Reconstruction of long-term changes of the underwater light field in large shallow lakes Peipsi and Võrtsjärv, North-East Europe

• Published in: Proceedings of the Estonian Academy of Sciences Vol. 62; no. 3; pp. 202 - 212
• Main Authors: Toming, K, Nõges, P, Arst, H, Kõiv, T, Nõges, T
• Format: Journal Article
• Language: English
• Published: Tallinn Teaduste Akadeemia Kirjastus (Estonian Academy Publishers) 01-01-2013
• Subjects: Freshwater
• ISSN: 1736-6046; 1736-7530
• DOI: 10.3176/proc.2013.3.06

The main objective of our study was to reconstruct the multi-decadal changes of the underwater light field in two large, shallow and polymictic Estonian lakes Vortsjarv and Peipsi in order (i) to assess the potential role that light limitation may have had on phytoplankton growth in the past and (ii) to get an insight into the factors driving underwater light climate in shallow turbid lakes in the long term. We reconstructed the long-term variations of the diffuse attenuation coefficient of water (K sub(d,PAR)) in the photosynthetically active region (PAR, 400-700 nm) partly on the basis of measured beam attenuation spectra and partly using regression analysis. From K sub(d,PAR) we calculated the depth of the euphotic zone (z sub(1%)) and the mean light availability in the mixed layer (E sub(mix)). The reconstructed time series of these bio-optical parameters gave a plausible picture of the long-term development of light conditions in the two lakes studied, which was in accordance with their eutrophication history and changes in their water levels. Better light availability in both lakes generally coincided with years of a low water level, and the coincidence was more distinct in the shallower Vortsjarv. Values of E sub(mix) revealed a probable light limitation in Peipsi in autumn and in Vortsjarv throughout the year.Original Abstract: Meie too peamine eesmark oli taastada veealuse valgusvalja mitme aastakumne pikkused muutused kahes suures madalas ja polumiktilises Eesti jarves - Vortsjarves ning Peipsi jarves: a) et hinnata valguse limitatsiooni moju futoplanktoni kasvule minevikus, b) vaadelda, mis mojutavad veealust valgusvalja madalates segunenud jarvedes pikal ajaskaalal. Me rekonstrueerisime vee difuusse norgenemise koefitsiendi (K sub(d,PAR)) pikaajalise varieeruvuse fotosunteetiliselt aktiivses spektri piirkonnas (PAR, 400-700 nm), kasutades nii moodetud kiirguse norgenemise spektreid kui ka regressioonanaluusi. K sub(d,PAR)-ist arvutasime omakorda eufootilise tsooni sugavuse (z sub(1%)) ja jarvevee segunenud kihi keskmise valguse kattesaadavuse (E sub(mix)). Eelnimetatud biooptiliste parameetrite rekonstrueeritud aegread andsid Peipsi ja Vortsjarve pikaajalistest valgustingimuste muutustest toeparase pildi, mis oli kooskolas eutrofeerumise ajaloo ning veetaseme varieerumisega nimetatud jarvedes. Veealused valgustingimused olid uldiselt paremad madalaveelistel aastatel. E sub(mix)-i vaartused naitasid, et Peipsi jarves on valgus limiteerivaks teguriks suurema toenaosusega sugisel ja Vortsjarves kogu aasta jooksul.