Temperate Shallow Lakes Research Articles

Connectivity of temperate shallow lakes to groundwater in the Pampean Plain, Argentina: A remote sensing and multi-tracer approach

The Pampean Plain in Argentina is one of the most extended plain regions of the world with numerous wetland systems and very shallow lakes. Certain anomalies related to geomorphology and shallow lakes’ hydrology are particularly present in a sector of the southern Pampean Plain, specifically, in the southwestern limit of the Quequén Grande River Basin. Here, shallow lakes can be defined as upland-embedded wetlands, characterized by the lack of observable hydrological connectivity due to the absence of surface water flows. The aim of this study was to identify groundwater connectivity of Pampean shallow lakes considered as upland-embedded wetlands and classify them according to their water presence frequency over time and their water source origin based on natural tracers (18O–H2O, 2H–H2O, electrical conductivity and Cl− content). In this work, a methodological framework involving a multitemporal analysis of the Normalized Difference Vegetation Index (NDVI) and the Normalized Difference Water Index (NDWI) derived from Landsat data, in addition to field data is proposed. According to water presence frequency, shallow lakes were classified in four categories: occasionally (89.0 %), regularly (6.7 %), usually (1.9 %) and permanently (2.3 %) with water. Obtained results allowed identifying shallow lakes having low, moderate or high mineralized water due to both different evaporation degree and water sources.

Winter Decomposition of Emergent Macrophytes Affects Water Quality under Ice in a Temperate Shallow Lake

Decomposition of emergent macrophytes is now recognized as an internal nutrient source for shallow lakes. Temperate lakes always experience seasonal ice cover in winter, but the influences of emergent macrophytes decomposition on water quality have rarely been examined under ice. Here, we conducted an incubation experiment to investigate winter decomposition of two common emergent macrophytes species (Typha orientalis and Phragmites australis) and its influences on water quality in the Hengshui Lake, North China. Mesocosms simulating a lake ice regime were incubated in the field for 120 days in winter and were treated with and without plant material addition. Water quality was monitored through dissolved oxygen (DO), dissolved organic carbon (DOC), total nitrogen (TN), total phosphorus (TP), ammonium nitrogen (NH4-N), and nitrate nitrogen (NO3-N). We found that both species were significantly decomposed in winter and that the majority of mass loss occurred in the first 10 days of decomposition when the water surface of mesocosms were already frozen. The concentrations of DO rapidly dropped to values close to zero after plant material submergence. At the end of incubation, the concentrations of DOC, TN, and NO3-N in the mesocosms with plant material addition were significantly higher than initial concentrations. In contrast, the concentrations of DOC, TN, TP, NO3-N, and NH4-N in the mesocosms without plant material addition were equal to or less than initial concentrations. Our research suggests that winter decomposition of emergent macrophytes produces negative influences on water quality under ice that lasts for the whole winter.

Short-term decisions in lake restoration have long-term consequences for water quality

Ecological regime shifts from clear to turbid water states in shallow temperate lakes are quite well-investigated phenomena but critical time lags from human interaction with the lake and restoration activities are much less understood. This is a complex challenge for institutions who manage lakes but are usually less familiar with non-linear dynamics, slow and fast influences on water quality and how to manage those from a social-ecological perspective. We extend a well-known minimal model of shallow lake regime shifts to enable simulations over time with short- and long-term management measures (nutrient reduction, trawling, planting of aquatic vegetation). While we explore the mathematical conditions for ecological bistability, we also identify the necessary and sufficient extent of measures to restore the clear water state. Restoration scenarios evaluated by trajectories in the state space demonstrate the increased effectiveness from combined measures even when considering countereffective activities such as pike fishing. But, single measures alone may delay or even miss the overall restoration target. Our analysis demonstrates the importance of understanding transient dynamics where stable state analyses alone remain elusive about alternative ways to interact with bistability. We conclude that successful management of bistable systems, and particularly shallow temperate lakes, needs careful balancing between short-term improvements and long-term influence on the systems state.

Water exchange processes estimation in a temperate shallow lake based on water stable isotope analysis

ABSTRACT The estimation of lake hydrological characteristics such as evaporation/inflow ratios and water residence time becomes necessary for understanding cycling and potential retention of natural and anthropogenic substances into the lake. The aim of this work was to estimate the evaporation loss and the water residence time of a temperate shallow lake based on the water isotope mass balance approach. One representative freshwater temperate shallow lake from the Argentinian Pampa Plain was selected. Groundwater, lake and stream samples (N = 56) were collected for δ 2H-H2O and δ 18O-H2O determinations. Moreover, water level fluctuations of the lake and its inflow stream were recorded with data loggers. Both the δ 2H and δ 18O relationship and d-excess of lake water indicated evaporation. Water isotopes and daily stream flow data recorded in the inflow stream evidenced groundwater contribution to Los Padres Stream. Monthly evaporation as a fraction of inflow estimations of the lake water indicated that about 20–25 ± 12% lake water was lost through lake surface evaporation and revealed that hydrologic balances were regulated mainly by changes in water inflow rather than evaporation. A mean residence time of 1.11 ± 0.65 year was also calculated. This lake hydrological information is relevant for the region and crucial to improve water management plans.

Long-term effects of fish biomanipulation and macrophyte management on zooplankton functional diversity and production in a temperate shallow lake

Eutrophication still is one of the greatest anthropogenic impacts affecting aquatic ecosystems. Food web manipulations to reverse eutrophic conditions have been applied, mainly by reducing fish stocks. However, the long-term results of biomanipulation are still ambiguous. The objective of this study was to evaluate biomanipulation effects on zooplankton functional diversity and production during 24 years of monitoring a shallow temperate lake. Temporal variation in crustacean biomass (CB), zooplankton resource use efficiency (RUE), limnological variables, functional diversity indices and functional traits were assessed using the Mann–Kendall test. The effects of the limnological and functional variables on zooplankton production (CB and RUE) were analyzed by generalized least squares models (GLSs). We found decreasing trends for almost all the limnological variables, indicating a reduction in lake eutrophic conditions. We also observed a decreasing trend in functional traits related to pelagic habitat, filter-feeding mode, herbivore feeding preference and body size. The GLS results showed that functional dispersion and chlorophyll a are important determinants of zooplankton production. The effects of fish biomanipulation produced no significant temporal trends in zooplankton biomass. In fact, the increase in submerged macrophytes promoted littoral food webs, which was likely the main determining factor of the changes in zooplankton functional traits and production.

Mutual Facilitation Among Invading Nuttall's Waterweed and Quagga Mussels.

Nuttall’s waterweed (Elodea nuttallii) is the most abundant invasive aquatic plant species in several European countries. Elodea populations often follow a boom-bust cycle, but the causes and consequences of this dynamics are yet unknown. We hypothesize that both boom and bust periods can be affected by dreissenid mussel invasions. While mutual facilitations between these invaders could explain their rapid parallel expansion, subsequent competition for space might occur. To test this hypothesis, we use data on temporal changes in the water quality and the abundance of E. nuttallii and the quagga mussel Dreissena rostriformis bugensis in a temperate shallow lake. Lake Müggelsee (Germany) was turbid and devoid of submerged macrophytes for 20 years (1970–1989), but re-colonization with macrophytes started in 1990 upon reductions in nutrient loading. We mapped macrophyte abundance from 1999 and mussel abundance from 2011 onwards. E. nuttallii was first detected in 2011, spread rapidly, and was the most abundant macrophyte species by 2017. Native macrophyte species were not replaced, but spread more slowly, resulting in an overall increase in macrophyte coverage to 25% of the lake surface. The increased abundance of E. nuttallii was paralleled by increasing water clarity and decreasing total phosphorus concentrations in the water. These changes were attributed to a rapid invasion by quagga mussels in 2012. In 2017, they covered about one-third of the lake area, with mean abundances of 3,600 mussels m−2, filtering up to twice the lake’s volume every day. The increasing light availability in deeper littoral areas supported the rapid spread of waterweed, while in turn waterweed provided surface for mussel colonization. Quantities of dreissenid mussels and E. nuttallii measured at 24 locations were significantly correlated in 2016, and yearly means of E. nuttallii quantities increased with increasing mean dreissenid mussel quantities between 2011 and 2018. In 2018, both E. nuttallii and dreissenid abundances declined. These data imply that invasive waterweed and quagga mussels initially facilitated their establishment, supporting the invasional meltdown hypothesis, while subsequently competition for space may have occurred. Such temporal changes in invasive species interaction might contribute to the boom-bust dynamics that have been observed in Elodea populations.

Hydrological variability, zooplankton availability and the shift between planktivore-benthivore feeding behaviour in the visual predator fish, Odontesthes bonariensis

Hydrology is a key factor in the dynamic of zooplankton community in lakes with high water renewal rate. Crustaceans’ zooplankton biomass can sharply decline during flooding periods as consequence of wash-out effect. The hydrologically induced changes in zooplankton community may certainly affect the feeding behaviour of planktivorous fishes, but this has not been studied yet. The inland silverside, Odontesthes bonariensis (Valenciennes, 1835), is the main visual planktivorous fish in temperate shallow lakes of South America. We analyzed the diet composition and feeding strategy of O. bonariensis in a large run-of-the-river shallow lake, along two consecutive spring-summer periods with contrasting hydrological conditions. Water residence time varied one order of magnitude between flood and drought period. As it was expected, silversides showed size selective feeding on large cladocerans during drought period when zooplankton biomass was high. On the other hand, during high flushing period, zooplankton abundance decreased noticeable and silversides drastically changes its feeding behaviour from visual planktivore to benthivore. The analysis of the feeding strategy of O. bonariensis showed that this species largely behaves as a specialist irrespective of the hydrological conditions. However, the main involved prey varied from large Daphnia during low flow conditions to large Ostracoda during floods. Our study reveals that the variation in the availability of suitable prey induced by hydrological variability is associated with a drastic change in feeding grounds and prey selection of O. bonariensis. This flexible predatory behaviour may have consequences on food web dynamics of shallow lakes that are further discussed.

Water temperature dynamics and the prevalence of daytime stratification in small temperate shallow lakes

Small lakes are understudied compared to medium- and large-sized lakes, but have recently received increased attention due to their abundance and importance for global scale biogeochemical cycles. They have close terrestrial contact, extensive environmental variability, and support high biodiversity among them. Temporal and spatial variability of water temperature, oxygen, and stratification–mixing dynamics were examined during a year in nine small Danish lakes. We found that diel mean surface water temperatures were similar among lakes while the diel range decreased with increasing water depth. Vertical temperature stratification occurred on 47% of the days during the entire year and 64% of summer days, usually with daytime stratification and nocturnal convective mixing. The probability of daytime stratification increased with higher incident irradiance, higher air temperature, and lower wind speed. During spring, daytime stratification caused differences in oxygen saturation between surface and bottom waters. These findings offer new insights on the high variability of water temperature and oxygen in time and space in small temperate shallow lakes. The variable water temperature and the regular stratification–mixing processes will have a pronounced influence on biogeochemical cycles. Also, these features are expected to affect the performance and evolutionary process of organisms associated with small lakes.

Impact of lake‐catchment processes on phytoplankton community structure in temperate shallow lakes

AbstractResearch on lake‐catchment and biological processes is still innovative. Few reports on this topic exist in the scientific literature. This paper presents species richness, diversity, biomass composition of phytoplankton, and occurrence of cyanobacterial blooms in relation to complex lake‐catchment processes in four shallow lakes. The results revealed that phytoplankton species richness increased with increasing lake trophic state, whereas the opposite relationship was observed for phytoplankton biodiversity. In less eutrophic and deeper lakes, the majority of dominant taxa were representatives of mixed, meso to eutrophic small‐ and medium‐sized lakes. In highly eutrophic lakes, the dominant taxa were common representatives of shallow, very rich in nutrients, turbid, mix layers, and eu‐hypertrophic environments. Redundancy analysis showed that lake depth, flushing time, and conductivity were the most significant factors for phytoplankton development and composition. The hypertrophic, polimictic, and flow‐through Lake Syczyńskie was the most dynamic ecosystem among the lakes. Water mixing and high concentrations of P‐PO4 were beneficial for heavy blooms of the cyanobacterium Planktothrix agardhii observed in Lake Syczyńskie despite the high rate of water exchange. Contrary to the findings of some previous studies, P. agardhii seems to be resistant to flushing at very high loads and concentrations of nutrients (mostly of P‐PO4 and N‐NO3). Phytoplankton assemblages reflected the lakes' habitats, trophic status, and lake‐catchment processes and revealed complex factors that influenced the lakes' functioning. The use of phytoplankton as an indicator of lake‐catchment processes may help understand ecosystem dynamics, essential for the proper selection of management practices protecting aquatic systems against eutrophication.

Fish-mediated plankton responses to increased temperature in subtropical aquatic mesocosm ecosystems: Implications for lake management

Although it is well established that climate warming can reinforce eutrophication in shallow lakes by altering top-down and bottom-up processes in the food web and biogeochemical cycling, recent studies in temperate zones have also shown that adverse effects of rising temperature are diminished in fishless systems. Whereas the removal of zooplanktivorous fish may be useful in attempts to mitigate eutrophication in temperate shallow lakes, it is uncertain whether similar mitigation might be achieved in warmer climates. We compared the responses of zooplankton and phytoplankton communities to climate warming in the presence and absence of fish (Aristichthys nobilis) in a 4-month mesocosm experiment at subtropical temperatures. We hypothesized that 1) fish and phytoplankton would benefit from warming, while zooplankton would suffer in fish-present mesocosms and 2) warming would favor zooplankton growth but reduce phytoplankton biomass in fish-absent mesocosms. Our results showed significant interacting effects of warming and fish presence on both phytoplankton and zooplankton. In mesocosms with fish, biomasses of fish and phytoplankton increased in heated treatments, while biomasses of Daphnia and total zooplankton declined. Warming reduced the proportion of large Daphnia in total zooplankton biomass, and reduced the zooplankton to phytoplankton biomass ratio, but increased the ratio of chlorophyll a to total phosphorus, indicating a relaxation of zooplankton grazing pressure on phytoplankton. Meanwhile, warming resulted in a 3-fold increase in TP concentrations in the mesocosms with fish present. The results suggest that climate warming has the potential to boost eutrophication in shallow lakes via both top-down (loss of herbivores) and bottom-up (elevated nutrient) effects. However, in the mesocosms without fish, there was no decline in large Daphnia or in total zooplankton biomass, supporting the conclusion that fish predation is the major driver of low large Daphnia abundance in warm lakes. In the fishless mesocosms, phytoplankton biomass and nutrient levels were not affected by temperature. Our study suggests that removing fish to mitigate warming effects on eutrophication may be potentially beneficial in subtropical lakes, though the rapid recruitment of fish in such lakes may present a challenge to success in the long-term.

Assessing macrophyte seasonal dynamics using dense time series of medium resolution satellite data

The improved spatial and temporal resolution of latest-generation Earth Observation missions, such as Landsat 8 and Sentinel-2, has increased the potential of remote sensing for mapping land surface phenology in inland water systems.The ability of a time series of medium-resolution satellite data to generate quantitative information on macrophyte phenology was examined, focusing on three temperate shallow lakes with connected wetlands in Italy, France, and Romania.Leaf area index (LAI) maps for floating and emergent macrophyte growth forms were derived from a semi-empirical regression model based on the best-performing spectral index, with an error level of 0.11 m2 m−2. Phenology metrics were computed from LAI time series using TIMESAT to analyze the seasonal dynamics of macrophyte spatial distribution patterns and species-dependent variability. Particular seasonal patterns seen in the autochthonous and allochthonous species across the three study areas related to local ecological and hydrological conditions.How characteristics of the satellite dataset (cloud cover threshold, temporal resolution, and missing acquisitions) influenced the phenology metrics obtained was also assessed. Our results indicate that, with a full-resolution time series (5-day revisit time), cloud cover introduced a bias in the phenology metrics of less than 2 days. Even when the temporal resolution was reduced to 15 days (like the Landsat revisit time) the timing of the start and the peak of macrophyte growth could still be mapped with an error of no more than 2–3 days.

Phytoplankton Functional Groups Variation and Influencing Factors in a Shallow Temperate Lake.

The present study was carried out in Luoma Lake, a shallow lake in temperate eastern China. Based on a two-year study, the dynamics of phytoplankton functional groups and influencing factors were analyzed. A total of 178 taxa were identified and sorted into 20 codons, according to the phytoplankton functional group classification. In order to find the environmental factors driving phytoplankton variations, fifteen groups were analyzed in detail using redundancy analysis. Groups P (Fragilaria crotonensis), X2 (Chlamydomonas globosa, C. microsphaera and Chroomonas acuta), and MP (Navicula rotaeana) were dominant during low temperature periods, whereas groups X2, S1 (Pseudanabaena limnetica), and W1 (Euglena sp.) were dominant during high temperature periods. Water temperature, total phosphorus, and ammonium were the significant driving factors explaining phytoplankton succession. Furthermore, total phosphorus and ammonium could be broadly used in risk management for potential algal blooms in Luoma Lake.

Hypoxia within macrophyte vegetation limits the use of methane-derived carbon by larval chironomids in a shallow temperate eutrophic lake

Methane-derived carbon (MDC) can subsidize lake food webs. However, the trophic transfer of MDC to consumers within macrophyte vegetation is largely unknown. We investigated the seasonality of δ13C in larval chironomids within Nelumbo nucifera (Gaertn.) and Trapa natans var. Japonica (Nakai) vegetation in the shallow, eutrophic Lake Izunuma in Japan. Over the past several years, N. nucifera has rapidly expanded across more than 80% of the lake surface. Prior to the expansion of N. nucifera (2007–2008), a previous study reported extremely low larval δ13C levels with peak sediment methane concentrations in August or September. After the expansion of N. nucifera (2014–2015), we observed extreme hypoxia as low as or lower than 1 mg l−1 among the macrophyte coverage during June and August. During August and September, no larvae could be found among N. nucifera, and larvae in T. natans showed relatively high δ13C levels (> − 40‰). In contrast, larvae were markedly 13C–depleted (down to − 60‰) during October and November. The renewed supply of oxygen to the lake bottom may stimulate MOB activity, leading to an increase in larval assimilation of MDC. Our results suggest that macrophyte vegetation can affect the seasonality of MDC transfer to benthic consumers under hypoxic conditions in summer.

Lake size, macrophytes, and omnivory contribute to food web linkage in temperate shallow eutrophic lakes

Physical, chemical and biological processes facilitate cross-habitat connections in lakes, prompting food webs to be supported by different subsidies. We tested the hypothesis that the pelagic food web is subsidized by littoral resources and fish foraging behaviour plays a major role in carbon flux and on food web structure in shallow hypereutrophic lakes. We performed a fish diet and carbon and nitrogen isotope analyses to predict the linkage between littoral and pelagic habitats in three shallow hypereutrophic lakes. Lakes differed in morphology, fetch, macrophyte composition and width of the littoral zone. δ13C signals of seston differed among lakes, but were similar to other producers. Macroinvertebrates and fish carbon signatures were more enriched in the lake co-dominated by emergent and submerged vegetation. Fish foraging behaviour indicates that more than the 80% of the carbon that sustain adult fish was channelled from the littoral. In conclusion, littoral carbon were relevant and sustain, in part, food web in these shallow lakes. Factors like the extension of the littoral zone, lake morphometry, and the dominance of multi-chain omnivorous fish facilitate the connection among lake compartments and the transference of littoral carbon to lake food web.

Response of Submerged Macrophyte Communities to External and Internal Restoration Measures in North Temperate Shallow Lakes.

Submerged macrophytes play a key role in north temperate shallow lakes by stabilizing clear-water conditions. Eutrophication has resulted in macrophyte loss and shifts to turbid conditions in many lakes. Considerable efforts have been devoted to shallow lake restoration in many countries, but long-term success depends on a stable recovery of submerged macrophytes. However, recovery patterns vary widely and remain to be fully understood. We hypothesize that reduced external nutrient loading leads to an intermediate recovery state with clear spring and turbid summer conditions similar to the pattern described for eutrophication. In contrast, lake internal restoration measures can result in transient clear-water conditions both in spring and summer and reversals to turbid conditions. Furthermore, we hypothesize that these contrasting restoration measures result in different macrophyte species composition, with added implications for seasonal dynamics due to differences in plant traits. To test these hypotheses, we analyzed data on water quality and submerged macrophytes from 49 north temperate shallow lakes that were in a turbid state and subjected to restoration measures. To study the dynamics of macrophytes during nutrient load reduction, we adapted the ecosystem model PCLake. Our survey and model simulations revealed the existence of an intermediate recovery state upon reduced external nutrient loading, characterized by spring clear-water phases and turbid summers, whereas internal lake restoration measures often resulted in clear-water conditions in spring and summer with returns to turbid conditions after some years. External and internal lake restoration measures resulted in different macrophyte communities. The intermediate recovery state following reduced nutrient loading is characterized by a few macrophyte species (mainly pondweeds) that can resist wave action allowing survival in shallow areas, germinate early in spring, have energy-rich vegetative propagules facilitating rapid initial growth and that can complete their life cycle by early summer. Later in the growing season these plants are, according to our simulations, outcompeted by periphyton, leading to late-summer phytoplankton blooms. Internal lake restoration measures often coincide with a rapid but transient colonization by hornworts, waterweeds or charophytes. Stable clear-water conditions and a diverse macrophyte flora only occurred decades after external nutrient load reduction or when measures were combined.

The structuring role of submerged macrophytes in a large subtropical shallow lake: Clear effects on water chemistry and phytoplankton structure community along a vegetated-pelagic gradient

It is well known that submerged macrophytes exert positive feedback effects that enhance the water transparency, stabilizing the clear-water state in shallow temperate lakes. However, the structuring effect of macrophytes on the food web of subtropical and tropical ecosystems is still poorly understood. In this study we investigated the influence of dense submerged vegetation beds on the water chemistry and phytoplankton structure along a littoral-pelagic gradient of large subtropical shallow lake in southern Brazil. Seasonal monitoring was carried throughout one year following along a submerged vegetated-pelagic transect in order to analyze the effects of macrophyte’s coverage (percentage of volume infested- PVI) on the water chemistry and phytoplankton community structure. Clear variations on nutrient concentration and phytoplankton biomass/composition could be observed permanently along the transect. Nutrients as orto-phosphate (PO4−) and bicarbonate increased linearly towards the pelagic zone, whereas dissolved organic carbon and humic substances decreased linearly as PVI decreased. Concomitantly, a significant increase in the phytoplankton biomass was observed outwards from the submerged vegetation bed. In the vegetated area, small species (C-R strategists), unicellular flagellates were selected; whereas in the pelagic zone, larger (K-selected) species of cyanobacteria occurred, especially representatives of the functional groups M, LO, SN, S1 and K. Such results indicate that the macrophytes and inherent metabolism, such as potential excretion of dissolved organic compounds with allelochemicals and nutrient uptake from water column influence the structure of the phytoplankton community reducing also significantly the biomass of cyanobacteria within the dense submerged vegetated zone. Because of the continuous growth of macrophytes over the year in low latitude systems, their feed-back effect pattern tends to also dictate a different role in ecosystem dynamics and structure of the food web. These findings contribute to the management and conservation of subtropical and tropical lakes.

Disentangling food webs interactions in the littoral of temperate shallow lakes

Elucidation of food web interactions provides better understanding of ecosystem functioning, indicates anthropogenic impacts which often cause alterations in environmental conditions and detoriorations in feeding network in terrestrial and aquatic ecosystems. The importance of microfauna and macroinvertebrates in littoral zones of shallow waters has poorly been studied regarding their trophic interactions. This study compares invertebrate assemblage and food web interactions among epiphyton, zooplankton and benthic macroinvertebrates across structural heterogeneity in the littoral zone of three temperate shallow water bodies. Submerged and structurally-complex stands of Ceratophyllum demersum inhabited higher abundance of cladocerans and supported higher diversity and biomass of epiphytic protozoans and invertebrates in comparison to floating-leaved Nuphar lutea stands. Analysis of the ecosystem functioning illustrated the increased biomass of macroinvertebrate predators and a preference of predation over planktonic crustaceans in more complex macrophyte stands. Food webs displayed higher complexity and size with habitat heterogeneity, while epiphyton and zooplankton shared the important fraction in food resources among the invertebrate trophic network. Results of food web modelling indicated zooplankton and epiphyton as more vulnerable to invertebrate predation in the complex submerged macrophytes than in floating-leaved macrophyte stands. Integrated community, ecosystem and food web approaches in explanation of complex trophic interactions in the littoral zones justified an increase in diversity and food-web functional complexity with structural heterogeneity of microhabitats.

Phytoplankton response to fish-induced environmental changes in a temperate shallow pond-type lake

AbstractSince 1967, the temperate, shallow, pond-type Lake Warniak has been subjected to different biomanipulation methods including the introduction of common carp,Cyprinus carpioL., grass carp,Ctenopharyngodon idella(Val.), silver carp,Hypophthalmichthys molitrix(Val.), and bighead carp,Hypophthalmichthys nobilis(Richardson) and then their removal in an effort to control macrophytes and phytoplankton. Recently, pilot stocking with predatory fish, particularly pike,Esox luciusL., has also been conducted. Hence, an examination of the long-term response patterns of phytoplankton to multiple fish-induced stressors was undertaken. In recent years,Charadomination (2000-2004) has helped to stabilize a clear-water state, high/good ecological status, and meso-eutrophic conditions. After the disappearance of Charales in 2004, the rapid, unstable changes in phytoplankton biomass, structure, and biodiversity suggested a shift toward a turbid-water state. As a result, the phytoplankton assemblages changed from those dominated by cryptophytes Y+X2+X1+LO(2000-2004) through those dominated by cyanobacteria K (2005-2008), dinoflagellates LO+Y (2009-2011), and cryptophytes Y+LO+F+X2 (2012), to those dominated by diatoms D+K+P+A (2013-2014) with representative taxa that occur in nutrient-rich and/or nutrient-poor water bodies. The 1967-2014 changes indicated that four periods, two with clear-water state and two with turbid-water state, alternately, one after the other, resulted from different fish pressure. Higher autochthonous fish biomass was usually accompanied by lower phytoplankton biomass. In contrast, the introduction of Cyprinidae fish had a stimulating effect on summer phytoplankton dominated by cyanobateria. Among the nutrients, only phosphorus played an important role.

Primary producers or consumers? Increasing phytoplankton bacterivory along a gradient of lake warming and browning

AbstractEukaryotic phytoplankton form the basis of aquatic food webs and play a key role in the global carbon cycle. Many of these evolutionarily diverse microalgae are also capable of feeding on other microbes, and hence simultaneously act both as primary producers and consumers. The net ecosystem impact of such mixotrophs depends on their nutritional strategy which is likely to alter with environmental change. Many temperate lakes are currently warming at unprecedented rates and are simultaneously increasing in water color (browning) due to increased run‐off of humic substances. We hypothesized that the resulting reduction in light intensity and increased bacterial abundances would favor mixotrophic phytoplankton over obligate autotrophs, while higher temperatures might boost their rates of bacterivory. We tested these hypotheses in a mesocosm experiment simulating a gradient of increasing temperature and water color in temperate shallow lakes as expected to occur over the coming century. Mixotrophs showed a faster increase in abundance under the climate change scenario during spring, when they dominated the phytoplankton community. Furthermore, both bacterial abundances and rates of phytoplankton bacterivory increased under future climate conditions. Bacterivory contributed significantly to phytoplankton resource acquisition under future climate conditions, while remaining negligible throughout most of the season in treatments resembling today's conditions. Hence, to our knowledge, we here provide the first evidence for an increasing importance of bacterivory by phytoplankton in future temperate shallow lakes. Such a change in phytoplankton nutritional strategies will likely impact biogeochemical cycles and highlights the need to conceptually integrate mixotrophy into current ecosystem models.

Interplay between climate and hydrogeomorphic features and their effect on the seasonal variation of dissolved organic matter in shallow temperate lakes of the Southern Andes (Patagonia, Argentina): a field study based on optical properties

AbstractThis study analyses the effects of the interplay between climate seasonality and hydrogeomorphic (HGM) lake features on dissolved organic matter (DOM) properties in two neighbouring shallow lakes of Andean Patagonia with different connectivities. The survey was conducted over three years at the end of the wet and dry seasons, assessing the seasonal and interannual variation of dissolved organic carbon (DOC) concentration, whole‐lake DOC mass, and DOM quality, through chromophoric and fluorescent DOM (CDOM and FDOM, respectively) properties. During the wet season (fall–winter), precipitation and runoff increased water discharge, water level, and inputs of terrestrial DOM with high aromaticity, humic content, and high molecular weight in both lakes. Contrastingly, during the dry season (spring–summer), in which photodegradation promoted by high irradiance and stagnant conditions drove DOM transformation, nonhumic, low‐molecular‐weight DOM prevailed. Both lakes displayed synchronicity in their DOC mass and CDOM and FDOM properties, indicative of similar responses to climate forcing, although the overall impact was modulated by their HGM features. Conversely, DOC concentration showed asynchronous responses between lakes, due to the higher intensity of the dilution or evapoconcentration processes in the connected lake, highlighting that DOC concentration is not always sensitive to climate‐driven forces. Overall, this study emphasizes the importance of variables other than DOC concentration, like whole‐lake DOC mass, DOM quality, and HGM features, to better understand the effect of climate variability on DOM dynamics. Our results allow inferring the potential impact of an environmental scenario characterized by lower precipitation and sustained warming on DOM dynamics in Northern Andean Patagonia.