Response of boreal lakes to episodic weather-induced events

Weather-induced episodic mixing events in lake ecosystems are often unpredictable, and their impacts are therefore poorly known. The impacts can be short-lived, including changes in water temperature and stratification, but long-lasting effects on the lake’s biology may also occur. In this study we used automated water quality monitoring (AWQM) data from 8 boreal lakes to examine how the episodic weather-induced mixing events influenced thermal structure, hypolimnetic dissolved oxygen (DO), fluorometric chlorophyll estimates (Chl-a), and lake metabolism and how these events varied in frequency and magnitude in lakes with different characteristics. Rise in wind speed alone had an effect on the lakes with the weakest thermal stability, but a decrease in air temperature together with strong wind induced mixing events in all lakes. The return period of these mixing events varied widely (from 20 to 92 d) and was dependent on the magnitude of change in weather. In lakes with strong stability, thermal structure and hypolimnetic DO concentration were only slightly affected. Weather-induced mixing in the upper water column diluted the surface water Chl-a repeatedly, whereas seasonal maximum occurred in late summer on each lake. Although Finnish lakes have been characterized with stable stratification during summer, we observed many substantial mixing events of relatively short return periods relevant to both chemical and biological properties of the lakes

Automated water quality monitoring of humic lakes by using the optical properties of water

Automated water quality monitoring (AWQM) is becoming increasingly common in lakes worldwide. The history of AWQM is relatively short and standard calibration procedures for the measured variables are largely yet to be established. The use of optical AWQM sensors, developed in oceanic environments, raises new questions on the diverse effects which humic compounds may have on the automated optical measurements in inlands waters. The focus of this thesis was to characterize the effects of coloured dissolved organic matter (CDOM) on optical in situ measurements of organic matter (OM) and chlorophyll (Chl) in lakes with varying humic content, and to use AWQM data as a part of traditional monitoring and independently to study current topics in limnology; weather-related episodic events on lake mixing and the effect of hypolimnetic oxygenation (HLO) on the nutrient conditions in a humic lake. The data were collected from AWQM stations on lakes in Finland and Ireland and involved both AWQM and discrete data from open water season. The study on the quality of CDOM revealed that OM fluorometers represented the humic content in water column reliably. However, OM fluorometers did not detect changes in the quality of OM. In situ Chl fluorometer was affected by background OM. The effect was less important when OM remained at constant level. Effects of weather-related episodic events on lake mixing and on Chl-patterns studied with AWQM showed Finnish lakes to face multiple short-lived mixing events during stratified period. The Chl-a content in the lakes varied accordingly. A case study conducted on Jyväsjärvi showed that HLO had a significant effect on the temperature structure of the lake but had little long-term effect on its trophic statu

Automaattisten mittalaitteiden käyttö vesistöseurannassa : esimerkkinä Jyväsjärven tutkimuslautta Aino

Vesistöseurannan automatisointi lisää huomattavasti saatavilla olevan tiedon määrää, mutta samalla automaattisten aineistojen laadunvarmennuksen tärkeys korostuu. Jyväskylän yliopiston tutkimuslautta Aino mittaa jatkuvatoimisesti mm. veden happipitoisuuden ja lämpötilan vertikaaliprofiilia Jyväsjärvellä. Lautan tulosten luotettavuutta tutkittiin automaattisten mittalaitteiden sekä manuaalisen mittauksen avulla. Lautalla mitattiin myös virtausten suuntaa ja nopeutta ADCP-virtausmittarilla. Virtaustiedot tarkensivat kerrostumisen purkautumisen kuvaamiseen käytettyjä lukuja. Eri mittalaitteiden tuottamat happipitoisuustulokset poikkesivat merkitsevästi toisistaan. Hapen automaattisessa mittaamisessa tulisikin erityisesti huomioida laitteiden epävarmuus kalibroinnin ja tekniikan suhteen. Mittauslautan pitkäaikainen vertikaaliprofiiliaikasarja tuo esiin ilmiöitä, joita perinteisellä näytteenotolla ei havaita, kuten kerrostumisen muodostuminen ja ajoittainen purkautuminen sekä tuulen aiheuttama päällysveden kuluminen. Mittausaseman tulokset ovat kuitenkin paikallisia mittauksia, eikä niiden avulla voida kuvata koko järven tilaa

Calibration of in situ chlorophyll fluorometers for organic matter

Organic matter (OM) other than living phytoplankton is known to affect fluorometric in situ assessments of chlorophyll in lakes. For this reason, calibrating fluorometric measurements for OM error is important. In this study, chlorophyll (Chl) fluores cence was measured in situ in multiple Finnish lakes using two sondes equipped with Chl fluorometers (ex.470/em.650–700 nm). OM absorbance (A420) was measured from water samples, and one of the two sondes was also equipped with in situ fluorometer for OM (ex.350/em.430 nm). The sonde with Chl and OM fluorometers was also deployed continuously on an automated water quality monitoring station on Lake Konnevesi. For data from multiple lakes, inclusion of water colour estimates into the calibration model improved the predictability of Chl assessments markedly. When OM absorbance or in situ OM fluorescence was used in the calibration model, predictability between the in situ Chl and laboratory Chl a assessments was also enhanced. However, correction was not superior to the one done with the water colour estimate. Our results demonstrated that correction with water colour assessments or in situ measurements of OM fluorescence offers practical means to overcome the variation due to OM when assessing Chl in humic lakes in situ

Negligible effect of hypolimnetic oxygenation on the trophic state of Lake Jyväsjärvi, Finland

Hypolimnetic oxygenation by pumping oxygen-rich surface water to the hypolimnion (HLO) is a commonly used tool for the restoration of nutrient-loaded dimictic lakes. However, in recent years its effectiveness has been questioned. In this case study we evaluated monitoring data covering a period of 23-years to show that, although experimental cessation of HLO drastically changed the lake's temperature and dissolved oxygen regimes, it did not significantly affect its trophic status. Thus, we recommend that the limited financial resources available are better directed towards further lowering the lake's external phosphorus load than continuing HLO.peerReviewe

Empirical Estimation of Accumulation-Induced Change in Gill Net Catch-ability : Mind the Observation Errors

We analyzed cumulative catches for 24 h gill net exposures divided into 4*6 h, 2*12 h and 1*24 h soak time treatments to estimate the reduction in its catchability due to accumulation of fish. The effects of loss of catch during net lifting, disturbance effect and fouling were eliminated as far as possible to reveal the true effect of accumulation. First we applied simple nonparametric and parametric tests in comparison of treatments. As expected, considerable reduction in catchability took place along with the increase in soak time, indicated by significantly lower total 24 h catches from longer soaks in comparison with shorter ones. The reduction was more pronounced for roach than for perch. Further, we compared a functional relationship regression (FRR), admitting correctly observation error variance also in the x-axis variable, with ordinary least squares regression (OLS) in modelling the relationship between cumulative 24 h catches for different treatments. We estimated the between-replicates proportional observation error variance within a treatment and found it to be similar in different treatments. Therefore the variance ratio could be assumed to be close to 1 enabling the use of major axis solution FRR. In this particular case the incorrect use of OLS obviously gives a seriously biased result, exacerbating the negative effect of accumulation for high x-axis values in comparison with FRR. We recommend the use of FRR for any analysis comparing different notoriously low precision fish abundance proxies.peerReviewe