Seaweed to the Rescue
How Aquatic Plants Can Clean Up Shrimp Farming
A popular science article
Introduction
In an era where the global appetite for seafood continues to grow, shrimp farming has emerged as a crucial industry to meet demand. However, this boom comes with a significant environmental cost: nutrient-rich wastewater that can pollute coastal waters and harm aquatic ecosystems. Imagine the waste produced by a large farm—uneaten feed, shrimp feces, and metabolic byproducts—all washing into the environment.
Did You Know?
This wastewater is loaded with excess nitrogen and phosphorus, which can cause algal blooms and create oxygen-depleted "dead zones" where marine life cannot survive 1 7 .
But what if the solution to this problem has been growing in the ocean all along? Recent scientific investigations are turning to a natural and sustainable ally—seaweeds—to biodegrade shrimp farm wastes and usher in a new era of cleaner aquaculture.
The Problem with Shrimp Farm Wastewater
Shrimp farming, particularly in its intensive form, generates a considerable amount of pollution. The primary sources of this waste are:
Excess Feed and Fertilizers
Shrimp feed is high in protein, but as much as 80% of the nitrogen and phosphorus from the feed is not retained by the shrimp. Instead, it dissolves in the water or settles at the pond bottom as organic waste 7 . Fertilizers used to promote algae growth further add to the nutrient load.
Metabolic Wastes
Shrimp naturally excrete toxic nitrogenous substances, with ammonia being the most harmful. In high concentrations, ammonia can damage shrimp gills, increase susceptibility to disease, and, when discharged, severely degrade the quality of the receiving water bodies 7 .
This combination of pollutants increases the biological and chemical oxygen demand of the water, threatening not only the farmed shrimp but also the surrounding environment 1 7 . Addressing this issue is critical for the sustainability of the industry.
Seaweeds: Nature's Water Purifiers
Seaweeds, or macroalgae, are not just passive inhabitants of the ocean; they are powerful biofilters. They absorb dissolved inorganic nutrients like ammonia, nitrates, and phosphates directly from the water, using them for their own growth 8 . This natural ability makes them ideal candidates for cleaning aquaculture effluent.
By integrating seaweed into shrimp farming, a process known as Integrated Multi-Trophic Aquaculture (IMTA), the waste from one species (shrimp) becomes a valuable resource for another (seaweed) 8 . This creates a more circular and sustainable system where waste is reduced, and a new, marketable crop is produced.
Traditional System
Linear model: Inputs → Shrimp → Waste Output
IMTA System
Circular model: Inputs → Shrimp → Seaweed (uses waste) → Multiple Outputs
A Closer Look: The UNH IMTA Experiment
A compelling experiment conducted by researchers at the University of New Hampshire (UNH) provides a clear example of how this integration works in practice 2 5 .
Methodology: Shrimp, Seaweed, and Oysters Unite
The research team set up a closed recirculating system to test the effectiveness of different combinations of species in reducing nutrient levels. They introduced Pacific white shrimp into tanks and then studied three different treatment setups:
Setup 1
Shrimp with Seaweed
Non-native red seaweed was added to absorb nitrogen.
Setup 2
Shrimp with Seaweed and Oxygenator
This setup tested if increased oxygen changed the dynamic.
Setup 3
Shrimp with Seaweed and Oysters
Native oysters, which are natural water filterers, were introduced alongside the seaweed.
The researchers then monitored the levels of various nitrogen compounds—ammonia, nitrite, and nitrate—in the water over a 30-day period 2 5 .
Results and Analysis: A Clear Winner Emerges
The results were striking. The treatment combining shrimp, seaweed, and oysters demonstrated the most significant reduction in nitrogen levels over time 5 . The oysters helped control the production of nitrogen, while the seaweed absorbed and stored it, creating a synergistic effect that neither could achieve alone.
Economic Benefits
This integrated approach not only cleans the water but also offers economic benefits for farmers. The seaweed and oysters can be harvested and sold, providing additional revenue streams and making the overall operation more profitable and sustainable 2 . Michael Chambers, a research professor at UNH, highlighted the potential, noting that shrimp could be grown sustainably in controlled environments "inside a barn, greenhouse or even a basement to provide fresh seafood to local restaurants at a premium price" 2 .
Beyond Seaweed: Other Promising Bio-Remediators
The principle of IMTA is flexible, and scientists are exploring other potent duos for wastewater cleanup. Another significant study investigated the combined culture of polychaete worms and halophytes (salt-tolerant plants) for treating effluent from a shrimp recirculating aquaculture system (RAS) 4 .
The researchers tested two system designs: one where the worms and plants were cultured together in a single tank, and another where they were kept in separate tanks. The results showed that the single polyculture tank with the ragworm Hediste diversicolor and the halophyte Salicornia ramosissima (saltwort) achieved excellent bioremediation, reducing particulate organic matter by 74-87% and dissolved inorganic nutrients by 56-65% 4 . This design also had the advantage of requiring half the operational area of the separated system, making it more efficient to implement.
| Parameter Remediated | Reduction Efficiency | Key Extractive Organism |
|---|---|---|
| Particulate Organic Matter (POM) | 74% - 87% | Polychaete Worm (Hediste diversicolor) |
| Dissolved Inorganic Nitrogen (DIN) | 56% - 64% | Halophyte (Salicornia ramosissima) |
| Dissolved Inorganic Phosphorus (DIP) | 60% - 65% | Halophyte (Salicornia ramosissima) |
74-87%
Reduction in Particulate Organic Matter
56-64%
Reduction in Dissolved Inorganic Nitrogen
60-65%
Reduction in Dissolved Inorganic Phosphorus
The Scientist's Toolkit: Key Components for IMTA Research
Setting up an IMTA experiment requires specific biological and technical components. Below is a list of essential "research reagents" and their functions in studying the biodegradation of shrimp farm waste.
| Component | Type/Example | Primary Function in the System |
|---|---|---|
| Fed Species | Pacific White Shrimp | The primary crop; its metabolic waste and uneaten feed create the nutrient-rich effluent to be treated. |
| Macroalgae | Red Seaweed, Gracilaria, Ulva | Absorbs and stores dissolved inorganic nutrients (ammonia, nitrate, phosphate) from the water. |
| Filter-Feeder | Native Oysters | Filters particulate organic matter from the water, complementing the action of the seaweed. |
| Deposit-Feeder | Polychaete Worms (Hediste diversicolor) | Consumes solid waste and uneaten feed from the bottom of the tank or pond. |
| Halophyte | Salicornia ramosissima | Salt-tolerant plant that absorbs dissolved inorganic nutrients from the effluent water. |
| Recirculating System | Tanks, Pumps, Filters | Creates a controlled, closed-loop environment to grow species together and monitor water quality. |
Biological Components
These living organisms work together to create a balanced ecosystem:
- Shrimp (fed species)
- Seaweed (nutrient absorber)
- Oysters (filter feeders)
- Polychaete worms (deposit feeders)
- Halophytes (salt-tolerant plants)
Technical Components
The infrastructure needed to create and maintain the IMTA system:
- Recirculating tanks
- Water pumps and filters
- Aeration systems
- Monitoring equipment
- Water quality testing kits
Conclusion: A Sustainable Future for Aquaculture
The evidence is clear: using seaweeds and other extractive species to biodegrade shrimp farm waste is a powerful and promising solution.
Economic Value
The research from UNH and others demonstrates that we can not only mitigate the environmental impact of shrimp farming but also create additional value and increase profitability for farmers.
Sustainable Future
As these IMTA technologies continue to be refined and adopted, they pave the way for an aquaculture industry that is not just a source of food, but a responsible steward of the aquatic environment.
The humble seaweed, therefore, is more than just a plant; it is a key to unlocking a more sustainable and harmonious relationship with our oceans.