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Effects of thermocline deepening on lake plankton communities

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Theory predicts, and some evidence demonstrates that in lakes, the depth of the thermocline can have a large structural influence on the spatial distribution, and strongly influences the composition of plankton communities. However, experimental assessments of responses of the planktonic food web to thermocline depth have not yet been done at the whole-basin scale. We conducted an experiment wherein we artificially lowered the thermocline in an isolated basin of a three-basin lake, maintaining another isolated basin as a control. The vertical distribution and taxonomic composition of both phytoplankton and zooplankton were monitored throughout the summer months. Greater phytoplankton production, especially in the epilimnion, attributable mainly to increases in the chlorophytes was observed with thermocline deepening, but at the deepest thermoclines, production was limited. Total zooplankton biomass was unaffected by thermocline depth, suggesting top-down control by predators. Zooplankton biomass peaks were less pronounced in the manipulated basin, but tended to follow the thermocline whether at its normal position or as it was deepened. Zooplankton composition was significantly altered by large increases in densities of predatory cyclopoid copepods and rotifers; taxa commonly found in turbulent environments. Overall, both phytoplankton and zooplankton communities demonstrated important shifts in structure and composition in response to thermocline deepening.

La théorie prédit et certaines données démontrent que la profondeur de la thermocline dans les lacs peut avoir une grande influence structurale tant sur la répartition spatiale que sur la composition des communautés planctoniques. Cependant, les réactions du réseau alimentaire planctonique à la profondeur de la thermocline n'ont pas encore été étudiées à l'échelle d'un bassin entier. Nous avons mené une expérience dans laquelle nous avons abaissé artificiellement la thermocline dans un bassin isolé dans un lac qui comporte trois bassins, tout en maintenant un autre bassin isolé comme témoin. Nous avons suivi la répartition verticale et la composition taxonomique à la fois du phytoplancton et du zooplancton au cours des mois d'été. Nous avons observé une production accrue du phytoplancton, particulièrement dans l'épilimnion, attribuable principalement à l'accroissement des chlorophytes lors de baisse de la thermocline; aux profondeurs les plus grandes de la thermocline, la production est cependant réduite. La biomasse totale du zooplancton est inaffectée par la profondeur de la thermocline, ce qui laisse croire à un contrôle descendant par les prédateurs. Les pics de biomasse du zooplancton sont moins prononcés dans le bassin manipulé, mais ils ont tendance à suivre la thermocline, qu'elle soit à sa profondeur normale ou plus profonde. La composition du zooplancton est significativement modifiée par des forts accroissements de densité des copépodes cyclopoïdes et des rotifères prédateurs, des taxons couramment retrouvés dans les environnements turbulents. Globalement, les communautés phytoplanctoniques et zooplanctoniques montrent toutes deux des changements importants en réaction à la baisse de la thermocline.

Document Type: Research Article

Publication date: February 1, 2011

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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