Disentangling food quantity and quality effects in zooplankton response to P-enrichment and UV radiation

Manuel Villar-Argaiz, Francisco José Bullejos, Juan Manuel Medina-Sánchez, Eloísa Ramos-Rodríguez, José Antonio Delgado-Molina and Presentación Carrillo

Limnol. Oceanogr., 57(1), 2012, 235-250 | DOI: 10.4319/lo.2012.57.1.0235

ABSTRACT: In a 32-yr record in oligotrophic Lake La Caldera (Sierra Nevada, Spain) biomass of zooplankton was strongly correlated with precipitation, aerosol deposition intensity, and ultraviolet radiation (UVR). The food associated-effects of these factors for zooplankton growth were tested in field-laboratory experiments with the aim of separating the effects of food quantity from those of food quality at low food conditions, where there is good evidence to support the existence of food quality effects. Manipulation of nutrients generated a large food quantity gradient that exerted the strongest effect on zooplankton growth, with no significant role of UVR. Growth curves were fitted to a saturation function that reached a plateau at increasing seston levels of ca. 250, 500, and 1000 µg C L-1 for the rotifer Keratella cochlearis, the copepod Mixodiaptomus laciniatus, and the cladoceran Daphnia pulicaria, respectively, and after which growth decreased in M. laciniatus. Nutrients and to a lesser extent UVR also affected seston quality, which had a minor effect on zooplankton growth. K. cochlearis growth was strongly related to the P content of seston, whereas M. laciniatus and D. pulicaria growth were positively correlated with a P-normalized ω3-polyunsaturated fatty acid index (ω3-PUFA:P). The increase in seston associated with more intense and frequent atmospheric depositions would adversely affect copepods, but improve growth of C-limited cladocerans and P-limited rotifers in pristine ecosystems of the Mediterranean region.

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