Highlight
University of Florida students and faculty researching Florida springs suggest alternative explanations for the algal bloom / nitrogen-enrichment hypothesis
Figure 1. Healthy stand of submerged aquatic vegetation in Weeki Wachee River.
Achievement/Results
A team of researchers from the University of Florida’s IGERT in Adaptive Management, a National Science Foundation supported Integrative Graduate Education and Research Training (IGERT) program is learning that declines in the biotic integrity of Florida’s springs have no simple cause and effect relationship. Geologists estimate that there are more than 700 springs in Florida, representing the largest concentration of freshwater springs on Earth. Over the past three decades many of Florida’s springs have experienced significant declines in biotic integrity as exhibited by marked decreases in cover of submerged aquatic vegetation (Fig. 1) and increases in filamentous algae (Fig. 2). Coinciding with these changes were changes in water chemistry, particularly nitrate concentrations, which has lead many scientists and resource managers to blame the changes on the increased nitrate concentrations in the discharging aquifer waters. However, UF scientists and students, working as an interdisciplinary team of ecologists, engineers and geologists have other ideas.
Three IGERT trainees, Marie Kurz (Geological Sciences), Dina Liebowitz (School of Natural Resources and Environment), and Sean King (Environmental Engineering Sciences), along with their faculty advisors, Jonathan Martin (Co-PI of the IGERT program), Matthew Cohen, and Mark Brown (PI of the IGERT program), as well as several other UF faculty are investigating the changes in Florida Springs. Using a combination of field data collection, experimental mesocosms, and simulation modeling the team provides an alternative explanation for spring ecosystem change involving, a combination of lower overall discharges, the loss of invertebrate grazers (snails) due to decreased dissolved oxygen (DO) in discharging water, and increased human recreation.
In a recent paper in Ecological Applications (Heffernan et al. 2010) the team of scientists and engineers have shown that while the N-limitation hypothesis appears well founded on broadly supported aquatic eutrophication models, several observations from Florida springs are inconsistent with this hypothesis in its present simplified form. New analyses of existing data indicate that dissolved oxygen (DO) has declined dramatically in many Florida springs over the past 30 years, and that DO and grazer abundance are better predictors of algal abundance in springs than are nutrient concentrations. Three lines of evidence and experimentation suggest that the changes in Florida springs are more complex than simply increased nitrate nitrogen. First evaluating data from 60 sites within 28 Florida springs they have shown that there is no relationship between nitrate concentration and algae cover. Second, historic observations of the establishment of macroalgal mats shows that establishment often lags behind observed increases in nitrate by more than a decade. Nutrient enrichment experiments within springs showed only minimal response to nitrate enrichment.
The team developed simulation models (Fig. 3) to test the hypothesis that mechanisms other than nutrient enrichment may result in algae domination of springs. One model tested the hypothesis the human disturbance and loss of grazers were the key stressors to cause a regime shift from SAV to algae and that nutrients alone are not sufficient since flowing water systems like springs generally exhibit minimal response to nutrient enrichment. The models were simulated for a period of 15 years testing increases in nutrient concentrations on algae production and dominance with little correlation to the observed quantities of algae in spring runs. Then the model was simulated including random occurrences of human disturbance and low DO individually and in combination. The simulation result suggested that nutrient enrichment indeed had little effect, and that individually human disturbance and low DO (resulting in die off of gravers) had some effect, but in combination caused a regime shift from SAV to algae dominance.
The team hypothesizes a suite of plausible causal pathways that link proximate mechanisms of change in the spring systems to broader environmental dynamics (Fig. 4). The available data are far from conclusive concerning the nutrient enrichment hypothesis, yet the empirical case for the DO-grazer hypothesis is somewhat stronger despite the relative inattention paid to the latter. While they stress that it is not the objective of their study to dissuade policy makers and resource managers from controlling nitrate nitrogen, especially in light of its ecotoxicological and human health effects as well as the sensitivity of downstream estuarine ecosystems to nitrogen enrichment, they suggest that effective protection of springs may be compromised if only nutrient enrichment is considered the agent of change. For one thing, changes in spring chemistry may take decades to appear once nitrogen loading to the springshed is implemented, due to the very long turn over times of the deep aquifer. As long as the nutrient hypothesis is the dominate narrative used to explain change in spring ecosystems, other more plausible causes are not being studied which may be easier and less costly to control. Effective adaptive management of spring ecosystems will require an approach that experimentally addresses a much wider range of proximate and ultimate mechanisms (Fig. 4) that determine the biotic structure of spring ecosystems.
Literature Cited
Heffernan, J.B. D. M. Liebowitz, T.K. Frazer, J. M. Evans, and M. J. Cohen. 2010. Algal blooms and the nitrogen-enrichment hypothesis in Florida springs: evidence, alternatives, and adaptive management. Ecological Applications, 20(3), 2010, pp. 816-829.
Address Goals
First, we recognize the importance of basic knowledge. While our research may be considered applied scientific research and is important in the realm of science policy, it is generating basic ecological knowledge of the relationships between drivers of change and shifts in ecosystem function sometimes called regime shifts. Basic understanding of the causal agents of regime shifts have general applicability in many fields of science. Our research suggests that such shifts are potentially more easily explained by the interaction of drivers rather than a single driver.
Second our research on spring ecosystems recognizes and embraces the increasingly interdisciplinary nature of generating new knowledge and in applying existing disciplinary knowledge within an interdisciplinary context.
Third, the contributions we are making to the current stock of knowledge and understanding of ecological systems is the result of our collaborative efforts between several disciplines.
Fourth we recognize that a central issue to understanding the impacts of climate change, a central issues of the global society of the 21st century, may be enhanced by a more through understanding of ecological changes that result from changing temperature, rainfall patterns and drought cycles that while long term in their ultimate affects may be discerned in much shorter time frames in the steady-state spring ecosystems.
Fifth, our research on spring ecosystems is important in the policy realm and as a result we have stressed expanding scientific literacy of the general population, using their concern over the declining health of spring ecosystems, a very visible and important resource in north central Florida as a teaching moment about the place of science with in the policy framework. While our study is particular to Florida springs, the explicitly hypothetico-deductive mode of synthesis employed here is broadly useful as an approach to adaptive management in general.
