Seasonal variability and Po River plume ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, C05S90, doi:10.1029/2007JC004370, 2008

Seasonal variability and Po River plume influence on biochemical properties along western Adriatic coast Mauro Marini,1 Burt H. Jones,2 Alessandra Campanelli,1 Federica Grilli,1 and Craig M. Lee3 Received 5 June 2007; revised 21 November 2007; accepted 2 January 2008; published 21 May 2008.

[1] The influence of the Po plume on the northern Adriatic Sea was observed during two

seasons in 2003 under distinct physical forcing regimes. During the winter, the plume was cool, low in both salinity and chlorophyll, but with higher chlorophyll concentrations occurring along the plume boundary. The plume mixed deeply in the water column in response to the strong wind forcing. The northern Adriatic and the Po plume cooled significantly during the observational period, and therefore salinity alone was the best discriminator of water mass variability. In contrast to the strong forcing of the winter period, the late spring was characterized by weak wind forcing, and below-average Po River discharge (600 m3/s) which was about one third of the typical discharge for this period. As in winter, salinity was again the best discriminator of water mass variability. The Po plume advected southward along the Italian coast and in some locations portions of the coastal plume were transferred offshore in filament-like features. However, the one observed filament was quite low in chlorophyll and was quite thin vertically, extending downward less than 5 m from the surface. The spring observations provide a distinct contrast in the effects of the physical forcings of river flow and wind stress from two different seasons. The strong winter forcing resulted in deep mixing of the plume despite its low salinity and buoyancy, whereas the weak summer flow under weak winds resulted in a very shallow plume (15 mM are observed where salinity is >38. The distribution of

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MARINI ET AL.: BIOCHEMICAL PROPERTIES IN WESTERN ADRIATIC COAST

Figure 4. Winter surface field of: (a) temperature (°C), contour interval is 0.5; (b) salinity, contour interval is 0.5; (c) DIN (mM), contour interval is 2; (d) orthosilicate (mM), contour interval is 1; and (e) orthophosphate (mM), contour interval is 0.05. (f) Rectangles indicate the Po (A) and Pesaro (B) transects. The dots represent the sampling points. 6 of 18

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Figure 5. Sea surface temperature map from the NOAA-17, GOS-ISAC-CNR (4 June 2003). orthosilicate does not appear to be controlled by river inputs but by the active consumption by phytoplankton as reported by Cozzi et al. [2002]. [26] The highest chlorophyll a concentrations from bottle samples, nearly 15 mg L 1, occur near surface in the lowsalinity water and decrease monotonically offshore (Figure 10, bottom). The near-surface region where chlorophyll is high corresponds with the region where orthosilicate concentrations are quite low. Moderate concentrations of 1 –2 mg L 1 are observed near the bottom at the offshore end of the transect. It is not clear from this hydrographic section whether this near-bottom chlorophyll results from sinking from the surface, or perhaps it is the edge of a subsurface chlorophyll maximum that is typical of regions away from the direct influence of the Po plume. [27] About 330 km southeast from the Po River delta in the southern part of the central basin, a small filament distinguishable in ocean color (Figure 6) was observed extending offshore toward the east from the coastal region. A hydrographic section was obtained through the feature to determine its characteristics (Figure 7f). In the section, the filament is evident as a near-surface salinity minimum where salinity is less than 38.5 (Figure 11). The thickness of this low-salinity filament is less than 15 meters. Despite the evidence from the ocean color image, near-surface chlorophyll within the filament is low, 37.5 Pula Survey (Surface) S < 38.15 S > 38.15 Pesaro Survey (Surface) S < 38.2 S > 38.2 a

Temperature, °C 7.06(12) ± 0.34 8.71(21) ± 0.54

Salinity

Si(OH)4, mM

DIN, mM

PO4, mM

DIN/Si, mM

Chl a, mg L

1

37.02(12) ± 0.32 4.03(12) ± 1.12 6.51(12) ± 3.69 0.17(12) ± 0.12 1.51(12) ± 0.59 0.88(12) ± 0.20 37.91(21) ± 0.11 2.20(21) ± 0.90 1.50(21) ± 0.83 0.11(21) ± 0.06 0.74(21) ± 0.42 0.79(20) ± 0.19

10.08(13) ± 0.46 37.84(13) ± 0.14 3.77(13) ± 0.51 1.97(13) ± 1.07 0.18(13) ± 0.12 0.53(13) ± 0.31 0.63(13) ± 0.05 12.17(12) ± 0.27 38.32(12) ± 0.03 1.95(13) ± 0.24 0.97(13) ± 0.29 0.15(13) ± 0.11 0.50(13) ± 0.15 0.45(13) ± 0.04 9.62(15) ± 0.76 37.94(15) ± 0.18 2.30(15) ± 0.94 2.17(15) ± 1.22 0.24(15) ± 0.07 1.07(15) ± 1.05 0.91(15) ± 0.32 11.39(13) ± 0.55 38.38(13) ± 0.07 1.60(13) ± 0.52 1.06(13) ± 0.60 0.22(13) ± 0.07 0.76(13) ± 0.61 0.63(13) ± 0.13

Number of data given in parentheses, ±standard deviation.

the Po delta, whereas orthosilicates and DIN concentrations decrease (Table 1). South of the Istrian Peninsula, a strong front separated colder, fresher, and more DIN-rich water on the north from warmer, saltier, less DIN-rich water south of the front (Table 1). This front marks the eastern extension of the Po plume driven by strong Bora winds on either side of the plume. Orthophosphate concentrations do not show much difference between the zones, indicating little or no contribution from the Po River runoff. 4.2. Late Spring Characterization of Western Adriatic Coast [33] During the late spring cruise strong stratification characterized the water column of the northern Adriatic (Figure 10). A SeaWiFS image of chlorophyll from 4 June shows the southward extension of the Po plume along the western boundary within the flow of the WAC (Figure 6). Despite below average flow from the Po River, buoyancy driven flow was evident along the western boundary of the basin consistent with results from models and observations [e.g., Kourafalou, 1999; Poulain, 2001]. [34] In order to evaluate the influence of Po River runoff on biochemical properties of surface waters from three regions along the western Adriatic are compared: the Po plume area and the coastal regions off Pescara and Vasto (Figure 7). As in winter, salinity was used to discriminate between the different water masses. Similar to the winter case, temperature/salinity structure indicates that temperature is highly variable (Figure 13). The T/S plots in Figure 13 include a similar set of observations as in Figure 12 to provide an overall sense of the T/S variability in the system. The blue dots indicate temperature and salinity data from the ship’s near-surface uncontaminated water system, the green dots show data from the towed vehicle’s broad-scale survey (similar area to the winter broad-scale survey) on 26– 29 May, and the red dots indicate profile data from the CTD/rosette profiler during the Po plume hydrographic transect on 8 June 2003. The effects of solar insolation on upper layer heating are apparent in the increase of temperature in the low-salinity plume water. For example, the temperature at salinity of 32 in the Po plume increased by 4°C from the period of the broad-scale survey in late May to time of the Po plume transect on 8 June (Figure 13). As in winter, two water masses are identified from the late spring cruise: fresher water where salinity is generally less than about 38.2 is indicative of the influence of river inputs along the western boundary, and more saline water, greater than 38.2, is present offshore (Figures 7 and 13 and Table 2). The area influenced most directly by the Po River discharge is

characterized by salinities less than 37.5 (Figure 13). As in winter, salinity increased toward the east, but in contrast to the winter, strong vertical stratification constrained this mainly to the surface layer. [35] In the fresher water of the Po plume (salinity 35.7 (Figure 13). DIN/orthosilicates ratio were variable, higher in the more saline water than in the fresh water. However, DIN/orthosilicate ratios were much higher in summer than in winter period when DIN/ Si ratios were nearly all less than 2.5. The much larger DIN/ Si ratios in the spring are most likely due to greater consumption of orthosilicate by diatom phytoplankton groups in response to high light availability near the surface and stratification. Diatoms were a dominated the phytoplankton community of the Po plume (I. Cetinic, personal communication, 2005). In the bottom plot in Figure 10 the maximum surface chlorophyll a of about 15 mg L 1 corresponded to a minimum of orthosilicate concentration as observed by Socal et al. [2002]. [36] The Pescara area showed larger differences in the water properties between the surface coastal water and the surface offshore water (Table 2). Orthosilicate and DIN concentrations decreased from near the coast toward offshore and DIN/orthosilicates ratio increased. In contrast to winter when concentrations of nutrients decreased southward from the Po River plume, concentrations off Pescara were higher than concentrations in the Po plume area. [37] In the most southern region off Vasto, a filament extending from the coast toward offshore was evident in the SeaWiFS image (Figure 6). In situ measurements across the filament (Figure 11) indicate that it contains less saline surface water than the water into which it advects (Table 2, average S = 36.54 ± 0.87). Although the filament is generally characterized by water less than saline than 38.55, the salinity of hydrographic samples from the filament were 35.7 Pescara Survey (Surface) S < 37.5 S > 37.5 Vasto Survey (Surface) S < 37.42 S > 37.42 a

Temperature, °C

Salinity

Si(OH)4, mM

DIN, mM

PO4, mM

DIN/Si, mM

23.95(11) ± 1.94 34.18(11) ± 1.97 1.37(11) ± 0.86 2.74(11) ± 2.89 0.19(11) ± 0.21 4.19(11) ± 6.79 23.00(18) ± 0.81 36.58(18) ± 0.46 1.75(17) ± 1.53 1.93(17) ± 1.98 0.13(17) ± 0.03 27.41(17) ± 71.60

Chl a, mg L

1

6.01(9) ± 4.49 2.97(2) ± 1.89

21.61(3) ± 1.12 20.29(6) ± 0.48

36.21(3) ± 0.83 38.74(6) ± 0.02

2.50(3) ± 0.58 0.15(6) ± 0.18

4.64(3) ± 1.08 2.95(5) ± 2.06

0.20(3) ± 0.02 0.09(6) ± 0.04

1.88(3) ± 0.39 0.81(3) ± 0.31 130.15(5) ± 121.69 0.15(6) ± 0.03

24.40(7) ± 1.70 25.52(9) ± 0.39

36.54(7) ± 0.87 37.85(9) ± 0.43

2.43(7) ± 153 1.86(7) ± 1.28

3.36(7) ± 1.86 1.59(7) ± 1.58

0.14(7) ± 0.12 0.12(7) ± 0.09

1.72(7) ± 1.18 0.87(7) ± 0.47

Number of data given in parentheses, ±standard deviation.

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0.50(3) ± 0.02 0.32(4) ± 0.11

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discharge and high phytoplankton uptake of nutrients during the spring, indicated by the relatively high chlorophyll concentrations within the Po plume. [40] During the winter cruise the Po plume was more clearly defined extending northeastward toward the Istrian Peninsula in response to strong Bora winds from Trieste and Senj, and higher, but typical, river discharge rates. During the late spring cruise, although the Po plume spread more broadly, the area south of the Istrian Peninsula appeared less influenced by the Po plume (Figures 7c and 7d) perhaps because of below average river discharge rates and the absence of strong wind forcing. The middle Adriatic shows less influence from the Po River. During springtime local river inputs contribute to the nutrient concentrations along the coast [Marini et al., 2002; Campanelli et al., 2004]; in particular the Pescara and Vasto areas are characterized by nutrient concentrations similar to the Po plume area (Table 2). Nutrient and chlorophyll concentrations are highest in the western coastal areas of the Adriatic during both seasons.

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rapidly alongshore and in offshore filaments because of the lack of nutrients to sustain phytoplankton growth. [44] DIN/orthosilicate ratios were much typically less than 2 – 3 during the winter when phytoplankton growth was small. Low phytoplankton growth rates were probably the result of cold temperatures, low incident light, high attenuation of light in the plume, and deeper mixing than in spring. During the late spring the DIN/orthosilicate ratios were much higher. The very high ratios in spring reflected the phytoplankton uptake under conditions of high available light and stratification. [45] Acknowledgments. We are grateful to the crews of R/V Knorr, R/V G. Dallaporta and Paola Fornasiero for their assistance with sample collection. The research was supported by the ‘‘Dynamics Of Localized Currents and Eddy Variability In The Adriatic’’ (DOLCEVITA) program funded by the U.S. Office of Naval Research (award N000140210854 to B. Jones), NATO, the Croatian Ministry of Science and Technology and the Italian Ministry of the Environment and Ministry of Universities and Research. Winter SST and SeaWiFS chlorophyll were provided by Robert Arnone (Naval Research Laboratory, Stennis Space Center).

References

5. Conclusions [41] The biochemical characteristics of water masses in the northern Adriatic and the western boundary of the Adriatic have been presented. Because temperature is highly responsive to seasonal heat fluxes in the shallow northern region of the Adriatic, salinity is a better discriminator of water mass variability and mixing in this region. [42] During winter the extent and shape of the Po plume appeared to respond to Bora winds, extending northeastward toward the Istrian Peninsula carrying highs concentrations of DIN and orthosilicate. In general, nutrient concentrations were negatively correlated with salinity, nutrients increasing with decreasing salinity. Little accumulation of phytoplankton biomass was observed within the Po plume, and nutrient concentrations tended to be transported offshore with the freshwater. The Western Coastal Layer, observed in the Pesaro transect, showed a nutrient concentrations decreasing and salinity increasing toward offshore. Coastal advection transported freshwater, nutrients, and suspended material southward, dominated by physical mixing, and with limited phytoplankton growth. [43] In late spring, the coastal waters along the western boundary were characterized by lower salinity water near the coast and saltier water offshore, as in winter period. Unlike the winter period, the onshore-to-offshore gradient is confined vertically mainly to the surface layer because of strong stratification. During the spring cruise, low Po River discharge (625 m3/s) and weak wind forcing resulted in a broadly spreading, vertically stratified river plume, where high phytoplankton abundance contributed to rapid depletion of nutrients. In the central part of the Adriatic basin a filament extended offshore from the coast. In situ measurements showed that this filament was characterized by lower salinity and temperature, and higher concentrations of orthosilicates and DIN relative to the surrounding water. Alongshore advection of the plume does occur, but because of the weak mixing, relatively slow advection, nutrients are relatively low in the advected plume, phytoplankton biomass is elevated relative to the offshore water, but decreases

Artegiani, A., D. Bregant, E. Paschini, N. Pinardi, F. Raicich, and A. Russo (1997a), The Adriatic Sea general circulation. Part I. Air-sea interactions and water mass structure, J. Phys. Oceanogr., 27, 1492 – 1514. Artegiani, A., D. Bregant, E. Paschini, N. Pinardi, F. Raicich, and A. Russo (1997b), The Adriatic Sea general circulation. Part II: Baroclinic Circulation Structure, J. Phys. Oceanogr., 27, 1515 – 1532. Boldrin, A., S. Miserocchi, S. Rabitti, M. M. Turchetto, V. Balboni, and G. Socal (2002), Particulate matter in the southern Adriatic and Ionian Sea: Characterisation and downward fluxes, J. Mar. Syst., 33 – 34, 389 – 410. Brzezinski, M. A. (1985), The Si-C-N ratio of marine diatoms—Interspecific variability and the effect of some environmental variables, J. Phycol., 21, 347 – 357. Campanelli, A., P. Fornasiero, and M. Marini (2004), Physical and chemical characterization of the water column in the Piceno Coastal Area (Adriatic Sea), Fresenius Environ, Bull., 13(5), 430 – 435. Cozzi, S., M. Lipizer, C. Cantoni, and G. Catalano (2002), Nutrient balance in the ecosystem of the north western Adriatic Sea, Chem. Ecol., 18(1 – 2), 1 – 12. Dorman, C. E., et al. (2007), February 2003 marine atmospheric conditions and the bora over the northern Adriatic, J. Geophys. Res., 112, C03S03, doi:10.1029/2005JC003134. Holm-Hansen, O., C. J. Lorenzen, R. W. Holmes, and J. D. H. Strickland (1965), Fluorometric determination of chlorophyll, J. Cons. Cons. Int. Explor. Mer, 30(1), 3 – 15. Hopkins, T. S. (1992), The structure of Ionian and Levantine Seas, Rep. Meteorol. Oceanogr. 41(II), pp. 35 – 56, Harvard Univ., Cambridge, Mass. Hopkins, T. S., A. Artegiani, F. Bignami, and A. Russo (1999), Water-mass modification in the Northern Adriatic: A preliminary assessment from the ELNA data set, in The Adriatic Sea, edited by T. S. Hopkins et al., Ecosyst. Res. Rep 32, pp. 3 – 23, Eur. Comm., Brussels. Kourafalou, V. H. (1999), Process studies on the Po River plume, north Adriatic Sea, J. Geophys. Res., 104(C12), 29,963 – 29,985. Lee, C. M., et al. (2005), Northern Adriatic response to a wintertime bora wind event, Eos Trans. AGU, 86(16), 157, 163, 165. Marini, M., P. Fornasiero, and A. Artegiani (2002), Variations of hydrochemical features in the coastal waters of Monte Conero: 1982 – 1990, Mar. Ecol., 23(1), 258 – 271. Orlic, M., M. Gacic, and P. E. La Violette (1992), The currents and circulation of the Adriatic Sea, Oceanol. Acta, 15(2), 109 – 124. Orlic, M., M. Kuzmic, and Z. Pasaric (1994), Response of the Adriatic Sea to Bora and Sirocco forcings, Cont. Shelf. Res., 14(1), 91 – 116. Poulain, P. M., (2001), Adriatic Sea surface circulation as derived from drifter data between 1990 and 1999, J. Mar. Syst., 29, 3 – 32. Poulain, P. M., and B. Cushman-Roisin (2001), Circulation, in Physical Oceanography of the Adriatic Sea, edited by B. Cushman-Roisin et al., pp. 67 – 109, Kluwer Acad., Dordrecht, Netherlands. Poulain, P. M., and F. Raicich (2001), Forcings, in Physical Oceanography of the Adriatic Sea, edited by B. Cushman-Roisin et al., pp. 45 – 65, Kluwer Acad., Dordrecht, Netherlands. Poulain, P. M., V. H. Kourafalou, and B. Cushman-Roisin (2001), Northern Adriatic Sea, in Physical Oceanography of the Adriatic Sea, edited

17 of 18

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by B. Cushman-Roisin et al., pp. 143 – 165, Kluwer Acad., Dordrecht, Netherlands. Raicich, F. (1996), On the fresh water balance of the Adriatic coast, J. Mar. Syst., 9, 305 – 319. Redfield, A. C., B. H. Ketchum, and F. A. Richerds (1963), The influence of organism on the composition of sea water , in M. N. Hill (Ed.), The Sea, edited by M. N. Hill, pp. 26 – 77, Wiley-Intersci., Hoboken, N. J. Socal, G., A. Pugnetti, L. Alberighi, and F. Acri (2002), Observations on phytoplankton productivity in relation to hydrography in the northern Adriatic, Chem. Ecol., 18(1 – 2), 61 – 73. Strickland, J. D. H., and T. R. Parsons (1972), A Practical Handbook of Seawater Analysis, Bull. Fish. Res., vol. 167, 310 pp., Fish. Res. Board of Can., Ottawa. U.N. Educational, Scientific and Cultural Organization (1988), The acquisition, calibration and analysis of CTD data: A report of SCOR WG 51, UNESCO Tech. Pap. Mar. Sci., 54, 59pp.

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Vilibic, I. (2003), An analysis of dense water production on the North Adriatic shelf, Estuarine Coastal Shelf Sci., 56, 697 – 707. Zavatarelli, M., F. Raicich, D. Bregant, A. Russo, and A. Artegiani (1998), Climatological biogeochemical characteristics of the Adriatic Sea, J. Mar. Syst., 18, 227 – 263. Zore-Armanda, M. (1963), Les masses d’eau de la mer Adriatique, Acta Adriat., 10(3), 5 – 88. A. Campanelli, F. Grilli, and M. Marini, National Research Council, Institute of Marine Sciences, Largo Fiera della Pesca, 2 Ancona I-60125, Italy. ([email protected]; [email protected]; m.marini@ismar. cnr.it) B. H. Jones, University of Southern California, AHF B30, Los Angeles, CA 90001, USA. ([email protected]) C. M. Lee, University of Washington, Henderson Hall, 1013 N. E. 40th Street, Seattle, WA 98105, USA. ([email protected])

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