Costa Rica Case Study: Overview

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Article by Jacqueline Boivin and Dr. Tayler Clarke.


  • Tayler Clarke (University of Costa Rica)
  • Christian Birkel (University of Costa Rica)
  • Ingo Wehrtmann (University of Costa Rica)
  • Astrid Sánchez (University of Costa Rica)


The Costa Rica case study aims to take a closer look at the Gulf of Nicoya (Golfo de Nicoya on the Nicoya Peninsula). Due to the enclosed nature of this gulf, it is susceptible to contamination from various sources, especially those that arrive by river runoff. These sources include, industrial, urban, agriculture and aquatic transport1

In recent years, the ability of local fishermen to catch an adequate amount of fish has been severely impacted. This is concerning given that 11,000 artisanal fisherman and their families live in this area and provide 65% of small scale fishery landings between 1995 and 20052,3.

According to Costa Rica case study researcher Tayler Clarke from the University of Costa Rica4, one particularly damaging factor is contamination from fertilizer. This fertilizer originates from farm lands and runs downstream in rivers and into the gulf. The impact of this fertilizer is that it leads to algae blooms and these lead to “coastal hypoxia” which causes coastal dead zones. These dead zones are caused from lack of oxygen which impacts the survival of fish and other species. This in turn negatively impacts fisheries resources, local economies and fisheries food security.

Fig 1. Algae. Image Credit: Alexey Stiop, Adobe Stock

The urgency of this situation is compounded by a range of different factors that are coming together to cut off oxygen to the fish that reside in the bottom layers of the estuary.

According to Clarke, more fertilizer is being used due to the reduction in farmlands square footage, and this is what is projected to happen under the climate mitigation plans for 2050. This fertilizer then runs downstream into the gulf, which is enclosed in nature. Along with ocean layers mixing, and the fertilizer acting as a food source, massive algae blooms appear. These blooms start to draw oxygen from the bottom layers of the estuary, and the bacteria that consume the algae also take oxygen away from the fish on the bottom layers. There is also a lack of mixing that causes the oxygen to decline in the bottom.

Dr. Clarke states, “so the combination of the lack of mixing and the oxygen consumption at night and the bacteria just eating all the algae makes it so there’s less oxygen for all the fish in the bottom. And so that’s why there’s lots of coastal dead zones like you hear about in the Gulf of Mexico.”

The other contributing factor is climate change, which has forced lutjanids, also known as snappers, to head for cooler and deeper waters. And this has been noticed by the local fisherman.

Fig 2. Graphical abstract from

The case study leads, such as Tayler Clarke, Ingo Wehrtmann, Astrid Sánchez and Christian Birkel from the University of Costa Rica, plan to work with the local fishing community to determine what a desirable future looks like in this gulf. An interdisciplinary team has been assembled which includes hydrologists, marine biologists, social scientists and nutrition experts.

They plan to work closely with the local fishing communities to co-develop future social economic and policy scenarios. The plan is to create models that will help outline future projections, such as desirable future outlooks with options of how to get there. They will also work with experts on the other case studies, such as the Netherlands case study, where they will learn more about nature inclusive and circular farming.