Phytoplankton in Catfish Ponds

H. Steven Killian
Extension Fisheries Specialist

Introduction

Phytoplankton are microscopic photosynthetic organisms found suspended in water (Figure 1).  Phytoplankton are found in most bodies of fresh and salt water, including channel catfish ponds.

Soon after a fish pond is dug and filled, samples of different species of phytoplankton can be found.  These new ponds can be seeded with phytoplankton through such sources as wading birds, turtles, and wind-blown spores.

Many species of phytoplankton will reach a new pond, but only certain ones will survive and flourish.  The species that colonizes the new pond depends upon the suitability of the environment for growth and, to some respect, chance.  It is not uncommon for ponds that are constructed side-by-side with similar soil types and the same water sources to contain very different populations of phytoplankton.

Water Quality

Phytoplankton, more than any other organism, directly affect a number of critical water quality variables in commercial catfish ponds.  The phytoplankton community drives the dissolved oxygen, carbon dioxide and pH cycles and directly affects the concentration of nitrogenous water products such as ammonia and nitrite.

Phytoplankton produce oxygen during photosynthesis.  The oxygen is then released into the water.   Dissolved oxygen concentrations reach their highest concentrations in the late afternoon.  During the night, all organisms, including phytoplankton, consume oxygen (Table 1).  This consumption of oxygen results in the lowest concentration of dissolved oxygen around sunrise (Figure 2).

The carbon dioxide cycle is the opposite of that of oxygen.   Phytoplankton take up carbon dioxide during the light hours for photosynthesis.   The total amount of photosynthesis in the pond overshadows the total pond respiration during the day.  Therefore, there is little or no carbon dioxide in the pond in the afternoon and evening.

Respiration of all pond organisms, including phytoplankton, results in the release of carbon dioxide.  This continues during the dark hours while photosynthesis is not occurring.  As a result, there is a net surplus of carbon dioxide in the pond during the night and concentrations peak around sunrise (Figure 3).

The carbon dioxide cycle directly affects the pH cycle.  High carbon dioxide levels produce low pH readings, and conversely, low concentrations of carbon dioxide in the water result in higher pH readings.  The size of the pH shift in a culture pond depends on the buffering capacity of the water which is referred to as alkalinity. 

Water with low alkalinity will have large swings in pH, while ponds that are highly buffered (high alkalinity) may only produce slight changes in pH over a 24-hour period (Figure 4).  Ponds with low levels of total alkalinity will have overall lower pH readings, and water with higher concentrations of alkalinity will have higher pH levels.

The pH has a significant effect on the toxicity of total ammonia.  Total ammonia is composed of toxic unionized ammonia (NH₃) and nontoxic ionized ammonia (NH₄).  At high pH levels, total ammonia is shifted more towards its toxic unionized component.  Conversely, when pH levels drop, the trend is reversed and total ammonia is shifted back toward its nontoxic ionized form (Figure 5).

Phytoplankton utilize nitrogenous waste products in the form of ammonia and nitrite as nutrient sources for growth.  This serves to maintain the concentrations of total ammonia and nitrite in ponds at low or moderate levels during periods of active phytoplankton growth.  The phytoplankton community serves the function of a waste manager in ponds.

The nutrient enrichment of the culture pond in the form of added feeds has a direct bearing on the phytoplankton community.   The standing stock of catfish increases greatly during the growing season.   This increase in catfish biomass requires more pounds of feed/acre/day to maximize production.

The continual loading of nutrients into culture ponds during the spring, summer and fall results in a proliferation of the phytoplankton community.  More than 80 percent of all the nitrogen and phosphate originally contained in fish feeds is excreted into the water and is not utilized for growth by catfish.

During periods of high feeding rates, the food available for phytoplankton growth is enormous.  These nutrients are found dissolved in the water for only a short period of time before the phytoplankton utilizes them for growth and reproduction.

A high density of phytoplankton is not desirable in culture ponds.  Ponds with heavy "blooms" of phytoplankton exhibit wide shifts in dissolved oxygen concentrations from day to night with readings at sunrise approaching levels that can be stressful to catfish (Figure 6).  These ponds are also more likely to have dissolved oxygen depletions during cloudy weather.  Moderate blooms of phytoplankton are desirable, but these are hard to maintain with the high feeding rates in fish culture today.

Phytoplankton Management

To date there is no effective way to manage a phytoplankton bloom other than reducing nutrient input in the form of feeds.   This will limit the nutrients (phosphorus, nitrogen) available for growth and reproduction of the phytoplankton community and will result in a moderate phytoplankton density.  In a catfish pond, this is around 40 to 50 pounds of feed/acre/day.   (This is not an economical option for most catfish producers.)

Other methods have been used to try to control phytoplankton densities but have been completely ineffective.  Studies have shown that the use of copper sulfate (CuSO₄) to selectively thin the bloom does much more harm than good.  Thinning the phytoplankton bloom in this manner results in very poor water quality and overall reduced fish production.

The use of dyes to shade out dense phytoplankton blooms has been studied.  Dyes probably result in poorer water quality throughout the growing season.  Dyes may also select for undesirable blue-green algae in treated ponds.

Phytoplankton Die-Offs

Phytoplankton density tends to increase steadily through the growing season.  Communities in individual ponds, however, often exhibit a marked periodicity in concentrations.  It is not uncommon for phytoplankton communities to crash even with high nitrogen and phosphorus concentrations.  Certain species of blue-green algae are known for their sudden crashes.  However, many other different types of phytoplankton also exhibit these wide swings in population densities.   Some possible causes of these fluctuations include changes in temperature, pH, Carbon dioxide, light intensity, nutrient concentration, disease and the release of phytoplankton toxins by other organisms including competing species of plankton.

A phytoplankton crash will adversely affect critical water quality variables.  After a crash, dissolved oxygen levels will decrease, pH will decrease, but carbon dioxide, ammonia, and nitrite levels will increase.   The relative severity of these changes is affected by a number of factors such as water temperature, phytoplankton density and the percent of the phytoplankton community that died.

A new phytoplankton bloom will develop in a few days after a crash.  The phytoplankton that die will decompose and release nutrients, which will generate a new bloom.  The surviving phytoplankton will multiply rapidly to reach pre-crash levels.  The most important thing fish farmers must do after a die-off is mechanically supply dissolved oxygen to the pond to maintain the fish population until the new bloom is established.

Off-Flavor

Phytoplankton are also responsible for a major problem facing catfish producers: off-flavor.  A major cause of off-flavor in catfish is certain species of blue-green algae.  These species of blue-green algae produce compounds which are absorbed by catfish and impart a bad flavor to the flesh.   This off-flavor causes the fish to be unmarketable for a period of time (few days - weeks).  For more information on off-flavor, refer to the Fact Sheet 9051, Off-Flavor.

Conclusion

Channel catfish ponds are solar collectors.  Phytoplankton use the sun's energy through photosynthesis to produce oxygen which is released into the pond.  Phytoplankton utilize nitrogenous waste products as nutrient sources for growth.  Without phytoplankton, the culture of channel catfish in ponds would be impractical.  The energy cost would be too great to mechanically add all the dissolved oxygen and remove nitrogenous waste products needed to economically produce channel catfish in ponds.

Understanding  the role of phytoplankton and its relation to water quality will help producers make critical management decisions.  Advances in our knowledge of phytoplankton dynamics are essential to the future of catfish production.

H. Steven Killian is an Extension fisheries specialist, University of Arkansas at Pine Bluff, located in Lake Village.

FSA9070-5M-9-93-S502