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1. The biomanipulation of Lake Vesij€ arvi in southern Finland putatively changed the lake ecosystem resulting in a clear-water state, but the dynamics behind the observed change remained unverified. 2. We used a general analytical approach to identify abrupt transitions in the long-term monitoring data of Lake Vesij€ arvi. Methods included the identification of breakpoints in chlorophyll a (response variable), in total phosphorus (TP; key driver) and in large-bodied cladocerans (feedback mechanism of biomanipulation). In addition, we characterised the frequency distribution and temporal variance of the response variable, as well as the interaction between TP and chlorophyll a. 3. According to long-term records, biomanipulation resulted in two distinct states: (i) a eutrophic state, where chlorophyll a and TP were linearly correlated and (ii) a mesotrophic state, where the key driver and response variables were not significantly correlated. The changed relationship between the key driver and response variables, together with the increase in the body size of cladocerans, suggested a hysteresis-type transition phase. However, the frequency distribution and the temporal variance of chlorophyll a agreed also with a threshold-type of transition. 4. Zooplankton seemed to be more important in maintaining the clear-water state than previously reported. Our results highlight the role of long-term monitoring data and the characterisation of abrupt transitions in ecosystem interactions as tools for sustainable management of lake ecosystems.

The effectiveness of different monitoring methods in detecting temporal changes in water quality depends on the achievable sampling intervals, and how these relate to the extent of temporal variation. However, water quality sampling frequencies are rarely adjusted to the actual variation of the monitoring area. Manual sampling, for example, is often limited by the level of funding and not by the optimal timing to take samples. Restrictions in monitoring methods therefore often determine their ability to estimate the true mean and variance values for a certain time period or season. Consequently, we estimated how different sampling intervals determine the mean and standard deviation in a specific monitoring area by using high frequency data from in situ automated monitoring stations. Raw fluorescence measurements of chlorophyll a for three automated monitoring stations were calibrated by using phycocyanin fluorescence measurements and chlorophyll a analyzed from manual water samples in a laboratory. A moving block bootstrap simulation was then used to estimate the standard errors of the mean and standard deviations for different sample sizes. Our results showed that in a temperate, meso-eutrophic lake, relatively high errors in seasonal statistics can be expected from monthly sampling. Moreover, weekly sampling yielded relatively small accuracy benefits compared to a fortnightly sampling. The presented method for temporal representation analysis can be used as a tool in sampling design by adjusting the sampling interval to suit the actual temporal variation in the monitoring area, in addition to being used for estimating the usefulness of previously collected data.

The body size of aquatic invertebrates is, to a great extent, dependent on ambient temperature, but size distributions are also determined by other factors like food supply and predation. The effect of temperature on organisms is formulated in the temperature–size hypothesis, which predicts a smaller body size with increasing temperature. In this study, the effect of temperature on the subfossil remains of three littoral Cladocera (Alona affnis, A. quadrangularis, and Chydorus cf. sphaericus) was investigated. Exoskeletal remains of these species can be found in large numbers in lacustrine sediments and over a wide north–south range in Europe. The total length of both headshield and postabdomen for A. affinis and A. quadrangularis and carapace length for C. cf. sphaericus were measured to observe their response to changes in latitude and temperature. A different response to ambient temperature in the growth of body parts was observed. The size of the headshields of both Alona species and of the carapace of Chydorus was significantly larger in colder regions as opposed to warm ones. It turned out that the postabdomen was not a good predictor of ambient temperature. While the sizes of all remains increased with latitude, the sizes of the Alona remains was smaller in the mountain lakes of the Southern Carpathians than in other cold lakes, in this case in Finland, a fact indicative of the importance of other factors on size distribution. This study demonstrates that a morphological response to climate is present in littoral cladocerans, and, therefore, changes in the length of headshield and carapace may be used as a proxy for climate changes in paleolimnological records.