The influence of castanhão reservoir on nutrient and suspended matter transport during rainy season in the ephemeral Jaguaribe river (CE, Brazil)

Abstract

Measurements of nutrient and suspended matter concentrations and loads entering and leaving the Castanhão reservoir during the rainy season were carried out to assess the influence of this large reservoir on land-sea fluvial transport in the ephemeral Jaguaribe river basin. Spatial variation indicated statistically significant attenuation of concentrations only for total phosphorous and suspended matter across the reservoir. Strong retention of nutrients and suspended matter loads by the reservoir was observed with average trapping efficiency of 89% for dissolved silicon, 98% of soluble reactive phosphorus, 71% for ammonium, 87% for total nitrogen, 98% for total phosphorus and 97% for suspended matter compared to the reservoir inflow. The dam operational procedure defined by the ephemeral conditions of the river reduced water releases compared to reservoir inflow and induced strong retention of nutrient and suspended matter loads within the reservoir when fluvial transfer occurs in this semiarid watershed.Keywords: dam, semi-arid, nutrient budget, sediment retention, impoundment.Influência do açude Castanhão no transporte de nutrientes e materiais em suspensão durante a estação chuvosa no intermitente rio Jaguaribe (CE, Brasil) ResumoMedidas de concentrações e fluxos de nutrientes e material particulado em suspensão de entrada e saída do Açude Castanhão foram efetuadas para determinar a influência de um grande reservatório no transporte fluvial entre o continente e o oceano na bacia hidrográfica intermitente do Rio Jaguaribe. A variação espacial indicou uma atenuação estatisticamente significativa ao longo do reservatório somente para as concentrações de fósforo total e material particulado em suspensão. Os resultados indicam uma forte retenção de nutrientes e do material particulado em suspensão com eficiências de retenção de 89% para sílica dissolvida, 98% para fósforo solúvel reativo, 71% para amônia, 87% para nitrogênio total, 98% para fósforo total e 97% para o material particulado em suspensão em relação aos fluxos de entrada no reservatório. A rotina de operação da barragem definida pela condição intermitente do Rio Jaguaribe reduz a descarga de água comparada com a entrada no reservatório e induz forte retenção do fluxo fluvial de nutrientes e material particulado em suspensão pelo reservatório.Palavras-chave: barragem, semiárido, balanço de nutrientes, retenção de sedimentos, represamento de águas.

Relative effects of nutrient emission from intensive cage aquaculture on the semiarid reservoir water quality

“…By comparing the N emission from the cage aquaculture during the producing cycle (12 t) and the fluvial flow from the main contributor Jaguaribe River during the same period (733 t per 6 months) (Molisani et al 2013), we observed that the cage farm contributed with 2 % of 7-7.7 N while river loads and upstream nitrogen sources represented most of N to the reservoir. However, river and aquaculture N inflow (745 t) and regulated dam outflow (85 t) budget showed that the reservoir retained 660 t or 88 % N showing the importance of management of both in-lake (cage aquaculture) and land-based (agriculture, urbanization, industry, cattle) activities for the Castanhão reservoir (Fig.…”

“…Data from Guo and Li (2003) indicate that effluents from cage aquaculture may cause negative effects in areas from the cage to a distance of 20 m or more outwards. During the sampling period, the water inflow into the Castanhão reservoir varied from 13 to 131 m 3 /s while the outflow over the dam was artificially controlled between 5 and 11 m 3 /s (Molisani et al 2013). Considering only the inflow and outflow rates and reservoir volume, an oversimplified water residence time on the reservoir was 4000 days.…”

“…Nutrient loads from the main tributary (the Jaguaribe river) and controlled dam outflow were also measured during sampling period to determine the relative importance of emission of nutrients from the cage aquaculture compared to the wholereservoir nutrient budget. Reservoir loads were obtained from nutrient concentration and water discharge measurements (Molisani et al 2013).…”

Limnology of the largest multi-use artificial reservoir in NE Brazil: The Castanhão Reservoir, Ceará State

“…Natural sources include atmospheric deposition and the denudation of soils; whereas major anthropogenic sources are irrigated agriculture, husbandry, fish aquaculture and urban runoff and solid waste disposal. Of those, only aquaculture released nutrient directly to the reservoir water column (Molisani et al 2013, Avelino 2015, Santos et al 2016. The comparison between estimates of the natural and anthropogenic nutrient loads to the watershed of the Castanhão reservoir indicates that anthropogenic sources correspond to almost all emissions of nitrogen and phosphorus to the reservoir (Table I).…”

Hydrochemistry and trophic state change in a large reservoir in the Brazilian northeast region under intense drought conditions

“…Previous observation on nutrient balance in the Castanhão reservoir showed a net retention of over 97% of the total fluvial input of N and P within the reservoir resulting in nutrient accumulation mostly in the hypolimnion (Molisani et al, 2013). With decreasing water depth, wind action can break the stability of the water column, thus favoring water column mixing processes and the resuspension of bottom material and the consequent increase in nutrient concentrations.…”