Richard C. Dugdale (1928–2026): The Passing of a Giant

For over 65 years, the study of the oceanic nitrogen cycle has been shaped by the contributions of Richard (Dick) Dugdale (Fig. 1). Part engineer, part tinkerer, part biologist, part modeler, Dick's imprint on the study of nitrogen has tentacles in methodologies, as well as in concepts and processes that have become mainstays of chemical and biological oceanography. After his early beginnings as a limnologist, the ocean called—first upwelling regions and later estuaries—and he never looked back. One of the backbone techniques for those who measure nitrogen processes is the application of the stable isotope 15N, either as a natural tracer or as an enrichment tracer. It was Dick's associations with his major professor Arthur Hasler and colleague Robert Burris, who had pioneered the use of 15N tracers in soils, that led to the first application of 15N in aquatic systems. The method was applied in Lake Mendota, Wisconsin, and unnamed lakes in Alaska, and demonstrated for the first time that N2 fixation occurs in open waters (Dugdale et al. 1959). With Dick's engineering mind and his love of instrument techniques, he and colleagues were able to develop the method to extract N2 on a vacuum line—a laborious technique at the time that was not for the faint of heart. His early work on applying 15N to the study of N2 fixation by Trichodesmium yielded the first direct measurements of N2 fixation by this organism, a process that was doubted by many at the time (Dugdale et al. 1961). The application of 15N in these early studies, after multiple cruises around Bermuda, led Dick, together with his colleague John Goering (with insight from John Ryther), to conceptualize that the different forms of nitrogen in the ocean, nitrate and ammonium, have different sources and fates. They described that nitrate, as the dominant oxidized form of nitrogen, represented “new” nitrogen to a system via upwelling or other allochthonous sources, whereas ammonium and urea represented the “regenerated” sources of nitrogen from pathways such as zooplankton excretion and bacterial remineralization (Dugdale and Goering 1967). Their reasoning was that nitrate is transported by mixing and upwelling and therefore provides new nitrogen to the surface waters. In contrast, ammonium, because of its source being predominantly in the euphotic zone, cannot enhance production, but can maintain it. This concept laid the foundation for development of the concept that the upward flux of nitrate is about equal to the downward export of organic matter, or export production (Eppley and Peterson 1979). Quantifying the “biological pump,” the export of carbon to the deep sea, has taken on enormous importance in understanding oceanic carbon cycling and ultimately how it may change with climate—or how it might be manipulated anthropogenically to reduce atmospheric CO2. The application of 15N to quantify the uptake of different forms of nitrogen brought out the biologist in Dick. Using his quantitative understanding of chemical reaction kinetics and biological enzyme kinetics, Dick conceptualized that the uptake of nitrogen should also follow a model analogous to Michaelis–Menten kinetics (Dugdale 1967). His interest in growth rates and limiting nutrients was pursued in a series of early chemostat studies and enzyme studies with his graduate students. While controversy inevitably developed with regard to whether uptake represented growth and the extent to which uptake and growth may be coupled or uncoupled, or even controlled by the same factors, there is no question that saturating response curves for nutrient uptake permeate to this day virtually all biological and biogeochemical models of nitrogen processing and algal growth. A few years after the publication of the new and regenerated concept and the applications of kinetic models to uptake rates, Dick, together with then colleague and spouse Janie MacIsaac, published a finding that would prove to be important throughout the remainder of his scientific career: the concept that nitrate uptake can be inhibited (repressed) by the uptake of ammonium (Dugdale and MacIsaac 1971; MacIsaac and Dugdale 1972). This finding was important because this process could put a limit on new production. This concept was an important area of physiological study by many throughout the 1990s and 2000s. Indeed, in his later years, while working on the problem of why productivity in the San Francisco Bay Estuary is comparatively low, the notion that nitrate uptake could be inhibited or repressed by ammonium discharged from a sewage treatment plant was hotly debated (Dugdale et al. 2007, 2012, 2013). Subsequent supporting data acquired using 15N tracer techniques contributed to the decision to build a new wastewater treatment plant in the upper San Francisco Bay Estuary with advanced effluent treatment. Dick was, always, a modeler at heart. Early applications of quantitative approaches involved slide rules and then punch cards and basic computers, but later models became much more sophisticated (Dugdale et al. 1989). With colleagues including Fei Chai, the Carbon, Silicate and Nitrogen Ecosystem (CoSiNE) model was developed (Dugdale et al. 2002) and has since been applied from the Pacific Basin to coastal California (Chai et al. 2007; Liu et al. 2018). Even in his final years (at age 96!), Dick was modeling productivity of San Francisco Bay Estuary (Dugdale et al. 2024). While nitrogen was Dick's primary biogeochemical love, silica shared space in his heart. Following his conceptual new and regenerated production model for nitrogen, Dick and colleagues opined on the “silica pump” (Dugdale et al. 1995). With interest developing in the mid-1990s of the factor (or factors) that may limit productivity in the high-nutrient-low chlorophyll (HNLC) regions of the oceans, Dugdale and Wilkerson (1991) described how the silica pump, in which silica is exported from the euphotic zone via diatom sinking, may lead to silica limitation. This may occur as the process of silica regeneration does not operate via the same pathways or on the same time scale as nitrogen regeneration (Dugdale and Wilkerson 1998). This notion also developed from the recognition that diatoms do biology differently and their productivity is central to the productivity of the sea. Dick was recognized as a global leader throughout his career. While balancing cruises and other research demands, Dick also served as President of ASLO, then named the American Society of Limnology and Oceanography, 1975–1976. He was the recipient of the ASLO Hutchinson medal in 1991 in recognition of his work on nutrient uptake kinetics and his introduction of the concept of “new” and “regenerated” primary production. Other honors include recognition as a Fellow of the American Geophysical Union, Fellow of the American Association for the Advancement of Science, Fellow of the California Academy of Sciences, and an Honoris Causa award from the University of Marseilles. Trying to capture Dick's 65-year career and his distinguished contributions in a short essay is near impossible. The hope is that current researchers and students recognize his broad shoulders on which we stand. The snippets described here provide only mere glimpses into the enormity of his contributions to biological and chemical oceanography. Dick provided his own summary of his accomplishments in an autobiography as his career was sunsetting (Dugdale 2018). He will be greatly missed by his extended family, as well as his enormous academic family. I have had the personal pleasure of working with Dick and his wife, Frances Wilkerson, on several projects and have benefitted enormously from the development of the 15N technique which I have applied over many years and in many systems. I have learned enormously from Dick's work, and I will miss his wisdom. I witnessed first-hand the controversies that emerged with his insights, ranging from debates about the role of nitrogen fixation in Trichodesmium to uptake vs growth kinetics, to the importance of silica limitation in HNLC regions, to the role of ammonium repression of nitrogen uptake as science understanding developed. I have no doubt that Dick, now in his final home, meeting once again with Janie MacIsaac (d. 1982) and Vera Alexander (d. 2023), will continue to discuss nutrients and growth regulation with former student Paul Harrison (d. 2016), defend his insights with colleagues such as Dick Barber (d. 2023), Dick Eppley (d. 2023), and Jim McCarthy (d. 2019), and debate the limiting factors of the San Francisco Bay estuary with Jim Cloern (d. 2025).