. The vitrifying bacteria grow
on the surface of the biofilter substrate and to some extent on all production
system components including pipes, valves, tank walls, etc. While nitrite-nitrogen
is not as toxic as ammonia-nitrogen, it is harmful to aquatic species
and must be removed from the system. Concentrations of nitrite-nitrogen
should not exceed 0.5 mg/l for long periods of time.
Nitrate-nitrogen (NO3) - fortunately,
Nitrobacter bacteria, which are also present in most biological
filters, utilize nitrite-nitrogen as an energy source and produce nitrate
as a by-product. Nitrates are not generally of great concern to the aquaculturist.
Studies have shown that aquatic species can tolerate extremely high levels
(greater than 100 mg/l) of nitrate-nitrogen in production systems. Nitrate-nitrogen
is either flushed from a system during system maintenance operations (such
as settled solids removal or filter backwashing) or denitrification occurs
within a treatment system component such as a settling tank. Denitrification
is mainly due to the metabolism of nitrate-nitrogen by anaerobic bacteria
producing nitrogen gas that is released to the atmosphere during aeration
processes.
Sulfate - the most common form of
sulfur in seawater is sulfate. Concentrations can vary with the nature
of the geological materials in the watershed and with hydrological conditions.
Pure seawater normally contains around 885 mg/l sulfate.
Heavy Metals - according to the recommendations
of the United States Environmental Protection Agency (EPA), safe levels
of cadmium, chromium, copper, lead, and zinc are 10, 100, 25, 100, and
100 micrograms/liter (ug/L) respectively. Most of these metals form a
dietary requirement at very low levels (less than 10 ug/L) while being
toxic at high concentrations.
Terrain & Topography
Relief & Elevation - relief is
defined as the difference between the elevation at a given location on
site and the elevation at sea level. Given the high risk of hurricanes
in this region, we viewed relief as one of the most critical site variables.
Tidal surges of 10-15 feet are not uncommon during a direct hit by a hurricane.
Risk is heightened by the fact that this a coastal operation having a
high level of exposure. Operations of this type must adhere to strict
site selection and design guidelines. Having sufficient elevation above
sea level is therefore critical. According to UNESCO, any coastal site
that has an elevation less than 10 feet above mean sea level is considered
to have high vulnerability to storm surge from hurricanes. Sites with
an elevation of 10-16 feet above mean sea level are said to have medium
vulnerability, while those above 16 feet have low vulnerability. Our goal
was to seek a site that had at least 10 feet of natural elevation above
mean sea level and build up the site to a final elevation of around 15
feet.
Land Surface Configuration - is geometrical
and includes both slope and surface shape. The gradient limits for a "nearly
level" slope classification are between 0-3%. Land conforming to the "nearly
level" slope classification is considered ideal for shrimp farming. In
other words, land that has a 1-foot change in elevation over 100 feet
is perfect for aquaculture.
Erosion & Drainage - erosion
is the detachment and movement of soil material. Erosion can be a natural
process but it can also be accelerated improper land development. Depending
on the local landscape and weather conditions, erosion may be very slow
or very rapid. We evaluate erosion risk for any given site based on landscape
features, meteorological data, soil classification, natural erosion history,
and contemplated project development. Soil profiles, buried soils, deposits
of wind-blown material, and other evidence that material has been moved
and redeposited are all helpful in understanding the natural erosion history
at a site. Water erosion results from the removal of soil material by
flowing water.
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