Paleolimnological record as an indication of incipient eutrophication in an oligotrophic subtropical coastal lake in Southern Brazil.

Abstract

Paleolimnology of lake sediments can be a powerful tool to assess various aspects of lake history and catchment change through elemental, isotopic and molecular analysis of the sedimented organic matter (OM). In this sense, the objective of the present study was to investigate the source, depositional history and preservation of OM in the sediments of two different sites in Peri Lake (southern Brazil) to better understand the nature and direction of environmental changes. Therefore, two sediment cores were sampled and analysed for total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) concentrations and elemental ratios, and stable isotope ratios of C and N (δ(13)C and δ(15)N). Both cores showed similar general tendencies, with increasing amounts of OM (range 1-35%), TOC (2.55-258.40 mg g(-1)), TN (0.30-25.97 mg g(-1)) and TP (0.03-4.72 mg g(-1)) from the bottom toward the top more recent layers. TOC:TN ratios (range 8.1-14.7) showed a slight decrease in recent times and indicated a mixture of allochthonous and autochthonous contribution to the OM, with predominance of the last source. TN:TP (range 0.2-51.3) indicated a condition of potential limitation by P in general. Both δ(13)C (range -25.58 to -20.85) and δ(15)N (range 2.6 to 7.1) showed a decreasing pattern toward the top of the cores, in opposition to macronutrient concentration. Differences in the depth variation pattern between the two cores were associated to the marginal location of one of the cores. The results suggest that nutrients and primary production are increasing in the lake.