Document Type

Thesis

Degree Name

Master of Science (MSc)

Department

Biology

Program Name/Specialization

Integrative Biology

Faculty/School

Faculty of Science

First Advisor

Jason Venkiteswaran

Advisor Role

Academic Advisor

Second Advisor

Helen Baulch

Third Advisor

Sonya Havens

Fourth Advisor

Derek Gray

Abstract

Freshwater eutrophication contributes towards the development of cyanobacterial harmful algal blooms (cHABs), but nutrient availability alone does not fully explain variation in phytoplankton community composition or physiological performance. While phosphorus (P) availability has been known to regulate phytoplanktonic biomass, iron (Fe) is an important micronutrient supporting Fe-heavy processes within cyanobacteria (i.e., nitrogen fixation and photosynthetic electron transport). Sediment Fe availability in freshwater ecosystems is regulated by reduction oxidation (redox) conditions at the sediment-water interface (SWI), where nitrate (NO3-) has been found to maintain oxidizing conditions and prevent the reduction of Fe(III) to more readily bioavailable soluble Fe(II), thus reducing Fe-derived sediment release. However, whether changes in Fe availability through the manipulation of NO3- concentrations are sufficient to suppress cyanobacterial dominance, and how variation in Fe supply affects phytoplankton photosynthetic efficiency remains uncertain. This thesis investigated how NO3- and Fe interrelations influenced phytoplankton at two varying biological scales at the IISD-Experimental Lakes Area (IISD-ELA) in Northwestern Ontario, Canada.

Mesocosm experiments in artificially eutrophic Lake 303 evaluated phytoplankton responses to calcium nitrate (Ca(NO3)2) enrichment. Ca(NO3)2 additions established distinct NO3- gradients but were insufficient alone to produce a corresponding decline in dissolved Fe (FeDiss). NO3- altered species-specific competitive relationships; redistribution of biomass among existing eutrophic taxa, increased community evenness, and reduced dominance without producing group-level replacement. Thus, NO3- was an ecological filter that reorganized competitive hierarchies within groups rather than driving larger changes in phytoplanktonic community composition primarily through shifts in the relative abundance of existing taxa.

Maximum photosystem II (PSII) quantum efficiency (Fv/Fm) was evaluated across five boreal lakes differing in trophic status, morphometry, nutrient enrichment, Fe availability, and phytoplankton community composition. Intracellular Fe (FeInt) alone weakly predicted Fv/Fm (R2 = 0.05, p = 0.21), whereas FeInt:Biomass exhibited a significant saturating relationship (R2 = 0.51, p < 0.001), approaching a maximum efficiency of ~0.48. FeInt:Chl a exhibited a similar but weaker saturating relationship (R2 = 0.34, p < 0.0005), indicating that FeInt relative to phytoplankton biomass better explained variation in PSII efficiency than FeInt relative to pigment content. Despite lower FeDiss concentrations in an NO3- enriched L303, PSII efficiency remained variable, while low FeInt:Biomass values provided evidence of FeInt limitation of PSII despite substantial phytoplankton biomass accumulation. Neither lake identity nor cyanobacterial dominance altered the relationship ii between normalized FeInt and Fv/Fm. Thus, NO3- may influence PSII indirectly by shifting community composition along an Fe limitation continuum rather than altering the physiological dependency of PSII on Fe.

These findings demonstrate the complexity of NO3- and Fe interactions which may impact phytoplankton community structure and physiological function. Rather than directly suppressing cyanobacteria or photosynthetic performance, with both biological scales being mediated by seasonal, physical, and lake-specific environmental conditions. These results highlight the limitations of single-nutrient understanding of cHAB control and the importance of integrating sediment biogeochemistry, community compositional dynamics, and physiological dependency of Fe demand when attempting to predict phytoplankton responses within eutrophic lakes.

Convocation Year

2027

Convocation Season

Spring

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