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Bio-optical estimates of primary productivity in Scandinavian lakes Jan – Erik Thrane Master Thesis Department of Biology Program for Marine Biology and Limnology UNIVERSITETET I OSLO 01.06.2012 1 © Jan-Erik Thrane 2012 Bio-optical estimates of primary productivity in Scandinavian lakes. Jan-Erik Thrane http://www.duo.uio.no/ Trykk: Reprosentralen, Universitetet i Oslo 2 Index Abstract ...................................................................................................................................... 5 Preface ........................................................................................................................................ 6 Introduction ................................................................................................................................ 7 Possible fates of solar irradiance impinging on the surface of a lake .................................... 7 Estimation of primary productivity ........................................................................................ 9 Materials and methods ............................................................................................................. 12 Sampling ............................................................................................................................... 12 Background ...................................................................................................................... 12 Selection of lakes for sampling ........................................................................................ 12 Sampling period ............................................................................................................... 12 Field sampling and in situ measurements ............................................................................ 13 Light measurements ......................................................................................................... 13 Water samples .................................................................................................................. 14 Vertical CTD profiles ....................................................................................................... 15 Calculation of mixing depth (zmix) ................................................................................... 16 Measurement of in vivo absorbance spectra ......................................................................... 16 Sample preparation and storage ....................................................................................... 17 Depigmentation of filters ................................................................................................. 18 Correction for scattering and calculation of algal in vivo absorbance coefficients .......... 19 Absorbance spectra of dissolved organic matter (DOM) ..................................................... 20 Sample preparation and storage ....................................................................................... 20 Measurements and calculation of absorbance coefficients for DOM .............................. 20 Absorbance spectra of water ................................................................................................ 21 Measurements of chlorophyll a in seston ............................................................................. 21 Chlorophyll a standard solutions ...................................................................................... 21 Sample preparations and measurement ............................................................................ 22 3 Other water chemical analyses ............................................................................................. 23 Calculation of in situ attenuation coefficients ...................................................................... 23 Calculation of the relative amount of incoming PAR absorbed by algae, dissolved organic matter, detritus and the water itself ...................................................................................... 24 Bio-optical estimation of areal gross primary productivity in lakes .................................... 25 Incoming solar irradiance ................................................................................................. 26 Light absorbed by the algal community .......................................................................... 28 Quantum yields of the Photosystem II ............................................................................. 29 AquaPen procedure .......................................................................................................... 30 Modelling of QY .............................................................................................................. 31 Light utilized for photochemistry ..................................................................................... 32 Results ...................................................................................................................................... 35 Spectral decomposition of total light absorbance ................................................................ 38 PAR absorbed by algal pigments in relation to other absorbers ...................................... 40 Primary productivity ............................................................................................................ 43 Estimates of primary productivity .................................................................................... 44 Discussion ................................................................................................................................ 48 The importance of algae and dissolved organic matter (DOM) as light absorbers and consequences for lake productivity ...................................................................................... 48 Factors influencing the estimates of PP ............................................................................... 49 Are the PP estimates plausible? ........................................................................................... 51 Other limitations of the procedure ....................................................................................... 52 Scattering .......................................................................................................................... 52 Distribution of algae with depth ....................................................................................... 53 The use of fluorescence measurements to estimate photosynthetic rates ........................ 54 Conclusion ................................................................................................................................ 56 Literature cited ......................................................................................................................... 57 Appendix .................................................................................................................................. 63 4 Abstract Introduction: The absorption of photons in the water column of a lake can generally be attributed to four main components: algae, dissolved organic matter (DOM), non-living particulate organic matter (detritus), and the water itself. Competing with the other components for photosynthetic active radiation (PAR), algae use the absorbed photons for photosynthesis. In the first part of this thesis, the relative absorbance of PAR by the different components will be addressed. In the second part, a procedure for estimating areal primary productivity (PP) from optical measurements will be attempted. Materials and methods: 75 lakes in southern Norway and Sweden were sampled by airplane during a 4 week period (summer 2011). Absorbance coefficients of algae, DOM and detritus across the PAR spectrum were measured in the lab and combined with a spectrum of incoming solar irradiance (I0) to calculate the relative absorbance of photons constituted by the different components. Pulse Amplitude Modulated (PAM) fluorescence measurements of quantum yield of Photosystem II were used to estimate photosynthetic efficiency and combined with daily average values of PAR from July 2011, vertical profiles of scalar PAR, and the absorbance properties of the water column, to obtain maximum estimates of areal primary productivity in the lakes surveyed. Results: DOM was the dominant absorber (mean: 44 %, range: 16 % – 64 %), followed by algae (mean: 16 %, range: 6% – 31 %), and detritus (mean: 17%, range: 5 % – 39 %). Absorbance by algae was negatively related to absorbance by DOM (P < 0.0001, R2 = 0.4). Estimates of areal PP ranged from 136 to 1597 mg C m-2 day-1, with a mean of 600 mg C m-2 day-1. The estimates were positively related to total phosphorus content, and negatively affected by total organic carbon. Estimating areal PP down to different depths indicated that the epilimnion was totally-absorbing in most lakes. Discussion: The competition for light between algae and DOM seems to be a limiting factor for areal primary productivity in the surveyed lakes. The method for estimating areal PP from optical measurements is somewhat biased, but proposes a time-efficient procedure for obtaining maximum estimates. 5 Preface First, I want to thank all the heroic “field limnologist” from the COMSAT-team; Tom Andersen, Dag O. Hessen, Johnny Håll, Serena Rasconi, Marcia Kyle, Marcus Lindholm, and Robert Ptacnik for some great weeks last summer, when sampling data for the project. Other people that