Forest Die-Back Modified Plankton Recovery from Acidic Stress

Abstract

We examined long-term data on water chemistry of Lake Rachelsee (Germany) following the changes in acidic depositions in central Europe since 1980s. Despite gradual chemical recovery of Rachelsee, its biological recovery was delayed. In 1999, lake recovery was abruptly reversed by a coincident forest die-back, which resulted in elevated terrestrial export of nitrate and ionic aluminum lasting *5 years. This re-acidification episode provided unique opportunity to study plankton recovery in the rapidly recovering lake water after the abrupt decline in nitrate leaching from the catchment. There were sudden changes both in lake water chemistry and in plankton biomass structure, such as decreased bacterial filaments, increased phytoplankton biomass, and rotifer abundance. The shift from dominance of heterotrophic to autotrophic organisms suggested their substantial release from severe phosphorus stress. Such a rapid change in plankton structure in a lake recovering from acidity has, to the best of our knowledge, not been previously documented.

Constraints on the biological recovery of the Bohemian Forest lakes from acid stress

“…This is definitely not the case in the Bohemian Forest lakes. Despite a slight calcium decline observed in some lakes since 1984 (Kopáček et al ., ; Oulehle et al ., ; Vrba et al ., ), its concentrations in all the lakes have always been far below those (1.4–2.2 mg Ca L −1 ) reported for boreal lakes by Jeziorski et al . ().…”

“…Kopáček et al ., ) and the data suggested a gradual increase in pH and remarkable declines in Al concentrations in CN, CT, PL and PR lakes (Fig. ), as well as in RA (Vrba et al ., ). Despite the similar pH (~4.5), the lakes differed in Al t concentrations, with values >1 mg L −1 in CT and PL, ~1 mg L −1 in CN, ~0.7 mg L −1 in RA and only 0.5–0.6 mg L −1 in PR in the mid‐1980s.…”

“…The low‐Al lakes (except PR) were also inhabited by large populations of P. major in 1999 and 2003 (Table S4). Despite the large decline in the abundances of rotifer species in 2007, Microcodon clavus showed a marked increase and exceeded even the abundances of crustaceans in the pelagic zone of the high‐Al RA (for details, see Vrba et al ., ). The crustacean abundance increased in 2003 and slightly decreased in 2011 (Fig.…”

“…In addition, the increase in DOC concentration is likely to have reduced light availability for the phytoplankton in some lakes. The most pronounced reduction in the euphotic zone in RA did indeed cause remarkable changes in the vertical distribution of phytoplankton (for details, see Vrba et al ., ). Our study also suggests an overall increasing top‐down control exerted by filter‐feeding zooplankton in the recovering pelagic communities.…”

“…The actual Al concentrations have apparently controlled the timing of biological recovery by forming the barrier that lagged the recovery of the high‐Al lakes. For instance, Ceriodaphnia quadrangula first appeared in CN in 1997, but in closely adjacent CT one decade later and in RA in 2009, only after the same threshold of both Al t and Al i concentrations was reached (Vrba et al ., ). Moreover, the harmful Al i effect was recognised as the critical bottleneck preventing reproduction of quillwort populations – Isoëtes echinospora in PL until 2004 (Čtvrtlíková et al ., ) and I. lacustris in CN till present (Čtvrtlíková, Znachor & Vrba, ).…”