Rethinking Darwin's coral reef paradox and the ubiquity of "marine oases"

Coral reefs are frequently described as "oases in marine deserts" for thriving in nutrient-depleted oceans.^1,2,3,4 This contrast is often termed "Darwin's paradox,"^5,6,7,8 which allegedly originates from Charles Darwin's coral reef work.⁹ Decades of research exploring these paradoxical dynamics led to groundbreaking findings in ecophysiology,^5,10 ecology,^2,11 oceanography, and biogeochemistry.^4,12,13 However, the historical foundations and scientific generality of the paradox remain unevaluated. Here, we demonstrate that Darwin's paradox is a misnomer. Indeed, the fundamental knowledge required to formulate this idea did not exist at the time of Darwin's 1842 coral reef treatise: its earliest references date, instead, to the 1940s-1950s. Given the frequent use of the term, and the "marine oasis" analogy, which implies high productivity despite nutrient-poor waters as a hallmark of reefs worldwide, we (1) compare reef productivity across ecosystems and (2) assess how globally widespread oligotrophic, low-nutrient reef conditions are. Our findings support earlier work placing coral reefs among the most productive ecosystems on Earth. However, relatively few coral reefs exist in oligotrophic waters; 80% of them occur in more productive mesotrophic and eutrophic conditions. Globally, median chlorophyll-a near reefs is 0.19 mg m^-3, twice the median for tropical oceans and double the upper oligotrophic threshold. Reefs range across the tropical ocean spectrum of phosphate, nitrate, iron, and silicate concentrations but are disproportionally common in moderate levels of these vital nutrients. Thus, coral reefs as oases in marine deserts are not the norm, highlighting the need to recognize environmental variability and both intrinsic and extrinsic pathways sustaining reef productivity.