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Conditions favoring dinoflagellate bloom development are most often observed in the coastal areas of Emilia Romagna. Here nutrient loads are higher than in all other parts of the basin due to freshwater inputs of the Po River and of minor rivers along the coast. Peak flows in spring and autumn, and less frequent, summer pulses, lead to enhanced phytoplankton production in general, and, since 1975, dinoflagellate blooms particularly (Table). The typical seasonal distribution in this region proceeds as follows: low water temperatures prevent bloom expression in the winter. After early spring diatom increases, dinoflagellate blooms generally commence in March-April. The principal contributors are Noctiluca miliaris and Glenodinium lenticula, followed by early summer blooms of extitProrocentrum micans, P. scutellum, Scrippsiella trochoidea, and Gymnodinium foliaceum. In summer-fall, Gymnodinium corii, Linguilodinium polyedra, and Heterocapsa rotundata prevail. In most cases, they occur in patches along the axes of the coastal belt, decreasing in intensity from north to south. The patchy distribution of the blooms also reflects variability in small tributary discharge in the area, with blooms localized to mouths of the small rivers.
Other regions of the basin and coastal bays are typified by bloom-forming species with regional variability in distributions of bloom-forming taxa more frequently observed in the early summer (Table). For example, in June 1977, a bloom of N. miliaris was noted in the Gulf of Trieste, persisting one week and concentrated in the inner and more confined part of the harbor. The bloom was triggered by a sudden increase in surface temperature (2-3°C in a few days) associated with an increase in tributary runoff of nutrient-rich freshwater. In the same summer, some of the same taxa noted above in the seasonal distribution of bloom forming taxa, G. corii and L. polyedra, were observed along the Emilia Romagna coast, lasting from July 19th to the 27th, followed in August by a P. micans red tide. In June 1980, another bloom of N. miliaris occurred but was far more widespread, affecting the whole northern portion of the Adriatic, from the coast of Istria to Venice. This taxon was not observed along the Emilia Romagna coast where another P. micans bloom was note. At the end of summer, frequently only one bloom-forming species has impacted the entire whole northern basin. L. polyedra was observed in September 1978 and again from August-September 1982; late summer densities were generally lower than densities reported for early summer blooms.
Occasionally, blooms have been noted late in the year. In fall, 1984, a Gymnodinium bloom was observed throughout the northern Adriatic Sea, persisting from September until December and resulted in chl a levels extgreater 700 mg m-3. The bloom developed along the Emilia Romagna coast after a collapse of dense blooms of diatoms and later of L. polyedra, and extended 20 km offshore. New nutrient influx was responsible for the bloom: heavy precipitation led to high freshwater inputs and associated new nutrient loads from the scouring of river bed nutrient stocks that had accumulated in a meteorologically calm period prior to mid-August.
Dinoflagellate blooms along the Emilia Romagna can contribute significantly to total biomass, at least in the northern most part of the basin directly influenced by the Po River. For example, during the August 1981 bloom of G. corii, chlorophyll concentrations exceeded 1000 mg m-3 and densities reached 2.3 x 108 cells L-1. Similarly, chlorophyll concentrations exceeded 40 mg m-3 during an August 1986 bloom of Peridinium trochoideum.
The G. corii bloom covered the Emilia Romagna region from the Po River delta to Mount Conero with highest chl a concentrations noted in the northern portion of the region, declining to extgreater 200 mg m-3 in southern areas. Although productivity data are not available, these biomass levels were likely responsible for very high productivity along the entire coast. Oxygen concentrations in surface waters were supersaturated (>170%) and anoxia typified bottom waters of the area. The bloom and its associated oxygen-rich surface waters were subsequently displaced offshore by the upwelling of anoxic bottom waters generated by strong offshore winds.
Annual blooms of individual bloom-forming species are not typical of most of the Northern Adriatic Sea. Only isolated bays on the coast and near-shore waters south of the Po River delta have conditions favoring frequent bloom activity. These conditions include: (1) a near continuous supply of new nutrient from the Po and small rivers, (2) tidal- or meteorologically-induced mixing of remineralized nutrients, vegetative cells and resting stages from the shallow bottom, (3) persistent, stable fronts (convergences) between the nutrient-rich plume and oligotrophic offshore water, and (4) wind-induced upwelling of deep, nutrient-rich bottom water from intermediate or deep basin water immediately to the east. In most years, summer river discharge is minimal so that nutrients delivered in the eastward-spreading summer plume are quickly diluted, restricting bloom development to the western-most portions of the basin. Nutrient uptake at depth via vertical migration provides little substrate for maintaining bloom populations due to nutrient-depleted intermediate water. Further, remineralized nutrient pools as well as "seed populations" in deep bottom waters are rarely introduced into surface waters due to strong vertical stratification and depth, thereby restricting surface production to new nutrient in the plume and surface regeneration in increasingly oligotrophic water. Thus, blooms in the basin would reflect new nutrient inputs and tend to be short-lived due to rapid exhaustion of nutrient pools.
Dinoflagellate blooms occur most frequently along the western margin of the northern Adriatic, probably as a consequence of the combined affects of the discharge of buoyant, nutrient-rich water and physical processes that confine the discharges from the Po and smaller rivers to the western margin under most circumstances. Three bloom periods characterize the region: 1) following the late spring-early summer diatom bloom period when surface nutrients are depleted and the water column has become thermally stratified, dinoflagellates are able to bloom by migrating between nutrient-rich, sub-thermocline water and the surface layer where light levels are higher; 2) during summer, surface waters remain nutrient depleted and dinoflagellates rapidly respond to local nutrient enrichment from river discharges or coastal upwelling; 3) rainfall-induced freshets in autumn lead to brief diatom blooms followed, as in spring, by blooms of dinoflagellates. The duration of spring and summer blooms appear to be limited by nutrient availability while autumn events appear to be more dependent on stable meteorological conditions and perhaps intense recycling of local production.
The October 1984 Gymnodinium bloom exemplifies this latter process. Intense precipitation in mid-August brought new nutrients previously accumulated in river beds into the coastal basin leading to an initial diatom bloom, subsequently replaced by the Linguilodinium. In September Linguilodinium was in turn replaced by Heterocapsa in the north and Gymnodinium in the south. With winds blowing seaward (Libeccio), the bloom spread over a large area, later invading the whole northern basin.
A coastal plume of less saline, nutrient-rich water is always found south of the Po River delta, with strongest flow and widest extent from autumn to spring. With surface warming and summer thermohaline circulation, much of the Po River's plume extends eastward into the basin from the eastern delta, confining some of the southern flowing coastal plume on shore as it moves to the southeast. Nutrients from terrestrial drainage are utilized and recycled in the water column of the coastal area and partially retained in its sediments. Dissolved nutrient concentrations are maintained at relatively high levels with an apparently positive balance between the inputs and outputs from the system. Phytoplankton is found in the plume and along discontinuities resulting from thermohaline circulation and the salt gradient between the coastal plume and more oligotrophic basin waters. Localized blooms result from spatial and temporal heterogeneity in nutrient inputs, vertical stratification, and frontal convergence. Few if any blooms are initiated from recycled nutrients delivered from the bottom although perpetuation of existing blooms has been suggested.
Extension of the Po River plume into the oligotrophic basin of the Northern Adriatic in the low discharge summer period or, alternatively, during winter Bora, should also lead to aperiodic blooms, reflecting increased buoyancy, nutrient enrichment as well as dinoflagellate accumulation at convergences between the lower salinity plume and basin surface waters. However, although elevated chlorophyll levels are reported during the eastern extensions of the plume, there is only one study containing taxonomic information (Gymnodinium, see above) that dinoflagellates dominated plume autotrophic biomass.
On smaller scales, red tides of May-June, 1977, 1980, 1981, and 1983 in the Gulf of Trieste always followed Isonzo River freshets. The river's discharge constitutes the only significant freshwater source for the shallow Gulf of Trieste, with low flows in early winter and summer and high flows in early spring and autumn. Episodic short-lived pulses of high flow may occur in late spring and summer as a result of local storms. Heavy rains in this period have led to high river discharge lowering surface salinity (from >37 to <32) and an enrichment of Gulf surface waters with new nutrients, primarily as nitrates. At the same time, sunny weather can increase surface temperature by 2-3 °C. These hydrological conditions have led to optimal conditions for dinoflagellate growth, with critical depth exceeding the mixing depth yielding a diluted and warm surface layer favoring bloom development. Blooms are more frequently noted in the inner, less hydrodynamic portions of the Gulf, and persist for less than a week followed generally by dispersion from winds from the land, mainly the Bora (ENE) [414].
However there are evidences that significant blooms, particularly red tides occurred with greater regularity in the 1970s and 1980s than in recent years. More recent data collected during late 1980s and 1990s have shown a decrease of phytoplankton biomass as chl a in summer and autumn, due to a decrease frequency of dinoflagellate blooms. These changes can be attributed to a reduction of phosphorus loads with progress in wastewaters treatment in the region, and to the replacement of polyphosphate in detergents throughout Italy. This latter measure resulted in reduced phosphorus concentrations in Po River waters and thereby impacted the receiving waters of the northern Adriatic [214].
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