where N
n
= nitrification g N pond
− 1
d
− 1
. Values of N
n
were averaged to provide monthly estimates.
Net mineralization of feed N was estimated from the difference between feed N and in fish N at harvest, ON accumulation in bottom soil, and ON in pond water
at draining. The appropriate equation is M = FN − RN + ON
s
+ ON
w
, 6
where M = net feed N mineralization g pond
− 1
, FN = feed N g pond
− 1
, RN = N recovered in fish g pond
− 1
, ON
s
= ON accumulation in soil g pond
− 1
, ON
w
= ON in water at draining.
2
.
5
. Nitrogen budget Budgets were prepared for each pond by summing the gains and losses of N.
Inputs were fish stock, feed, intentional water additions, rainfall, and N
2
fixation. Losses resulted from fish harvest, outflow, accumulation in soil, seepage, NH
3
volatilization, and denitrification. Data for individual ponds were averaged to provide a single N budget.
3. Results and discussion
3
.
1
. Feeds and fish production An average of 3525 kg ha
− 1
of fish were harvested. Survival averaged 94, and the mean feed conversion ratio was 1.4. At stocking, small fish contained 21.6 9
1.4 dry matter and the dry matter was 9.17 N. Harvested fish contained 30.5 dry matter, and N concentration was 7.8 Table 1.
Table 1 Dry matter and total nitrogen concentrations 9 SE for channel catfish at stocking and at harvest.
Average weights, survival, feed application, and feed conversion ratio of live fish harvested from four, 400-m
2
ponds at the Auburn University Fisheries Research Unit, Auburn, Alabama USA also are provided
Variable Small fish at stocking
Fish at harvest Feed
Feed application kg pond
− 1
198 9 5 141 9 13
Fish harvest kg pond
− 1
1.4 9 0 Feed conversion ratio
94 9 2 Survival
94.3 9 0.3 30.5 9 0.5
Dry matter wet weight 21.6 9 1.4
7.80 9 0.14 9.17 9 0.76
4.85 9 0.1 Total nitrogen dry weight
Table 2 Average water budget of four, 400-m
2
channel catfish ponds on the Auburn University Fisheries Research Unit, Auburn, Alabama USA
a
August Variable
September June
October Total
July Inflow
5.1 Rainfall
9.6 16.9
37.8 20.1
6.2 85.0
85.0 45.3
Initial filling 27.8
21.9 64.8
Water addition 34.6
4.4 10.7
32.9 31.5
106.3 187.6
16.9 100.00
Total Outflow
14.1 17.5
2.1 13.7
56.2 8.8
30.9 Evaporation
2.5 4.9
12.6 4.9
0.8 25.7
14.2 Seepage
2.1 3.0
8.1 1.5
14.7 8.1
Overflow 85.0
85.0 46.8
Draining 18.3
16.7 34.8
23.9 87.9
181.6 100.0
Total
a
Values are in centimeters of water depth.
3
.
2
. Water budget Overflow and seepage averaged 14.7 cm and 25.7 cm, respectively Table 2.
Overflow was 15 and seepage was 27 of water losses. Evaporation was the greatest water loss from ponds. Terms in the water budget were similar in
magnitude to those reported by Boyd 1982 and Gross et al. 1998 for ponds on the FRU.
3
.
3
. Nitrogen gains Feed was 94.3 dry matter and the dry matter contained 4.85 N Table 1. An
average of 198 kg of feed containing 9.1 kg N average of 171 mg m
− 2
d
− 1
was applied to each pond. Feed accounted for 87.89 of N entering ponds.
Water used to initially fill ponds and rain accounted for 4.65 and 3.67 of N inputs, respectively. Pipe inflow and small fish at stocking accounted for 1.91 and
1.83, respectively, of the N input Table 3. Nitrogen fixation was negligible Table 3 and measurable only in June when N
input in feed was small : 15 g feed N pond
− 1
d
− 1
. In the laboratory Jiwyam, 1996 and in fertilized fishponds El Samara and Olah, 1979; Lin et al., 1988 N
2
fixation by symbiotic bacteria and blue-green algae is suppressed by increasing concentrations of TAN. Findlay et al. 1994 and Hendzel et al. 1994 showed that
a molar ratio of TN:TP above 13 completely inhibited N
2
fixation. They suggested that a low TN:TP ratio causes phytoplankton to become N deficient and gives
blue-green algae capable of fixing N
2
a competitive advantage over other phyto- plankton species. The TN:TP ratio ranged between 10 – 52 Gross et al., 1999b, but
in June, when N
2
fixation was detectable, the average ratio was 11.5 Table 4. Fish
8
A .
Gross et
al .
Aquacultural
Engineering
24 2000
1 –
14
Table 3 A nitrogen budget for channel catfish ponds on the Auburn University Fisheries Research Unit, Auburn, Alabama USA. This budget provides average
gains and losses for nitrogen g pond
− 1
during a growing season 30 May to 9 October 1997 in four, 400-m
2
ponds each stocked with 550 fish fed to satiation with a 28 crude protein diet
September October
Total June
Variable July
August N Gains
480 4.65
Initial water 480
189 189
1.83 Fish stock
9,069 87.89
716 3,716
2,812 Feed
436 1,389
169 378
3.67 62
51 96
Rain 14
197 1.91
32 85
66 Pipe inflow
5 0.05
5 Nitrogen fixation
10,318 Total
100.0 1,293
1,483 2,948
3,878 716
N Losses harvested at end of study
3,354 31.53
Fish at harvest 23
167 1.57
Overflow 17
23 104
1,482 13.93
Draining for harvest drained at end of study
2,400 22.57
Loss to pond bottom calculated for entire period
17 3
56 0.53
Seepage 5
4 27
1,331 12.51
a
536 229
460 Ammonia volatilization
b
106
a
120 1,846
17.36 608
434 684
Denitrification 1,260
7,239 10,636
100.0 Total
865 247
1,025
a
Missing data.
b
Data from Gross et al. 1999a.
ponds on the FRU usually have abundant populations of blue-green algae capable of fixing N Boyd, 1990, so a shortage of potential N
2
fixing organisms is not a likely explanation for the lack of N
2
fixation in this study.
3
.
4
. Nitrogen losses An average of 3354 g N pond
− 1
or about 31.53 of the N input from all sources was recovered in fish at harvest Table 3. These results agree with reports by
Avnimelech and Lacher 1979, Boyd 1985, Green and Boyd 1995 regarding the proportion of feed N recovered in fish at harvest. Nitrogen losses in effluents during
overflow and at draining accounted for 1.57 and 13.93 of N losses, respectively, and were equivalent to 41.2 kg N ha
− 1
Table 3. Unlike ponds on the FRU that are drained annually at harvest, commercial catfish ponds in the United States are
only drained at 6- to 8-year intervals Boyd and Tucker 1995. Nitrogen losses during pond draining would be much smaller per unit area in commercial ponds
than in ponds of this study.
Accumulation of TN in pond soil averaged 2400 g pond
− 1
46 mg m
− 2
d
− 1
and accounted for 22.57 of the N loss Table 3. Bottom soil N was mainly associated
with organic compounds because organic C increased in the soil between May and October and the increase in inorganic N was much less than the increase in TN
Table 5.
Table 4 Average concentrations of total nitrogen TN and total phosphorus TP and their ratio in channel
catfish ponds
a
TN m moles l
− 1
Date TP m moles l
− 1
TN:TP 12.3
June 0.123
0.010 23.9
July 0.191
0.008 25.6
0.009 August
0.230 September
35.0 0.280
0.008 0.310
0.006 October
51.7
a
Data are from Gross et al. 1999b. Table 5
Average concentrations 9 SE for different nitrogen N fractions and total carbon C on two dates in bottom soils of four channel catfish ponds at the Auburn University Fisheries Research Unit, Auburn,
Alabama USA
October 1998 May 1998
Variable mg kg soil
− 1
0.94 9 0.09 0.51 9 0.11
Nitrate N 0.13 9 0.01
Nitrite N 0.02 9 0.01
49.3 9 4.3 120.0 9 1.6
Total ammonia N 1500 9 100
1900 9 100 Total N
Total C 9000 9 800
14500 9 900 Organic N
1450 9 96 1780 9 98
Fig. 1. Rates of nitrate production by nitrification, nitrate loss through denitrification, and nitrate uptake by phytoplankton between 2 June and 3 October 1997 in four, 400-m
2
channel catfish ponds on the Auburn University Fisheries Research Unit, Auburn, Alabama. Vertical bars represent standard
errors. Table 6
Monthly averages 9 SE of morning 6.30–8.00 h and afternoon 17.30–19.00 h pH, and morning dissolved oxygen DO and temperature in four, 400-m
2
channel catfish ponds at the Auburn University Fisheries Research Unit, Auburn, Alabama USA
Afternoon pH DO mg l
− 1
Water temperature °C Morning pH
6.8 9 0.6 7.3 9 0.2
25 9 1.5 June
8.6 9 0.4 July
8.8 9 0.4 5.0 9 0.8
27.1 9 1.9 7.4 9 0.2
8.7 9 0.3 7.4 9 0.3
5.2 9 0.4 26 9 1.4
August 5.7 9 0.4
23.7 9 2.5 8.6 9 0.4
September 7.0 9 0.3
6.0 9 0.6 21 9 1.9
October 6.8 9 0.5
8.3 9 0.3 2.8–7.0
21–29 8.3–10.6
Ranges 6.7–7.5
Denitrification occurred throughout the study Fig. 1 and the estimated average was 38 mg N m
− 2
d
− 1
with individual values up to 80 mg N m
− 2
d
− 1
. The estimated N loss through denitrification averaged 1846 g pond
− 1
or 17.36 of the N loss Table 3. Denitrification rate varies with temperature, pH, abundance of
denitrifying bacteria, and concentrations of NO
3
N, organic C, and DO Harg- reaves, 1995. Dissolved oxygen concentrations in pond waters were greater than 3
mg l
− 1
, and it is unlikely that much denitrification occurred in the water column. Denitrification proceeds fastest at temperatures between 25 – 35°C Boyd and
Tucker, 1998. Water temperatures in ponds ranged between 22 – 30°C and were above 25°C for more than 80 of the growing season Table 6 suggesting that low
or high temperature in the surface layer of sediment was not a limiting factor. Organic C is abundant in catfish soils because of sedimentation of dead phyto-
plankton Munsiri et al., 1995. Optimal pH for denitrification is between 6 – 8 Boyd and Tucker, 1998, and the pH of anaerobic sediment in ponds of the FRU
is usually between 6 – 7 Masuda and Boyd, 1994b. The most likely factor influenc- ing denitrification rate was NO
3
N availability. Concentrations in the water column were often below 0.2 mg l
− 1
and seldom exceeded 5 mg l
− 1
Gross et al., 1999b. There was little direct input of NO
3
N to ponds in water sources, and NO
3
N for denitrification was probably derived mainly from NO
3
N produced in nitrification. Also, Hargreaves 1997 suggests that denitrification rate is closely linked to a
supply of NO
3
N from nitrification in surface layers of sediment. Average denitrifi- cation rate was highest in July and August when DO concentrations were lowest
and water temperatures were highest Table 6. Denitrification rates in this study Fig. 1 were within the range commonly encountered in eutrophic aquatic systems
Hargreaves, 1995.
The measured ammonia volatilization averaged 1331 g pond
− 1
25 mg m
− 2
d
− 1
and accounted for about 12.51 of N loss from ponds Table 3. This is equal to 33 kg Nha and is nearly as great as the amount of N lost in effluents Table 3.
Nitrogen in seepage accounted for about 0.53 of the N loss from ponds. Bottom soils of fishponds are generally compacted and contain about 10 – 30 clay
to reduce seepage. Clay particles are negatively charged and can adsorb NH
4 +
that is the most abundant form of inorganic N in pond water. Nitrate is negatively
charged and subject to leaching because it is not retained on clay particles. Nevertheless, the mean 9 SE for percentages of NO
3
N and TAN seeping through 20-cm long bottom soil cores in the laboratory study of seepage were 10.6 9 3.8 and
51.0 9 8.3. Apparently, NO
3
N leached less than TAN because it was denitrified by soil microorganisms.
Despite the rather crude techniques used for estimating N losses through seepage and denitrification, the N budget was fairly accurate because all terms were based
on actual measurements of inputs and outputs, and the N gains 10 318 g pond
− 1
were approximately equal to N losses 10 636 g pond
− 1
. A previous N budget for channel catfish ponds Boyd, 1985, it was assumed that N
2
fixation and N accumulation in soil were negligible. Losses of N via denitrification and NH
3
volatilization were estimated by subtracting the sum of all other measured N losses from the measured N inputs. This difference was 6920 g N for 400-m
2
ponds and represented 57.3 of the N loss. In the present study, 22.6 of N loss resulted from
accumulation of N in soil, and denitrification and NH
3
volatilization combined accounted for 29.9 of the loss. These three variables account for 52.5 of total N
loss which is roughly the same proportion calculated for denitrification plus NH
3
volatilization in the earlier study. However, the present study shows that accumula- tion of N in bottom soils, NH
3
volatilization and denitrification are all significant factors in the N dynamics of ponds.
3
.
5
. Ammonia mineralization and nitrification The mineralization of feed N to TAN by fish and microorganisms was estimated
as 3123 g pond
− 1
or 59 mg m
− 2
d
− 1
. Much N mineralization resulted from fish
metabolism and the remainder was effected by microbial activity in the water column and bottom soil. Assuming that all TAN mineralized from feed reached the
water, it would equal a daily TAN input of 0.069 mg l
− 1
. The mineralization rate for feed N was less than the actual rate of N mineralization in the ponds.
Phytoplankton is continually turning over, and ON in dead plankton is mineralized to TAN and recycled internally. Also, some of the ON in residual soil organic
matter present at the beginning of the study also was mineralized. Thus, nitrifica- tion and other processes removed much TAN from the water or the TAN
concentration would have been much greater than observed.
Nitrification was measurable in the water column throughout the study Fig. 1, and the estimated average was 70 mg N m
− 2
d
− 1
. Possibly there was additional nitrification in the surface layer of bottom soil Hargreaves, 1995; Word, 1996 that
was not included. The mineralization rate of TAN from feed was 58.7 mg N m
− 2
d
− 1
that is less TAN than needed for the observed rate of nitrification. Additional TAN was available from N recycling within ponds, and concentrations of TAN in
pond waters ranged between 0.2 and 2.5 mg l
− 1
during the study Gross et al., 1999b.
Denitrification was estimated to average 38 mg N m
− 2
d
− 1
, and phytoplankton removed NO
3
N from pond water at a rate of 24 mg N m
− 2
d
− 1
. In addition, the concentration of NO
3
N in pond water at harvest equaled a daily accumulation of 1.5 g N m
− 2
d
− 1
. Thus, most of the NO
3
N produced in nitrification can be accounted for.
4. Conclusions
