DOI: 10.5061/dryad.4b8gthtbb
title: Data from: Climate change-driven regime shifts in a planktonic food web

description: Predicting how food webs will respond to global environmental change is
 difficult because of the complex interplay between the abiotic forcing and
 biotic interactions. Mechanistic models of species interactions in
 seasonal environments can help understand the effects of global change in
 different ecosystems. Seasonally ice-covered lakes are warming faster than
 many other ecosystems and undergoing pronounced food web changes, making
 the need to forecast those changes especially urgent. Using a seasonally
 forced food web model with a generalist zooplankton grazer and competing
 cold-adapted winter and warm-adapted summer phytoplankton, we show that
 with declining ice cover, the food web moves through different dynamic
 regimes, from annual to biennial cycles, with decreasing and then
 disappearing winter phytoplankton blooms and a shift of maximum biomass to
 summer season. Interestingly, when predator-prey interactions were not
 included, a declining ice cover did not cause regime shifts, suggesting
 that both are needed for regime transitions. A cluster analysis of
 long-term data from Lake Baikal, Siberia supports model results, revealing
 a change from regularly occurring winter blooms of endemic diatoms to
 less-frequent winter bloom years with decreasing ice cover. Together, the
 results show that even gradual environmental change, such as declining ice
 cover duration, may cause discontinuous or abrupt transitions between
 dynamic regimes in food webs.
description: <strong>Description of methods used for
 collection/generation of data</strong>: Data have
 been collected by researchers from Irkutsk State University at least
 monthly, usually every 7-10 days, in depth profiles from the surface to at
 least 250 m at a single main station approximately 2.7 km offshore from
 Bol’shie Koty in the Southern Basin. Ice conditions prevent collection in
 some months, usually January and May. Phytoplankton
 were sampled along depth profiles with the Van Dorn bottle at 0, 10, 50,
 100, and 200 m depths and were preserved before settling in Utermöhl
 chambers. In 1973 there was a change in the preservation methods from the
 use of formalin to a Lugol’s solution. Counts from samples before 1973 are
 considered representative for diatoms because they are equally well
 preserved in both, but not for other phytoplankton groups. Only diatom
 counts from the upper 50m are included in this dataset.
 Single zooplankton samples were collected with a closing plankton
 net (37.5 cm diameter, 100 μm mesh) from depth layers of 0-10, 10-25,
 25-50, 50-100, 100-150, 150-250, and 250-500 m. Zooplankton samples were
 fixed in formalin throughout the duration of the long-term monitoring
 program with greatest consistency of temporal and spatial sampling
 occurring from 1955 forward. Both phytoplankton and zooplankton were
 identified and counted at the species level, and copepods were enumerated
 by age class. Only Epischurella baikalensis counts from the upper 25 m are
 included in this dataset, samples from the 25–50 m depth layer were
 excluded from our analyses because sampling frequency was not consistent
 at this depth layer across the time series. See also
 Hampton SE, Gray DK, Izmest’eva LR, Moore MV, Ozersky T (2014) The Rise
 and Fall of Plankton: Long-Term Changes in the Vertical Distribution of
 Algae and Grazers in Lake Baikal, Siberia. PLoS ONE 9(2): e88920.
 doi:10.1371/journal.pone.0088920 <strong>Methods
 for processing the data</strong>: For the
 analysis, diatom cell count data were converted to corresponding biovolume
 values (µm³/L) based on reported cell sizes (Rioual & Mackay 2005,
 Olenina et al. 2006, Belykh et al. 2006, Genkal & Bondarenko 2006,
 Kremer et al. 2014 and Database on Biovolume Metrics from Academy of
 Natural Sciences of Drexel University 2001). Count data
 (ind/m²) for nauplii, copepodite and adult stages of the Lake Baikal
 dominant zooplankton species Epischurella baikalensis were converted to
 (ind/L). Count data were then converted to corresponding biomass data (mg
 dry weight/L) based on body size reported by Afanasyeva (1998),
 multiplying number of individuals by 0.00027 for nauplii, by 0.008 for
 copepodites and by 0.1 for adults. Total diatom
 biovolume and total Epischurella biomass per sampling date were calculated
 for data from the upper layer (0-50m and 0-25m respectively).
description: See ReadMe file for detailed information on dataset.