EGU21-14046, updated on 19 Apr 2021
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Short-term changes of anthropogenic eutrophication with precipitation in tropical coastal waters (Guanabara Bay, Rio de Janeiro, Brazil)

Tainan Fonseca1,2, Roberta Bittencourt Peixoto1,3, Luana Pinho4, Leticia Cotrim da Cunha4,6, Ricardo Pollery5, and Humberto Marotta1,2,3
Tainan Fonseca et al.
  • 1Ecosystems and Global Change Laboratory (LEMG-UFF), Biomass and Water Management Research Center (NAB-UFF), Universidade Federal Fluminense (UFF), Av. Edmundo March, s/n°, Niterói, RJ, Brazil, 24210-310
  • 2Physical Geography Laboratory (LAGEF-UFF), Department of Geography, Graduate Program in Geography, Universidade Federal Fluminense (UFF), Av. Gal. Milton Tavares de Souza, s/n°, Niterói, RJ, Brazil, 24210-346
  • 3Department of Geochemistry, Universidade Federal Fluminense (UFF), Av. Edmundo March, s/n°, Niterói, RJ, Brazil, 24210-310
  • 4Chemistry Laboratory (LABOQUI) - Department of Chemical Oceanography, Graduate Program in Oceanography, Universidade do Estado do Rio de Janeiro, Rua São Francisco Xavier, 524, Pavilhão João Lyra Filho, 4º andar, sala 4008 Bloco E, - Rio de Janeiro, RJ, B
  • 5Multi-User Environmental Analysis Unit – UMMA. Biology Institute, Universidade Federal do Rio de Janeiro (UFRJ), Prédio Inter bloco A-F. Av. Carlos Chagas Filho, 373 - Ilha do Fundão – Cidade Universitária, Rio de Janeiro, RJ, Brasil, 21941-971
  • 6Brazilian Ocean Acidification Research, Universidade Federal do Rio Grande, Rio Grande, RS, Brazil, 96203-000

Eutrophication in coastal waters caused by non-treated urban discharges has been considered one of the most important effects of global change. At tropical latitudes, nutrient dynamics may be especially intense due to increased metabolic responses supported by high temperatures and solar incidence throughout the year. In addition, short-term variations, such as in rainfall and the tidal regime, may determine important changes in nutrient concentrations and the subsequent trophic status of coastal waters, which are still neglected especially during nocturnal periods due to common logistical constraints. Here, we assessed 24-h variations of water quality during the winter season in a tropical eutrophic bay that receives large inputs of nutrients from non-treated urban effluents (Guanabara Bay, RJ, Brazil). We measured concentrations of dissolved forms of nutrients (nitrate, nitrite, N-ammoniacal, phosphate, and silicate) and carbon (DOC), and oxygen (DO) associated with temperature, salinity, and pH in surface waters each 2h over two daily cycles (July and August 2018). Water samples for nutrients and DOC were preserved for later analysis, while other variables were measured in the field. A biomonitoring system with a submersible pump was used to collect surface coastal waters without bubbling, and along a 70 m pipe from the beach to the field lab. In turn, meteorological data were obtained from a city weather station located ~6 Km from the sampling area. The monthly accumulated precipitation with respect to the 24-h cycle in July was ~70% lower than in August (58 and 16 mm, respectively), although only that in July has showed a rainfall event during the sampling period. As a result, average DOC and N-ammoniacal concentrations in surface waters were ~50% lower, while nitrate, silicate and DO concentrations ~56, 164 and 50 % higher, respectively, during the 24-h cycle in August compared to July. Also, waters were slightly more basic and less saltier in August, contrasting with similar average values of phosphate concentrations and temperature between both sampling periods. Finally, DO concentrations indicated an intense metabolism, varying from a peak of supersaturation with high solar incidence to net autotrophy (2 pm) to undersaturation values as a proxy of net heterotrophy after the nocturnal period (6 am). In conclusion, this short-term study showed that higher monthly accumulated precipitation may dilute high DOC and N-ammoniacal concentrations in coastal aquatic ecosystems undergoing anthropogenic eutrophication. On the other hand, silicate and nitrate concentrations might be related to higher runoff inputs from the watershed. The event of precipitation in July also confirmed a drastic increase in nitrate concentrations, likely due to inputs from the watershed. Therefore, our findings reveal the complexity of accumulated and immediate effects of rainfall on nutrient levels in tropical coastal waters, which  highlight the importance of biomonitoring studies specially in urban areas.

How to cite: Fonseca, T., Bittencourt Peixoto, R., Pinho, L., Cotrim da Cunha, L., Pollery, R., and Marotta, H.: Short-term changes of anthropogenic eutrophication with precipitation in tropical coastal waters (Guanabara Bay, Rio de Janeiro, Brazil), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14046,, 2021.

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