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Water transparency affects the thermal structure of lakes, and within certain lake depth ranges, it can determine whether a lake mixes regularly (polymictic regime) or stratifies continuously (dimictic regime) from spring through summer. Phytoplankton biomass can influence transparency but the effect of its seasonal pattern on stratification is unknown. Therefore we analysed long term field data from two lakes of similar depth, transparency and climate but one polymictic and one dimictic, and simulated a conceptual lake with a hydrodynamic model. Transparency in the study lakes was typically low during spring and summer blooms and high in between during the clear water phase (CWP), caused when zooplankton graze the spring bloom. The effect of variability of transparency on thermal structure was stronger at intermediate transparency and stronger during a critical window in spring when the rate of lake warming is highest. Whereas the spring bloom strengthened stratification in spring, the CWP weakened it in summer. The presence or absence of the CWP influenced stratification duration and under some conditions determined the mixing regime. Therefore seasonal plankton dynamics, including biotic interactions that suppress the CWP, can influence lake temperatures, stratification duration, and potentially also the mixing regime.

Background: Mortality is a main driver in zooplankton population biology but it is poorly constrained in models that describe zooplankton population dynamics, food web interactions and nutrient dynamics. Mortality due to non-predation factors is often ignored even though anecdotal evidence of non-predation mass mortality of zooplankton has been reported repeatedly. One way to estimate non-predation mortality rate is to measure the removal rate of carcasses, for which sinking is the primary removal mechanism especially in quiescent shallow water bodies., Objectives and Results: We used sediment traps to quantify in situ carcass sinking velocity and non-predation mortality rate on eight consecutive days in 2013 for the cladoceran Bosmina longirostris in the oligo-mesotrophic Lake Stechlin; the outcomes were compared against estimates derived from in vitro carcass sinking velocity measurements and an empirical model correcting in vitro sinking velocity for turbulence resuspension and microbial decomposition of carcasses. Our results show that the latter two approaches produced unrealistically high mortality rates of 0.58-1.04 d(-1), whereas the sediment trap approach, when used properly, yielded a mortality rate estimate of 0.015 d(-1), which is more consistent with concurrent population abundance data and comparable to physiological death rate from the literature., Ecological Implications: Zooplankton carcasses may be exposed to water column microbes for days before entering the benthos; therefore, non-predation mortality affects not only zooplankton population dynamics but also microbial and benthic food webs. This would be particularly important for carbon and nitrogen cycles in systems where recurring mid-summer decline of zooplankton population due to non-predation mortality is observed.

The contribution of dead zooplankton biomass to carbon cycle in aquatic ecosystems is practically unknown. Using abundance data of zooplankton in water column and dead zooplankton in sediment traps in Lake Stechlin, an ecological-mathematical model was developed to simulate the abundance and sinking of zooplankton carcasses and predict the related release of labile organic matter (LOM) into the water column. We found species-specific differences in mortality rate of the dominant zooplankton: Daphnia cucullata, Bosmina coregoni and Diaphanosoma brachyurum (0.008, 0.129 and 0.020 day−1, respectively) and differences in their carcass sinking velocities in metalimnion (and hypolimnion): 2.1 (7.64), 14.0 (19.5) and 1.1 (5.9) m day−1, respectively. Our model simulating formation and degradation processes of dead zooplankton predicted a bimodal distribution of the released LOM: epilimnic and metalimnic peaks of comparable intensity, ca. 1 mg DW m−3 day−1. Maximum degradation of carcasses up to ca. 1.7 mg DW m−3 day−1 occurred in the density gradient zone of metalimnion. LOM released from zooplankton carcasses into the surrounding water may stimulate microbial activity and facilitate microbial degradation of more refractory organic matter; therefore, dead zooplankton are expected to be an integral part of water column carbon source/sink dynamics in stratified lakes. [ABSTRACT FROM AUTHOR]

Zooplankton populations can at times suffer mass mortality due to non-predatory mortality (NPM) factors, and the resulting carcasses can be captured by sediment traps to estimate NPM rate. This approach assumes sinking to be the primary process in removing carcasses, but in reality, carcasses can also be removed by ingestion, turbulent mixing and microbial degradation in the water column. We present mathematical formulations to calculate NPM from sediment trap data by accounting for carcass removal by processes in addition to sinking, and demonstrate their application in a study in Lake Shira, Russia. Carcass abundance of the major calanoid copepod Arctodiaptomus salinus decreased with depth, indicating the effect of carcass removal from the water column. The estimated NPM values (0.0003-0.103 d-1) were comparable with previously reported physiological death rates. We further used independent data to partition carcass removal due to detritivory, turbulent mixing and microbial degradation. Estimated ingestion by the amphipod Gammarus lacustris could account for the disappearance of copepod carcasses above the traps. Wind-driven turbulence could also extend the carcass exposure time to microbial degradation. Collectively, these water column processes would facilitate the remineralization of carcasses in the water column, and diminish the carcass carbon flux to the benthos. [ABSTRACT FROM AUTHOR]

Zooplankton carcasses are ubiquitous in marine and freshwater systems, implicating the importance of non-predatory mortality, but both are often overlooked in ecological studies compared with predatory mortality. The development of several microscopic methods allows the distinction between live and dead zooplankton in field samples, and the reported percentages of dead zooplankton average 11.6 (minimum) to 59.8 (maximum) in marine environments, and 7.4 (minimum) to 47.6 (maximum) in fresh and inland waters. Common causes of non-predatory mortality among zooplankton include senescence, temperature change, physical and chemical stresses, parasitism and food-related factors. Carcasses resulting from non-predatory mortality may undergo decomposition leading to an increase in microbial production and a shift in microbial composition in the water column. Alternatively, sinking carcasses may contribute significantly to vertical carbon flux especially outside the phytoplankton growth seasons, and become a food source for the benthos. Global climate change is already altering freshwater ecosystems on multiple levels, and likely will have significant positive or negative effects on zooplankton non-predatory mortality. Better spatial and temporal studies of zooplankton carcasses and non-predatory mortality rates will improve our understanding of this important but under-appreciated topic. [ABSTRACT FROM AUTHOR]

Understanding the fate of organic carbon (OC) inputs in aquatic ecosystems is of primaryimportance for comprehending their functioning, in particular in relation to their carboncycle. A large part of terrigenous OC, coming from the watershed, enters aquatic ecosystemsin the form of refractory organic matter (ROM), which are difficult to oxidise. Recent studiesin the literature have suggested, however, that ROM mineralisation rates may be enhanced inpresence of labile organic matter (LOM), making ROM compounds easier to degrade, with aconsequent alteration of the carbon budget in those systems. This process refers to as primingeffect. In lakes, zooplankton carcasses represent an abundant and high quality LOM substrate formicroorganisms that has the potential of priming ROM mineralisation in water column andcreating new and overlooked pathways in the lake carbon cycle. To investigate this, it iscrucial to measure the in-situ sedimentation rate and residence time of dead zooplankton, as afunction of hydrodynamic conditions, microbial decomposition and organisms’characteristics (such as size and density). Current velocimetry acoustic methods cannot measure, however, velocities of singlemillimetre-sized carcasses because particles are too small and measurements are not accuratedue to the low correlation of the acoustic pings. To overcome this, we propose an inexpensiveparticle tracking velocimetry (PTV) system to estimate the instantaneous 3D velocity ofparticles in the field. From the data it is possible to measure sinking rates of deadzooplankton and potentially infer carbon fluxes in lakes in relation to the primingeffect. The PTV rig consists of two GoPro cameras arranged in a frontal parallel arrangementand installed in a waterproof case. The case is mounted on a stainless-steel frame with alaminate blank screen placed in front of the cameras to restrict the system sampling volume.A torch was also attached on the frame to enhance lighting underwater conditions. Thecameras are set up to acquire stereoscopic films of sinking particles at 48 frames per secondand calibrated in a calibration tank to estimate their distortion and rectification parameters. Asoftware package was also developed with new stereo algorithms to sync the videos,detect and track carcasses, as they move in the water, and measure their velocity andlength. In this study, we also present early results of sedimentation rates from the PTV system,estimated from 2 deployments in summer 2018 in the mesocosm of Lake Stechlin (Germany)at 7m. Finally, the settling rates are compared with (1) those inferred from the Stokes’ lawand (2) the daily settling rates estimated from sedimentation traps installed in the mesocosmat the same depth. [ABSTRACT FROM AUTHOR]

Using the carcass sinking rate and density determined in laboratory for several freshwater zooplankton species, we developed a model of zooplankton carcass sinking as affected by turbulence and stratification. The model was subsequently used to estimate the residence time of zooplankton carcasses in the water column of Lake Stechlin, a typical temperate dimictic lake in northeastern Germany. The residence time varied among the different species and was strongly affected by thermal stratification. At the peak of summer stratification, the carcasses stayed up to 5d in the 70m-deep water column before reaching the lake bottom. Residence time was long enough that zooplankton carcasses could serve as an important matter and energy source for bacteria in the lake's pelagic zone and hence have the potential to significantly affect aquatic carbon and nutrient cycling. The proposed model of sinking rates, based on physically sound relationships, can be easily applied to other passively sinking particles, and be integrated into large ecosystem models. [ABSTRACT FROM AUTHOR]