In highly seasonal systems, the emergence of planktonic resting stages from the sediment is a key driver for bloom timing and plankton community composition. The termination of the resting phase is often linked to environmental cues, but the extent to which recruitment of resting stages is affected by climate change remains largely unknown for coastal environments. Here we investigate phyto- and zooplankton recruitment from oxic sediments in the Baltic Sea in a controlled experiment under proposed temperature and light increase during the spring and summer. We find that emergence of resting stage differs between seasons and the abiotic environment. Phytoplankton recruitment from resting stages were high in spring with significantly higher emergence rates at increased temperature and light levels for dinoflagellate and cyanobacteria than for diatoms, which had highest emergence under cold and dark conditions. In comparison, copepod hatchlings were most abundant in summer and emergence was not affected by increased temperature and light levels. These results show that activation of plankton resting stages are affected to different degrees by increasing temperature and light levels, indicating that climate change affects plankton dynamics through processes related to resting stage termination with potential consequences for bloom timing, community composition and trophic mismatch.

Twenty-four sediment cores were collected with a Multicorer (K.U.M. Umwelt und Meerestechnik, Kiel GmbH), fitted with 4 plexiglass tubes measuring 10 cm (inner diameter) and subsampled with plexiglass tubes (25 cm high, 7.4 cm inner diameter with a surface area of 0.004 m²) from a depth of 21 m in the northern Baltic Sea proper (58°50´N, 17°33´E). The same tubes were used for the microcosm experiments. The ratio of sediment to overlaying bottom water (451 ml) in each subsample was about 1:1. The smaller tubes (hereafter called microcosms) were kept cold and dark from collection until being placed in a temperature-controlled room set to in situ bottom temperature for 24 h to allow any suspended material to settle to the bottom. Before deployment of the multicorer, bottom water was collected with a Niskin bottle, and temperature, oxygen and light intensity was measured at 20 m depth by use of a CTD (Sea & Sun Technology GmbH). Sampling was conducted once in the late June 2019 (summer) and early March 2020 (spring). Light and oxygen measurements at 20 m were 2.73 µE m^-2 and 7.26 ml l^-1 in spring, and 1.69 µE and 8.83 ml l^-1 in summer, respectively. In comparison, light values below the water surface were 471 µE m^-2 in spring and 1042.46 µE m^-2 in summer, respectively. 

Before the experimental start, water overlaying the sediment was siphoned off with syringes as close as possible to the sediment, leaving approximately 5 mm of water. Siphoning off the initial water did, however, not completely remove adult Eurytemora individuals in summer (Fig. S1). Thus, the experiment does not allow for separation of Eurytemoranauplii recruitment from the sediment or hatching from subitaneous eggs and are excluded from the analysis. Sterile, filtered (0.2 mm) bottom water from the collection site was drip-fed onto polystyrene foam disc, placed in each microcosm, to avoid resuspension of the sediment. After re-filling the microcosms with water, they were covered with parafilm with small holes in order to minimize evaporation but also to allow for gas exchange and light aeration through a single thin nylon tube per microcosm. Care was taken to allow minimal aeration of the upper few centimeters of the water to avoid any resuspension of the sediment. The 24 microcosms were placed in incubators divided into four abiotic factor treatments, with six replicates each: control CTRL was kept dark and at in situ bottom temperature (3 °C in spring and 9 °C in summer), elevated temperature (T), kept dark and subjected to an increase of +2 °C, light (L), subjected to the same temperature as CTRL and to weak green light emitting 0.5 mmol m^-2 s^-1 (Nascimento et al., 2008), and a combination of elevated temperature of +2 °C and weak green light (T+L). Treatment T was covered with a thick, dark plastic sheet to block all possible light from entering. A +2 °C temperature increase was based on average projected temperature increase for the Baltic Sea by the end of this century and weak green light representing underwater light change due to loss of sea ice (HELCOM, 2021).

Incubation of the experiment lasted for two weeks and samples were taken at day 7 and day 14 of the experiment. For this, the overlaying water in each microcosm was removed with syringes, treated with acid Lugol’s solution and placed in clear flasks covered with parafilm to prevent evaporation. Samples were allowed to settle for 6 days, reduced to < 100 ml by siphoning off the water surface and the remaining part transferred to clear 100 ml PVC bottles for storage at 4 °C. At the end of the experiment, sediment from all microcosms was sieved through a 500 mm mesh for identification of benthic macrofauna to determine potential impacts through bioturbation or predation, which did not differ between treatments. All macrofauna retrieved was living, suggesting that the sediment remained well oxygenated throughout the experimental duration. 

Identification and quantification of phytoplankton cells followed the Utermöhl method (Edler and Elbrächter, 2010) with a sedimentation chamber of 100 ml. All samples were allowed to settle for 48 h according to standard guidance (HELCOM, 2017). Phytoplankton were identified to either class (dinoflagellates < 20 mm), order (dinoflagellates > 30 mm), genus (diatoms and cyanobacteria) or dinoflagellate cysts, using http://nordicmicroalgae.org/galleries/HELCOM-PEG (2021). Phytoplankton cell density was calculated per L and carbon content per m² for comparison with plankton monitoring data and flux estimations from benthic to pelagic systems, respectively. Carbon content are based on Olenina et al. (Olenina et al., 2006). Due to the relatively high taxonomic level of dinoflagellates and to avoid overestimation of C content, all C content calculations for cells > 30 mm, was based on a size class of cells with a size < 30 mm. Zooplankton were identified to genus level using Telesh et al. (2015). Nauplii biomass was calculated on naupliar stages I-III based on Gorokhova et al. (2016).