Changes in nutrient pools during an experimentally simulated wet—dry cycle in the Delta Marsh, Manitoba, Canada

Summary

The MERP nutrient studies summarized in Tables 2 and 3 show that pools associated with macrophytes dominate the overall nutrient budgets during all stages of the wet-dry cycle, except during the lake stage when algal, surface water and pore water pools are the largest nutrient pools. During the regenerating stage, macrophyte pools contain 60% of the N and 70% of the P found in the major pools. During the lake marsh stage, macrophyte pools contain only 15–25% of the N and 20–33% of the P. For N and P, the highest total mass of nutrients in the major pools was found during the regenerating and degenerating stages and the lowest during the lake stage, except possibly for total P which might be lowest during the dry marsh stage (Tables 2 and 3).

Macrophytes control the sizes of nutrient pools in three ways: (1) their annual primary production which is highly dependent on water levels; (2) their interactions with algae and invertebrates; and (3) their litter production. The latter is particularly important because litter can tie up nutrients; can provide a substrate for algae and invertebrates; and can impact on recruitment of annual and emergents during the dry marsh stage. Although not measured directly, our data suggest that the macrophyte litter compartments, both above- and below-ground, play a major role in regulating the cycling of nutrients in these wetlands. Changes in water levels create large inputs of macrophyte litter particularly during the transition from dry marsh to the regenerating marsh stage and again during the degenerating marsh stage. During the transition from dry marsh to regenerating marsh, this litter is mostly above-ground annual litter and some of its N and P is quickly released by leaching to the water column (Wrubleski et al. 1997a), and thus is readily available to algae during the first year of the regenerating stage while most of the rest is slowly released due to mineralization and becomes available to expanding emergent populations. During the degenerating and lake marsh stages, mostly below-ground litter accumulates, and it decomposes very slowly as long as the wetland remains flooded (Wrubleski et al. 1997b). Nutrients stored in below-ground litter during these stages seem to be rapidly released during the dry marsh stage enabling the high production of annuals that characterizes this stage. In other words, as macrophyte production is reduced by deep and/or continuous flooding, nutrient uptake is reduced during the degenerating and lake stages, and nutrients become “locked” in the sediment pools. This results in an overall decline in the wetland's productivity. Only the re-establishment of the macrophytes during the dry marsh stage will “unlock” the nutrient in the sediments pools and re-establish the high rates of overall production characteristic of stages of the wet-dry cycle when the vegetation is dominated by emergents.

It must be emphasized that actual wet-dry cycles are highly variable both in the length of various stages and in water depths and rates of change of water depths during flooded stages. Only during the two most disparate stages of the wet-dry cycle, the dry marsh and lake marsh stages, are nutrient pools normally expected to resemble those in tables 2 and 3. The MERP water-level treatments were applied in the form of a step function with very rapid transitions from one phase to another. During an actual wet-dry cycle, water levels would change as a more-or-less continuous function, although fairly abrupt changes are not impossible, particularly in the transition from the dry marsh stage to the regenerating stage. It is also possible for water-level changes during a cycle to affect only part of the marsh with shallow areas not much affected (see van der Valk & Davis 1979).

Algal research during MERP in the experimental cells and elsewhere in the Delta Marsh (Goldsborough & Robinson 1996, Robinson et al. 1997a,b) demonstrated that algae also play an important role during various stages of the habitat cycle. Although algal biomass may not reach the levels found in macrophytes, the high production and rapid turn-over of algal cells results in annual algal production that may equal or exceed macrophyte production. Algae also may affect nutrient budgets and cycling through oxygenation of the water column, production of dissolved organic material, mediation of the flux of nutrient from the sediments to the overlying water (primarily epipelon), and sediment formation (see Goldsborough & Robinson 1996 for more details). This varied role of algae in the nutrient cycling of wetlands, and prairie wetlands in particular, requires further investigation.