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DIURNAL VARIATION IN FISH TANKS WITH TWO DIFFERENT AERATION SYSTEMS AND ONE CONTROL TANK

Lúcia Helena Sipaúba-Tavares1, Maria Luiza R. de Souza2 & Sérgio do Nascimento Kronka3

1Centro de Aqüicultura - Universidade Estadual Paulista - Jaboticabal - SP - Brasil

2Departamento de Zootecnia - UEM - Maringá - PR - PG em Aquicultura - UNESP/CAUNESP - Brasil

3Departamento de Ciências Exatas - Universidade Estadual Paulista - Jaboticabal - SP - Brasil

1Mailing address: Centro de Aqüicultura - UNESP, Via de Acesso Prof. Paulo Donato Castellane km 05, CEP 14870-000, Jaboticabal, SP, Brasil.


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ABSTRACT

Two different kinds of aeration systems were used during 24 hours (diurnal variation) to evaluate the effect an aeration system could have on the quality of water in fish tanks with a continual water flow. An air diffuser and a water fountain were used and compared to a control tank (without artificial aeration). There were no significant differences in the variables studied (P>0.05) between the control tank and that with an air diffuser, with chlorophyll a, temperature and total CO2 as exceptions. The tank with the water fountain was significantly different (P<0.05) from the other two tanks for all variables except pH, that had similar values for the three tanks. Temperature and total CO2 values did not show significant differences (P>0.05) for the tank with an air diffuser. When comparing samples collected from the surface and the bottom of the tanks, no significant differences (P>0.05) were apparent regarding the limnological parameters analysed.

Key-words: Aeration systems, fish tanks, limnological parameters, diurnal variation


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INTRODUCTION

With the continual increase of world production, artificial mechanisms are used in an attempt to gain greater productivity in restricted space areas.

The water quality in shallow artificial cultivation systems can be maintained with a continual flow of water or an increase in oxygen concentration. This can be obtained with artificial aeration systems (4).

 

 

The main purpose of continual flow is to carry away the excess organic material deposited at the bottom of the tanks, consequently decreasing the decomposition processes that consume oxygen. A build up of organic material leads to an accumulation of ammonia and nitrite in concentrations that are toxic to fish (11).

Artificial aeration systems provide greater oxygen availability, besides reducing manganese and iron concentrations which are insoluble in oxygenated water. Aeration disrupts thermal stratification permitting greater productivity of the system (18).

According to Thomforde and Boyd (14), continuous aeration can help reduce metabolites in the water, increasing the diffusion to the atmosphere of ammonia, carbon dioxide and other dissolved gases by way of the air/water interface. However, continuous mechanical aeration maintains high concentrations of suspended solids, reducing water transparency and decreasing primary production in the tank.

There is little information on the effect of mechanical aeration in aquaculture. According to Thomforde and Boyd (14), systems that discharge great volumes of air under high pressure are the best for fish cultivation; however, continual use leads to an increase of nitrogenous compounds when compared to those used only in emergency periods.

There are currently several types of aerators on the market. In this study a water fountain system was used, which promotes vertical and horizontal water movement. The other system used was an air diffuser placed at the bottom of the tank, creating a bubble stream throughout the water column.

The objective of this study was to evaluate the effect, on limnological parameters, of two different aeration systems (with an air diffuser and a water fountain system) in fish tanks with a continual water flow, comparing them to a tank without artificial aeration (control). Water samples were collected over a 24 hour period, although the tanks were only aerated at night.

MATERIAL AND METHODS

This experiment was conducted at the Aquaculture Center of UNESP, situated at 21o15’22" S and 48o15’58" W, at a height of 595 meters and 5 km from the nearest urban perimeter. Three tanks were used, each with 4.5 m by 8.5 m and 1.0 m in depth, with a continual water flow. One tank had no artificial aeration being the control (A1), while the other two had artificial aeration systems (A2 and A3). For the aeration of tank A2 an air diffuser was used, consisting of a 2 HP motor that aspirated atmospheric air that was distributed in the water column as bubbles from the bottom to the surface. The other system used was a water fountain (A3), consisting of a pump with a 0.5 HP motor that could suck the water at 60 cm of depth and direct it to one of the sides of the tank. There, two PVC tubes (3/4) that were approximately one meter high and that had air diffusers on the ends, sprayed water into the tanks like a fountain (Fig. 1). The aeration systems were kept working from 12:00 p.m. to 06:00 a.m..

Figure 1. Schematic of the aeration systems used in this study where: A= air diffuser and B= water fountain (1- PVC tube with air diffuser; 2-PVC tube for water suction; 3-PVC tubes (3/4) for water outlet).

Water samples were taken at 3 hour intervals, using a Van Dorn bottle with a 5 liter capacity, always at the same place in the tank but at two different depths: surface (0.20 m) and bottom (1.10 m). Water samples were taken from the source, water from a well located near the tanks.

 

The following limnological variables were analysed:

- temperature: determined with a digital Corning PS 16 thermometer;

- pH: obtained using a digital Corning PS 15 pH meter;

- conductivity: determined with a digital Corning PS 17 conductivity meter;

- dissolved oxygen: determined by the Winkler method (3);

- alkalinity: determined according to Golterman et al. (3);

 

- inorganic forms of carbon: calculated according to Mackereth et al. (7);

- dissolved nutrients: nitrite, nitrate, orthophosphate and total phosphorus analyses were made according to Golterman et al. (3), and the ammonia analysis according to Koroleff (6);

- chlorophyll a: determined according to the technique described in Nush (10), using hot ethanol as the solvent.

 

 

The data obtained was submitted to a variance analysis and mean values were compared using the Tukey test, with a 5% probability level.

RESULTS AND DISCUSSION

The use of mechanical aeration has avoided sudden falls in dissolved oxygen concentrations, where oxygen levels fall below 2 mg/L during night periods. In the mornings the systems are turned off when phytoplankton are able to restart photosynthesis. This way fish stress is reduced (14).

The dissolved oxygen values in the tanks studied were mainly above 4 mg/L (Fig. 2), except the tank that had the water fountain aeration system (A3) which did not show significant differences (P>0.05) between the different depths (Table 1).

Figure 2. Dissolved oxygen, pH, temperature and conductivity fluctuations in three fish tanks, with different aeration systems (A1 = control - no artificial aeration; A2 = air diffuser; A3 = water fountain) and in the supply channel, for a period of 24 hours.

Table 1. Mean and F test values, variation coefficient and Tukey test results for aeration systems and depths.

 

 

In each column, for each factor, mean values followed by the same letter do not differ between themselves according to the Tukey test (P>0.05).

**- significant (P<0.01) * - significant (P<0.05) NS - not significant (P>0.05)

Fluctuations in dissolved oxygen values are more pronounced in shallow systems by the influence of continual water

 

flow, residence time of the water and biological processes, increasing this variable and removing carbon and hydrogen sulfite with iron precipitation (8).

The water source and the channel leading from it affected the water quality. At this stage, various nutrients were already present in the water and the channel turbulence provided a certain degree of aeration (Fig. 2). In shallow systems the biological processes are much more pronounced and photosynthesis becomes the main factor responsible for variations in oxygen levels.

No significant differences were observed (P>0.05) when comparing the oxygen levels in tanks A1 (without aeration) and A2 (air diffuser) with mean values of 5.32 and 5,69 mg/L respectively, both differing from A3 (water fountain) with a mean value of 3.99 mg/L (Table 1).

No significant differences (P>0.05) were observed between the two depths analysed, surface and bottom, for any of the limnological parameters analysed in this study (Tables 1 and 2).

Table 2. Mean and F test values, variation coefficient and Tukey test results, for aeration systems and depths.

 


 

In each column, for each factor, mean values followed by the same letter do not differ between themselves by the Tukey test (P>0.05).

** - significant (P<0.01) * - significant (P<0.05) NS - not significant (P>0.05)

In experiments performed by Sipaúba-Tavares et al. (11), two depths of 30 and 60 cm were compared and no significant differences in limnological values were evident. The aforesaid experiment compared two tanks, one stocked with tilapia fish and the other with a variety of fish although no difference was observed between the different depths, differences were observed between the two tanks, which the authors reasoned was due to management, hydraulic residence time, and the feeding behavior of the cultivated species.

Some authors (9,13) have observed that, in shallow systems, climatic factors have a direct influence on water temperature.

The temperature in the tanks studied showed a slight decrease at night, tending to increase during the day. With aeration, there tends to be less stratification of temperature within the water column and thus nigh time temperatures only decreased slightly (Fig. 1). The tanks were of small dimensions and had a continual flow of water leaving through the bottom. This allowed a greater liberation of hypolimnetic water that could interfere with the environment and decreased the tanks temperature, especially at nigh time when a greater cooling of the water occurs.

The conductivity of water did not show any variation between the tanks, showing constant concentrations throughout the study of about 30 m .S./cm. The water source also had a direct influence on conductivity, as it presented similar or equal concentrations to the tanks (Fig. 2).

Thomforde and Boyd (14) observed that aerated tanks present higher conductivity concentrations when compared to those without artificial aeration.

The pH of water varied from slightly acid to alkaline, without significant differences (P>0.05) between the tanks (Table 1).

There is a close relationship between carbonate levels and environmental pH and these are inversely proportional to the bicarbonate and calcium levels in the water (15).

Bicarbonate was the dominant form of inorganic carbon, showing an inverse relationship with the pH in the tanks studied (Figs. 2 and 3).

Figure 3. Alkalinity, bicarbonate, free CO2 and total CO2 fluctuations in three fish tanks, with different aeration systems (A1 = control - no artificial aeration; A2 = air diffuser; A3 = water fountain) and in the supply channel, for a period of 24 hours.

The forms of inorganic carbon in both of the aeration systems were significantly different (P<0.05); however, the tank with no artificial aeration and the one with the air diffuser did not show significant differences (P>0.05). Regarding total CO2, the tanks that had artificial aeration systems showed similar (P>0.05) results, both significantly superior to the control tank (P<0.05) and free CO2 never exceeded 4 mg/L of CaCO3; meanwhile, carbonate concentrations showed negligible values in the environment (Table 2).

Alkalinity concentrations were similar in behavior to the forms of inorganic carbon, without significant differences between A1 and A2 (P>0.05). However there were significant differences when compared to A3 (P<0.05) (Table 2). Alkalinity and inorganic carbon, were influenced by the water source that showed concentrations very close to those of the tanks (Fig. 3).

Environments that have a dominance of bicarbonate have a tendency to affect alkalinity concentrations since it can work as a base or an acid (1). On the other hand, alkalinity is directly related to the decomposition of organic material and, consequently, produces great quantities of CO2 leading to an increase in CaCO3.

 

 

Sediment acts as a link or source of ammonia, nitrite and nitrate for the water column; however, the concentrations of these three elements decrease with time because of their fast assimilation by algae (2). Of the nitrogenous compounds, ammonia was dominant in the system since its concentration in cultivation tanks is

associated mainly to fish excretion, alimentary residues, decomposition of organic material, nitrification and denitrification processes, sediments and others (Fig. 4).

Figure 4. Ammonia, nitrite and nitrate fluctuations in three fish tanks, with different aeration systems (A1= control - no artificial aeration; A2 = air diffuser; A3 = water fountain) and in the supply channel, for a period of 24 hours.

 

The concentrations of ammonia in the tanks studied were higher in A3, showing significant differences (P<0.05) for A1 and A2, the same occurring for nitrate and nitrite (Table 1).

In systems that receive great quantities of nutrients, nitrate is the final product of the nitrification process, becoming the major source of inorganic nitrogen in the environment (16). Besides, nitrate is highly mobile in soil that suffers as an influence of sediment mixture and suspension (17).

Low concentrations of nitrite, or even its absence in the environment, can be associated to the incomplete oxidation of ammonia or its quick use by the phytoplanktonic community (Fig. 4).

An inverse relationship between phosphorus and chlorophyll a was evident, indicating the assimilation of this component by the algae biomass that has the ability to stock this element. Algae has an ample capacity to assimilate organic substrates, showing a competitive advantage when compared to obligatory photoautotrophs organisms (5,12).

Tank A3 was significantly different (P<0.05) from A1 and A2 , which did not differ (P>0.05), for total phosphorus and orthophosphate; while chlorophyll a showed significant differences when comparing the three tanks (P<0.05), with higher values for A2, followed by A1 and A3 (Table 1 and Fig. 5).

Figure 5. Total phosphorus, orthophosphate and chlorophyll a fluctuations in three fish tanks, with different aeration systems (A1= control- no artificial aeration; A2= air diffuser; A3= water fountain) and in the supply channel, for a period of 24 hours.

According to the results obtained in this study, the use of mechanical aeration systems is not necessary in fish tanks of small dimensions (4.5 by 8.5 m and 1.0 m in depth), since the continual flow of water sustains adequate conditions for cultivation. However, if the use of aerators becomes necessary because of a high stock density, the use of an air diffuser type of aerator is advisable as it does not cause harsh water movement.

ACKNOWLEDGEMENTS

The authors would like to thank Silvia R.L. de Laurentiz and Regina Helena Sant’Ana de Faria for their help in the field and laboratory work.


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RESUMO

Variação nictemeral em tanques de piscicultura com dois sistemas de aeração e um tanque controle. Com o objetivo de avaliar o efeito dos sistemas de aeração na qualidade da água em tanques de piscicultura de fluxo contínuo em um período de 24 horas, foram utilizados dois tipos de aeradores, um chafariz e outro compressor radial comparando-os com o controle (sem aerador). Em relação as variáveis estudadas, não apresentaram diferenças significativas (P>0,05) entre o tanque controle e o de compressor radial, com exceção da clorofila a, temperatura e CO2 total. O tanque com chafariz diferiu significativamente (P<0,05) dos outros dois tanques, com exceção do pH que foi similar entre os três tanques. Foram também realizadas coletas entre superfície e fundo, não apresentando diferenças significativas (P>0,05) entre esses dois estratos para as variáveis limnológicas analisadas.

Palavras chave: sistema de aeração, tanques de piscicultura, parâmetros limnológicos, variação nictemeral.


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