The significance of waterfowl feces as a source of nutrients to algae in a prairie wetland

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Purcell, Sara L.
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In situ enclosures in Delta Marsh, Canada were used to determine the responses of planktonic and benthic algae, and submersed macrophytes, to repeated additions of waterfowl feces from mallard ducks (Anas platyrhynchos) and Canada geese (Branta canadensis). Two types of feces loading were examined. In 1995, two feces pulses were added to duplicate enclosures, four weeks apart. The treatments were: 1) a high feces load (containing 2.421 g/m2N and 0.806 g/m2P), 2) a low feces load (0.242 g/m2N and 0.0806 g/m2P), and 3) untreated controls. In 1996, eight weekly additions of waterfowl feces were added to triplicate enclosures, representing the same cumulative load as added to each high feces load enclosure in 1995. The treatments were: l) high feces load (0.722 g/m2 N and 0.240 g/m2P) and 2) untreated controls. In the high feces load enclosures in both 1995 and 1996, total P, soluble reactive P and ammonia-N in the water column increased significantly after feces application. However, biomass, productivity and particulate P of algae (phytoplankton, epiphyton, metaphyton, and epipelon) did not respond significantly to nutrients released from the waterfowl feces, even when loading exceeded estimated natural values (438.9 g/m2 versus 1.28 g/m2 wet weight, respectively). These results contradict previous studies which showed that waterfowl do make substantial contributions to the nutrient requirements of wetland algae and macrophytes. I propose that at least three hypotheses may explain the mitigated response to waterfowl feces additions, as follows: 1) feces were deplete in N, relative to P, as compared to the ambient N:P ratio in Delta Marsh, suggesting that feces provided insufficient quantities of the growth-limiting resource; 2) fecal nutrients were not available for algal uptake because they were rapidly adsorbed in the sediments; 3) feces additions did stimulate algal production but the increase was immediately and entirely transferred to consumers so an increase in invertebrates with treatment concealed the algal response.