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The Guadiana estuary, located between Portugal and Spain, has the fourth largestdrainage basin of Iberian river systems. Up to 75% of the catchment area has been regulated by damssince the early 1970s. During the 1980s and 1990s, an increasing occurrence of summer cyanobacteriablooms dominated by the potentially toxic Microcystis spp. was reported. In an effort to understandthe causes of recurrent noxious blooms in the Guadiana estuary, nutrients (phosphorus, nitrogen[nitrate, nitrite and ammonium], and silicon [silicic acid], chlorophyll a and phytoplanktonabundance, specific composition and biomass were evaluated during field surveys, from April 1997to March 1998. A pattern of successive blooms of different phytoplankton assemblages was observedthroughout this period. Diatoms (nano-sized, chain-forming) dominated an early spring bloom (max.18 × 103 cells ml–1) in March and April. Following the decrease in diatom abundance, a chlorophytebloom (max. 11 × 103 cells ml–1) and then a cyanobacteria bloom (>6 × 104 cells ml–1) quickly followedduring late spring through to early summer. From July to September, a major cyanobacteria bloomdominated by the potentially toxic Microcystis spp. (> 4 × 105 cells ml–1) developed in the freshwaterzone. The results indicate that high winter loads of nitrogen and phosphorus led to the depletion ofsilicate (down to as low as 0.2 μM) during the early spring diatom bloom, which conditioned the successivephytoplankton assemblages during the remaining productive period. Low monthly dischargerates during spring and summer further provided an environment with low Si:N and N:P relativeavailability which, coupled with high water-column temperature (>21°C), seemed to favour thedominance of cyanobacteria over chlorophytes during the summer

Role of transient silicon limitation in the development of cyanobacteria blooms in the Guadiana estuary, south-western Iberia