Relationships of Dissolved Oxygen with Chlorophyll-a and Phytoplankton Composition in Tilapia Ponds

doi:10.4236/ijg.2013.45B008

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International Journal of Geosciences, 2013, 4, 46-53

http://dx.doi.org/10.4236/ijg.2013.45B008 Published Online September 2013 (http://www.scirp.org/journal/ijg)

Relationships of Dissolved Oxygen with Chlorophyll-a and

Phytoplankton Composition in Tilapia Ponds

Kornkanok Kunlasak1,2, Chanagun Chitmanat1, Niwooti Whangchai1,

Jongkon Promya1, Louis Lebel2

1Faculty of Fisheries Technology and Aquatic Resources, Maejo University, Chiang Mai, Thailand

2Unit for Social and Environmental Research (USER) Faculty of Social Science, Chiang Mai University, Ch iang Mai, Thailand

Email: [email protected]

Received July 2013

ABSTRACT

This study investigated the relationships among the parameters of dissolved oxygen, chlorophyll-a and phytoplankton

composition in tilapia ponds. Each pond (a total of 18 ponds) was sampled once in the dry, winter season between J an-

uary and March and again early in the rainy season between May and June. The data were analyzed by examining cor-

relations among parameters as affected by season, altitude and culture system. Observations were made at sites located

in 5 selected provinces of northern Thailand: Chiangrai, Chiangmai, Phayao, Lampang and Nakornsawan. Mean eleva-

tion of these areas range from 25 to 582 meters above sea level (masl) and were categorized into low (<400 masl) and

high (>400 masl) elevation sites. Ponds were 0.8 - 2.0 m deep, 0.16 - 0.64 ha in area and could be further categorized

into high and low input systems.Mean air temperature in winter ranged between 16.5˚C - 35.8˚C while mean water

temperature ranged between 25.5˚C - 27.1˚C. In rainy season, air temperature ranged between 22.0˚C - 3 7. 3˚C an d wa-

ter temperature ranged between 29.4˚C - 31.8˚C. The amount of chlorophyll-a in both seasons were comparable (p >

0.05), but chlorophyll-a in high input system was significantly higher (p < 0.05) than in low input ponds. Only weak

correlation was found between chlorophyll-a, DOmax and DOmin. Multifactor-ANOVA was used to analyze the differ-

ence of total bacteria and filamentous cyanobacteria in ponds based upon elevation, culture systems and season. Result

shows that there is a significant interaction observed between elevation, culture system and season (p < 0.05). Species

diversity and composition of phytoplankton in fish ponds in 2 seasons revealed the presence of 90 genera of phytop-

lankton under all 7 divisions. Divisions Chlorophyta and Cyanophyta had the most number of genera identified in both

seasons with Pediastrum spp., and Scendesmus spp., and Anabaena spp. as dominant genera/genus, respectively.

Keywords: Dissolved O xygen; Chlorophyll-a; Phytoplankton Composition; Tilapia Ponds; Elevation, Season

1. Introduction

Increased demand for high protein food and apparent

declines in capture fisheries together have helped drive

rapid expansion of the aquaculture industry in the past

two decades. Th e aquacultur e industry howev er is facing

challenges such as high cost of inputs and climatic

changes. Climate effect such as increase in temperature is

observed to increase disease transmission, deplete oxy-

gen, increase incidence of harmful algal blooms in ponds

to mention a few [1].

Tilapia culture is one of the major aquaculture indus-

tries in Thailand with river cage and earthen pond cul-

tures were both practiced. However, some tilapia cage

farmers have switched from river to earthen pond due to

difficulties with extreme water flows and poor water

quality. There are substantial differences in tilapia cul-

ture systems among places depending on various con-

straints and opportunities such as topography (lowland

and upland) and availability of water and alternative nu-

trient inputs [2]. Elevation above sea level, for instance,

influences air and water temperature in pond culture [3].

Physical, chemical and biological water quality in fish

pond ecosystem influences growth and survival rates as

well as reproduction and likelihood of disease infection

[4]. Dissolved oxygen (DO), in particular, is an important

factor for fish respiration and phytoplankton dynamics.

DO content typically correlates with phytoplankton den-

sity in fish ponds. Main tenance of phytoplank ton popula-

tions at desired levels is an important but difficult aspect

of fish pond management. Many fish culture manuals

stress that an algal bloo m must be maintained to improve

oxygen levels, to prevent macrophyte growth, and to

provide natural foods, either directly or indirectly, for

fish in the pond [5-7]. At the same time, however, uncon-

trolled algal growth causes many serious problems for

aquaculturists [8]. Thus, proper management of phytop-

K. KUNLASAK ET AL.

lankton growth is a major goal of modern pond aquacul-

ture [7-10].

While phytoplankton affects water quality in several

ways [7,8], the difficulty and importance of managing

phytoplankton stem primarily from the complex rela-

tionship between algal dynamics and dissolved oxygen

levels. Phytoplankton are the major source of dissolved

oxygen in fish ponds as well as—directly as consumers

and indirectly as the source of detritus upon which most

bacterial respiration is b ased—the major sink for oxygen

[11,12]. Dissolved oxygen levels depend primarily on the

relative magnitudes of photosynthetic oxygen generation

and total plankton respiration [13]. Given the complexity

of this relationship and the importance of dissolved oxy-

gen to aquacultural production [14], the interaction be-

tween dissolved oxygen and phytoplankton biomass

should be e xamined in de t a il.

Water quality in aquaculture ponds is influenced by

management and other external factors such as culture

system, stocking densities, water exchange practices and

sources, and fertilizer application. The main objective of

this research was to determine the association between

chlorophyll-a, phytoplankton composition, and dissolved

oxygen in tilapia ponds from different culture systems in

winter and rainy seasons. This information will be then

used to develop techniques to manage phytoplankton and

dissolved ox yge n dynamics in fish ponds .

2. Materials and Methods

2.1. Study Sites

In this study, data was collected in the dry, winter season,

between January and March and in the early rainy season

between May and June 2013. Observations were made in

18 ponds at sites located in 5 selected provinces of

northern Thailand: Chiangrai, Chiangmai, Phayao, Lam-

pang and Nakornsawan. Mean elevation of these areas

range from 25 to 582 meters above sea level (masl).

Ponds were 08 - 2.0 m deep and 0.16 - 0.64 ha in size at

different elevations above sea level (low: <400 masl and

high: >400 mas l) and employed different cultu re systems:

high input (high load of nutrient resulting either from

intensive feed ing or from manure fertilization) and low

input (low load of nutrient where feeding was sporadic).

2.2. Water Sampling and Analysis

Pond water samples were assessed to determine levels of

algal biomass, total bacteria and other water quality pa-

rameters. Dissolved oxygen was monitored in situ along

with pH, temperature, turbidity and conductivity using

the multi-meter TOA DKK WQC-22A (Japan). Water

samples were collected for nutrient analyses (nitrate-N,

nitrite-N, total ammonia-N, and orthophosphate-P) and

chlorophyll-a determination in the laboratory following

standard methods [15]. For hydro-biological analysis,

phytoplankton was sampled by filtration of 5-L pond

water with a net of 10-μm mesh. Samples were concen-

trated in a 30-mL plastic bottle and immediately pre-

served with 1 mL Lugol’s solution. Species and count of

phytoplankton were determined using an Olympus BH2

microscope with the aid of a 1000 oil immersion objec-

tive. Total bacterial analysis was done using the pour

plate technique as described in ISO, 1999 and SCA, 2002

[16,17]

2.3. Statistical Analysis

Analysis of Variance (ANOVA) was used to compare

water pond parameters across the two seasons and eleva-

tion groups. Paired sample T-test was used to compare

the differences of water quality variables between the

two seasons, elevation groups and culture systems.

3. Results and Discussion

3.1. Effect of Season on the Relationship between

Chlorophyll-a and Dissolved Oxygen

Monitoring of tilapia ponds was carried out during winter

and early rainy season. Mean chlorophyll-a concentra-

tions, DOmax and DOmin were comparable between the

two seasons (Tabl e 1 ). Mean DO ( max and min) concen-

trations were not significantly different between the two

seasons. A significant negative correlation was found

between chlorophyll-a and DOmin in both seasons (Fig-

ure 1). There was no association with DOmax. The fre-

quency of DOmin levels b elow 1-ppm threshold value (red

line in figures) were comparable between the two sea-

sons.

As a general rule of thumb, DO level in ponds corre-

lates with the amount of chlorophyll-a which results from

phytoplankton biomass. In water bodies when the

amount of phytoplankton increases, the amount of chlo-

rophyll-a increases as well [18] and so does DO due to

algal photosynthesis during daylight (positive relation-

ship with DOmax). However, as phytoplankton biomass

Table 1. Means (±SD) of chl orophyll-a, DOmax and DOmin in

winter and rainy season.

Parameter Season

Winter Rainy

Mean Chlorophyll-a (µg·L−1) 177.8 ± 174.0ns 204.8 ± 220.1ns

Mean DO max (mg·L−1) 7.78 ± 1.94ns 7.59 ± 4.60ns

Mean DO min (mg·L−1) 1.96 ± 2.19ns 1.12 ± 1.68ns

Values with the same letter superscripts in rows are not significantly differ-

ent (P > 0.05) .

K. KUNLASAK ET AL.

Figure 1. Correlation of DOmax/DOmin versus chlorophyll-a for winter (a) and rainy (b) season.

increases, respiration during nighttime can deplete DO

concentrations to critical values (negative relationship

with DOmin). The lack of relationship of chlorophyll-a

with DOmax and significant negative association with

DOmin in both seasons could be explained that DO was

affected not only by algal photosynthesis but also by aq-

uatic respiration, oxidative decomposition of organic

compounds (from fish and animal wastes), water ex-

change rate and artificial pond aeration (in the case of

commercial ponds). Differences in culture practices

among ponds surveyed in the study may have also influ-

enced relationships and will now be considered in more

details.

3.2. Effect of Culture System on the Relationship

between Chlorophyll-a and Dissolved

Oxygen

Mean chlorophyll-a concentration was significantly low-

er in tilapia ponds which adopt low input culture system

compared to those that receive high inputs (Table 2).

Mean DO (max and min) concentrations of ponds re-

ceiving low and high inputs were not significantly dif-

ferent. A significant positive correlation between chlo-

rophyll-a and DOmax was observed in high input ponds

but not low-input ponds (Figure 2). A significant nega-

tive correlation of chlorophyll-a with DOmin was found in

low-input ponds, bu t not high input ponds (Figure 2).

High input ponds, which comprise both surveyed com-

mercial and integrated fish farming ponds, are expected

Table 2. Means (±SD) of chlorophyll-a, DOmax and DOmin in

high and low input-based culture systems.

Parameter Culture System

High input L ow input

Chlorophyll-a (µg·L−1) 273.0 ± 199.3ª 45.1 ± 26.8ᵇ

DO max (mg·L−1) 8.20 ± 3.46ns 6.78 ± 3.10 ns

DO min (mg·L−1) 0.65 ± 1.22 ns 3.20 ± 2.08 ns

Values wit h the same letter superscripts in rows are no t significantl y differ-

ent (P > 0.05) .

to have higher chlorophyll-a concentrations compared to

low input (su bsistence) ponds due to their higher nutrient

loads. Commercial tilapia farms usually adopt intensive

culture system of production with high stocking rates

which rely heavily on th e use of commercial f eeds rather

than on natural food. As fish waste and uneaten feed en-

ter the pond system, nutrients accumulate in the bottom,

released into the water and taken up by phytoplankton

bloom. Similarly, integrated ponds (with p ig and chicken)

produce high loads of nutrients from manure fertilization

for the purpose of producing natural food for the fish.

The utilization of organic manure as the principal nu-

trient input to the pon d is a traditional management pra c-

tice in freshwater fish farming in Thailand.

In high input culture system, changes in DOmax slightly

follow adjustments in chlorophyll-a concentration show-

ing a weak positive relationship. However, the negative

correlation observed in low input culture system between

K. KUNLASAK ET AL.

Figure 2. Correlation of DOmax/DOmin versus chlorophyll-a for hi gh input (a) and low input ( b) culture syst em.

chlorophyll-a and DOmax contradicts the general rule and

no explanation can currently be found with regards to

this result.

3.3. Effect of Elevation on the Relationship

between Chlorophyll-a and Dissolved

Oxygen

Mean chlorophyll-a, mean DOmax and DOmin in ponds

surveyed from low elevation sites were not significantly

different from ponds in high elevation as shown (Table

3). In low elevation sites, chlorophyll-a and DOmax was

positively correlated with each other, whilst chloro-

phyll-a and DOmin were correlated negatively (Figure 3).

In high elevation sites chlorophyll-a was significantly

negatively correlated with DOmin (Figure 3).

The present study covered a wide range of tilapia pro-

duction systems (commercial, integrated and subsistence)

at low elevation sites. Water quantity is not a major con-

straint, but these areas may acquire other impacts making

changes to culture systems e.g. floods, capital cost or

market access. These impacts may cause farmers to

switch from commercial to integrated system (fish and

pig or chicken/multiple fish species) to reduce production

cost but the quality of tilapia maybe affected in terms of

off-flavor due to uncontrolled cyanobacterial growth in

high-nutrient waters of an integrated system. The inte-

grated system of culture could blend well with the re-

source-poor small-scale aquaculture farmers especially

Table 3. Means (±SD) of chlorophyll-a, DO max and DO

min at <400 and >400 masl.

Parameter Elevation, masl

<400 >400

Chlorophyll-a (µg·L−1) 219.0 ± 222.0ns 155.6 ± 153.1ns

DO max (mg·L−1) 7.19 ± 4.28ns 8.27 ± 1.71ns

DO min (mg·L−1) 1.00 ± 1.65ns 2.27 ± 2.19ns

Values wit h the same letter superscripts in rows are not significantly differ-

ent (P > 0.05) .

those dependent on aquatic resource for their livelihoods

since this strategy would make them more resilient to

climate variability. On the o ther hand, to resist the impact

of flooding in low elevation sites, the farmer can streng-

then and increase the height of the perimeter dikes. The

farmer can likewise deploy nets on the top of the dykes

so that when a flood occurs, the fish remain in the ponds.

At high elevation sites, it could be the proper area for

subsistence system since water scarcity, fingerlin gs qual-

ity and lower temperature are major conditions.

Higher elevation sites and high input systems were

more likely to have DOmin values below the threshold

level of 1 ppm than at lower elevation sites and low input

systems, respectively (Table 4). Chlorophyll-a in high

input system were significantly higher than in low input

system. Seasonal and elevation differences were not sig-

nificant (Table 4).

K. KUNLASAK ET AL.

Figure 3. Correlation of DOmax/DOmin versus chlorophyll-a for <400 masl (a) and >400 masl (b) sites.

Table 4. Percentage (%) of tilapia ponds with DOmin below t he threshold value (1 ppm) and effect of season, culture system

and elevation on c hlorophyll-a.

Factor Mean Chlorophyll-a (µg ·L−1) Number of ponds (%)

Total (n) No. with <1 p p m D O mi n.

Season

Winter 177.8 ± 174.0ns 18 9 50.00

Rainy 204.9 ± 220.1ns 15 10 66.67

Culture system

high input 273.0 ± 199.3ª 22 17 77.27

low input 45.1 ± 26.9ᵇ 12 2 16.67

Elevation

<400 218.9 ± 222.0ns 18 13 72.22

>400 155.6 ± 153.1ns 15 6 40.00

Values with different letter superscripts in the column are signifi cantly different (P < 0.05).

3.4. Effect of Elevation, Culture System and

Season on Total Bacterial Load

Mean total bacteria concentration in pond water at high

input system in low elevation sites (<400 masl) was sig-

nificantly higher (P < 0.05) than in high elevation sites

(>400 masl) whilst in low input systems at high elevation

sites mean total bacteria concentration was significantly

higher (P < 0.05) than at low elevation sites (Table 5).

Multifactor-ANOVA was used to evaluate differences

in total bacterial concentration in ponds based upon ele-

vation, culture system and s eason. Significant interaction

among elevation, culture system and season for total

bacteria concentration were observed (Figu re 4).

In general, temperature varies inversely with elevation.

This is one of the reasons why lower bacteria activity

was seen in higher altitude ponds (>400 masl). Mean

K. KUNLASAK ET AL.

Table 5. Total bacteria (×103) (±SD) in ponds at different

elevation i n both culture sys t em .

Culture System Elevation, masl

<400 >400

High input 7.37 ± 5.30ª 1.17 ± 1.11ᵇ

Low input 0.29 ± 0.16ᵇ 0.97 ± 0.59ª

Values with different letter superscripts in rows are significantly different (P

< 0.05).

Figure 4. Total bacteria in ponds at different elevation and

culture sy s t em in both seasons.

temperatures at low elevation sites are higher by 2 de-

grees than at high elevation sites. High organic loading

also increased nutrient and bacterial levels in high input

pond systems at <400 masl. Commercial and integrated

culture systems (high input systems) have higher fish

stocking density and feeding rate and therefore the

amount of organic matter and waste (including excess

uneaten feeds) are expectedly high for these systems

which increase bacterial load in water. This could deplete

DO from water during the decomposition of these mate-

rials at the bottom. Farms, especially those which grow

tilapia in commercial ponds like the one in Phayao (>400

masl), use aerators to counteract DO depletion during

nighttime.

However, for low input pond system at high elevation

sites, bacterial level is higher than the hotter low eleva-

tion sites. This observation contr adicts the result obtain ed

for high inp ut pond system. T emperatu re has b een sh own

to be one of the limiting factors for bacterial growth par-

ticularly in relatively low temperatures (<12˚C - 15˚C)

although many of bacteria are adapted to colder envi-

ronment, hence, the result. It is also possible that subsis-

tence ponds in higher altitudes had higher organic matter

content than in lower altitude ponds which could have

significantly contribute to the bacterial load in the water.

3.5. Phytoplankton Density and Effect of

Elevation, Culture System and Season on

Filamentous Cyanobacteria Biomass

For the assessment of diversity and composition of phy-

toplankton in the surveyed ponds, a total of 90 genera of

phytoplankton from 7 divisions were identified covering

the 2 sampling seasons. Divis ion C hl oro phy t a ( 49 ge ne ra,

54.4%) was the most abundant followed by Bacillario-

phyta (13 gene ra, 14.4%), Cyanophyta (10 genera,

11.1%), Cryptophyta (7 genera, 7.8%), Chromophyta (5

genera, 5.6%), Euglenophyta (5 genera, 5.6%), and Pyr-

rhophyt a (1 genus, 1.1% ) whi ch wa s fo und onl y in winte r.

In terms of phytoplankton density, Cyanophyta (cyano-

bacteria) dominated the pond waters in winter whilst

Chlorophyta were the prevalent group during rainy sea-

son. There were no significant differences in total bio-

mass of both Cyanophyta and Chlorophyta between sea-

sons (Figure 5). Among the phytoplankton identified in

the study, Pediastrum spp. and Scendesmus spp., both

chlorophytes, and Anabaena spp., a cyanobacteria, dom-

inated the pond waters.

Mean density of filamentous cyanobacteria was sig-

nificantly higher (P < 0.05) in high input system at low

elevation sites (<400 masl) than in high elevation sites

(>400 masl). However, result for low input system shows

otherwise, where density of filamentous cyanobacteria

was observed to be significantly higher (P < 0.05) at high

elevation sites as opposed to low elevation sites (Table

6).

Figure 5. Phy t opl an kt on densitie s in ponds in 2 seasons.

Table 6. Cell density (×103) (±SD) of filamentous cyanobac-

teria in ponds at different elevation in both culture system.

Culture System Elevation, masl

<400 >400

High input 7.96 ± 5.37ª 1.43 ± 0.1ᵇ

Low input 0.34 ± 0.16ᵇ 1.78 ± 0.6ª

Values with different letter superscripts in rows are significantly different (P

< 0.05).

K. KUNLASAK ET AL.

Multifactor-ANOVA was used to analyze the differ-

ence of filamentous cyanobacterial biomass in ponds

based upon elevation, culture system and season (Figure

6). Results show significant interaction s among elevation,

culture system and season for filamentous cyanobacteria.

Water temperature plays a significant role in the dis-

semination of phytoplankton [19]. Seasonal variability

changes algal density and diversity. The study shows that

the amount of phytoplankton in rainy season is generally

higher than in winter, except for Chlorophyta, which

conforms to the amount of chlorophyll-a in the ponds.

Furthermore, the quantity of filamentous cyanobacteria

in high input system was higher than in low input system

owing to the huge amount of nutrients present in the

former. The water temperature during the study ranged

between 28˚C - 32˚C. High fish stocking densities

(>10,000 fish ha−1) and feeding rates (exceeding 70 kg

ha−1·d−1) resulted in high waste loading rates that often

caused excessive eutrophication in fish ponds, leading to

the proliferation of cyanobacteria [20] in high input sys-

tem, especially during the rainy season. Generally, cya-

nobacteria require higher temperature than any other al-

gae in order to increase its growth rate [21,22] hence, the

high temperature during rainy season supports highly

visible blooms of cyanobacteria, in particular, Anabaena

spp. One major implication with filamentous cyanobac-

teria , s u ch as An aba e na spp., dominating the pond waters

is the production of off-flavors, which could be acquired

by the fish and adversely affect market demand.

3.6. Managing Chlorophyll-a and Dissolved

Oxygen in Aquaculture Ponds

Chlorophyll-a and dissolved oxygen are relevant para-

meters to be managed in an aquaculture pond system.

Phytoplankton, which is indexed by chlorophyll-a, can

develop conspicuously in pond waters, leading to eu-

trophic conditions, especially during climate-driven

low-water periods. Similarly, dissolved oxygen is impor-

tant to the health of aquatic ecosystems and a key indi-

cator in determining water quality. Uncontrolled phytop-

lankton growth (high chlorophyll-a) can be a serious

problem for the farmers as it takes more oxygen out of

the water during the night than what remains in solution

from daytime photosynthesis. Moreover, phytoplankton

die offs can increase bacterial decomposition and the

reduction in normal oxygen production can lead to oxy-

gen depletions, high ammonia levels, and stressed or

dead fish. To alleviate this problem mechanical aeration

must be applied to meet the increased demand for oxygen

and prevent oxygen depletion and subsequent fish losses

or stress [22]. In high input systems nutrient reduction is

arguably the best strategy to reduce the incidence of al-

gae, especially harmful cyanobacterial blooms [23]. Thus,

maintaining the suitable amount of phytoplankton in fish

pond is important.

An important limitation of this study was the restric-

tion of sampling to just two time periods. Observations

over more months and multiple years are needed to fully

the effects of seasons and other climate-driven factor s on

chlorophyll-a and dissolve d o xygen relationships .

5. Conclusion

The relationships between chlorophyll-a, and DO max or

DO min were complex, being effected by season, nu-

trient inputs and elevation. Taken together findings sug-

gest that other factors other than algal photosynthesis

were involved. As would be expected chlorophyll-a was

much higher in high input than in low input ponds. Sig-

nificant interactions were observed for total bacteria and

filamentous cyanobacteria between elevation, culture

system and season. Divisions Chlorophyta and Cyano-

phyta had the most number of genera identified in both

seasons with Pediastrum spp., and Scendesmus spp., and

Anabaena spp. as dominant genera/genus, respectively.

6. Acknowledgements

The work was carried out with the aid of a grant from the

International Development Research Centre, Ottawa,

Canada, as a contribution to the AQUADAPT project.

Figure 6. Density of filamentous cyanobacteria in ponds at different elevation and culture system in 2 seasons.

0.00

2000.00

4000.00

6000.00

8000.00

10000.00

12000.00

<400

>400

Cells mL

-1

Elevation, mas l

high input winter

high input rainy

low input winter

Filamentous Cyanobacteria

K. KUNLASAK ET AL.

Special thanks to Redel Gutierrez of Maejo University

for editing this paper.

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