Whole Ecosystem Evidence of Eutrophication Enhancement by Wetland Dredging in a Shallow Tropical Lake

Abstract

The purpose of this research was to assess the effects of dredging performed in a marginal wetland colonized by aquatic macrophytes on eutrophication of the adjacent shallow tropical lake (Imboassica Lake, Brazil). The river mouth of the Imboassica River that drains into Imboassica Lake had been densely colonized by aquatic vegetation dominated by Typha domingensis (Pers.) when it was dredged. Total and dissolved nitrogen and phosphorus concentrations were measured monthly over 13 years at four stations in the Imboassica river-lake system. Dredging activities reduced phosphorus and nitrogen retention at the river mouth and subsequently increased these nutrient stocks in the lake waters. Nutrient retention by non-dredged wetland was estimated to be ca. ) for phosphorus. Our whole-lake approach suggested that dredging might intensify rather than mitigate eutrophication in shallow tropical lakes when the removal of aquatic macrophytes is coupled to the persistence of anthropogenic nutrient inputs from the watershed.

Historic carbon burial spike in an Amazon floodplain lake linked to riparian deforestation near Santarém, Brazil

“…It has been shown that wetlands in the warm tropics are some of the most productive biological communities in the world (Neue et al, 1997), representing an important sink for nutrients (Marotta et al, 2009) and carbon (Peixoto et al, 2016;Sanders et al, 2017) as well as sources of organic substrates for carbon gas production in inland waters (Marotta et al, 2010). However, these wetland ecosystems are also highly threatened by land use activities, especially from deforestation, development of agricultural land and soil degradation (Junk, 2013;Lucas et al, 2014).…”

Catastrophic shifts in the aquatic primary production revealed by a small low-flow section of tropical downstream after dredging

“…Despite similar TSS and nutrients in waters, St 60 showed a substantially lower pelagic GPP and phytoplankton biomass than St 30. The nutrient availability by sewage discharges (Marotta et al, 2009a) and absence of shading by riparian vegetation (Wallace et al, 1997) could stimulate the floating aquatic vegetation (e.g. S. auriculata), conditions that were observed in the Imboassica Stream after dredging.…”

“…The Imboassica Stream (22º 25' S e 42º 55' W) is a small fourth order tropical stream with low flow and about 14 km in length, the main tributary of the Imboassica Lagoon watershed (area = 55 km 2 ; Marotta et al, 2009a) in the North of Rio de Janeiro state (Southeast of Brazil, Figure 1). The low flow of this stream reflects the low altitudinal gradient from the spring to the mouth, as 90% of the extension is between 0 and 40 m (maximum altitude 120 m).…”

“…Before the dredging activities, the mouth of the Imboassica Stream into Imboassica Lagoon had showed an intermittent water column and dense colonization by the emergent aquatic macrophyte Typha domingensis Pers (Marotta et al, 2009a). …”

“…The subsequent depth reduction followed by water quality and biodiversity loss has made shallow streams particularly sensitive to these anthropogenic disturbances (Casatti et al, 2006;Rörig et al, 2007). Also, the silting up of shallow inland waters may be still intensified by the growth of aquatic primary producers (Moreira-Turcq et al, 2004), especially under eutrophic conditions (Marotta et al, 2009a). In this way, dredging is a common man-made intervention to mitigate silting up processes (Bemvenuti et al, 2005;Jiang and Shen, 2006).…”