To analyze how water quality is monitored, understanding key components of water is an essential prerequisite. This series of newsletters will explore the key characteristics of both freshwater and seawater (e.g. solids, dissolved gas, temperature and pH), water balance in fish, and optimal physical and chemical ranges across species and production systems.
In our previous newsletter, we covered solids in water. This week, let’s turn our attention to dissolved gases.
List of Dissolved Gases
Measured in parts per million (ppm) or milligrams per liter (mg/L), the most common dissolved gases on aquaculture farms include:
This article will focus on gases related to the respiration cycle, i.e. oxygen and carbon dioxide.
How does fish and shrimp breath?
Similar to humans, as fish and shrimp breath (known as aerobic cellular respiration), oxygen reacts with glucose to form ATP (i.e. tiny nuggets of energy) and creates carbon dioxide and water as byproducts.
Unlike humans, fish and shrimp must pump water across their gills to allow sufficient quantities of molecular oxygen through their gill lamellae. Water contain less oxygen than air. Thus, fish gills are compacted with blood vessels to increase surface area and maximize air exchange. Around 75% of oxygen that passes through a fish's gills is extracted, according to the American Museum of Natural History. Once oxygen molecules enters the blood stream, it binds with pigment hemoglobin in fish and pigment hemocyanin in shrimp.
The amount of oxygen carried and offloaded by hemoglobin and hemocyanin is determined by dissolved oxygen (DO) tension in water and hemolymph (i.e. blood fluids). Hence, the amount of oxygen consumed by fish or shrimp is a function of its size, feeding rate and activity level, in addition to the temperature, depth and salinity of surrounding water.
Dissolved Oxygen (DO)
Expressed as mg of oxygen/litre of water (mg/L) and essential for respiration and decomposition, dissolved oxygen (DO) is a critical limiting variable in intensified aquaculture.
How much oxygen can water hold?
Concentration of oxygen in water varies with temperature, salinity and depth. Per scientific standards, 7.54 mg/L of molecular oxygen is required to saturate freshwater at standard atmospheric pressure (760 milliliters of mercury) and 30ºC. Water saturated with DO has an oxygen tension of ~160 mm Hg. The concentration of oxygen at saturation varies with water temperature, salinity and pressure, however, the tension of oxygen-saturated water remains constant.
General rules of thumb:
Solubility of oxygen decreases as temperature increases i.e. warmer surface water requires less DO to reach 100% saturation than does deeper, cooler water
DO decreases exponentially as salt levels increase. At the same pressure and temperature, saltwater holds about 20% less dissolved oxygen than freshwater
DO increase as pressure increases. Water holds more DO under greater hydrostatic pressures
Ranging from 9 mg/L near the poles to 4 mg/L near the equator in surface waters, ocean holds less oxygen than freshwater due to higher salinity.
Where does DO come from?
There are two main contributors of DO:
Atmospheric diffusion, which is slow and can be artificially amplified via surface turbulence
Photosynthesis, which is the major contributor to DO
Photosynthesis & Daily DO Fluctuation
Daily fluctuation of oxygen saturation occurs in open systems with presence of phytoplankton or plants. During daylight, production of oxygen by photosynthesis exceeds loss of oxygen via respiration and diffusion. This may lead to supersaturation of DO, and reaches concentration up to 10 mg/L or higher near surface waters. At night, photosynthesis stops and DO concentration reaches the lowest point right before day break. As the key biological activity, photosynthesis regulates dissolved oxygen, carbon dioxide, pH cycles and nitrogenous waste products.
How much oxygen does a fish need?
Warmwater species have a greater capacity to offload oxygen to tissues than coldwater species. Thus, coldwater species require higher DO concentration. According to Global Seafood Alliance, average oxygen consumption rates for adult fish ranges between 200 and 500 mg oxygen/kg fish/hour.
Regulated by the Water Quality Standard Acts, states in the US have differing minimum DO concentration requirements. For example, in Michigan, minimum DO required is 7 mg/L for cold-water fisheries and 5mg/L for warmwater species. In Colorado, “Class 1 Cold Water Aquatic Life” needs minimum 6 mg/L, and “Class 1 Warm Water Aquatic Life” requires at least 5 mg/L. For most farmed aquatic species, 3 mg/L is considered the bare minimum.
At a given temperature, depth and salinity of water, the amount of oxygen consumed by fish or shrimp is a function of its size, feeding rate and activity level.
Small fish uses more oxygen than larger fish due to higher metabolic rates. For example, 10g channel catfish were reported to use 1,050 mg oxygen/kg fish/hour, while 500g fish used only 480 mg oxygen/kg fish/hour. Oxygen consumption is heavily influenced by fish activities. An hour after feeding, channel catfish consumed 680 mg oxygen/kg/hour and drops to 380 mg/kg after overnight fasting. Tilapia forced to swim against a 60-cm/second current consumed oxygen twice as fast as those swimming against a 30-cm/second current (Global Seafood Alliance).
Consequences of Unusual DO Levels
Low DO levels can result lower reproduction frequencies and fish death. DO level < 6 mg/L prevent reproduction in salmon. At sea, coastal fish avoid and abandon areas with DO < 3.7 mg/L (Fondriest).
Supersaturation of DO (level sustained > 115%-120% air saturation) for extended time period can result in gas bubble disease and fish death.
Carbon Dioxide
Respiration and limestone bearing rocks contribute to carbon dioxide in water. Carbon dioxide is acidic and reacts with minerals to form bicarbonate in water (HCO3-). This directly impact pH levels in water (*pH will be covered in upcoming newsletters). Solubility of carbon dioxide increases with decreasing temperature in water.
Water bearing healthy fish population generally have < 5 mg/L of free carbon dioxide. Up to 10 - 60 mg/L of carbon dioxide can be tolerated in conditions where DO concentration is high.
In intensive pond fish culture, carbon dioxide levels may fluctuate between 0 mg/L in the afternoon to 15 mg/L at daybreak. In RAS systems, carbon dioxide concentration can easily topple 20 mg/L which impacts fish’s ability to uptake oxygen. High carbon dioxide concentration reduces the diffusion rate of CO2 into water via a fish’s gill and cause hypercapnia and acidosis (i.e. low pH) in fish.
Treatment with lime, calcium oxide or calcium hydroxide are common methods to remove free carbon dioxide in water. This can be done in two ways:
Input well or spring water from limestone bearing rocks and induce aeration to release excess gas
Add calcium carbonate (CaCO3) or sodium bicarbonate (Na2CO3).
References
Respiration in Fish | How do fish breathe underwater? | Oxygen - Solubility in Fresh and Sea Water vs. Temperature
The Fish Site - Water quality: a priority for successful aquaculture
The Fish Site - How to achieve good water quality management in aquaculture
The Fish Site - A Fish Farmer's Guide to Understanding Water Quality - Part 1
The Fish Site - A fish farmer's guide to understanding water quality - Part 2
The Fish Site - The Importance of Measuring Carbon Dioxide in Aquaculture
Global Seafood Alliance - Dissolved oxygen is a major concern in aquaculture. Here’s why.
Global Seafood Alliance - Dissolved oxygen requirements in aquatic animal respiration
Global Seafood Alliance - Carbon dioxide: Waste, nutrient
Lucas John S et al. Aquaculture : Farming Aquatic Animals and Plants. Third ed. Wiley Blackwell 2019
Fondriest Environmental, Inc. “Dissolved Oxygen” Fundamentals of Environmental Measurements. 19 Nov. 2013. Web.
Great summary, thanks for this.
High dissolved CO2 in salmon culture systems can result in nephrocalcinosis - calcium deposition in the kidney so always good practice to check levels frequently especially in RAS systems. 🐟