An inverse relationship between net ecosystem production and nutrient availability has been observed in karstic, oligotrophic wetlands, a “paradox of production.”  Carbon flux and nutrient cycling in these ecosystems is dominated by calcium carbonate-precipitating microbial communities analogous to marine stromatolites. Comprised of photoautotrophs, heterotrophs and perifauna, these complex micro-ecosystems efficiently sequester and recycle inorganic carbon and nutrients to attain significant biomass. Within the naturally occurring range of phosphorus (P) concentrations, stromatolite production increases with increasing P availability, the limiting nutrient in karstic wetlands. However, upon chronic exposure to above-ambient P concentrations the stromatolites disintegrate, commonly resulting in an inverse relationship of net ecosystem production to P availability.

Because karstic wetlands are exposed to P enrichment through cultural development in their watersheds, we are examining ecosystem response to P at a variety of spatial and temporal scales. In this study, we compare the direction and slope of the relationship of production to P availability at three scales. At the smallest scale, stromatolites are transferred to chemostats and exposed to elevated P for two weeks, at the mid-scale, we enriched the natural marsh with P for five years in 100-m long flumes, and at the large-scale, we took advantage of a “natural experiment” where decades of P-enrichment have caused strong P-gradients to develop.  We measured gross and net production and respiration using a variety of methods, including gas-exchange and biomass harvesting.

Results/Conclusions

At small scales, P enrichment stimulated both autotrophic and heterotrophic activity, although mass-specific net production decreased because P<R.  At the mid-scale, P enrichment stimulated both biomass-specific autotrophic and heterotrophic production and P>R, although stromatolite biomass declined.  At the large-scale, P enrichment reduced gross production and respiration, resulting in a strong negative aerial relationship between net production and P availability.

We conclude that the spatial and temporal scale of manipulation affects the interpretation of the relationship between P availability and ecosystem production.  At small scales, P-stimulated bacterial consumption of ectopolysaccarides and increased dissolved inorganic carbon dissolves the stromatolite matrix. At mid-scales, the autotrophic community shifts upon enrichment to species having greater biomass-specific production rates, although stromatolites disappear. At large scales, the displacement of stromatolites and shading by less productive macrophytes causes the persistent negative paradoxical relationship commonly reported for karstic wetlands.