Circulation and Flux of Suspended Particulate Matter in Ilha Grande Bay, SE Brazil

Transcription

1 Journal of Coastal Research SI ICS2009 (Proceedngs) Portugal ISSN Crculaton and Flux of Suspended Partculate Matter n Ilha Grande Bay, SE Brazl J. V. Guerra and F. L. M. Soares Dept. of Geologcal Oceanography State Unversty of Ro de Janero (UERJ), Ro de Janero , Brazl jose.guerra@gmal.com ABSTRACT GUERRA, J. V. and SOARES, F. L. M., Crculaton and flux of suspended partculate matter n Ilha Grande Bay, SE Brazl. Journal of Coastal Research, SI 56 (Proceedngs of the 10th Internatonal Coastal Symposum), Lsbon, Portugal, ISSN Ilha Grande Bay (IGB) s located n the southern tp of Ro de Janero state, SE Brazl. It s freely connected to the Atlantc ocean. In ts central area les a large, east-west trendng sland, separated from the contnent by a narrow and deep feature, the Central Channel. IGB s eastern end s connected to Sepetba Bay, a shallow, hghly polluted envronment and a potental source of fne-graned, contamnated sedment. The chef motvaton of the present study was to dentfy scenaros conducve to the advecton of suspended partculate matter (SPM) from Sepetba Bay nto IGB through the Central Channel where an anchor staton was repeatedly occuped for hours n December 2005, September 2006 and March, Aprl and July The data set ncludes contnuous measurements of water column currents; SPM concentraton determned through fltraton of water collected at 3-6 ponts from the water surface to the near-bed area; vertcal profles of seawater temperature and salnty. Samplng was carred out both under neap and sprng tde condtons. Current speed usually remaned under 0.5 m s -1, except durng the passage of a cold front n December Seawater propertes suggest that the Central Channel s mostly flled by shelf waters; SPM concentraton was low, rarely exceedng 20 mg L -1. Wthn the doman of our observatons, fluxes of SPM were strongly controlled by meteorologcal condtons and by the presence of low-frequency oscllatons (nternal seches?). Results support the epsodc occurrence of condtons that favor the ntroducton of partcles orgnated n Sepetba Bay nto the Central Channel. ADDITIONAL INDEX WORDS: coastal oceanography, acoustc current meters, coastal bay INTRODUCTION Ilha Grande Bay (heren IGB) les n the southern tp of Ro de Janero state, SE Brazl; t s freely connected to the Atlantc Ocean except n ts central regon where t les Ilha Grande Island, separated from the contnent by the Central Channel (Fgure 1). The eastern end of IGB s connected to Sepetba Bay, a hghly polluted envronment and a potental source of fne-graned, contamnated suspended partculate matter (SPM) for nstance, by znc (MIRANDA et al., 2007). Contrarly to Sepetba Bay, IGB receves no sgnfcant nput of freshwater from rvers and landsldes are currently the major sources of slcclastc sedments. IGB s shorelne s hghly jagged and coastal mountans reach the coastlne, leavng lttle space for coastal plan evoluton. Based on earler bathymetrc surveys and on the textural characterstcs of surfcal sedments, MAHIQUES (1987) suggested the segmentaton of IGB nto 3 sectors: Eastern (10-25 m deep, covered by medum to coarse sands), Western (20-40 m deep, covered by fne sands), and the Central Channel, a natural feature that can reach depths greater than 50 m and that s covered by slts and clays. Economc actvtes carred on n IGB range from envronmentallyconscous toursm to nuclear power plants; t harbors also one of Brazl s largest shpyards, several small prvate marnas, and an ol termnal. Despte ts economc and ecologcal mportance, IGB has not been the subject of systematc oceanographc studes and most of the current knowledge reles on solated surveys conducted n the 70 s and 80 s. The chef motvaton of the present study was to dentfy condtons that mght be conducve to the ntroducton of SPM from Sepetba Bay nto IGB through the Central Channel. In ths paper, such condtons are represented by the occurrence n the Central Channel of westward flows that can, potentally, transfer contamnated SPM orgnated n Sepetba Bay to Ilha Grande Bay. Fgure 1: Locaton of Ilha Grande Bay (Ro de Janero state, SE Brazl) 1350

2 Flux of suspended-sedment, Ilha Grande Bay METHODS Measurements were conducted at a fxed staton located n the northern edge of the Central Channel (Fgure 1); local depth s 26 m. The same staton was repeatedly occuped for 18 to 25 hours n December 2005, September 2006 and March, Aprl and July Instrumentaton ncluded a downward-lookng 600 khz Teledyne-RDI Acoustc Doppler Current Profler (ADCP) that, besdes current velocty and drecton, sampled the echo ntensty; the ADCP was programmed to contnuously collect data every 5 seconds at 0.5 m depth ntervals throughout the water column; the data set has been corrected for local magnetc declnaton and averaged at 30-s and 5-mn tme ntervals (n ths paper only the latter are used). Water column profles of temperature and conductvty were sampled wth a SeaBrd SeaCat19 CTD, ntegrated wth an optcal backscatter sensor (D&A Instrument Co. OBS-3) placed at 10 cmab; the samplng rate was 0.5 Hz and whenever possble, water column profles were obtaned every hour. Water were usually collected every three hours wth a 3-lter Van Dorn bottle at 3 to 6 depths (near-surface, near-bed and md-depth) and fltered n the laboratory through pre-weghed Mllpore flters ( =47 mm, pore sze = 0,45 m) to determnaton of SPM concentraton (n mg L -1, precson of 0,1 mg). Wnd speed and drecton were measured at varyng rates at nearby locatons. Fluxes of suspended-sedment were computed only at depths where SPM concentratons were obtaned drectly from water, by multplyng each 5-mn mean value of the velocty components u (E-W) and v (N-S) by the concentraton c,.e., uc = u c + u c and vc = v c + v c, where ( uc, vc ) are the total fluxes, ( u c, v c ) are the mean fluxes and ( u c, v c ) are the fluctuatng components. Due to the low temporal resoluton of water samplng, each value of SPM concentraton was appled to a tme nterval spannng half the tme between the moment each sample was collected (for nstance, water collected at 15:00 and 18:00 were used to compute fluxes between, respectvely, 13:35-16:30 and 16:35-19:30). In ths paper, only data from December 2005, Aprl and July 2007 wll be dscussed (Table 1). Table 1: Bref descrpton of each data set. Anchor staton December 2005 Aprl 2007 Duraton (h) 18 neap 20 sprng July sprng Tde Wnds Measurements strong from SW followed by NE/ESE moderate from NE followed by N/NE N/NE RESULTS ADCP+ CTD+water ADCP+ CTD+water ADCP+ water December 2005 Measurements were carred out between 19:30 of December 6 and 13:30 of December 7. Shortly after the begnnng of the measurements, the study area was reached by a cold front accompaned by strong wnds from the SW (wth speeds of up to 12 m s -1 ) and a ranstorm whch lasted 4 hours. Values of salnty (Fgure 2) reflect the nfluence of the ranwater; at the end of the measurements, the water column was agan flled up by more salne shelf waters. Due to operatonal problems, only two water samplng profles could be obtaned. Concentraton of SPM remaned farly stable throughout the samplng perod (Fgure 3A) except for an ncrease near the water surface from 6 mg L -1, before the storm (20:00), to 19 mg L -1 hours after the storm ended (09:00). In the near-bed area, the hghest concentraton ( 33 mg L -1 ) was documented at 20 mbs (meters below the surface) durng an epsode of easterly currents (not shown). Throughout the water column, mean flows were drected to the SE; mean values decreased towards the bottom although currents as fast as 54 cm s -1 were documented n the near-bed area (Table 2). SPM fluxes (Fgure 4, Table 3) show a domnance of transport to the SE, followed by fluxes to WNW- NW; the hghest extreme values (range) were found at 20 mbs (Table 3). July 07 Fgure 2: Seawater temperature and salnty measured n December 2005 (X), Aprl 2007 (o) and July 2007 (encrcled ). The observed larger scatterng of values from December 2005 s due to the passage of a cold front accompaned by a ranstorm that perssted for 4 hours. In July 2007, only one CTD profle could be obtaned. psu = Practcal Salnty Scale. Aprl 2007 Measurements were carred out between 18:00 of Aprl 2 and 14:00 of Aprl 3 under farweather condtons. The upper water column ( up to 8 mbs) was warmer (24-28 C) and fresher (salnty between 34.4 and 35, Practcal Salnty Scale) than the remanng water column. Ths upper layer was followed by another, approxmately 7 m thck, where temperature/salnty steadly decreased/ncreased wth depth; from 15 mbs up to close to the bottom, the water column was nearly homogeneous (T 18 C, S 35.6). Thrty-two water were collected; samplng occurred every 3 hours at 4 to 6 depths. Near-surface concentraton of SPM remaned low throughout the samplng perod (mean value of 0.7 mg L -1 ). An nterestng feature observed n the profles of SPM concentraton s the varable poston of the hghest value: ether at 15 mbs, or at 20 mbs or stll near the bottom (Fgure 3B). At all depths, mean current velocty was 1-2 orders of magntude lower than n December 2005 and extreme values were, n general, 2-4 tmes lower 1351

3 Guerra and Soares (Table 2). Mean flows were drected to NW, except at 15 mbs (to ENE). Fluxes of SPM were neglgble, usually 1-2 orders of magntude smaller than n December 2005 (Table 3), partcularly n the upper water column ( 0-15 mbs); the hghest values were assocated wth the peak concentratons observed at varable depths (Fgure 3B); n these occasons, transport of SPM occurred preferentally to NNW (Fgure 5). A B C Fgure 3: Concentraton of suspended partculate matter (n mg L -1 ) obtaned from fltered water. A December 2005; B Aprl 2007; C July start of the campagn; the water column was farly unform wth S (near-bed near-surface salnty) equal to 5 and T (near-surface near-bed temperature) equal to 0.46 C (see clusterng of ponts n Fgure 2). Twenty-seven water were collected every three hours at 3 depths; SPM concentraton was low and, except for one sample out of 27, stayed below 10 mg L -1 (Fgure 3C). Profles of SPM concentraton exhbt a pattern of ncreasng values towards the near-bed area, not showng the md-water hghs observed n December 2005 and Aprl The magntude of the recorded current veloctes was analogous to those documented n Aprl 2007 (Table 2); nonetheless, the mean values were usually lower reflectng the fact that durng extended perods current veloctes stayed below 5 cm s -1 ; n the near-bed area, mean flows were drected to NE. Fluxes of SPM closely reflect the combnaton of sluggsh flows and low SPM concentraton and were mnmal but for the near-bed area where transport of SPM occurred mostly to ESE and NW (Table 3, Fgure 6). CONCLUSIONS Short data sets of water temperature and salnty, water column current drecton and velocty, and concentraton of SPM presented n ths paper document the varablty of SPM transport patterns that take place n the Central Channel of Ilha Grande Bay, SE Brazl. Despte the lmted extenson of the avalable tme seres, fluxes of suspended-partculate matter could be nvestgated under dstnct meteorologcal and tdal condtons. Results pont out to: 1) the mportant role played by cold fronts (and the assocated southwesterly wnds); 2) the lttle nfluence of sprng/neap tdes (already partally predcted by a few modelng studes such as those of SIGNORINI, 1980 and FRAGOSO, 1999); and 3) the occasonal occurrence of lowfrequency oscllatons (nternal seches?) that apparently nfluence or even control the vertcal dstrbuton of SPM n the lower water column (depths greater than 15 m). However, due to the lmted duraton of the measurements, t remans unknown how frequently and under whch condtons the documented patterns of crculaton and of flux of SPM occur n the Central Channel area. Nonetheless, the data set provdes evdence of the epsodc occurrence of favorable condtons to the ntroducton and deposton of fne-graned sedment from Sepetba Bay nto the Central Channel regon. Such condtons were more conspcuous n Aprl 2007 when flows drected to NNW were assocated wth hgher concentratons of SPM, resultng n the only sgnfcant events of SPM transport throughout the water column, n partcular below 15 m deep. MIRANDA et al. (2007) have documented the assocaton between SPM and znc n collected close to the watercolumn surface n Sepetba Bay where the authors report values of up to 6.5 mg of znc per gram of SPM. Although no analyses for znc concentraton were conducted n the collected for ths study, the observed patterns of crculaton and SPM concentraton suggest that contamnated sedment mght be ntroduced nto Ilha Grande Bay durng the epsodes of NNWdrected flows. Ths hypothess wll be tested n future studes. July 2007 Measurements were carred out between 12:00 of July 4 and 13:00 of July 5 under farweather condtons (Table 1). Due to techncal problems only one CTD profle could be taken at the 1352

4 Flux of suspended-sedment, Ilha Grande Bay Table 2: Mean and range (between brackets) of current velocty components (n cm s -1 ). N-surface = near-surface, N-bed = near-bed, mbs=meters below the surface. Postve values ndcate flows drected to the north and to the east. Component December 2005 Aprl 2007 July 2007 N-surface 15 mbs 20 mbs N-bed N-surface 15 mbs 20 mbs N-bed N-surface 15 mbs N-bed N-S (v) [-48.9/4.0] [-45.5/16] -8.6 [-43.5/22] -5.1 [-45/20.9] 3.7 [-6.5/13.9] 0.9 [-9/11.5] 2.3 [-10.6/10] 2.5 [-10.4/10.7] 0.6 [-10.3/9.9] -5 [-14/9.8] 0.3 [-16/14] E-W (u) 30.0 [-15/ 65.7] 8.5 [-11.6/30] 3.0 [-23.6/47] 2.8 [-32/54.4] -1.2 [-11/26.6] 3.1 [-16/16.4] -1.0 [-11/22.4] -0.9 [-10/21.7] 1.4 [-16.9/15.4] 1.5 [-13/17] 0.3 [-16.9/20.9] v Fgure 4: Flux of suspended partculate matter n December 2005 computed at 4 depths: near-surface (), 15 mbs (), 20 mbs (), and near-bed (v). Vectors ndcate the drecton currents are gong to. Table 3: Mean and range (between brackets) of total flux of suspended-sedment (n g m -2 s -1 ). N-surface = near-surface, N-bed = nearbed, mbs = meters below the surface. Postve values ndcate fluxes to the north and to the east. Component December 2005 Aprl 2007 July 2007 N-surface 15 mbs 20 mbs N-bed N-surface 15 mbs 20 mbs N-bed N-surface 15 mbs N-bed vc -0.9 [-1.8/0.1] -1.2 [-6.3/1.5] -2.2 [-14.2/3.5] -0.4 [-3.3/1.5] 0.03 [-0.07/] [-0.9/0.4] [-0.5/1.5] [-0.6/2.0] 0.02 [-0.1/0.3] [-0.3/] [-2.3/1.2] uc 1.1 [-0.5/2.3] 0.9 [-0.8/3.8] 0.7 [-5.8/8.9] [-2.4/4] [-0.1/0.1] [-1.3/1.2] - [-1.4/0.9] - [-1.4/0.4] 0.02 [-0.03/] [-0.6/0.3] 0.05 [-2.3/3.1] 1353

5 Guerra and Soares v Fgure 5: Flux of suspended partculate matter n Aprl 2007 computed at 4 depths: near-surface (), 15 mbs (), 20 mbs (), and near-bed (v). Vectors ndcate the drecton currents are gong to Fgure 6: Flux of suspended partculate matter n July 2007 computed at 3 depths: near-surface (), 20 mbs (), and near-bed (). Vectors ndcate the drecton currents are gong to LITERATURE CITED FRAGOSO, MR, Estudo numérco da crculação marnha da regão das Baías de Sepetba e Ilha Grande (RJ). Dssertação de Mestrado. Insttuto Oceanográfco. Unversdade de São Paulo. 115p. MAHIQUES, MM, Consderações sobre os sedmentos de superfíce de fundo da Baía de Ilha Grande, Estado do Ro de Janero. Dssertação de Mestrado. Insttuto Oceanográfco, Unversdade de São Paulo. Vols. 1 e 2. MIRANDA, TC; PATCHINEELAM, SR; PATCHINEELAM, SM; ARARIPE, D; GUERRA, JV; MELO, GV; CYPRIANO, EF, Dstrbução de znco assocado ao materal partculado em suspensão ao longo de um cclo de maré na Baía de Sepetba, Ro de Janero. In: Anas do XI Congresso Braslero de Geoquímca, 2007, Atbaa. p SIGNORINI, SR, A study of the crculaton n Bay of Ilha Grande and Bay of Sepetba. Part II, An assessment to the tdally and wnd-drven crculaton usng a fnte element numercal model. Bolm. Inst. Oceanogr., S. Paulo, 29 (1): ACKNOWLEDGMENTS Thanks to the Brazlan fundng agency CNPq that provded a research grant (#481072/2004-7) to JVG and a scholarshp to FLMS (Pbc-CNPq). The authors are also deeply ndebted to the several students who took part n the feld actvtes and contrbuted wth ther hard work and resourcefulness, n partcular Edgard Vllarnho Garca Neto, Leonardo Gama Félx, Roberto Freres de Olvera and Gabrel Morera Menezes. 1354