Competition and coexistence of phytoplankton …

1 AQUATIC MICROBIAL ECOLOGYA quat Microb EcolVol. 31: 241 248, 2003 Published April 3 INTRODUCTIONE nvironmental variability plays an important role inecological communities. Nonequilibrium environmen-tal conditions were set forth to explain the paradox ofthe plankton, where many species of phytoplanktoncoexist in a relatively homogeneous environment(Hutchinson 1961, Richerson et al. 1970). One of theimportant aspects of nonequilibrium environmentalconditions is fluctuation in resource levels. Theoreticalinvestigations show that variable resource supply canpromote coexistence and increase diversity (Levins1979, Armstrong & McGehee 1980, Hsu 1980), thussupporting Hutchinson s explanation of the paradox ofthe plankton (1961).

2 Experimental studies with phyto-plankton showed that fluctuations in major limitingnutrients can alter competitive interactions andincrease diversity (Turpin & Harrison 1979, Sommer1985, Suttle et al. 1987, Grover 1988, 1990). light is a major essential resource in aquatic systemsand is highly variable in both time and space. How-ever, surprisingly little attention has been paid to theeffects of fluctuating light supply on phytoplanktoncompetition. Brzezinski & Nelson (1988) consideredcompetition between 2 diatoms for ammonia underfluctuating light of non-limiting levels.

3 They found thata fluctuating light regime facilitated coexistence ofboth species, while constant light conditions led tocompetitive exclusion of one of the species. Similarly,van Gemerden (1974) demonstrated that 2 strains ofsulfur bacteria were able to coexist under variable lightconditions while competing for a substrate. However,no studies have looked at the effects of fluctuatinglight at limiting levels on competitive interactionsamong , I examine how fluctuating light at limitinglevels affects competitive interactions in freshwaterphytoplankton. light -limited conditions are not un-common in natural waters and may result from either adeep mixed layer, high background turbidity ( dis-solved organic carbon [DOC] or suspended particles)and/or high biomass of phytoplankton .

4 In eutrophic Inter-Research 2003 *Email: and coexistence of phytoplankton underfluctuating light : experiments with two cyanobacteriaElena Litchman1, 2,*1 University of Minnesota, 1987 Upper Buford Circle, St. Paul, Minnesota 55108, USA2 Present address:School of Biology, 310 Ferst Drive, Georgia Institute of Technology, Atlanta, Georgia 30332, USAABSTRACT: The effects of light fluctuations on phytoplankton Competition were examined in theexperiments with 2 freshwater cyanobacteria, Anabaena flos-aquaeand Phormidium luridum. Lightregimes had the same average irradiance of 50 mol photons m 2s 1and included constant light , high-low light fluctuations of 8 and 24 h periods and light :dark fluctuations of a 24 h period.

5 A mechanisticmodel of light Competition was used to predict competitive outcomes under these light regimes. Theparameter values were obtained experimentally for constant light conditions. In the experiments,A. flos-aquaewas rapidly excluded under constant light but persisted under fluctuating light . Themodel predicted well the dynamics and the outcome of Competition under constant light butperformed poorly under fluctuating light . The results indicate that light fluctuations may change thedynamics and outcome of Competition and slow competitive exclusion and also that Competition underfluctuating light cannot necessarily be predicted from the constant light monoculture WORDS: Cyanobacteria Competition Irradiance FluctuationsResale or republication not permitted without written consent of the publisherAquat Microb Ecol 31: 241 248, 2003lakes, dense cyanobacterial blooms often create light -limited conditions where algae compete for light bymutual shading (Klemer 1985, Reynolds 1987).

6 Com-petition for light among phytoplankton under constantirradiance has been investigated in the experiments ofMur et al. (1977) and Huisman et al. (1999). Theyshowed that a species capable of reducing incidentlight to the lowest level (Iout, the incident light level atthe bottom of the water column; sensu Huisman &Weissing 1994) wins Competition . At the same time, innatural waters, even in severely light -limited environ-ments, more than 1 species can be found simultane-ously, cyanobacterial blooms can consist of 2 or 3species ( Anabaenasp., Aphanizomenonsp. andMicrocystissp.)

7 Even if a single species dominates(Brock 1985). Could it be possible that fluctuations inlight promote coexistence of several species underlight-limited conditions as was demonstrated theoreti-cally for limiting resources in general ( Levins 1979,Armstrong & McGehee 1980) and confirmed experi-mentally for nutrient-limited phytoplankton (Turpin &Harrison 1979, Sommer 1984)? I investigate this hypo-thesis in experiments with 2 cyanobacteria. The 2 spe-cies frequently occurring in eutrophic lakes weregrown together in constant and fluctuating light underlight-limited conditions and their dynamics were moni-tored.

8 Following Huisman & Weissing (1994), competi-tive abilities of each species were determined bygrowing them in monoculture and estimating Ioutatequilibrium. A model of light Competition was used topredict competitive outcomes under each light AND METHODSE xperimental setup. To investigate the effect of tem-poral variation in light supply on Competition for lightbetween phytoplankton , pairwise Competition experi-ments were performed. Two species of freshwatercyanobacteria, Anabaena flos-aquae(Lyng.) Br bisson(American Type Culture Collection Clone 22 664)(hereafter Anabaena) and Phormidium (University of Texas Culture Collec-tion Clone 426) (hereafter Phormidium), were grownunder 4 different light regimes.

9 Anabaenaoften formsextensive blooms and the genus Phormidiumis similarto the genus Oscillatoriain its ecological niche and canbe either planktonic or benthic (Whitford & Schu-macher 1984). The average incoming irradiance wasthe same in all treatments, 50 mol photons m 2s 1;however, temporal patterns of light supply differed, were the constant light treatment, square-wavefluctuations between 15 and 85 mol photons m 2s 1with 8 or 24 h periods, and light :dark fluctuationsbetween 0 and 100 mol photons m 2s 1 with a 24 hperiod. In all 3 fluctuating regimes, periods of low andhigh irradiance were of equal duration.

10 Algae weregrown in 1 l Erlenmeyer flasks (380 ml culture volume)at 20 C and gently shaken several times a day. Eachtreatment had 3 replicates. Treatments were assignedrandomly to environmental chambers. Flask positionswere determined so that the irradiance levels at thesurface of the flasks were within 1 to 2 mol photonsm 2s 1of the required levels as measured with a quan-tum scalar sensor (Biospherical Instruments QSL-100). light was provided by cool white fluorescent tubes(Philips); flasks were illuminated from all sides exceptthe bottom. Fluctuations in irradiance were imposedby periodically turning on and off additional strength WC freshwater medium (Guillard 1975)with 2 standard concentration of NaHCO3was usedto achieve high filament densities and light -limitedconditions for both species as well as Competition forlight by mutual shading.