Pergamon PII: S0025-326X(97)00115-X Marine Pollution Bulletin, Vol 34, No. 12, pp. 1001 1005, 1997 © 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0025 326X/97 $17.00+0.00 Impacts of Domestic Sewage Effluent on Phytoplankton from Bedford Basin, Eastern Canada YOULIAN PAN* and D. V. SUBBA RAOt Habitat Ecology, Marine Environmental Sciences Division, Department of Fisheries and Oceans, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia, B2Y 4A2, Canada The impact of domestic sewage effluent (SE) on the dynamics of phytoplankton assemblages from Bedford Basin was evaluated in the laboratory. Phytoplankton production and chlorophyll a increased proportionally with SE enrichment. Phytopinnkton species composition also changed. The potentially harmful diatoms, Pseudo- nitzschia spp., present initially in small numbers (600 cells 1-1) in Bedford Basin seawater, and became dominant (3- 5x106 cells 1-1) when the seawater was enriched with 0.5-5% untreated SE. With higher proportions of SE, other harmful species such as Fragilaria spp. and Euglena spp. became dominant (7-15 x 106 and 2.2x 104 cells 1-1, respectively). Treatment of SE with UV light or activated charcoal seems to favour growth of benign species, such as Chaetoceros socialis, Skeletonema costatum and Thalas- siosira spp., but not harmful species such as Pseudo- nitzschia spp. Further research on UV treatment of domestic sewage is recommended. © 1997 Elsevier Science Ltd. All rights reserved Keywords: sewage; phytoplankton; UV light; charcoal; Pseudo-nitzschia; species succession. There is evidence of a worldwide increase in the occurrence of harmful algal blooms (Anderson, 1994) and some have suggested that one factor in the increase is eutrophication of coastal waters caused, in part, by sewage discharge (Smayda, 1990). Bedford Basin is a temperate embayment on the Canadian Atlantic coast It receives 5.6 million gallons per day of primary municipal sewage from Bedford and Sackville via the Mill Cove Pollution Control Plant located at the head of Bedford Basin (Fig. 1). Despite its high exchange rate with Atlantic water (Platt and Conover, 1971), it remains a highly eutrophicated embayment that can be regarded as a continuous culture system for marine *Author for correspondence at: Institute for Marine Biosciences, National ResearchCouncilof Canada, 1411OxfordSt, Halifax, Nova Scotia, B3H 3Z1 Canada• E-mail: YOULIAN.PAN@NRC.CA tCurrent address:Mariculture and FisheriesDept., KISR,P.O. Box 1638, 22017 Salmiya,Kuwait. organisms (Platt, 1975). Recently, monospecific blooms of dinoflagellates have been reported in summer, for example Gonyaulax digitale (Amadi et al., 1992) and Dinophysis norvegica (Subba Rao et al., 1993). The potential for winter blooms is unknown• In this study, Bedford basin seawater taken in February and March was incubated in the laboratory with various concentra- tion of sewage effluent (SE) to study the effect of SE on phytoplankton biomass, production and species com- position. Materials and Methods Sewage effluent Secondary sewage effluent (SE) was collected from the outflow of Mill Cove Pollution Control Plant (Fig. 1) at 1 day before each experiment and filtered through I ...... :,.\ • .~:) "'.,: N ".'" I I Sackville river •.... . i'.. )... • ....... 0 1 I ~ I Kilometres 44' t • . . ... • ~ . 44° % ~ 42' ~ - B edford Insti.t.ut.e.._ I i " I • . • . . ". . . . "': "i- " 40' • ...:.:• ..'." I I I I 40' 63o38 ' 36 Fig. 1 Map of Bedford Basin showinglocation of sampling station ( I ) and Mill Cove PollutionControl Plant 1001