Relationships between macroalgal biomass and microbiological quality of water in a phytotreatment pond Luigi Vezzulli 1, *, Marco Bartoli 2 , Daniele Nizzoli 2 , Giorgio Fanciulli 1 , Pierluigi Viaroli 2 & Mauro Fabiano 1 1 Dipartimento per lo Studio del Territorio e delle sue Risorse (DIP.TE.RIS), Universita ` di Genova, Corso Europa 26, 16132, Genova, Italy 2 Dipartimento di Scienze Ambientali, Universita ` di Parma, Parco Area delle Scienze 33A, 43100, Parma, Italy (*Author for correspondence: E-mail: vezzulli@dipteris.unige.it; Tel.: +39-010-353-8069; Fax: +39-010-353-8140) Key words: microbiological quality, bacteria, macroalgae, phytoremediation, aquaculture. Abstract The microbiological quality of wastewater in phytotreatment ponds with foliose macroalgae can be influenced by biofilm formation on thallus surface. This hypothesis was tested with an in situ experiment which was carried out in a pond with Ulva spp. receiving wastewater from a land based fish farm at Piombino (Italy). The total bacterial load (TBL) was determined in the inflowing and outflowing waters and a multifactorial design was employed to investigate the effect of different macroalgal biomass. Microbiological analysis revealed a high TBL in the water column (18.4 ± 7.4 · 10 8 cells ml )1 ). TBL of inlet water was significantly correlated with quantity and quality of particulate organic matter (POM) of inflowing water, whereas no correlation was found between TBL and POM in the outlet water. A signif- icant decrease in the POM concentration was detected within macroalgal ponds, due to the mechanical action of thalli which favoured POM sedimentation. Nevertheless, great TBLs were found in the outlet water. These findings suggest that TBL probably depended upon macroalgae. Indeed high bacterial density was found on macroalgal thallus surface (10 8 cells cm 2 ). Furthermore, high plate counts of faecal bacteria (faecal enterococci) were determined on thallus surface (40 CFU cm 2 ) and outlet water (11 886 ± 3984 CFU 100 ml )1 ) supporting the evidence that macroalgae negatively affect the microbiological quality of treated water. Bacterial activities in terms of exoenzymatic rates and secondary production were two folds higher in the water within macroalgal beds, than in the open water. These preliminary results suggest that high macroalgal biomass represents a ‘hot spotÕ of bacterial density and activity that may affect microbi- ological quality of the treated water. Bacterial control of inlet water and management of macroalgal biomass through periodic removal are essential for a more efficient treatment of wastewater in phytotreatment ponds. Introduction The use of phytotreament ponds for the treat- ment of aquaculture wastewater is a low cost and widely employed technique (Redding et al., 1997; Brix, 1999; Tanner, 2001; Vymazal et al., 2001; Lin et al., 2002; Schulz et al., 2003). Due to their high net growth rates, nutrient uptake capacity and resistance to environmental stress, seaweeds are the most employed primary pro- ducers in treatment ponds (Laliberte´ et al., 1994). Recent studies have shown that ponds management through periodic harvesting of macroalgal biomass could be an efficient way to remove significant amounts of nutrients and to increase wastewater quality (Bartoli et al., this Hydrobiologia (2005) 550:211–219 Ó Springer 2005 P. Viaroli, M. Mistri, M. Troussellier, S. Guerzoni & A.C. Cardoso (eds), Structure, Functions and Ecosystem Alterations in Southern European Coastal Lagoons DOI 10.1007/s10750-005-4380-2