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Nitrogen often limits how much life the ocean can support, but a small group of microbes can pull nitrogen gas straight out of the water and convert it into a form other organisms can use. Most research has focused on the sunlight-dependent version of this process; we know far less about microbes that can do it in the dark.

I worked on a modeling study of the Gulf of Aqaba, in the northern Red Sea, to figure out how much these different nitrogen-fixing microbes — light-dependent and light-independent — matter to the local nutrient balance. We built simulations of the region’s chemistry and compared them against real measurements throughout the water column. A model without any nitrogen fixation matched near-surface chlorophyll but got the deeper nutrient chemistry wrong. Only a model including both light-dependent and light-independent nitrogen fixers reproduced the full pattern — evidence that nitrogen fixation happening in the dark is a real, necessary part of this ecosystem’s nutrient budget, not a minor detail.

Every spring, huge blooms of phytoplankton erupt across the North Atlantic and other mid- and high-latitude oceans, feeding the marine food web and helping pull carbon into the deep ocean. Scientists have long disagreed about what triggers them: one camp argues it’s driven by light, as the surface ocean layer shallows in spring; another argues it’s driven by predators, when there are simply too few grazers left to keep pace with phytoplankton growth.

I built and tuned a simple ocean ecosystem model to test both ideas directly, running controlled experiments that isolated each mechanism. The result: neither explanation fully works alone. Both light and grazing pressure matter, and what actually triggers a bloom depends on the interplay between them, a more nuanced answer than either camp’s original theory.

Ocean temperature shapes marine life in ways both obvious and subtle — it affects metabolism, survival, and ultimately how many fish, invertebrates, and other organisms a stretch of ocean can support. Understanding that relationship is key to predicting how ecosystems respond as the ocean keeps warming.

Early in my career, I studied how temperature shapes shallow-water marine life around Isabela and Fernandina, the westernmost Galápagos islands. This region is unusual: strong upwelling brings cold, nutrient-rich water to the surface, supporting cold-water species found nowhere else nearby, including penguins and flightless cormorants. I found that different parts of the region respond to temperature differently — communities in a deep channel were most sensitive to vertical temperature differences, while communities nearer the equatorial current responded more to seasonal swings.

Later, with colleagues, I looked at how temperature and fishing pressure together affected two economically important species, sea cucumber and spiny lobster. After accounting for fishing itself, we found warmer sea temperatures were followed, a year or two later, by higher catches of both — most visibly after the strong 1997–98 El Niño (Defeo et al., 2013).

Defeo, O., Castrejón, M., Ortega, L., Kuhn, A., Gutiérrez, N., & Castilla, J. C. (2013). Impacts of Climate Variability on Latin American Small-scale Fisheries. Ecology and Society, 18(4).
Edgar, G. J., Banks, S., Fariña, J. M., Calvopiña, M., & Martínez, C. (2004). Regional biogeography of shallow reef fish and macro‐invertebrate communities in the Galapagos archipelago. Journal of Biogeography, 31(7), 1107-1124.
Kuhn, A.M. (2010). Influencia de la temperatura del mar sobre comunidades rocosas submareales de la Reserva Marina de Galapagos. Tesis para la obtencion del titulo de Licenciatura en Oceanografia. Escuela Politecnica del Litoral. Guayaquil – Ecuador.