Once you’re familiar with the taxonomy and range of seabirds (check out a detailed introduction on seabirds here) it is important to understand why seabirds play such a crucial role in marine health and ecosystem function. Despite their small size and population abundance compared to the vastness of Earth’s oceans, seabirds play a crucial part in energy and nutrient transfer for marine systems and are contributors to the success of many marine habitats^[1]IOC World Bird List 12.1. (1969) doi:10.14344/IOC.ML.12.1.^[2]Seabirds. https://www.noaa.gov/seabirds..

What is Cross-Ecosystem Energy and Nutrient Transfer?

To understand how exactly seabirds are impacting the nutrient contents in the ocean, we need to understand cross-ecosystem transfer. Cross-ecosystem transfers refer to the movement of energy or nutrients across ecosystem boundaries^[3]Kowarik, C., Martin-Creuzburg, D. & Robinson, C. T. Cross-Ecosystem Linkages: Transfer of Polyunsaturated Fatty Acids From Streams to Riparian Spiders via Emergent Insects. Frontiers in Ecology … Continue reading. This movement is facilitated through three main vectors: (1) physical forces like wind and water, (2) biotic agents such as vertebrates, or (3) the movement of prey and consumers between habitats^[4]Gounand, I., Little, C. J., Harvey, E. & Altermatt, F. Cross-ecosystem carbon flows connecting ecosystems worldwide. Nat Commun 9, 4825 (2018).. The best way to understand these flows in an applied example is to observe food chains.

Figure 1: Trophic levels^[5]1.Reichle, D. E. Chapter 7 – Food chains and trophic level transfers. in The Global Carbon Cycle and Climate Change (ed. Reichle, D. E.) 95–117 (Elsevier, 2020). … Continue reading

As you move up the trophic levels of a food chain from the primary producers (i.e., organisms that make their food, like plants), to the tertiary or quaternary consumers (i.e., top predators in an environment) there is an exchange of energy and nutrients. However, with each level up on the food chain, only 10% of the energy is passed on.

This energy transfer occurs as follows:

1.  Primary producers like algae use sunlight to carry out photosynthesis and create organic compounds 
2. Many fish species consume algae, and through this consumption, receive only 10% of the available energy. The other 90% is lost to metabolic processes such as heat production. 
3. When a bird catches and eats the fish, it receives only 10% of the energy from the fish, (which as we know was only 10% of that from the algae)^[6]Wise, O. Trophic Levels Part 1: Why they Matter. Ocean Wise https://ocean.org/blog/trophic-levels-part-1/ (2021).. This limited energy transfer means that fewer organisms can be supported in higher trophic levels for a given ecosystem. 

Nutrient transfers parallel food chain energy transfers because as organisms are consumed, decomposed, or engage in biological activities such as excretion, there is a constant flow of nutrients between and within ecosystems. Ultimately the nutrients will cycle back to the primary producers and the system restarts. The vast extent of seabird habitats, coupled with the amount of guano produced, fish and other organisms eaten, and breeding extent means that these birds are heavily involved in marine food chains, and subsequently influential in energy and nutrient transfer systems^[7]Benkwitt, C. E., Carr, P., Wilson, S. K. & Graham, N. A. J. Seabird diversity and biomass enhance cross-ecosystem nutrient subsidies. Proceedings of the Royal Society B: Biological Sciences 289, … Continue reading.

Seabird Guano – Who gives a crap?

Guano, or bird poop, is a major source of nutrient input for marine systems. Due to its high contents of nitrogen, phosphate, and potassium, bird poop is considered a highly effective fertilizer that helps reinforce the success of marine plants and photosynthesizing organisms. The constant production of guano by birds aids in the fluctuation of nutrient levels and trophic organization. Although biomass and nutrients are eaten initially, waste from the birds restores them, creating a cyclical transfer of nutrients within an ecosystem^[8]Grant, M. L., Bond, A. L. & Lavers, J. L. The influence of seabirds on their breeding, roosting and nesting grounds: A systematic review and meta‐analysis. J Anim Ecol 91, 1266–1289 (2022).. When a bird produces guano, the nutrients (especially nitrogen) re-enter the ocean, and plant and phytoplankton growth is supported and increased. This increased growth in turn rejuvenates the food chain and increases the ability for carbon uptake and photosynthesis, enhances marine productivity, and makes seabirds significant facilitators of trophic variation in different ecosystems^[9]Parsons, M. et al. Seabirds as indicators of the marine environment. ICES Journal of Marine Science 65, 1520–1526 (2008).^[10]Hentati-Sundberg, J. et al. Fueling of a marine-terrestrial ecosystem by a major seabird colony. Sci Rep 10, 15455 (2020).. Because of its contribution, it is estimated that seabird guano creates a $1 billion industry each year as it provides the necessary nutrients to support fish, enabling the success of global fisheries and other resource extraction from the ocean^[11]Economic value of seabird poop exceeds $1 billion annually • Earth.com. https://www.earth.com/news/economic-value-of-seabird-poop-exceeds-1-billion-annually/.. 

Nutrient Impacts on Marine Ecosystems

Almost 300 species of birds reside in and around the arctic. The region acts as a major breeding ground due to coastal upwelling zones making food abundant. With large populations of birds nesting and breeding in the region, there is an influx of nitrogen into the water and sediments; however, studies on these environments have revealed that the influx of nitrogen from guano, coupled with the increase of sunlight in spring and summer months, leads to even higher levels of marine primary production^[12]Finne, E. A., Varpe, Ø., Durant, J. M., Gabrielsen, G. W. & Poste, A. E. Nutrient fluxes from an Arctic seabird colony to the adjacent coastal marine ecosystem. Polar Biol (2022) … Continue reading. Nitrogen, which is oftentimes a limiting nutrient (i.e. it occurs in the least quantity and therefore is responsible for restricting growth) is prevalent in bird zones^[13]What Is a Limiting Nutrient? https://chemtech-us.com/articles/what-is-a-limiting-nutrient/.. When those nutrients are coupled with 24-hour sunlight, the growth of plankton and algae are unrestricted^[14]Shatova, O., Wing, S. R., Gault-Ringold, M., Wing, L. & Hoffmann, L. J. Seabird guano enhances phytoplankton production in the Southern Ocean. Journal of Experimental Marine Biology and Ecology … Continue reading. Svalbard, a Norwegian archipelago in the arctic ocean is a key site for this increased primary production as it is a major breeding ground for arctic birds. The breeding season aligns with this increase in sunlight, and so arctic birds play a crucial role in kick-starting primary production in these areas^[15]Borboroglu, P. G. Penguin Health Equals Ocean Health | Smithsonian Ocean. https://ocean.si.edu/ocean-life/seabirds/penguin-health-equals-ocean-health..

In addition to their impacts in arctic waters, seabirds have significant ecosystem impacts in tropical waters, specifically on coral reefs. Guano-derived nitrogen is assimilated into corals to contribute to their nitrogen requirements. Nitrogen is a limiting nutrient in tropical waters as well, so this influx provides support for coral growth and aids other ecosystem processes such as fish population stability, and primary production^[16]15.  Lorrain, A. et al. Seabirds supply nitrogen to reef-building corals on remote Pacific islets. Sci Rep 7, 3721 (2017).. Without these populations of seabirds in tropical areas, many researchers hypothesize there would be a decline in the abundance of coral reefs due to the lack of extra nutrients in the water^[17]Seabird declines threatening coastal nutrient supply. https://www.nhm.ac.uk/discover/news/2022/april/seabird-declines-threatening-coastal-nutrient-supply.html.. Through this support, seabirds could be considered an ecosystem engineer level of structure for these reef systems, meaning that they play a huge role in the development and continuation of the ecosystem structure. In addition to supporting the development of coral reefs directly, seabirds have many indirect effects on the food chain. As nutrients enter the water and primary production increases, other plankton-eating animals (like small fish) increase in population size. Many species of small fish are crucial in maintaining the balance between plankton and coral populations^[18]17.  Graham, N. A. J. et al. Seabirds enhance coral reef productivity and functioning in the absence of invasive rats. Nature 559, 250–253 (2018).. They eat the plankton preventing it from growing rampant in these high-nutrient environments, and also supply energy for higher trophic levels. Overall, we’ve observed a very developed system in tropical water that oversees the input of nitrogen and phosphorus from birds, and the output of stronger coral reefs, more organismal diversity, and a better-balanced ecosystem^[19]Rowe, J. A., Litton, C. M., Lepczyk, C. A. & Popp, B. N. Impacts of Endangered Seabirds on Nutrient Cycling in Montane Forest Ecosystems of Hawai’i. Pacific Science 71, 495–509 (2017)..

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