276
10.4 Trends, Some Historical References
and Comparison of Data
F.X. Niell et al.
The long-term development of algal production during the past decades
can be shown by a few comparative studies. Our impression is that in
recent years changes in biomass of Ulvaceae are not significant in nonpolluted areas. Perhaps their overall production increased, however, the
standing crop remained constant, at least on the rocky shores of Asturias, Galicia and southern Spain. Species richness is decreasing in
intertidal communities, but the species that are disappearing are the
small ones, so the diversity, computed from biomass values, and the
standing crop seem not to change.
A second point of reference is the polluted area in the Ria of Pontevedra, where the first quantitative survey of Ulvaceae in relation to
pollution was made in 1974 (Niell and Buela 1976; Niell and Pazo 1978).
Ten years later, the biomass has clearly decreased (Fig. 10.3), and maximal values are half those of the past. However, the biomass of the whole
community of intertidal Fucaceans has increased (Fig. lOA). It is difficult
to explain these phenomena, but present trends of recuperation of biomass in the area are evident.
Seawards in this study area, there is a zone of continous sediment
perturbation due to active fishing of bivalves using spades and similar
tools. Although sediment turbation by women's shell-collecting activities
releases large amounts of nutrients from the sediment to the water, no
floating green algae have been observed.
In the Palmones river, the proliferation of Ulvaceae is recurrent, and
fast increases of biomass have been observed year after year (GarciaSanchez and Hernandez, unpubl.). Southern Spain is a semi-arid area,
where rains are scarce, but very heavy (the last heavy rains took place in
1984 and 1989, and in the autumn of 1993). The resulting floods remove
all the biomass of macrophytes from the estuary, favouring the
dominance of Ulva, and causing the disappearance of Zostera noltii, the
dominant seagrass in the estuary. Turbidity of the water in Palmones is
high with an attenuation coefficient of 1.5 m - 1, on clear summer days
(2500 IlE m -2 s -1). The irradiance at 2 m water depth for example would
be only 15 IlE m -2 s - 1, which is close to the light compensation point of
photosynthesis in Z. noltii (Jimenez et al. 1987; Perez-Llorens and Niell
1993). Measurement of attenuation by the laminar thalli of Ulvaceae give
an average attenuation coefficient of 1.6 per blade (Niell, unpubl.), which
means that three to four thalli of Ulva would decrease light to values
10.4 Trends, Some Historical References
and Comparison of Data
F.X. Niell et al.
The long-term development of algal production during the past decades
can be shown by a few comparative studies. Our impression is that in
recent years changes in biomass of Ulvaceae are not significant in nonpolluted areas. Perhaps their overall production increased, however, the
standing crop remained constant, at least on the rocky shores of Asturias, Galicia and southern Spain. Species richness is decreasing in
intertidal communities, but the species that are disappearing are the
small ones, so the diversity, computed from biomass values, and the
standing crop seem not to change.
A second point of reference is the polluted area in the Ria of Pontevedra, where the first quantitative survey of Ulvaceae in relation to
pollution was made in 1974 (Niell and Buela 1976; Niell and Pazo 1978).
Ten years later, the biomass has clearly decreased (Fig. 10.3), and maximal values are half those of the past. However, the biomass of the whole
community of intertidal Fucaceans has increased (Fig. lOA). It is difficult
to explain these phenomena, but present trends of recuperation of biomass in the area are evident.
Seawards in this study area, there is a zone of continous sediment
perturbation due to active fishing of bivalves using spades and similar
tools. Although sediment turbation by women's shell-collecting activities
releases large amounts of nutrients from the sediment to the water, no
floating green algae have been observed.
In the Palmones river, the proliferation of Ulvaceae is recurrent, and
fast increases of biomass have been observed year after year (GarciaSanchez and Hernandez, unpubl.). Southern Spain is a semi-arid area,
where rains are scarce, but very heavy (the last heavy rains took place in
1984 and 1989, and in the autumn of 1993). The resulting floods remove
all the biomass of macrophytes from the estuary, favouring the
dominance of Ulva, and causing the disappearance of Zostera noltii, the
dominant seagrass in the estuary. Turbidity of the water in Palmones is
high with an attenuation coefficient of 1.5 m - 1, on clear summer days
(2500 IlE m -2 s -1). The irradiance at 2 m water depth for example would
be only 15 IlE m -2 s - 1, which is close to the light compensation point of
photosynthesis in Z. noltii (Jimenez et al. 1987; Perez-Llorens and Niell
1993). Measurement of attenuation by the laminar thalli of Ulvaceae give
an average attenuation coefficient of 1.6 per blade (Niell, unpubl.), which
means that three to four thalli of Ulva would decrease light to values