The fate of streamwater nitrate entering littoral areas of an Amazonian floodplain lake : the role of plankton, periphyton, inundated soils, and sediments / Luiz Fernando Alves.

Tese (Ph.D.) University of Maryland, College Park, 1993

Lake Calado, a open body of water on the Amazon River floodplain, receives a substantial nitrate supply from adjacent upland watersheds. The inflow of local water onto the floodplain is delineated by the development of an important mixing front of stream and lake water, the stream/lake interface (SLI). Nitrate appears to be depleted within the SLI as a result of biological uptake and retention near the interface. The flooded forest (high water) and the adjacent lateral arm of the lake low water) are two habitats at the interface of the uplands and the floodplain, which are strategically located to intercept inflowing nitrate. I measured the relative importance of losses of nitrate by periphyton, plankton, inundated soils, and sediments in the flooded forest and in the lateral arm compared to the nitrate delivered by a local watershed (Mota Brook). The inundated soil was the dominant sink in the flooded forest, contributing 72 % of the total nitrate removal, while uptake by the periphyton and plankton accounted for only 6%. Across the lateral arm, the sediments acted as a net source of nitrate, adding ca. 10% to the nitrate input by local runoff, and the lake plankton removed almost half (44%) of the combined nitrate input. The flooded forest appears to be a more effective sink for incoming nitrate than the lateral arms along the interface of the upland and the floodplain at Lake Calado. Extrapolating the results at Mota Brook to the rest of Lake Calado, nitrate removal at the SLI represented a relatively small proportion (27%) of the total nitrate input. The low removal rates computed at larger spatial scales were probably due to errors in scaling from a small watershed to the total upland drainage of Lake Calado. For instance, the ratio of the total upland drainage to interface area (SLI) increased by 2-5 times, which increased the nitrate loading to the interface at larger scales. Extrapolation of the data may have underestimated the actual area of the SLI and consequently the potential nitrate removal by the sinks investigated.