Conservation Maven

Palustrine wetland with rooted emergent vegetation. Image credit, Howard Weamer.

Working in conservation, most of us have been trained to view biodiversity as an essential  aspect of a successfully functioning ecosystem.  But new research from wetland scientists at Halmstad University in Sweden challenges this common assumption. The study has provided evidence indicating that vegetative biodiversity in wetlands can actually inhibit nitrogen removal -- a desirable and vital wetland process. The study showed no support for vegetative biodiversity supporting ecosystem function.

One highly beneficial function of wetlands is their ability to remove nitrates from nutrient rich river water before it enters the ocean. For years, eutrophication has been a major environmental concern due to the fact that it causes numerous “dead zones” within our oceans. Wetlands filter out these nutrients preventing harmful algal blooms and preserving the dissolved oxygen for its rightful recipients: the aquatic fauna.

Methods
The study examined nitrogen removal (denitrification) and vegetative species diversity (Shannon diversity method) in 18 experimental wetlands. The researchers left six of those wetlands unplanted to develop freely. In six of the wetlands they added tall emergent vegetation (Phragmites australis, Glyceria maxima, & Phalaris arundinacea) and in the other six they added plantings of submerged vegetation (Elodea Canadensis, Myriophyllum alterniflorum, and Ceratphyllum demersum). An inlet pipe released water with 11 milligrams of total nitrogen per liter into the wetlands.  Water escaping an outlet pipe was then tested for nitrogen concentrations. The wetlands were monitored over the course of 3 years.

Results
Although the unplanted wetlands initially had the lowest biodiversity, as the study progressed, the other twelve wetlands gradually became less and less diverse. Where the tall emergent species were planted, they ultimately dominated by outcompeting the previously established plants. While these wetlands became the least diverse, they contained the densest vegetation and the water leaving them contained lower nitrogen concentrations than the submerged vegetation wetlands (P < 0.001) or the free development wetlands (P = 0.002).

There are several explanations for these results: The tall vegetation wetlands (1) provided higher amounts of organic matter needed for the denitrifying bacteria, (2) offered more litter for the bacteria to attach to, and/or (3) contained dense vegetation that limited sunlight from reaching the water column, promoting anaerobic conditions necessary for the denitrification process. 

Implications
While ecosystem managers commonly set a goal of species heterogeneity, low-diversity wetlands, dominated by tall plant life, seem to aid nitrogen removal. To eliminate detrimental nitrogen concentrations, managers may want to reconsider the common practice of thinning dominant species to promote biodiversity. This low-diversity may actually benefit ecosystem functioning!

--Reviewed by Evyan Borgnis

Weisner, Stefan, & Thiere, Geraldine (2009). Effects of vegetation state on biodiversity and nitrogen retention in created wetlands: a test of the biodiversity-ecosystem functioning hypothesis Freshwater Biology DOI: 10.1111/j.1365-2427.2009.02288.x