Nutrient cycling in a strongly acidified mesotrophic lake

Abstract

In contrast to the well‐known nutrient transformations in circumneutral lakes, acidified water bodies may exhibit significant changes in nutrient cycling due to changes in their chemistry and biology. In‐lake cycles of carbon (C), nitrogen (N), and phosphorus (P) were evaluated in the strongly acidified Plesné Lake (Central Europe). The lake tributaries had high concentrations of dissolved organic C (DOC), inorganic N, and total phosphorus (TP), resulting in annual averages of 644, 52, and 0.72 µmol L−1, respectively. Because of the absence of fish and largely reduced zooplankton, Plešne’ Lake has a “simplified” food web. The mass balance of nutrients was based on the major external inputs (tributaries and atmospheric deposition), internal sources and transformations (primary and bacterial production, biological decomposition of sedimenting seston and sediments), sedimentation, and outputs. External inputs of total organic C (TOC), total N (TN), and TP into Plesné Lake were 7,244, 864, and 8.6 mmol m−2 yr−1, respectively. Net primary production of particulate C (Cpart) and extracellularly released DOC were 3,205 and 613 mmol m−2 yr−1, respectively. Bacterial Cpart and total inorganic C production were both 1,518 mmol m−2 yr−1. Of the total internal and external inputs of TOC, TN, and TP, the in‐lake processes removed 4,551 (50% respiration, 40% sedimentation, and 10% photooxidation), 211 (74% sedimentation and 26% denitrification), and 4.6 (100% sedimentation) mmol m−2 yr−1, respectively. Compared to circumneutral lakes, nutrient cycling differed as follows: (1) Liberated orthophosphate from sedimenting seston was converted from a liquid to a particulate phase by colloidal aluminum (Al) in the hypolimnion and deposited. Similar abiotic P immobilization with Al removed 1.6 mmol m−2 yr−1 from the whole‐water column, thus reducing by ~20% the pool of potentially bioavailable P and contributing to a severe P limitation of biomass. (2) The cessation of nitrification due to long‐term water acidification led to an atypical situation in which the lake became a net source of NH4+ (30 mmol m−2 yr−1) because dissimilative liberation of NH4+ exceeded its assimilation.