Dissolved inorganic nutrients and chlorophyll a in an estuary receiving sewage treatment plant effluents: Cachoeira River estuary (NE Brazil)

Abstract

Sampling was conducted monthly during a transition period between the dry and rainy seasons in order to evaluate the effectiveness of a municipal sewage treatment plant (STP) in eutrophication control. STP effluent and fluvial input data were also estimated. In the dry period, high concentrations of nutrients, chlorophyll a (up to 360 μg L(-1)), and anoxia in bottom waters were observed in the upper portion of the estuary. Nitrate was scarce during the dry months, although high concentrations were observed at the river sources and the upper estuary. The N:P and Si:P molar ratios were usually below 16:1, and the Si:N ratio was higher than 1:1. The fluvial inputs were a greater source of nutrients to the estuary than the STP, but nutrient loading by these effluents were also important in contributing to the eutrophication of the upper estuarine zone, especially in the dry season when symptoms were more intense.

SPATIAL AND TEMPORALVARIATION OF DISSOLVED INORGANIC NUTRIENTS, AND CHLOROPHYLL-α IN a TROPICAL ESTUARY IN NORTHEASTERN BRAZIL: DYNAMICS OF NUTRIENT REMOVAL

“…Differently from NID and silicate, the phosphate in the Cachoeira River estuary did not show any clear relationship with the rainfall. Moreover, the behavior of phosphate along the salinity gradient corresponds to that observed in previous studies, where the highest concentrations were measured in oligohaline waters (SILVA et al, 2013;SOUZA et al, 2009). Except in May, a widespread removal of phosphate was observed along the estuary.…”

“…However, a comparison of our dataset with the chlorophyll-a values measured in previous studies in this same estuary (SILVA et al, 2013;SOUZA et al, 2009) The uptake of nutrients (N, P) by the phytoplankton contributed to a decrease in the concentrations of these elements in the Cachoeira River estuary, generating the high chlorophyll-a values measured in this system. The normalized mixing diagrams show that this is the main DIN removal mechanism in August and September.…”

“…3). High temperatures were already recorded in the inner portion of this estuary, even during high spring tide (Silva et al, 2013). The pH values showed little variation throughout the year (Fig.…”

“…The STP's input of dissolved inorganic nutrients into the estuary was estimated to be about 0.7 x 106 mmol d -1 (DIP) and 1.1 x 106 mmol month-1 (DIN). Most of this DIN input was in the form of ammoniacal nitrogen, which also contributed to some of the high concentrations observed in the estuary (SILVA et al, 2013). During the dry season the nutrient inputs from the STP were of the same order of magnitude as the fluvial ones, but slightly greater than the DIN.…”

“…Unfortunately, the only survey made before the STP came into operation was conducted in February 2000. SILVA et al (2013) showed that these trends continued in surveys carried out during a transition from the dry to the rainy period in [2003][2004]. Low DIN:DIP and ammoniacal nitrogen in the lower estuary indicated phytoplankton limitation by N, while eutrophication in the inner estuary increased (up to 368 Chl-a µg/L).…”