3. Productivity Paradox
FCE scientists revealed how human-induced nutrient enrichment in the Everglades and
Caribbean wetlands affect the "productivity paradox" in which an extraordinarily high level
of algal growth supports far fewer aquatic animal consumers than expected. Understanding
this dynamic is critical to the restoration of the Everglades ecosystem.
This graphic compares the biomass structure of the Everglades aquatic food web to other
aquatic ecosystems, showing how the large mass of primary production in the Everglades
(primarily periphyton) does not translate to a large mass of benthic invertebrates and
fish, as would be expected from studies in other aquatic ecosystems.
Source: Gaiser, E., J. Trexler and P. Wetzel. In Press. The Everglades. In Batzer, D. and A. Baldwin (eds). Wetland habitats of North America: Ecology and Conservation Concerns. Berkeley: University of California Press.
FCE researchers have found that productivity in the Everglades, and other limestone-based
Carbbean wetlands, is dominated by extraordinarily productive algal mats, despite extreme
nutrient limitation. This phenomenon has been called a "productivity paradox" (Gaiser et al.
2011). This production would be expected to support a large biomass of aquatic primary
consumers but does not (Turner et al. 1999). Instead, algal mats are highly inedible and
aquatic consumers rely on less energetically efficient detrital pathways (Sargeant et al.
2010). Top predators (i.e., wading birds, large fish, alligators) are supported by the
seasonal concentration of these prey animals during the dry season. Long-term research at
FCE has been necessary to determine how interannual variation in wet and dry-season
hydrology and water quality influence algal mat production, grazer abundance and predator
efficiency. Through international collaborative research in other Caribbean wetlands, we
have been able to show that this phenomenon is not just distinctive of the Everglades, but
of limestone-based Caribbean wetlands in general.
Long-term observational data have been coupled with long-term manipulations of the limiting
nutrient, phosphorus, to determine its influence on the productivity paradox and trophic
cascades. Our research indicates that anthropogenic nutrient enrichment erases the unusual
pyramid of biomass by temporarily increasing algal edibility, thereby increasing production
of aquatic consumers. However, phosphorus enrichment ultimately leads to a loss of algal
mats, which reduces the structural refuge for aquatic animals, increasing their
vulnerability to predation. This shift in algal and consumer biomass and structure is
initiated at all phosphorus levels above background levels, and leads to an altered
ecosystem state at rates dependent on the nutrient loading rate.
This research is of critical importance to hydrologic restoration of the Everglades and for
protection of Caribbean wetlands, as it describes how hydrology controls food web structure
directly and through interactions with the loading of the limiting nutrient.
This graphic shows the effects of long-term phosphorus enrichment in
Everglades wetlands. FCE scientists have shown that exposure to phosphorus
at any level above background initiates a cascade of changes, from a loss of
natural periphyton communities, to replacement by more palatable non-mat
forming algae, then to an open-water state that eventually leads to the
spread of invasive cattails.
For further reading:
Credit: Gaiser, E., J. Trexler and P. Wetzel. In Press. The Everglades. In Batzer, D. and A. Baldwin (eds). Wetland habitats of North America: Ecology and Conservation Concerns. Berkeley: University of California Press.
For further information:
Gaiser, E.E., P. V. McCormick, Scot E. Hagerthey and A. D. Gottlieb. 2011. Landscape patterns of periphyton in the Florida Everglades. Environmental Science and Technology 41 (S1): In press.
Turner, A.M., J. C. Trexler, F. Jordan, S.J. Slack, P. Geddes, and W. Loftus. 1999. Targeting ecosystem features for conservation: Standing crops in the Florida Everglades. Conservation Biology 13: 898-911.
Sargeant, B., J. Trexler and E. Gaiser. 2010. Biotic and abiotic determinants of intermediate-consumer trophic diversity in the Florida Everglades. Marine and Freshwater Research. 61: 11-22.