Beavers build dams that create wetlands. Elephants forage and stomp to create grasslands out of former forests. Grizzly bears act as ambassadors between the sea and land by transferring nutrients from the ocean to forest floors through their consumption of salmon. Charles Darwin devoted an entire book to describing the ways in which earthworms maintain soil fertility. These examples only begin to illustrate the magnitude of life’s engineering impacts on the environment.
In the early twentieth century, oceanographer Alfred Redfield observed a pattern that caused him to suggest that the abundance of major nutrients in the ocean is governed by minuscule organisms that inhabit the uppermost layers of the water column. In other words, if the ocean were devoid of these primary producers, its nutrient content would be dramatically different. These phytoplankton photosynthesize in the well-lit euphotic zone of the ocean and then sink into the deep ocean where their organic content is remineralized into inorganic nutrients through respiration. The amount of organic material that sinks below the euphotic zone is referred to as export production.
Redfield studied the composition of thousands of marine biomass samples and discovered that, on average, these phytoplankton seemed to incorporate elements such as carbon, nitrogen, and phosphorus into organic matter in predictable ratios. Furthermore, the elemental composition of the phytoplankton reflected the distribution of these elements dissolved in the deep ocean. The stoichiometric formula for the composition of phytoplankton can be expressed as:
106 CO2 + 16 HNO3 + H3PO4 + 78 H2O ↔ C106H175O42N16P + 150 O2
(Anderson and Sarmiento, 1994)
When the phytoplankton sink to greater depths, bacterial organisms process the organic content on the left side of the formula above (C106H175O42N16P) and convert it to inorganic molecules such as carbon dioxide (CO2), nitrate (NO3–), and phosphate (PO43-). This transformation of organic to inorganic matter, which is known as remineralization, led Redfield to conclude that the ratios of elements in phytoplankton can set the ratio of dissolved nutrients in the sea. For example, the nitrogen: phosphorus (N:P) ratio of 16:1 in phytoplankton explains the approximately 16:1 ratio of dissolved nitrate to phosphate in the ocean. These stoichiometric patterns came to be known as the Redfield ratios and provide the foundation for several of the most important assumptions used in ocean biogeochemistry.
Various tenets and calculations employed in ocean biochemistry operate on the assumption that Redfield ratios in the modern ocean stay unchanging. However, research done by Markus Pahlow and Ulf Riebesell suggests that this may no longer be the case. In their paper Temporal Trends in Deep Ocean Redfield Ratios (2000), they discuss the possibility that Redfield ratios in the North Atlantic and Pacific Oceans are fluctuating in response to anthropogenic-driven increases in nutrient inputs.
Anthropogenic emissions can rework the concentration of nutrients available for uptake by phytoplankton in the surface oceans. Because phytoplankton can alter their composition in response to spikes or depressions in the abundance of available nutrients, sustained human inputs have the potential to significantly change the composition of marine phytoplankton and, subsequently, the surface ocean Redfield ratio. Through remineralization, these differences would transfer to the deep ocean and correspondingly change deep-water, or oxidative, Redfield ratios.
In their study, Pahlow and Reibesell compiled and compared deep-ocean oxygen and nutrient data from the National Oceanographic Data Center (NODC SD2) and the World Ocean Circulation Experiment (WOCE). Their data set encompassed 1173 stations at 447 locations and spanned the years 1947 to 1994. They looked at 2000 m vertical profiles of apparent oxygen utilization (AOU), phosphate, and nitrate concentrations, measured at fine spatial and temporal scales (1° longitude and latitude and within five years apart) to ensure comparability. Apparent oxygen utilization refers to the difference between observed dissolved oxygen concentrations and dissolved oxygen concentrations under ideal, saturated conditions. Because microbes consume oygen as a reactant for respiration, AOU serves as an indicator of the amount of respiration that has occurred, and correspondingly, the amount of nitrate or phosphate that has formed. In other words, higher AOU values signify higher rates of respiration and remineralization.
Observed changes in AOU and nutrient concentrations can result from numerous factors, including changes in remineralization, ocean circulation, or levels of preformed (non-oxidized) nutrients, as well as measurement errors. In order to examine remineralization effects in isolation, the scientists disregarded all other factors. They restricted their data set to water masses with matching temperature and salinity profiles over at least a 2000 m depth range, excluded data deemed questionable in the original data sets, and removed data from profiles that exhibited large random variation. They also standardized measurements from different data sets by correcting for differences at depths below the comparison depth range. From their compiled data set, Pahlow and Reibessel calculated average relative trends for AOU, and derived ratios of C:N, C:P, and N:P from AOU:N and AOU:P.
In the North Atlantic, the scientists observed ratio trends that are consistent with a shift from N to P limitation, an occurrence that can be explained by the coupling of increased anthropogenic emissions of nitrous oxide and accelerated deep-water renewal. Although these changes should have been accompanied by increased AOU, which was not witnessed in the North Atlantic, the researchers note that enhanced horizontal advection could have offset increasing AOU. Other processes are plausible as well; for example, diminished export production would produce similar effects as accelerated deep-water formation.
Pahlow and Reibesell show that increased deposition of atmospheric nitrogen can feasibly explain the observed N:P increase in the North Atlantic deep ocean. They calculated that a 1.9 ± 1.2 per mil per year increase in deep-water N:P ratio would correspond to additional nitrogen inputs of 21 ± 13 mmol m-2 yr-1. This falls within the estimated range of 10 to 47 mmol m-2 yr-1 of atmospheric nitrogen deposition over the North Atlantic. Since most of this aeolian nitrogen comes from human emissions, increases in anthropogenic inputs through agriculture, combustion, and industrial processes provide a cogent explanation for oxidative N:P increase.
In the Pacific, Pahlow and Reibesell observed an increase in AOU. Unlike the North Atlantic, much of the Pacific is limited by iron, not nitrogen. Anthropogenic activities that amplify the amount of bioavailable iron could support higher rates of carbon export that correspond with the observed AOU trend in the North Pacific. These include activities that lead to desertification, such as over-cultivation of desert lands. Desertification in central Asia serves as a source of iron into the North Pacific, as desert dust contains an appreciable amount of iron.
Additionally, because atmospheric sulfur content is positively linked to iron solubility, human emissions of sulfur into the atmosphere increase the bioavailability of atmospheric iron. Large inputs of iron, a limiting nutrient, would boost the amount of photosynthetic activity and production of organic content. In turn, larger fluxes of organic material into the deep ocean would support higher oxidation, or respiration, rates that would augment AOU levels. This positive trend in AOU has larger consequences, including changes in carbon sequestration. Pahlow and Reibesell estimated regenerated C in the North Pacific to equal roughly 300 x 109 metric tons. A 0.7 per mil per year increase in AOU would sequester an additional 0.2 x 109 metric tons per year.
Ultimately, Pahlow and Reibesell describe and provide a convincing argument for the possibility of evolving Redfield ratios in the deep ocean. Redfield ratios support the notion that organisms can determine the properties of their surroundings. In the oceans, phytoplankton set nutrient ratios that match the composition of their organic material. However, the relationship between phytoplankton and nutrient levels is dynamic. Phytoplankton can alter their behavior in response to fluctuations in nutrient levels, particularly for nutrients that are otherwise limiting in a system. In this study, Pahlow and Reibesell show trends in oxidative Redfield ratios that potentially illustrate a biogeochemical response to increased anthropogenic emissions over the past five decades.
While inconsistencies in oxidative ratios have previously been written off as analytical errors, the results of this study suggest that these deep-water ratios might warrant closer inspection. Given that Redfield ratios serve as the basis of countless biogeochemical assumptions, the implications of this study are profound. The conclusion that Redfield ratios in the modern deep sea might not be fixed and can instead show temporal trends does not, however, disprove that organisms can govern the traits of their environment. Rather, it provides another story of how humans are in many ways the ultimate ecosystem engineers – oftentimes overwhelming the natural processes that govern the properties of ecosystems at rates that don’t allow these systems to adjust. In this case, human-driven increases in atmospheric nitrogen and iron manifest themselves in changing Redfield ratios. Furthermore, these trends suggest that the biological cycles of elements such as marine carbon are currently not in steady state. This, in turn, has great implications for the role that the ocean plays in climate regulation.
M. Pahlow and U Riebesell, Temporal Trends in Deep Ocean Redfield Ratios, Science 287, 831-33 (2000).