Nutrient and Chlorophyll Dynamics in Pacific Central America (Panama) Luis D’Croz and Aaron O’Dea ABSTRACT. Strong wind jets from the Caribbean and the Gulf of Mexico cross Cen- tra lAmerica through topographic depressions in the cordillera during the borea lwinter, pushing Pacific coastal waters offshore, lowering sea levels at the coast, and causing coasta lupwelling .Where high mountains impede the winds ,this phenomenon does not occur. The Panamanian Pacific shelf is an excellent example of this variability. The coast is divided into two large areas, the Gulf of Panama and the Gulf of Chiriqui. To investi- gate hydrologica lconditions between the two gulfs ,we sampled the water column during upwelling and non-upwelling seasons in each region .In both gulfs during non-upwelling conditions ,surface-leve lnutrients are poor ,and the chlorophyl lmaximum occurs around 30 m where the thermocline intersects the euphotic zone. Oxygen-poor waters (<2 ppm) commonly occurred below the thermocline. During the dry season, wind strength in- creased and strong upwelling was observed in the Gulf of Panama. The thermocline rose and surface waters became nutrient enriched and chlorophyll a levels increased. Well- oxygenated waters were compressed to shallow depths. In the Gulf of Chiriqui, wind strength was weaker ,surface waters did not become enriched with nutrients ,and surface chlorophyll a remained low. We did observe a shallowing of the thermocline in the Gulf of Chiriqui, but in contrast to the Gulf of Panama, wind mixing was not strong enough to result in sea-surface cooling and nutrient enrichment. We postulate that the conver- gence of a shallow thermocline and internal waves in the Gulf of Chiriqui is the likely mechanism that causes pockets of deep water to occasionally migrate into surface waters, leading to restricted and ephemera lupwelling-like conditions .Although its effects upon shallow-water communities remain to be studied, we propose that the process may be more likely to occur during the boreal winter when the thermocline is shallower. INTRODUCTION One of the most pervasive hydrological events to influence the shelf wa- ters of Pacific Central America is upwelling. Intermittent or seasonal upwell- ing develops in the gulfs of Tehuantepec (Mexico), Papagayo (Costa Rica), and Panama (Legeckis, 1988; McCreary et al., 1989; Xie et al., 2005), driving exten- Luis D’Croz and Aaron O’Dea ,Smithsonian sive planktonic productivity and shaping the secondary production of biological Tropica Rl esearc hInstitute B, o x0843-03092, communities (Jackson and D’Croz, 1997; O’Dea and Jackson, 2002). Panam aR,epubl io cPfanam aC.orresponding The shelf waters along the Pacific coast of Panama are among the most dy- autho r LD:. ’Cro (zdcrozl@si.edu )M. anuscript namic in the region. Here, the coastal shelf is naturally divided into two large gulfs receive d1 3Ma y2008 a; ccepte d2 0Apr i2l 009. by the Azuero Peninsula: the Gulf of Panama (shelf area, 27,175 km”) and the Gulf 336 ° SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES of Chiriqui (shelf area, 13,119 km?) (Figure 1). The Gulf shows both wind speeds and chlorophyll content of surface of Panama experiences strong seasonal upwelling while the waters to be lower in the Gulf of Chiriqui than the Gulf of Gulf of Chiriqui exemplifies a non-upwelling environment Panama during the dry seasons (Pennington et al., 2006). (Dana, 1975; Kwiecinski and Chial, 1983). This distinc- However, the statement that upwelling does not occur tion is customarily explained using geographic differences in the Gulf of Chiriqui is supported by sea-surface data between the two gulfs. Seasonal upwelling in the Gulf of derived from satellite imagery analysis or from the mea- Panama develops during Panama’s dry season, correspond- surement of properties in the shallow section of the water ing to the boreal winter, when northeast trade winds cross column. Hydrological profiles of the water column have to the Pacific over low areas in the isthmian mountain range, documented the shoaling of the thermocline in the Gulf pushing warm and nutrient-poor coastal surface water off- of Chiriqui, yet there appears to be no clear association shore, lowering the nearshore sea level, and causing the up- between the physical forcing of this event with the wind- ward movement of colder and nutrient-richer deep water induced upwelling in the Gulf of Panama. Nevertheless, (Smayda, 1966; Forsbergh, 1969; Kwiecinski et al., 1975; the movement of pockets of cool water that bring nutri- D’Croz et al., 1991; D’Croz and Robertson, 1997). The es- ents into the upper layer may be a more common occur- tablished model proposes that because western Panama has rence in the Gulf of Chiriqui than previously suspected higher mountain ranges that block the winds, surface waters (D’Croz and O’Dea, 2007). in the Gulf of Chiriqui are not displaced out to the Pacific, It is therefore essential that we obtain detailed and and no upwelling as such occurs there. comparable hydrological data from both gulfs if we wish The structure of shallow biological communities be- to explain variability in biological communities along the tween the two regions supports this inference. Coral reefs, Pacific coast of the Isthmus of Panama today and through which respond poorly to upwelling conditions, are more geologic time (O’Dea et al., 2007). In this paper we ex- extensive in size in the Gulf of Chiriqui than in the Gulf of pand the information presented in our previous study Panama (Glynn, 1977; Glynn and Maté, 1997), whereas (D’Croz and O’Dea, 2007), adding new hydrological and small pelagic fish species from the Gulf of Panama repre- biological data from the Gulf of Chiriqui and the Gulf of sent a large proportion of the total estimated fishery re- Panama, and we further discuss the issue of whether up- source in the country (NORAD, 1988). Satellite imagery welling takes place in the Gulf of Chiriqui. P—PCoAS foSt Azue r“oSNazure ——— FIGURE 1 .Map of the Republic of Panama showing sampling sites .Red dots represent the location of the rainy season samplings in the Gulf of Panama (a = 18 December 2004) and in the Gulf of Chiriqui (a = 13 July 2003; b = 17 December 2004). Blue dots represent the location of the dry season samplings in the Gulf of Panama (a = 29 February 2000) and in the Gulf of Chiriqui (a = 1 March 2000; b = 13 April 2007). Yellow square sindicate the location o fthe meteorologica sl tations. NUMBER 38 °¢ 337 MATERIALS AND METHODS nutrient and chlorophyll a concentrations. Water samples STUD YAREA were collected using Niskin bottles during the dry season of the year 2000 (29 February to 1 March) and during Panama’s Pacific shelf is located from 07°30’ to the rainy season of the year 2004 (17 and 18 December). 09°01’N and 78°10’ to 82°52'W. The shelf is predomi- Three replicate water samples per selected depth were col- nantly occupied by low-salinity surface water, similar to lected at each site. Two liters of each individual replicate the water mass found over the center of the tropical Pacific water sample were immediately sieved through Nitex Ocean at about 10°N (Wyrtki, 1967; Fiedler and Talley, (350 jm) to exclude zooplankton and vacuum filtered on 2006). The climatology is governed by the Inter-Tropical Whatman GF/F filter (0.7 ym pore size) for chlorophyll a Convergence Zone (ITCZ), the position of which defines analysis. An aliquot from each filtrate was set apart for the the seasonal pattern of rainfall and winds. The rainy season determination of dissolved inorganic nutrients. Filters and develops between May and December when the ITCZ is water samples were stored frozen (—20°C) until analysis. located over or slightly to the north of Panama and winds Salinity is expressed using the Practical Salinity Scale (pss) are light and variable in direction. The dry season devel- indicated by UNESCO (1981). Results from the chloro- ops between January and March when the ITCZ moves phyll a analyses were used to check the calibration of the south of Panama, a time period characterized by pre- CTD’s fluorometer. The depth of the euphotic zone (1% dominating intense northeast trade winds. The mean an- incident radiation) was estimated from Secchi disk read- nual rainfall recorded at meteorological stations near the ings (Parsons et al., 1984). The light attenuation coeffi- coast (1999-2004) was 2,760 mm in the Gulf of Chiriqui cient was calculated as Ky = f/zs where zs is the Secchi (David) and 1,880 mm in the Gulf of Panama (Tocumen). depth and f = 1.4. Approximately 94% of the annual rainfall in both areas corresponded to the rainy season, the months of Septem- ANALYSI SO FSAMPLES ber and October being the rainiest in both regions. The estimated sizes of the drainage basins are 11,846 km? in Not later than two weeks after sampling, filters hold- the Gulf of Chiriqui and 33,828 km? in the Gulf of Pan- ing the phytoplankton were analyzed for chlorophyll a ama. River discharges into both gulfs typically follow the using the non-acidification fluorometric method (Welsch- seasonal trend described for rainfall. Detailed discussions meyer, 1994). Water samples were analyzed for NO3;— + on wind-stress, rainfall, and river discharge patterns are NO, (nitrate + nitrite), Si(OH), (silicate), and PO,3- presented in D’Croz and O’Dea (2007). The tidal regime (phosphate) by colorimetric methods using an Alpkem is semidiurnal, and the sea-level difference during spring Flow Solution IV automated analyzer. Minimum detection tides is 6 m (Glynn, 1972). limits were 0.02 wM for nitrate, 0.01 M for nitrite, 0.12 uM for silicate, and 0.02 1M for phosphate. SAMPLIN PGROCEDURES ANALYSIS AND PRESENTATION OF DATA Sampling research cruises were conducted in the gulfs of Panama and Chiriqui using the Smithsonian Tropical Water quality variables, namely temperature, salin- Research Institute’s R/V Urraca (see Figure 1). Samplings ity, dissolved oxygen, dissolved inorganic nutrients, and were scheduled to correspond with different times of the chlorophyll a, are presented graphically as profiles of the year, representing contrasting hydrological conditions (up- samplings. Overall differences in between the two gulfs welling and non-upwelling). Surface-to-bottom profiles for were assessed with the Mann-Whitney test (U) by taking salinity, temperature, dissolved oxygen, and chlorophyll a the median of each variable from samples collected in the were recorded with a CTD (conductivity, temperature, top 30 m of the ocean where the highest hydrological vari- depth) multiparameter profiler (Ocean Seven 316, Idro- ability occurred (Table 1). Water transparency data were naut Srl, Milano, Italy). Hydrological casts with the CTD compared using the paired ¢ test. We followed the practice corresponding to the dry season were carried out in both of taking the position of the 20°C isotherm to represents gulfs on 29 February 2000 and 1 March 2000 and in the the depth of the center of the permanent thermocline in Gulf of Chiriqui on 13 April 2007. Rainy season CTD the eastern Pacific Ocean (Wyrtki, 1964; Fiedler et al., casts were carried out in the Gulf of Chiriqui on 13 July 1991; Xie et al., 2005). Pearson correlations with Bonfer- 2003 and in both gulfs during 17 and 18 December 2004. roni adjustment were used to test statistical relationships The water column was sampled at discrete levels to study among variables. 338 ° SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES TABLE 1. Average value of hydrological variables in the top water column (30 m) in the gulfs of Panama (GP) and Chiriqui (GC); SE = standard error of the mean. Statistical tests were either Mann-Whitney U test or paired t test (*P < 0.05, **P < 0.01, ***P < 0.001, ns = nonsignificant). Dr yseason values Rain yseason values Hydrological GP GC Statistical GP GC Statistical variables (Mean = SE) (Mean = SE) value 2» (Mean = SE) (Mean + SE) value > Temperature (°C) W/O == 02 BIN = 02 16.0% 4 26.75 + 0.54 28.61 + 0.05 18.0 ns 4 Salinity (pss) © 34.18 + 0.29 32.98 + 0.29 IDO? & 31.67 + 0.64 30.48 + 0.38 3.0 ns Chlorophyll a (ug L~!) 1.82 + 0.65 0.83 + 0.65 4.0% a 0.23 = 0.13 0.18 + 0.06 8.5 ns 2 Dissolved oxygen (ppm) 3.45 + 0.27 4.78 + 0.27 IO? 2 3.98 + 0.16 4.38 = 0.01 4.0 ns # NO3~ (wM) 14.37 + 2.48 3.72 + 2.48 = 0.99 + 0.34 0.36 + 0.02 2.5 ns 4 PO,3- (wM) 1.08 + 0.21 0.39 + 0.21 0.43 + 0.07 0.24 + 0.03 4.0 ns 4 N:P ratio 12.82 + 1.10 7.77 = 1.10 2.11 + 0.36 1.49 + 0.10 3.0 ns 4 Si(OH),4 (uM) 8.99 + 1.03 4.40 + 1.03 5.40 + 0.71 4.87 + 0.47 13.0 ns 4 Secchi depth (m) 4.20 + 0.00 14.80 + 0.00 20.00 + 0.00 19.00 + 0.00 2.0 ns > Euphotic zone (m) 13.8 + 0.00 48.63 + 0.00 65.71 + 0.00 62.43 + 0.00 188.4 ns > M4ann-Whitn etUeyst. Pai rt e¢dst. ©ps p=sractic aslalini tsycale. RESULTS season, both regions experienced high freshwater dilu- THERMOHALI SNTERUCTURE _ tion in the upper-layer waters, with surface salinities below 30 on the pss (see Figure 2). The halocline was Both the Gulf of Panama and the Gulf of Chiriqui located at 60 m depth, coinciding with the thermocline. exhibit the typical tropical coastal ocean water struc- During the dry season, lower rainfall led to increased ture of cool deep waters leading upward to a shallow salinities in the surface waters of both gulfs (Figure 3). thermocline topped by warm surface waters. However, However, the effect was more striking in the Gulf of significant differences occur between the two gulfs with Panama as the halocline shoaled and salinity in surface respect to climatic variability. During the rainy season, waters reached 34. the thermal structure in both gulfs is remarkably similar In April 2007, the thermohaline structure in the (see Table 1). Sea-surface temperatures (SSTs) are invari- Gulf of Chiriqui departed drastically from the typical ably warm (27°-28°C), and the thermocline sits at ap- condition as the thermocline/halocline shoaled to 20 m. proximately 60 m (Figure 2). Despite this condition, however, SSTs remained warm During the dry season, thermal conditions become dis- (Figure 3c). similar between the two regions (Table 1). In our observa- tions, the thermocline in the Gulf of Panama rose sharply CHLOROPHYLL and nearly broke at the surface, resulting in a significant cooling of surface waters to 22°C (Figure 3a). Simultane- Concentrations of surface chlorophyll were always be- ously, the thermocline in Gulf of Chiriqui rose to around low 0.30 g/L in both gulfs during the rainy season (Table 30 m, compressing warm SSTs into shallow waters (Figure 1), but a deep chlorophyll maximum developed from 30 m 3b). However, the shoaling of the thermocline in Chiriqui to 50 m, lying above the thermocline (Figure 2). The deep was not as intense as that seen in the Gulf of Panama and chlorophyll maximum contained most of the chlorophyll a did not result in SST cooling. in the water column in both gulfs, concentrations reaching In general, salinity profiles in both regions revealed 1 we/L during the rainy season. The dry season upwelling a sharp gradient from high-salinity deep water to fresher changed this pattern in the Gulf of Panama, as the chloro- surface waters. Seasonal variability in surface salinities phyll maximum moved into shallower waters, where con- in both gulfs was very similar (Table 1). During the rainy centrations surpassed 4 wg/L (Figure 3a). Surface chloro- NUMBER 38 ¢ 339 Dissol voexdyg (epnpm) FY Ao s 4b. SG G Dissol voexdyg (epnpm) Dissol voexdyg (epnpm)0 2 eh & )7 Wo aya GG SS SS ee ee Ree ey eee eee See es Se Os fe Ce OQ | Chloroph (yau lLlg*) Chloroph y(aul lLg") Chloroph y(aul lLg") 0 1 2 34 5 0 1 3 4 5 0 1 2 3 4 5 EL te ee a | Re ve a ey AE SE |) al (ae UR (ane en Sali n(pitsys) Sali n(pitsys) Sali n(pitsys) 29 30 31 32 33 34 35 3629 30 31 32 33 34 #35 #36291 30 31 32 33 34 #35 36 a ee es ee ee ee ee ee ES EE Temper a(°tCu)re Temper (a°tCu)re 215) 18) 21) 24 278 30812) 415 Temper a(°tCu)re21 247727, 30012). 15; 18) 24 2AM 2 oO, (m)DepiStoha 225 Gulf of Panama G uColhf iriqui G uColhf iriqui 18/December/2004 13/July/2003 17/December/2004 250 (Rainy season) (R aseinayson) (R aseinayson) FIGURE 2 .Profiles of dissolved oxygen ,chlorophyl la ,salinity ,and temperature in the Gulf of Panama and the Gulf of Chiriqui during the rainy season. a = Gulf of Panama, 18 December 2004; b = Gulf of Chiriqui, 13 July 2003; c = Gulf of Chiriqui, 17 December 2004. phyll a remained at very low values in the Gulf of Chiriqui compressed the oxygenated waters into shallow depths during the dry season, but the deep chlorophyll maximum (Figure 3). Dissolved oxygen below this depth rapidly de- became remarkably intense at 30 m where concentration clined to less than 1 ppm (Figure 3a), whereas waters in reached 3 pg/L (Figure 3b). the Gulf of Chiriqui only became hypoxic below the 50 m oxycline (Figure 3b). No correlations were confirmed be- DISSOLVE DOXYGEN tween dissolved oxygen and temperature in any of these regions during the dry season. Dissolved oxygen profiles followed the typical pat- tern of well-oxygenated surface waters lying on top of DISSOLV ENDUTRIENTS deeper oxygen-poor waters. During the rainy season, se- vere hypoxic conditions (<2 ppm) were recorded below Both gulfs exhibit a strong vertical gradient of upwardly the strong oxycline, at 50 m and nearly coincident with decreasing nutrient concentrations. Nitrate in surface waters the thermocline (see Figure 2). Oxygen concentrations in was depleted in both gulfs during the rainy season, with val- waters above the thermocline were strongly correlated ues below 0.5 M (Figure 4). During the dry season, nitrate with temperature in both the Gulf of Panama (r = 0.91; concentrations at the surface were observed to increase 10 P < 0.001) and the Gulf of Chiriqui (r = 0.89; P < fold in the Gulf of Panama when the nutricline shoaled to 0.001) during the rainy season. This arrangement, how- around 10 m (Figure 5a). No similar surface enrichment ever, had strong seasonal variation in the Gulf of Panama was detected in the Gulf of Chiriqui, where a strong nutri- during the dry season, as the oxycline rose to 25 m and cline was developed at 60 m (Figure 5b). 340 e SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES Dissol voexdyg (epnpm) Dissol voexdyg (epnpm) OM Say 2 Sie 45 eG ie 10 eel 2 Ch Sy 7 es ee ee ee ee ee ee eee ee ee ee eee Chloroph y(aul lLg”) Chloroph y(aul lLg”) Chloroph y(aul lLg") 0 1 2 3 4 5 0 1 3) AW 5ia0) Bt | WE Sees bet nt tt nn nt Sali n(pitsys) Sali n(pitsys) Sali n(pitsys) 29 30 31 32 33 34 35 3629 30 31 32 33 34 35 36 29 30 31 32 33 34 35 36 es ee ee ee ee ee ——— a on onl Temperature (°C) Te’ mpera (t°uCr)e 12 #15 18 #219 24 27 3042 15 4 Temper a(°tCu)re21 24 27 3012 15 18 21 24 27 30 (m)Depth 22 5= |Gu lo fPfanama G uColhf iriqui G uColhf iriqui 29/February/2000 01/March/2000 13/April/2007 25 04(Dr yseason) (sDerayson) (sDerayson) FIGURE 3 .Profiles of dissolved oxygen ,chlorophyl la ,salinity ,and temperature in the Gulf of Panama and the Gulf of Chiriqui during the dry season. a = Gulf of Panama, 29 February 2000; b = Gulf of Chiriqui, 1 March 2000; c = Gulf of Chiriqui, 13 April 2007. Overall, the patterns of phosphate resembled those of related to temperature in both the Gulf of Chiriqui (r = nitrate, but concentrations were lower by an order of mag- —0.78; P < 0.001) and the Gulf of Panama (r = -0.97; P < nitude. Concentrations of phosphate in excess of 1 ~M were 0.002). In the dry season, nitrate in the Gulf of Panama was usually found below 30 m depth. Phosphate concentrations negatively correlated to temperature (r = —0.98; P < 0.044) in surface waters remained relatively low (<0.3 wM) in the and directly related to salinity (r = 0.98; P < 0.049). Nir- Gulf of Chiriqui during both climatic seasons (Figures 4b, trate was negatively correlated to temperature in the Gulf of 5b). However, phosphate enrichment of surface waters Chiriqui during the dry season (r = —0.89; P < 0.016), but clearly occurred in the Gulf of Panama during the dry sea- not to salinity (r = 0.67; P > 0.159). Phosphate was nega- son when the nutricline shoaled and phosphate concentra- tively correlated to temperature during the dry season in the tions in the top of the water column reached about 1.0 ~.M Gulf of Panama (r = —0.98; P < 0.038) and in the Gulf of (Figure 5a). Chiriqui (r = -0.97; P < 0.036). Dry season phosphate was Silicate profiles followed similar trends to that of the also correlated to salinity in the Gulf of Chiriqui (r = 0.98; nitrate and phosphate (Figures 4, 5). Although silicate IP & 05). concentrations were similar in surface waters in both gulfs The extremely low nitrate to phosphate ratios (N:P) during the rainy season, they doubled in the Gulf of Pan- suggest that phytoplankton growth in both regions was ama during the dry season (Table 1). under severe nitrogen limitation during the rainy season Dissolved nutrients in the upper 50 m had a high de- (Figure 6). The N:P ratio was below 2:1 in surface water gree of relationship with temperature and salinity. During and increased with depth, surpassing the value of 10:1 the rainy season, nitrate concentrations were inversely cor- below the depth of 50 m. During the dry season, N:P ra- NUMBER 38 ¢ 341 b Si-Silicate (uM) Si-Silicate (uM) 0 8 16 24 32 40 0 8 16 24 32 40 We P-Phosphate (uM) P-Phosphate (uM) (OG) (0s) 4-0) as) PL) 72s) KOO) (0) 4/0) ds) ZA) Ze) S10 N-Nitrate (uM) N-Nitrate (uM) 0} ey 0) (Sie Z0ee 25306 350 Ol 5) 10 15eeeZ20e 253005 (mD) ept1h00 120 140 160 180 “| Gulf of Panama Gulf of Chiriqui 18/December/200: 17/December/2004 (Rainy season) (Rainy season)200 FIGURE 4. Mean profiles of silicate (Si), phosphate (P), and nitrate (N) in the Gulf of Panama and the Gulf of Chiriqui during the rainy season. a = Gulf of Panama, 18 December 2004; b = Gulf of Chiriqui, 17 De- cemb e20r04. tios within the euphotic zone largely increased in both re- parency and shallow euphotic zone (14 m) observed in the gions, becoming closer to the N:P ratio of 16:1 suggested Gulf of Panama during the dry season upwelling. as favorable for phytoplankton growth (Redfield, 1958). WAT ETR ANSPARENCY DISCUSSION Water transparency was seasonably stable in the Gulf Our data on bottom-to-surface profiles reveal the dy- of Chiriqui but varied considerably in the Gulf of Panama namics of hydrological conditions along the Pacific coast of (see Table 1). Water transparency in both gulfs was higher Panama during times of both upwelling and non-upwelling. during the rainy season when the euphotic zone was ap- During the non-upwelling rainy season, both gulfs exhibit proximately 60 m deep, in contrast to the limited trans- extremely similar hydrological structures dominated by the 342 e SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES b Si-Silicate (uM) Si-Silicate (uM) 0 Gi-- 4G 24 2 40 0 8 16 24 32 40 en P-Phosphate (uM) P-Phosphate (uM) O10) O55 120 1:5) 62:0) 922558 3:0010) G'S) a0 1:5) 2107 O25 0 N-Nitrate (uM) N-Nitrate (uM) Oe Damn 0 159620772557 308 350m Om Om 0 15020) 255 S0mesS (mD) ep1th00 120 140 160 180— Gulf of Panama Gulf of Chiriqui 01/March/2000 200 (Dry season) (Dry season) FIGURE 5. Mean profiles of silicate (Si), phosphate (P), and nitrate (N) in the Gulf of Panama and the Gulf of Chiriqui during the dry season. a = Gulf of Panama, 29 February 2000; b = Gulf of Chiriqui, 1 March 2000. development of an intense thermocline at approximately cline typical of the eastern tropical Pacific Ocean (Enfield, 60 m. Surface waters tend to have low salinities and are 2001). As such, the seasonal movement of the thermocline warm and nutrient depleted. Low N:P ratios in surface represents a key source of nutrients for phytoplankton. waters during the rainy season suggest that phytoplank- Our sampling sites were far offshore and therefore silicate ton growth is strongly nitrogen limited. Consequently, the concentrations were not as high as previously reported for standing stock of chlorophyll a is maintained at relatively the inner shelf (D’Croz and O’Dea, 2007) even though the low levels in surface waters. Phytoplankton does however concentration of silicate in the Gulf of Panama is reported peak at subsurface levels as the nutrient-rich thermocline to be the highest in the eastern Pacific as a consequence of waters intersect the euphotic zone, increasing N:P ratios the intense runoff in the area (Pennington et al., 2006). and favoring algal growth. The strong inverse correlation During the dry season, the hydrological patterns of the between nutrients and sea temperature is consistent with two gulfs become dissimilar. In the Gulf of Panama strong the coincidence of a shallow thermocline and strong nutri- upwelling of cold deep waters into coastal and surface wa- NUMBER 38 ¢ 343 of Panama during the dry season but not during the rainy Gulf of Panama Gulf of Chiriqui N:P ratio N:P ratio season (D’Croz and O’Dea, 2007). 0 3 6 S12 15310 3 6 9 12 15 Data from the Gulf of Chiriqui are scant but did sug- gest that upwelling does not occur, because wind stress during the dry season is normally one-third of that of the Gulf of Panama (Kwiecinski and Chial, 1983) and it does not displace surface waters offshore. High mountain ranges running along western Panama impede the flow of northerly winds across to the Gulf of Chiriqui (see Fig- (m)Dep°thro) ure 1), whereas mountain ranges in central Panama are low, allowing strong wind jets to form toward the Gulf of Panama. Despite this clear distinction, our data show that similar hydrological changes to those that occur in the Gulf of Panama do take place in the Gulf of Chiriqui. Dur- ing the dry season, and concurrent with strong upwelling 4 Rain yseason in the Gulf of Panama, we observed deeper waters rise to- @ Dr yseason ward shallower depths in the Gulf of Chiriqui. This move- ment led to a substantial compression of the mixed layer FIGURE 6. Profiles of average nitrate to phosphate ratios (N:P) in and the corresponding rise of available nutrients within rainy (triangles) and dry (circles) seasons: a = Gulf of Panama; b = the euphotic zone, shifting the chlorophyll maximum Gul fo fChiriqui. above the shallow thermocline. Although direct evidence of prolonged surface water cooling was not observed, we postulate that cooling and nutrient-enrichment episodes ters drives significant changes in the hydrological properties in the Gulf of Chiriqui may occur and that their inten- of the water column. The thermocline migrates vertically sity is dependent upon the depth to which the thermocline upward, leading to cooling, increased salinity, and nutrient reaches in the eastern Pacific during the boreal winter. enrichment on surface waters. Surface N:P ratios become Nonetheless, the process is clearly much less intense than closer to the Redfield value and, as a result, phytoplankton that in the Gulf of Panama. Despite substantial shifts in growth intensifies, leading to a reduction in water clarity. deeper water conditions in the Gulf of Chiriqui, surface A shallow oxycline also develops and oxygen concentra- waters remain warm and nutrient poor, presumably be- tion below the oxycline is reduced, often leading to severe cause wind stress is not strong enough to cause the advec- hypoxic conditions. In contrast, the oxycline in the Gulf of tion of deep, cool, and nutrient-rich waters to the surface Chiriqui is deeper and deep water remains hypoxic. Low (D’Croz and O’Dea, 2007). However, ocean forces such as oxygen minima are nonetheless typical in the eastern tropi- internal waves might change the oceanographic structure cal Pacific as a combination of high algal growth at the in the Gulf of Chiriqui, causing brief periods of advection surface, a strong pycnocline that impedes the ventilation of of deep cold water to the surface layer (Dana, 1975). Long- waters below, and the sluggish circulation of deep waters term records from data loggers deployed in coral reefs give (Fiedler and Talley, 2006). The report of large filamentous evidence of such brief SST drops in the Gulf of Chiriqui Thioploca-like sulfur bacteria on shallow sediments in both that are possibly related to internal waves (D’Croz and regions strongly suggests that the inner shelf is exposed to O’Dea, 2007). This effect might be more evident as the episodes of reduced oxygen (Gallardo and Espinoza, 2007). internal waves approach the shallow coasts around the is- A significant relationship between wind-stress index lands in the Gulf of Chiriqui and may be more likely to (calculated from the sum of northerly winds) and sea level occur during times of thermocline shallowing. provides an explanatory mechanism for upwelling in the In conclusion, although the Gulf of Chiriqui does not Gulf of Panama (Schaefer et al., 1958; Legeckis, 1988; experience the intense seasonal upwelling characteristic Xie et al., 2005). Surface waters are displaced into open of the Gulf of Panama, deeper waters do migrate upward ocean by strong northerly winds during the dry season, in synchrony with Gulf of Panama upwelling. This move- and deep waters move vertically upward to replace them ment is probably caused by an overall shallowing of the (Fleming, 1940; Smayda, 1966; Forsbergh, 1969). Conse- thermocline across Central America. The difference in in- quently, wind stress is inversely related to SST in the Gulf tensity of upward movement of the thermocline between 344 e SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES the two gulfs strongly influences the phytoplankton com- .1977 .Cora lGrowth in Upwelling and Non-upwelling Areas munity, with seasonal blooms occurring in the Gulf of off the Pacific Coast of Panama. Journa lof Marine Research,35:567-585. Panama but not in the Gulf of Chiriqui. Deeper waters do Glynn, P. W., and J. L. Maté. 1997. Field Guide to the Pacific Coral nonetheless experience similar patterns of seasonal hydro- Reef so fPanama .Proceeding so fthe 8th Internationa lCora lReef graphic change, and shallow waters of the Gulf of Chiriqui Symposi u1m:1,45-166. can be exposed to brief pulses of cold and nutrient-rich Jackson J .B .C. ,and L .D’Croz .1997 .“The Ocean Divided.” In CentralAmerica: A Natura land Cultura lHistory ,ed. A. G. Coates ,pp. waters by advection. However, the effects of thermocline 38-71 N. ew Haven and London :Yale Universit yPress. migration and advection on the shallow-water communi- Kwiecinsk iB, .a, nd B C. hia l1. 983 A. lguno sAspecto sde la Oceanografia ties of the Gulf of Chiriqui remain to be studied in detail. de lGolfo de Chiriqui ,su Comparacion con e lGolfo de Panama.Revist ad eBiologi aTropica l3, 1:323-325. Kwiecinski ,B. ,A .L .Jaén ,and A .M .Muschett .1975 .Afloramiento en ACKNOWLEDGMENTS e lGolfo de Panama durante la Temporada de 1973 .Universidad Autonoma de México .Anales Centro de Ciencias de lMar y Lim- nolog 2ia:7, 3-80. Juan B. Del Rosario, Plinio Gondola, and Dayanara Legeckis ,R .1988 .Upwelling of fthe Gulfs o fPanama and Papagayo in Macias assisted in the collection and processing of the the Tropica lPacific during March 1985 .Journa lo fGeophysical samples. Sebastien Tilmans and Juan L. Maté reviewed the Resea r9c3h:,15485-15489. manuscript. Rainfall data were kindly provided by Empresa McCreary, J. P., Jr, H. S. Lee, and D. B. Enfield. 1989. The Responseo fthe Coasta Ol cean to Strong Offshore Winds :With Applications de Transmision Eléctrica $.A., Panama. We acknowledge the to Circulation in the Gul fo fTehuantepe cand Papagayo .Journa ol f participants, skipper, and crew of the R/V Urraca for their Mari nReesearc 4h7, :81-109. assistance during the cruises. The Government of the Re- NORAD .1988 .Surveys off the Pacific Coast of Centra lAmerica .Re-ports on Surveys with R/V Dr .Fridtjo fNansen .Institute o fMarine public of Panama granted the permits for the collections. Researc hB,erge nN,orwa yN:ORAD/UNDP/FAO. O’Dea ,A. ,and J .B .C .Jackson .2002 .Bryozoan Growth Mirrors Con- trastin gSeasona Rl egime sAcros sth eIsthmu so Pf anama P. alaeo, LITERATURE CITED 185:77-94.O'Dea, A., J. B. C. Jackson, H. Fortunato, J. Travis-Smith, L. 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Glynn ,P .W .1972 .“Observations on the Ecology o fthe Caribbean and Xie ,S.-P. ,H .Xu ,W .S .Kessler ,and M .Nonaka .2005 .Air—Sea Inter- Pacific Coas to fPanama.” In The Panamic Biota :Some Observa- action ove rthe Eastern Pacific Warm Pool :Gap Winds ,Thermo- tions Prior to a Sea-Leve lCanal ,ed .M .L .Jones .Bulletin of the cline Dome ,and Atmospheric Convection .Journa lo fClimate, Biologica Slociet oy Wf ashington 2,:13-30. 18:5-20. D’Croz, Luis and O’Dea, Aaron. 2009. "Nutrient and Chlorophyll Dynamics in Pacific Central America (Panama)." Proceedings of the Smithsonian Marine Science Symposium 38, 335–344. View This Item Online: https://www.biodiversitylibrary.org/item/131385 Permalink: https://www.biodiversitylibrary.org/partpdf/387364 Holding Institution Smithsonian Libraries and Archives Sponsored by Biodiversity Heritage Library Copyright & Reuse Copyright Status: In Copyright. 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