Limnetica 40

Deep-chlorophyll layers (DCL) in oligotrophic lakes contribute significantly to primary productivity, but the importance of this production for the rest of the food web and for other strata is unknown. In Laguna El Tejo, a sheltered 1.7-ha Spanish karst lake, chlorophyll levels were < 2 μg/L in the epilimnion but reached 10 μg/L in the metalimnion and upper hypolimnion where cyanobacterial picoplankton dominated. Particulate nitrogen levels were 2-14 times higher in the metalimnion than in the epilimnion and 42 % of the primary productivity occurred in the deeper strata where the DCL was located. To address the trophic importance of the high biomass and production in the deep chlorophyll layer, we injected ¹⁵NH₄⁺ and rhodamine into a 0.5-m strata (15-16 m) in the metalimnion of the lake. The ¹⁵NH₄⁺ taken up by the nitrogen-limited phytoplankton allowed us to measure the importance of biologically mediated transport whereas the rhodamine traced physical eddy diffusion. After 28 days 55 % of the ¹⁵N could be accounted for: 71 % remained in the metalimnetic seston (11-18 m), 10 % was in the hypolimnetic seston (18-20 m), 11 % was found above in the epilimnetic seston, and only 8 % had sedimented into the anoxic layer below 20 m. Only negligible amounts of rhodamine (corrected for degradation) moved beyond the 14-18 m strata in the lake, but bio-diffusivity of ¹⁵N was 3 times greater than the physically induced diffusivity of rhodamine. A mixing model indicated that the deep chlorophyll layer contributed only 1-2 % of the diet of epilimnetic macrozooplankton but 14-33 % of the diet of the meta-hypolimnetic zooplankton. The data indicate the overall importance for primary production and the sequestration of nutrients in the DCL, but relatively limited importance for the macrozooplankton in the lake.