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Lakes are important components for regulating carbon cycling within landscapes. Most lakes are regarded as CO2 sources to the atmosphere, except for a few eutrophic ones. Algal blooms are common phenomena in many eutrophic lakes and can cause many environmental stresses, yet their effects on the net exchange of CO2 (Fco2) at large spatial scales have not been adequately addressed. We integrated remote sensing and Eddy Covariance (EC) technologies to investigate the effects that algal blooms have on FCO2 in the western basin of Lake Erie-a large lake infamous for these blooms. Three years of long-term EC data (2012-2014) at two sites were analyzed. We found that at both sites: (1) daily FCO2 significantly correlated with daily temperature, light, and wind speed during the algal bloom periods; (2) monthly FCO2 was negatively correlated with chlorophyll-a concentration; and (3) the year with larger algal blooms was always associated with lower carbon emissions. We concluded that large algal blooms could reduce carbon emissions in the western basin of Lake Erie. However, considering the complexity of processes within large lakes, the weak relationship we found, and the potential uncertainties that remain in our estimations of Fco2 and chlorophyll-a, we argue that additional data and analyses are needed to validate our conclusion and examine the underlying regulatory mechanisms.

We review alternative hypotheses and associated mechanisms to explain Lake Victoria's Nile perch (Lates niloticus) takeover and concurrent reduction in haplochromines through a (re)analysis of long-term climate, limnological, and stock observations in comparison with size-spectrum model predictions of co-existence, extinction, and demographic change. The empirical observations are in agreement with the outcomes of the model containing two interacting species with life histories matching Nile perch and a generalized haplochromine. The dynamic interactions may have depended on size-related differences in early juvenile mortality: mouth-brooding haplochromines escape predation mortality in early life stages, unlike Nile perch, which have miniscule planktonic eggs and larvae. In our model, predation on the latter by planktivorous haplochromine fry acts as a stabilizing factor for co-existence, but external mortality on the haplochromines would disrupt this balance in favor of Nile perch. To explain the observed switch, mortality on haplochromines would need to be much higher than the fishing mortality that can be realistically reconstructed from observations. Abrupt concomitant changes in algal and zooplankton composition, decreased water column transparency, and widespread hypoxia from increased eutrophication most likely caused haplochromine biomass decline. We hypothesize that the shift to Nile perch was a consequence of an externally caused, climate-triggered decrease in haplochromine biomass and associated recruitment failure rather than a direct cause of the introduction. Nous examinons differentes hypotheses et les mecanismes associes pour expliquer la prise de controle des perches du Nil (Lates niloticus) dans le lac Victoria et la reduction concurrente des haplochrominiens en (re-)analysant des observations climatiques, limnologiques et sur les stocks a long terme par rapport aux predictions de modeles de spectre de tailles quant a la coexistence, a la disparition et aux changements demographiques. Les observations empiriques concordent avec les resultats du modele contenant deux especes interagissent dont les cycles biologiques coincident, respectivement, avec ceux de la perche du Nil et d'un haplochrominien generalise. Les interactions dynamiques pourraient avoir dependu de differences reliees a la taille sur le plan de la mortalite juvenile precoce : les haplochrominiens a incubation buccale echappent a la mortalite par predation durant les premiers stades de vie, contrairement aux perches du Nil, qui ont de minuscules oeufs et larves planctoniques. Si, dans le modele, la predation de ces dernieres par les alevins d'haplochrominiens planctonivores est un facteur stabilisateur pour la coexistence, une mortalite d'origine externe des haplochrominiens perturberait cet equilibre en faveur des perches du Nil. Pour expliquer la transition observee, la mortalite des haplochrominiens devrait etre beaucoup plus grande que la mortalite par peche qui peut, de maniere realiste, etre reconstituee a partir des observations. Des changements abrupts concurrents de la composition des algues et du zooplancton, la reduction de la transparence de la colonne d'eau et une hypoxie repandue decoulant de l'eutrophisation accrue ont vraisemblablement cause la diminution de la biomasse d'haplochrominiens. Nous postulons que, plutot que d'etre une consequence directe de l'introduction, le passage a une predominance des perches du Nil est la consequence d'une reduction de la biomasse d'haplochrominiens causee par des facteurs externes et declenchee par le climat, ainsi que de l'echec du recrutement associe. [Traduit par la Redaction], Introduction Invasions of non-native species into ecosystems become increasingly common through anthropogenic activities, facilitated by climate change (Cheung et al. 2009). For most invasions, little is known about interactions with [...]