Alkalinity and hardness: Critical but elusive concepts in aquaculture

Authors: BOYD, CLAUDE - Auburn University; Tucker, Craig; SOMRIDHIVEJ, B - Auburn University

Submitted to: Journal of the World Aquaculture Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/8/2015
Publication Date: 1/27/2016
Citation: Boyd, C.E., Tucker, C.S., Somridhivej, B. 2016. Alkalinity and hardness: Critical but elusive concepts in aquaculture. Journal of the World Aquaculture Society. 47:6-41.

Interpretive Summary: This paper reviews the concepts of alkalinity and hardness as they relate to aquaculture. Alkalinity is the acid-neutralizing capacity of water and in most waters the primary bases contributing to alkalinity are bicarbonate and carbonate derived from weathering of limestones. Hardness is the total concentration of divalent cations. Calcium and magnesium are the most common cations contributing to hardness and, like bicarbonate and carbonate, are often derived from limestones. Alkalinity buffers water against wide daily pH swings related to photosynthesis and respiration rates, but early morning pH of waters tends to increase with greater alkalinity. Biological productivity may be limited by low availability of inorganic carbon for photosynthesis in some waters – especially those of low alkalinity. Higher concentrations of alkalinity protect aquatic animals from toxicity of trace metal ions added to water intentionally or through pollution, because the ionic forms of metals are most toxic. The main effect of hardness seems to result from calcium, despite both calcium and magnesium being common hardness cations. Calcium precipitates carbonate as calcium carbonate to minimize pH rise in waters of pH above 8.3 when photosynthesis is progressing rapidly. Calcium also tends to block trace metal uptake by fish thereby lessening toxicity. In fish hatcheries, calcium concentration is important to egg development and hatching. Greater hardness also facilitates the flocculation and precipitation of suspended clay that causes turbidity. Alkalinity and hardness in freshwater ponds for sportfish should be 40 mg/L or more, but in food fish ponds alkalinity should be 60 mg/L or more. In culture of estuarine or marine species, alkalinity should be above 100 mg/L. Hardness in estuarine or marine water is much greater than in freshwater and seldom will affect aquaculture production. Low alkalinity and hardness in ponds usually are increased by additions of agricultural limestone or other liming materials. Enough liming material must be used to neutralize bottom soil acidity and provide a residual to dissolve and increase alkalinity and hardness.

Technical Abstract: Total alkalinity and total hardness are familiar variables to those involved in aquatic animal production. Aquaculturists – both scientists and practitioners alike – tend to have some understanding of the two variables and of methods for adjusting their concentrations. The chemistry and the biological effects of alkalinity and hardness, however, are more complex than generally realized or depicted in the aquaculture literature. Moreover, the thorough discussions of alkalinity and hardness – alkalinity in particular – found in most water chemistry texts tend to be presented in a rigorous manner and without any explanation of how the two variables relate to aquaculture. The purpose of this review is to provide a thorough but less chemically rigorous discussion of alkalinity and hardness specifically oriented towards aquaculture. Alkalinity and hardness are defined, their sources identified, and analytical methods explained. This is followed by a discussion of the role of the two variables in aquaculture, including relationships with carbon dioxide, pH, atmospheric pollution, ammonia and other inorganic nitrogen compounds, phytoplankton communities, trace metals, animal physiology, and clay turbidity. The roles of liming and other practices to manage alkalinity and hardness are explained. Throughout the review, emphasis is placed on interactions among alkalinity, hardness, water quality, and aquacultural production.