2012-11-05_HYDROLOGY - C1994082 (2)

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1�1 produce carbon dioxide as a product of respiration. During daylight, algae and under�•vater plants remove carbon dioxide from the water as part of the sunlight- driven process of photosyn- thesis. The relative rates of respiration and photosynthesis within the pond determine whether there is a net addi- tion or removal of carbon dioxide, and therefore whether pH falls or rises. Respiration rates are affected by water temperature and the biomass of plants, animals and microorganisms in the water and bottom sediment. Rates of photosynthesis are controlled primarily by sunlight intensity, plant biomass and water temperature. During the day, underwater photosyn- thesis usually exceeds respiration, so pH rises as carbon dioxide is extracted from the water. As the sun begins to set in late afternoon, photosynthesis de- creases and eventually stops, so pH falls throughout the night as respiring organ- isms add carbon dioxide to the water. When the sun rises, plants resume pho- tosynthesis and remove carbon dioxide from water, causing pH to rise again. The daily interplay of respiration and photosynthesis causes pH to cycle up and down during a 24 -hour period. In most aquatic environments, daily photosynthesis is about equal to respira- tion and pH will usually remain within a range tolerated by most animals. However, when plants or algae are growing rapidly, more carbon dioxide is removed each days photosynt iesis than is added each night by respiration. As a resu , p may rise to a normally high*�evels dunng e a ternoon and night (Fig. 1). This condition may last for many days, until photosynthesis decreases or respiration increases. Problems with high pH are common in fry nursery ponds and in ponds used to grow freshwater prawns JJ14acr•o- brachiurn rosenbergii) because fertiliza- tion practices used to prepare ponds for stocking are designed to promote fast - growing phytoplankton blooms that rapidly take up carbon dioxide. Unfor- tunately, the early life stages of fish and crustaceans are particularly susceptible to pH toxicity and juveniles are less able than older animals to "enviroregu- �y� == UCa11/_cU uapicuun ui pn cycring curing a s -day period in two ponds. In both ponds, pH rises during the day as carbon dioxide is removed through photosynthesis and falls at night (shaded vertical bars) as carbon dioxide is added to the water through respiration. The solid line represents pH changes in a pond where carbon dioxide taken up in photosynthesis is offset by carbon dioxide respired at night. The dashed line represents pH changes in a pond where more carbon dioxide is fixed in photosynthesis than is produced at night, and pH values increase from day to day. late" by moving to areas of lower pH in the pond (such as deeper waters). Although high pH is most common in recently filled and fertilized ponds, ponds with established phytoplankton blooms are also susceptible. Phyto- plankton in fertile aquaculture ponds often cycles through periods of bloom and collapse. When large numbers of algal cells die, the nutrients released during decomposition stimulate the growth of a new bloom. When plants are growing quickly, their rapid carbon dioxide uptake may cause high pH until the phytoplankton community comes to a new equilibrium. Extended episodes of high pH are particularly common in ponds where filamentous algae dominate the plant community. Ponds with filamentous algae usually have clear water, allow- ing sunlight to penetrate deep into the water column and promote intense photosynthesis by underwater or float- ing mats of algae. High pH in aquaculture ponds appears to occur more frequently and with greater severity in waters with low total hardness and moderate to high total alkalinity. The reason for this is not completely understood. Managing problems With high pH Managing high pH in aquaculture ponds is difficult and no specific man- agement practice is always successful. Difficulties arise because the term "high pH" describes not only a chemical property, but also the outcome of many interacting chemical and biological pro- cesses. As a measure of water's hydro- gen ion concentration, high pH can be corrected simply by adding an acid to increase that concentration. However, 'high pH" also describes the net result of many individual processes that add or remove carbon dioxide. Reducing pH with an acid does not alter these pro- cesses and, therefore, cannot address the underlying causes of high pH. So while adding an acid may temporarily reduce pH, high pH will probably occur again unless other environmental condi- tions also change. The long -term solution to high pH prob- lems in ponds is to alter pond biology so that the net daily carbon dioxide up- take is near zero. This can be done by reducing photosynthesis or increasing respiration. But changing the metabo- lism of the pond community is difficult because biological processes have con-