BLACKWATER RIVER

:''This article is about a classification of rivers. For rivers officially named ''Blackwater'', see Blackwater River.''
'Blackwater rivers' are rivers with waters colored like black tea to coffee. Most major blackwater rivers are in the Amazon River system and the Southern United States.

Amazon River tributary classified as blackwater

Manaus, the largest city on the Amazon River, as seen from a NASA satellite image, surrounded by the muddy Amazon River and the blackwater Negro River.

A blackwater river is one with a deep, slow-moving channel that flows through forested swamps and wetlands. The color results from the leaching of tannins from the decaying leaves of adjoining vegetation.

Comparison between white and black waters

Blackwater rivers of the world:

Amazonia

Orinoco basin

Southern United States

Northern United States

Comparison between white and black waters

Black and white waters differ significantly in their ionic composition, as shown in Table 1 below. Black waters have ionic concentrations not much greater than that of rainwater. They are, however, much more acidic and this results in black waters having an aluminium concentration greater than that of the more neutral white waters. The major difference is the concentrations of sodium, magnesium, calcium and potassium; these are very low in black waters. This has considerable ecological implications. Some animal groups, such as snails, need a lot of calcium with which to build their shells and so are not abundant in black waters. The lack of dissolved ions in black waters results in a low conductivity, similar to that of rainwater.
Black and white waters also differ in their planktonic fauna and flora. Tables 2 and 3 below compare the number of planktonic animals caught in black and white water localities only a few meters apart. In fact, the black water was not even as extreme an example as can be found in the Rio Negro system. However, it can be seen that the black water held far greater numbers of rotifers but fewer crustaceans and mites. These crustaceans are important foods for larval fish. The zones where the two waters mix are particularly attractive to ostracods and young fish. These mixing zones tend to have high numbers of animals. The high abundance of animals is shown clearly in Table 3 which compares the numbers of animals present in 10 litres of water in each habitat sampled.^[1]

Table 1: Mean ionic composition, specific conductivity (μS/cm), and pH in Amazon waters. Data from Ribeiro and Darwich (1993).
	Solimoes or Amazon River – whitewater.	Rio Negro – blackwater.
Na (mg/L)	2.3 ± 0.8	0.380 ± 0.124
K (mg/L)	0.9 ± 0.2	0.327 ± 0.107
Mg (mg/L)	1.1 ± 0.2	0.114 ± 0.035
Ca (mg/L)	7.2 ± 1.6	0.212 ± 0.066
Cl (mg/L)	3.1 ± 2.1	1.7 ± 0.7
Si (mg/L)	4.0 ± 0.9	2.0 ± 0.5
Sr (μg/L)	37.8 ± 8.8	3.6 ± 1.0
Ba (μg/L)	22.7 ± 5.9	8.1 ± 2.1
Al (μg/L)	44 ± 37	112 ± 29
Fe (μg/L)	109 ± 76	178 ± 58
Mn (μg/L)	5.9 ± 5.1	9.0 ± 2.4
Cu (μg/L)	2.4 ± 0.6	1.8 ± 0.5
Zn (μg/L)	3.2 ± 1.5	4.1 ± 1.8
Conductivity	57 ± 8	9 ± 2
pH	6.9 ± 0.4	5.1±0.6
Total P (μg/L)	105 ± 58	25 ± 17
Total C (mg/L)	13.5 ± 3.1	10.5 ± 1.3
HCO₃-C (mg/L)	6.7 ± 0.8	1.7 ± 0.5

Table 2: Different planktonic organisms collected in black (Japura) and white (Solimoes) waters. Data from Ribeiro and Darwich (1993).
Animal groups present	Black water	Mixed water	White water
Rotifera	284	23	0
Cladocera	5	29	43
Ostracoda	39	97	29
Calanoida	11	51	66
Cyclopoida	22	49	61
Chironomidae	0	3	3
Acari (mites)	0	0	2

Table 3: Number of planktonic organisms collected in 10L of black, white and mixed waters. Data from Ribeiro and Darwich (1993).
	'Black water'		'Mixed water'		'White water'
'Animal groups present'	'Open water'	'Forest'	'Open water'	'Forest'	'Open water'	'Forest'
Volvocaceae	42		38
Rotifera	87	5	34
Cladocera	6		5		8	1
Ostracoda	2	11	3		7
Calanoida	23	3	10
Cyclopoida	5	27	19	1	13	1
Mysidacea		1
Diptera					1
Acari (mites)			1		1
Larval fish			1		1