Pelagic production and biomass in the Adriatic Sea - ICM-CSIC

SCI. MAR., 60 (Supl. 2): 65-77

SCIENTIA MARINA

1996

THE EUROPEAN ANCHOVY AND ITS ENVIRONMENT, I. PALOMERA and P. RUBIÉS (eds.)

Pelagic production and biomass in the Adriatic Sea* SERENA FONDA UMANI Department of Biology, University of Trieste, Laboratory of Marine Biology, Strada Costiera 336, 34010 Trieste, Italy.

SUMMARY: On the basis of the available data on plankton production and biomass in the Adriatic Sea, three different areas may be delineated: a - Open waters of the central and southern Adriatic, characterized by an “oceanic” community. Primary production rates are low. Microphytoplankton and especially diatoms are abundant; microzooplankton is dominated by tintinnids; herbivorous copepods are dominant throughout the year and carnivorous species are also important. The plankton communities show high diversity and great stability. A “top down” control appeared to regulate the energy fluxes. b Open waters of the northern basin, characterized by neritic associations with moderate primary production rates and biomass. The species originating from the southern Adriatic and the Mediterranean are present throughout the year. A marked west-to-east gradient of production and biomass is present during the stratified period. The models of phytoplankton control appear to shift from “bottom up” to “top down” control, following the western-eastern gradient and changing seasonally. c - A coastal zone characterized by a neritic community of low diversity and high primary production and biomass. Nanoplankton primary production generally prevails. This production appear to be used mainly by ciliates other than tintinnids during the stratified period. Nanoplankton also represents an important food source for the abundant filter feeders. During the persistence of the frontal system, separating coastal from off shore waters, terrigenous inputs stimulate high primary production which is confined to inshore areas. Zooplankton grazing is not sufficient to control the produced biomass and transfer to offshore is restricted, resulting in eutrophication Key words: Primary production, plankton biomass and distribution, Adriatic Sea. RESUMEN: PRODUCCIÓN PRIMARIA Y BIOMASA EN EL ADRIÁTICO. – En base a los datos disponibles de producción planctónica y biomasa del Adriático, pueden delimitarse tres áreas distintas: a - Aguas de mar abierto del Adriático central y meridional, caracterizadas por una comunidad “oceánica”. Las tasas de producción primaria son bajas. El microfitoplancton y especialmente las diatomeas, son abundantes; el microzooplancton está dominado por los tintínidos; los copépodos herbívoros dominan a lo largo de todo el año y las especies carnívoras tienen también importancia. Las comunidades planctónicas muestran una diversidad alta y una gran estabilidad. Un control de “top down” parece regular los flujos de energía. b Aguas de mar abierto de la cuenca septentrional, caracterizadas por asociaciones neríticas con tasas de producción primara y biomasa moderadas. Las especies de origen sud-adriático y mediterráneo están presentes todo el año. Durante el período de estratificación existe un marcado gradiente oeste-este de produccción y biomasa. Los modelos de control del fitoplancton parecen cambiar de “bottom up” a “top down”, siguiendo el gradiente oeste-este y cambiando estacionalmente. c - Una zona costera caracterizada por una comunidad nerítica de baja diversidad y elevadas producción primaria y biomasa. Generalmente predomina la producción primaria del nanoplancton. Dicha producción parece ser utilizada principalmente por ciliados no tintínidos durante el período de estratificación. El nanoplancton representa asimismo una fuente importante de alimento para los abundantes filtradores. Mientras persiste el sistema frontal, que separa las aguas costeras de las oceánicas, los aportes terrígenos estimulan una elevada producción primaria, confinada a las aguas litorales. Su consumición por parte del zooplancton no alcanza a controlar la biomasa producida y la transferencia a alta mar es limitada, dando como resultado la eutroficación. (Traducido por los Editores). Palabras clave: Producción primaria, biomasa y distribución planctónicas, Adriático.

*Received July 16, 1995. Accepted August 26, 1996.

ADRIATIC PELAGIC PRODUCTION 65

INTRODUCTION The Adriatic Sea is a semi-enclosed and elongated basin, stretching roughly SE to NW for 800 km from the Straits of Otranto to the Gulf of Venice (Fig.1). The elongate shape and the presence of the great Dalmatian Archipelago create an extremely long coastline. The eastern coasts are high, rocky, and articulated. The western (Italian) coasts generally are sandy, flat and alluvional with the exception of the Gargano Peninsula and the Apulian coast in the southern part. Extensive lagoons characterized the northern part and the areas of Po River Delta. Based on the bathymetry and different oceanographic properties it is possible to distinguish three distinct areas: North, Mid and South Adriatic. The North Adriatic is the least deep (max. depth 100 m) of the three basins; its bottom topography and sediment composition are strongly influenced by lower sea level during the Pleistocene (Brambati, 1992). Along the Italian shore the sea bottom is characterized by pelitic sediments of terrigenous supplies. Sand, left over from Holocenic transgression, predominates in the central and southern part of the shelf.

Fig. 1. – The Adriatic Sea: bathymetry, high and low coasts and surface currents (from Brambati, 1992).

66 S. FONDA UMANI

Its biological characteristics are influenced by the morphology, meteorology and hydrodynamics of the area, and in particular by the supply of the Po river (Fonda Umani et al., 1992, Franco and Michelato, 1992). The water mass distribution undergoes remarkable seasonal modification due to two main factors: the wide fluctuation of the surface heat fluxes and the large volume of fresh water flowing into the shallow basin (Fonda Umani et al., 1992). During winter, when the total heat budget is negative (Hendershott and Rizzoli, 1976), the cold waters diluted by the western riverine inflow remain confined in a coastal belt and are separated from the offshore waters by a frontal system and flow southward (Franco, 1973; 1986). In the offshore area the waters are highly saline, being advected from the southern basins and they are actively mixed by wind - driven surface cooling and mechanical stirring (Fonda Umani et al., 1992). During summer, after the generation of a thermocline and the injection of Po river waters in the offshore area, heating and dilution processes generate a highly stratified water column in which there are three layers separated by strong density gradients (Fonda Umani et al., 1992). The Mid and South Adriatic are deeper basins (max. depth 270 m and 1200 m respectively); they are separated by the Pelagosa sill (160 m). Another sill (800 m) situated in the Channel of Otranto separates the South Adriatic from the Ionian Sea. The morphology of the central and southern basins has been conditioned by deposition processes which occurred during periods of marine regression. The basin bottom is mainly composed of pelitic sediments which have the typical mineralogical characteristics of their province: Dalmatian, Central Apennine or South - Augitic- Albanian (Brambati, 1992). The two basins are characterized by low rates of primary production and a general oligotrophic condition (Buljan, 1964). They receive the supply of both northern, dense winter waters and of cold and highly salty waters originating during winter in the eastern Mediterranean. Therefore in both basins three layers can be distinguished along the water column: a superficial layer of low density, influenced, along the western side, by the indirect effects of riverine dilution, an intermediate one with dense waters of Ionian origin and the deep one with dense water generated in winter (Buljan and Zore-Armanda, 1976). The distribution of dissolved nutrients is influenced by hydrodynamic features of the basin and shows marked seasonal differences (Tables 1 and 2).

TABLE 1. – Mean values and confidence intervals (parentheses) of five water masses in the northern Adriatic basin in September, 1987 (from Franco and Michelato, 1992). The regions are: A’ - river plume waters; A - coastal waters; B’ - surface waters of the proper basin; B - intermediate water; and C - deep layer. Nutrient concentrations are expressed as µM dm-3. Si is silicon in SiO4. Temp. (ºC)

Sal. (PSU)

η (kg m-3)

AOU (cm3 dm-3)

N-NH3 (µM dm-3)

N-NO2

N-NO3

P-PO4

Si-SiO4

Chl.a (µg dm-3)

A’

23.11 (0.60)

24.59 (4.76)

16.03 (3.53)

-0.62 (0. 42)

1.39 (0.98)

0.96 (0.42)

26.97 (18.23)

0.22 (0.19)

23.78 (13.87)

9.47 (3.08)

A

24.47 (0.11)

33.96 (0.20)

22.72 (0.14)

-0.44 (0.09)

0.76 (0.14)

0.17 (0.03)

2.65 (0.67)

0.08 (0.02)

3.30 (0.53)

2.56 (0.67)

B’

24.63 (0.7)

36.14 (0.09)

24.32 (0.08)

-0.24 (0.04)

0.48 (0.11)

0.03 (0.01)

0.13 (0.04)

0.07 (0.01)

1.58 (0.18)

0.63 (0.06)

B

21.53 (0.62)

37.89 (0.08)

26.54 (0.22)

-0.13 (0.48)

1.29 (1.06)

0.07 (0.04)

0.24 (0.17)

0.13 (0.06)

4.73 (2.35)

0.85 (0.22)

C

15.96 (0.62)

38.28 (0.03)

28.27 (0.15)

0.63 (0.39)

1.54 (0.46)

0.08 (0.03)

0.58 (0.18)

0.30 (0.20)

10.15 (2.26)

1.53 (0.38)

During the winter mixing period in the western belt, which is separated by the frontal system, nutrient concentration can be 3 - 10 times higher than in open waters (Franco and Michelato, 1992). During summer stratification, the increase of nutrients in the surface layer from river inputs is followed by a situation of depletion due to phytoplankton utilization and progressive transport via faecal pellets and sinking of particulate matter to the deeper layers. In the Northern Adriatic, particulate organic carbon (POC) and particulate organic nitrogen (PON) concentrations appear related mostly to the phytoplankton crop: the mean surface phytoplankton carbon is about 20% of total POC. During summer stratification POC range from 75 to 1701 µg l-1 and PON from 8.9 to 303 µg l-1 showing a decreasing pattern from surface to bottom layers and from east to west (Gilmartin and Revelante, 1991). In the Gulf

of Trieste mean POC concentration ranged from 610.9 at the surface to 444.9 µg l-1 at bottom layer. The mean C/N atomic ratio at the surface was 16.4 and at the bottom 8.5, indicating the prevalence of allochthonous riverine POC at the surface and autochthonous plankton at the bottom (Salvi et al., in press). Recently, Wassmann et al. (in press) calculated the vertical flux of organic carbon in the Gulf of Trieste: it ranged from 800 mg C m-2d-1 in June to 200 mg C m-2d-1 in September. Phytoplankton carbon comprised respectively 17 - 29% and 40 - 87% of the suspended POC, implying that the detritus fraction was lower in September. Lastly, Giordani et al., (1992) calculated by measuring in situ benthic fluxes that for the Adriatic as a whole about 85% of the carbon, 60% of the phosphorus, 40% of the fixed nitrogen, and 85% of the silica are recycled into the water column.

TABLE 2. – Mean values and confidence intervals (parentheses) of five water masses in the northern Adriatic basin in February, 1987 (from Franco and Michelato, 1992). The regions are: A’ - river plume waters; A - coastal waters; B - proper basin dense waters; and C - proper basin waters. Nutrient concentrations are expressed as µM dm-3. Si is silicon in SiO4. Temp. (ºC)

Sal. (PSU)

η (kg m-3)

AOU (cm3 dm-3)

N-NH3 (µM dm-3)

N-NO2

N-NO3

P-PO4

Si-SiO4

Chl.a (µg dm-3)

A’

5.29 (0.84)

32.78 (2.26)

25.88 (1.73)

0.02 (0.09)

17.83 (2.97)

0.82 (0.30)

33.81 (2.40)

0.60 (0.36)

16.03 (8.72)

4.43

A

5.21 (0.40)

36.73 (0.28)

29.02 (0.22)

0.04 (0.06)

1.52 (0.56)

0.49 (0.06)

10.56 (3.14)

0.18 (0.07)

6.78 (0.53)

1.42 (0.80)

B

6.92 (0.13)

38.00 (0.05)

29.79 (0.03)

-0.02 (0.06)

0.44 (0.11)

0.21 (0.04)

1.04 (0.28)

0.18 (0.05)

4.06 (0.23)

1.62 (0.30)

C

7.77 (0.07)

38.10 (0.03)

29.75 (0.02)

0.03 (0.04)

0.48 (0.21)

0.36 (0.07)

0.71 (0.14)

0.18 (0.03)

4.36 (0.22)

1.59 (0.22)

ADRIATIC PELAGIC PRODUCTION 67

PRIMARY PRODUCTION AND PHYTOPLANKTON BIOMASS The rivers flowing into the northern Adriatic are the major sources of external nutrient input, especially so during stratified periods (Franco, 1973; Degobbis and Gilmartin, 1990). Water mass exchange between the northern region and the remainder of the essentially oligotrophic Adriatic, as well as the major influence of Ionian water in the south, have a great influence on the productivity and standing crops of different sub-areas. A terrigenous supply of nutrients in some semi-enclosed bays and channels of the eastern coast and all along the western coast via run-off, influences productivity in a relatively narrow coastal belt. Consequently, biomass and production rates are spatially very variable. Buljan (1964) estimated the productivity of the Adriatic Sea on the basis of its hydrographic properties and suggested four productivity zones: open waters of the central and southern Adriatic with low production; the shallow northern Adriatic including a narrow coastal belt along the western coast and characterized by permanent high production; the area of moderate production occupying eastern coastal waters; and the limited zones with high production under strong coastal influences (lagoons and embayments) along the eastern and also western shores. Northern Adriatic The northern Adriatic has been recognized for many years as a region of high marine production at several trophic levels from phytoplankton to fish. A region of high but variable phytoplankton biomass and production was quantified off the delta of the River Po and related to the spreading of its plume (Franco, 1973; Gilmartin and Revelante, 1981), and a marked west to east gradient of the standing crop and production was observed (Smodlaka and Revelante, 1983). In Table 3 the mean values for chlorophyll a as well as maximum and minimum values for primary production (measured by 14C method) are reported for the eastern and western part (Fonda Umani et al., 1992). In the northernmost part, the Gulf of Trieste, the chlorophyll a biomass is moderate with a mean value around 1 µg dm-3 in the eastern section and around 1.6 µg dm-3 in the western section (Olivotti et al, 1986), while annual productivity varies from 42 to 53 g C m-2 y-1 (Faganeli et al., 1981; Monti, 68 S. FONDA UMANI

TABLE 3. – Chlorophyll a mean values and maximum and minimum of primary production (measured by 14C method) in western and eastern North Adriatic North Adriatic Western Eastern chlorophyll a production

max. min.

2.87 µg.dm-3 30 µg. C dm-3h-1