The Laurentian Great Lakes are the largest system of freshwater on earth containing 22% of the world’s supply. Although part of a single system, each lake shows substantial variation regarding physical, chemical and biological parameters. The main goals of this thesis were to characterize the nutrient status of natural phytoplankton communities while comparing several commonly used measurements of nutrient status and Chlorophyll a (Chl a) fluorescence measurements. The study sites include the western basin (WB), west-central basin (WCB), and central basin (CB) of Lake Erie, the Bay of Quinte in Lake Ontario, and Colpoys Bay in Lake Huron. Independent measures of nutrient status were assessed by measurements of nitrogen (N) debt, phosphorus (P) debt, particulate C:N:P ratios, and alkaline phosphatase activity (APA). Variable fluorescence of chlorophyll a was measured by pulse amplitude modulated (PAM) fluorometry and fast repetition rate (FRR) fluorometry in parallel with the independent measures. In 2005, the phytoplankton communities in Lake Erie were generally N deficient in May, P deficient in June, and neither N nor P deficient in September. The maximum dark adapted quantum yield (Fv/Fm) measured by PAM or FRRF was lower in May and June, and maximal in September, while the functional absorption cross section of photosystem II (σPSII) was maximal in May and June, and minimal in September. Relationships between the variable fluorescence indicators and independent measures of nutrient status showed strong associations with N or P deficient sites having low Fv/Fm and high σPSII. In 2006, the electron transport rate (ETR) and the initial slope (α) derived from the PAM fluorescence rapid light-response curves (RLC) were compared to independent measures and Fv/Fm measurements in Lake Erie. Relationships between ETR, α, independent measures of nutrient status, and Fv/Fm measurements revealed strong associations with nutrient status. Confirming previous reports, N deficiency was highest in the WB during isothermal conditions while P deficiency was highest in the CB during summer stratification. The fluorescence parameters generally decreased as the severity of N and P deficiency increased. N and P enrichment assays also revealed increased values of Fv/Fm, ETR, and α from N and P deficient samples over twenty-four hours. Additionally, spatial variability of P status was evaluated during summer stratification. Colpoys Bay, the most oligotrophic site, had the strongest P deficiency, and evidence for existence of P deficiency was weakest in the Bay of Quinte, the most eutrophic site. Nutrient enrichment assays revealed that all fluorescence parameters showed a positive response to P additions in oligotrophic sites, with no response in eutrophic sites. Community structure was also associated with nutrient status and Chl a fluorescence at all locations. In P deficient sites, nano-flagellates such as chrysophytes and cryptophytes were prevalent; cyanobacteria were dominant at sites that displayed N deficiency.

The Laurentian Great Lakes are the largest system of freshwater on earth containing 22% of the world’s supply. Although part of a single system, each lake shows substantial variation regarding physical, chemical and biological parameters. The main goals of this thesis were to characterize the nutrient status of natural phytoplankton communities while comparing several commonly used measurements of nutrient status and Chlorophyll a (Chl a) fluorescence measurements. The study sites include the western basin (WB), west-central basin (WCB), and central basin (CB) of Lake Erie, the Bay of Quinte in Lake Ontario, and Colpoys Bay in Lake Huron. Independent measures of nutrient status were assessed by measurements of nitrogen (N) debt, phosphorus (P) debt, particulate C:N:P ratios, and alkaline phosphatase activity (APA). Variable fluorescence of chlorophyll a was measured by pulse amplitude modulated (PAM) fluorometry and fast repetition rate (FRR) fluorometry in parallel with the independent measures. In 2005, the phytoplankton communities in Lake Erie were generally N deficient in May, P deficient in June, and neither N nor P deficient in September. The maximum dark adapted quantum yield (Fv/Fm) measured by PAM or FRRF was lower in May and June, and maximal in September, while the functional absorption cross section of photosystem II (σPSII) was maximal in May and June, and minimal in September. Relationships between the variable fluorescence indicators and independent measures of nutrient status showed strong associations with N or P deficient sites having low Fv/Fm and high σPSII. In 2006, the electron transport rate (ETR) and the initial slope (α) derived from the PAM fluorescence rapid light-response curves (RLC) were compared to independent measures and Fv/Fm measurements in Lake Erie. Relationships between ETR, α, independent measures of nutrient status, and Fv/Fm measurements revealed strong associations with nutrient status. Confirming previous reports, N deficiency was highest in the WB during isothermal conditions while P deficiency was highest in the CB during summer stratification. The fluorescence parameters generally decreased as the severity of N and P deficiency increased. N and P enrichment assays also revealed increased values of Fv/Fm, ETR, and α from N and P deficient samples over twenty-four hours. Additionally, spatial variability of P status was evaluated during summer stratification. Colpoys Bay, the most oligotrophic site, had the strongest P deficiency, and evidence for existence of P deficiency was weakest in the Bay of Quinte, the most eutrophic site. Nutrient enrichment assays revealed that all fluorescence parameters showed a positive response to P additions in oligotrophic sites, with no response in eutrophic sites. Community structure was also associated with nutrient status and Chl a fluorescence at all locations. In P deficient sites, nano-flagellates such as chrysophytes and cryptophytes were prevalent; cyanobacteria were dominant at sites that displayed N deficiency.

Over the last decades, the effects of climate change have become increasingly evident across natural environments. Apart from other areas, climate change poses a serious threat on water quality. More specifically, it is expected that the effects of global warming around the world will severely limit our ability to control the spread and occurrence of Harmful Algal Blooms (HABs) in the future. A HAB episode is characterised by rapid proliferation of algal biomass which can cause major implications on the environment, the ecosystems, on human health, the economy, and societies overall. One way of detecting the presence of algae is to determine the chlorophyll-a concentration levels in water. This project proposes an embedded system for early algae detection in water samples by means of chl-a fluorometry. The system makes use of a constructed sensor to detect chl-a fluorescence emission. Two versions of the sensor were designed and implemented, both of which were calibrated and then tested. Afterwards, the results were presented, and the system’s performance was evaluated and discussed. Lastly, it was concluded that the performance of the system was adequate for detecting a 50 μg/L limit of chl-a concentration, however, careful testing of the site is required for field applications in samples of natural water. / ­Under de senaste decennierna har effekterna av klimatförändringar blivit allt tydligare i naturliga miljöer. Förutom andra områden utgör klimatförändringarna ett allvarligt hot mot vattenkvaliteten.  I synnerhet förväntas effekterna av global uppvärmning över hela världen begränsa vår förmåga att kontrollera spridningen och förekomsten av skadliga algblomningar (HAB) i framtiden. En HAB-episod kännetecknas av snabb spridning av algbiomassa som kan orsaka stora konsekvenser för miljön, ekosystemen, människors hälsa, ekonomin och samhället i stort. Ett sätt att upptäcka förekomsten av alger är att bestämma klorofyll-a-koncentrationsnivåerna i vatten.  Detta projekt föreslår ett inbyggt system för tidig algedetektering i vattenprover med hjälp av chl-a fluorometri. Systemet använder en konstruerad sensor för att detektera chl-a-fluorescensemission. Två versioner av sensorn designades och implementerades, båda kalibrerades och testades sedan. Därefter presenterades resultaten och systemets prestanda utvärderades och diskuterades.  Slutligen drogs slutsatsen att systemets prestanda var tillräcklig för att detektera 50 μg/L-gräns för chl-a-koncentration, men noggrann testning av platsen krävs för fältapplikationer i prover av naturligt vatten.