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action, so that the persistence of green algal blooms can be expected only
in the estuaries. Northwestern estuaries have their origin in submersed
fluvial valleys which are quite deep, with an acute profile, very sharp
margins and steep slopes.
There is evidence that at present in the Rias the phytoplanktonic
blooms, basically formed by diatoms, are more frequent, compared to 30
years ago. They are probably the consequence of fertilization processes
caused by the input of the Central North Atlantic water, or may be due to
mixing processes or terrigenous and fluvial inputs of nutrients, which are
utilized faster by diatoms than by green algae. Diatoms sink fast,
sequestering nutrients at the bottom in the form of organic matter,
followed by mineralization. On the other hand, tidal water remains in the
Ria for a very short time (renewal time 8 to 10 days). When nutrients
have been exhausted by the diatoms and the levels of released dissolved
organic matter increase, dinoflagellates become dominant and can form
red tides, if hydrodynamic conditions are stable enough (Jimenez et al.
1992). If hydrodynamic conditions are unstable, big centric diatoms
prevail in the nutrient-poor, well-mixed waters. The more frequent
occurrence of red tides, together with diatom blooms, points to an
anthropogenic origin of nutrients, a result of increasing coastal
residential development, local industrial pollution, frequent recurrent
fires and increased erosion.Under these conditions, Ulvaceae are not able
to increase their biomass and are restricted to shallow areas and rocky
shores.
Our impression is that, at present, green tides are not a problem along
the Spanish Atlantic coasts, except in some restricted areas, and that the
increasing levels of eutrophication in recent years induce more frequent
diatom blooms and red tides. The accumulation of green algae is, at
present not as important as on the North Atlantic European coasts.
Acknowledgments. We are indebted to Dr. A. Borja (lEO, San Sebastian),
Dr. A. Juanes (University of Cantabria), Prof. R. Anad6n (University of
Oviedo}, Prof. A. Carballeira (University of Santiago) and Dr. A. Arias
(CSIC, Cadiz), for their comments on the presence of proliferation of
Ulvaceae in different areas of the Spanish Atlantic coast. This work has
been supported by project AMB93-1211 of the CICYT and the ENCE
(Pontevedra).
F.X. Niell et al.
action, so that the persistence of green algal blooms can be expected only
in the estuaries. Northwestern estuaries have their origin in submersed
fluvial valleys which are quite deep, with an acute profile, very sharp
margins and steep slopes.
There is evidence that at present in the Rias the phytoplanktonic
blooms, basically formed by diatoms, are more frequent, compared to 30
years ago. They are probably the consequence of fertilization processes
caused by the input of the Central North Atlantic water, or may be due to
mixing processes or terrigenous and fluvial inputs of nutrients, which are
utilized faster by diatoms than by green algae. Diatoms sink fast,
sequestering nutrients at the bottom in the form of organic matter,
followed by mineralization. On the other hand, tidal water remains in the
Ria for a very short time (renewal time 8 to 10 days). When nutrients
have been exhausted by the diatoms and the levels of released dissolved
organic matter increase, dinoflagellates become dominant and can form
red tides, if hydrodynamic conditions are stable enough (Jimenez et al.
1992). If hydrodynamic conditions are unstable, big centric diatoms
prevail in the nutrient-poor, well-mixed waters. The more frequent
occurrence of red tides, together with diatom blooms, points to an
anthropogenic origin of nutrients, a result of increasing coastal
residential development, local industrial pollution, frequent recurrent
fires and increased erosion.Under these conditions, Ulvaceae are not able
to increase their biomass and are restricted to shallow areas and rocky
shores.
Our impression is that, at present, green tides are not a problem along
the Spanish Atlantic coasts, except in some restricted areas, and that the
increasing levels of eutrophication in recent years induce more frequent
diatom blooms and red tides. The accumulation of green algae is, at
present not as important as on the North Atlantic European coasts.
Acknowledgments. We are indebted to Dr. A. Borja (lEO, San Sebastian),
Dr. A. Juanes (University of Cantabria), Prof. R. Anad6n (University of
Oviedo}, Prof. A. Carballeira (University of Santiago) and Dr. A. Arias
(CSIC, Cadiz), for their comments on the presence of proliferation of
Ulvaceae in different areas of the Spanish Atlantic coast. This work has
been supported by project AMB93-1211 of the CICYT and the ENCE
(Pontevedra).