A note on the seasonally shifting zone of high primary production in the Bay of Marajó, Pará, Brazil, 1983-1984

A note on the seasonally shifting zone of high primary production in the Bay of Marajó, Pará, Brazil, 1983-1984^*

H.O. Schwassmann^I; R.B. Barthem^II; M.L. Carvalho^II

^IUniversity of Florida, Department of Zoology, Gainesville, Florida, U.S.A. 32611

^IICNPq/Museu Paraense Emilio Goeldi, Depto. de Zoologia

ABSTRACT

To understand the hydrological and limnological conditions during the annual regime of high and low river flow, as well as the action of the tides, a series of 18 collecting trips were conducted in monthly and bimonthly intervals across the Bay of Marajó during the years 1983-1985. The Tocantins River provides more than 80% of the inflow into this bay and shows a much greater difference in water volume flow between high and low water season than the Amazon. The annual displacement of brackish water influence is thus more extensive Marajó Bay than in the Amazon estuary. During the dry season of low river discharge (September-December), traces of seawater are found to penetrate up to 90 km upriver in the Guamá River. The high degree of turbidity of inner estuarine waters impedes light penetration and results in the near absence of primary production in spite of ample nutrients. Where these turbid river waters mix with brackish estuarine waters of 2 to 4‰ salinity, flocculation and subsequent sedimentation causes visibility to increase from a few to sometimes 200 cm. The water in this zone assumes a bright green color due to phytoplankton. About 90% of the biomass consists of a polyhalobic diatom species, Coscinodiscus. Concomitant great reductions in silica and other nutrient concentrations are noted. During low river flow (September to December), this high production zone is located in the central part of Marajó Bay, whereas it lies outside of the bay over the continental shelf during high river discharge (February to April).

Key Words: Limnology, Amazon Estuary, Tidal Rivers, Estuary Hydrodynamics, Phytoplankton.

RESUMO

Uma série de 18 travessias, em intervalos mensais ou bimensais, na baía do Marajó nos anos 1983-1985, a fim de medir os parâmetros físico-químicos e coletar plancton, contribuiu para nosso conhecimento das condições hidrocinéticas e limnológicas durante o ciclo anual de vazão alta e baixa dos rios e das marés. Devido à diferença de vazão entre a cheia e a seca no rio Tocantins ser maior que no rio Amazonas, a entrada de água salobra na baía do Marajó é mais extensa que na foz do rio Amazonas. No período de seca há indícios de água marinha até 90 km na montante no rio Guamá. A turbidez das águas do estuário interno impede a penetração de luz, resultando em falta de produção primária, apesar dos amplos nutrientes. Na zona de encontro das águas túrbidas com as salobras (com salinidade de 2 a 4%), a matéria em suspensão agrega-se em flocos maiores que se sedimentam, o que aumenta a claridade da água (visibilidade de até 200 cm). A água nesta zona torna-se verde devido à produção de fitoplâcton, sendo que 90% dessa biomassa é constituída da diatomácea marinha Coscinodiscus. Coincidentemente, nota-se grande diminuição no teor de sílica e de outros nutrientes. Durante o período de vazão baixa, a zona de alta produtividade se encontra fora da baía do Marajó, nas águas de plataforma continental.

Palavras-chave: Limnology, Amazon Estuary, Tidal Rivers, Estuary Hydrodynamics, Phytoplankton.

Full text available only in PDF format.

Texto completo disponível apenas em PDF.

Acknowledgements

We thanks Drs. F.G. Nordlie, J.S. Lourenço, and F.C. Novaes for support and encouragement. The Zoology Department, University of Florida, provided most of the equipment and supplies, while the Goeldi Museum at Belém provided logistic and financial support through CNPq fellowships and grants. H.O.S. was on sabbatical leave from the University of Florida.

Recebido em 10/09/88

Aprovado em 06/06/89

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*

In memory of Dr. Walter Alberto Egler, Botanist and Director of the Emilio Goeldi Museum until his tragic death in August, 1961.

Publication Dates

• Publication in this collection
  15 June 2011
• Date of issue
  1988

History

• Received
  10 Sept 1988
• Accepted
  06 June 1989