BACTERIAL-COLONIZATION OF SESTON PARTICLES IN BRACKISH WATERS (RIA DE AVEIRO, PORTUGAL)

The frequency of attached bacteria, as percentage of total bacteria, in estuarine water of the Ria de Aveiro was determined over a seasonal range of temperature, seston, BOD, salinity and water depth. The frequency was inversely related to temperature only in the marine zone of the lagoon. No other associations could be established with environmental factors. The broad spring-summer peak of total bacteria was not apparent in attached bacteria which showed an erratic temporal profile with an average frequency of 9 % (range 1 to 49 %) of total planktonic bacteria. The bacterial coverage per unit area of particle surface was densest in small particles. The density of coverage decreased sharply to values corresponding to 22, 5 and 2 % in particles > 3 to 10, > 10 to 40 and > 40 to 140 mum in diameter respectively compared to the density of coverage of the > 1 to 3 mum size-class. Colonized seston and sediment particles exhibited similar bacterial numbers per particle of each size-class and did not show tidal, spatial or seasonal patterns of variation. It is suggested that bacteria only seldom attach to particles in the water column of this lagoon and that resuspension of bottom sediments is the main factor governing the frequency of colonized particles in surface water.