300
J. Romero et al.
Plankton together with particulate matter and sediments from the channels induce high turbidity with an average extinction coefficient of
0.4 m- l in the centre of the bay (Perez 1989). In contrast, in the northern
Fangar Bay, with comparable freshwater input and morphological features, but with a different residence time of fresh water (only 2 - 3 days
during spring-summer), primary production is mainly achieved by macrophytes, which cover more than 80% of the bottom, including the central basin. This is because rapid water exchange prevents accumulation
of planktonic biomass in the bay, and thus the water is much clearer, so
that dense macrophyte beds can develop.
The conditions described above, together with the fact that planktonic
growth is not continuous, but occurs in pulses, suggest the possibility of
anoxia events in the deep waters of the Alfacs Bay, particularly since a
saline stratification is present throughout the year. This possibility has
been carefully examined (Camp et al. 1992), both through field measurements and conceptual modelling. Field data report oxygen depletion only
in extreme and peculiar cases, usually less than once a year, after
phytoplankton blooms when winds do not blow and temperature is high.
In general, anoxia is prevented mainly by estuarine circulation, which
brings oxygen-saturated offshore waters, together with other processes
(diapycnal eddy diffusion, wind-driven vertical mixing).
Macroalgae in the Ebro delta have been studied by Martinez-Arroyo
(1990). Usually they are present in the shallow platforms, but with distinct local variations in cover. While in the southerr. and eastern shores
algal cover is low (0-5%), in the northern shore locally it can be as high
as 25-50%, with maximum values in the NE part (> 60%). Green algae
(i.e. Ulva sp.) dominate in the areas closer to the freshwater inputs, while
in other parts of the bay other species grow together with Ulva (Chondria tenuissima, Gracilaria verrucosa; see Ferrer (1993) for a detailed
study on Cystoseira barbata). In general, young thalli grow attached on
small hard substrata (pebbles, mollusc shells etc.), but soon become
free-living, mostly lying on the sediment or the seagrasses, and seldom
floating on the surface.
Biomass of macroalgae shows a very irregular annual pattern. Ulva sp.
seems to culminate in winter and late spring, reaching 250 g m -2, dry
weight (Fig. 12.3). These values are mostly found in the northern shore,
where for an area of about 2 km 2 , cover by this species is around 50%
during maximal development. However, it is difficult to distinguish the
biomass originating from local algal growth from the biomass advected
by winds and currents. Such accumulations usually disappear within
1 month, either dispersed by winds or due to decay (decay rates:
0.03-0.08 day-I).
J. Romero et al.
Plankton together with particulate matter and sediments from the channels induce high turbidity with an average extinction coefficient of
0.4 m- l in the centre of the bay (Perez 1989). In contrast, in the northern
Fangar Bay, with comparable freshwater input and morphological features, but with a different residence time of fresh water (only 2 - 3 days
during spring-summer), primary production is mainly achieved by macrophytes, which cover more than 80% of the bottom, including the central basin. This is because rapid water exchange prevents accumulation
of planktonic biomass in the bay, and thus the water is much clearer, so
that dense macrophyte beds can develop.
The conditions described above, together with the fact that planktonic
growth is not continuous, but occurs in pulses, suggest the possibility of
anoxia events in the deep waters of the Alfacs Bay, particularly since a
saline stratification is present throughout the year. This possibility has
been carefully examined (Camp et al. 1992), both through field measurements and conceptual modelling. Field data report oxygen depletion only
in extreme and peculiar cases, usually less than once a year, after
phytoplankton blooms when winds do not blow and temperature is high.
In general, anoxia is prevented mainly by estuarine circulation, which
brings oxygen-saturated offshore waters, together with other processes
(diapycnal eddy diffusion, wind-driven vertical mixing).
Macroalgae in the Ebro delta have been studied by Martinez-Arroyo
(1990). Usually they are present in the shallow platforms, but with distinct local variations in cover. While in the southerr. and eastern shores
algal cover is low (0-5%), in the northern shore locally it can be as high
as 25-50%, with maximum values in the NE part (> 60%). Green algae
(i.e. Ulva sp.) dominate in the areas closer to the freshwater inputs, while
in other parts of the bay other species grow together with Ulva (Chondria tenuissima, Gracilaria verrucosa; see Ferrer (1993) for a detailed
study on Cystoseira barbata). In general, young thalli grow attached on
small hard substrata (pebbles, mollusc shells etc.), but soon become
free-living, mostly lying on the sediment or the seagrasses, and seldom
floating on the surface.
Biomass of macroalgae shows a very irregular annual pattern. Ulva sp.
seems to culminate in winter and late spring, reaching 250 g m -2, dry
weight (Fig. 12.3). These values are mostly found in the northern shore,
where for an area of about 2 km 2 , cover by this species is around 50%
during maximal development. However, it is difficult to distinguish the
biomass originating from local algal growth from the biomass advected
by winds and currents. Such accumulations usually disappear within
1 month, either dispersed by winds or due to decay (decay rates:
0.03-0.08 day-I).