Ecology and Ecohydrology of Florida's Springs and Spring-Fed Rivers
Florida's springs are an unparalleled natural resource. Florida boasts the highest density of 1st magnitude springs in the world (i.e., those with discharge > 2.8 cubic meters per second), and some of the most storied springs in the world (Silver, Wakulla, Rainbow, Ichetucknee). My lab group became interested in springs several years ago, in part because of the pervasive and alarming ecological decline that has been documented there. As we've made progress understanding these systems, their environmental properties (unparalleled thermal, discharge, and chemical stability; extraordinarily clear water) make them absolutely incredible model systems, a fact reflected in their use in H.T. Odum's early work on ecosystem metabolism and trophic structure. They are pretty extraordinary places to visit, and we consider ourselves enormously fortunate to be able to call our time spent there "work". We've also been lucky to have three National Science Foundation grants made to support our work in springs.
Our work in springs really has three major foci. The first is on nutrient dynamics in rivers. Because of their optical, chemical and hydrologic properties, as well as high rates of benthic primary production, springs allow insight into the inner workings of lotic ecosystems that can't readily be replicated in systems with flashier hydraulics and less well constrained mass inputs. To be clear, these are large rivers that emerge, disequilbrated with surface ecological processes, directly from a deep aquifer where the water has been sitting for an average of 20 years. Our work has focused on nitrogen dynamics, taking advantage of a new breed of nitrate sensor based on UV optical absorbance, as well as new methods for assessing the natural abundances of N and O isotopes in nitrate. The sensor has made possible a major advance in the temporal resolution of measurements (ca. 1 per 2 seconds) that has allowed insight in diel variation. From that diel variation, we've extracted whole-ecosystem N metabolism estimates that correspond well with estimates of autotrophic N demand from diel dissolved oxygen measurements and presumed autotrophic stoichiometry. As we've refined that method, it has revealed all manner of novel insights, ranging from direct quantification of denitrification and assimilation to the marked and previously undescribed day-to-day coupling of GPP and denitrification. We've recently begun using the sensors in springs to understand the geomorphic controls on denitrification and assimilation, the role of changing primary producer composition on whole-ecosystem stoichiometry, and the longer term controls on N removal (we now have nearly 18 months of mostly continuous nitrate data in the Ichetucknee River. Further, we can use the sensors on a "Lagrangian" rather than "Eulerian" way, towing the sensor in the water in a boat, gathering highly spatially resolved information about longitudinal nitate dynamics from which we hope to accurately disaggregate benthic removal spatially. Finally, we have begun testing on the use of the nitrate sensor for open-channel eddy correlation based estimates of removal that offer a unique alternative to chamber based methods for understanding fine scale flux rates. Among the many outcomes of this particular line of inference is a conclusion that these spring rivers are not nitrogen limited, and that the widespread contention that links algal proliferation with nitrate enrichment actually has surprisingly little empirical support. More on that below.
A second focus of our work in springs is on ecohydraulics. Flow is these systems is clearly modified by the dense vascular plant beds that persist in healthy systems, and also by the enormous algal mats that now dominate in some degraded springs. We have used pulse injection conservative tracer tests to quantify the hydraulic properties of these rivers, and to understand the locations and magnitude of water storage. This information is crucial to understand the capacity of spring rivers to attenuate nutrients. It also suggests a potential autogenic feedback between primary production and channel morphology. These spring-fed rivers have a calcium carbonate bed, and interactions between rooted plants (and organic matter respiration resulting from them) and the carbonate matrix may be reciprocal. The tantalizing notion is that the plants are in some fundemantal way responsible for the shape and evolution of the river-bed landform.
Our third area of focus is on spring-river food webs. We have mounting evidence to suggest that algal proliferation across the population of springs has something to do with grazers, particulary gastropods. Ongoing research on this question has revealed fairly consistent evidence of thresholds in the relationship between algal biomass and grazer density; in other words, the relationship appears to be strongly non-linear. The implication is that there may be alternative stable states operating in these systems wherein a mostly algal-free state is replaced by one with abundant algae in response to grazer stress (press or pulse) and that that state can persist in time, even if the stressor is reduced. From a theoretical perspective, we hypothesize that these rivers are particularly top-dowhn regulated (i.e., grazers, not nutrients, control algal accumulation) because of the remarkable environmental stability that characterizes the springs. Working with faculty in Geological Sciences, we have begun to explore the possibility that one source of grazer stress is dissolved oxygen, variation of which occurs in response the "age" of the water, which is, in turn, a function of both climatic variation (older during dry periods) and the ongoing utilization of the Upper Floridan Aquifer (the source of most of Florida's springs) for anthropogenic uses.
