Chapter 7 - Heat Energy Balance in Coastal Wetlands

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From M. Cintia Piccolo, Heat Energy Balance in Coastal Wetlands.In: Gerardo M. E. Perillo, Eric Wolanski, Donald R. Cahoon, Mark M. Brinson, editors,

Coastal Wetlands: An Integrated Ecosystem Approach.Elsevier, 2009, p. 211. ISBN: 978-0-444-53103-2

ª Copyright 2009 Elsevier B.V.Elsevier.

Author's personal copy

C H A P T E R 7

HEAT ENERGY BALANCE IN COASTAL WETLANDS

Marıa Cintia Piccolo

Contents

1. Introduction 211

2. Mid-Latitudes 2153. Low Latitudes 2204. High Latitudes 222

5. Summary 225Acknowledgments 226References 226

1. INTRODUCTION

By virtue of its unusually high specific and latent heat, water standsapart from other substances in its thermal properties for supporting the life cyclesof a great number of plant and animal species that depend on wetlands. Wetlandplants alone exhibit a wide diversity of growth forms including emergent plants,submerged plants, floating-leafed plants as well as a combination of these leafforms within the same species (Sculthorpe, 1967). Given this variety, it is usefulnot only to understand how these organisms have adapted to energy budgets,but also their role in contributing to the distribution of energy flows at theecosystem level.

Relatively few studies have been conducted on heat exchanges in coastalwetlands. Yet, an understanding of how heat is transferred across air–sea–soilinterfaces is fundamental to predicting, for example, how coastal wetlandswill respond to global climate change. Coastal wetlands develop particularlysteep chemical and hydrological gradients as a result of their position betweencontinents and the ocean. Unlike vertical fluxes that dominate exchanges inupland ecosystems, the exchange of matter and energy in coastal wetlands iscomplicated by strong horizontal fluxes, particularly those driven by watermovement. This chapter reviews studies that have contributed to understandingof how heat energy is transferred across the different levels of these fluid andsolid interfaces, and how this energy affects different biological and chemicalprocesses.

Coastal Wetlands: An Integrated Ecosystem Approach � 2009 Published by Elsevier B.V.

211


Studies of the energy budget over an earth surface elucidate how solar energy islocally redistributed to create a particular microclimate (Kjerfve, 1978). The heatbudget equation (Oke, 1978)

RN = QHþQBþQEþQA ð1Þ

establishes that at any moment in time the available energy at the earth’s surface(net radiation, RN) must be equivalent to a combination of convectiveexchange to or from the atmosphere (sensible, QH, and latent heat, QE),conductive flux to or from the soil (QB), and incoming or outgoing advectiveflux (QA). Figure 1 shows the components of the heat budget equation in anidealized bare tidal flat.

The interface or boundary between the water and the air is dynamic. Matter andenergy are continuously being transferred across the air–sea interface in bothdirections. Air either gains or loses heat from the water depending on the tem-perature difference between the water surface and the overlying air. Water evapo-rates to contribute to atmospheric moisture and atmospheric water vapor condensesto form fog and clouds, and eventually precipitation. Vegetated wetlands alsoreceive and lose energy by radiation, conduction, convection, and evaporation.However, water lost by plant transpiration must be separated from salts in seawater,a process that comes at great cost to the plant (Teal and Kanwisher, 1970). Fewspecies are well adapted to do this, which explains in part the low species richness ofvascular plants in coastal wetlands.

Horizontal exchanges of matter and energy are affected by tides and wind.Of these, tides behave more predictably (Perillo and Piccolo, 1991). They areresponsible for the ebb and flow of water in all of the major groups of wetlands(seagrass meadows, mudflats, marshes, and mangroves). Winds, on the otherhand, are less predictable (except for sea breezes), and few studies focus on howwind affects the energy budget of coastal areas (Leal and Lavın, 1998; Castroet al., 2003). In any case, advective fluxes may originate with tides and/or withwind (Figure 1b). Furthermore, winds may serve to amplify or dampen theeffects of astronomic tides.

Atmosphere

RN QH QE QA

QB

RN QH QEQA

QB

QAWaterSoil

Atmosphere(b)(a)

Soil

Figure 1 Components of the heat balance equation at a bare tidal flat: (a) low tide and(b) high tide.

212 Marıa Cintia Piccolo


Annual heat flux cycles in water bodies usually follow the seasonal fluctuationsof incident solar radiation. Annual mean radiation is used to express the main sourceof heat energy, and has been measured in several studies to determine both themagnitude and the seasonality in coastal wetlands (Hovel and Morgan, 1999;Bianciotto et al., 2003; El-Metwally, 2004; Jacobs et al., 2004; Paulescu et al.,2006). Radiation on tidal flats is of particular interest because of its significanceto ecological processes. In a study of sediment temperatures on the Bay of Fundy(Canada), Piccolo et al. (1993) confirmed that radiation and tides are the driversof thermal behavior over and within tidal flats. Gould and Hess (2005) focusedon environmental radiation related to tides. They measured sediment exposurerate from environmental radiation on tidal flats using a high-pressure ionchamber so the shielding effects of the tidal cycle could be evaluated. Theyderived a theoretical model to predict the behavior of exposure rate as afunction of time. In addition, they developed an empirical formula to calculatethe total exposure on a tidal flat that requires measurements of only the slope ofthe tidal flat and the exposure rate when no shielding occurs (Gould and Hess,2005). The formula and the model can be applied to biological studies whereradiation exposure is needed.

Radiation can be used to estimate other parameters of the heat balanceequation. For example, Geostationary Operational Environmental Satellite-derived estimates of radiation to predict daily evapotranspiration (ET) inFlorida wetlands (USA) with the Penman–Monteith, Turc, Hargreaves, andMakkink models (Jacobs et al., 2004). These estimates agreed well with ETmeasured with an eddy correlation system. Also incoming radiation plays amajor role in chlorophyll production in plants in general; higher radiationinduces a reduction in the chlorophyll production rate for each of the studiedspecies.

With regard to incoming short-wave radiation (Qsw), linear relationships havebeen found between daytime incoming radiation and both net radiation [RN = 0.73,Qsw� 13.45 (W/m2)] and reflected radiation [QR = 0.079, Qswþ 3.3 (W/m2)]over a Spartina alterniflora salt marsh during the summer (Crabtree and Kjerfve,1978). They found that, on the average, net radiation was 70% and reflected radiationwas 9% of incoming radiation. In spite of potentially damaging effects of Qsw, Costaet al. (2006) found no evidence of differential sensitivity or resilience to UV-Bradiation between Salicornia species from low-mid-latitudes and a high-latitudepopulation in the Americas.

For many wetlands, ET is the major component of water loss. When consideredas its energy equivalent, latent heat flux, evaporation is a major energy sink (Wesseland Rouse, 1993; Souch et al., 1996). Yet despite numerous studies, evaporationfrom wetlands is little understood (Lafleur, 1990) and detailed studies of the physicalprocesses involved are geographically restricted (Souch et al., 1996). The latent heatflux term in Equation (1) is second only to the radiation flux. Most of the radiationenergy is used for evaporation. In spite of that, the hydrologic implications may beminor because water loss from coastal wetland sediment is typically and oftenreplaced by precipitation or by infiltration of flooding estuarine water (Harveyand Nuttle, 1995).

Heat Energy Balance in Coastal Wetlands 213


Soil properties play an important role in plant composition, productivity, andzonation in coastal ecosystems because plant species are differentially tolerant tosalinity and soil saturation (Adams, 1963; Pennings et al., 2005; Wang et al.,2007). Variations in salinity and temperature in the intertidal zone restrict thedistribution and abundance of marine organisms (Johnson, 1967). In tidal flats,species richness is higher where rates of salinity change are low (Sanders et al.,1965). In sandy intertidal areas where salinities fluctuate widely, the maximumnumber of species and individuals is small. This feature is not caused by sedi-ment salinity alone because temperature, oxygen content, and other environ-mental factors also vary by intertidal zone. There is a strong relationshipbetween evaporation and sediment salinity. Removal of plant cover, for exam-ple, induces higher soil temperature, increases pore water salinities, and lowerswater content, most likely due to the greater sun exposure and higher evapora-tion (Whitcraft and Levin, 2007). On sandy beaches, Johnson (1967) found thatan increase in evaporation caused an increase in the soil salinity in the first 20 cmof depth. Many of the effects of salt on plants occur as a result of water stress.Tolerance of plants to saline soils is due in part to biophysical, morphological,and biochemical adaptations. The narrow leaves characteristic of high marshspecies may be an adaptation to help regulate leaf temperature in times of lowlatent cooling (Maricle et al., 2007). Latent heat fluxes of coastal wetlands play asignificant role in plant zonation.

Surface sensible and latent heat fluxes can be used to predict coastal stormdevelopment and precipitation. Both affect wetland development. Surface latentflux provides a direct source of moisture needed for precipitation, while sensibleheat flux can affect the stability of the storm environment, thereby modulatingthe timing and amount of precipitation (Persson et al., 1999). Data from anymeteorological station placed almost anywhere across a wetland provide repre-sentative estimates of evaporation when atmospheric conditions are relativelyhomogeneous. Individual patches of vegetation in a wetland do not influenceoverlying atmospheric conditions significantly and evaporation can be estimatedusing the well-known formula of Penman–Monteith (Gavin and Agnew, 2003).However, Acreman et al. (2003), using the eddy correlation method to calculateevaporation from two types of wetlands, wet grassland and reed beds, in south-west England, demonstrated that the evaporation calculated by the Penmanpotential method did not represent actual evaporation. General agreement as tothe best method of estimating evaporation on different coastal wetlands is still tobe achieved.

Because different climates generate different types of coastal wetlands and,therefore, are the product of different heat energy balances, this chapter reviewsheat balance studies on a diversity of wetland types, and with a variety ofmethods. Two facts must be highlighted. First, there are very few relevant studiesand second, most studies were performed on tidal flats in estuaries, with verylittle work on other types (i.e., salt marshes and mangroves). The review ispresented in three sections to emphasize climate difference in the wetland studied.Since most of the investigations were carried out at mid-latitudes, the review startsin this region.



2. MID-LATITUDES

Tidal flats are affected by tides, winds, and sun radiation exposure. Heatbalance researches made at temperate tidal flats are generally based on the bulkformulation (Hsu, 1978; Smith, 1981; Smith and Kierspe, 1981; Vugts and Zim-merman, 1985; Harrison and Phizacklea, 1985; Piccolo and Davila, 1993; Piccoloet al., 1993, 1999; Beigt et al., 2003; Beigt and Piccolo, 2003). Two differentmethods to estimate the heat balance on tidal flats will be presented here. Bothmethods highlight the significant influence of the tidal forces on heat fluxes.

For instance, the annual heat exchanges that occur at an estuarine tidal flat in theBahıa Blanca Estuary, Argentina were studied (Beigt, 2007; Beigt et al., 2008). Heatfluxes were analyzed across the water–atmosphere and the sediment–atmosphereinterfaces at high and low tide, respectively. Different bulk aerodynamic formulaswere used to estimate the radiative and turbulent fluxes from available meteorolo-gical and oceanographic data. Net radiation (RN) was determined from incidentsolar radiation and temperature data using (Evett, 2002)

RN = Rsið1� �Þ � L"þL#ðW=m2Þ ð2Þ

where Rsi is the incident solar radiation, � is the albedo, L" is the terrestrial long-wave radiation [L"= "s�Ts

4], L# is the atmospheric long-wave radiation, "s is thesurface emissivity, � is the Stefan–Boltzmann’s constant (W/m2/K4), and Ts is thesurface temperature (K) (water or sediment temperature, depending on tidal stage).Atmospheric long-wave radiation (L#) is assessed using two different equationsdepending on temperature data. The Swinbank (1963) equation is used whentemperatures are over 0�C, while the Monteith (1973) equation is used for tem-peratures lower than 0�C and higher than �5�C.

Soil heat flux (QB) across the surface layer was determined from temperaturedata using the usual Fourier equation (QB =�� (DT/Dz), Oke, 1978), where T isthe sediment temperature (K), z is the depth (m), � is the thermal conductivity[�= KSC (W/m/K)], KS is the thermal diffusivity (m2/s), and C is the heat capacity(J/m3/K). Sensible heat flux was estimated by means of two different equationsdepending on the tidal stage. During tidal flat inundation, sensible heat flux isassessed using (Kantha and Clayson, 2000; Zaker, 2003)

QH = �cpðUa � UsÞCHðTw � TaÞðW=m2Þ ð3Þ

where � is the air density (kg/m3), cp is the specific heat of the air (J/kg/�C), Ua isthe wind speed at height z, Us is the wind speed at the water surface (m/s) (zero fora stationary surface), CH is the heat exchange coefficient (dimensionless), Tw is thetemperature at the water surface (�C), and Ta is the air temperature (�C). The heatexchange coefficient (CH) for the water–atmosphere interface was taken fromFriehe and Schmitt (1976).



During sediment exposure to atmospheric conditions, sensible heat flux wasestimated by (Evett et al., 1994; Evett, 2002)

QH = �cpðTs � TaÞDHðW=m2Þ ð4Þ

where DH(= k2U[ln(z/z0H)]�2) is the heat exchange coefficient (Kreith andSellers, 1975; Ma et al., 2003) (m/s), k is the von Karman’s constant, and z0H isthe roughness length for sensible heat flux (m) taken from Kreith and Sellers(1975).

Latent heat flux across the sediment–atmosphere and water–atmosphere inter-faces was estimated by the Penman–Monteith equation that is the potential eva-poration rate, namely, the evaporation rate that occurs when water availability isnot limiting (Wallace and Holwill, 1997). Although it is usually applied in agro-nomical studies of vegetated and bare soils, it has also been applied to salt marshes(Hughes et al., 2001) and tidal flats (Harrison and Phizacklea, 1985). Taking intoaccount that the sediments of the studied tidal flat are permanently saturated,assessing the potential evaporation was considered correct (Beigt, 2007). Theauthors estimated that annually, nearly 5,978 MJ/m2 of heat entered the tidal flatas incident solar radiation. Of this, only 2,954 MJ/m2 (49.4%) remained as availableenergy (RN). Winds and tides helped to add heat to the ecosystem (1,301 MJ/m2).The annual budgets of sensible and soil heat fluxes showed that both processesprovided heat energy to the tidal flat surface. Indeed, an amount of 947 MJ/m2 wastransferred to the surface as sensible heat, while the annual budget of soil heat fluxindicated an upward heat transfer of 25.2 MJ/m2. The total energy that entered thetidal flat was balanced by an equal heat loss (5,227 MJ/m2 = 2,127 mm) as evapora-tion (Beigt, 2007; Beigt et al., 2008). Surface heat fluxes through the air–seainterface for the coastal water of Kuwait was estimated by Sultan and Ahmad(1994) using the bulk formulas such as Beigt (2007) and showed similar annualbehavior. Even though Kuwait is representative of an arid region, both investiga-tions show that a nocturnal inundation generally heats the tidal flat sediment(previously cooled by long-wave emission), causing an upward circulation ofsensible heat. On the contrary, a tidal inundation at midday or early afternoonusually cools the sediment, with the resultant flow of sensible heat from the air tothe tidal flat.

The horizontal transport of heat or advection causes the addition (or subtrac-tion) of energy to (from) an ecosystem. The most common agent of this process iswind; however, tide must also be considered when studying a tidal flat. Tidalenergy generally acts as an “energy subsidy” to the coastal ecosystem, increasingits productivity as it increases the amount of energy which is capable of beingconverted to production (Odum, 1975). Advective heat flux is estimated as theresidual energy from the heat budget equation [Equation (1)]. The total advectiveflux is then divided according to the tidal height into two different fluxes(“advective flux at low tide” and “advective flux at high tide”). The former isdeveloped by winds, while tide is considered to be the main agent during tidalinundation. Atmospheric and tidal conditions regulate the heat exchanges. Tidal



inundation affects the direction and magnitude of sensible and soil heat fluxes.The 2003 annual heat budget showed that net radiation, advective, sensible and soilheat fluxes provided heat to the tidal flat surface and the most important heat fluxeswere net radiation and latent heat. They are followed (in order of magnitude) byadvective and sensible heat fluxes and finally by the soil heat flux (Beigt et al.,2008).

Few heat balance studies were carried out on salt marshes. Teal and Kanwisher(1970) calculated the energy balance for the plants growing in a marsh on CapeCod. They found that leaf temperature was well coupled to air temperature. If therewere no evaporation of water from the leaves, they would have been from 3.6�C to9.2�C above air temperature when heat gain equalled heat loss. Some of theirresults are shown in Table 1, where � is the Bowen ratio (= QS/QE). Latent heatfluxes were always greater than sensible heat ones. The same behavior is found inmost of the heat balances in coastal wetlands (Vugts and Zimmerman, 1985; Rouse,2000; Beigt et al., 2008). The net balance between ET and rainfall infiltration isbelieved to be important in controlling soil salinity, particularly in the less fre-quently flooded high marsh zone. To elucidate the biophysical effects of droughtand salinity on the interception and dissipation of solar energy in estuarine grasses,Maricle et al. (2007) studied leaf energy budget of 13 species. They found thatlatent heat loss decreased by as much as 65% under decreasing water potential (ameasure of the ability of a substance to absorb or release water relative to anothersubstance), causing an increase in leaf temperature of up to 4�C. Consequently,radiative and sensible heat losses increased under decreasing water potential. Sen-sible heat flux increased as much as 336% under decreasing water potential. Latentheat loss appeared to be an important mode of temperature regulation in all speciesand sensible heat loss appeared to be more important in high marsh species than inlow marsh ones (Maricle et al., 2007). In general, heat budget estimates showedsimilar annual patterns (Sultan and Ahmad, 1994; Roads and Betts, 2000; Hugheset al., 2001; Rutgersson et al., 2001; Finch and Gash, 2002).

Another method to calculate the heat budget of a tidal flat area indirectly fromdownstream observations of temperature and horizontal velocity in a tidal coursewas presented by Onken et al. (2007). The advective heat flux (Qa) in tidal channelsis monitored and then the heat excess or deficit for the catchment area is calculatedby integral methods. Instead of using the bulk formulation, a relationship betweenthe velocity and the volume flux is established. The heat budget of the upstreamregion is then determined by integrating the heat flux over one tide (Qtide). The

Table 1 Heat fluxes (W/m2) measured in a Spartina alterniflora salt marsh on cape code

Date RN QE QH �=QH/ QE

30 August 425.353 599.678 104.595 0.1730 August 439.299 522.975 97.622 0.1928 September 320.758 278.920 83.676 0.3010 November 355.623 355.623 104.595 0.29

Source: Modified from Teal and Kanwisher (1970).



heat budget of a water volume fixed in space is simply defined as the sum of the heatinput through its boundaries and the production of thermal energy inside thevolume (Onken et al., 2007). The direction of the heat flux components betweenthe tidal flat and the associated channels during all the different tidal stages isdescribed in Figure 2. At low tide, the bottom and the atmosphere exchange heatin terms of the following vertical heat flux

Qat = QswþQlwþQEþQH ð5Þ

where Qsw is the short-wave radiative heat flux and Qlw is the long-wave radiativeheat flux. The bottom heat content changes due to atmosphere–soil interaction(Figure 2d). The advective flux in the channel is zero because there are no currents(slack water). During flood tide, there is a positive flux of heat toward the tidal flatby means of the advective flux Qa (onshore flow). At the same time, the heatcontent of the water column is modified by the interaction with the atmosphericflux Qat, and the heat exchange Qb between the substrate and the water (Onkenet al., 2007). The high water condition (Figure 2b) is also characterized by zero heatadvection but the water temperature will change due to Qat and Qb fluxes. Theheat gain (or loss) Qtide of the water over one tidal cycle from low tide to low tidewas determined by measuring the terms in Equation (6)

Qtide =

Z tþlow

tlow

Qadt =

Z tþlow

tlow

ðQatþQbÞdt ð6Þ

Qb

Qb

Qb

Qat

Qat

Qat

Qat

Ebb

High

Flood

Low

(a)

(b)

(c)

(d)

Qa

Qa

Figure 2 Heat flux components on different tide stages (modified fromOnken et al., 2007).



where t is the time, tlow indicated the time of the low tide, and tþlow the time of thesubsequent one. Onken et al. (2007) developed in addition a simple model whichcan be used to determine the integral bottom heat flux of the tidal flats. Ananalytical estimate suggests that the sign of the budget is controlled by the tidalprism and the length of the drying period of the flats in the upstream region. Thismethod presents another option for a good estimate of the heat balance equationwithout using the bulk formulation.

Other studies, such as heat budget investigations performed in PauatahanuiInlet, a small (3.5 km2) New Zealand coastal inlet, indicates that the balance isessentially between solar and long-wave radiation, evaporation and advective heatexchange with the coastal waters (Heath, 1977). The role of the mudflat in the heatbalance is only minor. The temperature at the entrance to the inlet exhibits strongtidal fluctuations resulting from exchange with the coastal waters and a diurnalinequality produced by interaction of solar heating with the tidally controlledsurface area and volume. The simulated temperature record for a month, calculatedfrom a heat budget equation exhibits the effect of the tidal/solar interaction inproducing a 14.75-day pulse, variable diurnal inequality, and the generation ofhigh-frequency components (Heath, 1977).

Several studies in different coastal environments related to heat balance equationfocus on temperature fluctuations and/or applied numerical models. The thermalbehavior of the air, water, and sediments over a tidal flat was studied at Starr Point,Minas Basin, Bay of Fundy, Canada in July 1989 (Piccolo et al., 1993). Tempera-ture in the intertidal sediments showed rapid changes which occur principallyduring tidal inundation. Vertical gradients of 0.5� 10�2�C/m were found in theupper 0.25 m layer. The presence of large populations of Corophium volutatorincreased thermal diffusivity because of their vertical migration. Therefore, theheat was distributed faster and through a greater depth. Vugts and Zimmerman(1985) predicted daily mean water temperatures with heat balance calculations ofthe tidal flat areas of the Dutch Wadden Sea. The daily heat balance interacts withthe tidal cycle, resulting in a 15-day periodicity in the water temperature as well inthe bottom temperature. They showed that with a simple model and some mea-sured bulk parameters, it is possible to predict daily mean water temperatures fromsimple weather data measured at a nearby coastal station.

The traditional formulation of the SWIFT2D model has been applied tonumerous estuaries, bays, and harbors throughout the world. Swain (2005) mademodifications to expand SWIFT2D for applicability to shallow coastal wetlands.These modifications include the representation of spatially and temporally varyingrainfall and ET, wind sheltering owing to effects of emergent vegetation, andchanges in frictional resistance with depth. Dietrich et al. (2004) presented amodel-based method of determining the surface fluxes of heat and freshwater inthe near-shore coastal waters. The new method determines the fluxes as a residualwithin the framework of physically realistic and natural boundary conditions on thesea surface temperature and sea surface salinity.

On the basis of a balance model of the energy by surface waves in a coastal zoneand experimental data about surface flows in shallow and deepwater zones, Paninet al. (2006) developed a model of the heat–mass exchange of a coastal zone



reservoir with the atmosphere. The model allows the calculation of the values ofthe energy–mass exchange in the atmosphere boundary layer and the correspondinginteraction characteristics based on standard micrometeorological information. Forthe construction of the model and its verifications they used the direct measurementdata (eddy covariance) of the momentum, heat and humidity turbulent fluxes, aswell as the surface wave characteristics and the main microcharacteristics of air andwater. The model shows the intensification of the processes in a coastal zone incomparison with an open sea and allows the determination of the size intensifica-tion of flows at different distances from the coast.

3. LOW LATITUDES

Estimation of the heat budget in low latitudes (tropical and subtropical) isperformed as it is in mid-latitudes. Tropical wetlands are characterized by an increasein radiation energy and dense vegetation. Salt marshes are ideal to study plantcommunity patterns. Species interactions during colonization of bare patches aredifferent than those found in dense vegetation. The metabolic process of the plantcommunities exhibit different rhythms of intensity. They are regulated by variationsin environmental factors such as light and temperature. Another important vegetationparameter such as photosynthesis is known to be temperature sensitive (Hargrave,1969; Gallagher and Daiber, 1973). Since temperature affects respiration rates, anexogenous daily rhythm in respiration in salt marshes and mangroves would beexpected in response to temperature cycles (Gallagher and Daiber, 1973).

The presence of plants affects ecosystem-level processes such as hydrology,sedimentation rate and nutrient cycling (Bertness, 1988; Whitcraft and Levin,2007). Plant cover is a fundamental feature of many coastal marine and terrestrialsystems and controls the structure of associated animal communities. Studying theimpact of shading in salt marshes, a relationship between temperature, salinity,water content and macrofaunal density and diversity was determined by Whitcraftand Levin (2007). Increases in temperature and salinity and decreases in watercontent for Salicornia virginica were correlated with decreased macrofaunal density.Although heat balance studies are important to determine these temperature varia-tions in vegetated ecosystems, no specific measurements were found in theliterature.

On the other hand, estuaries in arid tropical regions differ significantly fromtheir temperate and wet tropical counterparts. First, river discharge into the estu-aries is often highly seasonal with very large flows in the wet season being followedby 5–10 months of negligible discharge. The second difference is that large areas ofsalt marshes, mangrove swamps and salt flats (where annual evaporation greatlyexceeds annual precipitation) often fringe these tropical arid estuaries (Ridd andStieglitz, 2002; de Silva Samarasinghe and Lennon, 2004) and they usually becomeshypersaline for much of the year.

Evaporation plays an important role in concentrating salts and nutrients insoils and groundwater in estuarine wetlands. This is particularly true in zones



of less frequent tidal inundation where the soil salinity depends on the balancebetween “evapoconcentration” of tidally supplied salts and rainfall orgroundwater flushing (Hughes et al., 2001). The more arid the climate, themore extreme this effect becomes. For example, large areas of mangrove-fringedsalt flats occur in the dry or mainly dry tropics. In contrast, salt marshes intemperate Australia seldom have unvegetated zones (Saenger et al., 1977).ET estimates are often the weak link in wetland water and solute balancemodeling (Hughes et al., 2001).

Latent heat flux is a major variable to calculate for salinity changes estimates overthe mangroves. The plant zonation in salt marshes is a consequence of localvariation in soil patch salinities (Bertness, 1991). The different soil salinities is dueto tidal flooding, annual variation in rainfall, ET and small-scale topographicfeatures which influence the drainage. Evaporation and ET increase the salinity ofthe swamp soils. A typical value of evaporation over open water in tropical areas is5 mm/day. Wolanski et al. (1980) and Wolanski and Ridd (1986) calculatedevaporation rate over mangroves at a rate of 2 mm/day. For salt flats, Hollinsand Ridd (1997) estimated a monthly average evaporation rate of 2 mm/daywith peak rates of 4–5 mm/day during spring tides (when the flats are saturated)falling to less than 1 mm/day when the salt flats form a hard surface crust. Thesalinity rate of change due to this effective evaporation rate E (Ridd and Stieglitz,2002) is given by

@S

@t=

ES

hð7Þ

where S is the salinity, E is the evaporation and h is the depth. Field data from fivearid estuaries fringed by mangroves and salt flats indicate that where a large area ofsalt flats and mangroves extends over the whole length of an estuary, the estuarybecomes completely inverse with salinity rising up to 55 within a couple of months(Ridd and Stieglitz, 2002). The estuarine evaporation rates due to the presence ofsalt flats and mangroves cause a rapid increase in salinity. The persistence of fresherwater in the upper reaches of this type of estuaries is likely to affect mangrovespecies assemblages (Ridd and Stieglitz, 2002).

Precipitation has a significant importance in determining the salinity of the soilin coastal wetlands. Mondal et al. (2001) developed a multiple linear and nonlinearregression model to predict topsoil salinity (S) for both moderately saline and salinesoils by using daily rainfall (P) and evaporation (E) as independent variables. Theprediction level was not significantly improved with a nonlinear model; therefore,they suggest using the following linear one:

S = 1:29077� 0:49831Pþ1:31230E ð8ÞAn important contribution to estimate the factors controlling the surface energy

budget in coastal wetlands was made by Shoemaker et al. (2005). Changes in heatenergy stored within a column of wetland surface water was calculated because thisvariable is a considerable component of the surface energy budget; a feature that isdemonstrated by comparing changes in stored heat energy with net radiation at



seven sites in the wetland areas of southern Florida, including the Everglades. Usingthe following simplified surface energy budget equation for wetlands

RN � ðW þGvegÞ= QEþQH ð9Þ

where Gveg is biomass storage (heat energy storaged in the vegetation) and W is thechange in heat energy stored in wetland surface water. The difference betweenEquations (1) and (9) is that the advection and soil heat flux (QAþQB) terms werereplaced by the energy stored term in wetland surface. The biomass energy storageterm was included to add the effect of the vegetation. This method estimateschanges in stored heat energy that overcome an important data limitation, namely,the limited spatial and temporal availability of water temperature measurements,because it is assumed that a change in surface water temperature reflects a change instored heat energy (Edinger et al., 1968). The new method was based on readilyavailable air temperature measurements and relies on the convolution of air tem-perature changes with a regression-defined transfer function to estimate changes inwater temperature. The convolution-computed water temperature changes areused with water depths and heat capacity to estimate changes in stored heat energywithin the Everglades wetland areas. These results can probably be adapted to otherhumid subtropical wetlands characterized by open water, seagrass and severalvegetation community types. The final discrete form of the transfer function andconvolution integral, used to compute mean vertical water temperature changesand, ultimately, changes in heat energy stored in a column of wetland surface water,takes the form (Shoemaker et al., 2005)

DTwi =XMj=0

kext

hi�j

ekex thi�j�DTai�j; j = 1; 2; 3; . . . M ð10Þ

where kex is a proportionality constant that describes the rate at which watertemperature responds to heat exchange processes (Edinger et al., 1968). The coeffi-cient � represents the fraction of air temperature change (DTa) that eventually causesan equivalent water temperature change (DTw) and is less than or equal to 1.0, and Mis the historical time step discretizing the surface water’s thermal memory. It isimportant to point out that some authors incorporate not only the characteristicwetland heat energy stored in the vegetation, but also that stored in the fauna. In theliterature several studies on the energy flow or trophic level production of coastalwetlands discuss the subject (Smalley, 1960; Teal, 1962; Wolff et al., 2000).

4. HIGH LATITUDES

High-latitude (subpolar and polar) wetlands are underlain by ice-rich perma-frost, which helps maintain wetland systems and also imparts special characteristicsto their energy and water balances. In North America, components of the radiationbalance decrease poleward (�1.8 W/m2/�latitude), whereas the poleward rate ofdecrease of temperature (i.e., between 50�latitude and 65�latitude: �1�C/�latitude)



and precipitation lessens (i.e., between 50�latitude and 65�latitude:�22 mm/�latitude)(Rouse, 2000). High latitudes are characterized by large annual changes in solar input.Albedo decreases strongly from winter, when the surface is snow-covered, to summer,especially in nonforested regions such as Arctic tundra and boreal wetlands. A primarycharacteristic of wetlands, in permafrost regions, is that the ice-rich frozen layer inhibitsvertical water losses so that ponded water can persist through much of the summer.Components of the radiation balance tend to decrease with increasing latitude (Rouse,2000; Rouse et al., 2004). During the 4-month summer of a high sub-Arctic wetland,net radiation is large and the latent heat flux dominates the energy cycle (Rouse, 2000).In winter, which typically lasts a minimum of 7 months, there is almost no evaporationbut there is sublimation loss. In winter, heat loss from the ground approximatelybalances negative net radiation. After the final departure of the snow in spring, there is achange in net radiation, evaporation and ground heat flux. A major requirement inhigh latitudes is documentation of the magnitudes of the forcing parameters, of whichthe most important is precipitation, in all its forms.

Synoptic weather systems play a major role in day-to-day energy and waterresponses to climate forcing. Presented in Table 2 is an example of the influence ofweather conditions on the energy balance of Hudson Bay coastal wetland. RN issimilar for both warm and cold overlying air masses. However, all other energybalance components are different. Under warm air mass conditions latent heat fluxis greater than sensible heat flux, as expected in low and mid-latitudes, but in thecold air condition, sensible heat flux is greatly enhanced (Rouse, 2000).

Harazono et al. (1996) found that energy partitioning at a coastal site nearPrudhoe Bay, Alaska, was strongly controlled by cold and warm air advection asobserved near the Hudson Bay coast (Lynch et al., 1999). Onshore winds advectedcold and humid air masses from the Arctic Ocean resulting in low air temperature, alarge temperature gradient between the land surface and the air and, therefore, ahigh sensible heat flux and low evaporative flux. Conversely, when offshore windsadvected warm and dry continental air to the site, the temperature gradientbetween the land surface and air was small, resulting in low sensible heat flux, butonly slightly higher evaporative flux than during onshore wind conditions (Lynchet al., 1999). The difference in energy partitioning was primarily due to larger heatgain of the open water ponds during the offshore conditions and to a minor increasein ground heat flux (Table 3). Yoshimoto et al. (1996) found a similar behaviour inenergy partitioning at Barrow, Alaska, during their 1993 field season. Lynch et al.

Table 2 Comparative energy balance under a warm air mass and a cold air mass at HudsonBay

Fluxes (W/m2) Warm air mass Cold air mass Warm/cold

RN 161 161 1.00QE 85 64 1.33QH 47 80 0.59QB 19 17 1.13

Source: Rouse (2000).



(1999) used a regional climate model in their study (ARCSyM) to simulated fluxesthat were within the range of measured fluxes (Table 3), but overestimated both netradiation and latent heat fluxes. The regional model also captured site to sitevariations quite well, which appear to be more sensitive to mesoscale meteorolo-gical conditions than to site characteristics.

A study on the effect of advection from the cold polar sea to a warmer terrestrialsurface in the coastal area of Hudson Bay was carried out by Weick and Rouse(1991). Three objectives were pursued: (i) to investigate the changes in thesurface energy balance along a transect perpendicular to the coast line of the bay,(ii) to identify local and regional effects on the energy balance along the transect and(iii) to document the use of a box model and the divergence and convergence ofenergy mass in the coastal boundary layer during onshore and offshore winds. WithEquation (1) the different fluxes and the Bowen ratio were estimated. The authorsdemonstrated that the largest advective influence on the turbulent fluxes occurswithin 10 km of the coast, with a 2.7-fold downwind decrease in the Bowen ratioand a 1.8-fold decrease during offshore winds (Table 4). This decrease is due bothto boundary layer adjustments to a new surface under onshore winds and tohorizontal and vertical convergence and divergence in the atmosphere under allwind conditions.

Different ET models applied to an Arctic coastal wetland near Prudhoe Bay,Alaska, during the summers between 1994 and 1996 were compared by Mendezet al. (1998). The objective was to gain a better understanding of ET in Arcticwetlands. Evaporation after spring snowmelt averaged 3.11 mm/day (obtained viathe WB). Latent heat flux was the dominant heat sink in wetlands, whereas sensible

Table 3 Monthly mean surface energy balance (W/m2) at Betty Pingo coastal site (70�180N,149�550W) for summer

RN QH QE QB

June 1995 128.1 39.6 61.0 27.5July 1995 122.6 57.2 38.1 27.3August 1995 75.7 14.7 48.6 12.4

Source: Lynch et al. (1999).

Table 4 Seasonal Bowen ratios (�) at four microclimate stations located at 0, 2.5, 9.4 and12.4 km from the coast for all wind conditions and for onshore and offshore winds

Wind conditions Site1at thecoast

Site 2(2.5 km)

Site 3(9.4 km)

Site 4(12.4 km)

All directions 0.95 0.79 0.58 0.58Onshore winds 1.10 0.93 0.64 0.67Offshore winds 0.71 0.58 0.48 0.43

Source: Weick and Rouse (1991).



heat flux dominated in the drier upland area. Differences between the formulationswere not significant.

5. SUMMARY

Heat budget studies in coastal wetlands although significant for the health ofthese ecosystems have been relatively poorly studied. The heat balance depends onthe site conditions, the latitude and the climate. While the latitude determines theamount of incoming radiation entering the ecosystem, the resultant net radiationalso depends on the site conditions that characterize the ecosystem. The netradiation represents the available energy that different processes, which characterizeany energy budget, will have. Net radiation energy is used by evaporation (latentheat flux) and by heat conduction between the air–water and/or air–substrate(sensible heat flux). Naturally, the wetland region climate determines the magni-tude of these fluxes (i.e., low latitudes wetlands receive more radiative energy). Thetypical vegetation of each ecosystem use part of that energy in specific biologicalprocesses. Therefore, the remaining heat is transported as advective heat flux eitherby wind or tides.

Temperate wetlands, especially tidal flats, are by far the sectors that havereceived more attention. Plant cover influences the microhabitat of the sedimentby controlling the amount of light reaching the sediment surface. Then significantdifferences might be found in heat balances between bare and vegetated marshes.These changes may induce changes in the sediment biotic community (Whitcraftand Levin, 2007). In low latitudes, besides the net radiation, the water balance andthe evaporation are the most significant processes that define the heat balance, andthen the temperature variations in the soils, coastal waters, and lower layers of theatmosphere. Temperature affects respiration rates and photosynthesis, influencesthe distribution and movements of fish, and also affects many important biologicalprocesses (number of eggs laid, incubation time, etc.). Therefore, heat balancestudies might help to understand temperature variations of wetlands and some ofthe processes that generate distribution patterns in coastal natural flora and faunacommunities.

In the arid climate estuaries of low latitudes, the relationship between salinityand evaporation is important. Although some formulas that relate both parametersare presented in this chapter, more measurements and experiments in diverseenvironments should be made. On the other hand, in high latitudes, because ofthe cold climate, numerical models are the most powerful tool to study the heatbalance.

Despite the described research on heat budget in coastal wetlands, many pointsremain to be investigated such as the monitoring of the heat balance components invegetated and bare coastal wetlands and its influence in plant community, theeffects of shading in temperature variation of wetlands soils and the effect of theheat balance in plant zonation and animal interactions. The results of such studieswould provide some insight in flora and fauna distribution, biodiversity and species



behavior in wetlands. Scientists are working with the bulk formulas and/or numer-ical models in several sites, time and space scales. But even though some work hasbeen done on the interaction between biological and physical processes, an exhaus-tive analysis of the basic interactions among flora, fauna and heat stored in coastalwetlands is still lacking. Future studies should investigate these interactions.

ACKNOWLEDGMENTS

Partial support for the work dealing to this paper was provided by grants byCONICET, Agencia Nacional de Promocion Cientıfica e InnovacionTecnologica, and Universidad Nacional del Sur. I would to thank the commentsand suggestions by the Eric Wolanski, Mark M. Brinson, Bjorn Kjerfve, W. BarclayShoemaker, and Reiner Onken.

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Chapter 7 - Heat Energy Balance in Coastal Wetlands