Influence of diel cycles of respiration, chlorophyll, and photosynthetic parameters on the summer metabolic balance of temperate lakes and rivers Marie-He ´ le ` ne Forget, Richard Carignan, and Christiane Hudon Abstract: Primary production, planktonic respiration, bacterial abundance, and chlorophyll a were measured in the epi- limnion of two Canadian Shield lakes and in two large rivers to establish their metabolic balance and to contrast oligotro- phic and oligo-mesotrophic systems. Pronounced diel respiration cycles were observed in all systems, with a minimum in the morning and a maximum in the evening. Respiration was positively correlated with water temperature, incident light, and chlorophyll a concentration. Diel cycles of chlorophyll a were discernible in both rivers and in the oligo-mesotrophic lake. Our results show that a single morning sampling underestimates both respiration and chlorophyll a, whereas a single sample taken at noon generally agrees better with the average of four daily measurements. In oligotrophic Lac Croche, the photosynthetic parameter a B remained constant throughout the day, whereas P m B increased by 12.5% between 0600 h and 1300 h. Ignoring the diel community respiration cycle resulted in a ~25% underestimation of daily respiration, whereas ignoring the P m B cycle induced a ~9% underestimation of the daily primary production. The net balance between production (P) and respiration (R) of the oligotrophic lake epilimnion remained positive during two summers, and the P:R ratio showed net autotrophy for all systems except for the oligotrophic river, which was slightly net heterotrophic. Re ´sume ´: La production primaire, la respiration planctonique, l’abondance bacte ´rienne et la chlorophylle a ont e ´te ´ mesur- e ´es dans l’e ´pilimnion de deux lacs du Bouclier canadien et dans deux grandes rivie `res afin d’e ´tablir leur bilan me ´tabolique et de comparer des milieux oligotrophes et oligo-me ´sotrophes. Des cycles journaliers de respiration ont e ´te ´ observe ´s dans les quatre milieux aquatiques avec un minimum en matine ´e et un maximum en soire ´e. La respiration e ´tait corre ´le ´e posi- tivement avec la tempe ´rature de l’eau, la lumie `re incidente et la concentration en chlorophylle a. Des cycles journaliers de la chlorophylle a ont e ´te ´ mis en lumie `re dans les deux syste `mes fluviaux et dans le lac oligo-me ´sotrophe. Le pre ´le `vement d’un seul e ´chantillon en matine ´e sous-estime la respiration et la chlorophylle a, tandis qu’un seul pre ´le `vement effectue ´a ` midi correspond ge ´ne ´ralement mieux a ` la moyenne de quatre mesures journalie `res. Dans le lac Croche qui est oligotrophe, le parame `tre photosynthe ´tique a B ne pre ´sentait pas de variation journalie `re, alors que P m B augmentait de 12,5 % entre 0600 h et 1300 h. Ignorer la variation journalie `re de la respiration engendre une sous-estimation de la respiration journal- ie `re de ~25 %, tandis que l’omission du cycle de P m B produit une sous-estimation ~9 % de la production primaire journal- ie `re. Le bilan net entre la production (P) et la respiration (R) de l’e ´pilimnion du lac oligotrophe est demeure ´ positif au cours de deux e ´te ´s de mesure et le rapport P:R indique que tous les syste `mes e ´taient autotrophes nets a ` l’exception de la rivie `re oligotrophe, faiblement he ´te ´rotrophe. Introduction The importance of allochthonous organic carbon as a source of energy in oligotrophic systems has been a subject of debate for many years. Some studies suggest that plank- tonic respiration commonly exceeds primary production in the epilimnion of oligotrophic lakes and in the photic zone of rivers (del Giorgio and Peters 1994; Coveney and Wetzel 1995; del Giorgio et al. 1997a), whereas others have con- cluded that the epilimnion of oligotrophic lakes and oceans are net autotrophic (Williams 1998; Carignan et al. 2000; Berman et al. 2004). Russ and collaborators (2004) found that an oligotrophic lake alternated between net autotrophy and heterotrophy according to the stratification conditions of the water column (Russ et al. 2004). Most studies have measured daily production and respira- tion from samples collected at a single set time without considering the possible influence of diel cycles in plank- tonic respiration and photosynthetic parameters on the esti- mation of metabolic balances. Diel respiration cycles are well documented in systems ranging from mesotrophic to eutrophic (Markager et al. 1992; Szyper et al. 1992; Teichert-Coddington and Green 1993), but have never been reported for oligotrophic environments because of the lack of precision of oxygen methods. Diel cycles of bacte- rial abundance (Fuhrman et al. 1985) and metabolism have, however, been reported on a few occasions (Straskrabova and Fuksa 1982; Chin-Leo and Benner 1991; Simon 1994). Marine phytoplankton biomass, estimated as chlorophyll a, has also been shown to follow diel cycles, with a maxi- Received 7 August 2008. Accepted 11 February 2009. Published on the NRC Research Press Web site at cjfas.nrc.ca on 29 June 2009. J20712 M.-H. Forget 1,2 and R. Carignan. De ´partement de Sciences Biologiques, Universite ´ de Montre ´al, C.P. 6128, succ. Centre- Ville, Montre ´al, QC H3C 3J7, Canada. C. Hudon. St. Lawrence Centre, Environment Canada, 105 McGill Street, 7th floor, Montre ´al, QC H2Y 2E7, Canada. 1 Corresponding author (e-mail: mforget@dal.ca). 2 Present address: Dalhousie University, Department of Biology, 1355 Oxford Street, Halifax, NS B3H 4J1, Canada. 1048 Can. J. Fish. Aquat. Sci. 66: 1048–1058 (2009) doi:10.1139/F09-058 Published by NRC Research Press Can. J. Fish. Aquat. Sci. Downloaded from www.nrcresearchpress.com by Environment Canada on 08/15/11 For personal use only.