Artículos

 

Soil heat exchange in Puerto Cuatreros tidal flats, Argentina

 

Intercambio de calor en el suelo en las planicies de marea de Puerto Cuatreros, Argentina

 

Débora Beigt¹* M. Cintia Píccolo¹'² Gerardo M. E. Perillo¹

 

¹ Instituto Argentino de Oceanografía, CONICET Camino La Carrindanga km 7 Casilla de correo 804 (8000) Bahía Blanca, Argentina. *E-mail: dbeigt@criba.edu.ar

² Universidad Nacional del Sur Departamento de Geografía 12 de Octubre y San Juan (8000) Bahía Blanca, Argentina.

 

Recibido en marzo de 2003;
aceptado en agosto de 2003.

 

Abstract

Some interaction processes at the sediment-water and sediment-atmosphere interfaces were analyzed for each season of the year using mass aerodynamic formulas. Data were collected from Puerto Cuatreros tidal flats (Bahía Blanca estuary, Argentina) during an entire year (2000). The soil temperature was measured every 10 min by a thermistor chain at three levels below the sediment surface (-0.05, -0.15 and -0.25 m). Water and air temperatures, solar radiation and meteorological data were registered simultaneously, resulting in annual means of 14.07°C (soil at -0.15 m), 13.69°C (air) and 14.51°C (water). Atmospheric and tidal conditions regulated the mudflat's thermal behavior. The soil temperature vertical profiles showed a diurnal and semidiurnal cycle due to the influence of these factors. The diurnal thermal amplitude at -0.05 m reached 14.6°C on 20 February 2000, but only 2.8°C on 1 July 2000. Most fluctuations of sediment temperatures were observed in the first 15 cm, with vertical gradients of 0.82°C cm^-1 during summer. In order to obtain the soil heat capacity, the granulometric composition of a sediment column was analyzed. Soil heat fluxes during a typical summer and winter day were compared. A net heat gain in the soil was observed during the summer day (+840.6 kJ m^-2) and an inverse situation was found in winter, with a diurnal heat exchange of-768.6 kJm^-2.

Key words: sediment heat flux, soil temperature, tidal flats, Bahía Blanca estuary.

 

Resumen

Se analizaron procesos de interacción sedimento-agua y sedimento-atmósfera para cada estación del año mediante fórmulas aerodinámicas de masa. Los datos fueron recolectados en la llanura intermareal de Puerto Cuatreros (estuario de Bahía Blanca) durante un período anual (2000). La temperatura del suelo se midió cada 10 min empleando una cadena de termistores localizados a tres niveles de profundidad: -0.05, -0.15 y -0.25 m. Simultáneamente, se registraron las temperaturas del agua y del aire, la radiación solar y datos meteorológicos. Los valores medios anuales fueron 14.07°C, 13.69°C y 14.51°C para sedimento a -0.15 m, aire y agua, respectivamente. La radiación solar y las mareas regularon el comportamiento térmico de la planicie de marea. Debido a la influencia de estos factores los perfiles verticales de temperatura del sedimento mostraron un ciclo diurno y semidiurno. La amplitud térmica diurna a -0.05 m alcanzó los 14.6°C el 20 de febrero, mientras que durante el I^o de julio Se observaron sólo 2.8°C de amplitud. Las mayores fluctuaciones en la temperatura del sedimento se registraron en los primeros 15 cm, desarrollándose gradientes verticales que alcanzaron los 0.82°C cm^-1 durante el verano. A fin de obtener la capacidad calorífica del suelo en estudio se analizó la granulometría de una columna de sedimentos. Se comparó el flujo de calor en el suelo durante un día típico de verano y uno de invierno evidenciándose una ganancia neta de calor durante la jornada estival (+840.6 kJm^-2) y una situación inversa en el invierno (-768.6 kJm^-2).

Palabras clave: flujo de calor en sedimentos, temperatura del suelo, planicies de marea, estuario de Bahía Blanca.

 

Introduction

Soil temperature is an essential parameter when evaluating physical and biological characteristics of coastal ecosystems. Temperature has a great influence on the population dynamics of pelagic and benthic communities inhabiting the intertidal zone. Soil temperature profiles result mainly from molecular diffusion of heat through the sediment, depending also on its thermal properties and atmospheric conditions (Harrison and Phizacklea, 1987a). In intertidal sediments in particular, solar radiation and tides are mechanisms that drive the thermal behavior of the tidal flats (Piccolo et al., 1993).

The Bahía Blanca estuary (fig. 1), situated in the southwest of Buenos Aires Province, Argentina, is considered the second largest estuary in the country after the Río de la Plata estuary. It is a mesotidal, coastal plain environment formed by a series of major NW-SE trending channels separating extensive tidal flats, low salt marshes and islands (Perillo and Piccolo, 1999). The total surface of the estuary is approximately 2300 km², of which 1150 km² correspond to the intertidal sector. The principal energy input into the system is produced by a semidiurnal tidal wave (Perillo et al, 2000).

Temperatures of muddy intertidal sediments have been analyzed in other parts of the world by different authors (Heath, 1977; Harrison and Phizacklea, 1984, 1985, 1987a, b; Piccolo et al, 1993). Although work has been reported on the physical characteristics and hydrography of the Bahía Blanca estuary, as well as on the estuarine circulation (Arango, 1985; Sequeira and Piccolo, 1985; Serman, 1985; Perillo etal, 1987; Piccolo et al, 1987; Piccolo and Perillo, 1990; Perillo and Piccolo, 1991), specific knowledge about thermal characteristics of the tidal flats is needed. Because organisms inhabiting the mudflats are mostly infaunal species, the study of the sediment's thermal behavior is essential for understanding this habitat. Previous research about this subject in the Bahía Blanca estuary was carried out by Piccolo and Dávila (1991), who studied the thermal characteristics of intertidal sediments in Ingeniero White harbour (Bahía Blanca estuary), determining the mean thermal diffiisivity in summer (5.7 x 10^-7 m² s^-1) and winter (3.5 x 10^-7 m² s^-1).

This study was performed at Puerto Cuatreros, located in the inner portion of the estuary, near its head (fig. 1). It forms part of the first stages of a larger interdisciplinary study focused on establishing relationships between temperature and biodiversity in the tidal flats of the estuary. The objective of this paper is to describe the characteristics of the sediment thermal behavior of the Puerto Cuatreros tidal flats.

 

Material and methods

Soil, water and air temperatures were measured every lOmin during the 2000 calendar year by employing a thermistor chain. The thermistors were located below the sediment surface (0.05, 0.15 and 0.25 m depth), in the water column (1 m depth during ebb tide), and in the air column (0.05 and 3 m height). The thermistor at 0.05 m in the air recorded water or air temperature depending on the tidal stage. Solar radiation was recorded by a pyranometer and tidal data were obtained by a tidal gauge, both installed in the study area. Meteorological data were gathered from an automatic weather station situated in Ingeniero White harbour. Although measurements were carried out during an entire year (2000), two days were analyzed in detail, a summer day and a winter day, each with the same tidal stage and a cloudless sky.

The soil heat flux (Qg) was determined from temperature data using (Oke, 1978):

where T is sediment temperature [K], z is depth [m], X is thermal conductivity [W m^-1 K^_1], Ks is thermal diffusivity [m² s^-1], and C is heat capacity [J m^-3 K^-1 10⁶]. Thermal diffusivity was calculated from the heat conduction equation (Kjerfve, 1978):

where Δt is the phase lag [s], ω is the diurnal oscillation frequency [2Π/P], and P is the period [s]. A 25-cm-deep sediment core was extracted in order to analyze the grain size distribution at the laboratory. The heat capacity was obtained from tables (Oke, 1978) and estimated as a mean value of saturated clay and saturated sand heat capacities, typical conditions of the tidal flat sediments in the study area.

 

Results and discussion

Sediment analysis

The results of the grain size analysis are shown in table 1. Clay was dominant in the sediment column, increasing with depth. Silt represented a large percentage in the composition of all the samples, while gravel was absent. A different behavior was obviously observed for sand, its presence decreasing gradually from the sediment surface. Due to the prevailing fine fraction, water is retained in the pore spaces and sediment was saturated or near saturation point.

Daily cycle

Diurnal and seasonal variations of soil, water and air temperatures were important. Annual mean values of soil (at -0.15 m), air and water temperatures were 14.07°C, 13.69°C and 14.51°C, respectively. Figure 2 shows the temperature waves in the air, water and sediments during a typical summer day (20 February 2000) and a typical winter day (1 July 2000). The thermistor recording the air/water temperatures depending on tidal stage indicates the moment when sediments were flooded.

On the days mentioned, the air temperature ranged from 10.4°C to 33.1°C (summer) and from 2.1°C to 9.9°C (winter), resulting in a thermal amplitude of 22.7°C on the summer day and of 7.8°C on the winter day. The soil temperature waves showed a similar trajectory to that of the air, with a time lag that increased with depth. The time lag was estimated to be 3 h40 min at -5 cm depth and 7 h 40 min at -15 cm on 20 February, and 1 h 40 min at -5 cm and 6 h at -15 cm on 1 July. Throughout the measuring period, time lags changed according to weather conditions (cloudiness, for example), but the two days taken as an example give an idea of the daily variation.

Changes in wave amplitudes were observed in depth and between seasons: the amplitudes were reduced at greater depth and a seasonal variation could be observed between summer and winter (amplitudes decreased faster on winter days). Damping depth (D) was estimated to be 9.5 cm on 20 February and 8.5 cm on 1 July (Monteith, 1973).

Besides seasonality, another factor that affected the soil temperature waves was the tidal inundation. This was more clearly evident during the summer day, when temperature gradients between air, water and sediment were greater. Due to solar radiation, the temperature wave of the upper sediment layer showed an absolute maximum between 14:00 and 17:30 (approximately). During the hours of flood tide, the curve showed two relative maximums, indicating that soil was receiving heat from the water. On 1 July the temperature wave at -5 cm presented a more extended maximum that lasted about 5.5 h (from 14:30 to 20:00). The heat transfer from water to soil during tidal inundation was the probable cause.

Sediment temperature vertical profiles (fig. 3) showed a diurnal and semidiurnal cycle due to the atmospheric and tidal influence. The main fluctuations of sediment temperature were found in the first 15 cm. Below this layer, temperature presented a smaller gradient with depth, with a daily mean amplitude (at -25 cm) of 1.4°C on 20 February and 0.66°C on 1 July. When tidal flats were exposed to solar radiation, especially at midday and early afternoon, important vertical gradients developed in the upper 15 cm, reaching mean values of 0.82°C cm^-1 during the summer day. During the night hours, a profile inversion was observed. However, on the winter day, due to the important reduction of solar radiation (about half at midday) and light hours, significant thermal gradients were not recorded at noon. Higher temperatures at depth than on the surface were observed, which indicates that the soil was cooling as it was exposed to a cold atmospheric layer.

The diurnal amplitude at -5 cm was 14.6°C on 20 February and only 2.8°C on 1 July. The contrast in the sediment temperature conditions between both seasons was also evident in the profile shape (fig. 3). While a typical summer day presented a funnel-shaped profile, a winter day remained at an almost constant vertical profile inversion that was reduced towards 15:30-17:30, the time at which the tidal flat was flooded and water infiltration through the sediment interstices acted as a heat transfer mechanism.

Soil heat flux

Heat flows through the sediment by thermal conduction and the rate of heat flux depends on the strength of the mean temperature gradient and the ability of the particular soil to transmit heat (soil thermal conductivity) (Oke, 1978). By convention, soil heat flux is positive when it moves downwards (heat gain) and negative when the circulation is upwards (heat loss). In this investigation, the biggest temperature fluctuations were observed between 5 and 15 cm depth, so this layer was selected in order to assess the sediment heat flux. The equation used to calculate the soil heat flux considered the following data:

• Temperature gradient (AT/AZ). Although temperature gradient was measured at a depth of-5 and -15 cm, it is necessary to take into account the sediment-air temperature gradient. On 20 February, the temperature gradient between the sediment upper layer (-0.05 m) and the air (3 m) was mainly around 4°C or 5°C, reaching almost 9°C at 10:40 (absolute values); however, on 1 July the difference was only 1°C or 2°C during most of the day. Obviously, these conditions had an influence on the heat flux through the study layer, resulting in a considerably larger heat flux on 20 February than on 1 July (fig. 4).

• Thermal conductivity (X). Thermal conductivity values were 1 W m¹ K~' on 20 February and 0.79 W nr' K"¹ on 1 July. According to the granulometry of the tidal flat sediments (table 1), heat capacity was estimated as a mean value of saturated clay and saturated sand heat capacities (Oke, 1978), resulting in 3.03 * 10⁶ J m ³ K¹. Thermal diffusivity, which varies with periodic changes of temperature and soil moisture content, was estimated at 3.31 x 10^-7 m² s^-1 and 2.61 x 10^-7 m² s^-1 for 20 February and 1 July, respectively. This range of values was similar to those estimated previously by Piccolo and Dávila (1991) in Ingeniero White: 5.7 x 10^-7 m² s^-1 in summer and 3.5 x 10^-7 m² s^-1 in winter (monthly mean values).

Figure 4 shows the heat exchange during 20 February, which had its maximum value in the hours of maximum radiation during ebb tide. The tidal influence was also observed in the fast reduction of heat flux during the hours of maximum height, indicating that heat exchange across the sediment-atmosphere interface was greater than across the sediment-water interface during the summer day, mainly due to the difference between water temperature and air temperature in the same conditions.

A net gain or storage of heat in the soil was shown by the prevailing positive values of flux on 20 February (diurnal heat balance of 840.6 kJ m^-2). On the contrary, during the winter day a net loss was observed, with a total heat exchange of-768.6 kJ m^-2. Positive values were small and only observed between 14:30 and 19:00, during tidal inundation.

In summary, the analysis of diurnal temperature variations and soil heat balance indicates that solar radiation and tides were factors that regulated the shape of the heat flux in the tidal flats. The season appeared to have determined the general direction of heat flux through sediment, resulting in a net gain during summer and a net loss in winter.

Although this is a preliminary study, the analysis of the soil thermal behavior shows that the ecosystem experiences important variations throughout the year. Hence, the next step will be to analyze the relationship between this annual heat variation in the sediment and the biodiversity of the tidal flats.

 

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