Water Column Interactions

Image credit: Olivia Johnson

The water column is the part of the ocean that extends from the surface to the seabed. This may also be referred to as the ‘pelagic zone’. The oceanic water column is the largest ecosystem on the planet.

Salmon aquaculture

Salmon aquaculture provides nutrient inputs directly into the water column via the release of dissolved and particulate waste products such as uneaten feed, fish waste, and metabolic by-products. These nutrients inputs can affect the water column in a variety of ways, with changes in water quality potentially flowing into shifts in biological communities.

Nutrient enrichment

Nutrients are essential for marine life and increased nutrients can generally be assimilated by the water column ecosystem without negative effects. However, high nutrient levels can lead to nutrient enrichment, which can cause excessive growth of algae (a process known as eutrophication).

Over time, this can disrupt the balance of local ecosystems, reducing water quality and affecting marine food webs.

Changes in water quality

In certain cases, waste inputs from salmon aquaculture may lead to reduced oxygen levels in the water column due to bacteria using up oxygen while breaking down organic matter.

Salmon aquaculture can result in increased turbidity (cloudiness) of the water, which can limit sunlight penetration and affect the growth of algae.

Effects on marine life

Changes in water chemistry and nutrient levels can affect the types of algae and other microorganisms in the water. For example, a rise in nitrogen levels can cause shifts in the plankton community, which can affect the entire marine food chain.

While aquaculture can contribute to local productivity by increasing food availability for some species, this is only beneficial up to a point. Excessive nutrient enrichment can lead to algal blooms, which may increase turbidity. To date, aquaculture activities in Tasmania have not been associated with any harmful or nuisance algal blooms (algal blooms that have negative ecological effects through reduction of oxygen or production of toxins).

These interactions mainly occur on a local (i.e. farm) scale and can vary considerably depending upon farming practices and the environment in which aquaculture occurs. More exposed environments with greater water movement experience greater dilution of nutrients.

Monitoring and Management Implications

To ensure sustainable aquaculture, our research points toward the following approaches to monitor and manage water quality:

Site Selection: choosing appropriate locations for farms is crucial. Farms in well-flushed, open areas where water currents can disperse waste are less likely to cause localised pollution.
Monitoring Programs: regular water quality monitoring and compliance with trigger levels are requirements for aquaculture operations in Tasmania. Monitoring helps detect early signs of environmental stress, allowing for timely intervention. This involves checking water quality parameters like dissolved oxygen, nutrient levels, and primary productivity.
Regulations and Management: in Tasmania, regulations are in place to limit the intensity of aquaculture operations and the amount of waste that can be released into the water column. Other management practices include optimising feed to reduce waste, managing farm layout, and using technologies such as aeration and integrated multi-trophic aquaculture to improve water quality.

Looking Ahead

The water column is a highly dynamic ecosystem, and modelling and field-based technologies are continually developing to better predict and capture the responses of this ecosystem to aquaculture. Shifts in water column health can have wide-reaching effects in the marine environment. As a result, understanding and managing aquaculture interactions with the water column can contribute to good water quality and the health of interconnected marine ecosystems.