the Grussai lagoon, and their relationship to ephemeral sand bar openings ... During the period when the sand bar was closed (isolated), the lagoon water was.
Hydrobiologia 368: 111–122, 1998. © 1998 Kluwer Academic Publishers. Printed in Belgium.
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Effects of sand bar openings on some limnological variables in a hypertrophic tropical coastal lagoon of Brazil M. S. Suzuki, A. R. C. Ovalle & E. A. Pereira Laborat´orio de Ciências Ambientais, CBB, Universidade Estadual do Norte Fluminense, Av. Alberto Lamego, 2000 Horto, 28015-620 Campos dos Goytacazes, RJ Brazil Received 12 March 1997; in revised form 2 December 1997; accepted 9 December 1997
Key words: tropical coastal lagoon, eutrophication, nutrients, anthropogenic impacts, sand bar opening, brackish water ecology
Abstract This study describes the spatial and temporal dynamics of several physical, chemical and biological variables in the Grussai lagoon, and their relationship to ephemeral sand bar openings and to a constant in natura waste water input. The spatial variation in pH, dissolved oxygen, electrical conductivity, total alkalinity and nutrients (e.g. soluble reactive silicate, soluble reactive phosphate and ammonium) was associated to the anoxic and nutrient rich groundwater discharge, the development of aquatic macrophytes, the biological activities of phytoplanktonic community and the marine influence. During the period when the sand bar was closed (isolated), the lagoon water was supersaturated with dissolved oxygen and exhibited high values of pH (8–10), total alkalinity (3.000–5.000 µeq l−1 ), and chlorophyll a contents (60-300 µg l−1 ), and had low values of dissolved nutrients (nearly undetectable). These suggest a biological processes dominance. When the sand bar was opened, there was an enrichment with dissolved inorganic nutrients (e.g. ammonium and soluble reactive phosphorus up to 120 and 5 µM, respectively) and a decrease in pH (below 8), total alkalinity (below 3.000 µeq l−1 ) and dissolved oxygen during the initial second to eight days. Subsequently there was a period when the physical and chemical characteristics of seawater prevailed. The lagoon returned to the pre-opening water conditions in a few days (∼ 10–20). This quick return implies highly efficient biological mechanisms. The high levels of chlorophyll a, total nitrogen and phosphorus in the water column indicate a high eutrophication stage in the Grussai lagoon during the sand bar closed periods.
Introduction Coastal lagoons have peculiar structural and functional characteristics as a consequence of their position between land and sea. Generally, they present large spatial and temporal changes in their environmental and biological variables caused by land influence, shallow depth and strong wind action (Barnes, 1980; Day & Yañez-Arancibia, 1982; YañezArancibia, 1986). Shallowness is usually responsible for a short nutrient turnover time (Nixon, 1981, 1982) that supports high primary productivity (Knoppers, 1993) and fish production (Yañes-Arancibia, 1981). In coastal lagoons the allochthonous nutrient input is generally higher than the output, and this causes
a natural eutrophication process. In recent decades, this natural eutrophication has been accelerated by anthropogenic activities such as domestic, industrial and agricultural sewage discharge (Rozemberg, 1985; Lanza, 1986; Comín et al., 1995). Within Brazil, the state of Rio de Janeiro is characterized by numerous coastal lagoons ranging from large freshwater systems (Feia Lake, 150 km2 ) to hypersaline environments (Araruama Lagoon, 220 km2 ). Most of them are small brackish waterbodies (1– 10 km2) which has been mainly studied for the relationship between metabolic processes and anthropogenic activities in their margins, particularly those related to organic pollution (Moreira & Knoppers, 1990; Knoppers et al., 1990; Knoppers et al., 1991;
112 Carmouze & Vasconcelos, 1992). Several lagoons in the northern region of the state of Rio de Janeiro are isolated from the sea by a thin sand bar that is sporadically opened by fishermen. When this happens, a large volume of water and materials are exchanged between the lagoon and the adjacent coastal water. The main purpose of the present study was to evaluate the impact of the sand bar opening process wich causes intense physical stress coupled with organic pollution in one of these lagoons.
Study area The Grussai lagoon is located in the municipality of São João da Barra, State of Rio de Janeiro (21◦ 430 S; 41◦ 020 W ). It is a shallow (∼ 1.2 m depth), long (∼ 8 km length) and narrow (∼ 100 m width) brackish water body. A thin sand bar isolates it from the sea (Figure 1). The dominant land use on the drainage basin of the lagoon are pasture inserted on a sequence of quaternary sand bars (restinga) (Lamego, 1945). Close to the sea, on its north portion, the marginal wetlands have been urbanized. The lagoon is submitted to various anthropogenic stresses, of which the most important are the input of in natura domestic sewage and the artificial openings of the sand bar (that usually occur in higher pluviometric period). The sand bar is oppened by shovels and drag machines and it is closed by natural process involving longshore sand transport, tidal fluctuation, winds and precipitation. The water level in the lagoon is sustained by direct rainfall and groundwater discharge which flows diffusely especially in the south portion. The south portion exhibits extensive meadows of aquatic macrophytes such as, Typha dominguensis Pers, Pontederia cf lanceolata L., Eichhornia crassipes (Mart.) Solms and Pistia stratiotes L.. The climate is tropical subhumid-dry with an annual mean precipitation of 1000 mm, and temperature ranging from 19 ◦ C (July) to 30 ◦ C (February). A pronounced pluviometric seasonality causes a wet and hot season from November to April, and a dry period with mild temperatures from May to October.
Material and methods The spatial sampling was performed on March 14, 1995 when the sand bar was closed and fifteen months after the last sand bar opening (S.B.O). Seven sampling stations were established throughout the lagoon
for water and bottom sediment collection (Figure 1). These stations were located in the following areas: I – colonized by macrophytes and heavily affected by groundwater discharge; II, III and IV – central parts of the lagoon surrounded by pasture and restinga; V and VI – sub-urbanized regions that receive domestic sewage input; and VII – near the sand bar. Groundwater was sampled on July 20, 1996, in a spring near station I. Between February 22, 1995 and February 07, 1996, weekly to monthly samplings of the water column were performed in station VI. Two S.B.O.s occurred during this sampling period. The first one ocurred from May 26 to July 01, 1995 and the second one between the 1st and 14th of December, 1995. Sampling during high and low tide periods was intensified while the sand bar was opened in order to follow short term changes in physical, chemical and biological variables. Station VI was selected because its proximity to the sand bar (800 m) facilitates the measurement of changes in water quality during S.B.O. periods. Furthermore, this station has direct influence from domestic sewage discharge. At each station, water samples were collected directly into plastic bottles at the surface (∼ 0.05 m below the water column top) and ∼ 0.1 m from the bottom. Bottom water was collected with a van Dorn bottle when the depth was more than 0.5 m. Variables as saturation of dissolved oxygen (O2 ), water temperature, electric conductivity and pH were measured from the water column with portable meters (Horiba oximeter OM-14, with temperature compensation and salinity correction, WTW conductivimeter LF 96 and pH meter Digimed DMPH-3). All of the water samples collected were transported to the laboratory and processed immediately. Total alkalinity was measured following Gran (1952) and this data were used to compute percentual saturation of dissolved inorganic carbon (CO2 ) (Carmouze, 1994). Sub-samples were filtered in duplicate through Whatman GF/C filter and kept at −20 ◦ C. These filters were used to determine chlorophyll a concentration using the Nusch & Palmer (1975) extraction method. The filtered sample were stored in plastic bottles, kept at −5 ◦ C, and used to determine dissolved inorganic N, P and Si nutrients. Total nitrogen (N) and phosphorus (P) concentrations were determinated from an unfiltered subsample. Total and dissolved nutrients were determined according to Grasshoff et al. (1983) and to Strickland & Parsons (1972), except for nitrite and nitrate, which were deter-
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Figure 1. Study area and location of the sampling sites.
mined using automatic flow injection analyser ASIA Ismatec System methods. Superficial (∼ 5 cm) bottom sediments were collected with Eckman’s drag in each of the seven stations. In the laboratory, samples were dried in an oven (50 ◦ C), grounded and homogeneized to particles smaller than 63 µm. A subsample of total sediment (0.5 g) was used to determine organic matter (O.M.) content through loss by ignition in a muffle furnance (450 ◦ C; 24 h). Another subsample (0.5 g) was digested in acid solution, according to the method described by Jackson (1958), to determine available phosphorus (avail. P) content. Total N determinations were performed in a CHNS/O Analyser (Perkin Elmer model 2400). Results Both shallow depth and local wind action do not allow water column stratification. Consequently, no significant chemical variation was observed between surface and bottom water (t-student test, p
