Modelling the impact of higher temperature on the phytoplankton of a boreal lake

We linked the models PROTECH and MyLake to test potential impacts of climate-changeinduced warming on the phytoplankton community of Pyhäjärvi, a lake in southwest Finland. First, we calibrated the models for the present conditions, which revealed an apparent high significance of internal nutrient loading for Pyhäjärvi. We then estimated the effect of two climate change scenarios on lake water temperatures and ice cover duration with MyLake. Finally, we used those outputs to drive PROTECH to predict the resultant phytoplankton community. It was evident that cyanobacteria will grow significantly better in warmer water, especially in the summer. Even if phosphorus and nitrogen loads to the lake remain the same and there is little change in the total chlorophyll a concentrations, a higher proportion of the phytoplankton community could be dominated by cyanobacteria. The model outputs provided no clear evidence that earlier ice break would advance the timing of the diatom spring bloom.peerReviewe

Spatial and seasonal variation in reproductive indices of the clupeids Limnothrissa miodon and Stolothrissa tanganicae in the Congolese waters of northern Lake Tanganyika

peer reviewedKnowledge on the reproductive biology of the endemic clupeids Limnothrissa miodon and Stolothrissa tanganicae, two main target species of the pelagic fisheries of Lake Tanganyika, is constrained by fragmented monitoring activities. Here, we investigate the nursing areas of L. miodon, the timing of reproductive activities of littoral and pelagic L. miodon, and the timing of reproductive activities of pelagic S. tanganicae in the Congolese waters of the northern end of Lake Tanganyika (Bujumbura sub-basin). Nursing areas were determined year-round (2009–2010) based on the presence of clupeid larvae at two sandy and two stony beaches. The gonadosomatic index (GSI) and the proportion of fish having ripe gonads were used to study variation in reproductive indices in space (littoral vs. pelagic zones) during one year (2013–2014), as well as in time (dry vs. rainy season) during three years (2013–2016). Larvae of L. miodon were more frequently encountered on sandy than on stony beaches. Mature L. miodon females were more abundant in the littoral than in the pelagic zone, while the proportion of mature males in both habitats was similar. Irregular, low amplitude peaks could be distinguished in the GSI and proportion of mature males and females, but averages only differed between the dry and the rainy season in males. In contrast, GSI and proportions of mature males and females in S. tanganicae were higher in the dry season than in the rainy season. The reproductive effort of males and females of S. tanganicae and littoral L. miodon, but not pelagic L. miodon, was strongly synchronized. Interestingly, reproductive investment was also synchronised between pelagic male L. miodon, and pelagic S. tanganicae. Our time series strongly supports the view that L. miodon reproduces year-round in the littoral zone, while reproduction in S. tanganicae is seasonal. For fisheries management, we recommend year-round protection of sandy beaches, which are the main breeding grounds for L. miodon. © 2022, Royal Belgian Institute of Natural Sciences. All rights reserved

Daphniaperformance on diets containing different combinations of high-quality algae, heterotrophic bacteria, and allochthonous particulate organic matter

Filter-feeding zooplankton in lakes feed on a mixture of phytoplankton, bacteria, and terrestrial particles and the proportions and nutritional value of these components can be highly variable. However, the extent to which food quality interacts with food quantity in affecting overall zooplankton performance is not yet fully resolved. Here we performed laboratory feeding experiments to test how the performance of the unselective filter feederDaphnia galeatawas affected if various quantities of high-quality food (the phytoplanktonRhodomonas) were diluted with low-quality food such as heterotrophic bacteria (Pseudomonas) or terrestrial detritus particles (t-POM) from the riparian zone of a boreal forest stream. We hypothesised: that increased proportions of bacteria and t-POM in the diet will lead to decreased survival, somatic growth; and reproduction ofDaphniadespite the presence of phytoplankton; that these effects are more pronounced for t-POM than for heterotrophic bacteria; and that this response is stronger when phytoplankton availability is low. Increasing the concentrations ofPseudomonasaffectedDaphniasurvival, growth, and reproduction negatively whenRhodomonaswas available at intermediate (0.37 mgC/L) and high (0.55 mgC/L) quantities. WhenRhodomonasquantity was low (0.22 mgC/L), the addition ofPseudomonasgenerally resulted in betterDaphniaperformance except at very high concentrations of the bacterium relative toRhodomonas. In contrast, the addition of t-POM was detrimental for overallDaphniaperformance at allRhodomonasconcentrations. Daphniaperformance was best described by a model including the interaction between food quality and quantity, with stronger negative effects onDaphniawhen high-quality food was supplemented with t-POM than withPseudomonas. The results indicate that the ability of zooplankton to use low-quality food is affected by the concurrent availability of high-quality food. Furthermore, food sources that can be used but do not fulfil dietary requirements of grazers (e.g. bacteria), may still provide nutritional benefits as long as other complementary food components are available in sufficient quantities to compensate for biochemical deficiencies. Therefore, we conclude that heterotrophic bacteria, but not peat layer t-POM, can be an important component of zooplankton diets in boreal lakes, especially if the concentration of phytoplankton is low

Recent ecological change in ancient lakes

Ancient lakes are among the best archivists of past environmental change, having experienced more than one full glacial cycle, a wide range of climatic conditions, tectonic events, and long association with human settlements. These lakes not only record long histories of environmental variation and human activity in their sediments, but also harbor very high levels of biodiversity and endemism. Yet, ancient lakes are faced with a familiar suite of anthropogenic threats, which may degrade the unusual properties that make them especially valuable to science and society. In all ancient lakes for which data exist, significant warming of surface waters has occurred, with a broad range of consequences. Eutrophication threatens both native species assemblages and regional economies reliant on clean surface water, fisheries, and tourism. Where sewage contributes nutrients and heavy metals, one can anticipate the occurrence of less understood emerging contaminants, such as pharmaceuticals, personal care products, and microplastics that negatively affect lake biota and water quality. Human populations continue to increase in most of the ancient lakes’ watersheds, which will exacerbate these concerns. Further, human alterations of hydrology, including those produced through climate change, have altered lake levels. Co‐occurring with these impacts have been intentional and unintentional species introductions, altering biodiversity. Given that the distinctive character of each ancient lake is strongly linked to age, there may be few options to remediate losses of species or other ecosystem damage associated with modern ecological change, heightening the imperative for understanding these systems

Occurrence of two-year cyclicity, "saw-blade fluctuation", in vendace populations in Finland

The tendency towards two-year cyclicity is considered typical of many Fennoscandian vendace populations, especially in fluctuation of recruitment, based on time series of individual lakes. We used two robust indicators to identify and quantify two-year cycles in vendace population proxy time series at different life-stages - spawning stock biomass (SB), density of newly hatched larvae (LD) and recruitment (REC) - from 22 Finnish lakes. Then we applied Fisher's meta-analytical test to assess the adequacy of the evidence to support the hypothesis that vendace population dynamics include two-year cyclicity. The results supported this hypothesis for REC but not for SB or LD. Yet. the indicators and test are conservative and time-series of SB and LD are shorter than those for REC. The appearance of cycles in REC is associated with high post-recruitment mortality, consequently practically only one spawning per cohort. Cycles may be typical for the recovery period from low abundance period also. Still, some populations with moderate post-REC mortality and non-cyclic SB abundance exhibited cycles in REC. Such dynamics presuppose the existence of more complex regulation based on the interaction of different life stages

Trophic structure of Lake Tanganyika: Carbon flows in the pelagic food web

peer reviewedThe sources of carbon for the pelagic fish production in Lake Tanganyika, East Africa, were evaluated in a comprehensive multi-year study. Phytoplankton production was assessed from seasonal in situ 14C and simulated in situ results, using on-board incubator measurements and knowledge of the vertical distributions of chlorophyll and irradiance. Bacterioplankton production was measured on two cruises with the leucine incorporation method. Zooplankton production was calculated from seasonal population samples, the carbon contents of different developmental stages and growth rates derived from published sources. Fish production estimates were based on hydroacoustic assessment of pelagic fish biomass and data on growth rates obtained from length frequency analyses and checked against daily increment rings of fish otoliths. Estimates for primary production (426-662 g C m-2 a-1) were 47-128% higher than previously published values. Bacterioplankton production amounted to about 20% of the primary production. Zooplankton biomass (1 g C m-2) and production (23 g C m-2 a-1) were 50% lower than earlier reported, suggesting that the carbon transfer efficiency from phytoplankton to zooplankton was low, in contrast to earlier speculations. Planktivorous fish biomass (0.4 g C m-2) and production (1.4-1.7 g C m-2 a-1) likewise indicated a low carbon transfer efficiency from zooplankton into planktivorous fish production. Relatively low transfer efficiencies are not unexpected in a deep tropical lake, because of the generally high metabolic losses due to the high temperatures and presumably high costs of predator avoidance. The total fisheries yield in Lake Tanganyika in the mid- 1990s was 0.08-0.14% of pelagic primary production, i.e. within the range of typical values in lakes. Thus, no special mechanisms need be invoked to explain the productivity of fisheries in Lake Tanganyika

Impacts of climate warming on the long-term dynamics of key fish species in 24 European lakes

Fish play a key role in the trophic dynamics of lakes. With climate warming, complex changes in fish assemblage structure may be expected owing to direct effects of temperature and indirect effects operating through eutrophication, water level changes, stratification and salinisation. We reviewed published and new long-term (10-100 years) fish data series from 24 European lakes (area: 0.04-5,648 km(2); mean depth: 1-177 m; a north-south gradient from Sweden to Spain). Along with an annual temperature increase of about 0.15-0.3A degrees C per decade profound changes have occurred in either fish assemblage composition, body size and/or age structure during recent decades and a shift towards higher dominance of eurythermal species. These shifts have occurred despite a reduction in nutrient loading in many of the lakes that should have benefited the larger-sized individuals and the fish species typically inhabiting cold-water, low-nutrient lakes. The cold-stenothermic Arctic charr has been particularly affected and its abundance has decreased in the majority of the lakes where its presence was recorded. The harvest of cool-stenothermal brown trout has decreased substantially in two southern lakes. Vendace, whitefish and smelt show a different response depending on lake depth and latitude. Perch has apparently been stimulated in the north, with stronger year classes in warm years, but its abundance has declined in the southern Lake Maggiore, Italy. Where introduced, roach seems to take advantage of the higher temperature after years of low population densities. Eurythermal species such as common bream, pike-perch and/or shad are apparently on the increase in several of the lakes. The response of fish to the warming has been surprisingly strong and fast in recent decades, making them ideal sentinels for detecting and documenting climate-induced modifications of freshwater ecosystems

Global data set of long-term summertime vertical temperature profiles in 153 lakes

peer reviewedClimate change and other anthropogenic stressors have led to long-term changes in the thermal structure, including surface temperatures, deepwater temperatures, and vertical thermal gradients, in many lakes around the world. Though many studies highlight warming of surface water temperatures in lakes worldwide, less is known about long-term trends in full vertical thermal structure and deepwater temperatures, which have been changing less consistently in both direction and magnitude. Here, we present a globally-expansive data set of summertime in-situ vertical temperature profiles from 153 lakes, with one time series beginning as early as 1894. We also compiled lake geographic, morphometric, and water quality variables that can influence vertical thermal structure through a variety of potential mechanisms in these lakes. These long-term time series of vertical temperature profiles and corresponding lake characteristics serve as valuable data to help understand changes and drivers of lake thermal structure in a time of rapid global and ecological change. © 2021, The Author(s)