Introduction Microalgae are important for the functioning of faculta- tive and maturation wastewater stabilization ponds. Their ability to produce oxygen by photosynthesis is vital to the ecology of the water environment. In a sym- biotic relationship between algae and bacteria, algae are needed to supply oxygen to aerobic, heterotrophic bac- teria, which in turn mineralize the organic wastes into nutrients used by the algae (Tchobanoglous and Burton 1991). Though the oxygenation function of microalgae is often the only one emphasized, Mara and Pearson (1986) and others point out that many of the microalgae present in ponds can also act like bacteria, growing het- erotrophically by mineralizing organic energy sources. Pearson et al. (1987) showed that for a wastewater stabi- lization pond strain of Chlamydomonas, the dark utiliza- tion (without photosynthesis) of acetate was about 30 % of the utilization by bacteria. In the presence of light this utilization increased to 70 % compared to bacteria. The corresponding figures for the population of Euglena were less than 1 % in darkness and only 2–3 % in light. 239 VATTEN · 4 · 04 SEASONAL MICROALGAE VARIATION IN A SUBARCTIC WASTEWATER STABILIZATION POND USING CHEMICAL PRECIPITATION Säsongsvariation av mikroalger i en norrländsk fällningsdamm by ERIK GRÖNLUND 1 , ERICA JOHANSSON 2 , JÖRGEN HANÆUS 2 & STEFAN FALK 3 1 Div. of Ecotechnology, Mid Sweden University, SE-831 25 Östersund e-mail: erik.gronlund@mh.se 2 Luleå University of Technology, SE-971 87 Luleå 3 Dept. of Natural and Environmental Sciences, Mid Sweden University, SE-851 70 Sundsvall Abstract Surface water microalgae samples were collected during the ice-free period in a small subarctic wastewater sta- bilization pond system, complemented with chemical precipitation during the winter period (fellingsdam), and serving 310 persons. In the primary pond microalgae dominance alternated between the Cryptophyte Cryptomonas and green algae (Chlorophyta). In the second and third pond the general pattern was that Cryptomonas dominated during summer but was replaced by green algae in autumn and the following spring. Estimations of the microalgae part of the effluent COD and phosphorus showed that microalgae dominated these parameters for only 3–4 weeks of 12 evaluated. This does not support the reasoning behind the European Union directive of the use of filtered samples for effluent BOD, COD and SS from stabilization ponds, in con- trast to other wastewater treatment methods. The reasons behind the EU’s procedure for ponds are based on the assumption that stabilization ponds convert “sewage BOD” to “algal BOD”. The results of this study sug- gest that further investigations of the microalgae function in subarctic wastewater stabilization ponds and fellingsdams should be conducted, before implementing the EC directive into Swedish law, or into similar laws in other countries with subarctic regions. Key words – phytoplankton, sewage, oxidation pond, lagoon, fellingsdam, cold climate, EC directive. Sammanfattning I en jämtländsk fällningsdamm, med belastningen 310 p.e., samlades mikroalg-prover in från 10–20 cm djup under den isfria perioden. Biomassan och förekommande algtyper undersöktes därefter. I primärdammen väx- lade dominansen mellan släktet Cryptomonas (Cryptophyta) och olika grönalger (Chlorophyta). I den andra och tredje dammen var det generella mönstret att Cryptomonas dominerade under sommaren men ersattes av grönalger under vår och höst. Uppskattningar av algernas andel av utgående COD och fosfor visade att algerna dominerade dessa parametrar bara under 3–4 veckor av 12 undersökta. Resultaten är inte i överensstämmelse med de antaganden som finns bakom EG-direktivet (91/271/EEC) om att använda filtrerade prover i biodam- mar för utgående BOD, COD och SS. Detta till skillnad från andra reningsmetoder där ofiltrerade prover an- vänds. Orsaken till denna särbehandling baseras på antagandet att biodammar omvandlar »avlopps-BOD» till »alg-BOD». Denna undersökning visar att mikroalgers roll i subarktiska bio- och fällningsdammar bör under- sökas mer, innan det aktuella EG-direktivet implementeras i svensk lag, eller motsvarande lagar i andra länder med subarktiska regioner. VATTEN 60: 239–249. Lund 2004