1. What is limiting marine heterotrophic microorganisms ? Johnny Berglund Marine ecology Department of Ecology and Environmental Science Umeå University

2. Aquatic food web by Kristina Wiklund, UMF

3. Some marine protists... Rhynchomonas nasuta Parvicorbicula sp.Plagioselmis prolonga Bodo saliens Copyright ©, Mats Kuylenstierna & Bengt Karlson Mesodinium rubrum 5m5m 5m5m Heterotrophic nanoflagellates (HNF) Ciliates

4. Study area: Gulf of Bothnia

5. Gulf of Bothnia Low salinity from 3 psu to 7 psu Mean depth of 40 m in B.Bay and 66 m in B.Sea Strong seasonality, ice-cover of 120 and 60 days High river runoff

6. Seasonal pattern in bacterial and HNF abundance week

7. Question: What is limiting growth of heterotrophic nanoflagellates (HNF)? - especially small flagellates <5  m Why? Because small flagellates (1-3  m) are most important predators on heterotrophic bacteria (Wikner & Hagström 1988 and others) Hence, small flagellates are an important link to higher trophical levels Definition (Osenberg & Mittelbach 1995): Limitation is the extent to which a population’s, or trophic level’s, per capita growth rate is depressed by the action of a particular factor, for example resource availability or predator risk.

8. Factor 1: ~1.5*10 6 ml -1 ~2.5*10 6 ml -1 Natural threshold for HNF growth Bacteria as resource

9. Samuelsson, K. & Andersson, A. 2002 Predation limitation in the pelagic microbial food web, AME in press 0.8 – 5  m flagellates most important predator on bacteria Heterotrophic bacteria HNF PROBLEM...to distinguish effect of predation and effect of reduced competition for bacteria (resource) Medium HNF f.w. <90<10<5 <0.8 f.w. <90<10<5 <0.8 Factor 2: thightly size structured food-chain Predation by large flagellates and ciliates

10. Hence, there are two factors limiting the heterotrophic nanoflagellates 1. resource 2. predation Lets setup an experiment to evaluate the importance of the factors

11. Limitation can be measured by quantifying the increase in a population’s per capita growth rate following the removal of a particular form of limitation, for example after the addition of a surplus resource or removal of predators. (Osenberg & Mittelbach 1995)  2 after removal of limitation  1 in natural conditions Time t1t1 t0t0 ln N 2 lnN 1 LnN 0  1 = (lnN 1 –lnN 0 ) / (t 1 -t 0 ) Limitation L =  2 -  1

12. + bacteria no addition < 90  m < 10  m < 5  m Experimental setup to determine limiting factors for HNF

13. Dialysis bags incubated in situ at 4 m depth Duration: 4 days with sampling every day Performed in June 2001

14. Results Highly significant (p<0.001) effect of adding bacteria (resource) Weak effect of predation (p<0.05) by large protozoans (10-90  m) (No effect of predation by large flagellates (5-10  m))

15. Results... Relativ importance (change in pop. Growth) resource 62 % and predation 38 % To evaluate possible seasonal variations a second experiment was performed in September 2001 -same experimental setup

16. Experiment 2 Results... Again significant (p<0.005) effect of adding bacteria Relativ importance of resource 81 % and predation 19 %, though effect of predation not significant

17. Conclusions HNF growth mostly resource limited microbial food web bottom-up regulated Modelling still to come....

18. Thank you for your attention! Acknoledgements: Agneta Andersson Kristina Samuelsson Thomas Kull Umeå Marine Science Center