Transforming excess nutrients in eutrophic coastal waters to marine protein for feeds

by Daniel Taylor and Jens Kjerulf Petersen, Danish Shellfish Centre, DTU Aqua, Denmark

Eutrophication and the struggle to control nutrient loss
Human population density and utilisation of lands for cultivating foods have intensified in coastal regions over the past century. This intensification has dramatically affected the biological and chemical processes in coastal ecosystems through an increase in the flow of nutrients, principally nitrogen and phosphorus, from the land and atmosphere to the sea.
 

Essentially, the more that the land is fed (fertilised) and the more we feed ourselves (food plus fuels), the more we feed coastal waters with nutrients caused by agricultural runoff, wastewater, and atmospheric deposition.

As coastal waters become over-enriched, biological functioning changes with often long-term consequences. One of many symptoms is the increased growth and concentration of phytoplankton (single celled plants).

Enriched waters can become so productive with phytoplankton growth that sunlight will not sufficiently reach depths to support aquatic plant life on the sea floor and valuable plant habitats like seagrass meadows are lost. Further increases in phytoplankton concentrations can lead to oxygen depletion, when dead phytoplankton cells decay on the sea bed.

For the past few decades, the improvement of coastal water quality has been a locus of policy development in many regions around the world. There have been impressive achievements in the implementation of these policies, notably in the improvement of wastewater treatment.

Nevertheless, many coastal water bodies, such as those in Northern Europe, including the Baltic proper, are still considered heavily affected by excess nutrient run-off and will likely continue to be so for years to come.

Nitrogen inputs into coastal waters originate from point sources (eg water treatment plants, fish farms), non-point/diffuse sources (eg agricultural lands, groundwater discharge), or atmospheric (eg volatilised ammonia or combustion by-product absorption).

Following modifications and improvements in water quality management programs, diffuse sources of nutrients are the most significant. Treatment methods designed to minimise nutrient introduction into coastal waters are abundant in implementation.

Classic examples of such treatment methods include restrictions in use of fertilisers, constructed wetlands, settling ponds, vegetative riparian buffer zones; and more recently, systems of ‘precision agriculture’.

Although much progress has been made in reducing nutrient flows into coastal waters, the efficiency of further implementation rapidly diminishes and are also often more expensive to implement.

Furthermore, decades of enrichment has a legacy of enhanced enrichment of sea floor sediments, which will be a persistent source of nutrients by multiple processes (termed ‘internal loading’), and can only be mitigated within the aquatic environment.

Read more HERE.

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